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TR-55 - calculates storm runoff volume, peak rate of discharge, hydrographs, and storage volumes required for floodwater reservoirs
TR-55 Description
Technical Release 55 (TR-55) presents simplified procedures to calculate storm runoff volume, peak rate of discharge, hydrographs, and storage
volumes required for floodwater reservoirs. These procedures are applicable in small watersheds, especially urbanizing watersheds, in the United States. First issued by the Soil Conservation Service (SCS) in January
1975, TR-55 incorporates current SCS procedures. This revision includes results of recent research and other changes based on experience with use of the original edition.
The major revisions and additions are:
- A flow chart for selecting the appropriate procedure.
- Three additional rain distributions.
- Expansion of the chapter on runoff curve numbers.
- A procedure for calculating travel times of sheet flow.
- Deletion of a chapter on peak discharges.
- Modifications to the Graphical Peak Discharge method and Tabular Hydrograph method.
- A new storage routing procedure.
- Features of the TR-55 computer program.
- Worksheets.
Introduction to TR-55
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
the 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
(NEH-4) (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 (Tc), which is the time is 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 Tc 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 used 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
Chapter 3 describes a method for estimating the parameters used to distribute the runoff into a hydrograph. The method is based on velocities of
flow through segments of the watershed. Two major parameters are time of concentration (Tc) and travel time of flow through the segments (Tt). 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. The method in Chapter 3 was chosen because it is
basic; however, other methods may be used.
Peak discharge and hydrographs
Chapter 4 describes a method for approximating peak rates of discharge, and Chapter 5 describes a method for obtaining or routing hydrographs. Both
methods were derived from hydrographs prepared by procedures outlined in Chapter 16 of NEH-4 (SCS 1985). The computations were made with a computerized SCS hydrological model, TR-20 (SCS 1983).
The methods in Chapters 4 and 5 should be used in accordance with specific guidelines. If basic data are improperly prepared or adjustments not
properly used, errors will result.
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 hydrograph 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.
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