Summary

The objective of this study was to provide watershed sedimentation information to decision makers to inform management decisions related to soil surface erosion, water quality, and reservoir capacity. An HEC-HMS model of the UNBRW was created with representation of physical basin characteristics based on watershed soil, terrain, land use, and precipitation data. The HEC-HMS model was calibrated and validated based on observed data collected within the watershed. Model results produced valuable sedimentation information including the following:

  • The sediment yield rate for each subbasin giving indication of the most erodible regions in the watershed.
  • Sediment time series (sedigraph) for each grain class to provide boundary conditions to hydraulic sediment models such as HEC-RAS.
  • Long-term aggradation and degradation trends for each reach to indicate reaches with higher erosion rates. This information could be used to assist in formulating possible corrective measures to enhance fish and wildlife habitat, water quality, flood control, and bank stability.
  • Average trap efficiency rate for each reservoir to calculate the reasonable life expectancy of each reservoir.

This information can be used to assist management officials in determining non-point source contributors in the watershed and TSS loading within the watershed, as well as establishing TMDLs to achieve water quality standards.

Key Takeaways

Key Steps in HEC-HMS

  1. Enable Basin Model sediment and the intended element erosion/sediment paths. Review grade scale, unit weight/density, gradations, and element-specific methods using current sediment properties and the global editors.
  2. Check the continuous hydrologic model before calibrating sediment. Set compatible meteorological inputs, evapotranspiration, initial conditions, and warm-up, and compare runoff volume and timing with observations.
  3. Configure erosion, reach sediment routing, and reservoir sediment for the site's processes. Use the current North Bosque tutorial as a setup companion, while preserving a copy of the historical example.
  4. Calibrate to compatible observed quantities, validate on independent periods where possible, and compare event behavior, accumulated load, grain fractions, and storage changes. Check sensitivity to transport choices, time interval, and reach discretization.
  5. Inspect total and grain-class results and the sediment budget. Export consistent flow/sediment boundaries for detailed hydraulic, sediment, or water-quality analyses.

Common Pitfalls

  • Grain classes, units, or data types silently mixed. Match the grade scale to the gradation range and distinguish concentration, mass rate, interval mass, and deposited volume. Check short tons versus metric tonnes and the density/porosity basis.
  • TSS assumed identical to SSC or total sediment. Verify laboratory and sampling methods and modeled grain fractions before comparing loads, particularly where sand is important (EM 1110-2-4000 (USACE, 2025), paragraph 4-13c; Gray et al., 2000).
  • A good accumulated-load fit hides process or numerical errors. Check event timing, plausible parameters, independent data, and time/space sensitivity. HEC-HMS reach cross-section geometry remains fixed during sediment routing; select detailed hydraulics or movable-bed analysis when the decision needs it.
  • Trap efficiency treated as reservoir life expectancy. Estimate remaining usable capacity and sediment volume over the relevant period, including density/porosity, deposition assumptions, and future inflow. Check against surveys; current reservoir-capacity options are model representations, not a substitute for those observations (EM 1110-2-4000 (USACE, 2025), Chapter 8).
  • Historical conditions extrapolated without checking future assumptions. Document changes in land cover, hydrometeorology, runoff, and sediment supply for applicable planning analyses (EM 1110-2-4000 (USACE, 2025), paragraph 1-15; ECB 2026-1 (USACE, 2026), paragraphs 4c and 4d and section 8).