Background for Surface Erosion and Sediment Routing Studies

Goals of Erosion and Sediment Control

Surface erosion, reservoir sedimentation, and in-stream sediment transport have become increasingly important in watershed management and natural resources conservation planning studies. Sediment processes also affect evaluation and implementation of water quality Best Management Practices (BMPs) and the evaluation of Total Maximum Daily Loads (TMDL's) (Pak, 2010).

The U.S. Environmental Protection Agency (EPA, 2003) has identified several soil erosion and sedimentation consequences:

Soil loss reduces nutrients and deteriorates soil structure, decreasing the land's productive capacity.
Suspended solids reduce sunlight available to aquatic plants, cover fish spawning areas and food supplies, smother coral reefs, impede filter feeders, and clog fish gills. Turbidity interferes with the feeding habits of certain fish species.
Decreased fish population and the unappealing, turbid water limit recreation opportunities. Turbidity also reduces visibility, making swimming more dangerous.
Deposited sediment reduces the transport capacity of streams, rivers, and navigation channels. Decreased capacity can increase flood frequency. Sediment can also reduce the storage capabilities of reservoirs and lakes increasing flood risk and dredging costs.
Chemicals including pesticides, phosphorus, and ammonium can be adsorbed and transported with sediment and can eventually be released from the sediment into the water.

The EPA (2003) established management practices that seek to control the delivery of Non-Point Source (NPS) pollutants to receiving water resources by:

Minimizing available pollutants (source reduction);
Retarding pollutants transport and/or delivery by either reducing flow, and thus the amount of pollutant transported, or depositing the pollutant; or
Remediating or intercepting the pollutant through chemical or biological transformation.

Surface erosion and sediment control studies help identify the source of NPS pollutants and provide information beneficial in establishing management practices that serve to reduce or mitigate surface erosion and sedimentation.

Surface Erosion and Sediment Routing Study Objectives and Outputs

While specific erosion and sediment control objectives are site and study specific, surface runoff and erosion models can be used as a tool to model sediment loads from pervious and impervious areas in a watershed and then route the sediment downstream while modeling erosion and deposition within river reaches and reservoirs (Pak, 2010; Gibson, 2010). Models are a useful tool for estimating flow and sedimentation in areas where observed data are unavailable, using data from watersheds with similar characteristics, or in conjunction with sensitivity and uncertainty analyses. Models can be especially useful for interpolating or extrapolating flow and sediment records in watersheds where observed data are available but not at the particular location of interest, or for predicting system response to projected future conditions.

Potential soil erosion and sedimentation study objectives include:

Estimate sediment time series (sedigraphs) for each grain class at selected locations/elements.
Estimate the volume of deposition or erosion within a reach element.
Predict accumulated sediment at the reservoir bottom and accumulated sediment discharge from a reservoir for a given analysis period.
Estimate changes in volume of eroded soil as a result of alternative land use management practices.
Compute the quantity and gradation of sediment produced from a watershed to generate a sediment load boundary condition for a more detailed river hydraulics and sedimentation model, such as HEC-RAS (Gibson, 2010).

Carefully constructed models designed to address these objectives can inform soil erosion and sediment management decisions. A calibrated sediment model with accurate field data can predict regional and long term sediment trends.

Authority and Procedural Guidance

USACE soil erosion and sediment transport studies are authorized by:

The Clean Water Act of 1972. This authority establishes a process to identify impaired waters and set TMDL's in an effort to achieve pollution reduction goals.
WRDA 1986, Section 1135. This section provides the authority to modify existing USACE projects to restore the environment and construct new projects to restore areas degraded by USACE projects.
WRDA 1986, Section 204. This section authorizes projects to protect, restore, or create aquatic and ecologically functional habitat including wetlands.

USACE guidance on soil erosion and sediment transport studies includes the following:

ER 1110-2-8153 (USACE, 1995) prescribes the procedure for conducting sedimentation investigations in support of hydrologic analyses, hydraulic design of civil works projects, and environmental impact analyses.
EM 1110-2-4000 (USACE, 1989) identifies typical sediment problems encountered in the development of projects in inland waters and presents appropriate procedures to resolve these problems.
TP-130 (HEC, 1990a) presents a case study of ephemeral channels in Central California and summarizes the procedures used for computing basin wide annual yield and single event sediment production.
TP-152 (HEC, 1996) provides a procedure for using land surface erosion computations to develop the inflow sediment load for a river sedimentation model.
PR-13 (HEC, 1990b) provides a procedure for computing basin wide annual yields and single event sediment production for an ephemeral channel.
IHD-12 (HEC, 1977) addresses the topics of river morphology, data collection and analysis, reservoir sedimentation, and aggradation and degradation in free flowing streams.

Study Procedures

Surface soil erosion and sediment routing studies typically require the following information: a calibrated precipitation-runoff model, soil data (such as the Soil Survey Geographic (SSURGO) database), land use data, field soil sample data, observed channel and reservoir sediment data, and channel cross section data. This information is utilized in the development, calibration, and validation of the surface soil erosion and sediment routing model.

Surface soil erosion and sediment routing modeling typically includes the following steps:

Perform a site investigation and review available data.
If data available are insufficient, prepare a field sampling program.
Examine published long-term discharge records and sediment gage records.
Prepare a calibrated precipitation-runoff model.
Select appropriate sediment modeling methods representative of the watershed.
Utilize regional studies and equations to estimate parameter values.
Calibrate the model to historical sediment data if available.
Analyze results to determine desired output values (e.g. mean annual sediment yield).
Validate the model if additional historical sediment data are available.
(Optional) For alternative analysis: Modify the watershed model to reflect changes in the watershed. Re-exercise the model with the same precipitation events. Compare the results to quantify the impact of the watershed changes.

The development, calibration, and validation of a watershed model used to analyze surface soil erosion and sediment transport is described herein.