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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Without proper design or as-buiIt information, it is difficult to assess the reliability and
level of protection of the private ring levee. Based upon available information,
however, the ring levee likely does not provide for a 100-year level of protection, and
certainly does not provide for a SOD-year level of protection.

In order to achieve 1oo-year protection for the two residential structures, the crest of the
levee would need to be at an elevation of at least 1339.5' (MSL) around the perimeter of
the property. The levee may need to continue in an easterly direction toward East
Street from the point where it ties in to existing ground on the northwest side. This
continuation would provide flood protection for the north side of the property. In
addition, it appears from photographs that there is a low spot between the southeast
end of the levee and East Street. An extension of the levee for a tie-in along East Street
may be sufficient. The portion of the existing levee that is at or above elevation 1339.5
would provide for 1oo-year protection. The existing level of protection in the current
configuration may be in the range of a 50-year frequency.

To achieve a SOO-year level of protection at this site, a ring levee would have to be
constructed to an elevation of at least 1343.5 (MSL). A SOO-year levee would likely not
be feasible do to construction costs and constraints of the existing roadway elevation
and the ground elevation of the surrounding topography.

The existing ring levee may meet Corps of Engineers criteria based upon available
information, however further investigations would have to be performed in order to
make a determination. It would be advisable to obtain results of compaction tests for
various locations along the levee so that the compaction results could be compared to
Corps criteria. It appears from the photographs that the top width of the Ievee may not
consistently be 10 feet, and the side slopes may be steeper than 3H to IV in some areas.
These issues would have to be verified through field measurements in order to
determine the exact geometry of the levee.

Overall, the private ring levee appears to be constructed in such a manner that it would
provide a significant benefit to the property owners during a large flood event. At this

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