[HISTORY: Adopted by the City Council of the City of El Paso 2003 by Ord. No. 792 (Ch. 16.34 of the 1979 Code). Amendments noted where applicable.]
No development shall be authorized in the City unless there is either an adequate stormwater outlet or the City has approved stormwater retention or detention facilities. The applicant shall show by his detailed calculations that the outlet is adequate, and the calculations must be acceptable to the City. In the event the City determines the outlet is not adequate, the developer shall construct retention or detention facilities. Such facilities shall be designed based on the design formula. In lieu of providing retention or detention facilities, the developer may be asked, at the option of the City, to pay a fee.
[Amended 2-6-2017 by Ord. No. 994]
In all developments not already served by an adequate existing or proposed stormwater detention/retention facility, the developer shall provide a stormwater detention/retention facility or, at the option of the City, the developer shall pay a fee in lieu of providing said detention/retention facility. In those locations already served by an existing stormwater detention/retention facility or where the City is contemplating constructing a detention/retention facility to serve upstream watersheds not yet fully developed, or where the developer cannot provide adequate storage capacity in a detention/retention facility within his/her development, at the option of the City, the developer shall pay a fee in lieu of providing said detention/retention facility, for the deficient volume thereof.
Facilities shall be designed so that local storm sewer systems and drainageways will discharge to proposed or existing detention/retention facilities, or to an adequate trunk line storm sewer and flood route, or drainageway feeding same, which discharges into a proposed or existing stormwater detention/retention facility. Where adequate storm sewer or drainageway capacity is not available to convey design runoff to an existing downstream detention/retention facility, an on-site detention/retention facility shall be required.
A combination of storage and controlled release of stormwater is required for all:
Nonresidential and residential developments.
[Amended 2-6-2017 by Ord. No. 994]
An exception to this requirement will be granted by the City only when runoff can be directed into a storm sewer, channel or natural drainageway with sufficient capacity to transport stormwater to an acceptable location.
The applicant shall not be required to provide detention storage for runoff from land other than his own except where it can be proven that flow from adjacent land has historically crossed or inundated the land of the applicant.
Where a subdivision plat or a planned unit development involves new lots which are more than three acres in size that are zoned for commercial or industrial purposes, the developer/owner/applicant may provide for storage and controlled release of stormwater runoff for the developed lots by sufficient and enforceable covenants upon the individual lots. The covenant shall require construction of on-site stormwater detention facilities upon each lot at the time of subsequent development of the lot. The form of covenant shall be approved by the City Attorney, and the substance of the covenant shall require construction of on-site stormwater runoff storage and controlled release of stormwater runoff from each lot in a manner approved by the City and in accordance with the provisions of this chapter.
Exceptions to the required stormwater management plan are:
Traditional agricultural land uses;
Improvement of existing roadways;
Modification of single-family residential or two-family residential structures which will continue to be used as single-family or two-family dwellings; or
Modification of existing structures or appurtenances, other than a single-family dwelling or two-family dwelling, which does not increase the amount of impervious area of the lot or parcel upon which the structure or appurtenance is constructed.
Unless otherwise approved by the City, formulas to be used in connection with the calculation of runoff volumes and allowable release rates reasonably expected from the minimum design storm shall be the Rational Method for total contributing areas of 20 acres or less and the soil conservation hydrograph method for areas greater than 20 acres.
For post-developed conditions, the amount of pervious and impervious land shall be determined and prorated using the following coefficients: Pervious (C = 0.30, CN = 61); Impervious (C = 0.90, CN = 98). The overland flow coefficient (n) shall not be greater than 0.15.
For pre-developed conditions, the runoff coefficients shall not be greater than C = 0.30 or CN = 69. The overland flow coefficient (n) shall not be less than 0.13.
Allowable release rate. The allowable release rate will be determined by the design formula using a minimum design storm with an average rainfall intensity associated with average recurrence interval of five years for the storm period calculated by the time of concentration as outlined by the latest technical letters of the Illinois State Water Survey for rainfall frequencies. Runoff coefficients or CN numbers used shall reflect the pre-developed condition of the watershed, as previously noted.
Storage volume when using the Rational Method. The storage volume, when using the Rational Method, will be the volume generated by the difference between average runoffs of fifty-year rainfall frequency in post-development condition and five-year rainfall frequency in pre-development condition, assuming such difference occurs for one hour. The average discharge is half of the peak discharge calculated by the Rational Method.
Storage volume when using the soil conservation hydrograph method. The storage volume, when using the soil conservation hydrograph method, will be determined from inflow hydrographs generated by the design formula using a minimum design storm with a range of rainfall intensities associated with an average recurrence interval of 50 years. The storage volume will be the difference between the inflow hydrograph and a designed outflow hydrograph that releases no more than the allowable release rate.
Hydrologic design calculations shall be reviewed by the City Engineer through one of the following: subdivision plan review; grading and drainage plan review; or construction plan review (typically public improvements).
Hydrologic design calculations shall include a contour map(s) clearly showing the design drainage area(s), stormwater facilities, storm sewers, swales, ditches, culverts, bridges and receiving stormwater facilities. Critical spot elevations, invert elevations, pervious and impervious areas shall be clearly indicated.
Submittals shall include design calculations, drainage basin area and surface types, rainfall data used.
Plans shall clearly indicate normal and high water elevations, design storage volume, minimum and maximum and typical slopes.
Detention basins shall release the fifty-year storage volume within 24 hours to 48 hours.
Emergency overflow routes shall be clearly designated for runoff in excess of the fifty-year storm.
Minimum outlet storm sewer size shall be six inches; smaller diameter restrictions (e.g., orifice plate or short pipe length) are acceptable.
Inlet and outlet pipes shall be provided with end sections and erosion protection.
Weirs, dams and specialized outflows shall be designed by a professional engineer registered in the State of Illinois.
Temporary erosion techniques shall be used as required to ensure a full stand of cover vegetation in minimum time.
Detention basin side slopes shall be 4:1 or flatter above normal pool, and shall be designed with permanent erosion protection consisting of grass, nongrass vegetation, or other permanent finish. Permanent erosion protection shall be aesthetically suitable to the development or existing surrounding land use.
Rooftop detention is not allowed.
A twenty-foot minimum setback shall be required from all property lines to the normal pool elevation.
Buildings within 100 feet of a detention basin fifty-year design high water shall have the lowest water entry point a minimum of two feet above the fifty-year design water elevation.
All earthen dams, whether public or privately owned and maintained, shall meet the following:
Freeboard. All earthen dams shall be constructed with a minimum of one foot of freeboard over the maximum anticipated water level in the emergency spillway.
All earthen dams shall include, at a minimum, an impervious compacted clay core. The core shall be of sufficient width to allow for mechanical compaction and of sufficient depth to penetrate the overburden of topsoil, keying into the underlying strata of virgin clay.
The developer shall be responsible for obtaining all construction permits required by governmental agencies.
Emergency spillway. All detention and retention facilities, whether publicly or privately owned and maintained, shall include an emergency spillway.
Emergency spillways shall, wherever feasible, be constructed in virgin ground.
Erosion protection. Emergency spillways shall be protected from the erosion with approved surface paving or vertical cutoff walls appropriate for the size of the impoundment and the height of the spillway crest.
Capacity. Emergency spillways shall be sized for no less than 75% of the maximum peak inflow into the basin.
Side slopes shall be a maximum of 4H:1V. If retaining walls are used, their height is limited to four feet. Retaining wall design and material type shall be approved by the City Engineer.
Dry-bottom basins shall have 2% minimum bottom slopes or 0.5 minimum bottom slopes or 0.5 minimum bottom slopes with underdrain systems as approved by the City Engineer.
Dry-bottom basins shall include a low-flow channel with some form of erosion protection, or a bypass sewer.
Above-water side slopes shall be a maximum of 4H:1V to one foot below normal water surface elevation. If retaining walls are used, their height is limited to four feet. Retaining wall design and material type shall be approved by the City Engineer.
Erosion protection shall be required at the water's edge on all residential developments. The protection shall extend from one foot below to one foot above the normal water level.
Wet-bottom basin design shall include an evaluation of soil permeability. A basin liner shall be included in the design if needed to ensure water retention to normal pool elevation.
All construction requirements shall conform to the applicable sections of the Standard Specifications for Road and Bridge Construction, Department of Transportation.
Engineering costs. The applicant will be responsible for all engineering costs for design of storm sewers, drainageways and detention facilities within his/her development.
Detention basins may be maintained and owned by the applicant, the lot owner or by the developer, or by a homeowners' association or by the City. The decision on ownership will be made by the City of El Paso at the time of submission of the preliminary plan.
Alternate methods of detention will be allowed pending review by the City of El Paso and its City Engineer. The alternate methods may be, but are not limited to, the following:
The applicant shall pay to the City of El Paso: