CWEMF Oct 2006

Bill ScharffenbergU.S. Army Corps of EngineersHydrologic Engineering Center

Hydrologic Modeling SystemHydrologic Modeling SystemHEC-HMSHEC-HMS

CWEMF Oct 2006

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.

CWEMF Oct 2006

HEC-HMS Background

CWEMF Oct 2006

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.

CWEMF Oct 2006

Gridded Simulation

CWEMF Oct 2006

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.

CWEMF Oct 2006

Diversion Methods

CWEMF Oct 2006

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.

CWEMF Oct 2006

Spillway Gates

CWEMF Oct 2006

• 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.

Channel Losses

rQQQ lossroute 1

CWEMF Oct 2006

Channel Losses

CWEMF Oct 2006

• 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.

Smith Parlange Loss Method

eKK sat

CWEMF Oct 2006

Smith Parlange Loss Method

CWEMF Oct 2006

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.

dtQQQ ttt 11

CWEMF Oct 2006

Nonlinear Boussinesq Baseflow

CWEMF Oct 2006

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/

CWEMF Oct 2006

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.