TM 5820-4/FM 88-5, Chap 4
c. Protection of military installations against
21. General. Hydrologic studies include a care-
floodflows originating from areas exterior to the
ful appraisal of factors affecting storm runoff to
installation will normally be based on 25-year or
insure the development of a drainage system or
greater rainfall, again depending on operational
control works capable of providing the required
requirements, cost-benefit considerations, and
degree of protection. The selection of design storm
nature and consequences of flood damage result-
ing from the failure of protective works. Justifi-
sought but also onthe type of construction con-
cation for the selected design storm will be pre-
templated and the consequences of storms of
sented, and, if appropriate, comparative costs and
greater magnitude than the design storm. Ground
damages for alternative designs should be in-
conditions affecting runoff must be selected to be
consistent with existing and anticipated arel de-
velopment and also with the characteristics and
d. Rainfall intensity will be determined from the
seasonal time of occurrence of the design rainfall.
best available intensity-duration-frequency data.
For areas of up to about 1 square mile, where only
Basic information of this type will be taken from
peak discharges are required for design and ex-
such publications as (see app A for referenced pub-
tensive pondig is not involved, computation of
runoff will normally be accomplished by the s-
Rainfall Frequency Atlas of the United States.
called Rational Method. For larger areas, when
Technical Paper No. 40.
suitable unt-hydrograph data are available or
Generalized Estimates of Probable Maximum
where detailed consideration of pondng is re-
quired, computation should be by uit-hydro-
for Puerto Rico and Virgin Islands. Tech-
graph and flow-routing procedures.
nical Paper No. 42.
Rainfall-Frequency Atlas of the Hawaiian Is-
22. Design storm.
lands. Technical Paper No. 43.
a. For such developed portions of military in-
stallations as administrative, industrial, and
housing areas, the design storm will normally be
Paper No. 47.
based on rainfall of 10-year frequency. Potential
TM 5-785/AFM 88-29/NAVFAC P-89.
damage or operational requirements may war-
These publications may be supplemented as ap-
rant a more severe criterion; in certain storage
propriate by more detailed publications of the En-
and recreational areas a lesser criterion may be
vironmental Data and Information Center and by
appropriate. (With concurrence of the using Serv-
studies of local rainfall records. For large areas
ice, a lesser criterion may also be employed in
and in studies involving unit hydrography and flow-
regions where storms of an appreciable magni-
routing procedures, appropriate design storms
tude are infrequent and either damages or oper-
must be synthesized from areal and time-distri-
ational capabilities are such that large expendi-
bution characteristics of typical regional rainfalls.
e. For some areas, it might reasonably be as-
b. The design of roadway culverts will normally
sumed that the ground would be covered with snow
be based on 10-year rainfall. Examples of condi-
when the design rainfall occurs. If so, snowmelt
tions where greater than 10-year rainfall may be
would add to the runoff. Detailed procedures for
used are areas of steep slope in which overflows
estimating snowmelt runoff are given in TM 5-
would cause severe erosion damage; high road fills
852-7/AFM 88-19, Chap 7. It should be noted, how-
that impound large quantities of water; and pri-
ever, that the rate of snowmelt under the range
mary diversion structures, important bridges, and
of hydro-meteorological conditions normally en-
critical facilities where uninterrupted operation
countered in military drainage design would sel-