Stormwater Calculations - Mathematics and Statistics for the Environment - page 777

25.6.3 tR-55 e stimating r unoFF m ethod
The U.S. Soil Conservation Service published the second edition of TR-55 in 1986, entitled Urban
Hydrology for Small Watersheds . The techniques outlined in TR-55 require the same basic data as
the rational method: drainage area, time of concentration, land use, and rainfall. The SCS approach,
however, is more sophisticated in that it allows the designer to manipulate the time distribution of the
rainfall, the initial rainfall losses to interception and depression storage, and the moisture condition of
the soils prior to the storm. The procedures developed by SCS are based on a dimensionless rainfall
distribution curve for a 24-hour storm. TR-55 presents two general methods for estimating peak dis-
charges from urban watersheds: the graphical method and the tabular method. The graphical method
is limited to watersheds whose runoff characteristics are fairly uniform and whose soils, land use,
and ground cover can be represented by a single runoff curve number ( RCN ). The graphical method
provides a peak discharge only and is not applicable for situations where a hydrograph is required.
The tabular method is a more complete approach and can be used to develop a hydrograph at
any point in a watershed. For large areas it may be necessary to divide the area into sub-watersheds
to account for major land use changes, analyze specific study points within sub-watersheds, or
locate stormwater drainage facilities and assess their effects on peak flows. The tabular method can
generate a hydrograph for each sub-watershed for the same storm event. The hydrographs can then
be routed through the watershed and combined to produce a partial composite hydrograph at the
selected study point. The tabular method is particularly useful in evaluating the effects of an altered
land use in a specific area within a given watershed.
Prior to using either the graphical or tabular methods, the designer must determine the volume of
runoff resulting from a given depth of precipitation and the time of concentration, t c , for the water-
shed being analyzed. The methods for determining these values will be discussed briefly in this
section. However, the reader is strongly encouraged to obtain a copy of the TR-55 manual from the
Soil Conservation Service to gain more insight into the procedures and limitations.
The runoff curve number (RCN) method is used to estimate runoff (described in detail in Section
4 of the National Engineering Handbook ). The runoff equation (found in TR-55 and discussed later
in this section) provides a relationship between runoff and rainfall as a function of the CN. The CN
is a measure of the land's ability to infiltrate or otherwise detain rainfall, with the excess becoming
runoff. The CN is a function of the land cover (woods, pasture, agricultural use, percent impervious,
etc.), hydrologic condition, and soils.
25.6.3.1 Limitations
• TR-55 has simplified the relationship between rainfall and runoff by reducing all of the
initial losses before runoff begins, or initial abstractions, to the term I a , and approximat-
ing the soil and cover conditions using the variable S , potential maximum retention. Both
of these terms, I a and S , are functions of the runoff curve number. Runoff curve numbers
describe average conditions that are useful for design purposes. If the purpose of the hydro-
logic study is to model a historical storm event, average conditions may not be appropriate.
• The designer should understand the assumption relected in the initial abstraction term,
I a . I a represents interception, initial infiltration, surface depression storage, evapotrans-
piration, and other watershed factors and is generalized as a function of the runoff curve
number based on data from agricultural watersheds. This can be especially important in
an urban application because the combination of impervious area with pervious area can
imply a significant initial loss that may not take place. On the other hand, the combina-
tion of impervious and pervious area can underestimate initial losses if the urban area has
significant surface depression storage. (To use a relationship other than the one established
in TR-55 , the designer must redevelop the runoff equation by using the original rainfall-
runoff data to establish new curve number relationships for each cover and hydrologic soil
group. This would represent a large data collection and analysis effort.)