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# HEC HMS kinematic

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```									HEC-HMS
Runoff Computation
Modeling Direct Runoff with
HEC-HMS
 Empirical 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
User-specified Unit
Hydrograph
   Basic Concepts and Equations

Qn=storm hydrograph ordinate
Pm=rainfall excess depth
Un-m+1=UH ordinate
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
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
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
   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
Parameters
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 Hydrograph
   Required Parameters:
   TC
Not

   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
watershed
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
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
 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 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
Baseflow

   Three alternative models for baseflow

   Constant, monthly-varying flow
   Exponential recession model

   Linear-reservoir volume accounting model
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
Baseflow
   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
Baseflow
   Exponential recession model
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
Baseflow:
   Exponential recession
model:
 Applied at beginning
and after peak of
direct runoff
 User-specified
threshold flow
defines when
recession model
governs total flow
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 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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