WSP01998

• File Number: 5970
• Description: Flood Protection Section - Miscellaneous Reports
• State: CO
• Basin: Statewide
• Date: 12/1/1972
• Author: CSU
• Title: Theory and Experiments in the Prediction of Small Watershed Response - Completion Report Series Number 42
• Water Supply Pro - Doc Type: Report/Study

O~1238

THEORY AND EXPERIMENTS IN THE PREDICTION OF
SMALL WATERSHED RESPONSE

The more complex modern society is becoming more involved with

problems associated with floods from small watersheds. The problem of

estimating flood peaks from small watersheds is an important element in

the design of storm sewers, highway drainage, diversion works, flood
retarding structures, bridges and culverts. The majority of such
hydraulic structures are constructed on small watersheds. Since small

streams have not been gaged as extensively in the past as have large

streams, more of the designs have to be prepared without the benefit of
stream flow records. There is a great need to develop more satisfactory
methods for obtaining design floods for these watersheds under the varied

changes taking place in the modern day life.

INTRODUCTION

In a small watershed the predominant storage element is the surface
detention storage. It is the storage element in the watershed which

produces the difference between the input - rainfall excess - and the

output - surface runoff. As the watershed increases in size, the

predominant storage element in the watershed becomes the channel storage.
The small watershed with its relatively important relationship to
the surface detention is therefore more sensitive to man made changes in

those things which influence the transit time of the overland flow.

Changes such as increasing or decreasing the surface roughness, the

length of overland flow, the overland flow slope are all changes which

should have a large effect on the flood response of the watershed to

storm rainfall.