1. Module 7: Unit Hydrograph Concepts Theodore G. Cleveland, Ph.D., P.E, M. ASCE, F. EWRI 26-28 August 2015 Module 7 1

2.  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 2

3.  FHWA-NHI-02-001 Highway Hydrology  Chapter 6, Section 6.1  Systems Approach  Input : Hyetograph  Transfer : Unit Hydrograph  Output : Total Runoff Hydrograph Module 7 3

4.  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 4

5. Watershed –Losses –Transformation –Storage –Routing Precipitation –Meterology, Climate  Runoff  Fraction of precipitation signal remaining after losses Supply historical Precipitation Know historical Discharge Infer the Transformation Module 7 5

6. Watershed –Losses –Transformation –Storage –Routing Precipitation –Meterology, Climate  Runoff  Fraction of precipitation signal remaining after losses Estimate the Transformation, Supply design or historical Precipitation Predict (estimate) Discharge. Module 7 6

7.  Typically divided into three components  Initial abstraction  Loss  Excess ▪This component becomes direct runoff Module 7 7

8.  Excess as series of pulses Intensity Time Intensity If pulses went immediately to the outlet, would expect direct hydrograph to have same shape. Q(t) = i(i)A Runoff Time But pulses are assumed uniform over whole area – close to outlet arrive sooner than far from outlet –Hence there is time re-distribution Module 7 8

9.  Excess as series of pulses Intensity Time Intensity If pulses went immediately to the outlet, would expect direct hydrograph to have same shape. Q(t) = i(i)A Runoff Time But pulses are assumed uniform over whole area – close to outlet arrive sooner than far from outlet –Hence there is time re-distribution Module 7 9

10.  Excess as series of pulses Intensity Time Intensity The unit hydrograph is the “function” that maps the time-distribution of pulses of excess precipitation to the time-distribution of direct runoff. Runoff/Intensity Time Module 7 10

11.  Total hydrograph is the algebraic combination (in time) of the direct runoff hydrograph and the baseflow hydrograph Runoff/Intensity Time Runoff/Intensity/Discharge Time Module 7 11

12.  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 12

13.  Direct runoff duration (time) is same for all uniform-intensity storms of same duration (time). Runoff/Intensity Time Runoff/Intensity Time Module 7 13

14.  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 Time Runoff/Intensity Time Module 7 14

15.  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 15

16.  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 16

17.  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 17

18.  Example 7 used NRCS DUH and a Clark Unit Hydrograph.  Parametric models are listed along with their parameters.  NRCS DUH : T lag (Timing only)  Clark : T c, and R (Timing and a storage-delay)  Snyder : T lag, C p (Timing and a peak rate factor)  Gamma : (User-Specified) T c, K (Timing and a shape factor) Module 7 18

19.  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 19

20.  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 20

21.  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 21

22.  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 22