Bill Scharffenberg U.S. Army Corps of Engineers Hydrologic Engineering Center

1. Hydrologic Modeling SystemHEC-HMS Bill Scharffenberg U.S. Army Corps of Engineers Hydrologic Engineering Center

2. HEC-HMS Background • HEC-HMS is a complete engineering hydrology simulation system with model components for meteorology, subbasins, river reaches, reservoirs, and diversion structures. • The subbasin component includes loss rate, surface transform, and baseflow subcomponents. Several model choices are available for each process. • The reach component includes several routing methods from simple empirical methods to sophisticated approximations of the dynamic wave equation. • The reservoir component can represent a dam using individual spillways, outlets, and other structures.

3. HEC-HMS Background

4. Gridded Simulation • Represent each subbasin as a collection of grid cells. • Gridded precipitation can come from radar rainfall, interpolated gage data, or atmospheric models. • For continuous simulation, gridded Priestley-Taylor evapotranspiration and gridded snowmelt. • Partially gridded loss methods use the same parameters and initial conditions but different boundary conditions for all grid cells in a subbasin. • Fully gridded loss methods use different parameters, initial conditions, and boundary conditions for each grid cell.

5. Gridded Simulation

6. Diversion Methods • The existing diversion element is limited to a user-supplied function of inflow. • Two new methods. • Lateral weir method uses a broad-crested spillway equation. • Pump station method uses a head-discharge pump. • Stage in the channel is computed from flow with a user-supplied stage-discharge curve. • Tailwater reductions are calculated with a second user-supplied stage-discharge curve representing flow characteristics on the "dry" side of the channel bank. • An optimization routine is used to compute the diversion flow for each time step, assuming the diversion is a point in the channel and there are no storage changes.

7. Diversion Methods

8. Spillway Gates • Reservoir element currently includes two spillways options without capability for gates: • Broad-crested spillway • Ogee spillway • Add option for radial or vertical gates on both spillway types. • Each spillway can have up to 10 gate controls. • Each gate control can have different parameters and includes the number of identically operating gates. • Initially the only option for controlling the gates is a fixed opening height for the entire simulation; enhancements are already planned for a future release.

9. Spillway Gates

10. Channel Losses • Percolation losses from the bottom of a streambed can be an important part of the water balance, especially in arid regions. • Constant loss method with equation: • Percolation loss method. • Compute inundation area and multiply by percolation rate. • Revise all routing methods to include losses. • Use convergence algorithm to account for losses in the calculation of routed flow.

11. Channel Losses

12. Smith Parlange Loss Method • The Smith Parlange model approximates Richard's infiltration equation with the principal assumption: • K approximation allows Richard's equation to be linearized while maintaining a reasonable functional relationship between K and water content Θ. • Significantly faster to solve than Richard's equation. • New research incorporated temperature affects. • Water density and viscosity. • Matric potential.

13. Smith Parlange Loss Method

14. Nonlinear Boussinesq Baseflow • Assumes an unconfined soil layer feeding baseflow. • Saturated at the end of a precipitation event. • Receives no recharge between events. • Requires Dupuit assumptions: • Hydraulic gradient equal to the slope of the water table. • Streamlines are horizontal, equipotential lines are vertical. • Resulting equation: • Parameters a and b in the equation can be computed using physical properties of the watershed.

15. Nonlinear Boussinesq Baseflow

16. Next Release • Targeted before end of 2006. • Development is 100% complete. • Testing is 90% complete. • Release decision expected soon. www.hec.usace.army.mil/software/hec-hms/

17. Upcoming Development Work • Surface erosion subcomponent will be added to the subbasin. • Build-up and wash-off method. • Modified universal soil loss equation (MUSLE). • Nutrient subcomponent will be added to the subbasin element for simulating nitrogen and phosphorus processes. • New statistical summaries for continuous simulations will help support ecosystem studies. • New parameter estimation methods and tools will use optimization to better define parameters used in continuous simulation. • New methods for snowmelt and frozen ground simulation.