Topic 5- SP 5C-Use of an S-graph to Change UHG Duration

In SP-5A, a unit hydrograph (UHG) with 1-hour time steps was developed for the Los

Angeles County Department of Public Works' Alhambra Wash above Klingerman St. in

Rosemead, CA, rainstorm on January 23 to 24, 2021. Assume this is a 1-hour unit hydrograph

from a 1-hour duration storm.

a) Develop an S-graph from the 1-hour UHG time steps (UHG from SP-5A). Plot the S-

graph.

b) In the next part we will develop a 30-minute duration (1/2-hour) unit hydrograph from the

1-hour duration unit hydrograph using the S-graph from part A. But to do this reasonably

accurately, you will need values for the ½ hour intervals in the S-graph. (The S-graph

from part a) has values only at 1 hour time steps. From the S-graph of part b), estimate

the values at the ½ hour intervals. Note they will not be "averages" based on linear

interpolation; you will have to look at the S-graph and estimate the intermediate values.

Develop a spreadsheet table of the S-graph with the additional ½ hour values and plot it.

c) Using the S-graph with the half-hour values from part b), offset (lag) the S-graph ½ hour

(the new UHG duration) and take the difference in cumulative flow values (SA -SB, as in

the notes and example). This can be done using a spreadsheet quite easily. Then, once

you have the SA -SB, values, adjust the values for the duration so that you have 1 inch of

runoff in the new ½ hour duration as done in the notes.