DES 606 : Watershed Modeling with HEC-HMS

DES 606 : Watershed Modeling with HEC-HMS Module 7: Unit Hydrograph Concepts Theodore G. Cleveland, Ph.D., P.E, M. ASCE, F. EWRI 21-23 October 2013 Module 7

Outline for Module 7 • Unit hydrographs are a tool used to explain the time re-distribution of excess precipitation on a watershed. • Used for design purposes to produce discharge estimates at a location. • Different unit hydrograph models • Empirical • Parametric Module 7

Hydrographs • FHWA-NHI-02-001 Highway Hydrology • Chapter 6, Section 6.1 • Systems Approach • Input : Hyetograph • Transfer : Unit Hydrograph • Output : Total Runoff Hydrograph Module 7

Hydrographs • Hydrograph Analysis • Measured rainfall and runoff to infer the transfer function. • Implies: Have DATA. • Hydrograph Synthesis • Physical properties of watershed used to postulate the transfer function. • Actual measurements not required – Produces an ESTIMATE Module 7

Module 7: Hydrograph - Analysis • Precipitation • Meterology, Climate • Runoff • Fraction of precipitation signal remaining after losses Supply historical Precipitation Know historical Discharge Infer the Transformation • Watershed • Losses • Transformation • Storage • Routing Module 7

Hydrograph - Synthesis • Precipitation • Meterology, Climate • Runoff • Fraction of precipitation signal remaining after losses Estimate the Transformation, Supply design or historicalPrecipitation Predict (estimate) Discharge. • Watershed • Losses • Transformation • Storage • Routing Module 7

Rainfall Hyetographs • Typically divided into three components • Initial abstraction • Loss • Excess • This component becomes direct runoff Module 7

Excess Hyetographs • Excess as series of pulses • If pulses went immediately to the outlet, would expect direct hydrograph to have same shape. Q(t) = i(i)A Intensity Time Runoff Time Intensity • But pulses are assumed uniform over whole area – close to outlet arrive sooner than far from outlet • Hence there is time re-distribution Time Module 7

Unit Hydrograph • Excess as series of pulses • The unit hydrograph is the “function” that maps the time-distribution of pulses of excess precipitation to the time-distribution of direct runoff. Intensity Time Runoff/Intensity Intensity Time Time Module 7

Runoff/Intensity Time Total Hydrograph • Total hydrograph is the algebraic combination (in time) of the direct runoff hydrograph and the baseflow hydrograph Runoff/Intensity/Discharge Time Module 7

Unit Hydrograph Assumptions • Direct runoff duration (time) is same for all uniform-intensity storms of same duration (time). • Two excess hyetographs of the same duration (time) will produce direct runoff hydrographs of the same duration (time) but with runoff rates proportional to the volumes (depth) of the excess hyetographs. • The time distribution of direct runoff from a given storm duration is independent of concurrent runoff form prior storms. Module 7

Unit Hydrograph • Direct runoff duration (time) is same for all uniform-intensity storms of same duration (time). Runoff/Intensity Runoff/Intensity Time Time Module 7

Unit Hydrograph • Two excess hyetographs of the same duration (time) will produce direct runoff hydrographs of the same duration (time) but with runoff rates proportional to the volumes (depth) of the excess hyetographs. Runoff/Intensity Runoff/Intensity Time Time Module 7

Unit Hydrograph • The time distribution of direct runoff from a given storm duration is independent of concurrent runoff form prior storms. Runoff/Intensity Time Runoff/Intensity Time Runoff/Intensity Time Module 7

Parametric Unit Hydrographs • HEC-HMS has several different UH models available (eg. NRCS DUH, Clark, etc.). • These models are described by “parameters” • NRCS (Tp or Tc) • Clark (Tc, R) • Selection of the parameters selects the shape of the UH and the time base (or time to peak). • The analyst can also enter an empirical (user specified) unit hydrograph. Module 7

Developing Unit Hydrographs • Follow FHWA methods • Tedious • Use HEC-HMS and parametric UH, adjust values of parameters to fit observed runoff • Comparatively less tedious • Use HEC-HMS and user supplied UH, adjust values of UH ordinates (and re-scale to maintain a unit response) to fit observed runoff • Possible less tedious Module 7

Other Unit Hydrographs • Example 7 used NRCS DUH and a Clark Unit Hydrograph. • Parametric models are listed along with their parameters. • NRCS DUH : Tlag (Timing only) • Clark : Tc, and R (Timing and a storage-delay) • Snyder : Tlag, Cp (Timing and a peak rate factor) • Gamma : (User-Specified) Tc, K (Timing and a shape factor) Module 7

Synthesizing Unit Hydrographs • Synthesis does not use rainfall-runoff data. • Uses measurements on the watershed to postulate parameters of a parametric unit hydrograph. • Transposing hydrographs (sort of a regionalization concept) is a form of synthesis. Module 7

Synthesizing Unit Hydrographs • The loss model and unit hydrograph model in TxDOT 0-4193-7 are “synthesis” methods. • Supply • AREA • SLOPE • LENGTH • CN • Estimate Loss and Gamma UH parameters • We will examine in workshop Module 7

Synthesizing Unit Hydrographs • Transposition methods are discussed in FHWA “Highway Hydrology” page 6-70 • Essentially same as conventional synthesis methods. • As an aside, Eqn 6.26 in FHWA HDS-2 is incorrect, the exponent in the equation is listed as “N” and in the legend is listed as “K”. The authors mean “K” in the equation. Module 7

Summary • Unit hydrographs map the excess precipitation signal to the outlet\ • Base-flow separation isolates the total discharge from the storm-induced discharge • Loss models are implicit; the unit hydrograph maps excess to the outlet • Example 7 inferred a UH from data Module 7

DES 606 : Watershed Modeling with HEC-HMS