Stormwater Calculations - Mathematics and Statistics for the Environment - page 785

TABLE 25.6D
Roughness Coefficient ( n ) for the Manning Equation—
Natural Stream Channels

n Value Range

Channel Lining From To

1. Clean, straight bank, full stage, no rifts or deep pools 0.025 0.030

2. Same as #1, but some weeds and stones 0.030 0.035

3. Winding, some pools and shoals, clean 0.033 0.040

4. Same as #3, lower stages, more ineffective slope and sections 0.040 0.050

5. Same as #3, some weeds and stones 0.035 0.045

6. Same as #4, stony sections 0.045 0.055

7. Sluggish river reaches, rather weedy with very deep pools 0.050 0.070

8. Very weedy reaches 0.075 0.125

Source: Adapted from Brater, E. and King, H., Handbook of Hydraulics , 6th ed.,
McGraw-Hill, New York, 1976.

25.6.4 tR-55 g raphiCal p eaK d isCharge m ethod
The graphical peak discharge method was developed from hydrograph analyses using TR-20 ,
Computer Program for Project Formulation: Hydrology (USSCS, 1983). The graphical method
develops the peak discharge in cubic feet per second (cfs) for a given watershed.

25.6.4.1 Limitations
The engineer should be aware of several limitations before using the TR-55 graphical method:

1. The watershed being studied must be hydrologically homogeneous; that is, the land use,
soils, and cover are distributed uniformly throughout the watershed and can be described
by one curve number.

2. The watershed may have only one main stream or flow path. If more than one is present
they must have nearly equal t c values so that one t c represents the entire watershed.

3. The analysis of the watershed cannot be part of a larger watershed study, which would
require adding hydrographs since the graphical method does not generate a hydrograph.

4. For the same reason, the graphical method should not be used if a runoff hydrograph is to
be routed through a control structure.

5. When the initial abstraction/rainfall ratio ( I a / P ) falls outside the range of the unit peak
discharge curves (0.1 to 0.5), the limiting value of the curve must be used.

The reader is encouraged to review TR-55 to become familiar with these and other limitations asso-
ciated with the graphical method.
The graphical method can be used as a planning tool to determine the impact of development
or land use changes within a watershed, or to anticipate or predict the need for stormwater man-
agement facilities or conveyance improvements. Sometimes the graphical method can be used in
conjunction with the TR-55 short-cut method for estimating the storage volume required for post-
developed peak discharge control. This short-cut method is found in Chapter 6 of TR-55. However,
it should be noted that a more sophisticated computer model such as TR-20 or HEC-1 or even the
TR-55 tabular hydrograph method should be used for complex, urbanizing watersheds.