(A) Stormwater, general requirements.
(1) Subdivisions, commercial projects, site plans and development projects shall make adequate provision for stormwater conductance and stormwater storage for the increased runoff due to development. The City Engineer shall make the determination of adequate provision.
(2) Stormwater systems shall be designed by a state-licensed professional engineer.
(3) The City Engineer may approve justified and substantiated approaches and designs for storm water in compelling situations. The Rational Method is an approved methodology, other types of analysis are acceptable as approved by the City Engineer.
(B) Rainfall hydrology. Rainfall data shall be based on NOAA Atlas 14 precipitation data in the vicinity of the project.
(C) Conveyance elements.
(1) Stormwater conveyance facilities (i.e., piping, ditches, culverts, and the like) shall be based on the ten-year storm event for post-development conditions.
(2) Structures are to be above 100-year flow/flooding conditions.
(3) Maximum flow in streets with curb and gutter conditions shall be based on flows being within eight feet of spread from the curb, and per inlet capacity conditions, but not to exceed 500 feet before interception by curb inlets or catch basins.
(4) The designer shall demonstrate that ditch or swale erosion will not be excessive.
(a) Velocities are to be kept to less than four fps. Piping systems may have higher velocities.
(b) Piping shall be designed to have a flowing full velocity of at least two fps.
(5) Minimum curb and gutter slopes shall be 0.5%; vertical curves shall not be utilized on slopes less than 1%; and minimum asphalt slopes shall be 1.2% with 1.5% being preferred.
(6) Outfall locations shall have adequate provision for erosion control.
(7) A primary matter to be determined upfront in the subdivision development process is whether storm drainage facilities shall be privately-maintained or city-maintained.
(a) In general, facilities in public street rights-of-way will be publicly maintained and those across private property will be privately maintained, which must have appropriate, perpetual access.
(b) Access shall be a minimum of 15 feet in width with an all-weather, year-round maintained surface, 20 feet is preferred.
(8) Storm drain lines that are considered part of the city’s storm drain system shall be reinforced concrete pipe (RCP), of appropriate class.
(a) Other types of pipe are only allowed on a case-by-case basis per approval by City Engineer.
(b) The minimum size for storm drain mains shall be 15-inch diameter.
(c) Public storm drain pipes shall not be curved.
(d) Storm drain lines shall have clean-out boxes, inlets, or manholes installed at all changes in grade or alignment, with a maximum distance of 500 feet between accesses.
(e) Curb inlets with throat openings are preferred over grated-inlet-only appurtenances.
(f) Detailed plans, calculations, and specifications shall be submitted for approval by the City Engineer for all storm drainage appurtenances.
(9) Runoff coefficients shall generally be C=0.95 for impervious hardscape and C=0.15 for flat area softscape.
(a) Values from published sources are permitted as approved by the City Engineer.
(b) Commercial site plans shall be based on 85% hardscape and 15% softscape unless demonstrated otherwise.
(10) Projects shall have responsibility to route existing or encountered ditch, irrigation, spring water, or tile drain flows in an acceptable manner through or around projects.
(D) Storm water storage.
(1) Subdivision, commercial projects, and site plans shall store on-site (or an approved alternative location) the increased runoff due to development.
(2) Historical upstream flows are permitted to be conducted through a project without detainage.
(3) Projects shall clearly and distinctly identify the downstream outlet.
(a) Where a major watercourse is located downstream, projects may pipe directly to such a course and not be required to do on-site storage.
(b) The basic premise is that projects may not unduly impact downstream intervening properties between the site and watercourse.
(c) Permission must be secured by the appropriate entity one is discharging to.
(4) In general, stormwaters shall not be conveyed in irrigation ditches, and irrigation waters shall not be conveyed in storm drain systems. Where historical storm flows have entered irrigation ditches, such may be considered and analyzed for strictly historical-only flow perpetuation by City Engineer, project engineer, and irrigation entity.
(5) Storm drainage basins are required for all development; however, residential developments of less than one acre are not required to have detention or retention, except when determined by the City Engineer.
(6) (a) For the purpose of this subchapter, the following definitions shall apply unless the context clearly indicates or requires a different meaning.
DETENTION FACILITIES. Storm facilities that have an approved outlet.
RETENTION FACILITIES. Storm facilities that do not have an approved outlet.
(b) Due to relatively low permeable soils in the city area, percolation is not considered an acceptable outlet, nor can be included in storm water storage calculations.
(E) Basin property, easement, and access.
(1) In general, the city does not assume responsibility for the ownership and maintenance of stormwater drainage basins for private development. In unique cases where a public basin is to be adopted and implemented, the public basin shall be located on a separate parcel dedicated to the city with an approved permanent access.
(2) Private basin ownership and maintenance shall be clearly delineated in a development agreement with the city, with well-delineated and understood parameters including enforcement provisions.
(a) Typically, private basins serving multiple lots shall be owned and maintained by a homeowners’ or landowners’ association.
(b) If deemed appropriate, a basin may be located on a private parcel with an appropriate easement with maintenance responsibilities delineated.
(c) Basins shall be constructed with sufficient, all-weather, drivable access to all structures, with an appropriate turnaround.
(d) Actual ownership and responsibility shall be specifically defined in the owner’s dedication, certificates, development agreements, long-term stormwater maintenance agreements, or by deed.
(3) Dry-well type sumps are not allowed except as approved by the City Engineer on a case-by-case basis.
(F) Basin volume.
(1) Detention basins shall be minimally based on the ten-year storm event with an allowable release rate of 0.1 cfs per acre.
(2) Retention basins are based on the 100-year, two-hour storm event.
(3) Due to relatively low permeable soils in the city area, percolation is not considered in basin volume calculations.
(4) All basin designs and calculations shall be included in a stamped/signed drainage report submitted to and approved by the City Engineer.
(5) Volume shall be measured to the internal (overflow) spillway elevation.
(6) Volume in roadside ditches or swales shall not be considered in the volume calculation for detention or retention basins.
(7) Required storage of water shall not be allowed to surface in parking lots except in extreme and compelling circumstances, generally involving existing developed situations.
(8) Calculations shall be based on the total acreage of development draining to the basin.
(9) Pass-through of off-site drainage through the development must be considered and will be allowed.
(G) Basin characteristics.
(1) Basin side slopes shall not be steeper than 4:1 (horizontal to vertical).
(2) It is preferred to have a bottom of basin minimum slope of 2%.
(3) It is preferred to have piping that allows base low-flow, historical flows to be directly discharged through basin structures rather than by necessity having to pass through the basin causing wet, soggy, or unstable conditions.
(4) Fencing of basins is to be determined on a case-by-case basis.
(5) Basins must be landscaped or have sufficient long-term vegetation, ground cover, or hardscape (cobble).
(a) A long-term weed-free environment is mandated.
(b) Public and private basin landscaping shall be included in an escrow or other financial agreement with the city, which shall extend through the one-year warranty period.
(c) Basin landscaping must be demonstrated to be permanently established for release of warranty monies, regardless of time period (i.e., it may be longer than one year).
(6) One foot of freeboard above spillway is required, and spillway is to be designed for 100-year event.
(7) Underground chambers are allowed on a case-by-case basis. Proper separation from groundwater and seasonal groundwater rise and fluctuation must be accounted for in the design.
(8) Low-impact development (LID) matters will necessitate implementation as required by state mandates.
(H) Floodplain areas.
(1) The City Council may, when it deems it necessary for the health, safety, or welfare of the present and future population of the area and necessary to the conservation of water, drainage, and sanitary facilities, prohibit the subdivision of any portion of the property which lies within the floodplain of any stream or drainage course.
(2) These floodplain areas shall be preserved from any and all destruction or damage resulting from clearing, grading, or dumping of earth, waste material, or stumps, except at the discretion of the Planning Commission.
(I) Dedication of drainage easements.
(1) General requirements. Where a subdivision is traversed by a watercourse, drainage way, channel or stream, there shall be provided a stormwater easement or drainage right-of-way conforming substantially to the lines of such watercourse, and of such width and construction or both as will be adequate for the purpose of high flow conveyance.
(2) Drainage easements.
(a) Where topography or other conditions are such as to make impractical the inclusion of drainage facilities within road rights-of-way, perpetual unobstructed easements at least 15 feet in width for such drainage facilities shall be provided across the property outside the road lines and with satisfactory access to the road. Easements shall be so indicated on the plat.
(b) Drainage easements shall be carried from the road to a natural watercourse or to other drainage facilities.
(c) Depending upon the size and type of drainage facilities, a wider easement drainage easement may be required per the City Engineer.
(d) When a proposed drainage system will carry water across private land outside the subdivision, appropriate drainage rights or easements must be secured and indicated on the plat.
(e) The developer shall dedicate, either in fee or by drainage or conservation easement, land on both sides of existing watercourses, to a distance to be determined by the Planning Commission. All said easements shall be deemed rights-of-way for lawful municipal purposes.
(Ord. 2020-006, passed 3-9-2021) Penalty, see § 150.999