(a) Required in Subdivisions and Developments.
(1) Every subdivision and land development shall be provided with a comprehensive stormwater management system which is adequate to serve the area and which meets the requirements of this chapter, the Stormwater Management Design Manual and other criteria of the City. There are two classifications of stormwater management systems which must exist:
A. The initial or minor drainage system shall provide for the collecting and transporting of runoff water from frequently occurring storms. Such system includes watercourses, streets, curbs, gutters, underground storm sewers, manholes, catch basins and culverts. This system's purpose is to lessen or eliminate inconveniences and safety and health hazards associated with frequent storms. Except where indicated otherwise, design criteria and requirements of this chapter are directed to the minor drainage system.
B. Major drainage system features shall ensure that stormwater runoff which exceeds the capacity of the minor drainage system has a route to follow that will not cause any loss of property or any loss of life. It must be recognized that the major drainage system exists even when it is not planned and whether or not physical facilities are intelligently located in respect to it.
(b) Classification of Drainageways/Waterways. The following classifications shall be applied for drainageways/waterways in determining design and operational capacity:
(1) Major drainageways are defined as those with a tributary area in excess of 351 acres. Such major drainageways shall be designed so that no structural damage shall occur when the average recurrence interval of 100 years is used for hydraulic analysis of the drainageways. Storm detention channel construction or other improvements may be necessary to pass the 100-year design storm through existing downstream sections of the drainageway.
(2) Secondary waterways are defined as those with a tributary area between 100 and 350 acres. Such secondary waterways shall be designed so that no structural damage shall occur when the average recurrence interval of fifty years is used for hydraulic analysis of the drainageways. Storm detention channel construction or other improvements may be necessary to pass the fifty-year design storm through existing downstream sections of the drainageway.
(3) Minor waterways are defined as those with ninety-nine acres or less of tributary area. Such minor drainageways shall be designed so that no structural damage shall occur when the average recurrence interval of twenty-five years is used for hydraulic analysis of the drainageways. Storm detention channel construction or other improvements may be necessary to pass the twenty-five year design storm through existing downstream sections of the drainageway.
In addition to or in lieu of the above noted standards, the City Engineer may utilize a water surface profile program, such as HEC-2 (Hydraulic Engineering Center Software Program, U.S. Army Corps of Engineers), to determine the appropriate size of system components and to provide the maximum amount of cost benefit for each area.
(c) Stormwater Runoff.
(1) Land uses which increase the runoff rate or volume shall be required to control the discharge rate of runoff prior to its release to off-site land. The purposes of this policy are to:
A. Permit development without increasing the flooding of other lands.
B. Reduce damage to receiving streams and impairment of their capacity which may be caused by increases in quantity and rate of water discharged.
C. Establish a basis for design of a storm drainage system on lands below undeveloped areas which will preserve the rights of property owners and assure the long-term adequacy of storm drainage systems.
(2) Developers are required to design improvements such that after development, the rate of stormwater runoff leaving the project area at strategic points is no more than if the project area had remained undeveloped. If necessary, detention/retention facilities shall be constructed to assure that this requirement is met.
(3) Where a portion of the project area contains existing development features, such as buildings, streets, parking lots, sidewalks, driveways, etc., and a permit application is made for additional development, the area to be considered in determining the rate of stormwater runoff shall be only that on which the additional development is sought.
(4) The City Engineer may reduce or waive requirements for an individual detention/retention basin if a common or regional basin of adequate design is available, or if the Stormwater Management Utility is reasonably certain that one will be constructed, and if the major drainage system from the project area to such common or regional basin is such that the public health, safety and welfare will not be in jeopardy.
(5) Stormwater management systems will be designed for the ultimate use of the land and shall be capable of conveying runoff originating upstream as determined by the City Engineer.
(d) Acquisition of Easements for Public Facilities on Private Property. In those case where components of a stormwater management system, designed to be operated as a public facility, lie outside the public right of way, easements of sufficient size, as determined by the City Engineer, shall be acquired from the owner of the property to assure access and maintenance.
(e) Detention/Retention Basins.
(1) Detention/retention basins serve to capture and, temporarily or on a long-term basis, store the surface water runoff which results from urban development. This storage allows for the release of the stormwater runoff at discharge rates which are acceptable to the receiving waterway.
(2) On-site provisions of detention/retention storage shall conform to the following standards:
A. The release rate and volume of storage shall be based on the 100-year storm interval design for post development conditions and be released at a five-year storm interval rate for pre-development conditions;
B. Recreational use should be maximized where possible and feasible.
(Ord. A-1947. Passed 3-14-94.)