Summary

The goal of this study was to determine which stream restoration method should be implemented. HEC-HMS, in conjunction with HEC-RAS, was used to model the watershed and channels to compare restoration alternatives. When altering channel hydraulics for restoration projects, consideration must be given as well to the flood-control purposes of the channel. By increasing the vegetation in the channel, the water surface elevation will likely increase for a given flow.

Key Takeaways

Key Steps in HEC-HMS

  1. Configure separate basin/routing scenarios for the without-project and restoration conditions. Keep inflow forcing consistent for a channel-only comparison, and document actual watershed changes when they are part of an alternative.
  2. Select and check routing geometry, roughness, and numerical controls. Test sensitivity to the simulation interval and reach discretization.
  3. Use event runs for flood-response comparisons. Use a calibrated continuous water-balance model where low-flow, seasonal, or duration outputs are required.
  4. Compare full hydrographs, volume, timing, and relevant duration metrics at the same locations. Ensemble Analysis can organize scenarios without assigning their probabilities.
  5. Export the required inflow time series to the hydraulic analysis. Where floodplain reconnection is an objective, use the hydraulic results to evaluate inundation frequency, duration, and extent. Provide documented flow-regime information for ecological assessment, including tools such as HEC-EFM where appropriate (ERDC SR-26-5 (Wiest et al., 2026), sections 2.4 and 2.5).

Common Pitfalls

  • One recurrence interval assumed to represent channel form. Retain the 2-year event for comparison if appropriate, but check channel-forming discharge using site evidence and multiple methods (EM 1110-2-4000 (USACE, 2025), paragraph 7-3h(3)).
  • Peak attenuation treated as ecological success. Check the hydrologic variables tied to measurable ecological objectives, including relevant seasonality and duration. Channel stability and reduced flood peaks do not establish the full range of ecological outcomes; select performance metrics for the site and restoration objective (EP 1105-2-70 (USACE, 2025), paragraphs 3-2 and 4-3; ERDC SR-26-5 (Wiest et al., 2026), sections 2.4 and 3).
  • Maximum roughness assumed conservative for every output. A lower downstream peak can accompany a higher local stage. Test relevant roughness and maintenance conditions and provide the hydrographs for hydraulic assessment (EM 1110-2-4000 (USACE, 2025), paragraph 7-14).
  • Event-model parameters used for seasonal flow assessment. The SCS Curve Number loss method is for event simulation. Select compatible continuous loss, evapotranspiration, canopy, and baseflow components and check the water balance.
  • Historical land use or frequency assumptions silently treated as future conditions. Define the future without-project baseline and document how hydrometeorological changes are assessed (EP 1105-2-70 (USACE, 2025), paragraph 4-3; ECB 2026-1 (USACE, 2026), paragraphs 4c and 4d).