FLOOD06759

• County: Statewide
• Community: Rock Rapids, Iowa
• Basin: Statewide
• Title: Technical Assistance Report Rock River at Rock Rapids Iowa
• Date: 9/1/1995
• Prepared For: US Army Corps of Engineers
• Prepared By: Greenhorne & O'Mara, Inc.
• Floodplain - Doc Type: Project

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draining into the waterway. 1bis is the case for the proposed levee located to the west
of the fairgrounds. The interior drainage area behind the proposed levee was
delineated on the 1971 Rock Rapids topographic mapping (5' contour interval). The
drainage area is roughly bounded by the old railroad embankment to the west, South
2nd Avenue to the north, South 8th Avenue to the south, and Tama Street and the
proposed levee to the east. The deIineation of the drainage area assumed that the
railroad embankment was high ground for the western boundary, and existing storm
sewers were not taken into consideration. The rational method was used to estimate a
peak discharge that would result from runoff within the interior drainage area, and a
1oo-year design frequency was arbitrarily chosen to size the required facilities.

The formula for the rational method is Q=CiA, where "Q" is the peak discharge in
(cubic feet per second) cfs, "c" is the dimensionless runoff coefficient, "i" is the rainfall
intensity in inches per hour for the computed time of concentration, and "A" is the
drainage area in acres. The drainage area (A) was estimated to be 40 acres by
planirnetering the delineated basin from the 1971 topographic mapping. The runoff
coefficient (C) was estimated to be 0.47 for a 1oo-year design frequency assuming a
deveIoped area with grass cover of less than 50 percent and flat slopes (0 to 2 percent).
In order to estimate the rainfall intensity (i), the time of concentration for the basin was
first estimated using SCS average velocity charts for paved surfaces with average slopes
of about 0.5 percent, which yielded a time of concentration of 34.5 minutes. A rainfall
intensity-duration-frequency curve for Sioux City, Iowa (Iowa Department of
Transportation) was used to obtain a 100-year rainfall intensity of 4.8 inches per hour
for the computed time of concentration. Application of the rational method with the
above parameters yielded a peak discharge value of approximately 90 cfs.

Culverts for the interior drainage were assumed to be circular reinforced concrete pipe
(RCP) and were sized with nomographs assuming inlet control. Due to the flat
topography behind the levee, a maximum headwater to diameter of pipe (HW /D) ratio
of 1.0 was assumed so as to minimize the backwater ponding depth. Based upon the
preceding criteria, four 30" diameter RCP's would be needed to effectively drain the
total 90 cfs peak flow. The four pipes are shown on plate 2 in groups of two along the
levee alignment. The pipe outlets would be fitted with "automatic" flap gates to
prevent floodwater in the Rock River from backing up through the pipes during high

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