Stormwater Calculations - Mathematics and Statistics for the Environment - page 787

point. This procedure allows a quick check to verify the timing of the peak flows and to decide if a
more detailed study is necessary.

25.6.5.1 Limitations
Some of the basic limitations of which the engineer should be aware of before using the TR-55
tabular method include the following:

1. The travel time ( T t ) must be less than 3 hours (largest T t in TR-55 , Exhibit 5).

2. The time of concentration ( t c ) must be less than 2 hours (largest t c in TR-55 , Exhibit 5).

3. The acreage of the individual sub-watersheds should not differ by a factor of 5 or more.

When these limitations cannot be met, the engineer should use the TR-20 computer program or
other available computer models that will provide more accurate and detailed results. The reader is
encouraged to review the TR-55 manual to become familiar with these and other limitations associ-
ated with the tabular method.

25.6.5.2 Information Needed
The following parameters are needed to compute the peak discharge of a watershed using the TR-55
tabular method:

1. Subdivision of the watershed into areas that are relatively homogeneous

2. The drainage area of each subarea (square miles)

3. Time of concentration ( t c ) for each subarea (hours)

4. Travel time ( T t ) for each routing reach (hours)

5. Weighted runoff curve number (RCN) for each subarea

6. Rainfall amount ( P ) for each specified design storm (inches)

7. Total runoff ( Q ) for each subarea (inches)

8. Initial abstraction ( I a ) for each subarea

9. Ratio of I a / P for each subarea

10. Rainfall distribution (I, IA, II, or III)

25.6.5.3 Design Parameters
The use of the tabular method requires that the engineer determine the travel time through the entire
watershed. As stated previously, the entire watershed is divided into smaller sub-watersheds that
must be related to one another and to the whole watershed with respect to time. The result is that
the time of peak discharge is known for any one sub-watershed relative to any other sub-watershed
or for the entire watershed. Travel time T t represents the time for flow to travel from the study point
at the bottom of a sub-watershed to the bottom of the entire watershed. This information must be
compiled for each sub-watershed.

Note: The data for up to 10 sub-watersheds can be compiled on one TR-55 worksheet ( TR-55
Worksheets 5a and 5b).

To obtain the peak discharge using the graphical method, the unit peak discharge is read off of a
curve; however, the tabular method provides this information in the form of a table of values, found
in TR-55 , Exhibit 5. These tables are arranged by rainfall type (I, IA, II, and III), I a / P , t c , and T t . In
most cases, the actual values for these variables, other than the rainfall type, will be different from
the values shown in the table. Therefore, a system of rounding these values has been established in
the TR-55 manual. The I a / P term is simply rounded to the nearest table value. The t c and T t values
are rounded together in a procedure that is outlined on pages 5-2 and 5-3 of the TR-55 manual. The
accuracy of the computed peak discharge and time of peak discharge is highly dependent on the
proper use of these procedures.