discharge hydrographs are computed using the same method

Short historic records may be unrepresentative with respect to

described for the interior discharge hydrographs. The exterior

giving good estimates of more rare events or combinations of

stage hydrograph is then defined by applying the exterior

events. Thus, 30 to 40 years of record may be inadequate to

discharge hydrograph to a rating curve at the interior ponding

derive stage-frequency results for rare events (1- to 0.2-percent

area primary outlet.

events). For this situation, the CSA method should be used to

define the more frequent events and the HEA method to help

(2) Hypothetical analysis for independent events. This

determine the rarer events. The resulting frequency relationship

procedure is applicable when floods affecting the interior area

may be a product of both approaches.

can be independent of floods that affect the exterior stages.

These areas are often relatively small interior areas located

along large rivers, lakes, or coastlines. One probabilistic

Hydrologic analysis techniques used in planning studies of

procedure applicable to the analysis of independent events using

interior areas vary in analytical concepts and procedures.

hypothetical rainfall is the coincident frequency method,

Unfortunately, the analysis is usually tedious and complex.

conceptualized in Figure 2-3. This method applies the total

Selection of techniques should be based on the type and phase

probability theorem to generate stage-frequency functions for

of the study; complexity and relative importance of the

interior areas affected by various combinations of interior and

coincident nature of flooding at the outlet; complexity of the

exterior flooding. Figure 2-4 defines the steps necessary to

hydrologic system; the nature of the flood damage,

perform the coincident frequency procedure.

environmental, and social factors pertinent to the study area; and

the experience of the analyst. The two techniques presented

(3) HEA applicability and limitations. HEA requires less

here are the continuous simulation approach and the

data than the continuous record technique. The analysis

hypothetical event approach; several variations exist with each.

generates hypothetical frequency hydrographs in which the peak

When working on a study, one should try to use everything

flow rate, runoff volume, and all durations are assumed to be

available from both methods. For example, the CSA may be the

statistically consistent with the percent chance exceedance

best method to use on a particular study; however, the

assignment of the rainfall events. This method overcomes the

continuous record precipitation is so short that an HEA analysis

potential lack of data problems of CSA. However, for many

is needed to include the larger, rarer events. To get the

study settings, interior and exterior flooding are not totally

minimum and maximum range of interior stages, an analysis of

dependent or independent.

both totally blocked and unblocked conditions is also

recommended.

data are available, but the number of years of record is short.

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