(A) Stormwater Collection and Retention Plan. The Stormwater Collection and Retention Plan shall include but not be limited to the following:
(1) The watershed boundaries, both on-site and off-site, shall be delineated with any existing drainage or irrigation structures, such as waste or delivery ditches, natural drainage channels and the like, and the proposed development’s impact on existing features;
(2) A topographic map which shows the location of the project area; a 1- or 2-foot contour interval shall be used as the base map for both existing and proposed. The map shall also show the location of the property with respect to the street system and other features such as existing and proposed stormwater retention basins and the like;
(3) Method of collection (surface and/or subsurface);
(4) Depth, side slopes and volume of retention basins;
(5) Calculations showing retention required and provided;
(6) Method of disposal of water within 36 hours;
(7) Areas tributary to each retention basin;
(8) The development’s low outfall elevation and location relative to city datum shall be explained and the location on all watershed and topographic maps shall be identified;
(9) The drainage pattern of all streets within and adjacent to the proposed development on the drainage maps shall be indicated;
(10) A preliminary retention basin plan, including size, depth and possible methods of draining the retention basin, shall be proposed;
(11) Areas within any 100-year floodplain, including any FEMA-designated floodplain, shall be indicated; and
(12) All stormwater plans and preliminary drainage reports shall be prepared and signed by a professional civil engineer who is registered by the State of Arizona.
(B) Drainage reports. Drainage reports shall be submitted for approval by the city with the site improvement plans. The report shall be typewritten on letter-size sheets, with necessary maps folded and inserted into the report in the proper order. All drainage reports shall be prepared and sealed by a qualified professional civil engineer registered in the State of Arizona. All elevations shown on the plans shall be referenced to a benchmark on the city datum unless otherwise approved by the city engineer.
(1) Preliminary drainage report. A preliminary drainage report shall be required for all commercial, industrial, multi-family and residential subdivision developments. The report must be presented with preliminary development plans. Complete engineering reviews shall not occur without this report, which shall be on separate, letter-size, typewritten sheets, with necessary maps and attachments. Handwritten comments on a plan sheet are not acceptable and shall be returned unapproved. The preliminary drainage report shall include, but not be limited to:
(a) Adequate maps that delineate the boundaries of on-site and off-site drainage areas;
(b) Maps or drawings that indicate the drainage pattern of all existing drainage, irrigation, structures, streets and the like, and proposed streets and building sites. The different critical points and where inlets/outlets are to be located shall be labeled. The inlets/outlets are to have the same numbers as the drainage areas draining to them;
(c) Each sub-area is to be analyzed for the peak flow generated by a 10-year and 100-year storm events. Use of the rational method is preferred for drainage areas of less than 160 acres. Hydrology and hydraulics design methods shall be consistent with the Maricopa County Drainage Design Manual, Volumes I, II and III;
(d) Storm drain design computations;
(e) Inlets shall be analyzed separately and catch basin computations shall be submitted;
(f) The off-site storm runoff flows shall be described in detail. Drainage area, calculated peak flows, and other pertinent runoff data must be clearly set forth. If the flow is in a defined channel, the channel can be improved. An 8-foot bottom to accommodate maintenance vehicles may be required. Special consideration of the use of open channels is to be given on each individual project by the city. Channels for the runoff from areas outside the development may be realigned through the new development. Special precautions at the point of discharge for flow rate, velocity, erosion and sedimentation shall be evaluated to prevent adverse effects on downstream properties;
(g) The retention volumes required by the formula defined in Vol. II, § 10-4-10 (B)(2)(b) :
Drainage area (square feet) x drainage area coefficient (C) x 0.2 x 1.1 (freeboard) = retention volume in cubic feet; and
(h) Proposed retention basin location(s), size(s) and means of stormwater conveyance and disposal shall be shown on the drainage map(s). Narrative discussion and supporting calculations shall be provided in the body of the report.
(2) Final drainage report. A final drainage report shall be required and accompanied with the drainage and retention agreement (Exhibit 10-4.1, following the text of this article) to be recorded. The final drainage report shall include but not be limited to:
(a) Cover sheet.
1. With submittal number, name and address of project, parcel or development for which the report is submitted;
2. Name, address and phone number of engineer and property owner; and
3. Seal, signature and date of State of Arizona professional civil engineer.
(b) A narrative with topographic maps that describe the location and condition of the property the project is located on (on-site conditions); and the upstream (off-site) watersheds, as well as any downstream constraints which affect the property;
(c) Calculations demonstrating required retention volume, tributary areas to each basin and volume provided, and basin grades, depth, high water elevation, pipe invert elevations, basin outfall elevation and side slopes;
(d) Retention basin percolation test results;
(e) Elevation and location of low outfall elevation for the development, sub-area or site improvement;
(f) Explanation of the effect of a basin overflow due to back-to-back storms or a storm greater than the design storm;
(g) Street capacity calculations for the 10-year and 100-year storm events;
(h) Time of concentration calculations;
(i) Storm drain piping and catch basin hydraulic calculations, including the hydraulic grade line (“HGL”);
(j) The routing of off-site flows through or around the proposed development;
(k) The city datum benchmark to which all site and facility elevations are referenced;
(l) Tailwater and backwater elevations at all culverts;
(m) Lowest floor elevations of all habitable structures;
(n) Method of disposing of retained stormwater within 36 hours, and provide shallow pit percolation test results and calculations;
(o) Inflow and outflow points at retention/detention basins and culverts;
(p) Points of concentration and intake point for catch basins, scuppers, channels and street intersections;
(q) Calculations of the retention basin(s) drain time, the number of drywells per city criteria and percolation test results;
(r) Drainage report based upon computerized hydraulic models, with all values and variables identified in the report. Software manuals and documentation shall be made available upon request by the city;
(s) Long-term maintenance responsibility shall specify the name, address, and phone number(s) of the person(s), firm(s) or agency responsible for ownership, operation, liability and maintenance of drainage improvements. Other documents where these responsibilities are documented (i.e., CCRs, final plats and the like);
(t) Catch basin V-depth calculations; and
(u) All hydrology and hydraulic calculations as required by the city that includes a summary of the following of each drainage area:
1. Retention required and provided;
2. Street capacity and depths; and
3. Catch basin and storm drain pipe capacity, HGL and velocities.
(Ord. passed - - ; Ord. 1511, passed 10-5-2021)