The Santa Barbara Urban Hydrograph Method (SBUHM) consists of two basic computational step...

The Santa Barbara Urban Hydrograph Method (SBUHM) consists of two basic computational steps:

(1) Compute rainfall excesses for each rainfall time step. Rainfall excesses (depth/time) from the pervious and impervious areas are multiplied by their respective areas and summed to obtain one flow value per time step for the entire catchment (the “instantaneous hydrograph”). Any convenient means may be used to compute rainfall excess, ranging from very simple runoff coefficients with depression storage [Eq.], to SCS curve numbers (Chapter 2), to the infiltration equations of Chapter 2. In this method, DCIA is often assumed to have no losses.

(2) The resulting instantaneous hydrograph is then routed through an imaginary linear reservoir to provide delay and attenuation. The linear reservoir has a time constant K = tc for the catchment. Routing is performed using the Muskingum method equations (Section 4.2), with Muskingum parameter x = 0 (for a linear reservoir) and Δt = time step of instantaneous hydrograph.

Use the data from Problem to perform SBUHM routing. Assume that the rainfall excess applies to the entire catchment area (so for this problem, there is no merging of runoff from pervious area and DCIA).

(a) Compute the instantaneous hydrograph at 30-min intervals, in units of cfs ≈ ac-in./hr.

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(b) Inferring a value for the time of concentration for the overall catchment from the given time-area data, compute Muskingum coefficients Q, Q, and C2, with Muskingum K = tc and x = 0 for the linear reservoir.

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(c) Perform the indicated flow routing through the hypothetical reservoir. Plot the inflow “instantaneous hydrograph” and outflow hydrograph on the same graph. Remember that the hydrographs begin with zero flow.

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(d) If you have worked Problem, compare the hydrographs by the two methods.

Equation

Problem

A catchment is to be simulated using the Clark model—that is, by routing using a time-area method (to produce hydrograph time delays), followed by routing through a linear reservoir (to produce hydrograph attenuation). The time-area and rainfall-excess data are given below:

(a) What is the total area of the catchment?

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(b) What is the time of concentration of the overall catchment?

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(c) Perform the indicated time-area routing.

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(d) The linear reservoir is to be designed (conceptually) such that the peak flow out of the reservoir is only 60% ( ± 0.5 cfs) of the inflow peak. Using the Muskingum routing method with x = 0, experiment with K-values to achieve this result.

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(e) Tabulate and plot (on the same chart) the time-area hydrograph (“inflow”) and the outflow hydrograph from the linear reservoir identified in part (d).