Final Report—CONCEPTS

DATA COLLECTION AND CONCEPTS MODEL DEVELOPMENT AND VALIDATION

Prepared by

Eddy J. Langendoen Watershed Physical Processes Research Unit U.S. Department of Agriculture – Agricultural Research Service National Sedimentation Laboratory, P.O. Box 1157, Oxford, MS. 38655

April 2013

Prepared as part of agreement #60-6408-8-088 (Enhanced Stream-Corridor Modeling Tools for Adaptive Management of Tahoe Basin Streams; P003) with:

USDA Forest Service Pacific Southwest Research Station 1731 Research Park Dr Davis, CA 95618

This research was supported using funds provided by the Bureau of Land Management through the sale of public lands as authorized by the Southern Nevada Public Land Management Act.

TABLE OF CONTENTS

TABLE OF CONTENTS ...... I LIST OF FIGURES ...... II LIST OF TABLES ...... IV 1. INTRODUCTION AND BACKGROUND ...... 1 1.1 Objectives and Scope ...... 2 1.2 Approach, Methodology, and Geographic Location of Research ...... 3 1.2.1 Quantifying the Effects of Vegetation and Bio-Engineered Treatments ...... 5 1.2.2 Near-Bank Groundwater Model: Dynamic Pore-Water Pressure ...... 5 1.2.3. Lateral Migration of Meanders ...... 5 1.2.4 Evaluating Performance of Restoration Treatments and Validating Model Results ...... 6 2. FIELD DATA COLLECTION ...... 8 2.1 Streambank hydrology ...... 8 2.1.1 Upper Truckee River...... 8 2.1.2 Trout Creek ...... 15 2.2 Resistance to Erosion of Channel Boundary Materials ...... 20 2.2.1 Trout Creek ...... 21 2.3 Channel Geometry of Trout Creek ...... 25 3. CONCEPTS MODEL ENHANCEMENTS ...... 40 3.1 Near-bank Groundwater Model ...... 40 3.2 Lateral Migration Model ...... 41 4. CONCEPTS MODEL VALIDATION ...... 43 4.1 Model Setup ...... 43 4.2 Model Results ...... 47 REFERENCES ...... 54

LIST OF FIGURES

Figure 1 Map showing locations of study reaches on the Upper Truckee River and Trout Creek...... 4 Figure 2 Map of monitoring sites on the Upper Truckee River. Symbols: blue triangle, water surface transducer; red plus, tensiometer; orange circle, groundwater transducer; pink square, snow staff gage; white square, acoustic distance sensor; blue pentagon, soil moisture probe...... 9 Figure 3 Photos of the monitoring sites on the Upper Truckee River: (a) Site 1, (b) Site 2, (c) Site 3, and (d) Site 4...... 10 Figure 4 Observed precipitation, groundwater elevation (GWE), and pore-water pressure (PWP) at Site #1 on the Upper Truckee River...... 12 Figure 5 Observed precipitation, surface water (SWE) and groundwater elevations (GWE), and pore-water pressure (PWP) at Site #2 on the Upper Truckee River...... 13 Figure 6 Observed precipitation, surface water (SWE) and groundwater elevations (GWE), and pore-water pressure (PWP) at Site #3 on the Upper Truckee River...... 14 Figure 7 Observed precipitation, surface water (SWE) and groundwater elevations (GWE), and pore-water pressure (PWP) at Site #4 on the Upper Truckee River...... 15 Figure 8 Map of monitoring sites on Trout Creek. Symbols: blue triangle, water surface transducer; red plus, tensiometer; orange circle, groundwater transducer; pink square, snow staff gage; white square, acoustic distance sensor; blue pentagon, soil moisture probe...... 16 Figure 9 Photos of the monitoring sites on Trout Creek: (a) Site 1, (b) Site 2, (c) Site 3, and (d) Site 4...... 17 Figure 10 Observed precipitation, surface water (SWE) and groundwater elevations (GWE), and pore-water pressure (PWP) at Site #1 on Trout Creek...... 18 Figure 11 Observed precipitation, surface water elevation (SWE), and pore-water pressure (PWP) at Site #3 on Trout Creek...... 19 Figure 12 Observed precipitation, surface water and groundwater elevations, and pore-water pressure at Site #4 on Trout Creek...... 20 Figure 13 Location of data collection sites on Trout Creek to characterize the resistance to erosion of channel boundary materials...... 22 Figure 14 Grain-size distribution of bank-material samples collected along Trout Creek ...... 24 Figure 15 Grain-size distribution of riffles along Trout Creek. The riffles labeled PCx-x xx were sampled by SHG (2004). The first number indicates monitoring reach (cf. Figure 17), the second number indicates cross section, and the last number indicates year of sampling. The riffles labeled Trout #x were sampled in 2008; the number indicates sampling site (cf. Figure 13)...... 24 Figure 16 Grain-size distribution of pools along Trout Creek. The numbers in the legend indicate data collection site (cf. Figure 13)...... 25 Figure 17 Location of reaches and cross sections on Trout Creek monitored by Swanson Hydrology + Geomorphology (2004)...... 26 Figure 18 Observed changes in thalweg elevation of the reconstructed Trout Creek channel between 2001 and 2008...... 29 Figure 19 Observed changes in thalweg elevation along the reaches monitored by Swanson Hydrology + Geomorphology (2004) of the reconstructed Trout Creek channel between 2001 and 2008...... 30 Figure 20 Observed changes in cross section geometry along Reach 1 on Trout Creek...... 34

Figure 21 Observed changes in cross section geometry along Reach 2 on Trout Creek...... 35 Figure 22 Observed changes in cross section geometry along Reach 3 on Trout Creek...... 36 Figure 23 Observed changes in cross section geometry along Reach 4 on Trout Creek...... 37 Figure 24 Observed changes in cross section geometry along Reach 5 on Trout Creek...... 38 Figure 25 Observed changes in cross section geometry along Reach 6 on Trout Creek...... 39 Figure 26 Definition sketch of variables used to formulate the CONCEPTS sub-model of groundwater dynamics...... 40 Figure 27 Trout Creek modeling reach with model sections. Cold Creek discharges into cross section 152+05...... 44 Figure 28 List of soil profiles used in the CONCEPTS model of Trout Creek. For example, the soil profile between river stations 201+00 and 192+00 comprised four layers ranging from a medium loamy sand near the floodplain surface to a silt loam layer at depth...... 45 Figure 29 List of soils used in the CONCEPTS model of Trout Creek. The figure shows the properties for the “fine loamy sand” soil...... 45 Figure 30 List of sediment profiles used in the CONCEPTS model of Trout Creek. For example, the riffle profile between river stations 182+00 and 162+00 comprises three layers ranging from a coarse riffle layer at the surface to a lacustrine layer at depth...... 46 Figure 31 List of sediments used in the CONCEPTS model of Trout Creek. The figure shows the properties for the “riffle 182+00 – 162+00” sediment...... 46 Figure 32 Hourly discharge record used in the CONCEPTS model. Horizontal lines indicate, bankfull, five-year, and ten-year discharge...... 47 Figure 33 Simulated water surface profile for bankfull flow discharge of 2.5 m3/s...... 48 Figure 34 Simulated change in bed elevation for bankfull flow discharge of 2.5 m3/s...... 49 Figure 35 Simulated change in channel top width for bankfull flow discharge of 2.5 m3/s ...... 49 Figure 36 Simulated water surface profile for 5-year flow discharge of 5.6 m3/s...... 50 Figure 37 Simulated change in thalweg elevation for 5-year flow discharge of 5.6 m3/s...... 51 Figure 38 Simulated change in channel width for 5-year flow discharge of 5.6 m3/s...... 51 Figure 39 Simulated change in thalweg elevation between Oct 1, 2010 and Sep 30, 2009...... 52 Figure 40 Simulated change in channel top width between Oct 1, 2010 and Sep 30, 2009...... 53

III

LIST OF TABLES

Table 1 Discharges used in the CONCEPTS simulations...... 47

1. INTRODUCTION AND BACKGROUND

Substantial progress has been made on understanding and quantifying the contribution of stream channel erosion to sediment loading of Lake Tahoe (Simon et. al. 2003, Simon 2006). Streambank erosion has been estimated to account for about 25% of the total fine-sediment load entering Lake Tahoe (Simon, 2006). Much of this material emanates from unstable reaches of the Upper Truckee River, while Blackwood and Ward Creeks are also large contributors. Stream restoration has been accepted as an appropriate method to mitigate bank and channel instability, improve water quality, and contribute to other Basin Thresholds, with significant investment in planning, design, and implementation over several years. However, there are continuing needs for improved predictive modeling of channel processes with tools validated in the Lake Tahoe Basin at both watershed and project-specific scales, including representation of typical restoration features (channel, bank, and floodplain morphology, materials, hydrology, and vegetation). Modeling tools are needed that: 1) simulate channel response and quantify water quality performance of alternative restoration designs prior to approval and construction; and 2) estimate channel adjustments following implementation to inform adaptive management decisions.

As open systems, alluvial streams operate in a balance (dynamic equilibrium) between the upstream delivery of sediment and the transport capacity. Changes to this balance, either through direct modification of the channel or by indirect changes in sediment delivery or hydrology (i.e. land use changes), adjust the stream’s ability to transport sediment and consequently cause erosion and deposition. Thus, the best intended projects still represent “disturbances” to the fluvial system and must be designed/constructed to maintain a balance of flow energy and sediment transport so that stabilization of one reach does not lead to instability in adjacent reaches. This is referred to as sediment continuity. For instance, if sediment delivery from an upstream reach is cut off, it is possible that the imbalance between transport capacity and sediment delivery to the downstream reach will induce renewed streambed incision and associated streambank erosion.

On the Upper Truckee River for example, at least three projects along adjacent reaches were in the planning stages by different design groups in 2008, yet none of these projects explicitly analyzed for streambank stability or account for sediment contributed by bank failures in calculations of sediment continuity. Estimating future channel stability, migration, bank erosion, and resultant water quality for the project designs is hindered by the lack of geotechnical information and accepted tools. Similarly, restoration projects implemented on Angora Creek and Trout Creek have been monitored, but lack quantified information on performance of streambank and other restoration treatments, leading to the following questions regarding potential fine-load reductions from streambank erosion:

1. Are the restoration treatments performing as designed? 2. Why are some treatments more successful than others? 3. Have fine-loadings to the lake been reduced, and by what magnitude? 4. Have projects resulted in a properly functioning channel that supports habitat for native species?

There is a critical need for a comprehensive planning and prediction tool that deterministically incorporates bed and bank processes. Furthermore, with the widespread use of vegetation and bio-engineering techniques in restoration projects, data and numerical tools are required to account for the effects of riparian vegetation on geotechnical and hydraulic resistance of channel materials.

Two such tools were used previously in the Lake Tahoe Basin. The conservational channel evolution and pollutant transport system (CONCEPTS) model (Langendoen and Alonso, 2008; Langendoen and Simon, 2008) was used successfully to simulate sediment transport and channel changes on three streams tributary to Lake Tahoe: Upper Truckee River, Ward Creek, and General Creek (Simon et al., 2003). The deterministic, numerical model is unique in accounting for hydraulic and geotechnical processes that control streambank erosion in addition to bed processes and the routing of flow and sediment. The bank-stability algorithms are similar to those included in the Bank-Stability and Toe-Erosion Model (BSTEM) (Simon et al., 2000; 2011; Simon and Pollen, 2005), which was used along a reach of the Upper Truckee River to predict critical conditions for bank stability and the reinforcing effects of riparian vegetation (Simon et al., 2006).

These existing numerical tools are ideal for addressing the types of critical issues concerning stream-restoration design and performance aimed towards reducing fine-sediment loadings to Lake Tahoe. Model enhancements to address load reductions from specific treatments and projects would provide long-term benefits to basin agencies. Recent research has shown that the two most critical variables to accurately predicting bank strength, stability and channel-widening rates are cohesion and pore-water pressure (Langendoen and Simon, 2008). Vegetation plays a large role in controlling these variables. Work by Pollen and Simon (2005) using fiber-bundle theory and direct shear experiments of root-permeated soils showed that predictions of the magnitude of cohesion provided by root reinforcement were being overestimated by as much as 100% using conventional techniques. Enhancements to the CONCEPTS and BSTEM models to include a near-bank groundwater model, and quantification of the role of riparian roots on hydraulic resistance to erosion would improve predictive abilities. Addition of a lateral migration component to CONCEPTS would then complete a state-of the-art set of tools for adaptive management of streams tributary to Lake Tahoe. Field experiments of erosion resistance of bio- engineered treatments and numerical simulations of channel and treatment response could be validated with time-series data collected from selected streams in the Tahoe Basin.

1.1 Objectives and Scope

The primary objectives of the proposed study are to enhance and further validate the predictive, numerical models CONCEPTS and BSTEM to fully realize their potential as state-of the-art tools for stream management, in the Lake Tahoe Basin and elsewhere. The project-scale data collection and resultant model enhancement will specifically inform adaptive management strategies for local restored streams, and the validated models will be available for use in evaluating design and expected performance of proposed restoration projects. Specific study objectives include:

Quantifying the effects of riparian vegetation and bio-engineered treatments on the resistance of bank materials to hydraulic erosion and bank undercutting for inclusion into both models; Developing a near-bank groundwater model to integrate with CONCEPTS and BSTEM for the purpose of simulating pore-water pressures dynamically; Developing algorithms for CONCEPTS to simulate lateral migration of meandering channels in a deterministic fashion by accounting for hydraulic and geotechnical controls; and Validate the use of the CONCEPTS and BSTEM models at the project-scale for existing and restored reaches of selected Tahoe basin streams using time-series historical data on flow, sediment transport and channel geometry.

The underlying hypothesis of this research effort is that with improved definition of the effects of vegetation and other bio-engineered treatments on boundary resistance coupled with enhanced algorithms to simulate channel response, resource managers in the Lake Tahoe Basin will have the ability to accurately evaluate restoration strategies to reduce fine-sediment loadings to the lake with state-of the-art numerical tools.

1.2 Approach, Methodology, and Geographic Location of Research

The geographic scope of the presented research is selected reaches of the Upper Truckee River and Trout Creek (Figure 1). Trout Creek is included because it contains well documented restoration efforts that have been installed for several years.

The approach combines field and numerical experiments on the effects of vegetation, bio- engineered treatments and hydraulics in meander bends with the development and validation of process-based algorithms to better simulate pore-water pressure distributions, streambank erosion, lateral migration of meanders, and fine-sediment loadings. The approach builds on prior field data and modeling efforts in the same stream systems (Simon et al., 2003; Simon, 2006; Simon et al., 2006), and validation opportunities provided by post-project monitoring of implemented projects.

Figure 1 Map showing locations of study reaches on the Upper Truckee River and Trout Creek.

1.2.1 Quantifying the Effects of Vegetation and Bio-Engineered Treatments

To accurately evaluate the susceptibility of root-permeated and bio-engineered streambanks to erosion, channel widening and lateral migration, the hydraulic and geotechnical resistance of these features must be quantified. A series of field experiments were conducted using a root tensile-strength testing device (Abernethy and Rutherford, 2001) in conjunction with root mapping to quantify root reinforcement (Simon and Collison, 2002; Pollen and Simon, 2005) of meadow and other riparian vegetation throughout the bank profiles. Submerged jet-test devices (Hanson, 1990; Tolhurst et al., 1999) were used to determine hydraulic resistance (critical shear stress and erodibility) of bank sediments with and without riparian roots as well as bio- engineered treatments. Results of these field experiments provide values of critical shear stress ( c; stress for incipient motion of particles) and the erodibility coefficient (kd; erosion volume per unit of stress, per unit of time) that are important input parameters into both numerical models for predicting bank-toe erosion and undercutting (Hanson and Simon, 2001). This study is documented in the accompanying report by Bankhead et al. (2013).

1.2.2 Near-Bank Groundwater Model: Dynamic Pore-Water Pressure

Sub-models were developed or enhanced for BSTEM and CONCEPTS to simulate: (1) the spatial distribution of pore-water pressure in a streambank and along a stream-riparian corridor; and (2) the effectiveness of different riparian species in controlling streambank erosion by increasing the resistance of the bank to hydraulic and geotechnical forces. Data were collected at selected sites on Trout Creek and the Upper Truckee River to determine: permeability, soil texture, soil water retention curve, above- and below-ground plant biomass, volumetric soil water content, pore-water pressure, and groundwater table elevation. Data on groundwater levels and pore-water pressure distributions are available for some reaches (EDAW, 2006; Simon et al., 2006). Additional pore-water pressure data for model validation were obtained using digital tensiometers installed at various depths and distances from the channel along selected reaches representing a range of riparian-buffer systems (i.e., meadow vegetation, woody species). This approach was successfully used for Lemmon’s willow and Lodgepole pine along the Upper Truckee River (Simon et al., 2006). The data were used to: (1) develop species-specific data bases for BSTEM, and (2) test and validate the groundwater sub-models of BSTEM and CONCEPTS. Validation runs were conducted to compare simulated pore-water pressure distributions with time-series of pore-water pressure data obtained with the digital tensiometers along the study streams. The effects of different vegetation and bio-engineered treatments were then simulated with the validated models to evaluate and compare the effectiveness of various treatments. This report presents the CONCEPTS groundwater sub-model. The BSTEM model enhancements are documented in the accompanying report by Bankhead et al. (2013).

1.2.3. Lateral Migration of Meanders

To accurately predict channel evolution in sinuous streams and to evaluate restoration strategies that include re-meandering of channels, a model was developed to be used together with CONCEPTS to simulate the lateral migration and sediment loads of meandering streams. The RVR Meander model (Abad and Garcia, 2006) was enhanced with the process-based streambank erosion algorithms of CONCEPTS to provide a comprehensive, physically-based model of

lateral channel migration at engineering time scales. The combined model improves upon current, commonly used quasi two-dimensional models derived using perturbation analysis (e.g., Larsen, 1995; Sun et al., 2001) that are limited to cross sections of simple geometry with vertical banks linear transverse bed slope, at constant width. These models relate lateral retreat to near- bank velocity through an empirical erosion coefficient, and, therefore, are less useful for simulating lateral erosion rates of specific bends where materials and treatments may vary. By simulating the lateral distributions of flow and boundary shear stress, the enhanced model is able to predict the divergence of sediment-transport rates and bank-toe scour within sinuous streams. Shear stress distributions calculated by RVR Meander can be used to improve bank erosion estimates by CONCEPTS in meandering streams. The enhancements to the RVR Meander model and its application to Trout Creek are documented in the accompanying papers of Motta et al. (2011; 2012).

1.2.4 Evaluating Performance of Restoration Treatments and Validating Model Results

Validation of simulations with deterministic, numerical models such as CONCEPTS and BSTEM is best conducted with high-quality field data to define the available forces and boundary resistance that control the specific processes being simulated. Because the models simulate erosion by both hydraulic and geotechnical forces, field data quantifying channel dimensions, hydraulic resistance (particle-size for coarse sediments and jet tests for cohesive sediments) and geotechnical resistance (cohesion, friction angle bulk unit weight, pore-water pressure and root reinforcement) will be collected along the selected reaches. Fortunately, a wealth of data already exists along a number of the proposed study reaches of Trout Creek and on the Upper Truckee River, having been collected as part of earlier studies (Walck, 2001; Walck 2004; Simon et al., 2003; Swanson Hydrology and Geomorphology, 2004). Rapid geomorphic assessments were conducted along all of the study reaches to determine the relative stability and dominant geomorphic processes that are active along channels and treatments. This technique, which has been used at more than 300 sites in the basin (Simon et al., 2003) provides an index of channel stability and the performance of treated reaches.

Bed- and bank-material particle size data were collected at cross sections along previously un- sampled reaches as were geotechnical data to define shear strength (cohesive and frictional) of the bank materials. The latter were conducted in situ for at least two layers in each bank profile using a borehole shear-test (BST) apparatus (Lutenegger and Hallberg, 1981). Critical shear stress and the erodibility coefficient of fine-grained bank materials were measured in situ using submerged jet-test devices (Hanson, 1990; Tolhurst et al., 1999). Both of these types of instruments have been used successfully in previous data-collection efforts along Tahoe basin streams.

BSTEM simulations provide site specific predictions of appropriate streambank design and performance criteria using selected geometry, vegetation or other treatments. In contrast, CONCEPTS simulations provide quantifiable evaluations of the cumulative impact of all treatments within the reach on fine-load reduction as well as any downstream impacts on channel processes and morphology. The CONCEPTS simulations of the Trout Creek reach are for the purpose of validating model enhancements and for evaluating the performance of various restoration treatments. BSTEM simulations on Trout Creek and the Upper Truckee River were

conducted to validate the near-bank groundwater model and to test performance of specific bio- engineered treatments on streambank stability and reduction of fine-sediment loads. This report documents the CONCEPTS simulations. The BSTEM simulations are documented in the accompanying report by Bankhead et al. (2013).

2. FIELD DATA COLLECTION

Field data were collected to accurately characterize the forces acting on the channel boundary materials (represented by channel slope and cross-sectional geometry) and the resistance to erosion (boundary material texture, erodibility, shear-strength, and degree of saturation) of the channel boundary. The resistance to erosion of failed bank-material blocks is reported by Bankhead et al. (2013). Because of previous boundary material characterization studies (Simon et al., 2003; Simon, 2006), fewer data were collected along the Upper Truckee River study reaches.

2.1 Streambank hydrology

Streambank erosion can be greatly enhanced by soil water content of the bank soils. Soil water content is determined by the processes of infiltration from precipitation and snowmelt through the floodplain surface, lateral inflows from upland and stream, and evapotranspiration. Vegetation has been found to greatly modify local soil water content by both increasing evapotranspiration and infiltration (Simon and Collison, 2002). Stream restoration measures in the Tahoe Basin increasingly include management of the riparian zone through vegetative measures. Hence, four sites each on the Upper Truckee River and Trout Creek were instrumented to measure the movement of riparian water for various vegetation types. Sites were selected based on the presence of nearby existing groundwater wells. Instruments consisted of combinations of tensiometers to measure pore-water pressure, transducers to measure stream water surface elevation and groundwater elevation, soil moisture probes, and acoustic sensors and staff gages to measure depth of snowpack. Instruments were installed in October 2007, and data were collected to August 2009.

2.1.1 Upper Truckee River

Two sites (#1 and #2) were located on the Golf Course Reach and two sites (#3 and #4) were located on the Sunset Stables Reach of the Upper Truckee River (see Figure 2). Figure 3 shows a photograph of each site.

Site #1

SHG (2003) classified the vegetation at Site 1 as Lodgepole Pine Forest. The instrument installation consisted of (Figure 2): (1) a nest of four tensiometers located at 1 and 3 m from the bank edge and at depths of 0.3 and 1 m below the floodplain elevation, and (2) a transducer at well #14 monitored by the State of California Parks and Recreation.

Figure 4 compares the measured groundwater elevation and pore-water pressure. The pore-water pressure at a depth of 0.3 m is strongly correlated with precipitation events, whereas the pore- water pressure at a depth of 1.0 m is more strongly correlated with the groundwater elevation. During the spring snowmelt period groundwater elevation and pore-water pressure head are similar. During the dry summer and fall period, evapotranspiration provides almost 5 m of suction, which is equivalent to approximately 10 kPa of added cohesive strength to the bank material.

Figure 2 Map of monitoring sites on the Upper Truckee River. Symbols: blue triangle, water surface transducer; red plus, tensiometer; orange circle, groundwater transducer; pink square, snow staff gage; white square, acoustic distance sensor; blue pentagon, soil moisture probe.

Site #2

SHG (2003) classified the vegetation at Site 2 as Dry Meadow. The instrument installation consisted of ( Figure 2): (1) a nest of four tensiometers located at 1 and 3 m from the bank edge and at depths of 0.3 and 1 m below the floodplain, (2) a transducer at well #19 monitored by the State of California Parks and Recreation, (3) a snow staff, and (4) a surface water elevation transducer.

Figure 5 compares the measured surface water and groundwater elevations and pore-water pressure. The pore-water pressure at a depth of 0.3 m is strongly correlated with precipitation events and the water surface elevation, whereas the pore-water pressure at a depth of 1.0 m is more strongly correlated with the groundwater elevation. During the spring snowmelt period groundwater elevation and pore-water pressure head are similar, and the streambank profile is completely saturated. During the dry summer and fall period, the pore-water pressure at 0.3 m is in equilibrium with the surface water elevation, whereas the soil remains fairly wet at a depth of 1 m especially during and immediately following the snowmelt season. Evapotranspiration seems less important here, probably because the bank faces northeast.

(a) (b)

Figure 3 Photos of the monitoring sites on the Upper Truckee River: (a) Site 1, (b) Site 2, (c) Site 3, and (d) Site 4.

Site #3

The vegetation at Site 3 can be classified as Dry Meadow. The instrument installation consisted of (Figure 2): (1) a nest of four tensiometers located at 1 and 3 m from the bank edge and at depths of 0.3 and 1 m below the floodplain, (2) a tensiometer located 3 m from the bank edge at a depth of 1.5 m below the floodplain, (3) a transducer at a well monitored by the Tahoe Conservancy, (4) a soil moisture probe at a depth of 0.3 m below the floodplain, (5) a staff and acoustic distance sensor to measure depth of snowpack, and (6) a surface water elevation transducer.

Figure 6 compares the measured surface water and groundwater elevations and pore-water pressure. During the observation period groundwater elevation is strongly correlated and similar to the surface elevation except for the month of March. Ice dynamics may affect the surface water elevations during this period. The pore-water pressure at a depth of 0.3 m is strongly correlated with precipitation events and the groundwater elevation in Winter and Spring. The pore-water pressures at a depth of 1.0 and 1.5 m are mainly determined by the groundwater elevation. During the spring snowmelt period the top of the streambank profile remains unsaturated. During the dry summer and fall period, evapotranspiration provides about 4 m of suction (ca. 8 kPa of added cohesive strength) at a depth of 0.3 m and about 2 m (ca. 4 kPa of added cohesive strength) at a depth of 1 m. The significant increase in apparent cohesion at a depth of 1 m can be probably attributed to the fact that the bank faces west.

Figure 4 Observed precipitation, groundwater elevation (GWE), and pore-water pressure (PWP) at Site #1 on the Upper Truckee River.

Site #4

The vegetation at Site 4 can be classified as Dry Meadow Mixed with Willow. The instrument installation consisted of (Figure 2): (1) a nest of four tensiometers located at 1 and 3 m from the bank edge and at depths of 0.3 and 1 m below the floodplain, (3) a transducer at a well installed fairly close to the tensiometers, and (4) a surface water elevation transducer.

Figure 7 compares the measured surface water and groundwater elevations, and pore-water pressure. As at Site 3, groundwater elevation is strongly correlated and similar to the surface elevation during the observation period except for the month of March. The pore-water pressure at a depth of 0.3 m is strongly correlated with precipitation events and the groundwater elevation in Winter and Spring. The pore-water pressure at a depth of 1.0 is mainly determined by the groundwater elevation. During the spring snowmelt period the top of the streambank profile remains unsaturated. During the dry summer and fall period, evapotranspiration provides about 5 m of suction (ca. 10 kPa of added cohesive strength) throughout the top 1 m of the bank profile. The significant increase in apparent cohesion at a depth of 1 m compared to that at Site 3 may be caused by the increased transpiration provided by the willows.

Figure 5 Observed precipitation, surface water (SWE) and groundwater elevations (GWE), and pore-water pressure (PWP) at Site #2 on the Upper Truckee River.

Figure 6 Observed precipitation, surface water (SWE) and groundwater elevations (GWE), and pore-water pressure (PWP) at Site #3 on the Upper Truckee River.

Figure 7 Observed precipitation, surface water (SWE) and groundwater elevations (GWE), and pore-water pressure (PWP) at Site #4 on the Upper Truckee River.

2.1.2 Trout Creek

Three sites were located upstream of the Cold Creek tributary and one site was located downstream of the Cold Creek tributary (see Figure 8). Figure 9 shows a photograph of each site. All sites can be classified as Wet Graminoid Meadow. Site 3 has some willow at the bank’s edge.

Figure 8 Map of monitoring sites on Trout Creek. Symbols: blue triangle, water surface transducer; red plus, tensiometer; orange circle, groundwater transducer; pink square, snow staff gage; white square, acoustic distance sensor; blue pentagon, soil moisture probe.

(a) (b)

Figure 9 Photos of the monitoring sites on Trout Creek: (a) Site 1, (b) Site 2, (c) Site 3, and (d) Site 4.

Site #1

The instrument installation consisted of (Figure 8): (1) a nest of four tensiometers located at 1 and 3 m from the bank edge and at depths of 0.3 and 1 m below the floodplain, (2) a soil moisture probe at a depth of 1 m below the floodplain, (3) a surface water transducer, (4) a transducer at installed at a nearby groundwater well, and (5) a staff gage and acoustic distance sensor to measure snowpack depth.

Figure 10 compares the measured surface water and groundwater elevations and pore-water pressure. The tensiometers at a depth of 1 m did not work properly for most of the observation period (from April 2008). The tensiometer 1 m from the bank’s edge at a depth of 0.3 m was not working properly during the summer of 2008. Groundwater elevation is strongly correlated and similar to the surface water elevation except for the month of March. The pore-water pressure at a depth of 0.3 m is strongly correlated with precipitation events and the groundwater elevation in winter and spring. During the spring snowmelt period the top of the streambank profile remains unsaturated. The wet meadow vegetation does not provide any transpiration-induced strength (through suction) to the bank soil.

Figure 10 Observed precipitation, surface water (SWE) and groundwater elevations (GWE), and pore-water pressure (PWP) at Site #1 on Trout Creek.

Site #2

The instrument installation consisted of a nest of four tensiometers located at 1 and 3 m from the bank edge and at depths of 0.3 and 1 m below the floodplain (see Figure 8). The tensiometers did not work correctly, therefore the measured pore-water pressures are not reported here.

Site #3

The instrument installation consisted of (Figure 8): (1) a nest of four tensiometers located at 1 and 3 m from the bank edge and at depths of 0.3 and 1 m below the floodplain, and (2) a surface water transducer.

Figure 11 compares the measured surface water and pore-water pressure. The pore-water pressure at a depth of 0.3 m is strongly correlated with precipitation events and the water surface elevation. During the winter of 2008 the entire bank profile was saturated. The pressure head nearest to the stream (1 m from the bank’s edge) is almost identical to the water surface elevation, indicating that the soil water content is controlled by the water surface elevation in the stream and movement of water into the bank. During the summer of 2008, further away from the

bank, about 30 cm of suction (or approximately 0.5 kPa of added cohesive strength) is provided by the meadow vegetation in the upper part of the soil profile.

Figure 11 Observed precipitation, surface water elevation (SWE), and pore-water pressure (PWP) at Site #3 on Trout Creek.

Site #4

Figure 12 shows the measured pore-water pressure. The pore-water pressure at a depth of 0.3 m is strongly correlated with precipitation events and remains fairly high during the wet season and when snowpack is present. At a depth of 1 m, pressure head is generally lower than in the upper part of the streambank, indicating it is controlled by the water surface or groundwater elevation. During the spring snowmelt the upper part of the bank profile remains unsaturated. During the dry period, the meadow vegetation provides about 5 m of suction (approximately 10 kPa of added cohesive strength) in the upper part of the soil profile. The lower part of the Trout Creek floodplain seems to be drier than the area upstream of the Cold Creek confluence. This could be

attributed to a combination of: (1) patches of willow trees across the floodplain; and (2) lowering of the water surface elevation caused by channel incision (see section 2.3).

Figure 12 Observed precipitation, surface water and groundwater elevations, and pore-water pressure at Site #4 on Trout Creek.

2.2 Resistance to Erosion of Channel Boundary Materials

Geotechnical data to define shear strength (cohesive and frictional) of the bank materials was measured in situ for at least two layers in each bank profile using a borehole shear-test (BST) apparatus (Lutenegger and Hallberg, 1981). Critical shear stress and the erodibility coefficient of fine-grained bank materials was measured in situ using submerged jet-test devices (Hanson, 1990; Tolhurst et al., 1999).

The data collected along the Upper Truckee River are reported by Simon et al. (2003), Simon (2006), Simon et al. (2006), and Bankhead et al. (2013).

2.2.1 Trout Creek

Bank and bed-material samples, and data on resistance to erosion and failure were collected at 12 sites on Trout Creek (see Figure 13). Data are available on CD-ROM in the Microsoft Excel file named “Physical data.xlsx.”

The texture of the collected bank soil samples ranges from silt loams to coarse loamy sands (Figure 14). The soils are resistant to erosion by mass failure. Measured effective cohesion ranges from 0.7 kPa to 28.8 kPa (18 tests), with a mean cohesion of 11.3 kPa and a median cohesion of 10.2 kPa. The resistance to erosion is characterized by a critical shear stress and erodibility. The critical shear stress ranged from 0.2 Pa to 9.1 Pa (20 tests), with a mean value of 2.2 Pa and a median value of 1.4 Pa. This critical shear stress is similar to a stress needed to move a very coarse (1-2 mm) sand particle. The erodibility coefficient had a fairly constant value of 1E-5 m s-1 Pa-1.

SHG (2004) reported grain-size distribution of the riffles in 2001 and 2003 located within their six monitoring reaches (cf. Figure 17). Selected riffles were resampled in 2008. Grain-size analysis of the riffles along Trout Creek shows a downstream fining (Figure 15). The most upstream riffles from Site 1 through halfway Site 4 and 5 have a mean diameter of 38.1 mm (red line #1 in Figure 15). From this location to the confluence with Cold Creek the riffles have a mean diameter of 27.9 mm (green line #2 in Figure 15), whereas the riffles downstream of the confluence with Cold Creek have a mean diameter of 17.1 mm (blue line #3 in Figure 15).

Bed material in the pools is mainly comprised of medium and coarse sands (0.25 – 2 mm). There is little fine-grained (<63 m) sediment, indicating that these sediments are mainly transported as wash load. At certain locations where the bed has incised, such as near the confluence with Cold Creek and in the reach downstream of the Cold Creek confluence, a lacustrine layer containing about 25% clay has been exposed.

Figure 13 Location of data collection sites on Trout Creek to characterize the resistance to erosion of channel boundary materials. (Continued on next page).

Figure 13 Location of data collection sites on Trout Creek to characterize the resistance to erosion of channel boundary materials.

Figure 14 Grain-size distribution of bank-material samples collected along Trout Creek

Figure 15 Grain-size distribution of riffles along Trout Creek. The riffles labeled PCx-x xx were sampled by SHG (2004). The first number indicates monitoring reach (cf. Figure 17), the second number indicates cross section, and the last number indicates year of sampling. The riffles labeled Trout #x were sampled in 2008; the number indicates sampling site (cf. Figure 13).

Figure 16 Grain-size distribution of pools along Trout Creek. The numbers in the legend indicate data collection site (cf. Figure 13).

2.3 Channel Geometry of Trout Creek

The restored channel of Trout Creek was activated in 2001. Swanson Hydrology + Geomorphology monitored the geomorphic adjustment between 2001 and 2003 (SHG, 2004). Annual surveys were conducted of thalweg profile and four cross sections at six reaches (Figure 17). SHG (2004) found no significant adjustment of the channel had occurred by 2003. Some sand had accumulated along Reach #5.

In August 2008 the thalweg of the entire reconstructed channel and the 24 cross sections monitored by Swanson Hydrology + Geomorphology were resurveyed. Figure 18 and Figure 19 compare this survey with those reported in SHG (2004) and the channel design. It can be observed that the riffles downstream of the confluence with Cold Creek have been eroded between 2003 and 2008. This most likely occurred during the snow melt in 2006 when discharge consistently exceeded the five-year discharge (~ 5.6 m3/s at USGS gage #10336775 on Pioneer Trail, see also Figure 32). The riffles upstream of the confluence have remained fairly stable. Only some scour on the edges of the riffles has taken place. Pools have generally deepened by 0.3 to 0.6 m.

Figure 17 Location of reaches and cross sections on Trout Creek monitored by Swanson Hydrology + Geomorphology (2004). (Continued on next page).

Figure 17 Location of reaches and cross sections on Trout Creek monitored by Swanson Hydrology + Geomorphology (2004).

Figure 18 Observed changes in thalweg elevation of the reconstructed Trout Creek channel between 2001 and 2008.

REACH 1

REACH 2

Figure 19 Observed changes in thalweg elevation along the reaches monitored by Swanson Hydrology + Geomorphology (2004) of the reconstructed Trout Creek channel between 2001 and 2008. (Continued on next page).

REACH 3

REACH 4

REACH 5

REACH 6

Figure 19 Observed changes in thalweg elevation along the reaches monitored by Swanson Hydrology + Geomorphology (2004) of the reconstructed Trout Creek channel between 2001 and 2008.

Figure 20 to Figure 25 show the changes in cross-sectional geometry between 2001 and 2008 of the reaches monitored by SHG (2004). Channel width adjustment is minimal in the riffle sections. Only cross section 5A, which is located just downstream of the confluence with Cold Creek, experienced significant erosion of approximately 0.5 m, which appears to have occurred during the snowmelt of 2006.

Only three out of the twelve surveyed pool sections show significant erosion of the outer bank. Cross section 1B experienced about 1.5 m of erosion, cross section 1D experienced about 2 m of erosion, and cross section 4D experienced almost 2.5 m of erosion. The pool sections where widening occurred also experienced infilling of their pool. Deposition is also occurring in pool sections 2B, 4B, and 5D. The probability that streambank will occur at these sections may therefore increase. Further, these sections are located upstream of the bendway apex. Note that bank erosion on strongly curved bends (such as the Trout Creek bends) typically occurs at their upstream section.

Figure 20 Observed changes in cross section geometry along Reach 1 on Trout Creek.

Figure 21 Observed changes in cross section geometry along Reach 2 on Trout Creek.

Figure 22 Observed changes in cross section geometry along Reach 3 on Trout Creek.

Figure 23 Observed changes in cross section geometry along Reach 4 on Trout Creek.

Figure 24 Observed changes in cross section geometry along Reach 5 on Trout Creek.

Figure 25 Observed changes in cross section geometry along Reach 6 on Trout Creek.

3. CONCEPTS MODEL ENHANCEMENTS

Two sub-models were developed for CONCEPTS to simulate the effects of riparian groundwater and lateral channel migration on long-term channel morphology and sediment loads. The collected field data presented in the previous section were used to test these enhancements.

3.1 Near-Bank Groundwater Model

Figure 4 through Figure 7 and Figure 10 through Figure 12 show that: (1) groundwater and surface water elevations are closely correlated, and (2) soil water content above the groundwater table is in equilibrium with the water table except for the dry summer period. Little erosion of the channel boundary occurs during the summer and early fall period as flows are near base flow; hence the enhanced shear strength contributed by matric suction in the upper portion during this period is not important to the overall channel stability. As a result, the groundwater model for use in the Lake Tahoe Basin can be simplified and only needs to account for exchanges between surface water and groundwater through the bank face. In addition such model is very efficient for long-term simulations.

Figure 26 shows a definition sketch of the variables used to formulate the groundwater sub- model of CONCEPTS. Assuming a surface water elevation hs and groundwater elevation hg, the mass flux of water between the stream and riparian zone, q, is a function of the difference in elevations h = hg – hs. Seepage occurs if h > 0 and infiltration occurs if h < 0. Following Darcy, the flux of water between stream and riparian zone is defined as:

(1) where K is the saturated hydraulic conductivity of the bank soil. Assuming that this mass flux drains or fills the riparian zone over a length L, the water balance for the riparian zone is:

Figure 26 Definition sketch of variables used to formulate the CONCEPTS sub-model of groundwater dynamics.

(2)

Substituting Eq. (1) into Eq. (2) yields:

(3) where = K / L has the unit of time-1. The time scale 1 / can be perceived as a lag time of the groundwater table with respect to changes in surface water elevation. As surface water calculations are decoupled from the groundwater calculations, Eq. (3) can be written as:

(4)

Eq. (4) has an analytical solution. For a model time step t, the change in groundwater elevation can be calculated from:

(5)

3.2 Lateral Migration Model

To accurately predict channel evolution in sinuous streams and to evaluate restoration strategies that include re-meandering of channels, the RVR Meander model was enhanced with the CONCEPTS streambank erosion algorithms to accurately simulate the lateral migration and sediment loads of meandering streams (see accompanying paper of Motta et al., 2012). The enhanced RVR Meander model was tested on the observed migration of Trout Creek (Motta et al., 2011). One-dimensional (1D) sediment transport models such as CONCEPTS are not capable of simulating flow and sediment transport rates accurately in meander streams. However, CONCEPTS can simulate the correct bank erosion rates at the reach scale when used in combination with RVR Meander.

CONCEPTS uses an excess shear stress equation to calculate the rate of erosion of fine-grained streambank materials by flowing water:

(6) where E is erosion rate, kd is erodibility or detachment coefficient, is boundary shear stress, and c is critical shear stress below which no erosion occurs. The detachment coefficient and critical shear stress are soil properties, which can be measured in situ with devices such as the jet tester. Their values for the bank material on Trout Creek were reported in section 2.2.1. In the

following analysis we assume, for simplicity, homogeneous bank soils, that is kd and c do not vary along the streambank. The only parameter that varies is boundary shear stress .

The erosion rate along the outer bank of a meander bend, Ebend, is

(7) where bend is the actual boundary shear stress acting on the bank material. The shear stress bend can be accurately calculated by the RVR Meander computer model. Assuming the ratio

(8) where 1D is the bank shear stress predicted by a 1D model such as CONCEPTS, and substituting Eq. (8) into Eq. (7) yields

(9)

Eq. (9) shows that CONCEPTS can simulate the correct fluvial erosion in meander bends by modifying the resistance-to-erosion properties of the bank soils proportionally to the ratio , that is use:

(10)

The ratio can be calculated using RVR Meander.

4. CONCEPTS MODEL VALIDATION

4.1 Model Setup

Modeling Reach

The capabilities of the classic, one-dimensional version of CONCEPTS to evaluate the performance of stream restoration measures to reduce sediments eroded from the channel boundaries, were assessed for the Trout Creek Meadow Restoration Project completed in 2001. The modeled reach extended from river station 102+70 to 200+70 (Phase II stationing), see Figure 27. The reach comprises 172 model sections, which are typically located at each riffle and pool along the reach.

Boundary Materials

The data presented in Section 2.2.1 were used to characterize the boundary material properties. The data were analyzed to identify channel sections with similar characteristics. Figure 28 to Figure 31 show the bed and bank materials and their properties used by CONCEPTS. The soil erodibility properties for cross sections located in meander bends were modified using Eq. (10) in section 3.2.

Flows

The flow records at USGS gages 10336775 (TROUT CK AT PIONEER TRAIL NR SOUTH LAKE TAHOE, CA), 10336778 (COLD CK AT PIONEER TRAIL NR SOUTH LAKE TAHOE, CA), and 10336780 (TROUT CK NR TAHOE VALLEY, CA) located at the bridge crossing on Martin Avenue were analyzed to determine (1) hourly flow, (2) bankfull flow, and (3) five-year flow conditions at the upstream end of the modeling reach and Cold Creek tributary inflows for water years 2002 through 2009. Gages 10336775 and 10336780 bound the restored channel reach of Trout Creek.

The tributary inflow from Cold Creek is on average 65% of the discharge reported at gage 10336775. A discharge record for Cold Creek was then constructed by multiplying the flows at 10336775 by 0.65. Table 1 lists the discharge with 2- and 5-yr return period and its probability of exceedence. The bankfull discharge has a return period of about two years.

An hourly flow record at the upstream boundary of the modeling reach was constructed from the instantaneous flow record at gage 10336775 (Figure 32). Largest discharges (about 5-year recurrence interval) occurred during late December 2005 and the 2006 snowmelt season. Bankfull flow was exceeded during the snowmelt seasons of 2003, 2005, and 2006 only.

Figure 27 Trout Creek modeling reach with model sections. Cold Creek discharges into cross section 152+05.

Figure 28 List of soil profiles used in the CONCEPTS model of Trout Creek. For example, the soil profile between river stations 201+00 and 192+00 comprised four layers ranging from a medium loamy sand near the floodplain surface to a silt loam layer at depth.

Figure 29 List of soils used in the CONCEPTS model of Trout Creek. The figure shows the properties for the “fine loamy sand” soil.

Figure 30 List of sediment profiles used in the CONCEPTS model of Trout Creek. For example, the riffle profile between river stations 182+00 and 162+00 comprises three layers ranging from a coarse riffle layer at the surface to a lacustrine layer at depth.

Figure 31 List of sediments used in the CONCEPTS model of Trout Creek. The figure shows the properties for the “riffle 182+00 – 162+00” sediment.

10-yr flood

5-yr flood

bankfull

Figure 32 Hourly discharge record used in the CONCEPTS model. Horizontal lines indicate, bankfull, five-year, and ten-year discharge.

Table 1 Discharges used in the CONCEPTS simulations. Discharge in m3/s Return period Trout Creek upstream Cold Creek inflow %Exceedence 2-year 2.5 1.6 3.3 5-year 5.6 3.6 0.05

4.2 Model Results

Bankfull Flow

Figure 33 to Figure 35 show the water surface profile, change in bed elevation, and channel widening for bankfull flow. Note that the model results are plotted in model simulation days. As the %exceedence of the bankfull discharge is 3.3%, model simulation days need to be adjusted to reflect actual, real days. For example, an 80 day simulation of bankfull flow represents 80/0.033 ≈ 2,400 days (or 6.6 years). Such duration would approximately cover the 2001-2008 study period documented in section 2.

The predicted water surface profile is aligned with overbank elevation indicating that the used Manning n values, which range from 0.03 to 0.06, properly represent boundary roughness. Channel grade along the riffle sections shows negligible change, whereas the pool sections show significant scour. Most pool scour ranges from 0.3 to 0.6 m, which is similar to that observed and documented in section 2. Channel widening is zero except at a few locations where deposition has taken place on riffles. This agrees reasonably well with the observed widening documented in section 2.

The simulations show that the bankfull flow was unable to erode the riffles as was observed. This could partly explain why very little widening has been observed.

Figure 33 Simulated water surface profile for bankfull flow discharge of 2.5 m3/s.

Figure 34 Simulated change in bed elevation for bankfull flow discharge of 2.5 m3/s.

Figure 35 Simulated change in channel top width for bankfull flow discharge of 2.5 m3/s

5-Year Discharge

Figure 36 to Figure 38 show the water surface profile, change in bed elevation, and channel widening for 5-year flow discharge of 5.6 m3/s. The entire floodplain near the channel is covered by approximately 0.3 m of water. Channel grade along the most upstream riffle sections between river stations 200+70 and 163+15 shows negligible change, whereas the riffle sections downstream and immediately upstream of the confluence with Cold Creek have been completely removed causing the downstream reach to incise by approximately 1 m. This is very similar to that observed (cf. Figure 18). It seems that about 20 model simulation days were needed, however the gage record shows that the 5-year discharge was only exceeded for about 3 days. It can be concluded that the size of the bed material comprising the riffles was sized too small. The pool sections show significant scour. Even with the significant incision, channel widening is limited to a few locations where deposition has taken place on riffles. The bank soils and sod cover are sufficiently strong to withstand a discharge with a five-year return period for a limited time.

Figure 36 Simulated water surface profile for 5-year flow discharge of 5.6 m3/s.

Figure 37 Simulated change in thalweg elevation for 5-year flow discharge of 5.6 m3/s.

Figure 38 Simulated change in channel width for 5-year flow discharge of 5.6 m3/s.

Hourly Flow Record

Simulating pool-riffle systems, where the riffle bed-material is significantly larger than that of the pool, with a 1D model such as CONCEPTS may yield unrealistic results for long-term simulations. The simulated scour of the pools at bankfull conditions is too large, causing unrealistic deposition on the very coarse riffles. Therefore, it was decided too evaluate the restored reach on Trout Creek for the observed discharge record between 2001 and 2009 by only including the riffle sections. These sections are responsible for the vertical stability of the channel. Also, the above simulations have shown that simulated channel widening is minimal.

Figure 39 and Figure 40 show the simulated adjustment of bed elevation and channel top width. Both the simulated changes in bed elevation and channel top width are small. Significant incision was predicted in the non-restored section connecting two reconstructed reaches around model kilometer 2.5, where bed-material grain size is significantly smaller than that of the adjacent riffles. The difference in simulated bed elevation adjustment for the 5-year flow discharge (Figure 37) and that shown in Figure 40 is probably related to the duration at which the higher discharges were active.

Figure 39 Simulated change in thalweg elevation between Oct 1, 2010 and Sep 30, 2009.

Figure 40 Simulated change in channel top width between Oct 1, 2010 and Sep 30, 2009.

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