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HEC-HMS. Runoff Computation. 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

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hec hms


Runoff Computation

modeling direct runoff with hec hms
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

user specified unit hydrograph
User-specified Unit Hydrograph
  • Basic Concepts and Equations

Qn=storm hydrograph ordinate

Pm=rainfall excess depth

Un-m+1=UH ordinate

user specified unit hydrograph4
User-specified Unit Hydrograph
  • Estimating the Model Parameters

1. Collect data for an appropriate observed

storm runoff hydrograph and the causal


2. Estimate losses and subtract these from

precipitation. Estimate baseflow and

separate this from the runoff

user specified unit hydrograph5
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

user specified unit hydrograph6
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.

snyder s uh model
Snyder’s UH Model
  • Basic Concepts and Equations
snyder s uh model8
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 model9
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 model10
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
SCS UH Model
  • Basic Concepts and Equations
scs uh model12
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 model13
SCS UH Model
  • Estimating the SCS UH Model Parameters
clark unit hydrograph
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
clark unit hydrograph15
Clark Unit Hydrograph
  • Required Parameters:
    • TC


  • Time of Concentration!!!
  • Storage coefficient
clark unit hydrograph16
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
modclark method
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
modclark method18
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 method19
ModClark Method
  • Required parameters:
    • Gridded representation of watershed
    • Gridded cell file
    • Time of concentration
    • Storage coefficient
modclark method20
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
      • HEC’s standard hydrologic grid
      • GridParm (USACE)
      • Geo HEC-HMS
kinematic wave model
Conceptual model

Models watershed as a very wide open channel

Inflow to channel is excess precipitation

Open book:

Kinematic Wave Model
kinematic wave model22
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 equations
kinematic wave model23
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 simulation
  • 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
  • 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 data
Exponential recession model:

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
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