occur on an average of once during the design life of the structure. Engineers design structures to last a given
amount of time depending on the consequences of a culvert failure and the costs of designing it for greater
protection. For culverts on roads, highway departments usually use return periods of 10 to 50 years depending on
the importance of the road. For example, a culvert designed for a ten year period is expected to fail on an average
of once every ten years, since it has been designed to withstand a storm that will occur on an average of once
every ten years.
Remember that drainage structures are designed on the probable occurrence of a storm. Because larger storms
occur less frequently in nature, the longer the period of design is used, the longer the protection afforded.
Theater of Operations (TO) Design. In TO construction a two-year design life is usually assumed. In other
words, you select a two year storm as the design storm. This basically means that the culvert should fail on
average of once in its design life of two years. It would be unreasonable to base your construction on a 100-year
storm, since it would require more materials, a longer construction period, and of course high cost.
Pinpoint Data. The characteristics of rainfall vary considerably from one part of the world to another. Germany,
Korea, and Fort Leonard Wood, Missouri, do not receive as much rain as Honduras. Nor would one expect heavy
rains in North Dakota in December.
The best source of data for determining the intensity to be used in design is local records (pinpoint data). If
rainfall rates have been recorded for your project location and meteorologists have examined these records
statistically to formulate magnitudes and durations, then your staff weather officer (SWO) at division level would
be able to obtain this data. In the United States (US) this information would come directly from the weather
service or through the US Air Force.
Overseas, your SWO may be able to obtain this data from the local rainfall recording station; however, this data
may take some time to compile. Most likely you will be designing drainage structures in areas that do not have
rainfall data compiled. In this case, you will have to rely on the Army's world isohyetal map.
USING THE ISOHYETAL MAP WHEN PINPOINT DATA IS NOT AVAILABLE
When pinpoint data is not available for any location in the vicinity of the project site, use the isohyetal map (see
figure 1-4). Isohyets are lines of equal intensities drawn through geographic locations on a topographic map. Just
as a contour line represents equal elevations on a topographic map, isohyets represent equal intensities of rainfall.
The isohyetal map is based on a two-year design storm with a 60-minute duration. This two year design life
fulfills the needs of Army operations.