FLOOD03740

• County: Statewide
• Basin: Statewide
• Title: Adjustment of Peak Discharge Rates for Urbanization
• Date: 2/1/1979
• Prepared For: US
• Prepared By: COE
• Floodplain - Doc Type: Educational/Technical/Reference Information

and subscripts 2 and 50 designate tbe 2-yr and 50-yr recurrence intervals.
The preceding concept of taking ratios of precipitation ordinates was utilized
to develop representative storms ranging from about the l-yr event to the 500-yr
event. A representative So..yr event (with adjustments for drainage area size
according to Ref. 8) was used in conjunction with the multiplan.ratio option
of HEC-J. Loss rate functions were developed based on those used in the
reconstitution analysis for event I (Fig. 1) since the storm rainfall totals and
distribution patterns were relatively similar. Seven ratios of the S()..yr storm
event were computed for each of the land-use conditions analyzed (1940, 1950,
1960, and 1970) and the results plaited in Figs. 2(b) and 3.
Fig. 2(b) compares tbe rat.s of change of peak discharge witb time between
the specific storm events analyzed and the representative storm events developed.
Fig. 3 can be used to modify tbe recorded peak discharge for any panicular
year to a peak discharge consistent with 1970 conditions. From the results shown
it is apparent that urbanization has a significantly greater impact on the more
frequent events.
Stankowski (7) developed regional expressions, from data whicb included
streamflow records for the Rabway River Basin, relating the annual peak discharge
for a given recurrence interval to an index of man-made impervious cover.
Application of Stankowski's regional expression yields results that are in general
agreement with the HEC-l results shown in Fig. 3.
The methodology incorporated in this technical note has just recently been
applied in preliminary hydologic investigations. For example, tbe Red Run Drain
study (6) utilized similar procedures. In that study the basin model representing
past conditions was verified (where adequate data were available) by reconstituting
several historical events during each representative time frame.

SUMMARY AND CONCLUSIONS

Methods presented in this paper can be used as a guide to determine an
existing-condition discharge frequency curve of annual peaks (or peak discharge
rates above a given base) when utilizing a single event rainfall-runoff model.
The multiplan-ratio option of HEC-I permits 1he user to calculate systematicany
the hydrologic response of several storm events for a given set of land-use
conditions. The adjustment procedure described herein is also applicable when
predicting runoff for estimated future conditions.

ApPENDIX.-REFEAENCES

I. Anderson, D.O., "Effects of Urban Development of Floods in Northem Virginia:'
Water Supply Paper 2001, United States Oeological Survey, Washington, D.C., 1970,
pp. C-l-C-22.
2. H EC-J Flood HyJrograph Package, Hydrologic Engineering Center, Corps of Engineers,
Department of tbe Army. Jan.. 1913.
3. Hydrologic-Hydraulic Simulation. Railway River Basin, NtwJersey, Preliminary repon,
U.S. Army Engineer District, New York, N.Y., 1916.
4. Martcns. L. A" "Flood Inundation and Effects of Urbanization in Metropolilan
Charlotte. North Carolina," Water Supply Paper 1591. United States Geological Survey,
Washington, D.C.. 1968, pp. C-I-C-60.
5. Putnam, A. L.. "Effect of Urban Development on Floods in the Piedmont Province

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