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HEC HMS kinematic

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  Runoff Computation
Modeling Direct Runoff with
 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
   - represent possible physical
User-specified Unit
    Basic Concepts and Equations

          Qn=storm hydrograph ordinate
          Pm=rainfall excess depth
          Un-m+1=UH ordinate
User-specified Unit
    Estimating the Model Parameters
      1. Collect data for an appropriate
         storm runoff hydrograph and the
      2. Estimate losses and subtract these
         precipitation. Estimate baseflow and
         separate this from the runoff
User-specified Unit
    Estimating the Model Parameters
     3. Calculate the total volume of direct
        and convert this to equivalent
        depth over the watershed
      4. Divide the direct runoff ordinates by
         equivalent uniform depth
User-specified Unit
    Application of User-specified UH
      - In practice, direct runoff computation
        a specified-UH is uncommon.
      - The data are seldom available.
      - It is difficult to apply.
Snyder’s UH Model
    Basic Concepts and Equations
Snyder’s 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,

             tR=duration of desired UH,
             tpR=lag of desired UH
Snyder’s UH Model
   Basic Concepts and Equations
    - standard UH

    - for other duration

       Up =peak of standard UH, A=watershed drainage area
       Cp =UH peaking coefficient,C=conversion constant(2.75 for SI)
Snyder’s UH Model
    Estimating Snyder’s 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
SCS UH Model
    Basic Concepts and Equations
SCS UH Model
    Basic Concepts and Equations
      - SCS suggests the relationship

        A=watershed area; C=conversion constant(2.08 in SI)

        t=the excess precipitation duration;tlag =the basin lag
SCS UH Model
    Estimating the SCS UH Model
Clark Unit Hydrograph
    Models translation and attenuation of excess
        Translation: movement of excess from origin to
             based on synthetic time area curve and time of

        Attenuation: reduction of discharge as excess is
         stored in watershed
             modeled with linear reservoir
Clark Unit Hydrograph
    Required Parameters:
        TC

        Time of Concentration!!!

        Storage coefficient
Clark Unit Hydrograph
    Estimating parameters:
        Time of Concentration: T c
           Estimated via calibration
           SCS equation

        Storage coefficient
           Estimated via calibration
           Flow at inflection point of hydrograph
            divided by the time derivative of flow
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
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
ModClark Method
    Required parameters:
      Gridded representation of watershed
      Gridded cell file

      Time of concentration

      Storage coefficient
ModClark Method
    Gridded Cell File
        Contains the following for each cell in the
             Coordinate information
             Area
             Travel time index
        Can be created by:
             GIS System
             HEC’s standard hydrologic grid
             GridParm (USACE)
             Geo HEC-HMS
Kinematic Wave Model
    Conceptual model
        Models watershed as
         a very wide open
        Inflow to channel is
         excess precipitation
        Open book:
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 Manning’s
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

     Three alternative models for baseflow

         Constant, monthly-varying flow
         Exponential recession model

         Linear-reservoir volume accounting model

     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
     Exponential recession model
         Defines relationship of Qt (baseflow at time t)
          to an initial value of baseflow (Q0) as:

           Q t = Q 0 Kt

       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
     Exponential recession model
    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
    Exponential recession
       Applied at beginning
        and after peak of
        direct runoff
       User-specified
        threshold flow
        defines when
        recession model
        governs total flow
    Linear Reservoir Model:
       Used with Soil Moisture Accounting loss
        model (last time)
       Outflow linearly related to average storage
        of each time interval
       Similar to Clark’s watershed runoff
Applicability and Limitations
    Choice of model depends on:
       Availability of information
             Able to calibrate?
        Appropriateness of assumptions inherent in
         the model
             Don’t use SCS UH for multiple peak watersheds
        Use preference and experience

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