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Geomorphic Change, Hydrology, and Hydraulics of Caulks Creek, Wildwood, Missouri

Jessica Z. LeRoy, David C. Heimann, Kyle D. Hix, Charles V. Cigrand, Tyler J. Burk, B. Abernethy · U.S. Geological Survey

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Prepared in cooperation with the City of Wildwood

Geomorphic Change, Hydrology, and Hydraulics of Caulks Creek, Wildwood, Missouri

Scientific Investigations Report 2024–5079 Version 1.1, November 2024 U.S. Department of the Interior U.S. Geological Survey

Cover. Photograph showing a cut bank on a meander bend in Caulks Creek, with overhanging live roots, taken on March 2, 2022, by Jessica LeRoy, U.S. Geological Survey. Back Cover. Photograph showing a tall, nearly vertical cut bank on a highly sinuous meander bend in Caulks Creek, taken on March 28, 2023, by Mackenzie Marti, U.S. Geological Survey.

Geomorphic Change, Hydrology, and Hydraulics of Caulks Creek, Wildwood, Missouri By Jessica Z. LeRoy, David C. Heimann, Kyle D. Hix, Charles V. Cigrand, and Tyler J. Burk

Prepared in cooperation with the City of Wildwood

Scientific Investigations Report 2024–5079 Version 1.1, November 2024

U.S. Department of the Interior U.S. Geological Survey

U.S. Geological Survey, Reston, Virginia: 2024 First release: September 2024 Revised: November 2024 (ver. 1.1)

For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment—visit https://www.usgs.gov or call 1–888–392–8545. For an overview of USGS information products, including maps, imagery, and publications, visit https:// store.usgs.gov/ or contact the store at 1–888–275–8747. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner. Suggested citation: LeRoy, J.Z., Heimann, D.C., Hix, K.D., Cigrand, C.V., and Burk, T.J., 2024, Geomorphic change, hydrology, and hydraulics of Caulks Creek, Wildwood, Missouri (ver. 1.1, November 2024): U.S. Geological Survey Scientific Investigations Report 2024–5079, 118 p., https://doi.org/10.3133/sir20245079. Associated data for this publication: Dewitz, J., and U.S. Geological Survey, 2021, National Land Cover Database (NLCD) 2019 products (ver. 2.0, June 2021): U.S. Geological Survey data release, https://doi.org/10.5066/P9KZCM54. Heimann, D.C., Cigrand, C.V., Burk, T.J., Hix, K.D., and LeRoy, J.Z., 2024, Archive of hydrologic and hydraulic models used in the simulation of hydraulic characteristics of Caulks Creek, Wildwood, Missouri: U.S. Geological Survey data release, https://doi.org/10.5066/P9PBF12F. Hix, K.D., and LeRoy, J.Z., 2024, Topographic and bank erosion pin data used in monitoring of bank erosion in Caulks Creek, Wildwood, Missouri, 2022–23: U.S. Geological Survey data release, https://doi.org/10.5066/P138PRQL. Iowa State University, 2021, MRMS estimates—GIS raster in ERDAS imagine (.IMG) format: Iowa State University, Iowa Environmental Mesonet, https://mesonet.agron.iastate.edu/rainfall/. LeRoy, J.Z., and Hix, K.D., 2024, Archive of Bank Stability and Toe Erosion Model (BSTEM) simulations of Caulks Creek, Wildwood, Missouri: U.S. Geological Survey data release, https://doi.org/10.5066/P9STTC43. U.S. Geological Survey, 2023, USGS water data for the Nation: U.S. Geological Survey National Water Information System database, https://doi.org/10.5066/F7P55KJN. ISSN 2328-0328 (online)

iii

Acknowledgments This report was prepared with the support of the city of Wildwood. The authors also acknowledge the Metropolitan St. Louis Sewer District for their continued funding of the Caulks Creek at Chesterfield, Missouri, streamgage. Katherine Skalak and Kyle Juracek of the U.S. Geological Survey provided comments on a draft of this report which greatly improved the final product. We thank Henry Doyle, Joshua Keele, Everett Lasher, Mackenzie Marti, Joshua Piggott, Darrel Plank, and Kyle Sadowski of the U.S. Geological Survey for their assistance with field data collection.

Contents Acknowledgments��iii Abstract��1 Introduction��1 Purpose and Scope��3 Description of the Study Area��3 Part 1—Observations of Geomorphic Change in Caulks Creek��5 Methods for Monitoring Geomorphic Change��6 Terrestrial Lidar Scans��6 Total Station Surveys��9 Bank Erosion Pins��9 Study Reach 1 Results��10 Study Reach 2 Results��13 Study Reach 3 Results��13 Study Reach 4 Results��15 Study Reach 5 Results��17 Study Reach 6 Results��18 Bank Erosion Pin Observations��22 Spatial and Temporal Variability of Morphologic Change in Caulks Creek��23 Part 2—Hydrologic and Hydraulic Response of Caulks Creek to Design Storms��25 Methods for Estimating Precipitation-Derived Streamflow��25 Hydrologic Data��25 Hydrologic Model Development��27 Model Scenarios��27 Precipitation Data��28 Transformation Method��29 Loss Method��29 Base-Flow Method��29 Streamflow Routing Method��29 Hydrologic Structures��29 Hydrologic Model Calibration��30 Hydrologic Model Validation��30 Hydrologic Model Sensitivity��30 Methods for Computation of Water-Surface Profiles and Hydraulic Properties��31 Topographic and Bathymetric Data��31 Hydraulic Structures��31 Energy-Loss Factors��32 Hydraulic Model��32 Hydraulic Model Calibration and Validation��33 Hydraulic Model Sensitivity��33 Development of Hydraulic Property Raster Layers��34 Methods for Analysis of Hydraulic Property Raster Layers��35 Development of Channel Centerline and Transect Geometries��35 Extraction of Longitudinal Profiles of Velocity and Shear Stress and Comparison of Model Scenarios��35

Hydrologic and Hydraulic Modeling Results��35 Velocity and Shear Stress in Caulks Creek under Current Climate and Storage Conditions��35 Effects of Projected Climate Change on the Response of Caulks Creek to Design Storms��37 Effects of Additional Storage on the Response of Caulks Creek to Design Storms��40 Part 3—Simulations of Bank Retreat at Key Sites in Caulks Creek��45 Methods for Bank Retreat Simulations��45 Results of Bank Retreat Simulations��46 Summary and Conclusions��55 References Cited��57 Glossary��61 Appendix 1. Terrestrial Light Detection and Ranging Figures��62 Appendix 2. Peak Flow and Total Runoff Volume Tables��85 Appendix 3. Velocity and Shear Stress Tables for Current Conditions and Projected Climate Scenarios��95 Appendix 4. Velocity and Shear Stress Tables for Additional Storage Scenarios��105 Appendix 5. Grain Size Distributions for Sediment Samples��115

Figures 1. 2. 3. 4. 5. 6. 7. 8.

9. 10. 11. 12. 13. 14. 15. 16. 17.

Map showing location of the city of Wildwood and Caulks Creek study basin in St. Louis County, Missouri��2 Map showing locations of study reaches, bank erosion pins, and bank stability and toe erosion model sites��5 Graphs showing field data collection dates overlain on the hydrograph at U.S. Geological Survey streamgage Caulks Creek at Chesterfield, Mo��7 Photographs showing equipment used during each terrestrial light detection and ranging (t-lidar) survey��8 Photographs showing aerial views of study reach 1��11 Photographs showing views of study reach 1��12 Photographs showing aerial views of study reach 2��14 Photograph showing sections of a stormwater runoff pipe have collapsed into the channel in study reach 2 because the bank is eroded at the location indicated in figure 7G��15 Photographs showing aerial views of study reach 3��16 Photographs showing in-channel views of selected features of study reach 3��16 Photographs showing aerial views of study reach 4��18 Photographs showing aerial views of study reach 5��19 Photograph showing riprap along the left cut bank in study reach 5, where overland flow from a stormwater runoff pipe enters the channel��20 Photographs showing aerial views of study reach 6��21 Photographs showing views of study reach 6��22 Map and graph showing location and channel bank changes at bank pin locations��24 Map showing hydrologic and hydraulic model extents of the Caulks Creek study area, Wildwood, Missouri��26

vii

18.

19. 20. 21.

22.

23. 24. 25. 26.

27. 28. 29. 30. 31.

Graphs showing simulated and observed streamflow hydrographs from selected calibration high-flow events at the U.S. Geological Survey streamgage Caulks Creek at Chesterfield, Missouri��31 Graph showing sensitivity of hydrologic model output peak flows to selected model parameters and components��32 Image and graph showing channel characteristics of Caulks Creek��36 Graphs showing smoothed longitudinal profiles of transect-mean velocity and shear stress at the time of peak flow at study reach 3 for selected design storm scenarios under normal and wet antecedent response conditions and current climate conditions��38 Graphs showing smoothed longitudinal profiles of transect-maximum velocity and shear stress at the time of peak flow at study reach 3 for selected design storm scenarios under normal and wet antecedent response conditions and current climate conditions��39 Graphs showing smoothed longitudinal profiles at the time of peak flow at study reach 3 for variations on the CNII, 10-year, 6-hour design storm scenario��41 Graphs showing study area median values plotted against the peak flow at the downstream end of the study area for all scenarios��42 Graphs showing study area standard deviations plotted against the peak flow at the downstream end of the study area for all scenarios��43 Graphs showing near-bank shear stress at bank stability and toe erosion model locations for six design storm scenarios and for flows used in the initial deterministic simulations��47 Graph and photographs showing results and location of bank stability and toe erosion model simulations in study reach 1��50 Graph and photographs showing results and location of bank stability and toe erosion model simulations in study reach 2��51 Graph and photographs showing results and location of bank stability and toe erosion model simulations in study reach 4��52 Graph and photographs showing results and location of bank stability and toe erosion model simulations in study reach 4��53 Graph and photographs showing results and location of bank stability and toe erosion model simulations in study reach 6��54

Tables 1. 2. 3. 4. 5. 6.

Estimated peak-flow statistics for Caulks Creek at U.S. Geological Survey streamgage 06935830 based on an impervious area coverage of 23.4 percent��4 Terrestrial light detection and ranging and total station survey dates��7 Root mean square errors associated with scan-to-scan alignment and georeferencing the study reach point clouds��8 Locations of bank erosion pins included in this study��9 Long-term mean rates of bank retreat and widening in study reach 1 based on aerial photograph analysis, measured at 2015 bend apices��12 Long-term mean rates of bank retreat, channel widening, and narrowing of the meander bend neck in study reach 2, measured at present-day bend apices, based on aerial photograph analysis��15

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7. 8. 9. 10. 11.

12.

13.

14. 15.

16.

17.

18. 19. 20.

Comparison of point elevations surveyed around the two bridge piers in study reach 3��17 Description of U.S. Geological Survey streamgage Caulks Creek at Chesterfield, Missouri��27 Simulated high-flow events used in calibrating and validating the hydrologic and hydraulic models used in this study��27 Description of hydrologic and hydraulic model scenarios developed in the study��28 Simulated and observed hydrograph characteristics of the calibration and validation high-flow events at the U.S. Geological Survey streamgage Caulks Creek at Chesterfield, Missouri��30 Comparison of observed and simulated peak water-surface elevations for selected high-flow calibration and validation events at the U.S. Geological Survey streamgage Caulks Creek at Chesterfield, Missouri��33 Comparison of target and simulated water-surface elevations at continuous pressure transducer loggers for high-flow events on July 28, 2022, and August 4, 2022��34 Hydraulic model sensitivity analyses results��34 Comparison of peak-flow statistics for Caulks Creek from U.S. Geological Survey StreamStats to simulated peak flows for design storms under current climate conditions��37 Results of Pearson correlation tests for local channel width and the absolute value of centerline curvature with the transect-mean and transect-maximum velocity and shear stress for the normal antecedent response condition, 10-year, 6-hour scenario under current climate and storage conditions��40 Ranges in study area medians and standard deviations of the local percent differences between projected future climate conditions and current climate conditions for scenarios with existing storage��44 Ranges in study area medians and standard deviations of the local percentage differences between additional storage conditions and existing storage conditions��45 Summary of grain size analysis from samples collected at bank stability and toe erosion model (BSTEM) locations��48 Bank material properties used in bank stability and toe erosion model simulations��49

Conversion Factors U.S. customary units to International System of Units

Multiply

To obtain

Length foot (ft) mile (mi)

0.3048

meter (m)

1.609

kilometer (km)

Area square mile (mi2)

259.0

square mile (mi2)

2.590

hectare (ha) square kilometer (km2)

Volume cubic foot (ft3)

28.32

cubic decimeter (dm3)

cubic foot (ft3)

0.02832

cubic meter (m3)

acre-foot (acre-ft)

1,233

acre-foot (acre-ft)

0.001233

cubic meter (m3) cubic hectometer (hm3)

Flow rate foot per second (ft/s)

0.3048

meter per second (m/s)

cubic foot per second (ft3/s)

0.02832

cubic meter per second (m3/s)

International System of Units to U.S. customary units

Multiply

To obtain

Length centimeter (cm)

0.3937

inch (in.)

millimeter (mm)

0.03937

inch (in.)

meter (m)

3.281

foot (ft)

kilometer (km)

0.6214

mile (mi)

Area square meter (m2)

0.0002471

acre

square meter (m2)

10.76

square foot (ft2)

Volume cubic meter (m3)

35.31

cubic foot (ft3)

Flow rate meter per second (m/s)

3.281

foot per second (ft/s)

cubic meter per second (m3/s)

35.31

cubic foot per second (ft3/s)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows: °F = (1.8 × °C) + 32. Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows: °C = (°F – 32) / 1.8.

Datums Vertical coordinate information is referenced to the North American Vertical Datum of 1988 (NAVD 88). Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83) or Missouri East 2401 State Plane. Elevation, as used in this report, refers to distance above the vertical datum.

Abbreviations 2D

two-dimensional

three-dimensional

AEP

annual exceedance probability

BSTEM

bank stability and toe erosion model

CMIP

Coupled Model Intercomparison Project

Soil Conservation Service runoff curve number

DEM

digital elevation model

GNSS

Global Navigation Satellite System

HEC–HMS Hydrologic Engineering Center–Hydrologic Modeling System HEC–RAS Hydrologic Engineering Center–River Analysis System M3C2

Multiscale Model to Model Cloud Comparison tool

NAVD 88

North American Vertical Datum of 1988

NSE

Nash-Sutcliffe efficiency

n-value

Manning’s roughness coefficient

p-value

probability value

PBIAS

percentage bias

pressure transducer

storage coefficient

RCP

representative concentration pathway

recurrence interval

RMSE

root mean square error

standard deviation

t-lidar

terrestrial light detection and ranging

time of concentration

total station

USGS

U.S. Geological Survey

WETF

Watershed Erosion Task Force

water year

Geomorphic Change, Hydrology, and Hydraulics of Caulks Creek, Wildwood, Missouri By Jessica Z. LeRoy,1 David C. Heimann,1 Kyle D. Hix,1 Charles V. Cigrand,1 and Tyler J. Burk2

Abstract Caulks Creek is a small stream that flows through the city of Wildwood in western St. Louis County, Missouri. The U.S. Geological Survey, in cooperation with the city of Wildwood, has documented historical and recent geomorphic change along Caulks Creek, simulated the hydrologic and hydraulic response of Caulks Creek to a variety of design storm scenarios, and simulated bank retreat resulting from fluvial erosion and mass failure processes. Six study reaches were selected for monitoring geomorphic change based on known locations of erosion issues documented by the city of Wildwood. Recent short-term rates and patterns of geomorphic change in the study reaches, with a focus on bank retreat, were determined from repeat terrestrial light detection and ranging surveys and field observations of the six study reaches. Historical aerial photographs of the study reaches were analyzed to determine long-term rates of bank retreat and channel widening. In general, rapid bank retreat and widening was observed at the outer banks of meander bends and where banks are unforested. Short-term bank retreat varied substantially within individual study reaches, across the study area, and during the study period from no change to as much as 16 feet of retreat between consecutive surveys (5 to 8 months). The field surveys and visual observations indicated that bank retreat occurs episodically owing to a combination of fluvial erosion and mass failure processes, as well as freezing and thawing cycles. Long-term rates of bank retreat ranged from 0.6 to 4.4 feet per year. Hydrologic and hydraulic models of Caulks Creek were used to quantify the peak, volume, and timing of the flow response and the spatial distribution of hydraulic drivers of erosion (velocity and shear stress) along Caulks Creek for design storm scenarios that represent current (as of this publication) and projected future climate. The projected climate conditions resulted in higher peak flows compared to current conditions, including 6 to 21 percent for the year 2050 and 10 to 42 percent for the year 2099 at the downstream end of the study area. Additionally, for a given design storm, projected 1U.S. Geological Survey. 2Formerly with the U.S. Geological Survey and currently with the U.S.

Office of Surface Mining Reclamation and Enforcement.

climate change is predicted to result in faster flows with greater shear stress, as well as more within-stream variability in velocity and shear stress. Many factors affect the velocity and shear stress at a given location, but in general, somewhat fast velocities and high shear stresses tended to occur where the channel is relatively narrow and straight. The velocity and shear stress in the study reaches (known areas of widening and bank retreat) were not particularly high, at least in part owing to the relatively large widths and high sinuosity of the presentday channel in these reaches. The potential mitigating effect of adding runoff storage to the basin also was examined for a selection of design storm scenarios. Additional runoff storage was more effective at mitigating peak flows and total runoff volumes for higherfrequency, lower-intensity storms than for lower-frequency, higher-intensity storms. The additional storage also resulted in an overall reduction in velocity (by as much as 28 percent) and shear stress (by as much as 40 percent) in the study area. However, the effect of the additional storage on peak flows, total runoff volumes, velocity, and shear stress decreased with distance downstream through the study area. For the simulated scenarios, added runoff storage was effective at mitigating the increases in peak flows, total runoff volumes, velocity, and shear stress caused by projected climate change. Lastly, the bank stability and toe erosion model (BSTEM) was used to predict bank erosion and potential bank failure surfaces at five locations along Caulks Creek for a selection of design storm scenarios. The lower-frequency, highermagnitude design storms resulted in more bank retreat than the higher-frequency, lower-magnitude design storms, though the magnitude of the difference was site dependent. Although scenarios with additional storage were not directly simulated in BSTEM, it is likely that the additional storage would result in reduced bank retreat compared to the same design storm with existing storage, based on the hydraulic modeling results for scenarios with added runoff storage.

Introduction The Caulks Creek basin is a small, suburban watershed located in western St. Louis County, Missouri (fig. 1). The city of Wildwood, Mo., has identified erosion along Caulks

2 Geomorphic Change, Hydrology, and Hydraulics of Caulks Creek, Wildwood, Missouri Creek as a management priority because of the potential effects on stormwater and transportation infrastructure as well as residential, recreational, and commercial property. Erosion concerns along Caulks Creek have been identified as far back as Wildwood’s incorporation in 1995 (Bricker, 2019). In 2019, the city of Wildwood formed a Watershed Erosion Task Force (WETF) to assess issues related to the nine watersheds within the city, including erosion along Caulks Creek. The city of Wildwood has documented reports of erosion in the Caulks Creek basin from more than 40 residents (D. Rahn, Assistant City Engineer, written commun., 2021). Previous studies have also documented the history of erosion along Caulks Creek and, through analyses of aerial photographs and qualitative field observations, identified specific locations of concern

90°45'

90°40'

90°35'

along Caulks Creek (City of Wildwood, 2020; Hammer, 2020a,b). These previous reports highlight the role of urbanization in the Caulks Creek basin in contributing to bank erosion and describe some shared characteristics of the locations of concern. Many of the identified locations of concern were on the outer cut banks of meander bends and in deeply incised channels. Additional locations of concern were at bridge crossings, particularly where vegetation was cleared during bridge construction. Based on previous reports (City of Wildwood, 2020; Hammer, 2020a,b), the WETF and the city of Wildwood sought to quantitatively document bank erosion rates in Caulks Creek and understand the potential effects of future changes to the climate and storage conditions in the basin. Therefore,

90°30'

90°25'

90°20'

90°15'

90°10'

90°05'

Map area Study area

sso

uri

Riv

38°50'

MISSOURI ST CHARLES COUNTY

38°45'

MADISON COUNTY

City of Chesterfield 06935830

St Louis city

Caulks Creek Basin iR

ive

City of Wildwood

Caulks Creek

ssi

38°35'

ST LOUIS COUNTY

38°40'

ST CLAIR COUNTY

FRANKLIN COUNTY

IS IN O

JEFFERSON COUNTY

38°25'

ILL

38°30'

ssi

Bonhomme Creek

Base from U.S. Geological Survey, The National Map, 2021 World Geodetic System 1984 Web Mercator (Auxiliary Sphere) projection. Horizontal coordinate information referrence to the North American Datum of 1983

MONROE COUNTY

0 0

5 5

EXPLANATION 06935830

U.S. Geological Survey streamgage and identifier

Figure 1. Location of the city of Wildwood and Caulks Creek study basin in St. Louis County, Missouri.

10 MILES 10 KILOMETERS

Introduction 3 in 2021, the U.S. Geological Survey (USGS), in cooperation with the city of Wildwood, began a field monitoring effort to document the rates and modes of geomorphic change within six study reaches along Caulks Creek over a 2-year study period. Historical aerial photographs of the study reaches were analyzed to determine long-term rates of channel widening and bank retreat. Additionally, hydrologic, hydraulic, and bank retreat models were developed to examine the response of Caulks Creek to design storms that represent current (as of this publication) and projected future climate scenarios and the effect of additional runoff storage in the basin. The complementary monitoring and modeling efforts provide insight into the processes that drive erosion along Caulks Creek and how these processes may be affected by climate change and (or) additional storage.

Purpose and Scope The purpose of this report is to provide the city of Wildwood, Mo., with context for developing and evaluating measures aimed at mitigating the potential negative effects of future erosion along Caulks Creek through a combined monitoring and modeling study that quantifies recent and long-term rates of geomorphic change and examines the potential effects of climate change and (or) additional storage in the basin. This USGS report advances our understanding of the processes that drive fluvial erosion hazards and provides information to decision makers and the public to help manage resources to meet human and environmental needs (Evenson and others, 2013). Although the results of this study are specific to Caulks Creek, the methods and concepts are broadly applicable for future studies addressing fluvial erosion hazards along small streams. The report is structured in three parts that reflect the specific objectives of this study. The objective of the “Part 1—Observations of Geomorphic Change in Caulks Creek” section is to quantify and describe long-term mean and recent rates and modes of geomorphic change in six study reaches. Long-term rates of channel widening and bank erosion in the study reaches were determined from analysis of historical aerial photographs. Geomorphic change was monitored through repeat terrestrial light detection and ranging (t-lidar) scans and total station (TS) surveys in six study reaches that were selected based on the locations of concern identified by the city of Wildwood (City of Wildwood, 2020; Hammer, 2020a,b). A t-lidar scan or a TS survey was collected in each of the six reaches twice per year in 2022 and 2023. The repeat surveys were compared to determine rates and patterns of geomorphic change, with a focus on bank erosion rates. Additionally, bank erosion pins were installed throughout the study area and measured several times per year during the 2-year study period. The objectives of the “Part 2: Hydrologic and Hydraulic Response of Caulks Creek to Design Storms” section are to compare the hydraulic drivers of fluvial erosion in Caulks Creek under current versus predicted future climate scenarios

and to assess the effect of additional runoff storage in the Caulks Creek basin on the hydraulic drivers of fluvial erosion. Calibrated hydrologic and two-dimensional (2D) hydraulic models were used to simulate streamflows and hydraulics for 60 design storms including all combinations of the following: (a) storm recurrence intervals (RI), with annual exceedance probability (AEP) in parentheses: 2-year (0.5), 5-year (0.2), 10-year (0.1), 25-year (0.04), 50-year (0.02), and 100-year (0.01); (b) storm durations: 6-hour and 24-hour; and (c) climate scenarios: current climate and projected future climate conditions for years 2050 and 2099 under representative concentration pathways (RCPs) 4.5 and 8.5. RCP 4.5 is considered an intermediate climate-change scenario and RCP 8.5 is considered a “worst-case” climate-change scenario. Hydrologic and 2D hydraulic simulations were completed for all 60 design storms under normal and wet antecedent response conditions, resulting in 120 simulations. An additional 72 simulations were completed to examine the effects of additional storage for design storm scenarios representing current climate and the predicted future climate in 2050 under RCP 4.5. Lastly, the objective of the “Part 3—Simulations of Bank Retreat at Key Sites in Caulks Creek” section is to examine potential scenarios of bank retreat resulting from fluvial erosion and mass failure processes at five selected locations within the geomorphic change study reaches. A subset of the design storm scenarios was selected for simulating potential bank retreat at these five locations that includes all combinations of the following under normal antecedent response conditions, with no additional storage: (a) storm RI, with AEP in parentheses: 2-year (0.5), 10-year (0.1), and 100-year (0.01); (b) storm duration: 6-hour; and (c) climate scenarios: current climate and predicted future climate in 2099 under RCP 8.5.

Description of the Study Area This study focuses on the Caulks Creek basin in western St. Louis County, Mo. (fig. 1). The drainage area of Caulks Creek is 17.1 square miles (mi2) at the USGS streamgage 06935830 (Caulks Creek at Chesterfield, Mo.; USGS, 2023) and 19.3 mi2 at its confluence with Bonhomme Creek (USGS, 2022b). Prior to development, the Caulks Creek basin was characterized by steep, forested slopes with relief on the order of 150 feet (ft) (Hammer, 2020a). A wave of suburban development occurred in the 1990s, and by 2001, 18.1 percent of the Caulks Creek basin was covered with impervious surfaces (USGS, 2022b). By 2019, the impervious area had increased to 23.4 percent (Dewitz and U.S. Geological Survey, 2021). Development in the Caulks Creek basin was followed by stream incision and widening, in some cases resulting in an increase in channel cross-sectional area from 40 to 100 square feet to more than 800 square feet in less than a decade (Hammer, 2020a). The mean annual precipitation in Missouri Climate Division 2, which includes the city of Wildwood, was 41.35 inches during 1991–2020 (Midwest Regional

4 Geomorphic Change, Hydrology, and Hydraulics of Caulks Creek, Wildwood, Missouri Table 1. Estimated peak-flow statistics for Caulks Creek at U.S. Geological Survey streamgage 06935830 (Caulks Creek at Chesterfield, Missouri) from U.S. Geological Survey (2022b), based on an impervious area coverage of 23.4 percent. [AEP, annual exceedance probability]

Recurrence interval (years)

Flow (cubic feet per second)

2,200

20-percent AEP flood

3,430

10-percent AEP flood

4,500

4-percent AEP flood

5,680

2-percent AEP flood

7,090

1-percent AEP flood

100

8,150

0.2-percent AEP flood

500

11,400

Peak-flow statistic 50-percent AEP flood

Climate Center, 2024). Monthly precipitation tends to increase from January to May, then decrease from May to December (Midwest Regional Climate Center, 2024). The hydrologic and geomorphic character of Caulks Creek gradually changes from upstream to downstream. The upstream-most reaches of Caulks Creek are ephemeral, meaning they are dry most of the year and flow only in response to precipitation. Moving downstream, isolated pools of standing water begin to appear in the deepest scour holes, then become more and more common until they roughly trace the channel thalweg (deepest part of the river channel). The gradual transition into the perennially flowing lower reach of Caulks Creek occurs near the border of the city of Wildwood with the city of Chesterfield, Mo. At USGS streamgage 06935830 (fig. 1), the mean annual flow was 23 cubic feet per second (ft3/s) for water years (WYs) 1997 to 2015 (Granato and others, 2017). Annual peak flows at USGS streamgage 06935830 ranged from 1,120 to 7,940 ft3/s for the period of record (WYs 1972

to 1979 and 1997 to 2022; USGS, 2023), and estimated peakflow statistics at this streamgage location from USGS StreamStats are given in table 1 (USGS, 2022b). Analysis of annual peak flows at USGS streamgage 06935830 using the MannKendall’s trend test (Kendall, 1975) indicated no significant change during the period of record through 2022 (tau=−0.096, probability value [p-value] = 0.432). In the ephemeral upper reach of Caulks Creek, relatively straight sections are interspersed with high-sinuosity meander bends. The perennial lower reach is more consistently sinuous. Overall, the channel widens with distance downstream and the bed material gradually fines from a mix of angular chert cobbles, pebbles, and gravel to predominantly pebbles, gravel, and sand. The chert cobbles and pebbles are derived from coarse channel lag deposits in the banks (Hammer, 2020a,b). In some locations, large pieces of riprap have become dislodged from the bank and are located on the channel bed.

Part 1—Observations of Geomorphic Change in Caulks Creek 5

Part 1—Observations of Geomorphic Change in Caulks Creek

of concern identified by the city of Wildwood (City of Wildwood, 2020; Hammer, 2020a,b). The study reaches are in the ephemeral upper reach of Caulks Creek and are predominantly in meander bends or at bridges (fig. 2). An additional identified location of concern was not surveyed because of access constraints.

Six study reaches within the study area were selected for field monitoring of geomorphic change based on the locations

90°37'30"

90°37'

90°36'30"

38°38'30"

EXPLANATION Study reach Caulks Creek network

12 Bank erosion pin and identifier

Study Reach 6

Bank Stability and Toe Erosion Model site 38°38'

11 10

Study Reach 5

38°37'30

7 6

Study Reach 4

Study Reach 3 38°37'

Flow 4 3

Study Reach 2

38°36'30"

Caulks Creek Basin

Study Reach 1

Map area

38°36'

Base from Farm Service Agency, National Agricultural Imagery Program, 2020. Universal Transverse Mercator projection, Zone 15 North. Horizontal coordinate information referenced to the North American Datum of 1983

0 0

2 2

4 MILES 4 KILOMETERS

Figure 2. Locations of study reaches, bank erosion pins, and bank stability and toe erosion model (BSTEM) sites.

6 Geomorphic Change, Hydrology, and Hydraulics of Caulks Creek, Wildwood, Missouri

Methods for Monitoring Geomorphic Change Descriptions of historical geomorphic change and present-day geomorphology in the six study reaches were developed based on previous reports provided to the WETF (Hammer, 2020a,b), aerial photographs from the USGS Earth Resources Observation and Science Center archive of the National Aerial Photography Program and High-Resolution Orthoimagery (USGS, 2022a), the 2017 USGS National Elevation Dataset 1-meter (m) digital elevation model (DEM) (USGS, 2018), and visual observations made in the field in 2022 and 2023. Long-term (25-year) bank retreat rates were determined by measuring the distance from the bank to a fixed object (such as the edge of a road, sidewalk, or building) along a fixed angle in aerial photographs taken in 1990 and 2015. Similarly, long-term (25-year) changes in channel width were determined by measuring the distance between the banks in aerial photographs taken in 1990 and 2015 (measured perpendicular to the banks). The 1990 and 2015 aerial photographs were captured during leaf-off conditions, which facilitates identification of the channel banklines, in contrast to more recent imagery such as the 2020 aerial photographs. These long-term rates do not necessarily indicate a constant rate of change over time; it is likely that the change observed from one aerial photograph to the next occurred episodically. Unless otherwise specified, the bank retreat and widening rate measurements were made at the meander bend apex and may differ from the rates of change at other locations along the bend. These are point measurements of rates of bank retreat or widening and are not reach-averaged values. The bank retreat rate may exceed the widening rate at a particular location if bar deposition is occurring along the opposite bank during the same time period. The method used for the bank retreat and widening rate measurements minimizes potential errors owing to differences in the georectification of the two sets of aerial photographs. However, there is some subjectivity in identifying the bank position, particularly in older images with lower resolution, so these measurements are considered approximate. Short-term rates of geomorphic change were determined by comparing four repeat surveys of each study reach collected between early spring of 2022 and summer of 2023. Most of the study reaches were surveyed with t-lidar, whereas TS surveys were used in study reach 3 in 2023 to facilitate measurements of bed elevation in areas with standing water. Study reaches were surveyed in early spring 2022 (February/ March), summer 2022 (July/August), early spring 2023 (February/March), and summer 2023 (July) (fig. 3, table 2). The 2022 surveys bracketed several moderate flow events and a high-flow event in May 2022 (fig. 3A). Study reach 2 was surveyed after the WY 2022 annual peak flow in August 2022, but the other study reaches were surveyed prior to that event (fig. 3A). The 2023 surveys also bracketed several moderate flow events, including the annual peak flow in April 2023. Additionally, 14 bank erosion pins were installed in February/ March 2022 and were re-measured five times, with the final measurements in July 2023. Additional details on the t-lidar

scans, TS surveys, and bank erosion pin measurements are provided in the sections below, and these data are available in Hix and LeRoy (2024).

Terrestrial Lidar Scans During each t-lidar survey, a tripod-mounted FARO Focus3D X-130 scanner (fig. 4A) was used to generate threedimensional (3D) point clouds representing the bare earth surface of the bank face and portions of the channel bed. The FARO Focus3D X-130 scanner has a stated accuracy of 2 millimeters (mm) at a distance of 25 m (FARO Technologies, Inc., 2015). Each study reach required multiple t-lidar scans to cover the entire area of interest. Target spheres (fig. 4B) were set up throughout the field of view of the scanner to facilitate “stitching” adjacent scans together. Some of the target spheres were placed at known heights above surveyed control points and used to georeference the point cloud (assign real-world coordinates to individual points in the point cloud) during post-processing. The surveyed control points were established to Level II Global Navigation Satellite System (GNSS) quality (Rydlund and Densmore, 2012) using a Trimble R10 or R12 GNSS connected to the Missouri Department of Transportation Virtual Reference System, sometimes in combination with a Trimble M3 TS (Trimble Navigation Limited, 2013). Scans provided by the tripod-mounted t-lidar scanner and control spheres were grouped by study reach and postprocessed in CloudCompare version 2.12.4 (CloudCompare, 2022). The individual scans in each study reach were stitched together into point clouds by identifying the same spheres in adjacent scans using the Point-Pair Registration tool. The study reach point clouds were then georeferenced to the Missouri East 2401 state plane horizontal coordinate system and North American Vertical Datum of 1988 (NAVD 88) elevations using the Point-Pair Registration tool to align control point target spheres with the surveyed control points. The root mean square errors (RMSEs) associated with aligning the scans and georeferencing the study reach point clouds are given in table 3. The study reach point clouds were subsampled to a minimum spacing of 0.0328 ft to reduce processing demand using the space method in the Subsample tool. The subsampled study reach point clouds initially were classified using the CANUPO tool in CloudCompare (Brodu and Lague, 2012). Two classifiers were created, one for the February/March surveys and one for the July/August surveys, by manually sampling surface and nonsurface (for example, branches, leaves, grass) points within a subsection of a point cloud. The classifiers were then applied to split the study reach point cloud into surface and nonsurface point clouds. The initial classification was manually refined by removing points that were wrongly classified from one cloud and combining them with the correct cloud. Points representing infrastructure (for example, bridges, culverts, and pipes) were included in the surface point clouds, though bridges were later excluded from comparisons of the point clouds. The surface point cloud

Part 1—Observations of Geomorphic Change in Caulks Creek 7 104

A Bank pin installation

EXPLANATION 103

Study Reaches 4, 5, and 6

5-minute discharge Study Reach 2

Study Reaches 1, 3, 4, 5 and 6

Study Reaches 1, 2, and 3

Terrestral light detection and ranging survey Bank pin survey

102

Discharge, in cubic feet per second

101

100 Oct.

Nov.

Dec.

Jan.

Feb.

Mar.

Apr.

May

June

July

Aug.

Sept.

Aug.

Sept.

Water year 2022 104

All study reaches All study reaches

103

102

101

100 Oct.

Nov.

Dec.

Jan.

Feb.

Mar.

Apr.

May

June

July

Water year 2023

Figure 3. Field data collection dates overlain on the hydrograph at U.S. Geological Survey streamgage Caulks Creek at Chesterfield, Mo. (06935830). A, Water year 2022. B, Water year 2023.

Table 2. Terrestrial light detection and ranging (t-lidar) and total station survey dates. [t-lidar, terrestrial light detection and ranging]

Study reach

Survey 1 date (method)

Survey 2 date (method)

Survey 3 date (method)

Survey 4 date (method)

Study reach 1

February 14 and 16, 2022 (t-lidar)

July 14, 2022 (t-lidar)

March 1, 2023 (t-lidar)

July 25, 2023 (t-lidar)

Study reach 2

February 15, 2022 (t-lidar)

August 9, 2022 (t-lidar)

February 28, 2023 (t-lidar)

July 24, 2023 (t-lidar)

Study reach 3

February 16, 2022 (t-lidar)

July 12, 2022 (t-lidar)

February 27, 2023 (total station) July 25, 2023 (total station)

Study reach 4

March 2, 2022 (t-lidar)

July 13, 2022 (t-lidar)

March 1, 2023 (t-lidar)

July 27, 2023 (t-lidar)

Study reach 5

February 28, 2022 (t-lidar)

July 15, 2022 (t-lidar)

March 2, 2023 (t-lidar)

July 27, 2023 (t-lidar)

Study reach 6

March 1 and 2, 2022 (t-lidar) July 12 and 13, 2022 (t-lidar) February 27, 2023 (t-lidar)

July 26, 2023 (t-lidar)

8 Geomorphic Change, Hydrology, and Hydraulics of Caulks Creek, Wildwood, Missouri A

Figure 4. Equipment used during each terrestrial light detection and ranging (t-lidar) survey. A, Tripod-mounted FARO Focus3D t-lidar scanner. B, Control spheres used in georeferencing t-lidar scans. Photographs by Jessica LeRoy, U.S. Geological Survey. Table 3. Root mean square errors associated with scan-to-scan alignment and georeferencing the study reach point clouds. Study reaches 1, 2, and 6 were divided into sections, labeled alphabetically from upstream to downstream and noted below the study reach number in parentheses. [All values in table are measured in feet. RMSE, root mean square error; SR, study reach; --, no data]

SR1 (A–E)

SR1 (F–G)

SR2 (A)

Scan-to-scan alignment

0.057

0.059

Georeferencing

0.291

0.293

RMSE type

SR2 (B)

SR2 (C–D)

SR4

SR5

SR6 (A–C)

SR6 (D–E)

SR6 (F)

0.030

0.013

0.078

0.010

0.084

0.131

0.165

0.178

SR3

February/March 2022 surveys

July/August 2022 surveys Scan-to-scan alignment

0.009

0.006

0.005

0.007

0.006

0.009

Georeferencing

0.510

0.158

0.082

0.098

0.152

0.213

0.267

0.058

Scan-to-scan alignment

0.032

0.005

0.105

0.006

0.013

0.132

0.011

0.009

Georeferencing

0.287

0.058

0.131

0.086

0.108

0.306

0.048

0.088

February/March 2023 surveys

July 2023 surveys Scan-to-scan alignment

0.025

0.009

0.017

0.006

0.019

0.009

0.008

0.005

Georeferencing

0.447

0.160

0.066

0.378

0.628

0.066

0.147

0.106

0.127

0.090

Part 1—Observations of Geomorphic Change in Caulks Creek 9 normals, which describe the orientation of the surface at any given location, were then computed using the CloudCompare Normals tools (Compute Normals with a quadric surface function, Orient Normals with a Minimum Spanning Tree, and if needed, Invert Normals to change the normal orientation). The surface point clouds from consecutive surveys were compared using the Multiscale Model to Model Cloud Comparison (M3C2) tool (Lague and others, 2013). The M3C2 tool uses surface normals to measure the amount of surface change between point clouds, perpendicular to the local surface, in three dimensions. Therefore, the M3C2 tool can describe erosion and deposition of a complex 3D surface, like a river channel, where other methods may either introduce bias or limit detection of fine details (Lague and others, 2013). For study reaches 1, 3, 4, 5, and 6, the measured change between consecutive surveys was projected onto the surface cloud from the earlier survey for visualization. For study reach 2, the change was projected onto the surface cloud from the later survey owing to the substantial amount of change at that site. Any surface change values less than the maximum RMSE associated with the two surveys (table 3) were within the measurement error. Negative values of determined change in surveys indicated surface retreat outward from the channel (either bank retreat or bed degradation) and positive values of change indicated advance inward toward the channel (either bank advance or bed aggradation).

Total Station Surveys Study reach 3 was surveyed with a Trimble M3 TS (Trimble Navigation Limited, 2013) rather than t-lidar in February 2022 and July 2022 owing to water obscuring the bed change near the bridge from the previous t-lidar surveys. The TS was set up using GNSS surveyed control points.

Bed elevation points were measured around the bridge pier. Bed change around the piers was determined by comparing nearby points.

Bank Erosion Pins Bank erosion pins (2-ft lengths of 0.38-inch steel rebar) were placed at 14 locations throughout the study area (fig. 2, table 4). Two of the installed bank pins, both located downstream from the main study area, could not be found after installation and are not included in this analysis. Most of the bank erosion pins were located outside of the six study reaches, though one was located within study reach 4 and two were located within study reach 5. The bank erosion pin locations were largely determined by site access and the feasibility of inserting the pin into the bank face and are not intended to be a statistically representative sampling of the channel. The rebar was inserted part way up the bank face (table 4) until no more than 2.5 inches of the rebar remained visible. The tip of the pin represents a datum from which a change in the bank position can be measured. The distance from the tip of the pin to the bank face was measured on the top, bottom, upstream side, and downstream side of the pin, and these measurements were averaged to obtain a final measurement value. After their initial installation on February 28– March 3, 2022, the pins were remeasured on May 23, 2022; July 13–15, 2022; October 20, 2022; February 27–March 1, 2023; and July 24, 2023. If substantial erosion had occurred around a pin and the banks were not frozen, the pin was removed, reinstalled within 3 ft of the original location, and new measurements were recorded for comparison with the next survey. Pins were relocated rather than inserted farther into the bank after some initial testing indicated that attempting to push the pins in at their existing location was physically

Table 4. Locations of bank erosion pins included in this study. Bank pin number

Bank

Latitude

Longitude

Height above bank toe (feet)

Distance to top of bank (feet)

Left

38.600777

−90.615692

1.4

3.4

Right

38.601441

−90.616419

3.0

1.8

Left

38.613491

−90.617941

2.3

No dataa

Left

38.614312

−90.618261

1.2

0.3

Left

38.620419

−90.615348

2.3

5.1

Left

38.622451

−90.615069

1.8

7.0

Right

38.624261

−90.614579

1.4

4.0

Left

38.625433

−90.616665

5.9

5.0

Right

38.626058

−90.616615

6.7

5.1

Right

38.626796

−90.617190

3.1

3.8

Right

38.627067

−90.617317

1.9

3.9

Right

38.633406

−90.623156

5.4

No dataa

aTop of bank was inaccessible for measurement.

10 Geomorphic Change, Hydrology, and Hydraulics of Caulks Creek, Wildwood, Missouri difficult to do without causing the pin to shift in place, which hollowed out the sediment around the pin. The change in bank position was determined between consecutive pairs of measurements by subtracting the more recent measurement from the earlier measurement. Negative values indicate bank retreat away from the channel centerline, and positive values indicate bank advance inward toward the channel centerline.

Study Reach 1 Results Study reach 1 is a series of meander bends located upstream from the Clayton Road bridge crossing, between Woodlawn Chapel Presbyterian Church and Hope Montessori Academy (fig. 5). Study reach 1 is split into seven sections, A through G (fig. 5H), for use in the further discussion of the historical aerial photographs and the t-lidar survey results. The banks are mostly at the bottom of forested slopes, with the exception of the left bank (as viewed looking downstream) across from the Winding Trails tennis courts (section A in fig. 5H) and the right bank adjacent to Woodlawn Chapel Presbyterian Church (section F in fig. 5H), which are next to flat grassy fields. Bank erosion and bar growth have resulted in increased sinuosity, translation of bends downstream, and widening in this reach since the 1990s (Hammer, 2020b; fig. 5). Long-term bank retreat rates based on aerial photograph analyses in study reach 1 range from about 1.1 to 4.0 feet per year (ft/yr) and widening rates range from 0.8 to 2.7 ft/ yr at the 2015 apices of the meander bends (table 5). The fastest long-term bank retreat rate was observed in section C, where the downstream translation of bends has resulted in a complete change in orientation of the bend since 1990, such that the current (2023) outer bank is the left bank. Bank retreat also has occurred rapidly along the outer banks of the other bends in the reach, particularly along the fine-grained, unforested banks in sections A and F (fig. 5; fig. 6A). Large point bars also developed opposite the cut banks in sections A and F (labeled in fig. 5G). The banks in sections B–E are more heavily vegetated (fig. 6B) and include occasional bedrock outcroppings at the toe, though bank retreat has still occurred in these sections (table 5). In particular, field observations indicate that scour is occurring around stormwater runoff pipes in sections D (fig. 6C) and E (fig. 6D). Riprap boulders were installed along the left bank in the downstream portion of section E and in section G sometime between 1996 and 2002 (fig. 5C), which artificially narrowed the channel in these locations. Some of these boulders have become dislodged and are now located in the channel, though overall the channel bank position in this location has not changed substantially since the riprap was installed. T-lidar surveys of study reach 1 were completed on February 14 and 16, 2022 (sections F–G and sections A–E, respectively); July 14, 2022; March 1, 2023; and July 25, 2023. Consecutive surveys were compared to quantify bank retreat and bed aggradation/degradation. The comparisons of these t-lidar surveys are shown in the appendix 1 figures.

Throughout the study period, bank retreat was observed within sections A–F (including the nonriprapped portion of section E), whereas the riprapped portions of the banks in sections E and G were relatively stable (figs. 1.1–1.7). However, the amount of bank retreat varied substantially throughout the study reach and during the four t-lidar surveys. During the period of t-lidar monitoring, sections A and F showed the greatest amounts of bank retreat in study reach 1 along their outer banks at and downstream from the bend apices (figs. 1.1 and 1.6). In section A, the left bank retreated about 2 to 3 ft (local maximum retreat=4 ft) for February to July 2022, about 1 to 2.5 ft (local maximum retreat=4 ft) for July 2022 to March 2023, and about 1 to 3.5 ft (local maximum retreat=5 ft) for March to July 2023 (fig. 1.1). In section F, the left bank retreated about 2 to 5 ft (local maximum retreat=5.5 ft) for February to July 2022, about 0.5 to 1.5 ft (local maximum retreat=3.5 ft) for July 2022 to March 2023, and about 1 to 5 ft (local maximum retreat=6.3 ft) for March to July 2023 (fig. 1.6). In sections A and F, deposition also occurred along the bank toe or in the thalweg, which was below the area of bank retreat. This pattern may indicate that material from the top of the bank deposited on the bank toe or in the thalweg and had not yet been removed by flow in the channel. The banks of the meander bends in sections B, C, D, and E are at the bottom of heavily forested slopes, and accumulations of large woody debris and vegetation obscured parts of those banks during the t-lidar surveys. Where the banks were exposed enough for the t-lidar to penetrate, erosion was observed along the outer banks of the meander bends, including retreat and scour at the base of the banks and in the thalweg (section B right bank, fig. 1.2; section C left bank, fig. 1.3; section D right bank, fig. 1.4; and section E left bank, fig. 1.5). In particular, in section E, the t-lidar scans show the progression of scour around a stormwater runoff pipe, resulting in the pipe cracking and collapsing (figs. 1.5E–H, pipe is also pictured in fig. 6D). However, similar to the observations from aerial photographs, the riprapped portion of the left bank of section E (labeled in fig. 1.5A) was relatively stable during the study period. The changes in the channel bed observed in study reach 1 (figs. 1.1–1.7) can be interpreted to indicate active sediment transport during the flow events that occurred between surveys. In a general sense, the patterns of aggradation (increase of bed elevation) and degradation (decrease of bed elevation) in a river channel are tied to a combination of the spatial and temporal variations in the capacity of the flow to move sediment and the incoming sediment supply. Although measurable bed-elevation changes occurred throughout study reach 1 and in the other study reaches, it is unclear whether these changes indicate longer-term trends or whether they simply reflect the short-term variability in flows and sediment supply. Therefore, detailed descriptions of the bed-elevation changes have been excluded from this report, except where they may be related to specific process-based interpretations. For example, in section A, the deposition along the downstream edge of the point bar between July 2022 and July 2023 (fig. 1.1B and C) is

Part 1—Observations of Geomorphic Change in Caulks Creek 11 A

60 feet

40 feet

Riprap

Flow 195 feet

2004

2002 G

H CL

1996

1990

175 feet

185 feet

200 feet

Woodlawn Chapel Presbyterian Church

70 feet

80 feet

110 feet

Hope M onte Academ ssori y

Point bar

110 feet

E D

C B 150 feet

115 feet

134 feet

Point bar

95 feet

Winding Trails tennis courts

2010

2007 0 0

2015 500

100

1,000 FEET 200 METERS

2020

Figure 5. Aerial views of study reach 1 from U.S. Geological Survey (2022a). A, 1990. B, 1996. C, 2002. D, 2004. E, 2007. F, 2010. G, 2015. H, 2020.

12 Geomorphic Change, Hydrology, and Hydraulics of Caulks Creek, Wildwood, Missouri Table 5. Long-term mean rates of bank retreat and widening in study reach 1 based on aerial photograph analysis, measured at 2015 bend apices. [--, no data]

Apex long-term bank retreat rate (feet per year)

Apex long-term widening rate (feet per year)

3.2

1.6

1.1

0.8

4.0

1.9

2.6

2.7

Section

aRiprap was installed along the left bank of sections E and G, resulting in an artificially nar-

rower channel in 2015 compared to 1990, so no bank retreat or widening rates are reported for these sections.

Figure 6. Views of study reach 1. A, Section A left bank, viewed looking downstream. B, Section B right bank, viewed looking upstream. C, Stormwater runoff pipe on section D right bank, viewed looking toward the bank and slightly downstream. D, Stormwater runoff pipe on the left bank in section E, viewed looking downstream. Refer to figure 5 for section location. Photographs by Jessica LeRoy, U.S. Geological Survey.

Part 1—Observations of Geomorphic Change in Caulks Creek 13 consistent with the long-term observations of point bar growth concurrent with bank retreat and widening determined from the aerial photographs.

Study Reach 2 Results Study reach 2 is a series of highly sinuous meander bends located downstream from the Clayton Road bridge crossing, which are referred to as the “triple meander” in city of Wildwood and WETF documents (fig. 7; City of Wildwood, 2020). These bends are characterized by tall, nearly vertical cut banks that are undercut in some locations and prone to mass failures (Hammer, 2020a). Study reach 2 is split into four sections, A through D (fig. 7), for use in the further discussion of the historical aerial photographs and the t-lidar survey results. The long-term modes of bend migration and rates of bank retreat and meander neck narrowing in study reach 2 were derived from aerial photograph analysis (table 6, fig. 7). Overall, the channel in study reach 2 has widened since the 1990s. The section A bend has rotated in the downstream direction and may be beginning to develop into a doubleheaded meander (a bend with two local maxima in curvature, labeled in fig. 7E). The long-term mean bank retreat rate of the apex of the bend in section A is 4.4 ft/yr (table 6). Bank retreat in section A resulted in the collapse of a stormwater runoff pipe that empties into Caulks Creek (Hammer, 2020a; fig. 8, approximate location labeled in fig. 7G). The bend in section B has become double-headed during the last few decades. The long-term bank retreat rate at the apex of the bend in section B is 1.2 ft/yr (table 6). Prior to 2007, the bend in section C migrated toward Strecker Road and also rotated downstream. To protect Strecker Road from further erosion, the right cut bank of section C was laid back to a lower angle and the toe was covered with riprap in 2007–08, so bank retreat/widening rates are not reported for this section (fig. 7E). The long-term bank retreat rate at the apex of section D is 0.6 ft/yr. The necks of the meander bends in this reach (Necks A, B, and C, labeled in fig. 7H) have narrowed at rates ranging from 1.3 to 4.2 ft/ yr (table 6), prompting concerns about the potential for neck cutoffs in this reach (Hammer, 2020a). T-lidar surveys of study reach 2 were completed on February 15, 2022; August 9, 2022; February 28, 2023; and July 24, 2023 (figs. 1.8–1.11). Unlike the other study reaches, the second survey of study reach 2 occurred after the WY 2022 peak flow on August 4, 2022 (fig. 3). Widespread bank retreat was observed during the study period along the steep cut banks of sections A, B, and D of study reach 2. Some of the greatest rates of bank retreat observed in the Caulks Creek study area were documented in section A. In section A, bank retreat about 1 to 5 ft and as much as 8 to 16 ft in parts of the bend occurred along the right bank between February and August 2022 (fig. 1.8A). Such rapid bank retreat is likely the result of fluvial erosion destabilizing the bank and resulting in widespread mass failures. An incipient gully feature is

also present on the right bank (labeled in fig. 1.8) that eroded throughout the study period, forming a roughly triangular indentation on the bank. Although some aggradation occurred along the edge of the point bar (less than [<]1 ft), there was no major accumulation of sediment at the bank toe between February and August 2022, which may indicate that the eroded bank material was transported out of the bend during the highflow events. Between August 2022 and February 2023, an isolated mass failure caused bank retreat of as much as 8 ft and deposited a pile of sediment immediately beneath the failure (annotated in fig. 1.8B). Otherwise, bank retreat in section A between August 2022 and February 2023 was generally limited to the lower portion of the bank (approximately 1 to 3 ft). Substantial bank retreat of about 1 to 5 ft in section A occurred once again between February and July 2023, and a new mass failure resulted in as much as 8 to 14 ft in one location (fig. 1.8C). The location of the prior mass failure continued to erode, forming new accumulations of sediment at the bank toe. In section B, retreat of the left bank of about 1 to 2 ft and as much as 6 ft occurred between February and August 2022 (fig. 1.9A). Minimal change occurred in section B between August 2022 and February 2023, except within some isolated locations of erosion of the bank toe and bed scour (fig. 1.9B). Bank retreat resumed in section B between February 2023 and July 2023, generally about 1 to 2 ft and as much as 4 ft (fig. 1.9C). As noted above, section C was laid back to a lower angle and the toe was covered with riprap in 2007–08. Brushy vegetation and trees obscured much of the banks in section C during the t-lidar surveys. Along the lower portions of the section C banks where t-lidar data collection was possible, minimal change was observed during the study period (fig. 1.10). Some scour (0.5 to 2 ft) occurred in the thalweg near the section C bend entrance and at the bend apex between February and August 2022 and between February and July 2023. In section D, between February and August 2022, the lower portion of the left bank in section D retreated approximately 0.5 to 1.5 ft with a local maximum retreat of about 5.5 ft (fig. 1.11A). Minimal change occurred in section D between August 2022 and February 2023 (fig. 1.11B). Between February and July 2023, retreat of the left bank of about 0.25 to 1.5 ft was observed throughout section D (fig. 1.11C). A few isolated locations in section D showed retreat of about 4 ft between February and July 2023; where the retreat is high on the bank, this likely represents mass failures, and where retreat is low on the bank, this likely represents erosion of the bank toe during high-flow events.

Study Reach 3 Results Study reach 3 surrounds the Strecker Road bridge crossing at the apex of a broad, low-sinuosity meander bend near the intersection of Strecker Road with McBride Pointe Drive (fig. 9). When the bridge was constructed in 2002, riprap was placed at the bridge abutments and downstream

14 Geomorphic Change, Hydrology, and Hydraulics of Caulks Creek, Wildwood, Missouri C

Flow

200 feet 250 feet

272 feet

1990

1996

2004

2002 G

ST AD

D Neck C

95 feet 170 feet

Neck B

B Incipient downstream rotation Incipient double-heading

A Neck A

240 feet

AY TO

Stormwater runoff pipe

2007

2010 0 0

2015 500

100

1,000 FEET 200 METERS

2020

Figure 7. Aerial views of study reach 2 from U.S. Geological Survey (2022a). A, 1990. B, 1996. C, 2002. D, 2004. E, 2007. F, 2010. G, 2015. H, 2020.

Part 1—Observations of Geomorphic Change in Caulks Creek 15 Table 6. Long-term mean rates of bank retreat, channel widening, and narrowing of the meander bend neck in study reach 2, measured at present-day bend apices, based on aerial photograph analysis. [--, no data]

Apex long-term bank retreat rate (feet per year)

Apex long-term widening rate (feet per year)

Meander neck narrowing rate (feet per year)

4.4

3.8

1.3

1.2

1.0

3.2

4.2

0.6

0.0

Section

aThe right bank of section C was laid back to a lower angle and riprap was installed along the toe in section C in 2007–08, resulting in an artificial change in the position of the bank in 2015 compared to 1990.

Section of a stormwater runoff pipe

Figure 8. Sections of a stormwater runoff pipe have collapsed into the channel in study reach 2 because the bank is eroded at the location indicated in figure 7G (photographs by Jessica LeRoy, U.S. Geological Survey).

from the bridge along the left bank (fig. 9B), and the banks were planted with vegetation intended to help prevent erosion (Hammer, 2020b). This vegetation was subsequently removed (date unknown) and invasive shrubs began growing in its place (Hammer, 2020b). There is a remnant concrete structure in the thalweg of the channel on the upstream side of the bridge (fig. 10A), which dates back at least to 1966 and may have been a low water crossing (J. Vujnich, City of

Wildwood Director of Planning and Parks, written commun., 2021). There are two round bridge piers located in the center of the channel; scour holes have formed around these piers and the concrete footings of these piers are partially exposed (Hammer, 2020b; fig. 10B). Aerial imagery did not indicate substantial bank retreat or changes in channel width in study reach 3 (fig. 9). T-lidar surveys of study reach 3 were completed on February 16, 2022, and July 12, 2022. Standing water was present in the thalweg along the outer bank of the channel around and under the bridge during the t-lidar surveys, so no data are available in those areas for the 2022 t-lidar surveys. Furthermore, the banks on either side of Strecker Road bridge are heavily vegetated, which makes it difficult to detect ground points, resulting in limited data along the banks in the comparison between the two 2022 t-lidar surveys (fig. 1.12A). Downstream from the bridge, the channel bed showed 0.25 to 0.5 ft of degradation between February and July 2022 (fig. 1.12A). To better capture the channel bed around the piers, the 2023 surveys were completed using a TS on February 27, 2023, and July 25, 2023 (fig. 1.12B). The elevations of pairs of points located around the piers from the two surveys were compared (table 7). These comparisons indicated a small amount of aggradation (0.08 to 0.88 ft) around the piers between February and July 2023.

Study Reach 4 Results Study reach 4 is a short section of channel about 570 m downstream from study reach 3 (fig. 11). Study reach 4 is bounded on the left side by residential property and on the right side by a steep, tree-lined slope and Quaethem Drive. The left bank of study reach 4 has been eroding since at least 1990 (Hammer, 2020b), concurrent with the deposition of a bar on the right side of the channel. As a result, the channel has widened and gradually increased in sinuosity, forming an incipient meander bend. The incipient meander bend has grown in amplitude and translated downstream since the mid-2000s. The bar is separated from the right bank by a small chute channel that is visible in the 2017 DEM (not pictured)

16 Geomorphic Change, Hydrology, and Hydraulics of Caulks Creek, Wildwood, Missouri

1990 w

Flo

2002

2010

Riprap

2020

MCBRIDE POINTE DRIVE

Stu

c Rea

ent

ext

ST ER CK RE

D ROA

0 0

500 100

1,000 FEET 200 METERS

Figure 9. Aerial views of study reach 3 from U.S. Geological Survey (2022a). A, 1990. B, 2002. C, 2010. D, 2020.

Figure 10. In-channel views of selected features of study reach 3. A, View of Strecker Road bridge in study reach 3 from within the channel looking downstream. B, Close-up view of the bridge piers. Photographs by Jessica LeRoy, U.S. Geological Survey.

and was observed in the field in 2022 and 2023. At the apex of the incipient meander bend, the long-term mean bank retreat rate is 2.2 ft/yr and the channel is widening at a long-term mean rate of 1.8 ft/yr, based on aerial photograph analysis (fig. 11). A bank retreat rate of 4.4 ft/yr for 2007 to 2015 was calculated from the aerial photographs for the left bank at the

downstream end of the study reach, where the incipient bend is beginning to translate downstream (distances annotated in fig. 11E, F, and G). T-lidar surveys of study reach 4 were completed on March 2, 2022; July 13, 2022; March 1, 2023; and July 27, 2023 (fig. 1.13). Similar to observations from aerial photographs, retreat of the left bank of study reach 4 occurred throughout the study period. Between March and July 2022, approximately 0.5 to 1.5 ft of bank retreat occurred along the left cut bank, with some locations showing 2.5 to 3.5 ft of bank retreat (fig. 1.13A). Patches of deposition on the left cut bank may represent deposited blocks of failed bank material, similar to what is pictured in figure 1.13D. Between July 2022 and March 2023 and between March and July 2023, about 4 ft of erosion occurred mainly along the top of the left bank, whereas deposition occurred along the bank toe. This pattern may indicate that material from the top of the bank was deposited on the bank toe and had not yet been removed by flow in the channel. A single, collapsed tree was present in study reach 4 near the left bank for the duration of the study and prevents comparison of the scans in its immediate vicinity (labeled on fig. 1.13A–C and shown in fig. 1.13D). Although trees or other large woody debris can provide some protection to riverbanks, scour can occur around an isolated, collapsed tree (or other obstacle), owing to increased velocities and turbulence of the flow around the tree, similar to scour around a bridge pier. Farther downstream from the tree, deposition of the scoured material can occur in the low-velocity wake zone. Indeed, 2 to 3 ft of retreat of the left bank occurred near this tree between March and July 2022, with as much as 1.5 ft of bed aggradation downstream from the tree (fig. 1.13A). The bed aggradation downstream from the tree is in contrast to the widespread bed degradation (0.25 to 0.5 ft) throughout the reach from March to July 2022. Between July 2022 and March 2023, erosion of the bank near the tree continued (as much as 3 ft), though any effect of the tree on the pattern of bed deposition is difficult to discern from the widespread bed aggradation in the

Part 1—Observations of Geomorphic Change in Caulks Creek 17 Table 7. Comparison of point elevations surveyed around the two bridge piers in study reach 3 (Strecker Road bridge). [M, month; DD, day; YYYY, year]

Pier Upstream pier

Downstream pier

Point pair U1

Survey date (M/DD/YYYY)

Northinga (feet)

Eastinga (feet)

Elevation (feet)

Distance between points (feet)

Elevation difference (feet)

2/27/2023

1014411.88

786773.60

525.30

0.88

0.77

0.07

0.88

0.58

0.08

0.86

0.82

0.41

1.43

0.40

1.13

0.15

0.85

0.49

1.68

0.31

7/25/2023

1014411.33

786774.27

526.07

2/27/2023

1014410.20

786775.29

525.35

7/25/2023

1014410.26

786775.24

526.23

2/27/2023

1014409.38

786777.76

526.00

7/25/2023

1014409.94

786777.90

526.09

2/27/2023

1014415.44

786777.05

524.71

7/25/2023

1014414.90

786777.72

525.57

2/27/2023

1014421.45

786785.72

523.85

7/25/2023

1014421.69

786784.94

524.25

D2 D3 D4 D5

2/27/2023

1014423.17

786784.00

523.67

7/25/2023

1014424.59

786784.12

524.08

2/27/2023

1014425.68

786783.81

523.93

7/25/2023

1014424.59

786784.12

524.08

2/27/2023

1014426.61

786786.92

523.90

7/25/2023

1014426.90

786787.72

524.39

2/27/2023

1014424.28

786790.77

524.93

7/25/2023

1014423.72

786789.19

525.24

aMissouri East 2401 state plane coordinates, in feet.

study reach (fig. 1.13B). From March 2023 to July 2023, there was as much as 3 ft of bank retreat near the tree and an area of bed aggradation downstream from the tree (fig. 1.13C).

Study Reach 5 Results Study reach 5 surrounds the Strecker Road bridge crossing near the intersection of Strecker Road with Quaethem Drive and includes the meander bend upstream from the bridge (fig. 12). Study reach 5 is split into two sections, A and B (fig. 12H), for further discussion of the historical aerial photographs and the t-lidar survey results. The downstream limb of the study reach 5 meander bend (section A) has widened at a long-term rate of 3.7 ft/yr since 1990 owing to bank erosion and the concurrent development of a midchannel bar. Though erosion has occurred on both sides of the channel, the left bank is unforested and consists of fine-grained sediment and therefore has retreated rapidly at a long-term mean rate of 3.6 ft/ yr, according to aerial photograph analysis. Erosion of the left bank was likely accelerated by gullying associated with the outflow from a stormwater runoff pipe (labeled “Pipe outlet” in fig. 12H). Although an isolated pile of riprap was placed along the bank where the overland flow from the pipe enters the channel (labeled “Riprap pile” in fig. 12H and shown in fig. 13), this bank continues to actively erode (Hammer,

2020b). According to Hammer (2020b), the riprap may have accelerated erosion downstream owing to increased turbulence around the rock pile. Erosion and channel widening at and downstream from the Strecker Road bridge in study reach 5 (section B) also was noted as a location of concern owing to potential erosion effects on the bridge and to Dresser Hill Road (Hammer, 2020b). At some point between 2018 and 2019, vegetation was cleared and riprap was placed on the upstream and downstream sides of the bridge and directly on the bridge abutments of both banks, as well as in two isolated piles farther downstream along the left bank (Hammer, 2020b). Although it is not possible to assess changes occurring under the bridge from aerial photographs, the channel has widened upstream and downstream from the bridge since 1990 (fig. 12). In particular, the channel immediately downstream from the bridge has widened at a long-term rate of 1.1 ft/yr, largely owing to retreat of the left bank toward Dresser Hill Road. T-lidar surveys of study reach 5 were completed on February 28, 2022; July 15, 2022; March 2, 2023; and July 27, 2023. Between February and July 2022, the left bank in section A indicated generalized retreat of about 0.5 to 3 ft, as well as a local mass failure that resulted in 2 to 5 ft of bank retreat and deposited about 3 ft of sediment on the bank toe (fig. 1.14A). Between July 2022 and March 2023, there was

18 Geomorphic Change, Hydrology, and Hydraulics of Caulks Creek, Wildwood, Missouri B

55 feet

50 feet

1990

65 feet

1996

2004

F 100 feet

H 65 feet

110 feet 95 feet

85 feet

2007

75 ft

2002

95 feet

QUAETHEM DRIVE

Flow

2015

2010 0 0

500 100

2020 1,000 FEET

200 METERS

95 feet

Study Reach 4 extent

Figure 11. Aerial views of study reach 4 from U.S. Geological Survey (2022a). A, 1990. B, 1996. C, 2002. D, 2004. E, 2007. F, 2010. G, 2015. H, 2020.

widespread retreat of the left bank of section A of about 0.5 to 2.5 ft, with a local maximum retreat of about 4 ft (fig. 1.14B). Additionally, some of the sediment deposited by the prior mass failure was removed. Bank retreat between March 2023 and July 2023 was largely limited to the toe of the downstream half of the bank (0.5 to 2 ft), though removal of the sediment deposited by the mass failure in 2022 continued (fig. 1.14C). The t-lidar surveys indicated minimal change in section B of study reach 5, with the exception of some retreat of the left bank downstream from the Strecker Road bridge (about 0.5 to 2 ft each consecutive pair of surveys; fig. 1.15) and a localized mass failure of about 4 ft in the right bank upstream of the bridge (fig. 1.15C). The retreat of the left bank, downstream from the bridge, is consistent with the aerial photograph observations.

Study Reach 6 Results Study reach 6 is a large meander bend, followed by a comparatively straight section that ends at the Strecker Road bridge crossing near the intersection of Strecker Road and Church Road (fig. 14). Study reach 6 is split into six sections, A through F (fig. 14I), for the purpose of further discussion of the historical aerial photographs and the t-lidar survey results. Erosion of the left bank has occurred in section A, resulting in a long-term mean bank retreat rate of 1.2 ft/yr and a long-term mean widening rate of 0.7 ft/yr at the widest point

of the 2015 channel (labeled “1” in fig. 14G), as determined from aerial photographs. In section B, riprap was placed along the right cut-bank sometime prior to 2002 (labeled “Riprap” in fig. 14C), which initially narrowed the channel by about 20 ft, after which there was minimal change in the position of that bank. An access trail was built across the channel in section B sometime between 2015 and 2016 (labeled “Access trail” in fig. 14H). A large pile of riprap is present along the right bank at the location where the trail meets the channel, as well as a partially exposed utility hole (labeled “Riprap” in fig. 14I and shown in fig. 15A). Farther downstream, the left bank in section C also shows signs of active bank erosion, such as a near-vertical bank face, live exposed roots overhanging and emerging from the bank, and mass failures of bank material (Hammer, 2020b; and observed in the field, labeled “2” in fig. 14G). The long-term mean widening rate in this location is 1.2 ft/yr. There are two points of interest on the downstream limb of the large meander bend in study reach 6. A bank breach has developed on the left bank in section D, where flow has intermittently exited the main channel and deposited sand, gravel, and cobbles on the adjacent floodplain since at least 2014 (Hammer, 2020b; labeled “Breach” in fig. 14I). The apex of the right cut bank in section E (labeled “3” in fig. 14G) has eroded at a long-term mean rate of 2.3 ft/yr, according to aerial photograph analysis. The bank material in this location consists of a lower layer of coarse, noncohesive, cross-bedded

Part 1—Observations of Geomorphic Change in Caulks Creek 19 A

24 feet

Flow 35 feet

1990

2002 G

Riprap piles

2004 H Riprap

52 feet

THEM DRIVE

STRECKER

F QUAE

LL R HI SSE DRE ROAD

ROA D

ELore

1996

B Riprap pile

128 feet

A Pipe outlet

2007

2015

2010 0 0

500 100

1,000 FEET 200 METERS

2020

Figure 12. Aerial views of study reach 5 from U.S. Geological Survey (2022a). A, 1990. B, 1996. C, 2002. D, 2004. E, 2007. F, 2010. G, 2015. H, 2020.

channel lag deposits overlain by an upper layer of cohesive fine material (fig. 15B). Visual observations indicated a tendency for erosion of the coarse lower layer to undercut the bank. Hammer (2020b) identified the Strecker Road bridge crossing in section F of study reach 6 as another location of concern (fig. 14I). A new bridge was constructed in spring 2010 in this area and riprap was placed along both banks surrounding and underneath the bridge. The riprap placement on the right bank, upstream from the new bridge, narrowed the channel somewhat and therefore was not smoothly aligned with the bankline immediately upstream from the riprap placement (Hammer, 2020b). As a result, there is evidence of active bank erosion immediately upstream from the riprap on the right bank in this area, in the form of a near-vertical bank face with overhanging live exposed roots (“4” labeled in fig. 14G and shown in fig. 15C). The bridge has two wall piers (fig. 15D). Large woody debris was piled on the upstream side of the right pier during the field visits for this study.

T-lidar surveys of study reach 6 were completed on March 1, 2022 (sections A–C); March 2, 2022 (sections D–F); July 12, 2022 (sections A–C); July 13, 2022 (sections D–F); February 27, 2023; and July 26, 2023. Water in the thalweg in study reach 6 prevented t-lidar acquisition of the deepest part of the channel and the bank toe. Brushy vegetation also obscured the banks in parts of study reach 6. Even so, bank retreat was measurable with the t-lidar scans in all sections of the reach. In section A, there was widespread retreat of the left bank between March and July 2022, about 0.25 to 1.5 ft, with a local area of retreat of about 2 to 4.5 ft at the location labeled “1” in figures 14G and 1.16A. There was also an area of retreat observed on the right bank (2 to 5 ft) between March and July 2022 (fig. 1.16A), though much of the right bank was obscured by brushy vegetation. Between July 2022 and February 2023, there was 0.25 to 1.5 ft of retreat on both banks in section A (fig. 1.16B), but minimal retreat occurred between February 2023 and July 2023 (fig. 1.16C).

20 Geomorphic Change, Hydrology, and Hydraulics of Caulks Creek, Wildwood, Missouri

Figure 13. Riprap along the left cut bank in study reach 5, where overland flow from a stormwater runoff pipe enters the channel (refer to “Pipe outlet” in fig. 12H). Photograph by Jessica LeRoy, U.S. Geological Survey.

In section B, there is an actively eroding section of the right bank, upstream from the two riprapped areas. Between March 2022 and July 2022, 0.25 to 1.5 ft of erosion occurred on the lower part of the bank, whereas 0.25 to 1.5 ft of deposition occurred on the upper part of the bank (fig. 1.17A). Close inspection of the lidar data indicated that the bank advance observed on the upper part of the bank was mostly debris (leaves and sticks). That debris was subsequently removed between July 2022 and February 2023 (0.25 to 1.5 ft), but more debris accumulated on the bank between February and July 2023 (0.25 to 1 ft) (fig. 1.17B and C). Small amounts of change were observed along the riprapped portions of the right bank, which is again attributed to the accumulation and removal of debris. The left bank of section C includes a location noted as showing field evidence of active erosion (labeled “2” in fig. 14G and fig. 1.18A) as well as a heavily vegetated portion toward the middle of the section (fig. 1.18). The t-lidar surveys indicated active erosion upstream and downstream from the heavily vegetated portion between March 2022 and July 2022

(generally 0.25 to 1 ft, and as much as 3.5 ft), between July 2022 and February 2023 (generally 0.5 to 3 ft, and as much as 4 ft), and between February 2023 and July 2023 (generally 0.5 to 1.5 ft) (fig. 1.18). The patchy areas of bank advance are interpreted to be accumulations of debris (leaves and sticks) based on close inspection of the lidar data. Minimal bank erosion occurred in section D between March 2022 and July 2022, except for about 0.5 ft at the bank breach (fig. 1.19). Between July 2022 and February 2023, about 0.5 ft of bank retreat occurred (local maximum retreat=1 ft) and between February 2023 and July 2023, about 0.2 to 0.4 ft of bank retreat occurred (local maximum retreat=1 ft) in section D. Section E contains an actively eroding meander cut bank (fig. 1.20). Widespread bank retreat of about 0.5 to 1.5 ft (some patches of 2 to 3 ft) occurred in section E between March 2022 and July 2022, with local bank failures resulting in 5 to 12 ft of retreat. Some accumulation of sediment is visible below the large bank failure on the downstream end of the bend but most of the bank toe is hidden by standing

Part 1—Observations of Geomorphic Change in Caulks Creek 21

Flo

Riprap

1990 D

1996 E

2004

2007

2010 I

2002

STREC KER

Breach

Access trail

Riprap B A

2015

2016 0 0

500 100

1,000 FEET 200 METERS

2020

Figure 14. Aerial views of study reach 6 from U.S. Geological Survey (2022a). A, 1990. B, 1996. C, 2002. D, 2004. E, 2007. F, 2010. G, 2015. H, 2016. I, 2020.

22 Geomorphic Change, Hydrology, and Hydraulics of Caulks Creek, Wildwood, Missouri

Figure 15. Views of study reach 6. A, Riprap and utility hole on the right bank of the upstream limb of the large meander bend. B, Right bank of the downstream limb of the large meander bend. C, Right bank of the channel, upstream from the Strecker Road bridge crossing. D, The Strecker Road bridge crossing, viewed looking downstream. Photographs by Jessica LeRoy, U.S. Geological Survey.

water. Between July 2022 and February 2023, 0.5 to 2.5 ft of bank retreat occurred throughout the bend, and the local bank failure in the downstream end of the bend resulted in 5 to 8 ft of retreat. Lastly, between February 2023 and July 2023, 0.2 to 2.5 ft of bank retreat occurred throughout the bend, with 4 to 5 ft of retreat in the local bank failure in the downstream end of the bend. In section F, the t-lidar surveys indicate continued retreat of a portion of the right bank located upstream from the riprap placement (labeled “4” in fig. 14G), of about 0.25 to 0.5 ft from March to July 2022, 0.25 to 0.75 ft from July 2022 to February 2023, and 0.25 to 1.5 ft from February 2023 to July 2023 (fig. 1.21). The t-lidar surveys also indicated retreat of the riprapped part of the right bank (about 0.25 ft); however, the riprap was covered with brushy vegetation that increases the uncertainty of the comparison between the t-lidar surveys. The channel bed in study reach 6 went

through a cycle of mostly degradation from March 2022 to February 2023 followed by aggradation from February 2023 to July 2023.

Bank Erosion Pin Observations The bank erosion pin observations generally indicated bank retreat throughout the study area (fig. 16). Bank retreat may be caused by removal of sediment from the bank by flowing water or mass failures of the bank. The bank retreat measured at the bank pins during the whole study period (February/March 2022 to July 2023) corresponded to annual rates ranging from 0.002 to 0.75 ft/yr and averaging 0.28 ft/ yr (excludes pins 3 and 12, which showed net bank advance, and pin 4, which is missing data for part of the study period). The greatest bank retreat was observed at bank pins 6 (located

Part 1—Observations of Geomorphic Change in Caulks Creek 23 within study reach 4), 8, and 9 for the interval between July 2022 and October 2022 (0.63 to 0.75 ft over 3.54 months, corresponding to a rate of 2.14 to 2.54 ft/yr). This interval included several high-flow events, including the WY 2022 annual peak flow (fig. 3). Although most bank pins showed net retreat during the whole study period, many of the bank pins indicated bank advance for some of the measurement intervals. The bank faces in which the bank pins were installed were fairly steep and it is unlikely that fluvial deposition is occurring on these surfaces. Deposition of sediment on the bank may occur as a result of slumping or the mass failure of an overlying block of sediment; therefore, the observed bank advance may be tied to erosional processes. An alternate explanation for bank advance is swelling owing to the absorption of water in the sediment.

Spatial and Temporal Variability of Morphologic Change in Caulks Creek The analyses of historical aerial photographs indicated that bank retreat and channel widening has occurred since the 1990s in all the study reaches except study reach 3 (figs. 5, 7, 9, 11, 12, 14) at long-term mean rates ranging from 0.6 to 4.4 ft/yr. The t-lidar surveys, however, indicated that local, shortterm bank retreat rates vary considerably in space and time. The amount of bank retreat observed with the t-lidar surveys not only varied substantially among the study reaches but also varied along individual bank faces. Furthermore, bank retreat rates in any particular location differed between the consecutive surveys. These results highlight the episodic nature of bank retreat. In other words, bank retreat at a given location likely occurs in short, rapid bursts interspersed with periods of minimal change, rather than occurring at a consistent, steady pace. As a result, the local, short-term bank retreat can differ substantially from the long-term mean rates. The measurements made at the bank pins provide some additional context to the t-lidar survey, though such point measurements may not be representative of the surrounding reach, or even the local bank face. As indicated by the t-lidar scans, morphologic change along a bank face can be highly spatially variable. However, given the consistency of observed bank retreat across the bank pin locations, the results may indicate that bank erosion during the period from February/ March 2022 to July 2023 was widespread throughout the study area, albeit at an overall slower rate (0.002 to 0.75 ft/ yr) than the historical rate within the study reaches (0.6 to 4.4 ft/yr). Thus, bank retreat in the study area may be systemic and tied to the watershed-scale balance between the driving and resisting factors of erosion, whereas the differences in the local rates of bank erosion arise from local variations in this balance. The drivers of erosion include the hydraulic forces that entrain and transport sediment from the bank during flow events (referred to as “fluvial erosion”), which are affected by hydrologic conditions of the watershed, as well as gravitational forces acting on an exposed bank face that can result in

instability and collapse (referred to as “mass failures”). The resistance to fluvial erosion is largely dependent on the grain size and cohesion of the bank material, whereas resistance to mass failure is determined by the shear strength of the bank material (Knighton, 1998). Multiple factors affect shear strength, including the bank material characteristics (grain size and cohesion), pore-water pressure, the presence or absence of tension cracks, and strength added by riparian vegetation (Langendoen and Simon, 2008). Although roots can increase the shear strength of a bank, the weight of large trees may contribute toward bank destabilization; however, the phenomenon is complex and dependent on the relative depth of the roots compared to the bank height and the bank material cohesion (Abernethy and Rutherfurd, 2000a,b; Pollen 2007; Török and Parker, 2022). Observations from the six study reaches indicated that relatively high rates of bank retreat occurred at the outer banks of meander bends and where banks are unforested and the riparian vegetation is limited to shallow-rooted grasses. Typical bank retreat along actively eroding outer banks of meander bends in the study reaches was about 0.5 to 3 ft between consecutive surveys (approximately 5 to 8 months), though much greater values were measured at some locations (as much as 16 ft) that are likely due to mass failures. During the period of study, rapid bank retreat occurred on unforested banks in study reach 1, sections A and F; study reach 4; and study reach 5. However, bank retreat also occurred along banks located within heavily forested slopes (that is, study reach 1, sections B through E; study reach 2; and study reach 6), indicating that the processes that drive bank erosion are sufficient to overcome the resistance of the bank material in those forested sections as well. In particular, the greatest amount of bank retreat observed with the t-lidar scans occurred along the heavily forested outer bank of study reach 2, section A. The rapid retreat of that bank is hypothesized to be related to this bank being very tall and nearly vertical and the tight curvature of the meander bend. Even though the top of the bank is forested, the depth of the roots relative to the total height of the bank may be less than other locations where the banks are not as tall. It is possible the weight of the trees contributed to destabilizing the banks, but a detailed stability analysis is beyond the scope of this study. The tight curvature of the bends in study reach 2, section A may also promote rapid bank retreat, owing to curvature-induced secondary circulation during flow events, which tends to concentrate the highest velocities of a flow toward the outer bank toe (Dietrich, 1987). Wetting-drying and freeze-thaw cycles also have likely contributed to bank retreat in Caulks Creek. During early spring surveys, as air temperatures rose above freezing and the water in the bank sediment began to melt, small amounts of material were heard and seen falling off banks and mud was observed oozing down the banks. Wetting-drying and freezethaw cycles also can reduce the shear strength of bank material by contributing to the formation and growth of tension cracks (Knighton, 1998).

24 Geomorphic Change, Hydrology, and Hydraulics of Caulks Creek, Wildwood, Missouri 90°37'30" 38°38'30"

90°37'

90°36'30"

Study Reach 6

Bank pin 11

−1

Bank pin 9

5 38°38'

−0.3

38°37'30

Bank retreat and advance at bank pins, in inches

−10

Study Reach 5

Study Reach 4

6 5

Study Reach 3 38°37'

4 3

Bank pin 10

−5

11 10

Bank pin 12

−1

Bank pin 7

Bank pin 8

0.1

−5 −1

−10

Bank pin 5

Bank pin 6

5 0

− 0.3

−5 −1

−10

Bank pin 3

Bank pin 4

2 1

Current

Study Reach 2

0 −1

No data

−2 −3

2023

023

July 3 to

b./M ar. 2

to Fe

/Ma

2022

Feb.

Oct.

to O

2022 July

r. 20 2

2022 May

/Ma

r. 20 2

2 to

to J

2023

Feb.

Feb. /Ma

r. 20 2

3 to

July

ar. 2

022

b./M

to Fe 2022

ct. 2 to O

2022 July

4 MILE

Oct.

uly 2

to J

Feb.

Base from Farm Service Agency, National Agricultural Imagery Program, 2020. Universal Transverse Mercator projection, zone 15 North Horizontal coordinate information referenced to the North American Datum of 1983

/Ma

r. 20 2

2 to

38°36'

2022

May 2

−3

022

−2

uly 2

−1

−2 023

−1

022

1 0

May

1 0

022

Study Reach 1

Bank pin 2

2022

May

38°36'30"

Bank pin 1

ct. 2 022

−1

4 KILOMETER

EXPLANATION Bank retreat

EXPLANATION

Bank advance

Study reach Caulks Creek network

12 Bank erosion pin and identifier

Figure 16. Location and channel bank changes at bank pin locations. A, Bank pin locations. B, Observations of bank retreat and advance at each of the 12 bank pins in the study area. The y-axis scales differ among the 12 plots.

Part 2—Hydrologic and Hydraulic Response of Caulks Creek to Design Storms 25 Engineered bank protection, like riprap, is intended to increase the resistance of the banks to erosion. Riprap placements resulted in relatively stable banks in study reach 1, section E; study reach 2, section C (with adjustments to the bank angle); and study reach 3. However, riprap in study reach 5, section A has been largely ineffective as a bank protection measure and may even have exacerbated erosion along the surrounding bank, as the relatively small pile of riprap acts as a turbulence-generating obstacle to the flow. Similarly, although the riprapped portions of study reach 6, sections B and F have remained relatively stable, bank erosion has remained an issue immediately upstream from the riprap placements, which may be due to a combination of the steep banks in this area and (or) issues aligning the riprap with the local channel curvature. Furthermore, even in locations where the riprapped banks appear stable, pieces of riprap were present in the bed of the channel, indicating that it is possible for the concrete pieces to be mobilized by flows. The results of this study indicate that artificially stabilizing a bank with riprap may, in some cases, just transfer the erosion to another location. The combination of fluvial erosion and mass failure processes likely contributes to the episodic nature of bank retreat that was previously noted. Although the amount of fluvial erosion resulting from a given flow event is expected to roughly scale with the magnitude of the flow event, any flow or sequence of flows that generates sufficient bank toe erosion to destabilize the bank could trigger a mass failure that causes rapid bank retreat. Thus, a flow or sequence of flows that are not considered exceptional or extreme could lead to rapid bank retreat if the bank is near a stability threshold. The substantial bank retreat observed in study reach 2, section A in 2022 provides an example. Several flow events occurred between the two t-lidar surveys of study reach 2 (fig. 3), including the annual peak flow of WY 2022. However, the annual peak flow of WY 2022 (August 4, 2022; peak flow=2,350 ft3/s) was not an exceptional event; the annual peak flow was higher in 13 of the previous 26 WYs (since 1997; USGS, 2023). Despite this, erosion occurred along the entirety of the outer bank of study reach 2, section A during this period, including large mass failures that contributed to up to 16 ft of bank retreat.

Part 2—Hydrologic and Hydraulic Response of Caulks Creek to Design Storms Hydrologic and hydraulic models of the Caulks Creek basin were developed to assess the effects of projected future changes in precipitation and added runoff storage on peak flows and hydraulic properties. Probabilistic precipitation events were used to generate baseline condition streamflow hydrographs using a hydrologic model. These baseline model conditions were then subjected to precipitation modifications and changes in runoff storage. All generated hydrographs were

routed through a hydraulic model to generate profiles and define relative differences in hydraulic characteristics within the study reach. Developed and calibrated hydraulic models of the Caulks Creek study area were then used to estimate changes in hydraulic properties (velocity and shear stress) correlated with erosion potential. Tasks specific to the development of hydraulic property profiles for this study included: (1) acquiring and developing hydrologic and hydraulic models for the Caulks Creek study basin; (2) obtaining precipitation and streamflow information associated with measured high-flow events; (3) collecting water-level data during high flows at selected locations within the study reach during the study period; (4) calibrating parameters associated with the hydrologic model Hydrologic Engineering Center–Hydrologic Modeling System (HEC–HMS) computer program (U.S. Army Corps of Engineers, 2022a); (5) calibrating energy-loss factors (roughness coefficients) in the stream channel and floodplain associated with the Hydrologic Engineering Center–River Analysis System (HEC–RAS) computer program (U.S. Army Corps of Engineers, 2022c); (6) incorporating additional reservoir storage and projected modifications to future precipitation into the calibrated HEC–HMS model for probabilistic precipitation events; (7) computing water-surface profiles of various scenarios using the HEC–RAS model; (8) producing maps of hydraulic properties within the study reach, including velocity and shear stress; and (9) analyzing differences in hydraulic properties and estimated erosion potential under the various future condition scenarios.

Methods for Estimating Precipitation-Derived Streamflow The USGS streamgage Caulks Creek at Chesterfield, Mo., (06935830) is located near the downstream end of the Caulks Creek basin and the downstream end of the spatial extent of the hydrologic and hydraulic models (fig. 17). A precipitation-driven hydrologic model was developed to provide accurate streamflow hydrographs within the study reach to be used as inflows for the hydraulic model.

Hydrologic Data This study used hydrologic data from the USGS streamgage 06935830 (table 8; USGS, 2023) and six pressure transducers (PTs) that were installed to collect waterlevel data within the Caulks Creek study reach (fig. 17). Continuous stage and flow time series were collected at the USGS streamgage in accordance with techniques provided in Turnipseed and Sauer (2010). The stage at USGS streamgage 06935830 was converted to water-surface elevation using a

26 Geomorphic Change, Hydrology, and Hydraulics of Caulks Creek, Wildwood, Missouri 90°42'

38°40'

ST CHARLES COUNTY

90°40'

90°38'

90°36'

90°34'

Two-dimensional HEC – RAS model computation area 06935830

38°38'

k ee Cr ks PT 8 l au

PT 9

ST LOUIS COUNTY

PT 6 PT 5 PT 4 PT 3

38°36'

38°34'

Base map modified from Esri and its licensors, copyright 2023 World Geodetic System 1984 Web Mercator (Auxillary Sphere) projection. Horizontal coordinate information referenced to the North American Datum of 1983

0 0

1 1

EXPLANATION

[HEC–RAS, Hydrologic Engineering Center–River Analysis System; HEC–HMS, Hydrologic Engineering Center–Hydrologic Modeling System] Caulks Creek HEC–HMS model subbasins Bonhomme Creek HEC–HMS model subbasins Location of existing reservoir storage in HEC–HMS model Location of added reservoir storage in HEC–HMS model

PT 8 06935830

Water-level logger location and identifier U.S. Geological Survey streamgage and identifier

Figure 17. Hydrologic and hydraulic model extents of the Caulks Creek study area, Wildwood, Missouri.

2 MILES 2 KILOMETERS

Part 2—Hydrologic and Hydraulic Response of Caulks Creek to Design Storms 27 datum of 451.41 ft above NAVD 88. Stages at PT locations (fig. 17) were measured every 5 minutes using a submersible data logger, and data were corrected for atmospheric pressure changes using continuous (5-minute increments) barometric pressure data. The stage data at each pressure transducer location were converted to water-surface elevations referenced to NAVD 88 by adding the corresponding vertical datum. The time-series data for the PTs used in this study are available in a USGS data release (Heimann and others, 2024). A level IV survey (Rydlund and Densmore, 2012) procedure was used in acquiring reference elevations for PT locations in the study reach with a resulting vertical accuracy of reference points within 0.32 ft.

Hydrologic Model Development A hydrologic model was used to simulate peak flows for Caulks Creek using design rainfall events covering a range of magnitudes and probabilities of exceedance. The hydrologic model was obtained from WSP, Inc. (Alicia Williams, WSP, Inc., written commun., 2021) and was originally developed using the modeling system HEC–HMS version 4.2 and converted to version 4.9 (U.S. Army Corps of Engineers,

2022a). The HEC–HMS basin model (fig. 17) was calibrated and validated (interactively with the HEC–RAS model) using streamflow and stage time series from the USGS streamgage 06935830 for seven selected high-flow events (table 9). The calibrated HEC–HMS model was then used to simulate precipitation runoff from selected Atlas 14 probabilistic rainfall amounts (National Oceanic and Atmospheric Administration, 2021) of 2-, 5-, 10-, 25-, 50-, and 100-year RIs for the basin for 6- and 24-hour durations. The HEC–HMS model was run for single rainfall events (rather than as a continuous simulation), and subbasin components within the model were defined for transformation, loss, and recession components. The HEC–HMS model was then used to estimate streamflow time series at subbasin outlet and junction locations that were used to represent inflow boundary locations in the hydraulic model.

Model Scenarios A total of 192 hydrologic and hydraulic model scenarios were developed for this study (table 10). The scenarios included “current” and four “projected” climate conditions, “normal” and “wet” antecedent response conditions, and three

Table 8. Description of U.S. Geological Survey streamgage Caulks Creek at Chesterfield, Missouri (06935830). [Station location is shown in figure 17. Latitude and longitude are given in decimal degrees. USGS, U.S. Geological Survey; mi2, square mile; NAD 83, North American Datum of 1983; present refers to the time of publication (2024)]

USGS station name

USGS station number

Drainage area (mi2)

Latitude (NAD 83)

Longitude (NAD 83)

Period of continuous flow record (water yeara)

Caulks Creek at Chesterfield, Missouri

06935830

17.1

38.65466667

−90.59488890

1997–present (2024)

aWater year refers to the 12-month period beginning October 1 and ending September 30.

Table 9. Simulated high-flow events used in calibrating and validating the hydrologic and hydraulic models used in this study. [HEC–HMS, Hydrologic Engineering Center–Hydrologic Modeling System; HEC–RAS, Hydrologic Engineering Center-River Analysis System; ft3/s, cubic feet per second; CNII, Soil Conservation Service runoff curve number representing a “normal” antecedent response condition; CNI, Soil Conservation Service runoff curve number representing a “dry” antecedent response condition; CNIII, Soil Conservation Service runoff curve number representing a “wet” antecedent response condition]

HEC–HMS and HEC–RAS calibration/validation event

Antecedent response condition

Peak flow (ft3/s)

September 14, 2008, calibration

CNII

6,520

September 10, 2016, calibration

CNI

3,350

July 22, 2019, calibration

CNIII

5,130

July 28, 2022, calibrationa

CNIII

1,890

June 14, 2015, validation

CNII

2,640

August 9, 2020, validation

CNIII

4,140

August 4, 2022, validationa

CNII

2,440

aEvent includes high water mark data from six pressure transducers.

Geomorphic Change, Hydrology, and Hydraulics of Caulks Creek, Wildwood, Missouri

runoff storage conditions for the six identified probabilistic precipitation RIs and two durations. The current climate condition consisted of observed precipitation data, whereas future climate condition scenarios included adjustment factors to “current climate” precipitation representing RCP 4.5 and RCP 8.5 conditions for 2050 and 2099 (van Vuuren and others, 2011). Precipitation corrections for the projected climate condition scenarios were processed using the Coupled Model Intercomparison Project (CMIP) climate data processing tool version 2.1 (U.S. Department of Transportation, 2022) using the downscaled CMIP5 multi-model ensemble, Localized Constructed Analog dataset from the Bureau of Reclamation (2022). The reservoir storage scenarios were developed to assess the potential effects of a realistic representation of additional storage on hydrology rather than to serve as a prescriptive design for the mitigation of peak flows and erosion. “Existing capacity” scenarios do not include any additional storage. The

additional storage scenarios included “Additional, empty” (“detention” scenarios) and “Additional, 75 percent” (“retention” scenarios) with “empty” and “75 percent” referring to starting reservoir capacity of three additional reservoirs located in the upstream Caulks Creek basin. Further details on the reservoir storage scenarios are provided in the “Hydrologic Structures” section.

Precipitation Data Accurate precipitation data are needed as inputs to the hydrologic model to simulate the resulting runoff and streamflow. Current and historical precipitation data were unavailable for stations within the Caulks Creek basin and, therefore, hourly gridded Multi-Radar Multi-Sensor estimate (Iowa State University, 2021) data were used for the calibration and validation events.

Table 10. Description of hydrologic and hydraulic model scenarios developed in the study. [--, no data; CNII, Soil Conservation Service runoff curve number representing a “normal” antecedent response condition; CNIII, Soil Conservation Service runoff curve number representing a “wet” antecedent response condition; RCP, representative concentration pathway]

Climate condition Current conditions

Projected climate conditions

Antecedent response condition

6-hour duration

24-hour duration

Not applicable

CNIIa

Existing capacity

(2, 5, 10, 25, 50, 100)

CNII

Additional, emptyc

(2, 5, 10, 25, 50, 100)

CNII

Additional, 75 percentd

(2, 5, 10, 25, 50, 100)

CNIIIb

Existing capacity

(2, 5, 10, 25, 50, 100)

CNIII

Additional, empty

(2, 5, 10, 25, 50, 100)

CNIII

Additional, 75 percent

(2, 5, 10, 25, 50, 100)

CNII

Existing capacity

(2, 5, 10, 25, 50, 100)

Additional, 75 percent

(2, 5, 10, 25, 50, 100)

Existing capacity

(2, 5, 10, 25, 50, 100)

Additional, 75 percent

(2, 5, 10, 25, 50, 100)

CNII

Existing capacity

(2, 5, 10, 25, 50, 100)

CNIII

Existing capacity

(2, 5, 10, 25, 50, 100)

CNII

Existing capacity

(2, 5, 10, 25, 50,100)

(2, 5, 10, 25, 50, 100)

CNIII

Existing capacity

(2, 5, 10, 25, 50, 100)

CNII

Existing capacity

(2, 5, 10, 25, 50, 100)

CNIII

Existing capacity

(2, 5, 10, 25, 50, 100)

RCP 4.5–2050e

CNIII RCP 4.5–2099f RCP 8.5–2050g RCP 8.5–2099h

Additional storage condition

Simulated recurrence interval events (years) by selected storm durations

Projected climate condition

aA “normal” antecedent response condition corresponding to a Soil Conservation Service runoff curve number CNII condition. bA “wet” antecedent response condition corresponding to a Soil Conservation Service runoff curve number CNIII condition. cStorage scenario includes additional reservoir storage with a starting capacity at empty (100 percent capacity remaining). dStorage scenario includes additional reservoir storage with a starting capacity at 75 percent of maximum (25 percent capacity remaining). eRCP 4.5–2050 is the representative concentration pathway of 4.5 watts per square meter and target conditions in 2050. fRCP 4.5–2099 is the representative concentration pathway of 4.5 watts per square meter and target conditions in 2099. gRCP 8.5–2050 is the representative concentration pathway of 8.5 watts per square meter and target conditions in 2050. hRCP 8.5–2099 is the representative concentration pathway of 8.5 watts per square meter and target conditions in 2099.

Part 2—Hydrologic and Hydraulic Response of Caulks Creek to Design Storms 29 The same precipitation events were used as input to the hydrologic model for all 16 modeled scenarios (table 10) and were developed from the National Oceanic and Atmospheric Administration Atlas 14 point-precipitation frequency estimates (National Oceanic and Atmospheric Administration, 2021). The values represent a total rainfall for the specified duration. Distributed time series of precipitation based on these totals were developed by distributing the precipitation in 5-minute increments over the specified duration using the Frequency Storm precipitation distribution feature in HEC–HMS and the incorporated TP40 area reduction curve (Hershfield, 1961) adjustment.

Transformation Method Precipitation exceeding soil infiltration and storage is transformed into runoff in the developed HEC–HMS model using the Clark unit-hydrograph method (Clark, 1945). Two parameters needed to define this unit-hydrograph method are time of concentration (Tc) and a storage coefficient (R). The Tc is an estimate of the time of travel (in hours) it takes for precipitation runoff to travel from the most distant point in a subbasin to the subbasin outlet. R is used to account for storage (in hours) within the floodplain such as wetlands, reservoirs, and bridges that can produce flood-wave attenuation. Initial estimates of Tc were calculated using the TR–55 methodology (Soil Conservation Service, 1986). The R values for the subbasins were adjusted such that R/(Tc+R) = 0.3, corresponding to the approximate mean value for this relation in rural basins in west central Illinois (Straub and others, 2000). Values of Tc and R were then adjusted during model calibration.

Loss Method The Soil Conservation Service runoff curve number (CN) method was used to simulate precipitation losses (Soil Conservation Service, 1986; Natural Resources Conservation Service, 2004). The CN values were applied to each subbasin according to the TR–55 methodology (Soil Conservation Service, 1986), and represent hydrologic soil types, land uses and treatments, and antecedent response conditions. Using the Soil Conservation Service CN approach, antecedent response conditions are divided into three classes: CNI for “dry,” CNII for “normal,” and CNIII for “wet” antecedent response conditions (Natural Resources Conservation Service, 2004). The normal CNII (normal) and CNIII (wet) antecedent response conditions were used in this study. The initial CNII values used in the model were converted to CNIII using the following equation from Chow and others (1988):

23CN(II) ______________ CN(III) =    10 + 0.13CN(II)

(1)

Base-Flow Method The recession base-flow method (Chow and others, 1988) was used to simulate base flow within the basin. For this method, the HEC–HMS model requires three parameters: initial streamflow, a recession constant, and a ratio-to-peak constant. The initial streamflow was applied to the subbasin as a ratio of cubic feet per second per square mile. The recession constant represents the rate at which base flow recedes after a rainfall event. The ratio-to-peak constant is a threshold that indicates when to begin base flow on the recession limb of a hydrograph. These parameters were estimated and adjusted during the HEC–HMS model calibration.

Streamflow Routing Method The Muskingum-Cunge method (Cunge, 1969) of routing was selected within HEC–HMS to accurately represent floodplain storage and peak-flow attenuation conditions in model reaches incorporating multiple subbasin outflows. Generally, individual subbasin outflows were used as inflows in the HEC–RAS model except for subbasins that included reservoirs. In this case, HEC–HMS was used to simulate reservoir operations and the outflow from the multiple subbasins associated with the operation of the reservoir(s) as the inflow for HEC–RAS. The HEC–HMS and HEC–RAS models were run consecutively and iteratively for calibration and validation.

Hydrologic Structures A total of six reservoirs are represented in the baseline “existing capacity” hydrologic model of Caulks Creek. These structures have the potential to affect the timing and magnitude of peak flows along the stream. The reservoirs were modeled using the Elevation-Storage method (U.S. Army Corps of Engineers, 2022b) and a defined elevation-storage relation specific to each reservoir. Modeled additional storage scenarios included three additional reservoirs upstream of the study area reach and variations in reservoir starting capacity. The three additional reservoirs were located upstream from the study area such that the additional storage could potentially affect the magnitude of peak flows and maximum erosive forces contributing to documented areas of high bank instability. The three additional reservoirs were in areas with sufficient vacant ground to support the anticipated area of inundation associated with reservoir operations. The reservoirs were designed such that the target cumulative additional storage would be equivalent to that of the existing upstream-most reservoir (115 acre-feet; fig. 17) and have the emergency capacity (storage between outflow and emergency spillway) to contain a 25-year runoff event. All reservoir parameter information is contained in the hydrologic models used for analyses and provided in Heimann and others (2024).

30 Geomorphic Change, Hydrology, and Hydraulics of Caulks Creek, Wildwood, Missouri

Hydrologic Model Calibration The hydrologic model was manually (no automatic functions used) calibrated to observed streamflow data for four high-flow events between 2008 and 2022 and ranging from 1,890 to 6,520 ft3/s. The parameters CN, Tc, R, and base-flow recession were the primary parameters used in calibrating the simulated hydrograph peak and shape and were adjusted to match the corresponding hydrograph shape, magnitude, and timing for USGS streamgage 06935830. Final calibration results indicated similarity in simulated and observed hydrographs (fig. 18). The Nash-Sutcliffe efficiency (NSE) coefficient (Nash and Sutcliffe, 1970) and percentage bias (PBIAS; Gupta and others, 1999) statistics were used to assess model fit. Values of NSE can vary from negative infinity to 1. Values of 1 correspond to a perfect match between simulated and observed time series, whereas values <0 indicate the observed mean is a better predictor than the simulated values. The NSE of the four calibrated models ranged from 0.86 to 0.98, indicating that the simulated values were a good to excellent predictor of the observed hydrograph (table 11). The PBIAS is a measure of mean tendency of the simulated data to be larger or smaller than observed values. Values of PBIAS can vary from negative infinity to infinity with an optimum value of 0. The calibrated models yielded PBIAS values of 0.01 to −27.5, indicating an excellent to fair model fit (table 11). A

comparison of relevant hydrograph characteristics also indicated similarities, in general, between simulated and observed peak flow and time of peak (table 11).

Hydrologic Model Validation Three additional independent observed high-flow events representing varying peak flows and antecedent response conditions were used to validate the calibrated models. The NSE value of the validation events of the Caulks Creek model ranged from 0.81 to 0.99, indicating that the simulated values were a good to excellent predictor of the observed hydrograph (table 11). The PBIAS values of the validation runs were −8.4 to 3.9 and again indicated a good to excellent model fit (table 11). The simulated and observed peak flows and timing also were similar (table 11).

Hydrologic Model Sensitivity The primary hydrologic model parameters that were modified for calibration were Tc, R, and CN values. The sensitivity of the Caulks Creek simulated streamflows to these parameters in addition to precipitation, reservoir storage capacity, and HEC–RAS roughness coefficients (Manning’s n-values) were assessed through sensitivity analyses. The sensitivity was analyzed by modifying each of these parameters/components by +10 percent and −10 percent and determining the resulting

Table 11. Simulated and observed hydrograph characteristics of the calibration and validation high-flow events at the U.S. Geological Survey streamgage Caulks Creek at Chesterfield, Missouri (06935830). [ft3/s, cubic foot per second; hh, hour; mm, minute; NSE, Nash-Sutcliffe Efficiency; PBIAS, percentage bas; CNII, Soil Conservation Service runoff curve number representing a “normal” antecedent response condition; CNI, Soil Conservation Service runoff curve number representing a “dry” antecedent response condition; CNIII, Soil Conservation Service runoff curve number representing a “wet” antecedent response condition]

Peak flow (ft3/s)

Time of peak (hh:mm)

Simulated

Observed

Percentage difference

6,520

6,370

2.4

3,350

0.0

Simulated

Observed

Model performance metric Difference

NSE

PBIAS