Topic 5- SP 5B-Use of a Unit Hydrograph to Determine Storm Flow

In SP-5A, a unit hydrograph 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. Alhambra Wash drains a primarily urban area from

Pasadena through El Monte and has a watershed area of 15.2 square miles (9,728 acres).

a) Use the unit hydrograph to determine the peak rate of runoff from a storm which has the

following rainfall amounts:

Period 1 (first hour)

Period 2 (second hour)

Period 3 (third hour)

1.83 inches

3.75 inches

2.05 inches

Initial abstractions (first period only) = 0.25 inches

Infiltration data using Horton's Model: fo= 1.4 in/hr, fc = 0.25 in/hr, K = 3.8/hr