(A) General requirements. The city will not recommend for approval any plat of a subdivision which does not make adequate provision for storm or flood water run-off channels or basins. The storm water drainage system shall be separate and independent of any sanitary sewer system. Storm drainage shall be accommodated in the streets, gutters, or retention basins unless otherwise indicted by the City Engineer. Sub-surface drainage systems shall be discouraged, but where storm sewers are required, the cost shall be born by the developer for storm sewers to a size of 24 inches; if over 24 inches in size and determined to be for benefit outside of the subdivision, the costs may be shared, as determined by the City Council, but only for that portion determined to be of benefit outside the subdivision.
(1) Use of street for drainage. Curbed streets shall be designed and constructed to carry the storm water run-off from a ten-year storm between curbs. When peak flows form the designated storm exceed the street capacity, an open area drainage and retention system shall be designed to carry the excess storm water. The rational method based on the run-off factors in division (B) below shall be used to determine the peak flows. Fifteen minutes maximum may be used for the time of concentration for the runoff across the lots.
(2) Peak flows in streets. Peak flows from a 100-year storm shall be carried within the cross section between buildings (front yards and streets). The finish floor elevation of all buildings shall be one foot above the 100-year storm. All finished floors shall be a minimum of 14 inches across the low outfall of the site.
(3) Accommodation of upstream drainage areas. A culvert, gutter, or other drainage facility shall in each case be large enough to accommodate potential run-off from its entire upstream drainage area, whether inside or outside the subdivision. The City Engineer shall determine the necessary size of the facility, based on the provisions of the construction standards and specifications assuming conditions of maximum potential watershed development permitted by the zoning ordinance (Chapter 154 of this code.)
(4) Effect on downstream drainage areas. The City Engineer shall also study the effect of each subdivision on the existing downstream drainage facilities outside the area of the subdivision. Local government drainage studies together with the other studies as shall be appropriate, shall serve as a guide to needed improvements. Where it is anticipated that the additional run-off incident to the development of the subdivision will overload an existing downstream drainage facility, the City Council may withhold approval of the subdivision until provision has been made for the improvement of the potential condition in a sum as the City Council shall determine. No subdivision shall be approved unless adequate drainage will be provided to an adequate drainage will be provided to an adequate drainage watercourse or facility.
(5) Areas of poor drainage. Whenever a plat is submitted for an area which is subject to flooding, the City Council may approve the subdivision provided that the applicant fills the affected area of the subdivision to an elevation sufficient to place the elevation of streets and lots at a minimum of 12 inches above the elevation of the maximum probable 100-year flood, as determined by the City Engineer. The plat of the subdivision shall provide for an overflow zone along the bank of any stream or watercourse, of a width which shall be sufficient in times of high water to contain or move the water, and no fill shall be placed in the overflow zone nor shall any structure be erected or placed therein. The boundaries of the overflow zone shall be subject to approval by the City Engineer.
(6) Floodplain areas. The City Council may, when it deems it necessary for the health, safety, or welfare of the present and future population of the area and necessary to the conservation of water, drainage, and the sanitary facilities, prohibit the subdivision of any portion of the property which lies within any 100-year floodplain as determined in the zoning ordinance (Chapter 154 of this code.) These floodplain areas shall be preserved from any and all destruction or damage resulting from clearing, grading, or dumping of earth, waste material, or stumps, except at the discretion of the City Council.
(B) Retention requirements.
(1) Areas for retention. All water from a 100-year storm of a one-hour duration (approximately 2.4 inches), which falls within the area being developed, including the respective one-half of all abutting streets (whether or not it is a dedicated street which may exist by an improvement district and owned by the city) shall be retained within the boundaries of the developed land. No retention shall be allowed in public rights-of-way. The method of collection and retention shall be approved by the Department of Public Works. The method of retention calculation, drainage flows, and removal of storm water within 36 hours shall conform to division (B)(5) and (6) below.
(2) Retention basin capabilities. All retention basins shall have a design capacity to preclude a water depth in excess of three feet resulting from a 100-year, one-hour storm. The depth of retention basins shall be measured from lowest adjacent top of curb. In no event shall storm water stand in the retention basins longer than 36 hours. Drywells (or other approved methods of removing storm water within 36 hours) shall be required for any depth of retention over one foot, provided a volume greater than1,000 cubic feet is being retained in a single retention area. However equalizer pipes from smaller to larger retention areas may be required by the City Engineer.
(3) Dedication. No retention basins will be controlled or owned by the city unless dedicated as part of the city’s park and open space system. Non-dedicated retention basins shall be maintained by the owner. All retention basin areas shall be designated as easement areas for retention purposes and shall have a recorded protective covenant requiring perpetual maintenance.
(4) Residential lots for retention. On-lot retention is permissible in single-family developments to a maximum of six inches, but shall not be used for calculating development retention requirements.
(5) Retention calculations.
Total volume required = (2.4 in/12) x (drainage area) x (average run-off coefficient)
(6) Run-off factors.
(a) The run-off factor used in computation must be justified by a typical cross section of actual area calculation.
Concrete pavement (concrete, brick, etc.) | 0.95 |
Roofs/asphalt pavement | 0.85 |
Grass lawn | 0.15 |
Desert lawn or rock lawn | 0.70 |
(b) Infiltration into the dry well cannot be considered to reduce the size of the retention area. The property owner of record shall be responsible for the design, performance, operation, or maintenance of dry wells used with on-site retention. A percolation rate of 0.8 cfs per dry well over a 36-hour period shall be used in calculating the number of dry wells.
(7) Joint development. Two or more developers may join together to provide a common retention facility. A letter of agreement signed by all developers participating in the common retention area must be presented to the City Engineer, and the recorded plat shall indicate that the retention area is a joint facility. The joint retention area must meet all criteria as a single area.
(C) Dedication of drainage easements.
(1) General requirements. Where a subdivision is traversed by a watercourse, drainage way, channel, or stream, a storm water easement or drainage right-of-way shall be provided conforming substantially to the lines of the watercourse, and of the width and construction, or both as will be adequate for the purpose. Wherever possible it is desirable that the drainage be maintained by an open channel with landscaped banks and adequate width for maximum potential volume of flow.
(2) Drainage easements.
(a) Where topography or other conditions make impractical the inclusion of drainage facilities within road rights-of-way, perpetual, unobstructed easements at least 15 feet in width for the drainage facilities shall be provided across property outside the road lines and with satisfactory access to the road. Easements shall be included on the plat. Drainage easements shall be carried from the road to a natural watercourse or to other drainage facilities.
(b) When a proposed drainage system will carry water across private land outside the subdivision, appropriate drainage rights must be secured and indicated on the plat.
(c) The applicant shall dedicate, either in fee or by drainage or conservation easement, land on both sides of existing watercourses, to a distance to be determined by the City Council.
(d) Low-lying lands along watercourses subject to flooding or overflowing during storm periods, whether or not included in areas for dedication, shall be preserved and retained in their natural state as drainage ways.
(Prior Code, § 15-4-5) (Ord. O88-06-007, passed 10-13-1988)
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