(A) Storm sewers and appurtenant drainage structures shall be provided and constructed in compliance with the requirements contained in these regulations and in compliance with construction standards and specifications governing storm sewers and drainage facilities approved by the Council and on file with the City Engineer.
(B) Unless otherwise specifically designed, the developer of a subdivision is responsible for the following:
(1) All surface drainage within the subdivision;
(2) The improvement, enlargement or replacement of all drainage channels lying within the subdivision to accommodate potential runoff from the entire upstream drainage area, whether inside or outside the subdivision; and
(3) The maintenance of all floodways and flood fringe areas which have not been dedicated to the public.
(C) In making the determination of stream flow and runoff, the developer shall be responsible for having an engineer registered in the state prepare a drainage assessment of all the area of the subdivision, all upstream drainage areas affecting the sizing of all drainage channels and structures and all of the area affected by runoff resulting from development of the proposed subdivision, in accordance with the following provisions:
(1) The maximum flood to be used for the purpose of determining all runoff for the sizing of drainage channels and structures shall be contained in city specifications approved by the Council and on file with the City Engineer.
(2) Values used in formulas for runoff and size of drainage structures shall be based on maximum potential urbanization of the watershed, as set forth in the projected land use pattern contained in the comprehensive plan of the city.
(3) The calculation of stream flow and runoff characteristics of the subdivision shall be carried out in consultation with the City Engineer and the methodology and formulas used shall result in quantities which would not be less than those derived from the application of the following formulas:
(a) Runoff from all drainage areas shall not be less than that determined by the Rational Formula. In using the Rational Formula, 100-year storm frequency shall be used for rainfall intensity.
(b) The size of closed storm sewers, open channels, culverts and bridges shall not be less than that determined by the Manning Formula.
(4) The City Engineer shall review and approve all stream flow and runoff calculations as may be required of a developer.
(D) Prior to the approval of any plat of subdivision, the Council shall evaluate the drainage improvements proposed by the developer to determine whether the improvements are in accordance with the provisions of these regulations.
(1) The developer shall be required to install adequate storm sewer and drainage facilities in accordance with the requirements of these regulations connecting a public storm water drainage system, except as provided herein.
(2) The City Engineer shall evaluate the effect of each subdivision on existing downstream storm drainage facilities outside the area subdivision. Where it is anticipated that drainage improvements are necessary outside the boundaries of the subdivision or where existing downstream storm drainage facilities will be adversely affected by magnitude or velocity of storm waters to be carried by the proposed drainage system, the Council may withhold approval of the subdivision until provision has been made for the necessary drainage improvements as determined by the Council.
(3) In residential subdivisions containing four or more platted lots, in residential multiple-family subdivisions and in all non-residential subdivisions, underground storm sewer systems shall be constructed in accordance with the requirements of this chapter, except where the combined drainage area served by the proposed storm drainage facility exceeds 40 acres, in which case an open improved storm drainage channel may be used for the conveyance of storm water.
(4) In residential subdivisions containing three or fewer platted lots, the strict requirement for underground storm sewers may be waived by the Council, provided that the developer makes adequate provision for the disposal of storm water, subject to the specifications of the City Engineer and the final approval of the Council.
(E) The storm water drainage system shall be separate and independent of any sanitary sewer system. New or replacement water supply and sanitary sewage systems shall be designed and installed to minimize or eliminate infiltration of floodwaters into water systems or discharges from sewage systems into floodwaters. On-site waste disposal systems, where use is permitted, shall be designed, located and constructed to avoid impairment of them or contamination from them during flooding.
(F) Storm sewers, where required, shall be designed by the Rational and Manning Formulas or other methods as approved by the Council and a copy of design computations shall be submitted along with plans.
(G) All channels shall be sized in accordance with city specifications, shall be designed for self-cleaning and ease of maintenance, shall have sufficient hard surface along the flow line to prevent ponding of water and shall have design characteristics of alignment, materials of construction and cross-sectioned elements that will be hydraulically efficient and visually harmonious with the adjacent landscape.
(H) Open improved storm drainage shall be in accordance with the following:
(1) Where the combined drainage area served by the proposed storm drainage facility exceeds 40 acres, it is desirable that drainage be maintained by an open improved storm drainage channel, with sloped and sodded banks designed and constructed to control erosion. The banks shall be sufficiently wide to carry a 50-year maximum flood.
(2) Where special or unusual conditions make open channels hazardous or otherwise unfeasible, enclosed storm sewers may be required by the City Engineer.
(3) An open storm drainage channel shall not be located in a street easement, except under the following conditions:
(a) Where a paved street surface is located on both sides of the drainage channel to give access to abutting properties;
(b) Where lots are platted in a way that the drainage channel is located between the side or rear lot lines and the paved street, provided that there shall be no access constructed over the open drainage channel to the side or rear of lots so platted; and
(c) Adequate space adjacent to the channel has been dedicated as right-of-way to provide for maintenance of the paved drainage channel and its unpaved bank.
(I) Surface drainage shall be in accordance with the following:
(1) Surface drainage patterns shall be shown for each and every lot and block. In single-family residential, duplex or mobile home development, site grading shall be carried out in a manner that surface water from each lot shall flow directly to a storm sewer, improved channel, sodded swale or paved street without crossing more than four adjacent lots.
(2) Surface water collected on streets shall be diverted to storm drains at satisfactory intervals to prevent overflow of six-inch high curbs during a 25-year frequency rain for the area and grades involved. Regardless of flow, inlets shall be provided so that surface flow in gutters shall be diverted to storm sewers by inlets at intervals not exceeding 990 feet.
(J) Site improvement shall provide for the grading of all building sites and streets to an elevation where all lots, building areas and streets will not be subject to overflow and in a manner that will provide for the rapid runoff of all rainfall. Whenever possible, it is desirable that the improvements are carried out in a manner that will preserve and protect large trees and attractive physical features of the area.
(K) All storm drainage facilities, bridges, culverts and other drainage structures shall be constructed in accordance with the specifications and design criteria of the city when the city shall have present or future maintenance responsibility.
(Prior Code, § 13-541) (Am. Ord. 1974, passed 8-16-19; Am. Ord. 2025, passed 4-19-22)