When necessary the Public Works Director may require the owner or developer of a property to submit a stormwater management plan. The plan shall be required when it is determined that the existing natural or manmade drainage ways are not adequate to carry stormwater flow from a proposed development. Plans and specifications shall be prepared and certified by a professional engineer registered in the state.
(A) Design criteria. The design of stormwater runoff systems, structures and facilities shall be based on the following minimum standards which do not preclude the use of higher design standards.
(B) Applicability. The provisions of divisions (C) through (G) below shall apply to the following areas under development:
(1) All residential development of two acres or more and all commercial and industrial developments in excess of one-half acres;
(2) Any development where the percentage of the impervious area of the lot is 50% or greater; and
(3) Any development which, in the opinion of the City Engineer, lacks an adequate internal or external excess stormwater passageway.
(C) Runoff calculation.
(1) Design flows shall normally be calculated using the procedures outlined in the Soil Conservation Services, Technical Release No. 55 Urban Hydrology for Small Watersheds and the Iowa Users Guide and Supplement for the Technical Release No. 55. For drainage basins of 20 acres or less, the rational method may be used.
(2) In all cases, all areas are to be considered fully developed, in accordance with the city’s current comprehensive plan.
(D) Storm sewers. Storm sewers and intakes shall be designed for a five-year frequency storm in a manner that the flooded street width shall not exceed ten feet on each side from the face of the curb.
(E) Excess stormwater passageway. An excess stormwater passageway shall be provided for all developments. The passage shall have the capacity to convey through the proposed development the excess stormwater from the tributary watershed. The capacity of the excess stormwater passage shall be constructed in a manner as to transport the peak rate of runoff from a 100-year return frequency storm.
(F) Open channels. The size and shape of open channels shall be designed to meet the requirements of runoff, depth, side slopes, gradient and velocity limitations in accordance with the site conditions. Runoff shall be based on 100-year storm frequency. Manning’s formula as cited in most civil engineering handbooks, shall be used in hydraulic design of open channels. Channel banks shall be protected by use of low vegetation, rip-rap, turf reinforcement matting, or paving as design velocity dictates subject to the approval of the City Engineer.
(G) Easements.
(1) Drainage easements shall be provided for all conduits and those bypass channels where the 100-year runoff exceeds one cubic foot per second.
(2) Whenever any stream or water course is located in an area that is being subdivided, the subdivider shall dedicate a public right-of-way or drainage easement conforming substantially with the lines of the stream or water course and shall include the additional area adjoining both edges of the stream or water course that has been affected by damaging flood waters and/or inundated by the 100-year flood waters, as determined by the city engineer. Maintenance of the stream and banks shall be the responsibility of the landowners upon which the stream is located. This maintenance responsibility shall be clearly defined in a recorded document. The subdivider shall also provide reasonable public easements for access.
(3) It shall be noted on the final plat, “Owners of lots on which a drainage easement has been established as a stormwater passageway shall maintain the easement as a lawn, planted in grass and free of structures, fences, fill, bushes, trees, shrubs or other landscaping that would impede the flow of water.” In the event that the area established as a drainage easement is reshaped or otherwise restricted for use as a drainage easement, the city will cause the restrictions to be removed at the expense of the parties causing the restrictions.
(Ord. 2390, passed 10-28-2008; Ord. 2543, passed 9-27-2016) Penalty, see § 53.99