FLOOD09674

• County: Statewide
• Community: Nationwide
• Basin: Statewide
• Title: Hydrology for Urban Land Planning - A Guidebook on the Hydrologic Effects of Urban Land Use
• Date: 1/1/1968
• Prepared By: USGS
• Floodplain - Doc Type: Educational/Technical/Reference Information

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HYDROLOGY FOR URBAN LAND PLANNING-A GUIDEBOOK ON THE HYDROLOGIC

EFFECTS OF URBAN LAND USE

By LUNA B. LEOPOLD

This cir~ular attempts to summarize existing
knowledge of the effects of urbanization on
hydrologic factors. It also attempts to express
this know ledge in terms that the planner can
use to test alternatives during the planning
process. Because the available data used in
this report are applied to a portion of the
Brandywine Creek basin in Pennsylvania, this
.an be considered as a report on the basic hy-
rologic conditions of the Brandywine Creek
basin prior to the beginning of major urbani-
zation. Because the available data are not yet
adequate, this report can be considered as a
compilation of tentative suggestions in the
form of an explanatory, not a definitive, hand-
book.

The application of current knowledge of the
hydrologic effects of urbanization to the
Brandywine should be viewed as a forecast of
conditions which may be expected as urbaniza-
tion proceeds. By making such forecasts in
advance of actual urban development, the
methods can be tested, data can be extended,
and procedures improved as verification be-
comes possible.

PLANNING PROCEDURES AND HYDROLOGIC
VARIABLES

A planning document presented to a com-
munity for adoption must always be more sug-
gestive than coercive. This is true not only
because the planner is unable to foresee the
innumerable complications of actual develop-
.ent, but also because there are many detailed
lternatives which would accomplish generally
similar results. The planner is particularly
concerned with both the constraints and the

opportunities offered by the principal physi-
ographic characteristics of the area, especially
the location of hillslopes, soils, and streams.
The existing pattern of land use and the ac-
companying distribution of woods and agricul-
ture are parameters which over a period of
years may actually change, albeit slowly. Roads,
villages, industries, and other manmade fea-
tures are more or less permanent and exert
their greatest influence in their effect on
further development, especially through land
values.

Of particular concern to the planner are
those alternatives that affect the hydrologic
functioning of the basins. To be interpreted
hydrologically, the details of the land-use pat-
tern must be expressed in te,ms of hydrologic
parameters which are affected by land use.
These parameters in turn become hydrologic
variables by which the effects of alternative
planning patterns can be evaluated in hydro-
logic terms.

There are four interrelated but separable
effects of land-use changes on the hydrology of
an area: changes in peak flow characteristics,
changes in total runoff, changes in quality of
water, and changes in the hydrologic amenities.
The hydrologic amenities are what might be
called the appearance or the impression which
the river, its channel and its valleys, leaves with
the observer. Of all land-use changes affecting
the hydrology of an area, urbanization is by
far the most forceful.

Runoff, which spans the entire regimen of
flow, can be measured by number and by char-
acteristics of rise in streamflow. The many

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