HEC-HMSHEC-HMS

Runoff ComputationRunoff Computation

Modeling Direct Runoff Modeling Direct Runoff with HEC-HMSwith HEC-HMS Empirical modelsEmpirical models

- traditional UH models- traditional UH models

- a causal linkage between runoff and - a causal linkage between runoff and excess precipitation without detailed excess precipitation without detailed consideration of the internal processesconsideration of the internal processes

A conceptual modelA conceptual model

- kinematic-wave model of overland flow- kinematic-wave model of overland flow

- represent possible physical mechanism- represent possible physical mechanism

User-specified Unit User-specified Unit HydrographHydrograph

Basic Concepts and Equations Basic Concepts and Equations

QQnn=storm hydrograph ordinate=storm hydrograph ordinate PPmm==rainfall excess depthrainfall excess depth UUn-m+1n-m+1==UH ordinateUH ordinate

Mn

mmnmn UPQ

11

User-specified Unit User-specified Unit HydrographHydrograph

Estimating the Model ParametersEstimating the Model Parameters 1. Collect data for an appropriate 1. Collect data for an appropriate

observedobserved storm runoff hydrograph and the storm runoff hydrograph and the

causalcausal precipitationprecipitation 2. Estimate losses and subtract these 2. Estimate losses and subtract these

fromfrom precipitation. Estimate baseflow and precipitation. Estimate baseflow and separate this from the runoffseparate this from the runoff

User-specified Unit User-specified Unit HydrographHydrograph

Estimating the Model ParametersEstimating the Model Parameters

3. Calculate the total volume of direct 3. Calculate the total volume of direct runoffrunoff

and convert this to equivalent uniformand convert this to equivalent uniform

depth over the watersheddepth over the watershed

4. Divide the direct runoff ordinates by 4. Divide the direct runoff ordinates by the the

equivalent uniform depthequivalent uniform depth

User-specified Unit User-specified Unit HydrographHydrograph

Application of User-specified UHApplication of User-specified UH

- In practice, direct runoff - In practice, direct runoff computation withcomputation with

a specified-UH is uncommon.a specified-UH is uncommon.

- The data are seldom available.- The data are seldom available.

- It is difficult to apply.- It is difficult to apply.

Snyder’s UH ModelSnyder’s UH Model

Basic Concepts and EquationsBasic Concepts and Equations

Snyder’s UH ModelSnyder’s UH Model Basic Concepts and EquationsBasic Concepts and Equations

- standard UH - standard UH

- If the duration of the desired UH for - If the duration of the desired UH for the watershed of interest is significantly the watershed of interest is significantly different from the above equation,different from the above equation,

ttRR=duration of desired UH,=duration of desired UH, ttpRpR=lag of desired UH=lag of desired UH

rp tt 5.5

4Rr

ppRtt

tt

Snyder’s UH ModelSnyder’s UH Model Basic Concepts and EquationsBasic Concepts and Equations

- standard UH- standard UH

- for other duration- for other duration

UUpp=peak of standard UH, A=watershed drainage area=peak of standard UH, A=watershed drainage area

CCpp=UH peaking coefficient,C=conversion constant(2.75 for =UH peaking coefficient,C=conversion constant(2.75 for SI)SI)

p

pp

t

CC

A

U

pR

ppR

t

CC

A

U

Snyder’s UH ModelSnyder’s UH Model

Estimating Snyder’s UH ParametersEstimating Snyder’s UH Parameters

- C- Ctt typically ranges from 1.8 to 2.0 typically ranges from 1.8 to 2.0

- C- Cpp ranges from 0.4 to 0.8 ranges from 0.4 to 0.8 - Larger values of C- Larger values of Cpp are associated are associated

with smaller values of Cwith smaller values of Ctt

3.0)( ctp LLCCt

SCS UH ModelSCS UH Model

Basic Concepts and EquationsBasic Concepts and Equations

SCS UH ModelSCS UH Model Basic Concepts and EquationsBasic Concepts and Equations

- SCS suggests the relationship- SCS suggests the relationship

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

t=the excess precipitation duration;tt=the excess precipitation duration;t laglag=the basin lag=the basin lag

pp T

ACU

lagp tt

T

2

SCS UH ModelSCS UH Model

Estimating the SCS UH Model Estimating the SCS UH Model ParametersParameters

clag tt 6.0

Clark Unit HydrographClark Unit Hydrograph

Models translation and attenuation of Models translation and attenuation of excess precipitationexcess precipitation

Translation: movement of excess from Translation: movement of excess from origin to outletorigin to outlet

based on synthetic time area curve and time of based on synthetic time area curve and time of concentrationconcentration

Attenuation: reduction of discharge as Attenuation: reduction of discharge as excess is stored in watershedexcess is stored in watershed

modeled with linear reservoirmodeled with linear reservoir

Clark Unit HydrographClark Unit Hydrograph

Required Parameters:Required Parameters:

TCTC NotNot

Time of Concentration!!!Time of Concentration!!!

Storage coefficientStorage coefficient

Clark Unit HydrographClark Unit Hydrograph

Estimating parameters:Estimating parameters:

Time of Concentration: TTime of Concentration: Tcc

Estimated via calibrationEstimated via calibration SCS equationSCS equation

Storage coefficientStorage coefficient Estimated via calibrationEstimated via calibration Flow at inflection point of Flow at inflection point of

hydrograph divided by the time hydrograph divided by the time derivative of flowderivative of flow

ModClark MethodModClark Method

Models translation and attenuation Models translation and attenuation like the Clark modellike the Clark model Attenuation as linear reservoirAttenuation as linear reservoir Translation as grid-based travel-time Translation as grid-based travel-time

modelmodel Accounts for variations in travel Accounts for variations in travel

time to watershed outlet from all time to watershed outlet from all regions of a watershedregions of a watershed

ModClark MethodModClark Method

Excess precipitation for each cell is Excess precipitation for each cell is lagged in time and then routed lagged in time and then routed through a linear reservoir S = K * through a linear reservoir S = K * SoSo

Lag time computed by:Lag time computed by: ttcellcell = t = tcc * d * dcellcell / d / dmaxmax

All cells have the same reservoir All cells have the same reservoir coefficient Kcoefficient K

ModClark MethodModClark Method

Required parameters:Required parameters: Gridded representation of watershedGridded representation of watershed Gridded cell fileGridded cell file Time of concentrationTime of concentration Storage coefficientStorage coefficient

ModClark MethodModClark Method

Gridded Cell File Gridded Cell File Contains the following for each cell in the Contains the following for each cell in the

subbasin:subbasin: Coordinate informationCoordinate information AreaArea Travel time indexTravel time index

Can be created by:Can be created by: GIS SystemGIS System HEC’s standard hydrologic gridHEC’s standard hydrologic grid GridParm (USACE)GridParm (USACE) Geo HEC-HMSGeo HEC-HMS

Kinematic Wave ModelKinematic Wave Model

Conceptual modelConceptual model Models watershed Models watershed

as a very wide as a very wide open channelopen channel

Inflow to channel Inflow to channel is excess is excess precipitationprecipitation

Open book:Open book:

Kinematic Wave ModelKinematic Wave Model

HMS solves kinematic wave equation HMS solves kinematic wave equation for overland runoff hydrographfor overland runoff hydrograph

Can also be used for channel flow Can also be used for channel flow (later)(later)

Kinematic wave equation is derived Kinematic wave equation is derived from the continuity, momentum, and from the continuity, momentum, and Manning’s equationsManning’s equations

Kinematic Wave ModelKinematic Wave Model

Required parameters for overland flow:Required parameters for overland flow: Plane parametersPlane parameters

– Typical lengthTypical length– Representative slopeRepresentative slope– Overland flow roughness coefficientOverland flow roughness coefficient

Table in HMS technical manual (Ch. 5)Table in HMS technical manual (Ch. 5)– % of subbasin area% of subbasin area– Loss model parametersLoss model parameters– Minimum no. of distance stepsMinimum no. of distance steps

OptionalOptional

BaseflowBaseflow

Three alternative models for baseflowThree alternative models for baseflow

Constant, monthly-varying flowConstant, monthly-varying flow

Linear-reservoir volume accounting modelLinear-reservoir volume accounting model

Exponential recession modelExponential recession model

BaseflowBaseflow

Constant, monthly-varying flowConstant, monthly-varying flow

Represents baseflow as a constant flowRepresents baseflow as a constant flow

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

Flow may vary from month to monthFlow may vary from month to month

User-specifiedUser-specified Empirically estimatedEmpirically estimated Often negligibleOften negligible

BaseflowBaseflow Exponential recession modelExponential recession model

Defines relationship of Qt (baseflow at Defines relationship of Qt (baseflow at time t) to an initial value of baseflow (Qtime t) to an initial value of baseflow (Q00) ) as:as:

QQtt = Q = Q00KKtt

K is an exponential decay constantK is an exponential decay constant Defined as ratio of baseflow at time t Defined as ratio of baseflow at time t

to baseflow one day earlierto baseflow one day earlier QQ0 0 is the average flow before a storm is the average flow before a storm

beginsbegins

BaseflowBaseflow

Exponential recession modelExponential recession model

BaseflowBaseflow

Exponential recession modelExponential recession model Typical values of KTypical values of K

0.95 0.95 for Groundwaterfor Groundwater 0.8 – 0.9 0.8 – 0.9 for Interflowfor Interflow 0.3 – 0.8 0.3 – 0.8 for Surface Runofffor Surface Runoff

Can also be estimated from gaged Can also be estimated from gaged flow dataflow data

Baseflow: Baseflow: Exponential Exponential

recession model:recession model: Applied at Applied at

beginning and beginning and after peak of after peak of direct runoffdirect runoff

User-specified User-specified threshold flow threshold flow defines when defines when recession model recession model governs total flowgoverns total flow

Baseflow Baseflow

Linear Reservoir Model:Linear Reservoir Model: Used with Soil Moisture Accounting Used with Soil Moisture Accounting

loss model (last time)loss model (last time) Outflow linearly related to average Outflow linearly related to average

storage of each time intervalstorage of each time interval Similar to Clark’s watershed runoffSimilar to Clark’s watershed runoff

Applicability and Applicability and LimitationsLimitations

Choice of model depends on:Choice of model depends on: Availability of informationAvailability of information

Able to calibrate?Able to calibrate? Appropriateness of assumptions Appropriateness of assumptions

inherent in the modelinherent in the model Don’t use SCS UH for multiple peak Don’t use SCS UH for multiple peak

watershedswatersheds Use preference and experienceUse preference and experience