(A) Retention methods. Allowable methods of providing stormwater retention are:
(1) Surface storage. Where surface storage (i.e., the traditional retention basin) of the required retention is planned or provided, the basin(s) shall comply with the following:
(a) Irregular areas. Retention is to be accommodated within a limited number of well-defined, irregular-shaped areas. Pursuant to the Apache Junction City Code, Vol. I, multiple small retention basins are not acceptable for either public or private developments, unless approved by the city engineer for public developments, and by the development services project engineer for private developments, and are in conformance with city standards.
1. To count separate retention areas as one, they must have the same high water elevation and be connected by equalizer pipes.
2. On-lot retention is prohibited in all residential subdivisions.
3. Maximum depth of the required retention that shall be retained within a surface retention basin is 3 feet (see division (D)(1) below).
4. The high water elevation shall be lower than the adjacent grades of adjoining property unless an engineered berm is approved by the city.
(b) Within city right-of-way or PUE.
1. Retention facilities (surface basins, underground pipes, tanks and the like) are to be located within the private property or “on-lot” of the proposed land development. Right-of-way areas shall not be excavated, depressed or encroached upon without prior approval of the City of Engineer. When allowed, the retention facility shall conform to division (D)(1).
2. Retention area(s) including the high water elevation limits for the design storm can encroach into public utility easements (“PUE”) subject to:
a. Adequate protection and cover is maintained for any existing public utilities.
b. Buoyancy calculations for the public utilities, whether existing or proposed, are required when the pipe diameter is larger than 16 inches.
c. Where wastewater manholes exist or are planned within the high water limits, the manholes shall have a watertight frame and cover per MAG Standard Detail 423, and the rim shall be higher than the high water elevation of the design storm.
(2) Paved surface storage. In commercial and industrial areas, retention may be accomplished upon paved areas (e.g., impervious surfaces such as asphalt, concrete and the like). See division (E) of this section.
(3) Underground storage. Underground storage utilizes storage tanks, vaults, pipes and the like to place the required stormwater retention volume underground on the development site. Underground storage is permitted within residential and non-residential development.
(a) The design engineer shall consider the following during the design process for any project involving underground storage:
1. Venting of the structure;
2. Access to the underground structure for routine maintenance;
3. Structural loads, including any surface loads;
4. Buoyancy of the structure; and
5. Drainage of the structure within the required 36 hours.
(b) The engineer shall include in the drainage report for the project the justifications, design criteria, operational and maintenance information for the underground structure and any associated equipment such as pumps.
(c) See division (L) below for underground storage guidelines.
(B) Sizing.
(1) Basis of design.
(a) All retention/detention facilities shall be sized to retain the storm event falling over the entire project (gross area, including streets) (see division (B)(2)(a) and (b) below). For the purpose of determining the volume required, the project shall be considered to extend to the centerline of all existing and/or future streets on the exterior boundaries, unless it is unpractical to capture those flows (e.g., the street is separated from the project by a drainage channel), and to include all interior streets and other rights-of-way within the project.
(b) There shall be a minimum of 1 foot freeboard from the water surface elevation to the lowest building elevation and/or the gutter of the upstream streets.
(2) Volume required. The volume required for each retention/detention basin shall be calculated as follows:
(a) North of Baseline Avenue:
V R =1.1 x 0.2 (Cpost - Cpre) x A
where:
V R = Retention volume required (cubic feet)
1.1 = An additional 10% of retained volume to account for losses due to sedimentation, weed growth and the like
0.2 = 10-year, 24-hour depth of rainfall (2.4 inches/12)
Cpost = A coefficient relating the runoff to rainfall for the proposed construction (per FCDMC Drainage Design Manual, Volume 1, Table 3.2).
Cpre = A coefficient relating the runoff to rainfall for the existing condition (per FCDMC Drainage Design Manual, Volume 1, Table 3.2).
A = Drainage area, including ½ of all abutting streets (square feet)
(b) South of Baseline Avenue:
V R = DC x A
where:
V R = Retention volume required (cubic feet)
D = 100-year, 2-hour depth of rainfall per current NOAA Atlas Documents
C = a coefficient relating the runoff to rainfall (per FCDMC Drainage Design Manual, Volume 1, Table 3.2).
A = drainage area, including ½ of all abutting streets (square feet)
Drywell volumes shall not be included in the proposed storage capacities.
(3) Location.
(a) Retention/detention basins shall be located such that they can intercept the flows from all developed portions of the site not used to convey off-site flows.
(b) If the basin is located other than at the lowest point of the project, the developer’s engineer shall denote on the master drainage map the actual or effective drainage area. If portions of the project cannot drain to the primary basin, additional basins shall be added to retain runoff from these areas. Credit will not be given for providing volume in excess of that needed to retain the required storm event from a basin’s effective drainage area.
(C) Volume certification. The developer will provide the city with certified as-built dimensions of the basins and the actual volume of storage provided. This must be based on as-built topographic surveys made by either a civil engineer or land surveyor who is registered to practice in the State of Arizona. These as-built volumes must reflect permanent finished landscaping in place. The design engineer shall certify that the volumes provided meet or exceed the required design volumes per city ordinance and the approved drainage plan. The volume of storage provided must equal or exceed the approved design volumes before the city engineer will issue letters of acceptance for maintenance of any public facilities.
(D) Grading.
(1) Depths.
(a) Retention basins are not permitted within the city ROW. Where unusual circumstances are encountered, retention basins may encroach in city ROW with prior written approval from the city engineer. When retention basins are allowed to encroach, the basins shall not extend more than 10 feet into the city ROW and shall not exceed 1.5 feet of water depth within the city ROW, unless there is a fence or other similar protection to restrict access to the area.
(b) The overall average depth shall not exceed 3 feet without authorization of the city engineer. If granted, the basin must be fenced to prohibit access or a side slope of 8:1 shall be provided for a minimum distance of 25 feet, measured from the 100-year high water level.
(c) While it is the city’s intent that the “average” depth not exceed 3 feet, it is also the city’s intent that the basins be contoured to present an aesthetically pleasing appearance as determined by the city engineer. Therefore, up to 25% of the bottom area may be up to 4 feet deep.
(d) In no case shall the depth exceed 1 foot without a positive means of disposing of accumulated runoff.
(2) Slopes, side and bottom.
(a) Bottom. The bottom of all basins shall be sloped towards the discharge points. The minimum bottom slope shall be 0.5%.
(b) Side slopes.
1. Side slopes adjacent to public rights-of-way, or when there is pedestrian-type access to that portion of the basin, shall have a side slope of 6:1 or flatter.
2. Side slopes adjacent to walls, fences, hedges and the like (e.g., no or limited pedestrian-type access in that area) may have side slopes up to 4:1.
3. Retaining walls (e.g., vertical slopes) may be used in areas adjacent to permanent walls, fences and the like.
(3) Grading/landscaping/joint use as parks.
(a) 1. It is the intent of the city that retention/detention basins present an aesthetically pleasing appearance. The developer’s engineer shall contour the sides and bottoms of the basins to enhance appearance through varied slopes.
2. The developer and designer shall work with representatives of the city’s development services department and parks and recreation department to determine the need/desirability and feasibility of joint usage of the basin as a park site. If appropriate, the design shall provide for appropriate open areas for the recreational facilities. All design shall be approved by the city parks and recreation department.
(b) It is not the intent of these standards to dictate the specific details of the configuration to the designers; however, the following concepts will be used as the basis of reviewing the plans:
1. Curvilinear sides should be used in lieu of long stretches of straight lines.
2. Side slopes should be varied (e.g., start with 6:1 then change to 7-8:1 or more). With appropriate use of landscaping, side slopes can even be reduced to 4:1.
3. Bottom areas should contour to varying depths in lieu of uniform depth/slope.
(c) The tops and bottoms of side slopes shall be rounded off, generally over a distance of 5 feet each way of the curve point of intersection (“PI”).
(d) All landscaping within city ROW, easements or dedicated tracts shall be approved by the city parks and recreation department.
(E) Retention/detention in parking lots.
(1) Retention/detention in parking lots of multi-family developments is not allowed. All retention/detention of such developments shall be in landscaped areas or underground vaults.
(2) Retention/detention of runoff in parking lots of industrial/commercial developments is allowed, subject to the following guidelines:
(a) No more than 50% of the required storage volume may be retained/detained in parking areas. The balance shall be provided in landscaped areas and underground vaults. The tributary areas to each basin shall be noted on the master drainage map.
(b) No more than 50% of the required parking spaces shall be covered by stormwater retention/detention.
(c) Storage system shall be designed to store the first 30% of the required runoff volume off paved areas (to avoid nuisance water constantly ponding on the pavement).
(d) Depth of water shall not exceed 6 inches within the parking area, nor shall it exceed 0.15 feet at the midpoint of any parking space.
(e) Interference with pedestrian traffic will be minimized in the design of the storage facility.
(f) A continuous fire access lane shall be provided throughout the development, and it shall be free of ponded water from the retention areas.
(g) All parking spaces shall be accessible during periods when the basins are filled to capacity, without pedestrians having to cross ponded water deeper than 0.15 feet.
(3) Before final plan approval, an approved drainage report must show the calculated stormwater storage volume based on runoff from the 100-year, 2-hour storm, or the pre-approved 10-year, 24-hour storm for the Downtown Area and in-fill projects north of Baseline Avenue.
(F) Overflow/outfall.
(1) Each project shall be designed such that the “ultimate” outfall for all drainage in excess of the 100-year, 2-hour storm is routed to a public street, storm drain, drainage channel or natural watercourse. The outfall shall be accessible without draining over private property.
(2) If such an outfall does not exist, the project must provide an outfall.
(G) Overflow/conveyance.
(1) Off-project flows which historically flowed through the project may be routed through the project. Off-site runoff volumes shall not be allowed across private lots, streets or public/private access ways.
(2) Runoff volumes in excess of those required to be retained/detained may be routed directly through the outfall, although they shall be routed via the retention/detention facilities.
(H) Location/conflicts with existing utilities.
(1) Retention/detention facilities shall not encroach into existing easements for private utilities without written approval of the encroachment from all utilities using the easement.
(2) Retention/detention facilities shall not encroach into public ROW nor into public easements. If necessary, the developer shall relocate conflicting utilities into a new dedicated easement.
(3) The top of the retention/detention facilities (e.g., freeboard elevation) shall be at least 4 horizontal feet from any building or public roadway.
(4) Retention/detention facilities shall not be located within 20 feet of an active septic system nor within 100 feet of an active water well.
(5) A minimum 3 feet of cover (from the bottom of the basin to the top of the pipe) shall be maintained over water and sewer service lines.
(I) Disposal/discharge.
(1) All retention/detention facilities shall have a positive method of disposing of retained/detained runoff waters. All water so retained/detained shall be disposed of within 36 hours. Public streets are not considered an acceptable outlet for disposal of retained/detained runoff; however, they are considered an acceptable outlet for overflow. Only under special circumstances, with prior city engineer approval, should pump disposal methods be used.
(2) The minimum allowable pipe size for primary outlet structures is 12 inches. A headwall (MAG Standard Detail 501-4) and an access barrier shall be constructed at the outlet.
(3) Acceptable methods of disposal of accumulated stormwater runoff are:
(a) Positive gravity outlet.
1. Surface infiltration. A percolation test is required in each retention basin location to determine natural percolation. Test results shall be submitted to the city engineer for approval of the retention basin(s).
2. Drywells.
a. Drywells are permitted when no other means of disposal are available. Infiltration into the drywell cannot be considered to reduce the size of the retention area.
b. The property owner of record shall be responsible for the design, performance, operation or maintenance of drywells used with on-site retention.
c. Drywells shall comply with the ADEQ publication Guidance for Design, Installation, Operation, Maintenance, and Inspection of Drywells, and the additional requirements described herein. A copy of the application for registration by ADEQ of the proposed drywell(s) shall be submitted prior to approval of grading plans. Drywells shall comply with the ADEQ publication Guidance for Design, Installation, Operation, Maintenance, and Inspection of Drywells and the additional requirements described herein. A copy of the application for registration by ADEQ of the proposed drywell(s) shall be submitted prior to approval of grading plans.
e. Multiple drywell installations shall be located a minimum of 100 feet apart, unless waived by the city engineer, and a minimum of 20 feet away from a basin inlet.
f. All drywells draining runoff from more than 1 acre of pavement are to be a dual chamber type similar to the MaxWell dual chamber drywell (Torrent Resources) or approved alternate.
g. For any project involving hazardous materials, including fuels, the drywell must include the “Envibro” Drainage System (Torrent Resources) or approved alternate.
h. Drywells shall be located a minimum of 100 feet away from water wells and underground storage tanks (except stormwater underground storage tanks).
i. Top of the drywell grate shall be set 2 inches above the bottom elevation of the retention basin.
j. Number and type of drywells are to be called out on the engineering plans.
k. Each drywell will be percolation tested after installation and a 50% value of its disposal rate shall be used, up to but not exceeding 0.5 cfs. The percolation test results are to be submitted to the development services project engineer with the final as-built drawings of the stormwater collection and retention plan.
l. Drywells which encounter perched water shall be sealed in the perched water zone of the well.
m. All drywells shall be registered with the ADEQ and constructed by an ADEQ licensed contractor. The approved drywell registration shall be submitted to the city by the developer at the time as-builts are submitted. A tabulation showing drywell number, registration number, and percolation rate will be added to the grading plan coversheet before submitting as-builts.
n. The property owner of record shall be responsible for the design, performance, operation and maintenance of drywells used with on-site retention.
o. Drywells that cease to drain a project area in a 36-hour period shall be replaced by the maintenance authority with new ones.
p. Drywells are not to be located within public street ROW or private street roadway tracts unless authorized by the city engineer.
3. Storm drain discharge. Discharge to an existing storm drain with a maximum discharge of 1 cubic foot per second. A waiver is required.
4. Drainage channel discharge. Discharge to a drainage channel, either natural or man-made, of sufficient capacity to convey the anticipated flows from the tributary drainage area. A waiver is required.
5. Prohibited discharge. Water cannot be discharged into a city street, gutter or alley.
(b) Pump station (waiver required).
1. Permitted direction:
a. To an open channel, either natural or man-made;
b. Subsurface direction to a nearby storm sewer system with a maximum discharge of 1 cubic foot per second; or
c. Surface to a storm sewer system if pumped water can be discharged directly into a catch basin or other inlet.
2. Water cannot be discharged into a city street, gutter or alley.
3. Pump stations shall comply with the requirements of Chapter 9 of Volume II of the FCDMC Drainage Design Manual, except as noted below.
a. Pumping facilities shall be set at an elevation at or above the anticipated level of the 100-year event, considering that a total power failure may occur.
b. Pumps shall be capable of handling solids up to a maximum of 3 inches.
(J) Nuisance water. Each basin, particularly those used as a park, shall be graded such that there are one or more “sump” areas wherein runoff from the more frequent storms and nuisance runoff may be retained/detained without flooding the balance of the basin, with preference to surface percolation.
(K) Embankments.
(1) Detention or retention facilities should be constructed below the natural ground surface.
(2) The use of embankments to impound stormwater runoff requires prior approval by the development services project engineer. Embankments become small dams that can be a serious potential downstream flood hazard.
(3) If approval is obtained, all the design requirements shall comply with Section 8.2.3, Embankment Design Criteria, of Chapter 8 of Volume II of the FCDMC Drainage Design Manual.
(4) The developer must provide the city with as-built certification by a State of Arizona registered geotechnical or civil engineer experienced in dam technology, that the embankment was designed and constructed properly, is stable and will safely impound the design volumes of water.
(L) Underground retention storage requirements.
(1) Underground retention shall be permitted in residential and non-residential uses.
(2) Each request for underground storage will be evaluated on an individual basis and permitted if approved by the development services project engineer.
(3) The following guidelines are not meant to be exclusive and, based on the type of underground storage approved, may require additional data and details be submitted to the city engineer.
(a) Installation of corrugated metal pipe (“CMP”) shall be in accordance with MAG Standard Specification No. 621, MAG Standard Detail 510, and a note on strutting spacing, if required, demonstrating that they are within the manufacturer’s specification and recommendation for installation. Excavation, bedding and backfill shall be in accordance with MAG Standard Specification No. 601 and material per MAG Standard Specification No. 760;
(b) Provide the depth of groundwater and the depth of the proposed installation. Provide soil boring results to at least 10 feet below the bottom of the proposed drywell;
(c) Demonstration of a 50-year life of the installation (lining and coating must be specified). Aluminized CMP is usually good for more than 50 years, depending on the soil chemistry;
(d) Traffic/load bearing capacity of the installation. Pipe gauge and corrugation size of CMP and D-load for RCP must be specified;
(e) The storage pipe must drain into a dual chamber drywell. Include a detail of how the installation will be drained into the drywell. The standard drywell detail does not address anything other than normal installation. The sedimentation chamber and drain must be lower than the tank drain so the tank drains completely. The system must drain within 36 hours;
(f) Provide a backfill detail. Include material and compaction requirements, particularly under the haunches and to the springline of the pipe;
(g) Provide a minimum-30-inch diameter manhole shaft at each end of underground pipes up to 150 feet in length. Underground pipes longer than 150 feet require an additional minimum-30-inch manhole shaft every 150 feet or fraction thereof. All manhole shafts shall be equally spaced no more than 150 feet, measured from center to center.
1. A 30-inch manhole frame and cover can be used at grade, with a concrete collar where subject to wheel loads. Provide assurance that the pipe will structurally accommodate the manholes where there is traffic loading on the manhole.
2. The manhole shaft is to be 30 inches in unobstructed diameter. No projections or obstructions are permitted into the manhole shaft. If the manhole shaft is to be provided with a ladder, the distance from any part of the ladder, measured on the side of the ladder to the opposite wall of the manhole shaft, shall not be less than 30 inches. This may result in the necessity of a larger diameter manhole to accommodate a ladder. A detail of how the ladders are to be anchored, and the material to be used for the ladders in order to assure a 50-year life, must be specified.
(h) Provide assurance that the material used for the piping is suitable for the site’s soil (letter from the Soils Engineer);
(i) Specify water-tight manufactured joints;
(j) Provide end walls for pipe per manufacturer’s recommendation with a detail or MAG Standard Detail 427;
(k) Cover to be 3 feet minimum in traffic areas (or manufacturer’s recommendation); and
(l) RGRCP is suggested for strength and durability. Structural strength calculations based on subgrade capacity are required in areas subject to wheel loads.
(4) The above requirements are to be shown or specified on the plan or otherwise satisfactorily addressed. The requested assurances are to be provided for the city’s project file.
(M) Operation and maintenance.
(1) Maintenance of on-site detention or retention facilities within the city shall be the responsibility of the property owner.
(2) All retention basins that will be maintained by the city shall be improved by the developer per city standards for retention basin development, and installed prior to the city’s acceptance of the retention. Retention basins, when not privately maintained, shall be dedicated to the city in fee title as stormwater retention basins or drainage ROW. In the case where private retention basins receive water, other than that which falls upon the property and adjacent streets and/or alleys, the areas shall be designated as easement areas for retention purposes, and shall have a recorded restrictive covenant requiring perpetual maintenance.
(N) Retention basins within SRP transmission line ROW.
(1) Retention basins are to be designed to the latest SRP’s requirements.
(2) Developer shall submit drawings to the city and SRP for approval. SRP will consider both existing and planned future power facilities in their review. SRP approval letter shall be forwarded to the city engineer before final city approvals.
(Ord. passed - - ; Ord. 1511, passed 10-5-2021)