FLOOD07777

• County: Statewide
• Community: Southern California
• Basin: Statewide
• Title: Urban Sprawl and Flooding in Southern California
• Date: 1/1/1970
• Prepared By: USGS
• Floodplain - Doc Type: Educational/Technical/Reference Information

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Water in the Urban Environment

Urban Sprawl and Flooding in Southern California

By S. E. Rantz

ABSTRACT

The floods of January 1969 in south-coastal
California provide a timely example of the effect of
urban sprawal on flood damage. Despite recordbreaking,
or near recordbreaking, stream discharges, damage was
minimal in the older developed areas that are protected
against inundation and debris damage by carefully
planned flood-control facilities, including debris basins
and flood-conveyance channels. By contrast, heavy
~mage occurred in areas of more -recent urban sprawl
where the hazards of inundation and debris or landslide
damage have not been taken into consideration, and
where the improvement and development of drainage or
f1ood.control facilities have not kept pace with
expanding urbanization.

INTRODUCTION

Intense storms during the period January
l8-February 25,1969, produced
recordbreaking, or near record breaking floods in
southern California in late January and again in
late February. Particularly hard hit were the
urbanized areaS in the COastal region between
the Santa Ana and Santa Ynez Rivers (fig. I).
The loss of life and physical damage attributable
to the storms and ensuing floods in the report
area- 92 lives lost and an estimated physical
damage of $62 million in January alone-have
focused attention on the effect of urban sprawl
on flood damage. Urban sprawl is defined here
as the rapid expansion of suburban development
without complete planning for the optimum
.control and development of water and
associated land resources. The greatest damage

during the storms was sustained in areas of
urban sprawl where the hazards of inundation
and debris or landslide damage have not been
taken into consideration, and where the
improvement and development of drainage or
flood-control facilities have not kept pace with
expanded urbanization.

Because the floods in south-coastal California
provide a timely example of the effect of urban
sprawl, a brief case history of that region is
presented here. The discussions are related to
the floods of January 1969; the floods of
February 1969 were somewhat similar with
respect to both the magnitude of discharge and
the areal pattern of physical damage.

THE FLOOD PROBLEM IN
SOUTHERN CALIFORNIA

To better understand the flood problem in
south-coastal California, one should be familiar
with the characteristics of the stream channels
and the floodflows they carry. The upper reach
of a typical stream channel occupies the floor of
a steep mountain canyon. At the mouth of the
canyon near the base of the mountain is a
well-developed alluvial cone, a cone being a
built-up slope of water-transported rock debris
that extends from the canyon mouth down to
the alluvial valley floor. Intense mountain
storms cause swift sediment-laden streamflow in
the canyon. When the streamflow reaches the
flatter alluvial cone its velocity is reduced and

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