TR-55 (Technical
Release 55): Urban Hydrology for Small Watersheds
VERSION/DATE:
Version 2.10 July, 1998
OPERATING
SYSTEM: DOS
DISTRIBUTOR:
NRCS
DOCUMENTATION
(239 Pages):
Technical
Release 55 (TR-55): Urban Hydrology for Small Watersheds Technical Release 55 By Soil Conservation
Service Version 2.10, July 1998 (includes cover and table of contents from
June, 1988)
Technical
Release No. TR-55 CP Urban Hydrology for Small Watersheds Hydrology Training
Series By Soil Conservation Service
ABSTRACT
The
conversion of rural land to urban land usually increases erosion and the
discharge and volume of storm runoff in a watershed. It also causes other problems that affect soil and water. As part of programs established to alleviate
these problems, engineers increasingly must assess the probable effects of
urban development, as well as design and implement measures that will minimize
its adverse effects.
Technical
Release 55 (TR-55) presents simplified procedures for estimating runoff and
peak discharges in small watersheds. In
selecting the appropriate procedure, consider the scope and complexity of the
problem, the available data, and the acceptable level of error. While this TR gives special emphasis to
urban and urbanizing watersheds, the procedures apply to any small watershed in
which certain limitations are met.
EFFECTS
OF URBAN DEVELOPMENT
An
urban or urbanizing watershed is one in which impervious surfaces cover or will
soon cover a considerable area.
Impervious surfaces include roads, sidewalks, parking lots, and
buildings. Natural flow paths in the
watershed may be replaced or supplemented by paved gutters, storm sewers, or
other elements of artificial drainage.
Hydrologic
studies to determine runoff and peak discharge should ideally be based on
long-term stationary streamflow records for the area. Such records are seldom
available for small drainage areas.
Even where they are available, accurate statistical analysis of them is
usually impossible because of the conversion of land to urban uses during the
period of record. It therefore is
necessary to estimate peak discharges with hydrologic models based on
measurable watershed characteristics.
Only through an understanding of these characteristics and experience in
using these models can we make sound judgments on how to alter model parameters
to reflect changing watershed conditions.
Urbanization
changes a watershed's response to precipitation. The most common effects are reduced infiltration and decreased
travel time, which significantly increase peak discharges and runoff. Runoff is determined primarily by the amount
of precipitation and by infiltration characteristics related to soil type, soil
moisture, antecedent rainfall, cover type, impervious surfaces, and surface
retention. Travel time is determined
primarily by slope, length of flow path, depth of flow, and roughness of flow
surfaces. Peak discharges are based on
the relationship of these parameters and on the total drainage area of the
watershed, the location of the development, the effect of any flood control
works or other natural or manmade storage, and the time distribution of
rainfall during a given storm event.
The
model described in TR-55 begins with a rainfall amount uniformly imposed on the
watershed over a specified time distribution.
Mass rainfall is converted to mass runoff by using a runoff curve number
(CN). CN is based on soils, plant
cover, amount of impervious areas, interception, and surface storage. Runoff is then transformed into a hydrograph
by using unit hydrograph theory and routing procedures that depend on runoff
travel time through segments of the watershed.
For a
description of the hydrograph development method used by SCS, see chapter 16 of
the SCS National Engineering Handbook, Section 4-Hydrology (NEH4) (SCS
1985). The routing method (Modified
Att-Kin) is explained in appendixes G and H of draft Technical Release 20
(TR-20) (SCS 1983).
RAINFALL
TR-55
includes four regional rainfall time distributions. All four distributions are
for a 24-hour period. This period was
chosen because of the general availability of daily rainfall data that were
used to estimate 24-hour rainfall amounts.
The 24-hour duration spans most of the applications of TR-55.
One
critical parameter in the model is time of concentration (T,), which is the time
it takes for runoff to travel to a point of interest from the hydraulically
most distant point. Normally a rainfall
duration equal to or greater than T. is used.
Therefore, the rainfall distributions were designed to contain the
intensity of any duration of rainfall for the frequency of the event
chosen. That is, if the 10-year
frequency, 24-hour rainfall is used, the most intense hour will approximate the
10-year, 1- hour rainfall volume.
RUNOFF
To
estimate runoff from storm rainfall, SCS uses the Runoff Curve Number (CN)
method. Determination of CN depends on the watershed's soil and cover
conditions, which the model represents as hydrologic soil group, cover type,
treatment, and hydrologic condition.
Chapter 2 of this TR discusses the effect of urban development on CN and
explains how to use CN to estimate runoff.
TIME
PARAMETERS
The
method is based on velocities of flow through segments of the watershed. Two major parameters are time of
concentration and travel time of flow through the segments. These and the other parameters used are the
same as those used in accepted hydraulic analyses of open channels. Many
methods are empirically derived from actual runoff hydrographs and watershed
characteristics.
PEAK
DISCHARGE AND HYDROGRAPHS
Method
for approximating peak rates of discharge and a method for obtaining or routing
hydrographs are both methods that were derived from hydrographs prepared by
procedures outlined in chapter 16 of NEH4 (SCS 1985). The computations were made with a computerized SCS hydrologic
model, TR-20 (SCS 1983).
STORAGE
EFFECTS
Chapter
6 outlines procedures to account for the effect of detention- type
storage. It provides a shortcut method
to estimate temporary flood storage based on hydrologic data developed from the
Graphical Peak Discharge or Tabular Hydrograph methods.
By
increasing runoff and decreasing travel times. urbanization can be expected to
increase downstream peak discharges.
Chapter 6 discusses how flood detention can modify the hydrogmph so
that, ideally, downstream peak discharge is reduced approximately to the
predevelopment condition. The shortcuts
in chapter 6 are useful in sizing a basin even though the final design may
require a more detailed analysis.
LIMITATIONS
To save
time, the procedures in TR-55 are simplified by assumptions about some
parameters. These simplifications,
however, limit the use of the procedures and can provide results that are less
accurate than more detailed methods.
The user should examine the sensitivity of the analysis being conducted
to a variation of the peak discharge or hydrograph. To ensure that the degree of error is tolerable, specific
limitations are given in chapters 2 through 6. Additional general constraints
to the use of TR-55 are as follows:
The
methods in this TR are based on open and unconfined flow over land or in
channels. For large events during which
flow is divided between sewer and overland flow, -more information about
hydraulics than is presented here is needed to determine T,. After flow enters a closed system, the
discharge can be assumed constant until another flow is encountered at a
junction or another inlet.
Both
the Graphical Peak Discharge and Tabular Hydrograph methods are derived from
TR-20 (SCS 1983) output. Their accuracy
is comparable; they differ only in their products. The use of T. permits them to be used for any size watershed
within the scope of the curves or tables.
The Graphical method (chapter 4) is used only for hydrologically
homogeneous watersheds because the procedure is limited to a single watershed
subarea. The Tabular method can be used
for a heterogeneous watershed that is divided into a number of homogeneous
subwatersheds. Hydrographs for the
subwatersheds can be routed and added.
The
approximate storage-routing curves should not be used if the adjustment for
ponding is used. These storage-routing
curves, like the peak discharge and hydrograph procedures, are generalizations
derived from TR-20 routings.
END