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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requirements of the study results. It should be kept in mind that any data
for the basin which are available should be used in the calibration of the
hydrologic model of the basin and in the development of discharge-frequency
curves at gaged locations, By using all the resources available, including
data from regional sources, in conjunction with the analytical techniques
described here, the hydrologist can usually make reasonable determinations of
discha:rge-f:requency :relations fo:r ungaged basins,

1,2 Need for Guidelines

A lack of meteorological and hydrological data is one of the greatest
obstacles to accurate discharge-frequency analysis (Burnham, 1980), The
hydrologist is usually faced with the prOblem of having few rainfall
measuring sites and even fewer st:reamflow measuring stations in the basin
under study. Techniques are therefore required for predicting or forecasting
the peak flow rate and/or volume of runoff for events or situations of
interest that do not reply on historical records as a check.

Jlccurate prediction of streamflows is essential in the planning and
design of all types of water resource systems. Of major concern is the
prediction Df the magnitude of flood peaks and their frequency Df Dccurrence.
Information on peak flow is needed for the sizing of channels, bridge
waterways, storm drainage systems, reservoir spillways, and other hydraulic
structures. The frequency associated with a given flood flow permits the
selection of the appropriate level of probability for design. General areas
for which this information is used include: Design of urban drainage
systems, flood plain mapping, and design of structural and nonstructural
measures for reduction of flood damages.

~drologic studies to provide data for flood insurance studies require
flow and frequency estimates for a wide range of recurrence intervals. In
these studies, the 10-r 50-, 100-, 500-year (the 10-, 2-, 1-, 0.2- percent
events), and the Standard Project Flood are frequently determined. For
reservoir spillway design the largest expected floods are of greatest
concern, and the lOa-year, 50a-year, Standard Project Flood, plus the
Probable Maximum Flood magnitudes are required.

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