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TO: Zoning Board of Adjustment
FROM: Charlie Kuester,Planner
CC: Kelly Diekmann,Mark Lambert
DATE:December 5 2014
SUBJECT: Follow-up information for CUP for 1204 S. 4`1' Street
This memo is to provide information requested by two members of the Board following the
meeting on November 12. In addition, I added some information about the requirement to
extend the elevation 18 feet around the perimeter of a residential building.
Storm water management.
A question was asked about the storm water regulations of the City and how this site is meeting
it. Storm water management is governed by Chapters 5A and 5B of the Ames ordinance.
Chapter 5A requires the owner to use certain construction and stabilization methods to reduce
soil erosion during construction. Chapter 5B requires the owner to control the quantity of water
coming off the site during a rain event and improve the quality of that water during the initial
flush of runoff from the site.
The applicant provided a storm water management plan for this site, based on the full build out
of the parking lot and two apartment buildings. The storm water management plan, including the
storm water pollution prevention plan, met the requirements of 5A. Accordingly, a COSESCO
(Construction Site Erosion and Sediment Control) permit was issued. The applicant has also
obtained the required NPDES permit. He is in compliance with the requirements of Chapter 5A.
The applicant has also submitted a grading plan showing storm water management practices
intended to meet the requirements of Chapter 5B. Because of the constraints of the site, the
applicant sought waivers from:
• the requirement for a 100 foot buffer from the top of the bank of Squaw Creek,
• the requirement for a natural resource inventory,
• the requirement for extended detention,and
• the requirement for an elevation certificate.
The Municipal Engineer approved a waiver allowing for a 53 foot buffer, supplemented by
native plantings and a waiver to the extended detention requirements. All other requirements of
Chapter 5B have been met.
Planning and Housing Department 515 239.5400 nuuu 515 Clark Ave.P.C.Box 811
515.239.5404 y„x Ames,IA 50010
www Cityo`Arres or0
Impact of retaining wall on flood waters.
A question was asked whether the retaining wall will speed up water flow or make the ground
less able to absorb water in the event of flood conditions. The velocity of flowing water is
affected by the type of surface it flows over. For instance, a heavily wooded area offers greater
resistance to flowing water whixch would increase upstream water surface elevations. A smooth
concrete surface offers less resistance which would decrease upstream water surface elevations.
The hydraulic model used for the preparation of FEMA's Flood Insurance Rate Maps to
determine the extent of flood inundation accounts for the type of ground surface and assigns
values at each cross section. These cross sections occur about every 100 yards or so in urban
areas and every 500 yards or so in rural areas; therefore the resolution is very course. A
retaining wall that extends above the water surface level of a flood event would totally restrict
water movement in that direction. The velocity, though, would be influenced by the ground
surface.
As for the ability of the ground to absorb water during a flood event,saturated soils have little to
no absorption ability. City flood plain regulations do not prohibit impermeable surfaces in the
Floodway or Floodway Fringe. The storm water regulations require that water not leave the site
at rate faster than prior to development.
18 foot Perimeter Elevation.
Staff can provide more information on the requirement that residential structures be elevated 3
feet above the BFE and that the elevation extend 18 feet around the perimeter of the building.
The regulations note that elevation is the only allowed method for a residential structure.
(Commercial and other uses can be elevated or floodproofed.) The regulations require elevation
by compacted fill. It also allows, though, the use of other elevation techniques such as piles,
posts, piers, columns, or walls. The regulations require a Condition Use Permit from the zoning
board of a community for this alternative method.
The purpose of the elevation on fill is to allow for the safe evacuation of residents and to ensure
that flood waters do not scour, erode, or undermine the building foundation. A properly
designed flood wall will be protected from scouring and eroding. The board will need to
determine whether the safe evacuation of the residents can still occur if the perimeter elevation
is not met.
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