GREEN BOOK - Georgia Soil and Water Conservation Commission

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 c . After flow enters a closed system, the discharge
can be assumed constant until another flow is encountered
at a junction or another inlet.
1-4 (210-VI-TR-55, Second Ed., June 1986)
A-1-10
• 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 c 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 (chapter 5) 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 (chapter
6) should not be used if the adjustment for ponding
(chapter 4) is used. These storage-routing curves,
like the peak discharge and hydrograph procedures,
are generalizations derived from TR-20
routings.
GaSWCC