Basic Hydrology & Hydraulics: DES 601 Module 10 Response Models (Unit Hydrograph)

Basin response models Basin response models include unit hydrograph models documented in the HDM including: Snyder NRCS DUH Combined with routing the collective “model” is called a rainfall-runoff model Module 10

Response models Response models convert the excess precipitation signal into a direct runoff hydrograph at the point of interest Precipitation Losses Loss Model Excess Precipitation Response (Transform) Runoff Module 10

Response models Precipitation Precipitation Loss Excess Excess Runoff Tlag Excess Runoff TP Loss TD Time Module 10

Timing Strictly speaking, each unit hydrograph has a particular duration associated with it, D in the diagram That duration must coincide with the time step size used in discrete aggregation Thus a D-hour unit hydrograph is a response to a D-hour “pulse” of excess precipitation. The flow associated with that response is reported every D-hours until there is no further response (TD in the diagram) Module 10

Timing Each watershed has a characteristic response time, Tlag and TP in the diagram The characteristic time of the watershed is related to physical characteristics of the watershed- contributing area, slope, etc. The time step size for aggregation must the same as the duration, and the time-to-peak for the watershed must be an integer multiple of that value. Module 10

Estimating Timing The HDM presents several methods to estimate the characteristic time among these are: Kerby-Kirpich Overland and Channel Flow NRCS Method Sheetflow, Shallow Concentrated, Channel NRCS Upland Method Rule-of-Thumb Check Module 10

Kerby-Kirpich Kerby Equation for Overland Flow Can be used in smaller watersheds where overland flow is a substantial component of overall travel time. Overland flow length can be up to 1200’ Tc = K(LxN)0.467S-0.235 where: K = 0.828 (English units conversion coefficient) L = overland flow distance, in ft. N = Retardance Coefficient S = slope of overland flow path, ft/ft Module 10

Kerby Retardance Coefficient Time of concentration Kerby Retardance Coefficient Generalized Terrain Condition Dimensionless Retardance Coefficient (N) Pavement .02 Smooth, bare, packed soil .10 Poor grass, cultivated row crops, or moderately rough packed surfaces .20 Pasture, average grass .40 Deciduous forest .60 Dense grass, coniferous forest, or deciduous forest with deep litter .80 Module 10

Time of concentration Kirpich Equation for Channelized Flow Tc = KL0.770S-0.385 where: K = 0.0078 (English units conversion coefficient) L = overland flow distance, in ft. S = channel slope, ft/ft Module 10

NRCS Method for Time of concentration Time of concentration, tc = ∑L/V, where L is flow segment length and V is flow segment velocity. tc = tsheetflow +tshallowconcentrated+ tchannel flow Module 10

NRCS Method for Time of concentration Time of concentration, tc = ∑L/V, where L is flow segment length and V is flow segment velocity. tc = tsheetflow +tshallowconcentrated+ tchannel flow Module 10

NRCS Method for Time of concentration Time of concentration, tc = ∑L/V, where L is flow segment length and V is flow segment velocity. tc = tsheetflow +tshallowconcentrated+ tchannel flow Module 10

NRCS Method for Time of concentration Time of concentration, tc = ∑L/V, where L is flow segment length and V is flow segment velocity. tc = tsheetflow +tshallowconcentrated+ tchannel flow Module 10

NRCS Upland Method Module 10

Rule-of-Thumb Check Alternate “ballpark” method – applicable for preliminary design or as a reasonableness check of other methods: Tc = A0.5 where Tc = Time of Concentration, in hours A = Contributing watershed area in square miles. Module 10

Module 8

Summary The time step size for aggregation must the same as the duration, and the time-to-peak for the watershed must be an integer multiple of that value. The time-to-peak of the watershed is related to physical characteristics of the watershed- contributing area, slope, etc. Module 10