The following policies relate to required stormwater storage information to be provided with subdivision drainage studies submitted to the town for approval. When applicable, exceptions to these policies may be made at the discretion of the Town Engineer under special conditions.
(A) General policies.
(1) Stormwater detention/retention shall be incorporated within subdivisions, commercial or industrial developments. The detention/retention system shall have the effect of ensuring that, at a minimum, post-development 2-, 10- and 100-year 24-hour peak discharges from the development will not exceed the corresponding pre-development values.
(2) The use of rooftops as storage areas for runoff is not an acceptable method of meeting the detention/retention criteria of the town.
(3) Individual lot-storage systems within single-family residential developments are discouraged.
(4) Finished floors of structures shall be a minimum of one foot above the 100-year water-surface elevation of any adjacent detention/retention basin(s).
(5) No part of any storm water detention/retention facility for a private development shall be constructed within any public right-of-way.
(6) Detention/retention areas within subdivisions shall lie within a separate tract or easement dedicated for drainage management purposes.
(7) The use of dry wells is prohibited for draining detention or retention basins.
(B) Detention policies.
(1) The intent of a detention basin is to reduce the storm water peak flow leaving a site. Detention basins shall be designed to maintain the peak post-development flows to be at or below the pre-development flows for the 2-, 10- and 100-year 24-hour storms.
(2) The only outlet allowed in the bottom one-third by depth of a detention basin is a two-inch orifice. Above the bottom one third depth, the orifice shall be designed to restrict the post development flows to no more than the predevelopment flows for the 2-, 10- and 100-year 24-hour storms. The actual outlet pipe the orifice supplies shall be a minimum of 18 inches in diameter unless otherwise approved by the Town Engineer. For the purposes of this section, the bottom one-third of any detention basin shall not exceed one foot in depth.
(3) Gravity drainage of a detention basin must be used where available. If gravity drainage is not available, a pump system may be used. If a pump system is used, it shall conform to the following requirements:
(a) The pump system shall be privately owned, operated, and maintained.
(b) The pump system shall be equipped with an automatic control switch with a vertical float mechanism, or equivalent device to turn on and off automatically.
(c) The pump shall be located in an inlet box or wet well. The wet well shall be located at the edge of the detention basin where it can be maintained even if there is water in the detention basin.
(d) The wet well shall have a gate or other type of control device that can restrict the flow into the wet well.
(e) The pump discharge flow rate shall meet the requirements of § 152.31(B). Multiple staging of pumps may be required to meet such requirements.
(4) On-line detention facilities shall not be permitted on natural watercourses or watercourses that drain more than 100 acres upstream of the detention-basin outlet structure, unless written approval to do so is obtained from the Town Engineer.
(5) If a parking lot is used as a detention area, the maximum depth of ponded water within any parking area shall not exceed one foot for the 100-year storm. Planning of areas within a parking lot, which accept ponding, should be such that pedestrian inconvenience is minimized. Deeper areas should be confined to remote areas of parking lots, whenever possible. All detention areas shall be permanently signed to notify users of the potential flood risk in accordance with § 152.101 (B)(4).
(6) The minimum longitudinal slope permitted within parking-lot storage facilities is 0.005 foot per foot, unless concrete valley gutters are provided. With concrete valley gutters, a minimum longitudinal slope of 0.002 foot per foot may be permitted.
(7) It is desirable that all detention basins be emptied within 36 hours. However, the requirements of this Code may dictate that it will require more than 36 hours to empty the basin. In that case, the basin shall be emptied in no more than 72 hours. The drainage report shall indicate how long it will take to empty each basin.
(8) All detention basins shall have a minimum freeboard of 1 foot above the 100-year 24-hour peak stage in the basin.
(9) In addition to meeting the standard detention basin requirements, underground storm water detention shall meet the following requirements:
(a) Underground detention shall be designed with adequate access for inspection and maintenance.
(b) Final plats for subdivisions utilizing underground detention shall (1) state who is responsible for inspection and maintenance of the underground detention, (2) provide a funding source for such inspection and maintenance, and (3) have the following note, "The owner shall retain a registered civil engineer, or other personnel as approved by the Town Engineer, to provide annual inspections of all detention facilities and provide annual reports to the Town of Payson Town Engineer."
(c) The final improvement plans for all developments utilizing underground detention shall have the following note on such improvement plans, "The owner shall retain a registered civil engineer, or other personnel as approved by the Town Engineer, to provide annual inspections of all detention facilities and provide annual reports to the Town of Payson Town Engineer."
(C) Retention policies.
(1) Stormwater retention basins are generally not encouraged within the town, due to potential problems associated with long-term ponding of storm water. Retention basins shall be emptied by infiltration. The maximum depth allowed for a retention basin is one foot unless adequate percolation is provided.
(2) Retention basins shall be designed to reduce the peak discharge from the 2-, 10-, and 100-year 24-hour storm to no more than the pre-development peak discharge.
(3) Maximum disposal time for storm water runoff for retention facilities is 36 hours.
(4) Retention systems that utilize subsurface disposal of storm water shall incorporate some form of infiltration enhancement system such as gravel-filled engineered basin floors or trenches to minimize sealing of the basin bottom. A soils report, including a site-specific percolation test, shall be required in order to demonstrate that the basin will drain within the maximum disposal time of 36 hours. Percolation tests shall be performed in accordance with State of Arizona, ADEQ, Aquifer Protection Permit Rules, Chapter 9, Section R18-9-A310. Authority for ADEQ to establish rules in this regard comes from A.R.S. Title 49, Article 3. Approval of a site-specific percolation rate by the Town Engineer is discretionary. If the percolation rate is not acceptable, the basin must be drained by other acceptable means within the required 36-hour period. An Aquifer Protection Permit may be required by ADEQ for disposal of storm water using retention basins.
(D) Points of compliance.
(1) The initial point of compliance for the reduction of storm water flows shall be at all locations where flows exit the development site.
(2) Additional points of compliance downstream from the development property may be required by the Town Engineer.
(3) Post-development sheet storm water flows leaving a development shall not exceed the pre-development storm water sheet flows.
(E) Maintenance policies.
(1) A maintenance plan shall be prepared in conjunction with the detention/retention basin design that includes both scheduled and unscheduled maintenance activities and who is responsible for such maintenance. Scheduled maintenance includes such items as mowing, pruning, and trash removal that are to be performed on at least an annual basis. Unscheduled maintenance includes repairs, usually made necessary by storms and floods, which are discovered either during regularly scheduled annual inspections, or during inspections made after flooding. The maintenance plan shall be approved by the Town Engineer prior to approval of the final improvement plans. The owner of the detention/retention basin shall maintain records of inspections and maintenance and provide them to the town when requested.
(2) Maintenance ramps or other access shall be provided into detention/retention facilities in order to facilitate scheduled and unscheduled maintenance activities. Access easements shall be provided to all detention/retention facilities.
(3) Maintenance of local detention/retention facilities, provided in conjunction with new developments, shall be the responsibility of the private property owner or HOA. The town shall reserve the authority to periodically inspect privately owned detention/retention basins to ensure satisfactory maintenance is being provided. If the town identifies deficiencies that will affect the intended operation of the facility, the town will provide written notification by registered mail of such deficiencies to the responsible party and/or property owner. The maintenance entity will be given 30 calendar days from the date of receipt of written notification to correct the deficiencies. A variance to the 30-day repair period may be granted at the discretion of the Town Engineer based on progress or extenuating circumstances. Violations are subject to the penalties described in § 152.999.
(4) Final Plats, Development Plans and CC&R's shall have notes stating:
(a) Who shall be responsible for the operation, maintenance, and liability for detention/retention systems.
(b) The town may periodically inspect the detention/retention facilities to verify that scheduled and unscheduled maintenance activities are being performed adequately.
(F) Variances.
(1) A variance or variances to one or more of the requirements of this section may be granted upon a showing that both:
(a) On-site detention results in an increase in peak discharge at locations in the watershed downstream of the proposed project due to timing of peak discharges. There must be sufficient hydraulic capacity in the downstream drainage conveyance system to convey the proposed condition 100-year peak discharge to the town corporate boundary without damaging homes or property, either within or outside the town. The proposed condition peak discharges may not result in any increase in the future condition peak discharges listed in the PDMP; and
(b) The quality of the storm water runoff will not reduce the quality of any downstream waters.
(2) The Town Engineer may grant a variance to one or more of the requirements of this section if the developer provides downstream drainage improvements such that:
(a) The existing flows are directed into a channel or storm drain that has adequate capacity to handle the flows, and
(b) There is a downstream location that can be improved to enhance existing downstream drainage facilities and neighborhoods, and
(c) The quality of the storm water runoff will not reduce the quality of any downstream waters.
(3) Any variance requested pursuant to this subsection, must be requested in writing. If the Town Engineer approves such variance, such approval shall be in writing.
(4) If a variance is granted, the Town Engineer may assess the developer a fee equal to the estimated cost of constructing the on-site detention facilities. This fee will be deposited with the town prior to issuance of a grading permit. The fee will be placed in an impound account and used for future construction of regional detention or drainage conveyance facilities. The total amount of the fee will be based on the difference between the post-development and pre-development runoff volume for the site from a 100-year 24-hour storm. A standard fee based on dollars per acre-foot of excess runoff volume will be set by the Town Engineer, and revised on an annual basis.
(Res. 1637, passed 2-28-02; Am. Res. 2272, passed 5-17-07; Am. Ord. 714, passed 5-17-07; Am. Res. 2871, passed 9-17-15; Am. Ord. 872, passed 9-17-15)