Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

USDA, ARS, National Sedimentation 4/1/2014 Laboratory

P.O. Box 1157 Oxford, MS 38655

Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

Prepared by:

Eddy J. Langendoen and Mick E. Ursic

USDA, ARS, National Sedimentation Laboratory Oxford, MS 38655

Christian E. Frias and Jorge D. Abad

Department of Civil and Environmental Engineering, University of Pittsburgh Pittsburgh, PA 15261

Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

EXECUTIVE SUMMARY Large portions of the Big Sioux River and its tributaries in South Dakota are impaired because of increased levels of Total Suspended Solids. Stream bank erosion can be an important contributor of fine-grained sediment that are transported downstream in suspension. Further, bank failures result in channel widening and the loss of adjacent lands. The USDA-ARS National Sedimentation Laboratory (NSL) determined through an earlier study along the Big Sioux River that various types of bank- stabilization measures would be effective at reducing bank-erosion rates. What was unknown, however, was whether bed erosion (and then further bank erosion) will be initiated as a result of the reduction in sediment supply if successful bank-stabilization measures are undertaken at a large scale along the river. To determine this, a model that not only can dynamically adjust the bed and banks, but also routes flow and sediment needs to be applied.

This study used NSL’s CONservational Channel Evolution and Pollutant Transport System (CONCEPTS) channel evolution computer model in combination with the RVR Meander computer model that simulates the morphodynamics of meandering streams. Computer model simulations were conducted to:

1. Evaluate the effects of current bank stabilization measures between Dell Rapids and Sioux Falls on the morphology of the Big Sioux River along the same reach. 2. Identify unprotected locations with the highest bank erosion rates for future stabilization.

The forces acting on the stream boundary and the resistance to erosion of the boundary materials govern stream morphology. In general, the force exerted by the flowing water on the channel boundary depends on flow velocity distribution and boundary roughness. The resistance to erosion of sediment is represented by its particle size when cohesionless or by a critical shear stress and soil detachment coefficient (or erodibility coefficient) when cohesive. The latter properties of cohesive soils are themselves dependent on such properties as texture, density, and soil water content.

Data on cross-sectional profiles and resistance-to-erosion properties of channel boundary materials were collected in the field in collaboration with the State of South Dakota Department of Environment and Natural Resources and the South Dakota Association of Conservation Districts. Representative design discharges were used to provide hydraulic input to calculate the force exerted by the flowing water.

The geographic scope of the project is a 34-km long reach on the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota. The reach is fairly sinuous with an average sinuosity (ratio of channel length over valley length) of 1.6. The average channel slope is 0.4 m/km.

Page i Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

The bed material is sand dominated with the median bed material grain size along the study reach varying between 0.03 and 7.0 mm with a mean value of 1.4 mm. Both pool and riffle bed materials gradually coarsen from the upstream end of the study reach at Dell Rapids to the downstream end at Sioux Falls; the median grain size varied between 2.5 mm at the downstream end to about 0.4 mm at the upstream end of the study reach. These grain sizes are such that the bed is mobile along the entire study reach when mean flow depth exceeds approximately 0.5 m.

Bank material is cohesive except for the sediments/soils at depth, which consist of sands and gravels. The upper cohesive layer primarily comprises erodible loam and sandy loam soils, but percent clay is found as high as 55%. The critical shear stress required to erode these materials was fairly constant (about 10 Pa) for the lower 25 km of the study reach. The critical shear stress linearly reduced to . about 2 Pa at the upstream end of the study reach. The bank face materials (� = 4.1 10 � ) were . about twice as hard to erode as the bank toe materials (� = 8.1 10 � ). The erodibility coefficient

� (in m s-1 Pa-1) represents the rate at which bank-material particles are entrained by the flow when the boundary shear stress exerted by the flowing water exceeds the critical shear stress � of the bank soils.

It should be noted that the lower resistance to erosion measured at the upper end of the study reach was caused by having only a single measurement available. As a result, the simulated channel widening along the upper portion of the study reach was quite large. More measurements to characterize soil erodibility are required along the upper 2 km of the study reach.

The CONCEPTS computer model used to assess the morphologic adjustment of the study reach is one- dimensional, and therefore cannot accurately simulate flow hydraulics in meander bends that increase boundary shear stresses relative to those in straight channel sections. The computer model RVR Meander was used to determine: (1) the enhanced shear stresses exerted by the helical flow on the outer bank of meander bends, and (2) highest stream bank erosion potential. Further, the ratio of the shear stress on the outer bank of meander bends to that at the channel centerline was used to modify the bank soil erodibility employed by the CONCEPTS model.

RVR Meander simulated high shear stresses at 45 unprotected bends that may potentially lead to enhanced migration rates. Sixteen of these bends have exhibited significant migration between 1991 and 2012, and should be targeted for construction of bank protection works. Eleven of these bends are located between study transects 2100 and 3000. Along this section of the Big Sioux River several meander bends were cutoff between 1937 and 1991, which has resulted in increased bank erosion rates due to the channel adjustment caused by the local shortening of channel and consequent increased gradient.

The one-dimensional channel evolution computer model CONCEPTS was used to assess the effect of current bank protection measures on channel morphologic adjustment and to identify locations

Page ii Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota vulnerable for erosion along the Big Sioux River between Dell Rapids and Sioux Falls under a range of different flow conditions. Evaluated flow discharges ranged from bankfull (discharge with a 1.5- to 2- yr return period) to discharges with a return period of 50 years.

Changes in bed elevation did not vary much between the different flow scenarios or because of the presence of bank protection measures. There were small variations in pool elevations, some eroded slightly and some became slightly shallower, however the pool-riffle pattern was not affected much. This indicates there is enough transport capacity at higher flows to move the eroded bank materials downstream.

Channel top width increased significantly at a few locations. Bank erosion potential was categorized as minor, moderate, and severe. The minor erosion potential class consisted of locations that experienced erosion < 5 m for the 1.5-yr flow scenario, and which did not greatly increase for the larger flow scenarios. The moderate erosion potential class comprised locations that experienced erosion > 5 m for the 1.5-yr flow scenario, or < 5 m for the 1.5-yr flow scenario but with significantly increased erosion for the larger flow events. The severe erosion potential class consisted of locations that experienced top bank retreat exceeding 15 m for any flow scenario. 6.5 km (19.3%) of the study reach was classified as having minor erosion potential, 0.5 km (1.5%) of the study reach was classified as having moderate erosion potential, and 1.8 km (5.5%) of the study reach was classified as having severe erosion potential. It should be noted that these increases in channel top width were sometimes accompanied by deposition on the bed, which could have accelerated the rate of bank erosion. Severe erosion potential locations were: meander bends downstream of transect 3300, meander bends downstream of transect 2700, meander bend at transect 2000, meander bend at transect 900, meander bend midway transects 200 and 300, and the meander bend midway transects 100 and 200.

Three locations with very high erosion potential were identified by both RVR Meander (only examining the applied hydraulic forces) and CONCEPTS (examining both the applied hydraulic forces and resistance of the bank soils). These are: two meander bends downstream of transect 3300, a meander bend midway transects 2600 and 2700, and a meander bend midway transects 200 and 300.

Page iii Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

Page iv Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

Table of Contents

EXECUTIVE SUMMARY ...... I

TABLE OF CONTENTS ...... V

LIST OF FIGURES ...... VIII

LIST OF TABLES ...... XI

LIST OF ABBREVIATIONS AND UNITS...... XII

CONVERSION FACTORS ...... XIV

INTRODUCTION ...... 1 Problem statement ...... 1 Objective ...... 1 Study area ...... 1 Report organization ...... 2

MODEL DESCRIPTION ...... 5 CONCEPTS ...... 5 Hydraulics ...... 5 Sediment transport and bed adjustment ...... 5 Stream bank erosion ...... 6 Implementation of in-stream channel protection measures ...... 6 Streambed Restoration Measures ...... 6 Stream bank Restoration Measures ...... 7 Input data requirements ...... 7 RVR Meander ...... 9 Hydrodynamics and bed topography ...... 9 Bank erosion and meander migration ...... 9 Input data requirements ...... 10

MODEL DATA & SETUP ...... 13 Overview ...... 13 Field Data Collection ...... 13 Channel form ...... 13 Channel boundary materials ...... 15 Grain size distribution and bulk density ...... 15 Erosion-Resistance Data Collection: Submerged Jet Erosion Test Device ...... 15

Page v Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

Geotechnical Data Collection: Borehole Shear Tests ...... 17 Flow...... 19 Bank protection measures ...... 22 Model setup ...... 25 RVR Meander ...... 25 Geometry ...... 25 Design discharge ...... 26 Friction coefficient ...... 26 Scour factor ...... 27 CONCEPTS ...... 28 Cross-sectional geometry ...... 28 Boundary materials ...... 33 Flow ...... 39 Bank protection measures ...... 43

MODEL RESULTS ...... 45 Overview ...... 45 RVR Meander ...... 45 CONCEPTS ...... 58 1.5-yr return period scenario ...... 58 Thalweg elevation ...... 58 Channel width ...... 59 2-yr return period scenario ...... 59 Thalweg elevation ...... 59 Channel width ...... 65 10-yr return period scenario ...... 65 Thalweg elevation ...... 65 Channel width ...... 68 50-yr return period scenario ...... 68 Thalweg elevation ...... 68 Channel width ...... 70 Summary ...... 72

CONCLUSIONS ...... 75 Resistance to erosion properties of boundary materials ...... 75 Bed material ...... 75 Bank material ...... 76

Page vi Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

Shear stresses applied by the flow in meander bends ...... 76 Simulated channel morphologic adjustment ...... 77

REFERENCES...... 79

APPENDIX A. CHANNEL GEOMETRY ...... 83

APPENDIX B. CHANNEL BOUNDARY MATERIALS...... 97

APPENDIX C. BANK PROTECTION MEASURES ...... 111

APPENDIX D. CONCEPTS MODEL BANK MATERIAL DATA ...... 113

APPENDIX E. BIG SIOUX RIVER MIGRATION BETWEEN 1991 AND 2012 ...... 117

Page vii Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

List of Figures Figure 1 Study reach on the Big Sioux River, South Dakota...... 3 Figure 2 Data collection sites (labeled 100 through 4100) along the Big Sioux River study reach. The collected data were: cross-sectional geometry and bed and bank material properties...... 14 Figure 3 Schematic of jet-test device (Hanson & Simon, 2001)...... 17 Figure 4 Photographs of the scaled-down mini-jet submerged jet test device, used in situ to measure soil erodibility...... 18 Figure 5 Schematic representation of borehole shear test (BST) device used to determine cohesive and frictional strengths of in-situ streambank materials...... 19 Figure 6 Observed daily discharge at USGS Gaging Station #06481000 (BIG SIOUX R NEAR DELL RAPIDS, SD)...... 20 Figure 7 Flow duration curve calculated from daily discharge data at USGS Gaging Station #06481000 (BIG SIOUX R NEAR DELL RAPIDS, SD)...... 20 Figure 8 Peak flow frequency analysis output from the USGS PEAKFQ software (Flynn, Kirby, & Hummel, 2006) for USGS Gaging Station #06481000 (BIG SIOUX R NEAR DELL RAPIDS, SD)...... 21 Figure 9 Photos of bank protection measures installed along the Big Sioux River study reach: (a) rip rap lining at transect 1700; (b) rip rap lining with a bendway weir at its downstream end (transect 2700); (C) Bendway weirs located at the upstream end of the bank protection at transect 1200; and (d) Bendway weirs at transect 400...... 23 Figure 10 Thalweg slope of the study reach used by the RVR Meander model...... 26 Figure 11 Scour factor validation for Cross Section 500 ...... 27 Figure 12 Scour factor validation for Cross Section 1700 ...... 28 Figure 13 Scour factor validation for Cross Section 2700 ...... 28 Figure 14 Procedure to construct synthetic cross sections to improve planform coverage: (a) sample bed, bank, and floodplain regions for transect 500, and (b) example constructed cross section near transect 3900...... 30 Figure 15 Fitted grain size distribution of riffle bed material as a function of river station...... 35 Figure 16 Fitted grain size distribution of pool bed material as a function of river station...... 36 Figure 17 Resistance to erosion analysis: (a) smoothing spline fitted to the measured critical shear stress as a function of river station; (b) regression between critical shear stress and soil detachment coefficient for bank face materials; and (c) regression between critical shear stress and soil detachment coefficient for bank toe materials...... 38 Figure 18 Annual Flow scenarios of selected return periods to assess impacts of bank protection measures on the morphology of the Big Sioux River between Dell Rapids and Sioux Falls, SD: (a) 1.5-yr return period, (b) 2-yr return period; (c) 10-yr return period, and (d) 50-yr return period...... 42

Page viii Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

Figure 19 Flow duration curves for the four simulated flow scenarios...... 43 Figure 20 Locations of large near-bank bed shear stress zones 0, 1 and 2. Flow is from top to bottom...... 46 Figure 21 Location of large near-bank bed shear stress zone 3. Flow is from top to bottom...... 47 Figure 22 Locations of large near-bank bed shear stress zones 4, 5 and 6. Flow is from top to bottom...... 48 Figure 23 Locations of large near-bank bed shear stress zones 7, 8 and 9. Flow is from top to bottom...... 49 Figure 24 Locations of large near-bank bed shear stress zones 10 and 11. Flow is from top to bottom...... 50 Figure 25 Locations of large near-bank bed shear stress zones 12 and 13. Flow is from top to bottom...... 51 Figure 26 Locations of large near-bank bed shear stress zones 14, 15, 16, 17, 18, 19, 20, and 21. Flow is from top to bottom...... 52 Figure 27 Locations of large near-bank bed shear stress zones 22, 23, 24, 25, 26, 27, 28, 29, and 30. Flow is from top to bottom...... 53 Figure 28 Locations of large near-bank bed shear stress zones 31, 32, 33, 34, 35, 36, and 37a. Flow is from top to bottom...... 54 Figure 29 Locations of large near-bank bed shear stress zones 37b, 38, 39, 40, 41, and 42. Flow is from top to bottom...... 55 Figure 30 Locations of large near-bank bed shear stress zones 43 and 44. Flow is from top to bottom...... 56 Figure 31 Comparison of bed elevation adjustment with and without current bank protection along the Big Sioux River study reach for the 1.5-yr return period flow scenario: (a) thalweg elevation, (b) change in thalweg elevation for each scenario, and (c) difference in final thalweg elevation between the two bank protection scenarios...... 60 Figure 32 Comparison of top width adjustment with and without current bank protection along the Big Sioux River study reach for the 1.5-yr return period flow scenario: (a) top width, (b) change in top width, and (c) difference in final top width between the two bank protection scenarios. .... 61 Figure 33 Map showing the simulated bank erosion for the 1.5-yr runoff scenario with current bank protection measures...... 62 Figure 34 Comparison of bed elevation adjustment along the Big Sioux River study reach with current bank protection under the 1.5- and 2-yr flow scenarios: (a) thalweg elevation, (b) change in thalweg elevation, and (c) difference between final thalweg elevations for 2- and 1.5-yr flow scenarios...... 63

Page ix Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

Figure 35 Comparison of channel top width adjustment along the Big Sioux River study reach with current bank protection under the 1.5- and 2-yr flow scenarios: (a) top width, (b) change in top width, and (c) difference in final top width for 2- and 1.5-yr flow scenarios...... 64 Figure 36 Comparison of bed elevation adjustment along the Big Sioux River study reach with current bank protection under the 1.5- and 10-yr flow scenarios: (a) thalweg elevation, (b) change in thalweg elevation, and (c) difference in thalweg elevation for 10- and 1.5-yr flow scenarios. .. 66 Figure 37 Comparison of channel top width adjustment along the Big Sioux River study reach with current bank protection under the 1.5- and 10-yr flow scenarios: (a) top width, (b) change in top width, and (c) difference in final top width for 10- and 1.5-yr flow scenarios...... 67 Figure 38 Comparison of bed elevation adjustment along the Big Sioux River study reach with current bank protection under the 1.5- and 50-yr flow scenarios: (a) thalweg elevation, (b) change in thalweg elevation, and (c) difference in thalweg elevation for 50- and 1.5-yr flow scenarios. .. 69 Figure 39 Comparison of channel top width adjustment along the Big Sioux River study reach with current bank protection under the 1.5- and 50-yr flow scenarios: (a) top width, (b) change in top width, and (c) difference in final top width for 50- and 1.5-yr flow scenarios...... 71 Figure 40 Map of erosion potential along the study reach of the Big Sioux River simulated by the CONCEPTS model...... 74 Figure 41 Map of transect locations along the study reach on the Big Sioux River, South Dakota ...... 84 Figure 42 Locations of sampled bed and bank material...... 98 Figure 43 Locations of JET and BST tests...... 99 Figure 44 Map of bank protection measures (marked as red lines) along the study reach of the Big Sioux River, South Dakota...... 111 Figure 45 Closeup of digitized top-of-bank from the 2012 USDA NAIP imagery...... 117 Figure 46 Observed migration of the Big Sioux River study reach between 1991 and 2012. The plotted bank lines represent top of bank and were digitized from the 2003 and 2012 NAIP imagery. 118 Figure 47 Change in channel planform of the Big Sioux River study reach between transects 2200 and 3100 over the period 1937-2012. The background image is the 1937 aerial photo. The blue line approximates the 1937 channel. The red line represents the 2012 channel top-of-bank...... 119

Page x Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

List of Tables Table 1 Flow duration statistics for USGS Gaging Station #06481000 (BIG SIOUX R NEAR DELL RAPIDS, SD)...... 21 Table 2 flow discharge for selected recurrence intervals calculated by the USGS PeakFQ software (Flynn, Kirby, & Hummel, 2006)...... 22 Table 3 Bank protection measures located along the Big Sioux River study reach...... 24 Table 4 Model cross sections used by CONCEPTS...... 31 Table 5 Smoothing parameter � and corresponding R-square value for smoothing splines fitted to the grain size distribution of the bed material samples...... 34 Table 6 Values of the coefficient and exponent of the fitted power law function between soil

detachment coefficient �� and critical shear stress ��...... 37 Table 7 Correction factor � for soil erodibility parameters: critical shear stress and soil detachment coefficient...... 40 Table 8 RVR Meander model parameters ...... 45 Table 9 List of high shear stress zones (see Figure 19 to Figure 29) that have exhibit significant migration between 1991 and 2012 (see Figure 35)...... 57 Table 10 Summary statistics of simulated changes in thalweg elevation and channel top width for the four flow scenarios. The upper two km of the study reach were omitted because of unrealistic widening rates...... 73 Table 11 Composite cross-sectional geometry of the transects along the study reach on the Big Sioux River, South Dakota...... 85 Table 12 Sediment/soil sample location, fractional content of main textural size classes, and bulk density. Sample location key: B = bed, LB = left bank (internal), LF = left bank face, LT = left bank toe, RB = right bank (internal), RF = right bank face, RT = right bank toe, R = riffle, P = pool, and O = other...... 100 Table 13 Resistance to fluvial erosion parameters measured with the JET test device. Test location key: LF = left bank face, LT = left bank toe, RF = right bank face, and RT = right bank toe. Note that the values of the soil detachment coefficient were multiplied by 106 for presentation purposes...... 105 Table 14 Bank soil shear strength measured with the BST device. Test location key: LB = left bank and RB = right bank...... 107

Page xi Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

List of Abbreviations and Units 1D one-dimensional � curve fitting parameter (coefficient) � flow area, in square meters; m2 AASL Annual Average Sediment Loading; m3 km-1 yr-1 ARS Agricultural Research Service � channel width, in meters; m � curve fitting parameter (exponent) BST Borehole Shear Test BSTEM Bank Stability and Toe Erosion Model �′ effective cohesion, in kilopascals; kPa � apparent cohesion, in kilopascals; Pa � dimensionless friction coefficient cms cubic meters per second; m3 s-1 DEM Digital Elevation Model DENR South Dakota Department of Environment and Natural Resources D/S downstream � sediment particle median diameter of grains in the bed, in meters; m � erosion rate, in meters per second; m s-1 GPS Global Positioning System � acceleration due to gravity, in meters per square second; 9.81 m s-2 � flow depth, in meters; m JET Jet Erosion Test � soil detachment coefficient, in meters per second per Pascal; m s-1 Pa-1 �∗ length of channel centerline, in kilometers; km � length of valley centerline, in kilometers, km LiDAR Light Detection and Ranging m curve-fitting parameter in the van Genuchten (1980) equation � Manning’s roughness coefficient, in seconds per cubic root of a meter; s m-1/3. NSL National Sedimentation Laboratory � smoothing parameter of smoothing spline � � discharge, in cubic meters per second; m3 s-1 � hydraulic radius, in meters; m RGA Rapid Geomorphic Assessment � smoothing spline �∗ channel gradient, in meters per meter; m m-1

Page xii Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

� friction slope, in meters per meter; m m-1 � valley slope, in meters per meter; m m-1 SDACD South Dakota Association of Conservation Districts TSS Total Suspended Solids USDA U.S. Department of Agriculture USGS U.S. Geological Survey U/S upstream � scour factor � unit weight of water, in kilonewtons per cubic meter; kN m-3 � ratio between shear stress on the outer bank of a meander bend and that at the channel centerline (or that predicted by a 1D model) Ω sinuosity, in kilometers per kilometer; km km-1 � boundary (bed or bank) shear stress, in Pascals; Pa. � D boundary shear stress calculated by a 1D model such as CONCEPTS, in pascals; Pa � critical shear stress, in Pascals; Pa

Page xiii Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

Conversion Factors Multiply By To obtain Length millimeter (mm) 0.03937 inch meter (m) 3.281 foot kilometer (km) 0.6214 mile

Area square meter (m2) 10.764 square foot square kilometer (km2) 0.3861 square mile

Volume cubic meter (m3) 35.31 cubic foot

Flow meter per second (m s-1) 3.281 foot per second cubic meter per second (m3 s-1) 35.31 cubic foot per second

Mass kilogram (kg) 2.205 pound tonne, metric 1.102 ton (short) metric tonne per square kilometer per year 2.855 ton (short) per square mile per year (ton km-1yr-1)

Force per unit length kilonewton per meter (kN m-1) 5.710 pound-force per inch kilonewton per meter (kN m-1) 68.52 pound-force per foot

Stress pascal (Pa) 0.02089 pound-force per square foot (= newton per square meter, N m-2) kilopascal (kPa) 0.145 pound-force per square inch kilopascal (kPa) 20.89 pound-force per square foot

Unit weight kilonewton per cubic meter (kN m-3) 6.366 pound-force per cubic foot

Page xiv Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

INTRODUCTION

Problem statement The State of South Dakota and the East Dakota Water Development District (Strom, 2010) identified large portions of the Big Sioux River and its tributaries in South Dakota that are impaired because of increased levels of Total Suspended Solids (TSS). Stream bank erosion can be an important contributor of sediment. Bank failures result in channel widening and the loss of adjacent lands. The USDA-ARS National Sedimentation Laboratory (NSL) has determined through an earlier study along the Big Sioux River that various types of bank-stabilization measures would be effective at reducing bank-erosion rates (Bankhead & Simon, 2009). The Bank-Stability and Toe-Erosion Model (BSTEM) was used to compare erosion rates under existing and mitigated conditions. What is unknown, however, is whether bed erosion (and then further bank erosion) will be initiated as a result of the reduction in sediment supply if successful bank-stabilization measures are undertaken at a large scale along the river. To determine this, a model that not only can dynamically adjust the bed and banks, but also routes flow and sediment needs to be applied.

Objective This study’s objective is to determine the impact on channel geometry of ongoing bank stabilization efforts along the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota.

NSL’s CONservational Channel Evolution and Pollutant Transport System (CONCEPTS) channel evolution computer model was used to:

1. Evaluate the effects of current bank stabilization measures between Dell Rapids and Sioux Falls on the morphology of the Big Sioux River along the same reach. 2. Identify unprotected locations with the highest bank erosion rates for future stabilization.

Data on cross-sectional profiles and resistance-to-erosion properties of channel boundary materials were collected in the field in collaboration with the State of South Dakota Department of Environment and Natural Resources and the South Dakota Association of Conservation Districts. Representative design discharges were used to provide hydraulic input.

Study area The geographic scope of the project is a 34-km long reach on the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota (Figure 1). The reach is fairly sinuous with an average sinuosity (ratio of channel length over valley length) of 1.6. The average channel slope is 0.4 m/km. The bed material is sand dominated with the median bed material grain size along the study reach varying between 0.03

Page 1 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota and 7.0 mm with a mean value of 1.4 mm. Bank material comprises erodible loam and sandy loam soils, but percent clay is found as high as 55%. As the Big Sioux is a meandering stream, bank erosion is common and responsible for about 25% of stream loading (Bankhead & Simon, 2009), and exceeds rates of 10 m/yr at some locations. Snow-melt driven flows in late Spring can be quite large and remain close to bankfull conditions for extended periods of time.

Report organization This report is organized as follows:

1. Model Description section. This section summarizes the capabilities and data requirements of the CONCEPTS and RVR Meander computer models used in the presented study. 2. Model Data & Setup section. This section presents what data and how it is used by the models. 3. Results section. This section presents the results of the modeling effort. 4. Conclusions section. This section summarizes the main findings of the modeling effort.

Page 2 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

FIGURE 1 STUDY REACH ON THE BIG SIOUX RIVER, SOUTH DAKOTA.

Page 3 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

Page 4 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

MODEL DESCRIPTION This section presents the computer models CONCEPTS and RVR Meander that were used to study the effects of bank protection measures on the morphology of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota. It summarizes the science and data requirements of the models.

CONCEPTS The forces acting on the stream boundary and the resistance to erosion of the boundary materials govern stream morphology. In general, the force exerted by the flowing water on the channel boundary depends on flow velocity distribution and boundary roughness. The resistance to erosion is a function of boundary material properties such as texture, density, erodibility, and shear strength. These properties are significantly affected by the presence of riparian vegetation. The CONCEPTS computer model simulates these forces and their controls. The following sections very briefly discuss the science included in the model. More detail can be found in Langendoen & Alonso (2008), Langendoen & Simon (2008), and Langendoen et al. (2009b).

Hydraulics CONCEPTS models streamflow as one-dimensional (1D) along the channel’s centerline. Hence, it is limited to fairly straight channels; it cannot predict bar formation and channel migration. CONCEPTS simulates gradually-varying flow (described by the Saint-Venant equations) as a function of time along a series of cross sections representing stream and floodplain geometry. The governing system of equations are solved using the generalized Preissmann scheme, allowing a variable spacing between cross sections and large time steps conducive to long-term simulations of channel evolution. The implementation of the solution method contains various enhancements to improve the robustness of the model, particularly for flashy runoff events.

Sediment transport and bed adjustment Alluvial stream banks are typically composed of fine-grained deposits containing clays, silts, and fine sands (hereafter referred to as fines), which may overlay coarser relic point bars. Streambeds are more commonly composed of sands and gravels, resistant clay layers or bed rock. Therefore, the range in particle sizes being transported in alluvial streams may be quite large and the composition of the sediment mixture in transport may be quite different from that of the bed material if a majority of the sediments are fines transported in suspension. CONCEPTS therefore calculates sediment transport rates by size fraction for 14 predefined sediment size classes ranging from 10 μm to 64 mm.

CONCEPTS uses a total-load evaluation of bed-material transport and treats movement of clays and fine silts (<10 μm) as pass-through background wash load. The differences in transport mechanics of suspended and bed load movement are accounted for through non-equilibrium effects. The

Page 5 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota composition of bed surface and substrate is tracked, enabling the simulation of vertical and longitudinal fining or coarsening of the bed material.

Stream bank erosion CONCEPTS simulates channel width adjustment by incorporating the two fundamental physical processes responsible for bank retreat: fluvial erosion or entrainment of bank-material particles by flow, and bank mass failure due to gravity. Bank material may be cohesive or non-cohesive and may comprise numerous soil layers.

The detachment of cohesive soils is calculated following an excess shear-stress approach. An average shear-stress on each soil layer is computed. If the critical shear stress of the material is exceeded, entrainment occurs. CONCEPTS is able to simulate the development of overhanging banks.

Stream bank failure occurs when gravitational forces that tend to move soil downslope exceed the forces of friction and cohesion that resist movement. The risk of failure is expressed by a factor of safety, defined as the ratio of resisting to driving forces or moments. CONCEPTS performs stability analyses of wedge-type failures and cantilever failures of overhanging banks. The effects of pore- water pressure and confining pressure exerted by the water in the stream are accounted for.

Implementation of in-stream channel protection measures CONCEPTS is capable of evaluating restoration measures at individual cross sections and along entire reaches. This allows, for example, the determination of restoration measure placement or the length of protection needed. It should be noted that, because CONCEPTS is a 1D model it cannot simulate the complex three-dimensional flow near in-stream structures and the resulting local channel morphology. Three-dimensional effects are averaged over the distance between two consecutive cross sections. However, a 1D approach can adequately assess the long-term impact of restoration measures on channel stability.

STREAMBED RESTORATION MEASURES Streambed restoration measures are typically employed to stabilize the streambed and control channel grade. Common grade control measures are sills or drop structures that can be constructed of large stones, logs, or sheet pile weirs.

There are two methods to evaluate grade control measures using CONCEPTS. Both methods assume that the grade control measures are stable under the full range of imposed flow conditions. First, if the designed drop in bed elevation at the structure is rather small, such that the flow drowns the structure for medium to large runoff events, the bed rock elevation can be set to the level of the bed surface at the cross section with the grade control structure. This will prevent erosion below this elevation. Deposition is possible, and the deposited material can be eroded in the future, but the extent of erosion is then limited to the top of the grade control structure.

Page 6 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

The second method uses a drop structure element in CONCEPTS. This method should be used if the drop in bed elevation at the structure is significant. In this case free-fall conditions cause a significant energy head loss that may not be simulated adequately by the above method. This method simulates both free fall and drowned conditions at drop structures. Bed load will be captured by the structure as long as its invert exceeds the upstream bed elevation. Once bed elevation exceeds structure invert all sediment will pass the structure and no further deposition will occur upstream of the structure. The drop structure geometry is limited to a symmetrical trapezoidal cross section with a horizontal bottom.

STREAM BANK RESTORATION MEASURES Stream bank restoration practices can be placed anywhere on the bank by introducing layers that represent the erodibility of the protection measure. Hence, these bank protection measures could cover the toe only or protect the entire bank face. Similarly, the effects of riparian vegetation on top of the bank on stream bank erosion can be evaluated using different soil layers.

Protection against fluvial erosion A bank material must be introduced to represent the protected portion of the bank. The critical shear stress and erodibility coefficient for this bank material layer should characterize the resistance to erosion of the stream bank protection measure. For example, the critical shear stress could be set to the allowable shear stress used in tractive channel design. Chapter 8 of the National Engineering Handbook (Natural Resources Conservation Service, 2007) tabulates allowable shear stress values for many bank protection measures.

A number of protection measures, for example vegetation, root wads or vanes, deflect the flow away from the bank thereby reducing shear stresses exerted by the flow, which cannot be simulated accurately by a 1D model such as CONCEPTS. However, this could be represented by an equivalent increase in critical shear stress of the affected bank soils.

Bank Stabilization Measures Bank stabilization measures typically enhance soil shear strength. This could be done for example by improving drainage or by mechanical reinforcement provided by roots of riparian vegetation. The vertical distribution of root biomass of riparian vegetation is represented by introducing bank-material layers with varying cohesion values. The Riproot model of (Pollen-Bankhead & Simon, 2009) can be used to calculate the added cohesion due to plant roots.

Input data requirements CONCEPTS uses two types of input data: (1) input data that control the execution of the model (e.g., simulation start and end dates, simulated processes, and requested output); and (2) input data that characterize the modeled stream corridor. Different data are required to perform hydraulic routing, sediment routing, and stream bank erosion calculations.

Page 7 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

To perform hydraulic routing the channel and floodplain geometry are required and are represented by a series of cross sections. These data are typically obtained through channel surveys using standard methods such as level or total station. Flow resistance is parameterized using the Manning � friction factor. The user can input different Manning � values for streambed, left and right banks, and left and right floodplains. Manning � values are reported in literature and can be calibrated using observed water surface profiles or flow depths. Discharge has to be specified at the inlet of the study reach and at tributary inflow points. Time series of discharges can be obtained through measurements or generated using hydrologic computer models. A boundary condition at the model outlet is optional. The model calculates a looped rating curve internally based on local flow conditions. However, if water level at the downstream boundary is controlled externally, the user can specify a rating curve or a time series of water level elevation.

To simulate sediment transport and bed adjustment initial bed-material stratigraphy with grain size distribution and porosity for each stratigraphic layer is required. Bed material can vary along the stream but is assumed homogeneous across the stream. Bed material gradation can be determined by sampling the bed material. Entrainment of cohesive, fine-grained bed material is calculated using an excess shear-stress approach that requires the specification of a critical shear stress below which no erosion takes place and an erosion-rate or erodibility coefficient that represents the rate at which the cohesive bed material is eroded once the critical shear stress is exceeded. The resistance to erosion can be measured in situ using portable flumes or jet testers, or samples can be collected and tested in laboratory settings using annular flumes or flumes such as the Erosion Function Apparatus (Briaud, et al., 2001). At inflow locations fractional sediment transport rates have to be specified, which can be either measured or calculated using sediment transport relations.

Stream bank erosion calculations require the specification of bank-material stratigraphy, with its associated grain-size distributions, bulk density, resistance to erosion (critical shear stress and erosion-rate coefficient) values, and shear-strength (cohesion and friction angle) values. Most properties can be measured by collecting samples and consequent laboratory analysis. Resistance to erosion and shear strength properties can also be measured in situ using jet test and borehole shear test devices, respectively.

Validation and applications of CONCEPTS (e.g., Wells et al., 2007; Langendoen & Alonso, 2008; Langendoen & Simon, 2008; Langendoen et al., 2009a, 2009b) showed that it can satisfactorily predict and quantify: (a) the temporal progression of an incised stream through the different stages of channel evolution, (b) changes in thalweg elevation, (c) changes in channel top-width, and (d) bed-material grain size distribution. However, bed- and bank-material properties representing resistance to erosion and failure must be adequately characterized. It is highly recommended to perform a geomorphic analysis of the stream system to determine channel conditions and variations in sediments and soils along the stream. Such an analysis could be performed using the Rapid Geomorphic

Page 8 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

Assessment technique (e.g., Simon et al., 2002). Differences between observed and simulated evolution are commonly largest along reaches where either: model assumptions regarding flow and sediment transport (e.g., one-dimensional assumption) are inappropriate, as is the case in the late stages of channel adjustment; or assumptions regarding input data (e.g., channel geometry, water inflows, or bed- and bank-material properties) are required. The use of median and average values of critical shear stresses and effective cohesion generally provide good results. Because critical shear stresses typically vary greatly both between different soils and within a soil, users of the model should measure an adequate number of critical shear stress values for each soil in the bed and banks.

RVR Meander The current RVR Meander platform (Motta, Abad, Langendoen, & Garcia, 2012) (http://rvrmeander.org/) extends the capabilities of the original version of RVR Meander (Abad & Garcia, 2006) by merging it with the stream bank erosion submodel of CONCEPTS (Langendoen & Simon, 2008). RVR Meander is composed of modules to simulate hydrodynamics, bed topography, bank erosion, and migration of meandering rivers. It is available as a plugin for ArcGis version 10.x.

Hydrodynamics and bed topography The model for hydrodynamics and bed topography implemented in RVR Meander is analytical and obtained from linearization of the two-dimensional depth-averaged Saint Venant equations of motion. It follows the approach first developed by Ikeda et al. (1981), and adopts the secondary flow correction derived by Johannesson & Parker (1989a), who introduced an “effective centerline curvature” - the secondary current cell strength - which lags behind the local channel curvature and determines the bed transverse slope through a coefficient of proportionality named scour factor. Johannesson & Parker (1989a) and Camporeale et al. (2007) provide details of the analytical solution. Important model assumptions are: spatially- and temporally-constant channel width; bed topography is only a function of channel planform (no free response of sediment); and spatially-constant friction coefficient. The assumption of constant channel width during meander migration, while generally being supported by empirical observations (Ikeda et al., 1981) and adopted by many authors (e.g., Johannesson & Parker, 1989b; Zolezzi & Seminara, 2001), is a mathematical and physical simplification to obtain the analytical solution for the two-dimensional hydrodynamics and is not a result of modeling conservation of sediment mass.

Bank erosion and meander migration In the physically-based meander-migration approach in RVR Meander developed by Motta et al. (2012), simulated bank retreat is controlled by the resistance to hydraulic erosion and the occurrence of cantilever and planar failures (Langendoen & Simon, 2008).

Page 9 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

Hydraulic erosion requires that the local boundary shear stress exceeds the critical value to detach crumbs or peds rather than that related to the primary sediment particles, and is modeled with an excess shear stress relation. An average erosion distance is computed for each layer comprising the composite bank material. Shear stress distribution on banks along bends is influenced by factors such as: secondary flow strength, bank slope, width-to-depth ratio, difference in roughness between bed and bank, and bedform progression. In the case of a straight channel, integration of the streamwise, depth-integrated momentum equation over a portion of wetted perimeter of interest allows computing the averaged stress over that portion. Following this method, the shear stress acting on each of the bank material layers is obtained by scaling the shear stress at the toe, which is the computed bed shear stress at the bank with the linear hydrodynamic model), using the hydraulic radius of the flow area impinging on the layer. In spite of the shortcomings associated to these methods and their strict validity for straight channels, they are adopted for their simplicity and hence efficiency to perform medium- to long-term simulations of channel evolution.

Cantilever failure is the collapse of an overhanging slab of bank material formed by preferential retreat of more erodible underlying layers or simply by the erosion of the bank below the water level with respect to its upper, unsaturated portion. The occurrence of cantilever failure, for the case of shear collapse mechanism (Thorne & Tovey, 1981) considered here, is simply determined from geometrical considerations, once an undercut threshold is exceeded. The undercut threshold is defined as the ratio of bank material cohesion to unit weight.

In cohesive materials, mass failures of whole blocks may occur along a planar or curved failure surface. For high banks with mild slopes (slope lower than 60 degrees), the failure block typically slides along a curved slip surface, whereas steep banks tend to develop planar-failure surfaces that are often truncated by tension cracks. RVR Meander only considers the latter case, since eroding banks are often steep at the outer margins of meander bends. In the RVR Meander model, the planar failure is analyzed using a limit equilibrium method in combination with a search algorithm to find the failure block configuration with the smallest factor of safety (Langendoen & Simon, 2008). Factor of safety is the ratio of available shear strength to mobilized shear strength, and when smaller than one the bank is unstable. The method accounts for the effects of pore-water pressure on bank material shear strength, confining hydrostatic pressure provided by the water in the channel, and can automatically insert tension cracks if the upper portion of the failure block is under tension.

Input data requirements As RVR Meander is a simplified 2D model it inputs are limited:

Design or bankfull discharge. Channel dimensions such as width, depth, and slope. Cross-sectional geometry is typically trapezoidal.

Page 10 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

Channel centerline to determine channel curvature. Valley slope. Channel boundary roughness. Scour factor, which relates the transverse channel bed slope to local curvature. Similarly to CONCEPTS, resistance to erosion properties of the bank soils, i.e. erodibility and shear-strength parameters.

Page 11 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

Page 12 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

MODEL DATA & SETUP

Overview This section describes the preparation of the data used by RVR Meander and CONCEPTS computer models. Model data were measured in the field or synthesized. Data to characterize channel form and the resistance-to-erosion properties of the boundary materials were collected in collaboration with the State of South Dakota Department of Environment and Natural Resources (DENR) and the State of South Dakota Association of Conservation Districts (SDACD) during the Summer and Fall of 2011. Data were collected at 41 transects along the study reach (see Figure 2).

Field Data Collection

Channel form Cross-sectional geometry was measured and compiled by DENR and SDACD. Three sources were used to construct profiles of the cross section and adjacent floodplain:

1. Survey-grade Global Positioning System (GPS) with an accuracy of about 1 cm. This instrument was used to survey points on the bed and bank during low flow conditions and floodplain adjacent to the channel. 2. Ohmex SonarMite Echo Sounder mounted on a boat for underwater bathymetry. The manufacturer-reported range accuracy of this instrument is about 2.5 cm. Four passes were made across the river at each location. Bottom elevation was derived by subtracting the measured range from the water surface elevation. 3. Aerial Light Detection and Ranging (LiDAR) derived Digital Elevation Model (DEM) topography to extract floodplain elevation.

Photos of the transects and measured cross-sectional geometry are documented in “Appendix A. Channel geometry.”

Page 13 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

FIGURE 2 DATA COLLECTION SITES (LABELED 100 THROUGH 4100) ALONG THE BIG SIOUX RIVER STUDY REACH. THE COLLECTED DATA WERE: CROSS-SECTIONAL GEOMETRY AND BED AND BANK MATERIAL PROPERTIES.

Page 14 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

Channel boundary materials The erosion, deposition, and transport of bed material is controlled by the shear stress acting on the sediment grains in the channel bed and its size distribution. Erosion of fine-grained (cohesive) bank materials is a combination of scour by the flowing water and bank collapse. Fluvial erosion of fine- grained materials is controlled by the hydraulic shear acting on the bank soils and the erodibility of these soils. The bank soil erodibility in this study was measured using a Jet Erosion Test (JET). Bank stability is a function of the strength of the bank material to resist collapse under gravity, which requires measurements of the components of shearing resistance (or shear strength), which were measured in this study using a Borehole Shear Test (BST) device. The below sections present these bed and bank material properties. Measured channel boundary materials are documented in “Appendix B. Channel Boundary Materials.” The following sections present the measuring methods.

GRAIN SIZE DISTRIBUTION AND BULK DENSITY The rate at which sediment particles are eroded and transported is partly determined by their sizes. Samples were collected from the bed and bank at each measurement transect. The bank samples were collected at locations where JET and BST tests were performed.

Sediment samples were weighed, oven dried at 40.5 °C for 24 hrs, then reweighed. Select sediment samples were sieved to determine particle size distribution as percent mass of size for the following size fractions (all diameters � are in mm): finer than 64, 32, 16, 8, 4, 2 , 1.41, 0.5, 0.354, 0.25, 0.178, 0.125, 0.088, and less than 0.063. The pan remains (less than 0.063 mm) were pipetted to determine size breaks for fine (< 0.025 mm) and coarse (>0.025 mm) silt and clay (< 0.002 mm). Bulk density determinations were collected in aluminium rings pressed normal to the surface, excavated, cleaned, weighed, oven dried at 40.5 °C for 24 hrs, then reweighed.

Table 12 on page 22 in Appendix B lists the major size fractions (clay (� < 0.002 mm), silt (0.002 < � < 0.0063 mm), sand (0.063 < � < 2 mm, and gravel (2 < � < 64 mm)) of the analyzed bed and bank material samples.

EROSION-RESISTANCE DATA COLLECTION: SUBMERGED JET EROSION TEST DEVICE Resistance properties of the bank-toe and face are input properties of the bank erosion submodel of RVR Meander and CONCEPTS. Where materials are non-cohesive, a bulk particle size or particle count are sufficient to describe resistance properties. However, cohesive materials are not entrained into the water column predictably due to particle size, as a result of electro-chemical bonds between particles. Ariathurai & Arulanandan (1978) showed that the rate of erosion, �of cohesive materials can be predicted by an excess shear stress equation:

�(� − �) � > � � = 0 � ≤ � (1)

Page 15 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

where � is erosion rate (m s-1), � is bed shear stress (Pa), � is critical shear stress (Pa), and � is soil detachment coefficient (m s-1 Pa-1) representing the volume of material eroded per unit force and per unit time.

A submerged jet erosion test has been developed by the Agricultural Research Service (Figure 3) for testing the in situ erodibility of surface materials in the laboratory and in the field (Hanson G. , 1990). This device has been developed based on knowledge of the hydraulic characteristics of a submerged jet and the corresponding scour produced by the jet. Fitting the jet-test data to the logarithmic- hyperbolic method to determine equilibrium (final) scour depth described in Hanson and Cook (1997) establishes �. The soil detachment coefficient � is then determined by curve fitting measured values of scour depth versus time. Here, a miniature version of the jet apparatus (mini-jet) was used (Figure 4). The mini-jet apparatus consists of an electric submersible 950 GPH pump powered by a portable A/C generator, a scaled-down 0.15 m-diameter submergence tank with an integrated, rotatable 3.18 mm-diameter nozzle and depth gauge, and delivery hoses. The nozzle is submerged within a cylindrical tank that is driven into the in situ material. The initial height of the nozzle above the streambed is noted and can be easily adjusted prior to initiating a test. Changes in maximum scour are measured using a point gauge at specific time increments and an asymptotic regression fitted to the erosion curve to calculate an initial point of entrainment, or material critical shear stress.

Page 16 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

FIGURE 3 SCHEMATIC OF JET-TEST DEVICE (HANSON & SIMON, 2001).

GEOTECHNICAL DATA COLLECTION: BOREHOLE SHEAR TESTS To gather data on the internal shear strength properties of the banks, in-situ Borehole Shear Test (BST) devices were used (Figure 5). To properly determine the resistance of cohesive materials to erosion by mass movement, data must be acquired on those characteristics that control shear strength; that is, cohesion, angle of internal friction, pore-water pressure, and bulk unit weight. Cohesion and friction angle data can be obtained from standard laboratory testing (triaxial shear or unconfined compression tests), or by in-situ testing with a borehole shear test device (Lutenegger & Hallberg, 1981). The BST provides direct, drained shear-strength tests on the walls of a borehole. Advantages of the instrument include:

1. The test is performed in situ and testing is, therefore, performed on undisturbed material. 2. Cohesion and friction angle are evaluated separately with the cohesion value representing apparent cohesion (�). Effective cohesion (�′) is then obtained by adjusting � for strength provided by matric suction according to measured pore-water pressure with a mini tensiometer. 3. A number of separate trials are run at the same sample depth to produce single values of cohesion and friction angle based on a standard Mohr-Coulomb failure envelope.

Page 17 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

4. Data and results obtained from the instrument are plotted and calculated on site, allowing for repetition if results are unreasonable; and 5. Tests can be carried out at various depths in the bank to locate weak strata.

At each testing depth, a small core of known volume was removed and sealed to be returned to the laboratory. The samples were weighed, dried and weighed again to obtain values of moisture content and bulk unit weight, both required for analysis of stream bank stability. Bulk particle size samples were also taken at each depth and tested in order to classify materials.

FIGURE 4 PHOTOGRAPHS OF THE SCALED-DOWN MINI-JET SUBMERGED JET TEST DEVICE, USED IN SITU TO MEASURE SOIL ERODIBILITY.

Page 18 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

FIGURE 5 SCHEMATIC REPRESENTATION OF BOREHOLE SHEAR TEST (BST) DEVICE USED TO DETERMINE COHESIVE AND FRICTIONAL STRENGTHS OF IN-SITU STREAMBANK MATERIALS.

Flow

Shear stresses acting on the channel boundary (responsible for erosion) are a function of the near- boundary flow field and boundary roughness. In CONCEPTS boundary shear stress (�) is calculated as:

� = ��� (2) where � is unit weight of water, � is hydraulic radius, and � is friction slope

�� � = � (3) in which � is discharge and � is flow area. For given discharge and boundary roughness, CONCEPTS calculates flow area and hydraulic radius.

The US Geological Survey operates a gage near the upstream boundary of the study reach: USGS #06481000 (BIG SIOUX R NEAR DELL RAPIDS, SD). Historical daily discharge is available for the period May 1, 1948 to September 30, 2013 (Figure 6). Figure 7 shows the flow duration curve calculated from the daily discharge record; the flow duration statistics are listed in Table 1. Figure 7 shows that the daily discharges follow a log-normal distribution. Annual peak flow rates are available between April 6, 1949 and July 2, 2013 (Figure 8). Both daily and peak flow data were used to determine representative runoff events to simulate channel adjustment of the Big Sioux River study reach. Peak

Page 19 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota flow data were analyzed using the USGS PeakFQ software to determine bankfull flow (Flynn, Kirby, & Hummel, 2006). Discharges at selected exceedance probabilities are listed in Table 2.

FIGURE 6 OBSERVED DAILY DISCHARGE AT USGS GAGING STATION #06481000 (BIG SIOUX R NEAR DELL RAPIDS, SD).

FIGURE 7 FLOW DURATION CURVE CALCULATED FROM DAILY DISCHARGE DATA AT USGS GAGING STATION #06481000 (BIG SIOUX R NEAR DELL RAPIDS, SD).

Page 20 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

FIGURE 8 PEAK FLOW FREQUENCY ANALYSIS OUTPUT FROM THE USGS PEAKFQ SOFTWARE (FLYNN, KIRBY, & HUMMEL, 2006) FOR USGS GAGING STATION #06481000 (BIG SIOUX R NEAR DELL RAPIDS, SD).

TABLE 1 FLOW DURATION STATISTICS FOR USGS GAGING STATION #06481000 (BIG SIOUX R NEAR DELL RAPIDS, SD).

Percentage of time exceeded Discharge (cms) 1 139 5 51.8 10 29.4 25 10.4 50 2.97 75 0.878 90 0.340 95 0.170 99 0.0396

Page 21 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

TABLE 2 FLOW DISCHARGE FOR SELECTED RECURRENCE INTERVALS CALCULATED BY THE USGS PEAKFQ SOFTWARE (FLYNN, KIRBY, & HUMMEL, 2006).

Recurrence interval Flow discharge (yrs) (m3/s) 1.05 17.4 1.5 62.4 2 97.3 5 229 10 355 50 761 100 993

Bank protection measures About 6.8 km of banks along the Big Sioux River study reach are protected against erosion. A map of the protected bank sections (Figure 44) is included in Appendix “Appendix C. Bank Protection Measures.” Bank protection is mainly comprised of rip rap lining placed on the bank toe and lower portion of the bank face sometimes in combination with bendway weir D/S to deflect the flow away from the bank (example photos are shown in Figure 9a and b). Five sites are protected by bendway weirs (example photos are shown in Figure 9c and d). Table 3 lists the installed bank protection measures.

Page 22 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

FIGURE 9 PHOTOS OF BANK PROTECTION MEASURES INSTALLED ALONG THE BIG SIOUX RIVER STUDY REACH: (A) RIP RAP LINING AT TRANSECT 1700; (B) RIP RAP LINING WITH A BENDWAY WEIR AT ITS DOWNSTREAM END (TRANSECT 2700); (C) BENDWAY WEIRS LOCATED AT THE UPSTREAM END OF THE BANK PROTECTION AT TRANSECT 1200; AND (D) BENDWAY WEIRS AT TRANSECT 400.

Page 23 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

TABLE 3 BANK PROTECTION MEASURES LOCATED ALONG THE BIG SIOUX RIVER STUDY REACH.

Location Bank Type Length (ft) D/S 300 Left Rip rap 605 300 Left Bendway weirs 960 U/S 300 Right Rip rap 855 400 Right Bendway weirs 430 U/S 400 Right Rip rap 550 D/S 500 Left Bendway weirs 560 D/S 600 Left Barbs 330 600 Right Rip rap 420 U/S 600 Left Barbs 1090 700 Left Rip rap 795 U/S 700 Right Rip rap & D/S weir 925 1000 Left Bendway weirs 755 1100 Left Rip rap 605 D/S 1200 Right Rip rap 645 1200 Left Rip rap & U/S weirs 1085 U/S 1400 Right Rip rap & D/S weir 335 1500 Left Rip rap 1200 U/S 1500 Right Rip rap & D/S weirs 735 1600 Right Rip rap 705 1700 Left Rip rap 1470 U/S 1800 Left Rip rap 775 2200 Left Rip rap 840 2300 Right Rip rap & D/S weir 520 2400 Right Rip rap 530 2700 Right Rip rap & D/S weir 1795 U/S 2700 Left Bendway weirs 545 2900 Right Rip rap 595 3100 Left Rip rap 780 3200 Right Rip rap 815

Page 24 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

Model setup

RVR Meander The RVR Meander computer model was used to:

Calculate the increased boundary shear stresses acting along the outer bank of meander bends in order to adjust the resistance-to-erosion values of the bank soils used by CONCEPTS. Indicate what banks experience the largest shear stresses and should be targeted for future bank protection.

The below sections present the data used to setup the model. RVR Meander needs the following data to run (see also Section “Input data requirements” in the RVR Meander description section): 1) design discharge; 2) channel dimensions, such as width, depth, and slope; 3) channel centerline to determine channel curvature; 4) valley centerline along the length of the channel to determine valley slope; 5) channel roughness; and 6) scour factor.

Plots of model runs are presented in the next chapter “Model Results.”

GEOMETRY Channel form data required to execute RVR Meander are:

1. channel centerline to represent channel planform, and 2. cross section geometry.

These data were provided by DENR and the SDACD (see above section “Channel form”).

Cross section geometry The surveyed cross sections (Section “Appendix A. Channel geometry”) were used to calculate an average channel top width and depth of 40.5 m and 2.2 m, respectively, which were used as input to the RVR Meander model.

Channel planform The channel planform is represented by the channel centerline, which was digitized from the 2010 National Agricultural Imagery Program (NAIP) aerial imagery between the upstream and downstream ∗ ends of the study reach (cf. Figure 1). The length of the digitized channel centerline is � = 34.4 km, ∗ and the length of the valley centerline is � = 20.9 km. The average centerline sinuosity (Ω = � ⁄�) of the study reach is 1.65. This information was used to determine both mean valley and channel slopes (see below). Computational nodes, spaced one-half channel width apart, were uniformly distributed along the discretized channel centerline. The centerline (�,�)-coordinates were used to calculate channel curvature at each computational node.

Page 25 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

Channel slope The series of thalweg points of the surveyed cross sections was used to determine a mean thalweg ∗ profile of the river and channel slope � , see Figure 10. The mean channel slope is 0.4 m/km. The ∗ valley slope used in the RVR Meander simulations is then calculated � = � Ω = 0.66 m/km.

DESIGN DISCHARGE The discharge record from the USGS gaging station #06481000 (BIG SIOUX R NEAR DELL RAPIDS, SD) was used to determine the design discharge following Bennett et al. (2012). Their peak discharge frequency analysis yielded a 1.5-year return period discharge of 57.5 m3/s, which is assumed to represent the bankfull discharge (Leopold, 1994). This value is different than that computed using the USGS PeakFQ software (see section “Flow” above), however it is used here as the RVR Meander design discharge as this value was also used in the engineered log jams laboratory experiments (Bennett et al., 2012). The design discharge was imposed at the upstream boundary of the RVR Meander model (near Dell Rapids) and was kept constant throughout the model reach. As there are no significant inflows along the study reach, the assumption of a spatially constant discharge was reasonable.

FIGURE 10 THALWEG SLOPE OF THE STUDY REACH USED BY THE RVR MEANDER MODEL.

FRICTION COEFFICIENT The friction coefficient was calculated by using the same Manning n coefficient (� = 0.052) as reported in the progress report on the experimental study of engineered log jams for the Big Sioux (Bennett et al., 2012). The equation used to relate the manning coefficient to the friction coefficient used by RVR Meander is:

Page 26 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

�� � = �/ (4) where � is the gravitational acceleration (9.81 m/s2). Using an average flow depth � = 2.2 m and the above Manning n, yields a friction coefficient � = 0.02.

SCOUR FACTOR Though analytical expression exist for the scour factor �, this parameter is typically calibrated to match the cross-sectional shape of a given planform. A comparison between the modeled and the surveyed bathymetry of the Big Sioux River study reach was carried out. Tests were conducted for scour factor values of 2, 5, and 7. It was determined that a scour factor of 7 fits the field data best. Figure 11 to Figure 13 show the comparison between the modeled and measured transverse bed topography (using a scour factor of 7) for cross sections 500, 1700 and 2700, respectively.

438 437 436 435 434

Elevation Elevation [m.a.s.l.] 433 432 0 50 100 150 200 250 Distance [m]

Bathymetry RVRMeander Result

FIGURE 11 SCOUR FACTOR VALIDATION FOR CROSS SECTION 500

Page 27 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

443 442 441 440 439 438 437

Elevation Elevation [m.a.s.l.] 436 435 0 50 100 150 200 250 300 Distance [m]

Bathymetry RVRMeander Result

FIGURE 12 SCOUR FACTOR VALIDATION FOR CROSS SECTION 1700

445 444 443 442 441 440 439 Elevation Elevation [m.a.s.l.] 438 0 50 100 150 200 250 300 Distance [m]

Bathymetry RVRMeander Result

FIGURE 13 SCOUR FACTOR VALIDATION FOR CROSS SECTION 2700

CONCEPTS The CONCEPTS model was used to simulate the morphologic adjustment of the Big Sioux River study reach to bank protection measures under different flow scenarios. To carry out the simulation the CONCEPTS model requires characterization of channel form, boundary materials (i.e., resistance to erosion of the channel boundary), and flow. This sections presents how these parameters were developed.

CROSS-SECTIONAL GEOMETRY The channel geometry used for CONCEPTS must adequately represent the planform shape of the river as channel erosion differs between riffle (in between meander bends) and pool (at meander bend)

Page 28 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota sections. Unfortunately, the collected topographic data consisted of cross sections at approximately 1.0 km spacing (see section “Appendix A. Channel geometry”), which is too coarse to describe the channel planform. The number and locations of the additional cross sections were selected based on the planform configuration of the Big Sioux River study reach. The channel centerline used in the RVR Meander simulation was used to select a minimum of three cross sections per meander wavelength. A total of 92 new cross sections were obtained, which yield a total of 133 cross sections for the CONCEPTS model (Table 4).

In general, the geometry of a cross section can be divided in distinct sections: bed, bank, and floodplain (see Figure 14). The below procedure was followed to obtain the geometry for the new cross sections:

1) The bed profile was obtained from the transverse slope calculated by the RVR Meander model (see Model Setup section “RVR Meander”). The elevation of the thalweg point in the cross section was determined from the average thalweg profile (Figure 10).

2) The topographic profile of the floodplain section was obtained from the LiDAR data. Channel top width was measured from the 2010 aerial imagery. As RVR Meander used a constant channel width of 40.5 m (see above), the width of the bed profile in some cross sections was larger than the actual top width shown in the aerial imagery. For those cases, the top width from the aerial imagery was used to constrain the bed width.

3) The bank geometry was obtained from interpolation of surrounding surveyed cross sections. For most cases the banks were almost vertical and there was no need to carry out a process of interpolation. In those cases a straight line was used to connect the bank toe point to the bank top point.

Page 29 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

(a)

(b)

FIGURE 14 PROCEDURE TO CONSTRUCT SYNTHETIC CROSS SECTIONS TO IMPROVE PLANFORM COVERAGE: (A) SAMPLE BED, BANK, AND FLOODPLAIN REGIONS FOR TRANSECT 500, AND (B) EXAMPLE CONSTRUCTED CROSS SECTION NEAR TRANSECT 3900.

Page 30 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

TABLE 4 MODEL CROSS SECTIONS USED BY CONCEPTS.

Cross Model distance Cross Source Model distance Cross Source Model distance section ID Source (km) section ID (km) section ID (km) 1 4100 0.274 23 synthesized 9.259 45 2300 16.19 2 synthesized 0.639 24 3100 9.665 46 synthesized 16.373 3 4000 1.005 25 synthesized 10.08 47 synthesized 16.557 4 synthesized 1.4 26 3000 10.47 48 synthesized 16.74 5 3900 1.828 27 synthesized 10.872 49 2200 17.015 6 synthesized 2.248 28 2900 11.277 50 synthesized 17.198 7 3800 2.65 29 synthesized 11.673 51 synthesized 17.38 8 synthesized 3.05 30 2800 12.065 52 synthesized 17.564 9 3700 3.453 31 synthesized 12.461 53 2100 17.821 10 synthesized 3.853 32 2700 12.87 54 synthesized 18.003 11 3600 4.256 33 synthesized 13.28 55 synthesized 18.186 12 synthesized 4.657 34 2600 13.696 56 synthesized 18.369 13 3500 5.042 35 synthesized 14 57 2000 18.57 14 synthesized 5.463 36 synthesized 14.3 58 synthesized 18.752 15 3400 5.92 37 2500 14.523 59 synthesized 18.935 16 synthesized 6.331 38 synthesized 14.706 60 synthesized 19.118 17 3300 6.706 39 synthesized 14.889 61 1900 19.41 18 synthesized 7.09 40 synthesized 15.073 62 synthesized 19.592 19 synthesized 7.5 41 2400 15.347 63 synthesized 19.775 20 synthesized 7.94 42 synthesized 15.531 64 synthesized 19.958 21 synthesized 8.39 43 synthesized 15.714 65 1800 20.215 22 3200 8.842 44 synthesized 15.897 66 synthesized 20.397

Page 31 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

Cross Model distance Cross Source Model distance Cross Source Model distance section ID Source (km) section ID (km) section ID (km) 67 synthesized 20.58 90 synthesized 25.223 113 600 29.865 68 synthesized 20.763 91 synthesized 25.406 114 synthesized 30.048 69 1700 21.019 92 synthesized 25.589 115 synthesized 30.231 70 synthesized 21.201 93 1100 25.845 116 synthesized 30.413 71 synthesized 21.384 94 synthesized 26.027 117 500 30.669 72 synthesized 21.566 95 synthesized 26.21 118 synthesized 30.852 73 1600 21.84 96 synthesized 26.393 119 synthesized 31.035 74 synthesized 22.023 97 1000 26.666 120 synthesized 31.218 75 synthesized 22.206 98 synthesized 26.849 121 400 31.474 76 synthesized 22.39 99 synthesized 27.032 122 synthesized 31.657 77 1500 22.627 100 synthesized 27.215 123 synthesized 31.84 78 synthesized 22.81 101 900 27.452 124 synthesized 32.022 79 synthesized 22.993 102 synthesized 27.635 125 300 32.259 80 synthesized 23.176 103 synthesized 27.818 126 synthesized 32.442 81 1400 23.432 104 synthesized 28.001 127 synthesized 32.626 82 synthesized 23.615 105 800 28.275 128 synthesized 32.81 83 synthesized 23.797 106 synthesized 28.457 129 200 33.084 84 synthesized 23.98 107 synthesized 28.64 130 synthesized 33.267 85 1300 24.254 108 synthesized 28.823 131 synthesized 33.449 86 synthesized 24.437 109 700 29.025 132 synthesized 33.632 87 synthesized 24.62 110 synthesized 29.207 133 100 33.888 88 synthesized 24.803 111 synthesized 29.39 89 1200 25.04 112 synthesized 29.573

Page 32 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

BOUNDARY MATERIALS Profiles of bank soils and bed sediments must be assigned to each cross section. Each profile comprised one or more material layers and their characteristics: grain size distribution and resistance to erosion properties. The information needed for the bed sediment profile is grain size distribution, particle density and porosity for each bed-material layer. As channel boundary materials are generally heterogeneous because of spatially complicated deposition and erosion patterns, their resistance-to- erosion properties may vary greatly. Therefore, measured values of these properties may also vary greatly. For use by CONCEPTS, the scatter in the measured data has been reduced by deriving representative values for the boundary material properties.

Bed material As only surface bed-material samples were available, the bed was assumed to comprise vertically homogeneous materials. The grain size distribution was obtained from the field data collection (see Section “Grain size distribution and bulk density”), whereas the particle density and porosity were assumed to be 2650 kg/m3 and 0.5, respectively.

Bed material in meandering streams typically varies between riffles (in between meander bends) and pools (in meander bends). Initial plotting of the size distribution of various grain size classes showed differences between riffle and pool samples, and showed similarities between riffle samples or between pool samples. Hence, characteristic bed-material grain size distributions were derived separately for pools and riffles as a function of river station along the study reach. Breaks in grain size classes used to derive the grain size distribution were < 0.002 mm (clay to silt split), < 0.063 mm (silt to sand split), < 0.5 mm (medium sand split), < 2 mm (sand to gravel split), and < 16 mm (medium gravel split). Observed data were fit with a smoothing spline using the Matlab Curve Fitting Toolbox (The MathWorks, Inc., 2014). The smoothing spline (�) is constructed using a smoothing parameter �, and minimizes the function:

�� � (� − �(� )) + (1 − �) �� �� (5) where (�, �) are the observed data points. The smoothing parameter � is defined between 0 and 1, where � = 0 produces a least-squares straight-line fit to the data, while � = 1 produces a cubic spline interpolant (The MathWorks, Inc., 2014). The smaller the smoothing parameter the smoother the curve.

Table 5 lists the values of the smoothing parameters used to derive the smoothing spline fitted to the grain size distribution of the riffle and pool bed-material samples. Table 5 also list the coefficient of determination � , which is the measure of the goodness of fit.

Page 33 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

TABLE 5 SMOOTHING PARAMETER � AND CORRESPONDING R-SQUARE VALUE FOR SMOOTHING SPLINES FITTED TO THE GRAIN SIZE DISTRIBUTION OF THE BED MATERIAL SAMPLES.

Smoothing Grain size parameter � R-squared value Riffles < 0.002 mm 0.035 0.6288 < 0.063 mm 0.035 0.6503 < 0.5 mm 0.008 0.4015 < 2 mm 0.035 0.8195 < 16 mm 0.035 0.4647 Pools < 0.002 mm 0.008 0.7074 < 0.063 mm 0.008 0.6605 < 0.5 mm 0.008 0.6665 < 2 mm 0.004 0.3078 < 16 mm 0.004 0.2378

Figure 15 shows the fitted smoothing splines of grain size distribution for riffle bed material samples, while Figure 16 shows the fitted smoothing splines of grain size distribution for pool bed material samples. The grain size distributions used by CONCEPTS were then determined by obtaining the values from the smoothing spline for a certain distance along the stream. For example, for riffle cross section 22 (that is, transect 3200), which is located on 25.05 km upstream of cross section 133 (that is, transect 100, and the most downstream point in Figure 15 and Figure 16), the %finer for the various size classes are: 8.77% for � < 0.002 mm, 24.0% for � < 0.063 mm, 46.3% for � < 0.5 mm, 93.5% for � < 2.0 mm, and 100% for � < 16.0 mm. Intermediate %finer values were interpolated from the before-mentioned values using log-transformed particle diameter.

Page 34 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

FIGURE 15 FITTED GRAIN SIZE DISTRIBUTION OF RIFFLE BED MATERIAL AS A FUNCTION OF RIVER STATION.

Page 35 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

FIGURE 16 FITTED GRAIN SIZE DISTRIBUTION OF POOL BED MATERIAL AS A FUNCTION OF RIVER STATION.

Bank material The stratigraphy of the bank materials was characterized at each data transect by collecting samples from the borehole prepared for the BST tests to measure bank material shear strength. A maximum of three layers was used for the various soil profiles. The measured grain size distribution, bulk density, shear strength (BST), and erodibility (JET) data are summarized in Appendix “Appendix B. Channel Boundary Materials.” The procedures to assign these data to CONCEPTS model cross sections were:

1. The grain size distribution of BST test locations was used.

2. The bulk density was obtained from the dry density at BST test locations.

3. The bank material stratigraphy was assumed to be the same for both banks in each cross section.

Page 36 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

4. Erodibility parameters (� and �) were smoothed along the length of the study reach to reduce scatter.

5. Erodibility parameters (� and �) were adjusted in meander bends in order to account for the increased boundary shear stress acting on the outer bank, which cannot be simulated with a 1D model such as CONCEPTS.

Further details are presented in the below section. Appendix “Appendix D. CONCEPTS Model Bank Material Data” lists the bank material profile and properties assigned to each model cross section.

Erodibility

The critical shear stress � was smoothed along the length of the study reach using a smoothing spline to reduce the natural variability in the measured data (Figure 17a). The smoothing procedure used the same spline formulation as was used to smooth the bed material grain size distribution, Eq. (5).

The soil detachment coefficient was then calculated from the observed relationship between � and �. The erodibility of bank face and bank toe materials is typically different. Bank face materials represent the eroded (parent) bank material, which is typically more consolidated than the toe material that is composed of deposits from excess bed material or local, failed bank material. The deposited material on the bank toe typically is looser than that on the bank face. Hence, different �-� relationships were used for bank toe and bank face materials. A power law function, � = �� , was fitted through the measured values (Figure 17b and c). Table 6 lists the values of the coefficient and exponent of the two power law functions, which shows that the soil detachment coefficient of the bank toe material is about twice that of the bank face material.

TABLE 6 VALUES OF THE COEFFICIENT AND EXPONENT OF THE FITTED POWER LAW FUNCTION BETWEEN SOIL DETACHMENT

COEFFICIENT �� AND CRITICAL SHEAR STRESS ��.

Coefficient � Exponent � R-squared Bank face 4.08 × 10-4 -1.58 0.68 Bank toe 8.09 × 10-4 -1.44 0.43

Page 37 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

FIGURE 17 RESISTANCE TO EROSION ANALYSIS: (A) SMOOTHING SPLINE FITTED TO THE MEASURED CRITICAL SHEAR STRESS AS A FUNCTION OF RIVER STATION; (B) REGRESSION BETWEEN CRITICAL SHEAR STRESS AND SOIL DETACHMENT COEFFICIENT FOR BANK FACE MATERIALS; AND (C) REGRESSION BETWEEN CRITICAL SHEAR STRESS AND SOIL DETACHMENT COEFFICIENT FOR BANK TOE MATERIALS.

In meander bends the erodibility values � and � used by CONCEPTS have to be adjusted because CONCEPTS underpredicts the enhanced boundary shear stress on the outer bank caused by the helical flow. If we assume that the actual shear stress is a factor � greater than that computed by CONCEPTS

(�D), that is:

� = ��D (6) and substitute Eq. (6) into Eq. (1), yields a bank soil erosion rate in meander bends of

� = �(��D − �) (7)

Eq. (7) can be rewritten as

� = �(�D − �) (8)

Page 38 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

where � = �� and � = �⁄�. Hence, the correct rate of erosion of outer bank soils in meander bends can be calculated by increasing the soil detachment coefficient by a factor � and reducing the critical shear stress by the same factor �.

The correction factor � was calculated using the RVR Meander model. It was defined as the ratio between the calculated bed shear stress at the outer bank and the calculated bed shear stress at the centerline of the river. This ratio was termed normalized shear stress by Frias et al. (2011). The shear stress calculated at the channel centerline is that for uniform flow and therefore similar to the cross section average shear stress calculated by CONCEPTS. The distribution of the normalized shear stress calculated by RVR Meander is presented in the next section “RVR Meander.” Table 7 lists the values of � calculated by RVR Meander for CONCEPTS model cross sections located in meander bends.

FLOW The discharge record from USGS gaging station #06481000 (BIG SIOUX R NEAR DELL RAPIDS, SD) was used to determine four annual flow scenarios to study the impact of bank protection measures on the morphology of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota. The four flow scenarios were based on years containing a runoff event (hydrographs) with a peak discharge of 1.5-yr, 2-yr, 10-yr, or 50-yr return period (see Table 2 for the corresponding peak discharge). The selected annual hydrographs are for calendar years 2008 (1.5-yr return period), 1978 (2-yr return period), 1997 (10-yr return period), and 1969 (50-yr return period), see Figure 18. The 50-yr scenario represents the highest flow of record at the USGS gage.

The flow duration curve for each scenario is shown in Figure 19. The duration curves for the 2- and 5o-yr scenarios are very similar except for the greatest discharges. The annual runoff volumes for each flow scenario were as follows: 1.5-yr scenario, 406 million m3; 2-yr scenario, 428 million m3; 10-yr scenario, 1,320 million m3; and 50-yr scenario, 686 million m3.

The base flow was assumed to be 3 m3/s, which is the median daily discharge value. Discharges smaller than the base-flow discharge were set equal to the base-flow discharge. It was further assumed that the discharge occurred at noon of each day.

Page 39 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

TABLE 7 CORRECTION FACTOR � FOR SOIL ERODIBILITY PARAMETERS: CRITICAL SHEAR STRESS AND SOIL DETACHMENT COEFFICIENT.

Cross Model Soil erodibility section ID distance (km) correction factor � Outer bank 2 0.639 2 right 3 1.005 1.5 left 4 1.400 2 right 5 1.828 1.5 left 7 2.650 3 right 18 7.090 2 right 19 7.500 3 left 25 10.080 1.5 left 26 10.470 2.5 left 28 11.277 3 right 31 12.461 1.5 left 32 12.870 2 right 33 13.280 3 left 34 13.696 2 left 35 14.000 2 right 37 14.523 2 right 38 14.706 1.5 left 41 15.347 2.5 right 42 15.531 2 left 45 16.190 2.5 right 46 16.373 2.5 left 47 16.557 1.5 right 48 16.740 2 left 49 17.015 1.5 right 51 17.380 1.5 right 52 17.564 1.5 left 53 17.821 1.5 left 54 18.003 1.5 left 55 18.186 2 right 56 18.369 1.5 left 57 18.570 2 left 58 18.752 1.5 right 59 18.935 1.5 left 64 19.958 2 left 69 21.019 2 left

Page 40 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

Cross Model Soil erodibility section ID distance (km) correction factor � Outer bank 71 21.384 1.5 right 73 21.840 1.5 left 74 22.023 2 right 75 22.206 2 left 78 22.810 1.5 left 79 22.993 1.5 left 80 23.176 2 right 81 23.432 2 left 82 23.615 1.5 right 85 24.254 2 left 86 24.437 2 right 87 24.620 2 left 88 24.803 2 right 90 25.223 2 left 93 25.845 1.5 right 94 26.027 1.5 left 96 26.393 2 right 98 26.849 2.5 left 99 27.032 2 right 100 27.215 1.5 left 101 27.452 1.5 right 103 27.818 1.5 left 104 28.001 2 right 107 28.640 2 right 108 28.823 1.5 right 109 29.025 3 left 110 29.207 1.5 right 114 30.048 1.5 right 115 30.231 1.5 left 119 31.035 3 right 120 31.218 1.5 left 121 31.474 1.5 right 122 31.657 2 left 123 31.840 1.5 right 125 32.259 1.5 left 127 32.626 2 right 128 32.810 2.5 left

Page 41 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

Cross Model Soil erodibility section ID distance (km) correction factor � Outer bank 132 33.632 2.5 right 133 33.888 1.5 right

FIGURE 18 ANNUAL FLOW SCENARIOS OF SELECTED RETURN PERIODS TO ASSESS IMPACTS OF BANK PROTECTION MEASURES ON THE MORPHOLOGY OF THE BIG SIOUX RIVER BETWEEN DELL RAPIDS AND SIOUX FALLS, SD: (A) 1.5-YR RETURN PERIOD, (B) 2-YR RETURN PERIOD; (C) 10-YR RETURN PERIOD, AND (D) 50-YR RETURN PERIOD.

Page 42 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

FIGURE 19 FLOW DURATION CURVES FOR THE FOUR SIMULATED FLOW SCENARIOS.

BANK PROTECTION MEASURES Bank soil properties at model cross sections with bank protection measures were adjusted to prevent bank erosion to occur. The critical shear stress and soil detachment coefficient were set to 1,000 Pa and 0.0 m s-1 Pa-1, respectively. Soil cohesion was set to a relatively large value of 10 kPa, to prevent mass failure of the bank during periods with elevated groundwater tables. No changes were made to the constructed profile of the cross section.

Page 43 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

Page 44 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

MODEL RESULTS

Overview This section presents the results of the simulations carried out using the RVR Meander and CONCEPTS computer models. RVR Meander was used to determine: (1) the enhanced shear stresses exerted by the helical flow on the outer bank of meander bends, and (2) highest stream bank erosion potential. The ratio of the shear stress on the outer bank of meander bends to that at the channel centerline was used to modify the bank soil erodibility employed by the CONCEPTS model. The CONCEPTS model was used to study the effects of bank stabilization structures on bank and bed erosion. Model data and setup were described in detail in the previous section.

RVR Meander The model parameters used in the simulation are summarized in Table 8. To assess potential erosion zones, the bed shear stress vector was normalized with the centerline bed shear stress magnitude for each cross section (�normalized = �⁄ �centerline). The simulated normalized shear stresses are shown in Figure 20 to Figure 30 (downstream to upstream). The flow direction is from North to South. Results are plotted on top of the 2010 National Agricultural Imagery Program aerial imagery. These figures also show the locations of existing bank erosion control structures and the locations where bank erosion control structures may potentially be needed. The potential sites are located at areas with normalized shear stresses that exceed a value of 1.5. This value was selected because the majority of the existing bank erosion control structures are located in such regions. The latter could also be considered as a validation of the hydrodynamics of the model since the location of existing bank erosion control structures should match zones of high shear stress.

TABLE 8 RVR MEANDER MODEL PARAMETERS

Description Symbol Value Units Flow rate � 57.5 m3/s Channel width 2� 40.5 m Channel depth � 2.2 m Valley slope � 0.658 m/km Friction coefficient � 0.02 - Scour factor � 7 -

Page 45 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

FIGURE 20 LOCATIONS OF LARGE NEAR-BANK BED SHEAR STRESS ZONES 0, 1 AND 2. FLOW IS FROM TOP TO BOTTOM.

Page 46 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

FIGURE 21 LOCATION OF LARGE NEAR-BANK BED SHEAR STRESS ZONE 3. FLOW IS FROM TOP TO BOTTOM.

Page 47 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

FIGURE 22 LOCATIONS OF LARGE NEAR-BANK BED SHEAR STRESS ZONES 4, 5 AND 6. FLOW IS FROM TOP TO BOTTOM.

Page 48 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

FIGURE 23 LOCATIONS OF LARGE NEAR-BANK BED SHEAR STRESS ZONES 7, 8 AND 9. FLOW IS FROM TOP TO BOTTOM.

Page 49 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

FIGURE 24 LOCATIONS OF LARGE NEAR-BANK BED SHEAR STRESS ZONES 10 AND 11. FLOW IS FROM TOP TO BOTTOM.

Page 50 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

FIGURE 25 LOCATIONS OF LARGE NEAR-BANK BED SHEAR STRESS ZONES 12 AND 13. FLOW IS FROM TOP TO BOTTOM.

Page 51 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

FIGURE 26 LOCATIONS OF LARGE NEAR-BANK BED SHEAR STRESS ZONES 14, 15, 16, 17, 18, 19, 20, AND 21. FLOW IS FROM TOP TO BOTTOM.

Page 52 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

FIGURE 27 LOCATIONS OF LARGE NEAR-BANK BED SHEAR STRESS ZONES 22, 23, 24, 25, 26, 27, 28, 29, AND 30. FLOW IS FROM TOP TO BOTTOM.

Page 53 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

a

FIGURE 28 LOCATIONS OF LARGE NEAR-BANK BED SHEAR STRESS ZONES 31, 32, 33, 34, 35, 36, AND 37A. FLOW IS FROM TOP TO BOTTOM.

Page 54 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

b

FIGURE 29 LOCATIONS OF LARGE NEAR-BANK BED SHEAR STRESS ZONES 37B, 38, 39, 40, 41, AND 42. FLOW IS FROM TOP TO BOTTOM.

Page 55 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

FIGURE 30 LOCATIONS OF LARGE NEAR-BANK BED SHEAR STRESS ZONES 43 AND 44. FLOW IS FROM TOP TO BOTTOM.

Page 56 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

These high-shear-stress zones do not necessarily require bank erosion control structures because the analysis does not consider the resistance to erosion of the bank materials. For example, zones 5 and 6 in Figure 22 are located along a riparian forest. The historic migration of the study reach (see Appendix “Appendix E. Big Sioux River Migration Between 1991 and 2012”) shows that zone 5 may not require a bank erosion control structure as the riparian vegetation may reduce soil erodibility and increase soil shear strength sufficient to greatly reduce bank retreat; however, this is not the case for zone 6. High-shear-stress zones that have also exhibited significant migration are listed in Table 9.

TABLE 9 LIST OF HIGH SHEAR STRESS ZONES (SEE FIGURE 20 TO FIGURE 30) THAT HAVE EXHIBIT SIGNIFICANT MIGRATION BETWEEN 1991 AND 2012 (SEE FIGURE 46).

High shear stress zone Nearby transect 0 100 2 300 4 600 6 900 8 1300 15 2100 18 2200 22 2400 23 2500 24 2500 26 2600 27 2600 29 2600 32 2800 34 2900 37a 3000

One important factor to be considered in determining the location of potentially, new bank erosion control structures is the morphodynamic equilibrium of the channel. In case of a meander bend cutoff there will be a local hydraulic disequilibrium (caused by the shortening of the channel and subsequent increase in channel slope) rejuvenating the migration process. The 2003 and 2012 aerial imagery show that the study reach has not exhibited significant migration for most of its length (except for the stretch of river between transects 2000 and 3100). Moreover, the meandering channel can be considered to be fairly stable because the sinuosity is greater than 1.5 and the slope is 0.4 m/km. Therefore, any potential cutoff should be controlled to avoid a dramatic increase in the migration rates

Page 57 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota of the bends (cf. observed historic migration between transects 2000 and 3100 that appears to be caused by meander bend cutoff, see Figure 46 and Figure 47). Figure 29 shows a potential cutoff could occur at high shear zone 40 (the only such region in the study area). If erosion in this zone is not controlled, a cutoff may cause accelerated migration locally because the slope of the channel increases due to a shortening of the channel. The steeper gradient will change the hydrodynamics of the system, creating new bends and changing the existing planform of the system.

CONCEPTS This section presents the bed elevation and channel top width adjustment simulated by the CONCEPTS model for the annual runoff records containing runoff events that have peak discharges with return periods of 1.5, 2, 10, and 50 years (see Section “Flow” above for more details on the runoff scenarios). The model results show the effects of the current bank protection measures on the bed elevation and channel width for the 1.5-yr runoff event, and investigates the effects of larger flow discharges (> 1.5- yr return period) on bed elevation and channel top width. Note that the initial thalweg profile of the study reach on the Big Sioux River assumed a fairly constant slope of 0.4 m/km, which is not representative of meandering rivers that have lowered elevations in meander bends (at pools) and raised elevations at riffles relative to the mean bed elevation. As a result, CONCEPTS simulated scour up to 2 m at pool locations and deposition up to 1 m at riffle locations (see for example Figure 31). Figure 10 shows that these deviations from the mean thalweg profile agree well with those observed, which also vary by 1 to 2 m.

1.5-yr return period scenario

THALWEG ELEVATION Figure 31 shows the effect of the installed bank protection measures on bed elevation for the 1.5-yr flow scenario (i.e., bank full discharge). Figure 31a compares the simulated thalweg profiles, Figure 31b compares the simulated change in bed elevation, and Figure 31c plots the ratio of simulated bed elevation change with current bank protection structures to that without bank protection structures. Model simulations show that bank protection measures do not affect bed elevation significantly. The main difference in simulated bed elevation is located near model kilometer 12.87 or transect 2700 (cf. Figure 41 for transect location), which shows about 1.5 m of scour at the pool of transect 2700 but no change in bed elevation and deposition downstream of transect 2700 for the scenario without bank protection measures. The river is fairly active in this region, see also Appendix “Appendix E. Big Sioux River Migration Between 1991 and 2012.” The simulated increase in bank erosion in this area without bank protection measures (see Figure 32) prevents the simulated scour of the bed that occurs with the current bank protection measures in place. Some enhanced scour was simulated for the scenario without current bank protection measures at model kilometers 25.85, 32.26 and 33.63. At these locations the upstream river reaches exhibited greater widening without bank protection that results in

Page 58 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota a relative narrowing downstream compared to the scenario with bank protection measures and consequently a greater capacity to erode sediments from the bed.

CHANNEL WIDTH Figure 32 shows the effect of the installed bank protection measures on channel top width for the 1.5- yr flow scenario (i.e., bank full discharge). Figure 32a compares the simulated channel top width for each bank protection scenario, Figure 32b compares the simulated change in channel top width for each bank protection scenario, and Figure 32c plots the difference in simulated channel top width for the two bank protection scenarios.

Figure 32b shows that even with the current bank protection structures there is still significant bank erosion along about 10 km (~30%) of the study reach. The amount of widening at the majority of eroding cross sections is about 4 m. Figure 33 shows an aerial view of these eroding locations.

Without bank protection structures eight locations show increased bank erosion (defined here as an erosion rate greater than 1.0 m): model station 12.46 (cross section 31, midway transects 2700 and 2800), model station 25.04 (cross section 89, transect 1200), model station 25.41 (cross section 91, midway transects 1100 and 1200), model station 26.03 (cross section 94, immediately downstream of transect 1100), model station 28.82 (cross section 108, immediately upstream of transect 700), model station 31.47 (cross section 121, transect 400), model station 31.84 (cross section 123 midway transects 400 and 500), model station 32.81 (cross section 128, immediately upstream of transect 200), and model station 33.08 (cross section 129, transect 200).

2-yr return period scenario

THALWEG ELEVATION Figure 34 compares the effects of annual flow events containing peak discharges of 1.5-yr and 2-yr return period on thalweg elevation. Figure 34a compares the simulated thalweg elevation profile for each flow scenario, Figure 34b compares the simulated change in thalweg elevation for each flow scenario, and Figure 34c plots the difference in simulated thalweg elevation for the two flow scenarios.

The simulated final thalweg profiles are very similar (see Figure 34a and Figure 34b). On average the elevations are slightly larger for the 2-yr flow scenario. There is increased deposition at the upper 1.5 km of the study reach (cross sections 1 to 4, from midway transects 3900 and 4000 to transect 4100). Scour of the bed is reduced for model stations 13.28 (cross section 33, midway between transects 2600 and 2700) and 27.45 (cross section 101, transect 900), see Figure 34c. Figure 35c shows that at these locations channel top width has increased significantly.

Page 59 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

FIGURE 31 COMPARISON OF BED ELEVATION ADJUSTMENT WITH AND WITHOUT CURRENT BANK PROTECTION ALONG THE BIG SIOUX RIVER STUDY REACH FOR THE 1.5-YR RETURN PERIOD FLOW SCENARIO: (A) THALWEG ELEVATION, (B) CHANGE IN THALWEG ELEVATION FOR EACH SCENARIO, AND (C) DIFFERENCE IN FINAL THALWEG ELEVATION BETWEEN THE TWO BANK PROTECTION SCENARIOS.

Page 60 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

FIGURE 32 COMPARISON OF TOP WIDTH ADJUSTMENT WITH AND WITHOUT CURRENT BANK PROTECTION ALONG THE BIG SIOUX RIVER STUDY REACH FOR THE 1.5-YR RETURN PERIOD FLOW SCENARIO: (A) TOP WIDTH, (B) CHANGE IN TOP WIDTH, AND (C) DIFFERENCE IN FINAL TOP WIDTH BETWEEN THE TWO BANK PROTECTION SCENARIOS.

Page 61 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

FIGURE 33 MAP SHOWING THE SIMULATED BANK EROSION FOR THE 1.5-YR RUNOFF SCENARIO WITH CURRENT BANK PROTECTION MEASURES.

Page 62 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

FIGURE 34 COMPARISON OF BED ELEVATION ADJUSTMENT ALONG THE BIG SIOUX RIVER STUDY REACH WITH CURRENT BANK PROTECTION UNDER THE 1.5- AND 2-YR FLOW SCENARIOS: (A) THALWEG ELEVATION, (B) CHANGE IN THALWEG ELEVATION, AND (C) DIFFERENCE BETWEEN FINAL THALWEG ELEVATIONS FOR 2- AND 1.5-YR FLOW SCENARIOS.

Page 63 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

FIGURE 35 COMPARISON OF CHANNEL TOP WIDTH ADJUSTMENT ALONG THE BIG SIOUX RIVER STUDY REACH WITH CURRENT BANK PROTECTION UNDER THE 1.5- AND 2-YR FLOW SCENARIOS: (A) TOP WIDTH, (B) CHANGE IN TOP WIDTH, AND (C) DIFFERENCE IN FINAL TOP WIDTH FOR 2- AND 1.5-YR FLOW SCENARIOS.

Page 64 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

CHANNEL WIDTH Figure 35 compares the effects of annual flow events containing peak discharges of 1.5-yr and 2-yr return period on channel top width. Figure 35a compares the simulated channel top width for each flow scenario, Figure 35b compares the simulated change in channel top width for each flow scenario, and Figure 35c plots the difference in simulated channel top width for the two flow scenarios.

The increased runoff of the 2-yr flow scenario increased bank erosion significantly at the following locations: 4.6 to 17.8 m increase between model stations 0.639 and 1.4 (between cross sections 2 and 4, from midway transects 3900 and 4000 to midway transects 4000 and 4100), 2.2 m increase at model station 3.85 (cross section 10, midway transects 3600 and 3700), 2.7 m increase at model station 12.87 (cross section 32 at transect 2700), 4.0 m at model station 13.28 (cross section 33, midway transects 2600 and 2700), 2.2 m at model station 26.21 (cross section 95, midway transects 1000 and 1100), 12.4 m at model station 27.45 (cross section 101, transect 900), and 5.4 m at model station 33.08 (cross section 129, transect 200).

10-yr return period scenario

THALWEG ELEVATION Figure 36 compares the effects of annual flow events containing peak discharges of 1.5-yr and 10-yr return period on thalweg elevation. Figure 36a compares the simulated thalweg elevation profile for each flow scenario, Figure 36b compares the simulated change in thalweg elevation for each flow scenario, and Figure 36c plots the difference in simulated thalweg elevation for the two flow scenarios.

The mean change in bed elevation along the study reach is fairly similar for each flow scenario (see also below summary section). In general, there is increased deposition along the upper three km of the study reach (between transects 3800 and 4100), increased scour between model stations 5 and 7 km (between transects 3300 and 3500), filling in of the pool at model station 13.28 (midway transects 2600 and 2700), and increased scour (up to 1.7 m) between model stations 31.03 and 33.45 (between midway transects 400 and 500 and midway transects 100 and 200).

Page 65 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

FIGURE 36 COMPARISON OF BED ELEVATION ADJUSTMENT ALONG THE BIG SIOUX RIVER STUDY REACH WITH CURRENT BANK PROTECTION UNDER THE 1.5- AND 10-YR FLOW SCENARIOS: (A) THALWEG ELEVATION, (B) CHANGE IN THALWEG ELEVATION, AND (C) DIFFERENCE IN THALWEG ELEVATION FOR 10- AND 1.5-YR FLOW SCENARIOS.

Page 66 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

FIGURE 37 COMPARISON OF CHANNEL TOP WIDTH ADJUSTMENT ALONG THE BIG SIOUX RIVER STUDY REACH WITH CURRENT BANK PROTECTION UNDER THE 1.5- AND 10-YR FLOW SCENARIOS: (A) TOP WIDTH, (B) CHANGE IN TOP WIDTH, AND (C) DIFFERENCE IN FINAL TOP WIDTH FOR 10- AND 1.5-YR FLOW SCENARIOS.

Page 67 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

CHANNEL WIDTH Figure 37 compares the effects of annual flow events containing peak discharges of 1.5-yr and 10-yr return period on channel top width. Figure 37a compares the simulated channel top width for each flow scenario, Figure 37b compares the simulated change in channel top width for each flow scenario, and Figure 37c plots the difference in simulated channel top width for the two flow scenarios.

The increased runoff of the 10-yr flow scenario increased bank erosion significantly at the following locations: 5.7 m increase at model station 0.639 (cross section 2, midway transects 4000 and 4100), 17.7 m at model station 1.4 (cross section 4, midway transects 3900 and 4000), 6.7 m increase at model station 1.82 (cross section 5, transect 3900), 2.2 m increase between model stations 3.85 and 4.25 (cross sections 10 and 11, transect 3600 to midway transects 3600 and 3700), 5.4 m at model station 7.09 (cross section 18, immediately downstream of transect 3300), 2.7 m increase at model station 12.87 (cross section 32 at transect 2700), 11.9 m at model station 13.28 (cross section 33, midway transects 2600 and 2700), 5.2 m increase at model station 18.57 (cross section 57, transect 2000), 2.3 m at model station 23.98 (cross section 84, immediately upstream of transect 1300), 1.5 m at model station 25.84 (cross section 93, transect 1100), 2.2 m at model station 26.21 (cross section 95, midway transects 1000 and 1100), 11.9 m at model station 27.45 (cross section 101, transect 900), 8.5 m increase at model station 32.63 (cross section 127, midway transects 200 and 300), 8.3 m at model station 33.08 (cross section 129, transect 200), 2.8 m at model station 33.27 (cross section 130, immediately downstream of transect 200), and 13.3 m at model station 33.63 (cross section 132, immediately upstream of transect 100). Most widening locations correspond to segments of the study reach with deposition, except those at the lower end of the study reach (see above section).

50-yr return period scenario

THALWEG ELEVATION Figure 38 compares the effects of annual flow events containing peak discharges of 1.5-yr and 50-yr return period on thalweg elevation. Figure 38a compares the simulated thalweg elevation profile for each flow scenario, Figure 38b compares the simulated change in thalweg elevation for each flow scenario, and Figure 38c plots the difference in simulated thalweg elevation for the two flow scenarios.

Similarly to the 10-yr scenario, there is generally increased deposition along the upper three km of the study reach (between transects 3800 and 4100), increased scour between model stations 5 and 8 km (between transects 3200 and 3500), filling in of the pool at model station 13.28 (midway transects 2600 and 2700). However, the lower portion of the study reach exhibits some degradation instead of aggradation (between transects 100 and 900).

Page 68 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

FIGURE 38 COMPARISON OF BED ELEVATION ADJUSTMENT ALONG THE BIG SIOUX RIVER STUDY REACH WITH CURRENT BANK PROTECTION UNDER THE 1.5- AND 50-YR FLOW SCENARIOS: (A) THALWEG ELEVATION, (B) CHANGE IN THALWEG ELEVATION, AND (C) DIFFERENCE IN THALWEG ELEVATION FOR 50- AND 1.5-YR FLOW SCENARIOS.

Page 69 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

CHANNEL WIDTH Figure 39 compares the effects of annual flow events containing peak discharges of 1.5-yr and 50-yr return period on channel top width. Figure 39a compares the simulated channel top width for each flow scenario, Figure 39b compares the simulated change in channel top width for each flow scenario, and Figure 39c plots the difference in simulated channel top width for the two flow scenarios.

Figure 39 shows significant widening along most unprotected bends of the study reach. The simulated widening along the upper 2 km of the study reach seems unrealistic, which is partly caused by the fairly large deposition along this portion of the study reach. In addition, only one jet test measurement was available along the upper reach (transect 4000, see Table 13), which indicated an erodible soil:

� = 1.7 Pa and � = 0.22 mm/s Pa.

Simulated channel widening exceeded 5 m at the following locations: 13.3 to 20.5 m between model stations 6.71 (cross sections 17 to 19, transect 3300 to midway transects 3200 and 3300), 35.5 m at model station 13.28 (cross section 33, midway transects 2600 and 2700), 11.9 m at model station 15.53 (cross section 42, immediately downstream of transect 2400), 8.9 m at model station 15.71 (cross section 43, midway transects 2300 and 2400), up to 20 m increase between model stations 16.37 and 18.75 (between cross sections 46 and 58, from transect 2000 to 2300), 11.7 m at model station 22.21 (cross section 75, midway transects 1500 and 1600), 10.6 m at model station 23.43 (cross section 81, transect 1400), up to 23 m between model stations 27.45 and 28.27 (between cross sections 101 and 105, from transect 800 to transect 900), 9.6 m at model station 29.21 (cross section 110, immediately downstream of transect 700), 8.5 m at model station 29.86 (cross section 113, transect 600), 7.4 m at model station 31.66 (cross section 122, immediately downstream of transect 400), 6.4 m at model station 32.02 (cross section 124, immediately upstream of transect 300), 13.9 m at model station 32.63 (cross section 127, midway transects 200 and 300), 15.0 m at model station 33.08 (cross section 129, transect 200), and 27.9 m at model station 33.63 (cross section 132, immediately upstream of transect 100). Unlike the 10-yr flow scenario, where most widening locations corresponded to segments of the study reach with deposition, some segments showed both bed scour and widening for the 50-yr scenario, such as: downstream of transect 3300, between transects 2000 and 2300, between transects 1400 and 1600, between transects 600 and 700, and near transect 400.

Page 70 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

FIGURE 39 COMPARISON OF CHANNEL TOP WIDTH ADJUSTMENT ALONG THE BIG SIOUX RIVER STUDY REACH WITH CURRENT BANK PROTECTION UNDER THE 1.5- AND 50-YR FLOW SCENARIOS: (A) TOP WIDTH, (B) CHANGE IN TOP WIDTH, AND (C) DIFFERENCE IN FINAL TOP WIDTH FOR 50- AND 1.5-YR FLOW SCENARIOS.

Page 71 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

Summary Table 10 summarizes the mean and maximum changes in bed elevation and channel top width for the four flow scenarios evaluated using the CONCEPTS computer model. Changes in bed elevation do not vary much between the different flow scenarios or the presence of bank protection measures. There are small variations in pool elevations, some become slightly deeper or some become slightly shallower, however the pool-riffle pattern is not affected much. This indicates there is enough transport capacity at higher flows to move the eroded bank materials downstream. Note, it was assumed here that the supply of bed-material load at the upstream boundary increased accordingly.

On the other hand, channel top width at a few locations increased greatly. It should be noted that these increases in channel top width were sometimes accompanied by deposition on the bed, which could have accelerated the rate of bank erosion. Figure 40 shows a map of the erosion potential along the study reach. Erosion potential is classified as minor, moderate, and severe. The minor erosion potential class consists of locations that experience erosion < 5 m for the 1.5-yr flow scenario, which does not greatly increases for the other, larger flow scenarios. The moderate erosion potential class consists of locations that experience erosion > 5 m for the 1.5-yr flow scenario, or < 5 m for the 1.5- yr flow scenario but with significant increase for the larger flow events. The severe erosion potential class consists of locations that may experience erosion amounts exceeding 15 m for any flow scenario. Severe erosion potential locations are (Figure 40): meander bends downstream of transect 3300, bends downstream of transect 2700, meander bend at transect 2000, meander bend at transect 900, meander bend at transect 200, and the meander bend midway transects 100 and 200.

Page 72 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

TABLE 10 SUMMARY STATISTICS OF SIMULATED CHANGES IN THALWEG ELEVATION AND CHANNEL TOP WIDTH FOR THE FOUR FLOW SCENARIOS. THE UPPER TWO KM OF THE STUDY REACH WERE OMITTED BECAUSE OF UNREALISTIC WIDENING RATES.

Bed elevation Channel top width change change Annual Bank Peak Total runoff (m) (m) flow protection discharge volume scenario measures (m3/s) (millions of m3) Mean Maximum† Mean Maximum -2.14 1.5-yr No 79.9 406 -0.08 1.51 8.78 1.01 -2.10 1.5-yr Yes 79.9 406 -0.11 1.28 8.77 1.12 -1.41 2-yr Yes 142 428 -0.02 1.49 16.2 1.20 -2.57 10-yr Yes 445 1,320 -0.19 1.97 19.6 0.98 -2.11 50-yr Yes 991 686 -0.34 4.39 35.5 1.13 † The negative number indicates maximum scour, whereas the positive number indicates maximum deposition.

Page 73 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

FIGURE 40 MAP OF EROSION POTENTIAL ALONG THE STUDY REACH OF THE BIG SIOUX RIVER SIMULATED BY THE CONCEPTS MODEL.

Page 74 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

CONCLUSIONS Large portions of the Big Sioux River and its tributaries in South Dakota are impaired because of increased levels of Total Suspended Solids (TSS). Stream bank erosion can be an important contributor of sediment. Further, bank failures result in channel widening and the loss of adjacent lands. The USDA-ARS National Sedimentation Laboratory (NSL) determined through an earlier study along the Big Sioux River that various types of bank-stabilization measures would be effective at reducing bank- erosion rates. What was unknown, however, was whether bed erosion (and then further bank erosion) will be initiated as a result of the reduction in sediment supply if successful bank-stabilization measures are undertaken at a large scale along the river. To determine this, a model that not only can dynamically adjust the bed and banks, but also routes flow and sediment needs to be applied.

NSL’s CONservational Channel Evolution and Pollutant Transport System (CONCEPTS) channel evolution computer model was used to:

1. Evaluate the effects of current bank stabilization measures between Dell Rapids and Sioux Falls on the morphology of the Big Sioux River along the same reach. 2. Identify unprotected locations with the highest bank erosion rates for future stabilization.

The forces acting on the stream boundary and the resistance to erosion of the boundary materials govern stream morphology. In general, the force exerted by the flowing water on the channel boundary depends on flow velocity distribution and boundary roughness. The resistance to erosion is a function of boundary material properties such as texture, density, erodibility, and shear strength.

Data on cross-sectional profiles and resistance-to-erosion properties of channel boundary materials were collected in the field in collaboration with the State of South Dakota Department of Environment and Natural Resources and the South Dakota Association of Conservation Districts. Representative design discharges were used to provide hydraulic input to calculate the force exerted by the flowing water.

The study’s findings are presented in three sections below.

Resistance to erosion properties of boundary materials The resistance to erosion of sediment is represented by its particle size when cohesionless or by a critical shear stress and soil detachment coefficient (or erodibility coefficient) when cohesive.

Bed material Bed material is cohesionless. Both pool and riffle bed materials gradually coarsen from the upstream end of the study reach at Dell Rapids to the downstream end at Sioux Falls (Figure 15 and Figure 16). The streamwise-smoothed median grain size varied between 2.5 mm at the downstream end to about

Page 75 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

0.4 mm at the upstream end of the study reach. These grain sizes are such that the bed is mobile along the entire study reach when mean flow depth exceeds approximately 0.5 m.

Bank material Bank material is cohesive except for the sediments/soils at depth, which consist of sands and gravels. The upper cohesive layer primarily comprises sandy loam (34% of collected samples) and loam soils (19% of collected samples). The critical shear stress required to erode these materials was fairly constant (about 10 Pa) for the lower 25 km of the study reach (Figure 17a). The critical shear stress linearly reduced to about 2 Pa at the upstream end of the study reach. The bank face materials (� = . . 4.1 10 � ) were about twice as hard to erode as the bank toe materials (� = 8.1 10 � ), see Figure 17a and b.

Note that the low measured resistance to erosion at the upper end of the study reach was caused by having only a single measurement available. As a result, the simulated channel widening along the upper portion of the study reach was quite large. More measurements to characterize soil erodibility are required along the upper portion between study transects 3800 and 4100.

Shear stresses applied by the flow in meander bends The CONCEPTS computer model used to assess the morphologic adjustment of the study reach is one- dimensional, and therefore cannot accurately simulate flow hydraulics that increase boundary shear stresses in meander bends relative to straight channel sections. The computer model RVR Meander was used to determine: (1) the enhanced shear stresses exerted by the helical flow on the outer bank of meander bends, and (2) highest stream bank erosion potential. The ratio of the shear stress on the outer bank of meander bends to that at the channel centerline was used to modify the bank soil erodibility employed by the CONCEPTS model.

RVR Meander simulated high shear stresses at 45 unprotected bends that may potentially lead to enhanced migration rates (Figure 20 to Figure 30). Sixteen of these bends have exhibited significant migration between 1991 and 2012 (Table 9), and should be targeted for construction of bank protection works. Note that 11 of these bends are located between study transects 2100 and 3000.

One important factor to be considered in determining the location of potentially, new bank erosion control structures is the morphodynamic equilibrium of the channel. In case of a meander bend cutoff there will be a local hydraulic disequilibrium (caused by the shortening of the channel and subsequent increase in channel slope) rejuvenating the migration process. The 2003 and 2012 aerial imagery show that the study reach has not exhibited significant migration for most of its length (except for the stretch of river between transects 2000 and 3100). Moreover, the meandering channel can be considered to be fairly stable because the sinuosity is greater than 1.5 and the slope is 0.4 m/km. Therefore, any potential cutoff should be controlled to avoid a dramatic increase in the migration rates

Page 76 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota of the bends (cf. observed historic migration between transects 2000 and 3100 that appears to be caused by meander bend cutoff, see Figure 46 and Figure 47). Figure 29 shows a potential cutoff could occur at high shear zone 40 (the only such region in the study area). If erosion in this zone is not controlled, a cutoff may cause accelerated migration locally because the slope of the channel increases due to a shortening of the channel. The steeper gradient will change the hydrodynamics of the system, creating new bends and changing the existing planform of the system.

Simulated channel morphologic adjustment The one-dimensional channel evolution computer model CONCEPTS was used to assess the effect of current bank protection measures on channel morphologic adjustment and to identify locations vulnerable for erosion along the Big Sioux River between Dell Rapids and Sioux Falls under a range of different flow conditions. Evaluated flow discharges ranged from bankfull (discharge with a 1.5- to 2- yr return period) to discharges with a return period of 50 years.

Changes in bed elevation did not vary much between the different flow scenarios or because of the presence of bank protection measures (Table 10). There were small variations in pool elevations, some became eroded slightly and some became slightly shallower, however the pool-riffle pattern was not affected much. This indicates there is enough transport capacity at higher flows to move the eroded bank materials downstream.

Channel top width increased significantly at a few locations. Bank erosion potential was categorized as minor, moderate, and severe. The minor erosion potential class consisted of locations that experienced erosion < 5 m for the 1.5-yr flow scenario, and which did not greatly increase for the larger flow scenarios. The moderate erosion potential class comprised locations that experienced erosion > 5 m for the 1.5-yr flow scenario, or < 5 m for the 1.5-yr flow scenario but with significantly increased erosion for the larger flow events. The severe erosion potential class consisted of locations that experienced top bank retreat exceeding 15 m for any flow scenario. 6.5 km (19.3%) of the study reach was classified as having minor erosion potential, 0.5 km (1.5%) of the study reach was classified as having moderate erosion potential, and 1.8 km (5.5%) of the study reach was classified as having severe erosion potential (Figure 40). It should be noted that these increases in channel top width were sometimes accompanied by deposition on the bed, which could have accelerated the rate of bank erosion. Severe erosion potential locations were: meander bends downstream of transect 3300, bends downstream of transect 2700, meander bend at transect 2000, meander bend at transect 900, meander bend at transect 200, and the meander bend midway transects 100 and 200.

Three locations with very high erosion potential were identified by both RVR Meander (only examining the applied hydraulic forces) and CONCEPTS (examining both the applied hydraulic forces and

Page 77 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota resistance of the bank soils). These are: two meander bends downstream of transect 3300, a meander bend midway transects 2600 and 2700 and a meander bend midway transects 200 and 300.

Page 78 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

REFERENCES Abad, J. D., & Garcia, M. H. (2006). RVR Meander: A toolbox for re-meandering of channelized streams. Computer & Geosciences, 32(1), 92-101. doi:10.1016/j.cageo.2005.05.006

Ariathurai, R., & Arulanandan, K. (1978). Erosion rates of cohesive soils. Journal of the Hydraulics Division, 104(HY2), 279-283.

Bankhead, N., & Simon, A. (2009). Analysis of bank stability and potential load reduction along reaches of the Big Sioux River, South Dakota. National Sedimentation Laboratory. Oxford, Mississippi: US Department of Agriculture, Agricultural Research Service.

Bennett, S. J., Atkinson, J. F., Gallisdorfer, M. S., & Ghaneeizad, S. M. (2012). Progress Report: Experimental Study of Engineered Log Jams for Use in the Big Sioux River, SD. Buffalo, NY: SUNY Buffalo.

Briaud, J. L., Ting, F., Chen, H. C., Cao, Y., Han, S. W., & Kwak, K. (2001). Erosion Function Apparatus for scour rate predictions. Journal of Geotechnical and Geoenvironmental Engineering, 127(2), 105-113.

Camporeale, C., Perona, P., Porporato, A., & Ridolfi, L. (2007). Hierarchy of models for meandering rivers and related morphodynamic processes. Reviews of Geophysics, 45(1), RG1001. doi:10.1029/2005RG000185

Flynn, K. M., Kirby, W. H., & Hummel, P. R. (2006). User's manual for program PeakFQ, Annual Flood Frequency Analysis Using Bulletin 17B Guidelines. In U.S. Geological Survey Techniques and Methods Book 4 , Chapter B4 (p. 42). Reston, VA: US Geological Survey.

Frias, C. E., Abad, J. D., & Langendoen, E. J. (2011). Numerical simulation to evaluate the location of existing and potential new bank erosion control structures on the Big Sioux River, SD: Simulations with the RVR Meander computer model. Pittsburgh: The University of Pittsburgh.

Hanson, G. (1990). Surface erodibility of earthen channels at high stress. Part II - Developing an in situ testing device. Transactions of ASAE, 33(1), 132-137.

Hanson, G. J., & Simon, A. (2001). Erodibility of cohesive streambeds in the loess area of the midwestern USA. Hydrological Processes, 15(1), 23-38. doi:10.1002/hyp.149

Ikeda, S., Parker, G., & Sawai, K. (1981). Bend theory of river meanders. Part 1. Linear development. J. Fluid Mech., 112, 363-377. doi:10.1017/S0022112081000451

Johannesson, H., & Parker, G. (1989a). Velocity redistribution in meandering rivers. Journal of Hydraulic Engineering, 115(8), 1019-1039. doi:10.1061/(ASCE)0733-9429(1989)115:8(1019)

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Johannesson, H., & Parker, G. (1989b). Linear theory of river meanders. In S. Ikeda, & G. Parker (Eds.), River Meandering (pp. 181-212). Washington, DC: AGU.

Langendoen, E. J., & Alonso, C. V. (2008). Modeling the evolution of incised streams. I: Model formulation and validation of flow and streambed evolution components. Journal of Hydraulic Engineering, 134(6), 749-762. doi:10.1061/(ASCE)0733-9429(2008)134:6(749)

Langendoen, E. J., & Simon, A. (2008). Modeling the evolution of incised streams. II: Streambank erosion. Journal of Hydraulic Engineering, 134(7), 905-915. doi:10.1061/(ASCE)0733- 9429(2008)134:7(905)

Langendoen, E. J., Lowrance, R. R., & Simon, A. (2009a). Assessing the impact of riparian processes on streambank stability. Ecohydrology, 2, 360-369. doi:10.1002/eco.78

Langendoen, E. J., Wells, R. R., Thomas, R. E., Simon, A., & Bingner, R. L. (2009b). Modeling the evolution of incised streams. II: Model application. Journal of Hydraulic Engineering, 135(6), 476-486. doi:10.1061/(ASCE)HY.1943-7900.0000029

Leopold, L. B. (1994). A View of the River. Cambridge, MA: Harvard University Press.

Lutenegger, J. A., & Hallberg, B. R. (1981). Borehole shear test in geotechnical investigations. ASTM Special Publication 740, 566-578.

Motta, D., Abad, J. D., Langendoen, E. J., & Garcia, M. H. (2012). A simplified 2D model for meander migration with physically-based bank evolution. Geomorphology, 163-164, 10-25. doi:10.1016/j.geomorph.2011.06.036

Natural Resources Conservation Service. (2007). Stream Restoration Design. In National Engineering Handbook Part 654 (p. 1626). Washington, DC: US Department of Agriculture.

Pollen-Bankhead, N., & Simon, A. (2009). Enhanced application of root-reinforcement algorithms for bank-stability modeling. Earth SUrface Processes and Landforms, 34, 471-480. doi:10.1002/esp.1690

Simon, A., Bingner, R. L., Langendoen, E. J., & Alonso, C. V. (2002). Actual and reference sediment yields for the James Creek Watershed - Mississippi. National Sedimentation Laboratory. Oxford, Mississippi: US Department of Agriculture, Agricultural Research Service.

Strom, R. (2010). Central Big Sioux River Watershed project - Segment 1. Brookings, South Dakota: East Dakota Water Development DIstrict.

The MathWorks, Inc. (2014, March 1). MATLAB R2014a documentation. Retrieved from MathWorks Web site: http://www.mathworks.com/help/matlab/index.html

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Thorne, C. R., & Tovey, N. K. (1981). Stability of composite river banks. Earth SUrface Processes and Landforms, 6(5), 469-484. doi:10.1002/esp.3290060507

Wells, R. R., Langendoen, E. J., & Simon, A. (2007). Modeling pre- and post-dam removal sediment dynamics: The Kalamazoo River, Michigan. Journal of the American Water Resources Association, 43, 773-785. doi:10.1111/j.1752-1688.2007.00062.x

Zolezzi, G., & Seminara, G. (2001). Downstream and upstream influence in river meandering. Part 1. General theory and application to overdeepening. Journal of Fluid Mechanics, 438, 183-211.

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Page 82 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

APPENDIX A. CHANNEL GEOMETRY This appendix presents the cross-sectional geometry of the transects where field data to characterize channel form and boundary materials were collected. Figure 41 shows the location of the transects along the study reach on the Big Sioux River. Table 11 tabulates the cross-sectional geometry for each transect, which was constructed using:

1. Survey-grade Global Positioning System (GPS) with an accuracy of about 1 cm. This instrument was used to survey points on the bed and bank during low flow conditions and floodplain adjacent to the channel. 2. Ohmex SonarMite Echo Sounder mounted on a boat for underwater bathymetry. The manufacturer reported range accuracy of this instrument is about 2.5 cm. Four passes were made across the river at each location. Bottom elevation was derived by subtracting the measured range from the water surface elevation. 3. Aerial Light Detection and Ranging (LiDAR) derived Digital Elevation Model (DEM) topography to extract floodplain elevation.

Page 83 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

FIGURE 41 MAP OF TRANSECT LOCATIONS ALONG THE STUDY REACH ON THE BIG SIOUX RIVER, SOUTH DAKOTA

Page 84 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

TABLE 11 COMPOSITE CROSS-SECTIONAL GEOMETRY OF THE TRANSECTS ALONG THE STUDY REACH ON THE BIG SIOUX RIVER, SOUTH DAKOTA. XS 100 437

436

435

434

433

432 ELEVATION IN IN (NAD83) METERSELEVATION 431 350 400 450 500 550 600 650 STATION, IN METERS XS 200 440 439 438 437 436 435 434 433 432 ELEVATION IN IN (NAD83) METERSELEVATION 431 350 400 450 500 550 600 650 STATION, IN METERS XS 300 437

436

435

434

433 ELEVATION IN IN (NAD83) METERSELEVATION 432 350 400 450 500 550 600 650 STATION, IN METERS XS 400 438

437

436

435

434

433

432 ELEVATION IN ELEVATION(NAD83) METERS 431 350 400 450 500 550 600 650 STATION, IN METERS

Page 85 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

TABLE 11 COMPOSITE CROSS-SECTIONAL GEOMETRY OF THE TRANSECTS ALONG THE STUDY REACH ON THE BIG SIOUX RIVER, SOUTH DAKOTA. XS 500 438

437

436

435

434

433 ELEVATION IN IN (NAD83) METERSELEVATION 432 350 400 450 500 550 600 650 STATION, IN METERS XS 600 438

437

436

435

434 ELEVATION IN IN (NAD83) METERSELEVATION 433 350 400 450 500 550 600 650 STATION, IN METERS XS 700 439

438

437

436

435

434 ELEVATIONIN METERS (NAD83) 433 350 400 450 500 550 600 650 STATION, IN METERS XS 800 444 443 442 441 440 439 438 437 436 435 434 ELEVATION IN IN (NAD83) METERSELEVATION 433 350 400 450 500 550 600 650 STATION, IN METERS

Page 86 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

TABLE 11 COMPOSITE CROSS-SECTIONAL GEOMETRY OF THE TRANSECTS ALONG THE STUDY REACH ON THE BIG SIOUX RIVER, SOUTH DAKOTA. XS 900 444 443 442 441 440 439 438 437 436 435 434 ELEVATION IN IN (NAD83) METERSELEVATION 433 350 400 450 500 550 600 650 STATION, IN METERS XS 1000 443 442 441 440 439 438 437 436 435 ELEVATION IN IN (NAD83) METERSELEVATION 434 350 400 450 500 550 600 650 STATION, IN METERS XS 1100 440

439

438

437

436

435

434 ELEVATION IN ELEVATION(NAD83) METERS 433 350 400 450 500 550 600 650 STATION, IN METERS XS 1200 441

440

439

438

437

436

435 ELEVATION IN IN (NAD83) METERSELEVATION 434 350 400 450 500 550 600 650 STATION, IN METERS

Page 87 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

TABLE 11 COMPOSITE CROSS-SECTIONAL GEOMETRY OF THE TRANSECTS ALONG THE STUDY REACH ON THE BIG SIOUX RIVER, SOUTH DAKOTA. XS 1300 441

440

439

438

437

436

435 ELEVATION IN IN (NAD83) METERSELEVATION 434 350 400 450 500 550 600 650 STATION, IN METERS XS 1400 442

441

440

439

438

437

436 ELEVATIONIN METERS (NAD83) 435 350 400 450 500 550 600 650 STATION, IN METERS XS 1500 442

441

440

439

438

437

436 ELEVATION IN ELEVATION(NAD83) METERS 435 350 400 450 500 550 600 650 STATION, IN METERS XS 1600 446 445 444 443 442 441 440 439 438 437 ELEVATION IN IN (NAD83) METERSELEVATION 436 350 400 450 500 550 600 650 STATION, IN METERS

Page 88 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

TABLE 11 COMPOSITE CROSS-SECTIONAL GEOMETRY OF THE TRANSECTS ALONG THE STUDY REACH ON THE BIG SIOUX RIVER, SOUTH DAKOTA. XS 1700 443 442 441 440 439 438 437 436 ELEVATIONIN METERS (NAD83) 435 350 400 450 500 550 600 650 STATION, IN METERS XS 1800 452 451 450 449 448 447 446 445 444 443 442 441 440 439

ELEVATION IN IN (NAD83) METERSELEVATION 438 437 350 400 450 500 550 600 650 STATION, IN METERS XS 1900 446 445 444 443 442 441 440 439 438 ELEVATION IN IN (NAD83) METERSELEVATION 437 350 400 450 500 550 600 650 STATION, IN METERS XS 2000 444

443

442

441

440

439

438 ELEVATION IN IN (NAD83) METERSELEVATION 437 350 400 450 500 550 600 650 STATION, IN METERS

Page 89 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

TABLE 11 COMPOSITE CROSS-SECTIONAL GEOMETRY OF THE TRANSECTS ALONG THE STUDY REACH ON THE BIG SIOUX RIVER, SOUTH DAKOTA. XS 2100 444

443

442

441

440

439

438 ELEVATION IN IN (NAD83) METERSELEVATION 437 350 400 450 500 550 600 650 STATION, IN METERS XS 2200 444 443 442 441 440 439 438 437 ELEVATION IN IN (NAD83) METERSELEVATION 436 350 400 450 500 550 600 650 STATION, IN METERS XS 2300 444

443

442

441

440

439 ELEVATION IN IN (NAD83) METERSELEVATION 438 350 400 450 500 550 600 650 STATION, IN METERS XS 2400 446 445 444 443 442 441 440 439 ELEVATION IN IN (NAD83) METERSELEVATION 438 350 400 450 500 550 600 650 STATION, IN METERS

Page 90 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

TABLE 11 COMPOSITE CROSS-SECTIONAL GEOMETRY OF THE TRANSECTS ALONG THE STUDY REACH ON THE BIG SIOUX RIVER, SOUTH DAKOTA. XS 2500 445

444

443

442

441

440

439 ELEVATION IN ELEVATION(NAD83) METERS 438 350 400 450 500 550 600 650 STATION, IN METERS XS 2600 446

445

444

443

442

441

440 ELEVATIONIN METERS (NAD83) 439 350 400 450 500 550 600 650 STATION, IN METERS XS 2700 445

444

443

442

441

440

439 ELEVATION IN IN (NAD83) METERSELEVATION 438 350 400 450 500 550 600 650 STATION, IN METERS XS 2800 446

445

444

443

442

441

440 ELEVATION IN IN (NAD83) METERSELEVATION 439 350 400 450 500 550 600 650 STATION, IN METERS

Page 91 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

TABLE 11 COMPOSITE CROSS-SECTIONAL GEOMETRY OF THE TRANSECTS ALONG THE STUDY REACH ON THE BIG SIOUX RIVER, SOUTH DAKOTA. XS 2900 447 446 445 444 443 442 441 440 ELEVATION IN IN (NAD83) METERSELEVATION 439 350 400 450 500 550 600 650 STATION, IN METERS XS 3000 446

445

444

443

442

441 ELEVATION IN IN (NAD83) METERSELEVATION 440 350 400 450 500 550 600 650 STATION, IN METERS XS 3100 447

446

445

444

443

442 ELEVATION IN ELEVATION(NAD83) METERS 441 350 400 450 500 550 600 650 STATION, IN METERS XS 3200 447

446

445

444

443

442

441 ELEVATION IN IN (NAD83) METERSELEVATION 440 350 400 450 500 550 600 650 STATION, IN METERS

Page 92 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

TABLE 11 COMPOSITE CROSS-SECTIONAL GEOMETRY OF THE TRANSECTS ALONG THE STUDY REACH ON THE BIG SIOUX RIVER, SOUTH DAKOTA. XS 3300 448

447

446

445

444

443 ELEVATION IN IN (NAD83) METERSELEVATION 442 350 400 450 500 550 600 650 STATION, IN METERS XS 3400 448

447

446

445

444 ELEVATION IN IN (NAD83) METERSELEVATION 443 350 400 450 500 550 600 650 STATION, IN METERS XS 3500 451 450 449 448 447 446 445 444 443 ELEVATION IN IN (NAD83) METERSELEVATION 442 350 400 450 500 550 600 650 STATION, IN METERS XS 3600 448

447

446

445

444

443 ELEVATION IN IN (NAD83) METERSELEVATION 442 350 400 450 500 550 600 650 STATION, IN METERS

Page 93 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

TABLE 11 COMPOSITE CROSS-SECTIONAL GEOMETRY OF THE TRANSECTS ALONG THE STUDY REACH ON THE BIG SIOUX RIVER, SOUTH DAKOTA. XS 3700 448

447

446

445

444

443 ELEVATION IN IN (NAD83) METERSELEVATION 442 350 400 450 500 550 600 650 STATION, IN METERS XS 3800 448

447

446

445

444

443 ELEVATION IN IN (NAD83) METERSELEVATION 442 350 400 450 500 550 600 650 STATION, IN METERS XS 3900 448

447

446

445

444 ELEVATION IN IN (NAD83) METERSELEVATION 443 350 400 450 500 550 600 650 STATION, IN METERS XS 4000 449

448

447

446

445

444

443 ELEVATION IN IN (NAD83) METERSELEVATION 442 350 400 450 500 550 600 650 STATION, IN METERS

Page 94 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

TABLE 11 COMPOSITE CROSS-SECTIONAL GEOMETRY OF THE TRANSECTS ALONG THE STUDY REACH ON THE BIG SIOUX RIVER, SOUTH DAKOTA. XS 4100 453 452 451 450 449 448 447 446 445 444 ELEVATION IN IN (NAD83) METERSELEVATION 443 350 400 450 500 550 600 650 STATION, IN METERS

Page 95 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

Page 96 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

APPENDIX B. CHANNEL BOUNDARY MATERIALS Boundary material at the transects along the study reach was sampled for its resistance to erosion. Resistance to erosion of cohesionless materials is a function of particle size. Hence the grain size distribution of bed, bank, bank face and bank toe material was determine by collecting samples for laboratory sieve analysis. The resistance to erosion of cohesive materials is typically characterized by an excess shear stress relationship, Eq. (1), which states that erosion commences at a rate proportional to the soil detachment coefficient (�) if the shear stress exerted by the flowing water exceeds a critical shear stress of the soil (�). Critical shear stress and soil detachment coefficient were measured using a jet erosion tester (see Section “Erosion-Resistance Data Collection: Submerged Jet Erosion Test Device”). The resistance to mass failure is a function of soil shear strength (represented by cohesion and friction angle), which was measured using a bore-hole shear-test device (see Section “Geotechnical Data Collection: Borehole Shear Tests”).

Figure 42 shows a map of the study reach with the locations of samples collected for bed and bank material grain size analysis. Figure 43 shows a map of the study reach with the locations where JET and BST tests were performed. Table 12 lists the fractional content of the main textural size classes (clay, silt, sand, and gravel) and bulk density of sampled bed and bank material. Table 13 lists the measured resistance-to-fluvial erosion parameters (� and �) using the jet erosion test device. Table 14 lists the soil shear strength parameters (effective cohesion and angle of internal friction) measured with the borehole shear test device. Note that the borehole shear test device measures an apparent cohesion (that is effective cohesion + the strength provided by air pockets (suction) in the soil matrix). Effective cohesion was calculated by subtracting the estimated shear strength due to suction (measured with a miniature tensiometer) from the apparent cohesion.

Page 97 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

FIGURE 42 LOCATIONS OF SAMPLED BED AND BANK MATERIAL.

Page 98 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

FIGURE 43 LOCATIONS OF JET AND BST TESTS.

Page 99 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

TABLE 12 SEDIMENT/SOIL SAMPLE LOCATION, FRACTIONAL CONTENT OF MAIN TEXTURAL SIZE CLASSES, AND BULK DENSITY. SAMPLE LOCATION KEY: B = BED, LB = LEFT BANK (INTERNAL), LF = LEFT BANK FACE, LT = LEFT BANK TOE, RB = RIGHT BANK (INTERNAL), RF = RIGHT BANK FACE, RT = RIGHT BANK TOE, R = RIFFLE, P = POOL, AND O = OTHER.

Study Sample Sample Dry density Grain size distribution transect location depth (m) (g/cm3) % clay % silt % sand % gravel

100 B, R 0.72 0.47 48.17 50.64 100 RB 0.54 1.07 55.25 12.97 31.78 0.00 100 RB 1.45 31.51 32.03 32.73 3.73 100 RT 1.19 17.87 24.95 57.18 0.00 200 B, O 0.71 5.25 42.31 51.73 200 LB 0.60 1.47 12.80 12.50 74.63 0.07 200 LB 1.40 4.39 3.46 90.84 1.31 200 LF 17.38 1.36 81.19 0.07 300 B, P 1.09 0.20 80.37 18.33 300 LB 0.75 1.14 25.19 33.12 41.62 0.08 300 LB 1.50 1.26 45.01 27.99 26.98 0.02 300 LT 1.29 27.55 27.58 42.34 2.52 400 B, P 0.59 0.42 34.47 64.51 400 RB 0.50 1.23 20.07 25.13 54.80 0.00 400 RB 1.30 7.79 5.38 86.83 0.00 400 RT 1.18 31.46 23.05 19.34 26.15 500 B, O 1.02 0.36 53.45 45.17 500 RB 0.75 1.14 29.33 30.07 40.60 0.00 500 RB 1.75 1.20 31.01 38.32 30.68 0.00 500 RF 1.14 23.91 18.93 56.98 0.17 600 B, R 1.01 0.12 84.74 14.14 600 RB 0.70 1.24 19.59 29.39 51.02 0.00 600 RF 1.25 18.84 23.27 56.62 1.27 700 B, P 0.40 0.12 42.65 56.83 700 LB 0.80 1.28 24.46 29.46 46.09 0.00 700 LB 1.10 1.43 16.15 45.65 38.19 0.00 700 LB 1.95 7.41 5.44 87.15 0.00 700 LF 1.21 18.08 18.62 63.30 0.00 800 B, O 0.71 0.55 58.16 40.58 800 LB 0.60 1.39 21.07 23.05 55.88 0.00 800 LB 1.40 1.30 18.47 18.91 62.62 0.00 800 LB 2.30 13.52 9.52 76.96 0.00 800 LT 1.27 17.10 21.60 61.31 0.00

Page 100 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

Study Sample Sample Dry density Grain size distribution transect location depth (m) (g/cm3) % clay % silt % sand % gravel

900 B, R 0.68 0.16 40.39 58.77 900 LB 0.55 1.40 20.08 20.89 59.04 0.00 900 LB 0.90 4.81 1.86 91.10 2.23 900 LB 1.10 2.73 2.27 46.65 48.34 900 LF 1.09 16.05 15.91 68.04 0.00 1000 B, R 0.70 0.02 61.70 37.58 1000 LB 0.50 1.13 25.71 27.89 46.40 0.00 1000 LB 1.40 1.31 22.67 30.47 46.86 0.00 1000 LB 2.40 9.12 3.42 87.46 0.00 1000 LF 1.20 15.32 12.76 71.93 0.00 1000 LT 1.25 15.14 10.46 74.40 0.00 1100 B, O 1.21 0.25 74.08 24.46 1100 RB 0.50 1.49 9.97 9.63 80.40 0.00 1100 RB 0.90 1.11 26.38 19.28 54.34 0.00 1100 RF 1.22 28.52 30.14 41.32 0.02 1100 RT 1.37 16.11 18.67 65.22 0.00 1200 B, R 0.96 0.44 72.32 26.28 1200 LB 0.70 1.33 27.60 33.14 39.26 0.00 1200 LB 1.93 1.39 15.56 17.86 66.58 0.00 1200 LB 2.65 4.59 3.25 92.16 0.00 1200 LF 1.00 14.13 14.14 71.73 0.00 1200 LF 0.82 24.11 33.27 42.62 0.00 1300 B, P 1.60 1.55 80.76 16.10 1300 LB 0.50 1.37 15.66 17.79 66.56 0.00 1300 LB 1.50 1.47 6.83 5.23 87.92 0.02 1300 LF 1.35 12.54 12.23 75.22 0.00 1300 LT 1.22 15.16 14.53 70.18 0.13 1400 B, P 3.34 5.30 36.86 54.50 1400 LB 0.65 1.35 21.06 28.70 50.25 0.00 1400 LB 2.45 1.25 48.28 26.57 25.14 0.00 1400 LF 0.98 49.43 38.25 12.32 0.00 1400 LF 1.09 23.56 28.22 48.22 0.00 1500 B, P 0.80 0.16 91.63 7.40 1500 LB 0.50 1.38 27.71 31.86 40.43 0.00 1500 LB 2.10 1.55 12.83 11.40 75.77 0.00 1500 LB 2.90 7.59 8.47 82.27 1.67

Page 101 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

Study Sample Sample Dry density Grain size distribution transect location depth (m) (g/cm3) % clay % silt % sand % gravel

1500 LF 24.03 22.72 53.24 0.00 1500 LT 1.32 6.16 4.06 89.76 0.02 1600 B, R 2.40 4.53 77.77 15.30 1600 RB 0.62 1.15 19.42 29.48 51.10 0.00 1600 RB 2.70 1.35 28.29 38.30 33.41 0.00 1600 RB 3.30 3.65 2.91 93.44 0.00 1600 RF 1.12 21.20 45.00 33.81 0.00 1700 B, P 0.99 0.17 66.50 32.34 1700 LB 0.65 1.26 20.75 24.29 54.96 0.00 1700 LB 2.00 1.47 14.19 14.15 71.50 0.16 1800 B, R 0.88 0.42 60.32 38.38 1900 B, O 1.10 0.04 88.08 10.77 1900 LB 0.60 1.44 18.57 24.83 56.60 0.00 1900 LB 1.60 2.07 0.97 83.87 13.10 1900 LB 3.10 2.09 1.02 84.25 12.65 1900 LF 1.16 19.62 26.25 54.02 0.11 2000 B, P 0.56 0.12 37.96 61.36 2000 LB 5.06 3.12 91.81 0.00 2000 LB 0.30 11.29 11.88 76.83 0.00 2000 LB 0.50 1.41 3.53 2.95 93.50 0.02 2000 LB 1.80 20.23 23.69 56.08 0.00 2000 LT 1.30 16.16 14.55 69.27 0.02 2100 B, O 1.17 0.39 78.36 20.07 2200 B, P 0.66 0.45 49.30 49.59 2200 RB 0.76 1.31 18.76 22.13 59.06 0.05 2200 RB 2.00 1.31 26.36 24.83 48.81 0.00 2200 RF 1.23 18.21 16.46 64.70 0.63 2200 RF 1.38 21.87 20.17 57.29 0.67 2300 B, P 0.62 0.44 35.50 63.44 2300 RB 0.25 11.67 12.69 73.85 1.80 2300 RB 0.55 21.83 21.49 56.67 0.00 2300 RB 0.75 12.86 15.14 72.00 0.00 2300 RB 1.18 1.30 23.91 37.78 38.25 0.07 2300 RF 1.37 20.38 31.41 47.76 0.44 2400 B, P 8.72 17.23 64.72 9.34 2400 RB 0.50 1.00 28.28 49.23 22.50 0.00

Page 102 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

Study Sample Sample Dry density Grain size distribution transect location depth (m) (g/cm3) % clay % silt % sand % gravel

2400 RB 1.50 1.24 37.42 55.62 6.96 0.00 2400 RB 2.55 37.69 21.31 41.00 0.00 2400 RT 0.83 29.72 31.55 38.72 0.00 2500 B, R 0.54 0.16 36.78 62.52 2600 B, R 1.72 0.98 85.38 11.91 2600 RB 0.60 1.02 22.21 33.10 44.39 0.30 2600 RB 1.35 1.34 29.75 35.76 34.49 0.00 2600 RB 2.04 4.68 3.78 91.55 0.00 2600 RF 20.87 11.97 67.16 0.00 2600 RT 1.09 15.82 23.41 60.73 0.03 2700 B, P 0.85 0.11 61.96 37.08 2700 RB 0.55 1.33 12.93 7.43 79.51 0.12 2700 RB 1.10 6.65 6.41 86.95 0.00 2700 RB 1.65 1.35 18.64 55.04 26.32 0.00 2700 RB 1.80 4.32 2.89 92.62 0.17 2700 RF 7.92 8.93 83.15 0.00 2700 RF 21.07 27.42 51.50 0.00 2700 RT 1.42 6.09 3.84 89.95 0.11 2800 B, R 1.06 0.18 84.88 13.88 2800 LB 0.90 1.42 34.29 38.67 27.04 0.00 2800 LB 2.00 8.86 2.50 88.64 0.00 2800 LF 1.27 37.15 40.44 22.41 0.00 2900 B, P 0.45 0.00 56.72 42.82 2900 LB 0.55 0.93 24.67 17.91 57.42 0.00 2900 LB 1.70 13.92 18.97 67.11 0.00 2900 LF 1.23 29.71 33.67 36.62 0.00 3000 B, P 1.45 1.83 96.50 0.22 3000 LB 0.70 1.42 15.68 17.52 66.69 0.11 3000 LB 1.55 1.67 1.02 47.33 49.97 3000 LB 2.20 0.98 1.07 30.94 67.01 3000 LF 1.26 15.48 11.91 71.12 1.49 3100 B, P 0.94 0.31 65.12 33.63 3100 LB 0.95 1.19 19.73 17.72 62.55 0.00 3100 LF 1.27 18.82 23.75 57.43 0.00 3200 B, R 5.67 7.54 83.22 3.57 3200 RB 0.60 1.13 36.83 33.56 29.61 0.00

Page 103 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

Study Sample Sample Dry density Grain size distribution transect location depth (m) (g/cm3) % clay % silt % sand % gravel

3200 RB 1.60 1.25 24.24 31.35 41.18 3.22 3200 RF 1.30 36.79 23.85 38.57 0.79 3500 B, R 25.41 45.31 25.36 3.92 3500 LB 1.00 1.24 35.69 28.42 35.89 0.00 3500 LF 25.55 36.38 38.07 0.00 3600 B, O 11.04 27.57 60.34 1.05 3600 LB 0.60 1.25 20.42 21.89 57.69 0.00 3600 LF 17.10 18.68 64.22 0.00 3700 B, P 13.06 27.90 59.04 0.00 3700 LB 1.55 1.19 28.37 25.54 45.97 0.12 3700 LF 1.22 17.73 20.25 62.02 0.00 3800 B, P 5.36 7.37 85.07 2.21 3800 LB 1.00 1.45 31.59 31.03 37.03 0.35 3800 LF 31.15 31.23 36.80 0.82 3900 B, R 1.34 0.91 59.81 37.93 3900 LB 1.00 1.04 19.72 15.76 64.53 0.00 3900 LF 25.00 36.74 38.21 0.06 4000 B, R 5.36 13.22 73.76 7.66 4000 LB 1.50 1.25 16.28 14.23 69.49 0.00 4000 LB 0.75 1.38 11.03 11.45 77.51 0.00 4000 RF 11.43 12.44 76.10 0.03 4000 - 23.17 35.94 40.89 0.00 4100 B, R 1.54 1.78 79.80 16.88 4100 B, R 21.52 52.04 26.18 0.26 4100 RB 0.70 1.16 16.79 16.96 66.14 0.12

Page 104 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

TABLE 13 RESISTANCE TO FLUVIAL EROSION PARAMETERS MEASURED WITH THE JET TEST DEVICE. TEST LOCATION KEY: LF = LEFT BANK FACE, LT = LEFT BANK TOE, RF = RIGHT BANK FACE, AND RT = RIGHT BANK TOE. NOTE THAT THE VALUES OF THE SOIL DETACHMENT COEFFICIENT WERE MULTIPLIED BY 106 FOR PRESENTATION PURPOSES.

Soil detachment Critical shear stress coefficient x 106 Study transect Test location (Pa) (m/s Pa)

200 LF 7.27 34.8 200 LF 11.5 5.23 200 LF 13.2 19.5 300 LT 13.8 13.4 300 LT 12.4 122 500 RF 5.58 7.55 500 RF 5.48 18.0 600 RT 14.0 6.56 800 LT 18.3 11.1 800 LT 10.3 61.0 900 LT 10.7 37.5 900 LT 10.1 95.4 1000 LF 14.8 6.70 1000 LT 7.96 112 1000 LF 18.3 4.04 1100 RT 3.05 118 1100 RF 2.79 109 1200 LF 3.75 106 1200 LF 4.93 19.9 1200 LF 13.2 4.19 1200 LF 13.2 4.12 1300 LT 5.10 72.5 1300 LT 8.77 30.6 1400 LF 10.5 13.3 1400 LF 25.7 4.95 1500 LF 8.69 11.7 1500 LF 14.7 12.8 1600 LF 4.59 27.4 1600 LF 3.92 29.2 1900 LF 3.83 373

Page 105 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

Soil detachment Critical shear stress coefficient x 106 Study transect Test location (Pa) (m/s Pa)

2000 LT 15.1 3.23 2200 RF 4.70 20.0 2200 RF 10.6 3.86 2300 RF 14.9 2.84 2400 LT 15.1 8.21 2400 LT 14.3 7.10 2600 RT 3.41 65.9 2600 RF 1.53 24.3 2700 LF 2.89 128 2800 LF 7.19 55.6 2800 LF 15.1 9.77 2900 LF 15.5 6.47 3000 LF 14.7 3.50 3000 LF 16.9 5.55 3100 LF 15.1 9.77 3200 RF 11.5 8.68 3200 RF 11.6 3.36 3300 LF 7.71 7.14 3400 LF 4.56 16.7 3500 LF 9.24 13.8 3600 LF 4.84 18.7 4000 RF 1.69 224

Page 106 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

TABLE 14 BANK SOIL SHEAR STRENGTH MEASURED WITH THE BST DEVICE. TEST LOCATION KEY: LB = LEFT BANK AND RB = RIGHT BANK.

Apparent Matric Effective Study Test depth cohesion Friction angle suction cohesion transect Test location (m) (kPa) (°) (kPa) (kPa)

100 RB 0.75 17.6 15.0 31.6 12.0 100 RB 0.7 18.6 21.1 31.6 13.0 200 LB 0.75 4.27 24.1 10.9 2.35 300 LB 0.85 3.16 25.2 32.6 0 300 LB 1.55 5.90 17.6 19.8 2.41 400 RB 0.6 15.3 14.7 2.10 14.9 500 RB 0.95 17.5 15.8 45.0 9.54 500 RB 1.85 5.34 21.1 5.50 4.37 600 RB 0.75 13.1 14.2 14.1 10.6 600 RB 0.75 13.3 22.4 14.1 10.9 700 LB 1.25 5.34 22.3 7.30 4.05 800 LB 1.45 20.5 15.2 16.2 17.7 900 LB 0.65 9.42 10.4 3.66 8.78 900 LB 0.5 2.23 13.7 3.66 1.58 1000 LB 0.5 3.85 36.9 30.0 0 1000 LB 1.5 11.0 21.1 5.8 10.0 1100 RB 0.5 6.54 24.1 13.6 4.15 1100 RB 1.05 7.62 23.8 35.9 1.29 1200 LB 1.0 5.49 16.0 24.3 1.21 1200 LB 2.0 3.81 21.8 10.9 1.90 1200 LB 2.0 7.43 18.4 10.9 5.51 1300 LB 0.7 3.16 27.6 15.0 0.12 1300 LB 0.7 10.5 32.1 15.0 7.88 1400 LB 0.75 5.32 25.3 2.27 2.27 1400 LB 2.55 10.3 27.7 - 9.45† 1500 LB 0.6 13.8 23.8 34.4 7.75 1500 LB 2.1 19.8 34.2 15.4 17.1 1600 RB 0.75 8.69 21.1 22.6 4.70 1700 LB 0.7 18.7 11.5 31.6 13.1 1700 LB 2.15 16.1 21.9 5.45 15.2 1900 LB 0.75 12.1 21.8 21.6 8.24

Page 107 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

Apparent Matric Effective

Study Test depth cohesion Friction angle suction cohesion transect Test location (m) (kPa) (°) (kPa) (kPa)

2000 LB 0.8 13.5 31.4 16.1 10.6 2000 LB 0.8 9.91 26.2 16.1 7.08 2200 RB 0.9 12.4 15.6 19.3 9.00 2200 RB 2.1 6.5 26.6 19.1 3.13 2300 RB 1.3 8.54 33.0 24.9 4.16 2400 RB 0.55 22.1 15.5 53.9 12.5 2400 RB 1.7 15.6 19.5 45.0 7.64 2600 RB 0.6 13.4 32.8 33.1 7.59 2600 RB 1.35 7.61 32.4 28.0 2.68 2600 RB 1.4 6.89 21.7 28.0 1.96 2700 RB 0.8 6.45 24.4 23.0 2.40 2700 RB 1.55 10.1 19.1 14.8 7.52 2700 RB 1.6 11.7 31.8 14.8 9.05 2800 LB 0.8 3.48 17.1 39.8 0 2900 LB 0.7 10.4 33.6 23.9 6.21 2900 LB 0.75 19.8 30.2 23.9 15.6 3000 LB 0.85 8.27 23.6 31.7 2.68 3000 LB 0.85 9.31 31.4 31.7 3.73 3100 LB 1.0 8.77 31.8 15.6 6.01 3100 LB 1.05 7.12 24.5 15.6 4.36 3200 RB 0.65 27.3 24.0 54.7 17.6 3200 RB 1.65 3.93 29.0 3.50 3.31 3200 RB 1.65 5.83 19.6 3.50 5.21 3400 LB 0.5 2.87 35.0 - 0† 3400 LB 0.6 6.18 38.0 - 0† 3500 LB 1.0 19.3 30.0 - 17.9† 3600 LB 0.6 2.00 27.2 - 1.28† 3600 LB 0.6 11.4 29.6 - 10.7† 3700 LB 1.0 2.50 25.4 - 1.48† 3700 LB 0.7 2.96 34.9 - 1.62† 3800 LB 1.0 4.81 33.7 - 3.89† 3800 LB 1.0 2.94 34.6 - 2.02† 3900 LB 1.0 3.12 40.0 - 2.22†

Page 108 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

Apparent Matric Effective

Study Test depth cohesion Friction angle suction cohesion transect Test location (m) (kPa) (°) (kPa) (kPa)

4000 LB 0.7 5.79 38.7 - 4.88† 4000 LB 1.6 3.7 26.3 - 2.94† 4100 LB 0.7 17.2 34.6 - 15.2† 4100 LB 0.8 14.2 32.1 - 12.3† † Effective cohesion was calculated by estimating matric suction from the depth to the water table.

Page 109 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

Page 110 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

APPENDIX C. BANK PROTECTION MEASURES

FIGURE 44 MAP OF BANK PROTECTION MEASURES (MARKED AS RED LINES) ALONG THE STUDY REACH OF THE BIG SIOUX RIVER, SOUTH DAKOTA.

Page 111 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

Page 112 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

APPENDIX D. CONCEPTS MODEL BANK MATERIAL DATA Suction angle 10 degrees, particle density 2,650 kg/m3.

Soil Layer Critical detachment Study Layer height Bulk density shear coefficient Cohesion Friction transect number (m) (kg/m3) Porosity stress (Pa) (m/s Pa) (kPa) angle (°) 100 1 - 1,074 0.60 11.3 8.85E-06 12.0 15 200 1 1.30 1,471 0.60 11.2 8.97E-06 2.35 24 200 2 - 1,471 0.5 11.2 8.97E-06 1.00 30 300 1 - 1,200 0.56 11.0 9.23E-06 1.21 21 400 1 1.20 1,232 0.54 10.9 9.37E-06 14.9 15 400 2 - 1,400 0.5 10.9 9.37E-06 1.00 32 500 1 - 1,170 0.57 10.7 9.64E-06 6.96 18 600 1 - 1,239 0.54 10.6 9.79E-06 10.7 18 700 1 - 1,360 0.5 10.4 1.01E-05 4.05 22 800 1 2.00 1,394 0.48 10.3 1.02E-05 17.7 15 800 2 - 1,297 0.52 10.3 1.02E-05 5.0 25 900 1 0.75 1,403 0.48 10.0 1.07E-05 5.18 12 900 2 0.25 1,400 0.5 10.0 1.07E-05 1.00 30 900 3 - 1,400 0.5 10.0 1.07E-05 1.00 35 1000 1 0.90 1,132 0.58 9.7 1.13E-05 10.0 21 1000 2 0.80 1,312 0.51 9.7 1.13E-05 10.0 21 1000 3 - 1,400 0.5 9.7 3.07E-05 1.00 30 1100 1 0.80 1,491 0.45 9.4 1.18E-05 4.15 24 1100 2 1.90 1,110 0.59 9.4 1.18E-05 1.29 24

Page 113 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

Soil Layer Critical detachment Study Layer height Bulk density shear coefficient Cohesion Friction transect number (m) (kg/m3) Porosity stress (Pa) (m/s Pa) (kPa) angle (°) 1100 3 - 1,450 0.5 9.4 3.21E-05 1.00 30 1200 1 1.80 1,329 0.51 9.1 1.25E-05 1.21 16 1200 2 - 1,394 0.48 9.1 1.25E-05 3.71 20 1300 1 1.00 1,371 0.49 8.9 1.29E-05 4.00 30 1300 2 - 1,345 0.50 8.9 1.29E-05 1.00 32 1400 1 1.60 1,352 0.50 8.7 1.34E-05 2.27 25 1400 2 - 1,224 0.55 8.7 1.34E-05 9.00 28 1500 1 1.85 1,384 0.5 8.7 1.34E-05 7.75 24 1500 2 - 1,554 0.5 8.7 1.34E-05 17.1 34 1600 1 2.45 1,151 0.57 8.6 1.36E-05 4.70 21 1600 2 - 1,351 0.50 8.6 3.65E-05 4.70 21 1700 1 1.50 1,262 0.5 8.7 1.34E-05 13.1 12 1700 2 - 1,470 0.5 8.7 1.34E-05 15.2 22 1800 1 - 1,450 0.5 8.8 1.31E-05 10.0 22 1900 1 1.00 1,445 0.5 9.0 1.27E-05 8.24 22 1900 2 2.00 1,450 0.5 9.0 1.27E-05 1.00 30 1900 3 - 1,450 0.5 9.0 3.42E-05 1.00 35 2000 1 - 1,409 0.5 9.2 1.22E-05 8.85 29 2100 1 - 1,360 0.51 9.4 1.18E-05 9.00 23 2200 1 1.80 1,312 0.51 9.6 1.14E-05 9.00 16 2200 2 - 1,305 0.52 9.6 1.14E-05 3.13 27 2300 1 0.90 1,296 0.52 9.9 1.09E-05 4.16 33 2300 2 - 1,296 0.52 9.9 1.09E-05 4.16 33 2400 1 - 1,020 0.62 10.1 1.06E-05 10.1 18

Page 114 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

Soil Layer Critical detachment Study Layer height Bulk density shear coefficient Cohesion Friction transect number (m) (kg/m3) Porosity stress (Pa) (m/s Pa) (kPa) angle (°) 2500 1 - 1,024 0.55 10.2 1.04E-05 7.50 28 2600 1 0.80 1,024 0.62 10.4 1.01E-05 7.59 33 2600 2 1.10 1,341 0.50 10.4 1.01E-05 2.32 27 2600 3 - 1,420 0.59 10.4 2.78E-05 1.00 32 2700 1 1.20 1,326 0.51 10.6 9.79E-06 2.40 24 2700 2 0.55 1,423 0.47 10.6 9.79E-06 8.29 25 2700 3 - 1,423 0.47 10.6 2.70E-05 1.00 32 2800 1 - 1,424 0.47 10.8 9.50E-06 1.00 17 2900 1 1.60 933 0.5 10.9 9.37E-06 10.9 32 2900 2 - 1,420 0.5 10.9 9.37E-06 1.00 32 3000 1 1.35 1,420 0.5 10.9 9.37E-06 2.68 24 3000 2 0.65 1,420 0.5 10.9 9.37E-06 3.73 31 3000 3 - 1,420 0.5 10.9 2.59E-05 1.00 35 3100 1 - 1,193 0.56 10.7 9.64E-06 6.01 32 3200 1 1.35 1,127 0.58 10.4 1.01E-05 17.6 24 3200 2 - 1,247 0.54 10.4 1.01E-05 3.31 29 3300 1 - 1,240 0.54 9.0 1.27E-05 3.40 32 3400 1 - 1,240 0.54 8.3 1.44E-05 3.50 36 3500 1 - 1,238 0.54 7.5 1.69E-05 17.9 30 3600 1 - 1,255 0.54 6.7 2.02E-05 1.28 27 3700 1 - 1,189 0.56 5.9 2.47E-05 1.55 30 3800 1 - 1,450 0.46 5.0 3.21E-05 2.96 34 3900 1 - 1,044 0.61 4.1 4.39E-05 2.22 40 4000 1 1.20 1,383 0.49 3.3 6.19E-05 4.88 39

Page 115 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

Soil Layer Critical detachment Study Layer height Bulk density shear coefficient Cohesion Friction transect number (m) (kg/m3) Porosity stress (Pa) (m/s Pa) (kPa) angle (°) 4000 2 - 1,246 0.49 3.3 6.19E-05 2.94 26 4100 1 - 1,160 0.57 2.5 9.59E-05 13.7 33

Page 116 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

APPENDIX E. BIG SIOUX RIVER MIGRATION BETWEEN 1991 AND 2012 The observed migration of the Big Sioux River study reach was assessed for the period 1991-2012. The top-of-bank was digitized from the 1991 Digital Orthophoto Quadrangle (DOQ) imagery, 2003 USDA NAIP imagery and 2012 USDA NAIP imagery using the ArcGIS® 10.2.1 software from Esri. Figure 45 shows an example of the digitized 2012 top-of-bank. Figure 46 compares the 1991, 2003, and 2012 top-of-bank lines of the Big Sioux River study reach.

Figure 46 shows that the most active subreaches of the study reach is found between transects 2100 and 3000. Most bends along this subreach have migrated quite actively with migration distances exceeding 60 m (~3 m/yr) at some locations. It seems that this activity stems from meander cutoffs that have occurred somewhere between 1937 and 1991, which significantly shortened the channel and increased channel slope and hence increased hydraulic forces applied to the channel boundary (see Figure 47).

FIGURE 45 CLOSEUP OF DIGITIZED TOP-OF-BANK FROM THE 2012 USDA NAIP IMAGERY.

Page 117 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

FIGURE 46 OBSERVED MIGRATION OF THE BIG SIOUX RIVER STUDY REACH BETWEEN 1991 AND 2012. THE PLOTTED BANK LINES REPRESENT TOP OF BANK AND WERE DIGITIZED FROM THE 2003 AND 2012 NAIP IMAGERY.

Page 118 Bank Erosion and Stabilization of the Big Sioux River between Dell Rapids and Sioux Falls, South Dakota

FIGURE 47 CHANGE IN CHANNEL PLANFORM OF THE BIG SIOUX RIVER STUDY REACH BETWEEN TRANSECTS 2200 AND 3100 OVER THE PERIOD 1937-2012. THE BACKGROUND IMAGE IS THE 1937 AERIAL PHOTO. THE BLUE LINE APPROXIMATES THE 1937 CHANNEL. THE RED LINE REPRESENTS THE 2012 CHANNEL TOP-OF-BANK.

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