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CWEMF Oct 2006
Bill ScharffenbergU.S. Army Corps of EngineersHydrologic Engineering Center
Hydrologic Modeling SystemHydrologic Modeling SystemHEC-HMSHEC-HMS
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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.
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CWEMF Oct 2006
HEC-HMS Background
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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.
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CWEMF Oct 2006
Gridded Simulation
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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.
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CWEMF Oct 2006
Diversion Methods
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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.
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CWEMF Oct 2006
Spillway Gates
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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
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CWEMF Oct 2006
Channel Losses
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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
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CWEMF Oct 2006
Smith Parlange Loss Method
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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
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CWEMF Oct 2006
Nonlinear Boussinesq Baseflow
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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/
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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.