SWWRP Template PR

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Hydrologic Modeling SystemHEC-HMS
Bill Scharffenberg U.S. Army Corps of
Engineers Hydrologic Engineering Center
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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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HEC-HMS Background
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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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Gridded Simulation
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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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Diversion Methods
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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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Spillway Gates
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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.

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Channel Losses
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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 T.
• Significantly faster to solve than Richard's
  equation.
• New research incorporated temperature affects.
• Water density and viscosity.
• Matric potential.

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Smith Parlange Loss Method
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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.

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Nonlinear Boussinesq Baseflow
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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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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.