(A) Drainage and topography shall be a primary consideration of any subdivision.
(B) The preservation of natural flood areas, streams, washes, arroyos, rivers or ephemeral drainage courses shall be maintained, if possible, in their natural riverine environment. The limits of the ten (10) year flood event shall define the extent of the area of concern. The only exceptions are for roadway crossings and utility lines, if no other alternative exists. Any subdivision proposal which proposes the preservation of natural flood areas shall be considered superior to all others.
(C) Drainage layout and development shall meet all state and federal requirements to allow residents of Show Low to purchase flood insurance, to receive disaster relief, to obtain real estate loans and to minimize flood damage.
(D) Subdivision improvements that propose grading and/or grade changes shall not have an adverse impact on surrounding property. At the boundaries of the subdivision, all drainage and floodwaters shall be accepted and released so that the flow characteristics are minimally disturbed by providing appropriate entrance and exit transitions.
(E) All-weather access to all lots shall be provided during the regulatory flood and shall mean depths of flows over streets will not exceed one (1) foot to allow passage of emergency vehicles. The standard applies to both public and private streets.
(F) All drainage ways that convey fifty (50) cubic feet per second or more, during the one hundred (100) year flood event, shall be considered a regulated drainage course and shall be dedicated to the public with provisions for maintenance access ramps. Flows less than fifty (50) cubic feet per second shall be regulated for impacts to buildings and structures, particularly the placement of the finished first floor or basement, and shall be designated a common area or noted on the final plat as impacting a lot and the lot owner’s responsibility for maintenance.
(G) Design engineer shall provide sizing calculations for any required drainage pipes, including future driveway culverts.
(H) Drainage basins or watersheds with known flood hazards shall be designated as a critical basin. Within designated critical basins, all proposed subdivisions shall address on-site detention for the two (2), ten (10) and one hundred (100) year flood event to mitigate the post-development drainage to the pre-development levels. Each of the three (3) design storm events shall be routed through an outlet structure. A stage, storage, and discharge table shall be provided in the drainage study. The minimum stage increment shall be one-half (1/2) of a foot. The study shall include a table comparing the pre- versus post-development runoff for the three (3) design storm events. If the developer can demonstrate that on-site detention will exacerbate the downstream condition then the public works director may waive the requirement. Downstream constrictions and obstructions must be considered. If the proposed project is the re-development of an existing developed commercial or industrial parcel, the detention requirement may be waived by the public works director if the developer can demonstrate that the impervious area is reduced with the new development.
(I) A drainage study which addresses the hydrologic and hydraulic components relating to on-site and off-site drainage shall be developed and prepared by a registered Arizona professional engineer. The drainage study shall be approved before the street improvement plans and final plat are approved. If the subdivision will be developed in phases, a master drainage plan will be required.
(J) Storm water detention facilities are to be constructed prior to roadway grading.
(K) The drainage report must include a narrative section describing the project, the hydrologic and hydraulic methods used, a comparison of the pre- versus post-conditions, a discussion of hydrographs, downstream impacts, floodplain impacts, and a discussion regarding the proposed detention facilities, conclusions and recommendations, or a determination that detention is not required.
(L) The drainage report calculations must include hydrographs that illustrate the difference in the pre- and post-developed storm events (two (2), ten (10), and one hundred (100) year). Provide hydraulic design calculations for any required culverts, pipes and structures, roadside ditches, and detention basins and their outlet structures. If detention is required, pond routing calculations shall also be provided.
(M) The drainage report shall include exhibit maps of the tributary drainage areas for both pre- and post-development. The maps should show all drainage sub-basins and their areas and the calculated flows for each sub-basin.
(N) Exterior slopes of detention ponds shall not exceed 2:1. Exterior slopes exceeding 3:1 are strongly discouraged and any exterior slopes exceeding 3:1 shall be rip rap-armored. Every effort shall be made to keep interior slopes of detention ponds at 4:1 or flatter. In no case shall the interior slopes exceed 3:1. In cases where the interior slopes exceed 4:1 or the pond is in excess of three (3) feet deep at the emergency overflow, a six (6) foot high fence is required. The fence shall be constructed of materials that allow visual observation of the pond interior from the outside.
(O) In order to prevent erosion of slopes, interior and exterior slopes of storm water detention facilities (excluding those treated with rip rap) shall be planted or seeded with perennial plant materials. Prior to seeding, the soils shall be prepared or topsoil placed in order to promote growth. Mineral clay soils will not be acceptable for seeding.
(P) An operation and maintenance manual shall be provided for storm water detention facilities. The manual shall provide directions regarding how to properly maintain the facility. This must include instructions for weed and sediment removal. The frequency of the maintenance interval shall not exceed one (1) year. Sediment removal should be scheduled to take place just prior to seasonal monsoon rains. The operation and maintenance (O&M) shall be the responsibility of the homeowners association and the O&M manual shall be an attachment to the CC&Rs for the development. If a development does not have a homeowners association, the O&M manual will be required in the form of a recorded agreement. This agreement shall assign permanent responsibility for the operation and maintenance of the facility. (Ord. No. 2008-12, §§ 1, 2, 9-16-08; Ord. No. 2019-02, § 1, 2-5-19. 1976 Code § 12-3-5)