FLOOD02408

• County: Statewide
• Community: Nationwide
• Basin: Statewide
• Title: Design Hydrographs for Very Small Wathersheds from Rainfall
• Date: 7/1/1962
• Prepared By: USDA Hydrograph Laboratory, Civil Engineering Section Colorado State University
• Floodplain - Doc Type: Educational/Technical/Reference Information

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Chapter 1

INTRODUCTION

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Many efforts (19)* have been made towards
rationalizing flood flow estimation for very small
watersheds within the past five years. A large num-
ber of peak-flow estimates are still being made by
inadequate means. Firstly. there is a paucity of hy-
drographs observed on watersheds within the range
of one-third to four square miles. Secondly, the sub-
ject is confused by the many design methods cited in
handbooks without reference to their regional restric-
tion and limited basis.

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The time seems appropriate to associate the
flood hydrograph with the causative rainstorm. The
intensity-frequency-duration regime for rainfall in
the United States of America has been thoroughly in-
vestigated by the Weather Bureau (11). Designers
should be provided with methods for estimating flood-
hydrographs with commensurate accuracy.

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Objectives

The purpose of this study was to describe
flood hydrographs observed on small agricultural and
range land watersheds by a mathematical model with
three parameters. These hydrograph parameters
were then correlated to other numerical variables
which describe: the rainstorm causing each particu-
lar flood, the topographic characteristics of the
watershedt and the soil and current land use. If this
could be successfully achieved it would provide a
means of synthesizing flood hydrographs for ungaged
watersheds. The present work should be viewed as a
pilot study to test the validity of the approach on
readily available data. Its success should signal the
analysis of a large quantity of unpublished hydrologic
data. This pilot study could greatly reduce the ex-
pense of such a large -scale follow up by establishing
techniques and by showing which variables do not
w arr ant further study.

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The multiple regressions obtained at the con-
clusion of this study only represent tentative relations.
Their reliability can best be judged by seeing how
well they predict results in a new set of observed
data. Truer regressions with narrower confidence
intervals will doubtless arise as more and more data
be co me available.

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Delineations

The forty-seven hydrographs and hyetographs
used in this study were observed by the U. S. Agri-
cultural Research Service (23) on fourteen watersheds.
Arranged according to the code numbers given them

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*Numbers in parenthesis refer to items in the
"Bibliography. II

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by the Agricultural Research Service, all watersheds
are listed in Table 1. They were located in the
following eleven widely spaced states: Virginia,
Illinois, Iowa, Ohio, Wisconsin, Texas, Nebraska,
Arizona, New Mexico, Washington and Mississippi.

The largest watershed studied had an area of
2086 acres (3. 26 square miles). The smallest was
290 acres (0.45 square miles). Beyond this arbi-
trary size range, the next-smaller and next-larger
watersheds for which the A. R. S. had published data
were 187 acres (.292 square miles) and 4430 acres
(6.93 square miles). Thus one may generalize by
saying that the present study aimed at presenting re-
sults applicable to watersheds within the general size
range from one-third to four square miles.

Only floods caused by rainstorms were con-
sidered. This is justified on very small watersheds,
since the flood series (21) from these stations shows
the annual maxima to occur almost exclusively in the
summer months, and not to be due to snow melt.

Certain watersheds seem to produce a charac-
teristic double-peaked hydrograph. Although the
mathematical model fitted was single peaked, not all
the observed double peaks were excluded from the
study. The approach was rather to approximate the.m
with a broad-crested theoretical hydrograph whose
parameters could hopefully be related to the particu-
lar basin characteristics responsible for this unusual
watershed response (28, 29, 30). Only one hydro-
graph with clearly separated peaks was omitted from
the study. It was the result of a storm moving trans-
versely across a small belt of long twin tributaries.
The peak rate was considerably less than for the
three hydrographs actually used for this watershed.
From the point of view of predicting design floods its
omission seems desirable. Furthermore, uneven
areal distribution is an undesirable feature for this
study of very .small watersheds.

The basic approach was to use the total ob-
served runoff. That is to say. groundwater and
interflow were not subtracted as in the unit hydro-
graph attempt to isolate 11surface runoff. II A simple
adjustment was made to the observed hydrographs to
account for antecedent flow when this was present.

It should be stressed that the forty-seven
hydrographs originally selected from the available
fifty were retained for the remainder of the study.
The degree to which the mathematical model could
be fitted to the corrected "observedll hydrographs,
in no way affected their retention in the study. Like-
wise no further adjustments were made to the
corrected ltobservedl1 hydrographs in an attempt to
influence the regressions. Whatever deviations