(A) When a retention basin is utilized to handle storm water runoff in a commercial or residential subdivision, the following design standards shall be applicable:
(1) The minimum storage volume shall be computed by estimating the area under a hydrograph of a 100-year storm of two hours duration (1.22 inches per hour). In addition, the greater of one foot of freeboard or 25% additional basin capacity shall be provided in the facility.
(2) Maximum basin depth shall not exceed 3.5 feet unless otherwise approved by the Public Works Director. If a basin is approved to exceed the maximum depth, the facility shall be enclosed with a six foot high masonry block fence and a 12 foot wide gate with lock.
(3) Maximum side slope shall be 4:1 unless otherwise approved by the Public Works Director.
(4) An acceptable method of transporting water from the street into the basin shall be provided so as to preserve the integrity of the basin walls and floor.
(5) All residential streets shall be designed to carry the runoff from a ten-year, two hour storm between the curbs. In cases where peak flows from the design storm exceed the street capacity, storm sewers of sufficient size to carry the excess must be installed. The minimum pipe size for underground storm drainage shall be 12 inches. The peak flow from a 50-year storm must be carried within the cross-section between buildings (front yards and streets) for subdivisions served by retention basins. The floor elevations of all habitable space shall be above flood levels equivalent to a 100-year storm.
(6) All streets with curb shall have a minimum longitudinal slope as shown in the table below, designed in accordance with the Public Works Director approval.
Table 3: Street Slope
Minimum Slope
| |
Straight (400 feet radius and over) curb | 0.20% |
Curved (less than 400 feet radius) curb | 0.25% |
Valley gutter flow-line | 0.30% |
(7) Construction of the retention basin should follow the La Paz County Public Works Design and Construction Manual.
(8) Each retention basin shall be constructed so that storm water will not stand longer than five days after termination of the storm. Basin drainage shall be accomplished by infiltration or by controlled flow into an existing facility suitable to receive, transport and dispose of storm water. When controlled flow is proposed, the owner/developer and the town shall obtain approval from the owner of the existing facility, prior to formulating final drainage for the retention basin. Final drainage plans shall be subject to the review and approval of the Public Works Director.
(9) Subdivisions with more than one retention basin should interconnect all basins.
(10) The owner/developer shall maintain the portion of drainage improvements that fall within their subdivision following the La Paz County Public Works Design and Construction Manual.
(11) The owner of the property on which the retention basin is to be located shall grant a right to use said property for drainage purposes. Such grant shall be made by means of document which shall be approved by the Town Attorney and recorded in the office of the La Paz County Recorder and shall contain the following provisions:
(a) Legal description of the property to be used for storage purposes;
(b) Legal description of the property which is permitted to drain the basin;
(c) A statement that the owner is responsible for construction and maintenance of the basin in accordance with standards established by the Public Works Director;
(d) A statement that no habitable structure may be constructed within the basin. Statement that the property shall be used for storm water storage so long as required in the opinion of the Public Works Director;
(e) Such other provisions as are deemed by the Town Attorney to be necessary to effectuate the provisions of this chapter.
(B) In lieu of the requirements contained in division (A), the owner may dedicate the property to be used for storm water retention to the town for drainage and for public use. Such dedication shall become effective only upon acceptance of the basin by the Town Council. Conditions of acceptance by the town may include but are not limited to:
(1) The owner/developer shall include desert landscaping, rocks, decomposed granite, a drip irrigation system, and lighting as approved by the Public Works Director.
(2) Shall be located adjacent to arterial streets (mile and half-mile streets) and the geometrics of the basin shall be coordinated with the Public Works Director.
(3) With the approval of the Public Works Director, a retention basin may be located on other streets, which allow a direct storm sewer route to an arterial street. Additionally properly designed and sized inlets, manholes, storm sewer lines and all other appurtenances shall be constructed to a point where the storm sewer may be connected to a future storm main on an arterial street.
(C) When on-site retention is utilized in residential subdivisions, the following criteria shall be applicable:
(1) Minimum lot size for an on-site retention shall be one acre.
(2) A deed covenant prohibiting alteration of the final site ground elevations shall be required.
(3) The maximum design depth of storm water storage on any lot shall not exceed eight inches except as approved by the Public Works Director.
(4) The lot depth below the back edge of the sidewalk shall be computed using a 100-year storm of 24 hour duration with 3.05 inches of total rainfall. An allowance for evaporation and/or percolation shall not be incorporated into the computation for minimum lot depth. Storage volume shall also be provided for adjacent streets and alleys except for arterial streets.
(5) Each lot shall be provided with an earthen berm around the back and sides of each lot. The earthen berm shall be one foot higher than the adjacent final ground elevation of the lot.
(6) All residential streets shall have curb and gutter designed in accordance with the La Paz County Public Works Design and Construction Manual and Public Works Director approval. An appropriate means of transporting water into the storage area shall be provided.
(7) Slope away from building shall not exceed 21:1 for a distance of five feet out from foundation.
(8) Construction of the retention basin should follow the La Paz County Public Works Design and Construction Manual.
(9) Owner shall maintain the portion of drainage improvements that fall within their parcel following the La Paz County Public Works Design and Construction Manual.
(D) When on-site retention is utilized in commercial and industrial developments, the following criteria shall be applicable:
(1) All storm water from a 100-year storm of two hour duration (2.44 inches) must be retained on site.
(2) Dry wells will be permitted to drain surface retention areas. However, the infiltration cannot be considered to reduce the size of retention areas.
(3) The owner shall be responsible for the design, performance, operation or maintenance of dry wells or retention basins in commercial or industrial developments.
(4) Dry wells shall not be located within 20 feet of any building.
(5) Construction of the retention basin should follow the La Paz County Design and Construction Manual standards.
(6) Owner shall maintain the portion of drainage improvements that fall within their parcel following the La Paz County Design and Construction Manual standards.
(E) Combination storage shall consist of providing adequate storage volume for the appropriate design storm by a combination of on-site retention and a retention basin. All requirements and conditions outlined in the previous provisions of this section shall apply.
(Ord. 21-11, passed 10-26-2021)