HEC-HMS

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HEC-HMS

• Runoff Computation

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Modeling Direct Runoff with HEC-HMS

• Empirical models
• - traditional UH models
• - a causal linkage between runoff and excess
  precipitation without detailed consideration of
  the internal processes
• A conceptual model
• - kinematic-wave model of overland flow
• - represent possible physical mechanism

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User-specified Unit Hydrograph

• Basic Concepts and Equations
• Qnstorm hydrograph ordinate
• Pmrainfall excess depth
• Un-m1UH ordinate

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User-specified Unit Hydrograph

• Estimating the Model Parameters
• 1. Collect data for an appropriate observed
• storm runoff hydrograph and the causal
• precipitation
• 2. Estimate losses and subtract these from
• precipitation. Estimate baseflow and
• separate this from the runoff

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User-specified Unit Hydrograph

• Estimating the Model Parameters
• 3. Calculate the total volume of direct runoff
• and convert this to equivalent uniform
• depth over the watershed
• 4. Divide the direct runoff ordinates by the
• equivalent uniform depth

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User-specified Unit Hydrograph

• Application of User-specified UH
• - In practice, direct runoff computation with
• a specified-UH is uncommon.
• - The data are seldom available.
• - It is difficult to apply.

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Snyders UH Model

• Basic Concepts and Equations

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Snyders UH Model

• Basic Concepts and Equations
• - standard UH
• - If the duration of the desired UH for the
  watershed of interest is significantly different
  from the above equation,
• tRduration of desired UH,
• tpRlag of desired UH

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Snyders UH Model

• Basic Concepts and Equations
• - standard UH
• - for other duration
• Uppeak of standard UH,
  Awatershed drainage area
• CpUH peaking coefficient,Cconvers
  ion constant(2.75 for SI)

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Snyders UH Model

• Estimating Snyders UH Parameters
• - Ct typically ranges from 1.8 to 2.0
• - Cp ranges from 0.4 to 0.8
• - Larger values of Cp are associated with
  smaller values of Ct

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SCS UH Model

• Basic Concepts and Equations

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SCS UH Model

• Basic Concepts and Equations
• - SCS suggests the relationship
• Awatershed area Cconversion
  constant(2.08 in SI)
• ?tthe excess precipitation
  durationtlagthe basin lag

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SCS UH Model

• Estimating the SCS UH Model Parameters

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Clark Unit Hydrograph

• Models translation and attenuation of excess
  precipitation
• Translation movement of excess from origin to
  outlet
• based on synthetic time area curve and time of
  concentration
• Attenuation reduction of discharge as excess is
  stored in watershed
• modeled with linear reservoir

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Clark Unit Hydrograph

• Required Parameters
• TC

Not

• Time of Concentration!!!

• Storage coefficient

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Clark Unit Hydrograph

• Estimating parameters
• Time of Concentration Tc
• Estimated via calibration
• SCS equation
• Storage coefficient
• Estimated via calibration
• Flow at inflection point of hydrograph divided by
  the time derivative of flow

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ModClark Method

• Models translation and attenuation like the Clark
  model
• Attenuation as linear reservoir
• Translation as grid-based travel-time model
• Accounts for variations in travel time to
  watershed outlet from all regions of a watershed

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ModClark Method

• Excess precipitation for each cell is lagged in
  time and then routed through a linear reservoir S
  K So
• Lag time computed by
• tcell tc dcell / dmax
• All cells have the same reservoir coefficient K

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ModClark Method

• Required parameters
• Gridded representation of watershed
• Gridded cell file
• Time of concentration
• Storage coefficient

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ModClark Method

• Gridded Cell File
• Contains the following for each cell in the
  subbasin
• Coordinate information
• Area
• Travel time index
• Can be created by
• GIS System
• HECs standard hydrologic grid
• GridParm (USACE)
• Geo HEC-HMS

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Kinematic Wave Model

• Conceptual model
• Models watershed as a very wide open channel
• Inflow to channel is excess precipitation
• Open book

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Kinematic Wave Model

• HMS solves kinematic wave equation for overland
  runoff hydrograph
• Can also be used for channel flow (later)
• Kinematic wave equation is derived from the
  continuity, momentum, and Mannings equations

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Kinematic Wave Model

• Required parameters for overland flow
• Plane parameters
• Typical length
• Representative slope
• Overland flow roughness coefficient
• Table in HMS technical manual (Ch. 5)
• of subbasin area
• Loss model parameters
• Minimum no. of distance steps
• Optional

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Baseflow

• Three alternative models for baseflow

• Constant, monthly-varying flow

• Exponential recession model

• Linear-reservoir volume accounting model

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Baseflow

• Constant, monthly-varying flow

• User-specified
• Empirically estimated
• Often negligible

• Represents baseflow as a constant flow

• Flow may vary from month to month

• Baseflow added to direct runoff for each time
  step of simulation

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Baseflow

• Exponential recession model

• Defines relationship of Qt (baseflow at time t)
  to an initial value of baseflow (Q0) as
• Qt Q0Kt
• K is an exponential decay constant
• Defined as ratio of baseflow at time t to
  baseflow one day earlier
• Q0 is the average flow before a storm begins

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Baseflow

• Exponential recession model

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Baseflow

• Exponential recession model
• Typical values of K
• 0.95 for Groundwater
• 0.8 0.9 for Interflow
• 0.3 0.8 for Surface Runoff
• Can also be estimated from gaged flow data

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Baseflow

• Exponential recession model
• Applied at beginning and after peak of direct
  runoff
• User-specified threshold flow defines when
  recession model governs total flow

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Baseflow

• Linear Reservoir Model
• Used with Soil Moisture Accounting loss model
  (last time)
• Outflow linearly related to average storage of
  each time interval
• Similar to Clarks watershed runoff

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Applicability and Limitations

• Choice of model depends on
• Availability of information
• Able to calibrate?
• Appropriateness of assumptions inherent in the
  model
• Dont use SCS UH for multiple peak watersheds
• Use preference and experience