8.1.9 Hydrologic Data
The purpose of hydrologic data is to determine the stream discharge, flood magnitudes, and
duration and frequencies of flood prior to analysis of river behavior and design of the river
encroachments and crossings. Hydrologic data and hydraulic analyses should be documented
in report form for project development. After construction, the documentation would be helpful
in evaluating any damage from floods and failures, in the event they occur, and providing
background for any litigation which may arise as a consequence.
Sometimes a highway crossing and/or encroachment may have a significant effect on flood
hydrographs and a hydrologic analysis should be made to determine the level of significance.
This analysis would involve hydrograph development and flow routing within the zone of
influence of such highway structures.
The basic data needed are stream discharge data at the nearest gaging station, historical
floods and highwater marks. It is also desirable to prepare a drainage map for the region
upstream of the proposed highway project, with delineation of size, shape, slope, land use, and
water resource facilities such as storage reservoirs for irrigation and power and flood control
projects. It is desirable whenever possible to obtain flood histories of the river from residents
and accounts by the news media, particularly for events prior to stream gaging records.
Estimates of flood discharge can be made from these accounts which are valuable in
A flood-frequency curve is prepared from recorded stream flow data and augmented by
estimated discharges (using Manning's equation or equivalent) from high water marks.
Several methods ranging from sophisticated stochastic analysis to simple methods have been
developed. The greatest difficulty in constructing a flood-frequency curve is lack of sufficient
and reliable data. Approximate methods for extrapolating the range of flood-frequency curves
are available but are not discussed in detail here (see HDS-2, FHWA 1961).
A simple graphical method based on extreme value theory is reasonably satisfactory. The
method consists of ordering the annual peak flood discharges of record from the largest to
smallest, irrespective of chronological order. The annual (flood) discharge is plotted against its
recurrence interval on special probability (Gumbel, or other) paper. The recurrence interval, RI
is calculated from
in which n is the number of years of records, and m is the order (largest flood is ranked 1) of
the flood magnitude. Thus, the highest flood discharge would have a recurrence interval of n +
1 years and lowest would have a recurrence interval of (1+1/n) years. The U.S. Water
Resources Council (1981) has adopted the log-Pearson III distribution for use as a base
method for determining flood flow frequencies. Details of the method and plotting paper may
be obtained from the U.S. Geological Survey in Bulletin 17B.
When adjusting discharge records from a nearby gaging station to the project site, the flood
peaks are often prorated on the basis of drainage area ratios. Depending on drainage basin
characteristics, the exponent of the ratio varies from 0.5 to 0.8. Slope-area calculations for