(A) In addition to the installation of curbs or gutters along the streets as required by this Manual, storm sewers and other drainage appurtenances shall be constructed throughout the entire subdivision to carry off water from all inlets and catch basins, and be connected to an adequate outfall. The storm water drainage system shall be separate and independent of the sanitary sewer system and shall be in accordance with drainage laws of the city and the state. Specifically, the flow rate and velocity of post-development storm water runoff from the site shall not exceed the flow rate and velocity of pre-development run-off from the site. Storm water detention, retention or other BMP structures shall be designed, constructed and maintained to achieve this.
(B) During the final improvement plan phase, the entire subdivision shall be analyzed for the effects of the 100-year storm to assure that buildings are properly elevated (lowest structure opening one foot above the 100-year storm hydraulic gradeline) above the floods occurring beyond the design capabilities of the storm water structures. Trenches shall be backfilled as required herein.
(C) The developer shall be responsible for constructing adequate facilities for the control, collection, conveyance, acceptable discharge of storm water, other surface water and subsurface water which may be detrimental to the safe and convenient use of any portion of the area. The storm drainage system shall provide for runoff from the entire area of the subdivision. It shall take into account land outside the subdivision limits, which normally drains across the area of the subdivision as well as the effects of the subdivision upon downstream drainage systems. As primary focus, the drainage system for the subdivision shall make use of, protect and improve, as needed, the natural drainage system. Drainage facilities shall be designed and constructed in accordance with the standards and procedures specified in these regulations.
(1) Design storm. All storm drainage facilities shall be designed with sufficient capacity and freeboard where necessary to convey the peak rates of runoff from storms with the average return frequencies listed below.
(a) Two-year. Water quality BMPs which will be required in the future to comply with the city’s NPDES storm water Phase II permit requirements.
(b) Ten-year. All drainage facilities within road and easements from catch basins grates to outlet structures. Pavement encroachment, or maximum in-street spread, for drainage should be designed according to § 1-303.01 of the Illinois Department of Transportation Drainage Manual.
(c) Twenty-five year. All cross culverts under drives, minor and local streets, as well as improved streams, swales and ditches. Pavement encroachment, or maximum in-street spread, for drainage should be designed according to § 1-303.01 of the Illinois Department of Transportation Drainage Manual.
(d) Fifty-year. All cross culverts and bridges constructed under residential collector and arterial streets.
(e) One hundred year. All detention/retention basins and bridges.
1. The lowest opening elevation in all habitable buildings shall be the 100-year hydraulic gradeline (HGL) elevation, plus one foot.
2. Drainage ditches, open channels and natural drainageways shall have zero created head compared with predevelopment conditions at the upstream property line along said drainageway up to the 100-year flood frequency if any increase in flooding up to the 100-year frequency storm affects upstream structures or collector roads (or higher category roads), unless the developer acquires permanent flood easements to cover the area with increased flooding levels up to the 100-year flood event from the parties upstream whose structures and/or roads (collector level or higher) will experience an increase of flooding.
3. Drainage ditches, open channels and natural drainageways which have no potential to increase flooding upstream of structures or collector roads (or higher category roads) up to the 100-year event, shall not have a created head greater than 0.1 feet at the upstream property line of the project property along said drainageway unless easements are obtained from the upstream property owner to cover the increases of flooding up to 100-year event, or unless there is a storm sewer, culvert, bridge, dam or other drainage structure within 200 feet downstream of the upstream property line on said drainageway in which case the created head shall not be greater than 0.5 feet at the upstream property line of the project property along said drainageway unless easements are obtained from the upstream property owner to cover the increases of flooding up to 100-year event.
(2) Storm water discharge.
(a) The discharge of all storm water shall be into an established wetlands, watercourse or drainage structure as approved by the city. Where the discharge shall be into or through private property, proper easements or drainage rights in a form acceptable to the City Attorney shall be secured by the developer for the city.
(b) Permanent easements, at least 20 feet in width, shall be provided in all cases where storm drainage facilities are installed in land other than the street right-of-way. The centerline of pipes shall be no closer than five feet and the top of the slope for channels shall be no closer than five feet to the boundaries of the drainage easements. Easements shall also be provided for storm drainage facilities that may need to be installed in the future to serve underdeveloped land within the watershed that normally drains across the area of a proposed development. All easements shall be clearly delineated and described on the final plat.
(3) Other requirements.
(a) In all instances, unless otherwise waived, a drainage analysis map shall be submitted showing the tributary watershed area, sub-drainage basins, and the downstream area affected by runoff. Drainage computations shall consider the entire tributary area (on-site and off-site) of those drainage basins contributing runoff to all design points. See also § 155.035 of this chapter.
(b) Roadway underdrains shall be required where a soils report states this is needed, and their installation shall be performed by the developer and as directed by the city or its duly authorized agent, to protect the stability of the roadway.
(c) Suitable head wall or precast end sections shall be provided at the open end of any pipe. Culverts under streets shall have a minimum cover of 30 inches and shall be extended to a minimum of ten feet from the edge of pavement, unless otherwise approved by the City Engineer.
(d) Catch basins shall be provided so that no portion of any road shall drain in one direction more than 300 feet without catch basins on both sides of the road, unless otherwise approved by the City Engineer.
(e) The hydraulic capacity and the required size and slope of storm sewer pipes and channels shall be established by using the Manning equation. The hydraulic capacity of driveway and roadway cross culverts shall be established only after considering both the inlet and outlet control conditions. The lower of the two flow rates obtained shall be the actual rated capacity. The upstream backwater shall not encroach onto adjacent properties unless backwater is contained within existing watercourse or wetland limits, does not encroach upon roadway beyond existing IDOT standards or driveway areas, and the necessary drainage easements are secured from those affected property owners.
(f) House and foundation drains shall in no case be permitted to discharge onto a roadway surface. Discharge to existing wetlands, watercourses and storm drainage facilities shall be made. All such drainage connections shall be made prior to construction of, or be made so as not to cause damage to, pavement surface. Any damage to roadway or its appurtenances will be the responsibility of the property owner from whose property the drain is discharging.
(g) All pipes and drainage structures shall be thoroughly cleaned by the developer before acceptance by the city and again following final construction of homes or structures within the subdivision. The developer shall supply a letter of credit to secure such cleaning.
(h) Existing wetlands/watercourses proposed to receive storm drainage discharge shall be analyzed to determine the downstream effects on any watercourse or existing storm drainage system for its adequacy to receive the proposed drainage discharge. The extent to which downstream studies are conducted shall be commensurate with the probable impact of the proposed development. Where it is anticipated that the additional discharge resulting from the proposed subdivision will overload the existing downstream drainage system, the city may deny the subdivision until the applicant has adequately provided for improvements to the drainage system.
(i) Each lot on any final plat or final development plan shall have identified on the plat of record an elevation height that shall be the minimum elevation of the lowest opening (first floor, walkout basement or basement window) to adjacent grade. This elevation shall be determined and established by the elevation of the 100-year storm in any drainageway or structure adjacent to that lot or surrounding area that may subject that lot to potential flooding from any of those drainageways. That elevation will then be required to have one foot of freeboard. All lots shall be noted on the plat of record as either suitable or unsuitable for walkout basements.
(j) When the street side of any principal structure will be constructed below roadway elevation, the developer shall submit documentation from an engineer showing how they propose to protect the structures from storm water runoff in excess of the design capacity of the roadway.
(k) Detention basins shall be located on outlots maintained by the homeowner’s association. A special service area may be created for potential maintenance by the city should the homeowner’s association become defunct, unable or unwilling to provide sufficient maintenance of the detention basins.
(l) Additional design standards, as appropriate, may be required by the City Engineer.
(m) Scour protection shall be provided for all ditches and storm sewer discharges. The amount of scour protection shall be determined by the flow rate and velocity of the storm water in the drainage structure.
(Ord. 3319, passed 2-22-2005)