FLOOD04403

• County: Statewide
• Basin: Statewide
• Title: Hydrologic Analysis of Ungaged Waterways with HEC-1
• Date: 4/1/1981
• Prepared For: US
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Educational/Technical/Reference Information

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The approaches used include regional frequency analysis, continuous
precipitation-runoff analysis with historical precipitBtion data, and
single-event precipitation-runoff analysis using synthetic precipitation
data. These three general approaches are discussed in this chapter. There
are other widely used procedures, such as USGS nomographs, empirical
equations, and others. The reader is referred to McCuen, et al. (1977) for a
list of references on other methods.

2,1 Generalized Rainfall-Runoff Relations

Hydrograph analysis concepts may be applied to ungaged watersheds
through the development and application of generalized functions for
estimating the amount of precipitation lost due to interception and
infiltration (loss rates), unit hydrographs, and base flow. The unit
hydrograph is usually assumed to give a unique relationship between rainfall
excess and surface runDff for a basin regardless of storm size, losses, or
other factors (Hydrologic Engineering Center, 1973c). Because of its ease of
use, the unit hydrograph has received the most attention by hydrologists.
However, other methods of hydrograph generation are also becoming widely used
(Feldman, 1979), such as the kinematic wave approach to basin modeling, also
a feature of HEC-l, Both the unit hydrograph and the kinematic wave analyses
will be discussed in this report.

The determination of loss rates is a major problem because loss rates
are extremely variable and are dependent on both precipitation patterns and
basin characteristics. As a consequence, there is a tendency to adopt a
"criteria" approach where loss rates are chosen on the basis of policy rather
than being computed. The criteria approach is most appropriate for computing
large floods (from synthetic rainfall) r such as floods for spillway design,
since losses tend to be small relative to rainfall for large and rare
events. This approach is less appropriate when it is used to compute runoff
for a range of flood events for system performance evaluations, due in part
to the fact that different combinations of rainfall intensities and loss
rates can yield similar runoff quantities.

Relating loss rates and analytical loss-rate functions to soil type,
land use and cover, antecedent precipitation, and rainfall intensity can be

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