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).

Models constructed and checked for the intended application can inform sediment-management decisions. Define the modeled sediment fraction, spatial scale, analysis period, and data limitations before interpreting a result. A fit to accumulated load does not by itself establish correct event timing, grain-size distribution, or future performance. Use measured flow and sediment information, independent checks where possible, and sensitivity analysis to establish the useful scope of the results (EM 1110-2-4000 (USACE, 2025), Chapters 4 and 6 and paragraph 1-14).

Authority and Procedural Guidance

Applicable authority depends on the project and study purpose. The following laws provide water-quality context or examples of USACE restoration authority. Confirm current applicability and program requirements for the study; EP 1105-2-70 (USACE, 2025), paragraph 2-3, summarizes Civil Works aquatic ecosystem restoration authorities.

  • The Clean Water Act of 1972 provides the water-quality context for identifying impaired waters and developing TMDLs. The EPA TMDL overview (EPA, n.d.) describes that process; this water-quality context is distinct from the specific authority for a USACE project.
  • 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 1992, Section 204. This section authorizes projects to protect, restore, or create aquatic and ecologically functional habitat including wetlands.

The following USACE guidance is relevant to a watershed erosion and sediment-routing study:

  • ER 1110-2-8153 (USACE, 1995), paragraphs 7 and 8, addresses the sediment study work plan, data needs, study boundaries, model information, and reporting. Its staged approach should be matched to the project's decision needs.
  • EM 1110-2-4000 (USACE, 2025), paragraph 2-5, addresses the Sediment Studies Work Plan. Chapter 4 covers sampling and data interpretation; Chapter 6 covers sediment yield and budgets, including paragraph 6-9; Chapter 8 addresses reservoirs; Chapter 9 addresses numerical sediment-model selection and evaluation. Paragraph 1-14 addresses uncertainty, while Chapter 10 focuses on flood risk management applications.
  • EM 1110-2-1417 (USACE, 1994), Chapter 10, supports calibration of the precipitation-runoff model that drives sediment computations.
  • ER 1110-2-8154 (USACE, 2018), paragraphs 1-2 and 3-2 through 3-3, provides water-quality management and data-collection direction for applicable USACE Civil Works responsibilities. Coordinate sediment observations with the project's water-quality objectives.
  • EM 1110-2-5025 (USACE, 2015) addresses dredging and dredged material management and supersedes EM 1110-2-5027 (USACE, 1987). Appendix N contains monthly pan-evaporation charts based on 1931-1960 data; Appendix H in the newer manual concerns settling-column tests. The appendix change does not make these evaporation normals representative of a current study period.
  • EM 1110-2-4000 (USACE, 2025), paragraph 1-15, and ECB 2026-1 (USACE, 2026), paragraphs 4c and 4d and section 8, address changing hydrometeorological conditions relevant to applicable planning analyses. Check the bulletin's scope and exceptions; it expires 9 January 2028. Document future land cover, runoff, and sediment-supply assumptions rather than apply an unsupported uniform sediment multiplier.

Historical HEC studies remain useful context: TP-130 (HEC, 1990a) and PR-13 (HEC, 1990b) address sediment yield for ephemeral channels; TP-152 (HEC, 1996) discusses watershed erosion inputs to channel sedimentation models; IHD-12 (HEC, 1977) provides earlier sediment-transport background. Use current guidance and method documentation for a new study's setup and interpretation.

Study Procedures

Prepare a checked precipitation-runoff model, land-cover and terrain information, channel geometry and bed gradations, reservoir elevation-area/storage relationships, and representative sediment observations. The NRCS SSURGO resources (NRCS, n.d.) provide mapped soils information; supplement it with field evidence for erodibility and the grain-size distribution delivered to the stream. Record whether observations are suspended-sediment concentration (SSC), total suspended solids (TSS), bedload, deposited sediment, or a derived load time series, together with sample location, method, frequency, units, and uncertainty (EM 1110-2-4000 (USACE, 2025), Chapter 4). TSS and SSC laboratory results should not be assumed interchangeable (Gray et al., 2000).

A study workflow should connect the sediment budget to the management decision:

  1. Define the decision, modeled sediment fractions, reporting locations, analysis period, study boundaries, and required level of detail in the work plan. Identify source, transport, and deposition areas.
  2. Review existing observations and identify missing information. Plan sampling of the relevant suspended, bed-material, and deposited fractions and reservoir surveys before using sparse observations to constrain the model.
  3. Calibrate and check the hydrologic model for runoff volume, event timing, and seasonal behavior. Select a suitable evapotranspiration method and initial-condition warm-up for a continuous model.
  4. Select erosion, sediment routing, transport potential, settling, and reservoir methods that match the processes and grain sizes of the watershed. Prepare outlet and initial-bed gradations for their different roles.
  5. Calibrate sediment against compatible observations, progressing from upstream components to downstream reaches and reservoirs. Compare event behavior, accumulated mass, grain fractions, and storage changes as data allow; validate on independent periods where possible.
  6. Check the sediment budget and numerical sensitivity, including time interval, reach discretization, transport function, initial bed, and reservoir assumptions. Document unrepresented processes and residual uncertainty.
  7. Compare alternatives using consistent forcing when isolating a management change. Define future baseline assumptions explicitly, and provide flow and sediment boundaries to a more detailed hydraulic, sediment, or water-quality analysis when required.

Current HEC-HMS watershed sediment properties include grade scale, specific gravity, and sediment unit weight/density. Starting with version 4.14, individual subbasins, reaches, and reservoirs have erosion or sediment enable controls, and the applicable transport and settling methods can be specified by element. Use the global editors under Parameters | Erosion and Parameters | Sediment to inspect multiple elements. Check enable status throughout the intended routing network; a disabled element's zero sediment discharge is not a pass-through routing assumption.

Modified USLE remains an option for agricultural runoff-driven sediment yield. Its Threshold and Exponent settings influence event separation and sedigraph shape and should be checked with the hydrologic inputs and sediment observations. The current North Bosque tutorial demonstrates the version 4.14 setup using a reduced model; it is a companion workflow rather than a recalculation of the historical Case Study.

For reservoir studies, current Reservoir Capacity Method options can update elevation-area and elevation-storage relationships as sediment accumulates. Select an appropriate deposition representation and unit weight/density and porosity assumptions, and check predicted storage loss against surveys. For a spatially distributed subbasin application, the current 2D Sediment Transport method works with the 2D transform framework and requires its own mesh, data, and calibration; it is not an automatic substitute for the historical lumped model.

Use erosion and sediment results to examine total and grain-class time series and the storage terms available for each element. HEC-HMS reach sediment routing retains fixed channel cross-section geometry during a simulation; use a suitable detailed sediment model, such as HEC-RAS sediment transport, for questions requiring that model's spatial or movable-bed representation. Sediment outputs can support water-quality work but do not by themselves calculate nutrient fate, water-quality compliance, or a TMDL.