Storm water runoff from a proposed development site such shall be controlled to meet the following criteria:
(a) Two methods for calculating storm water runoff volumes and control measures are approved. They are the modified rational method and the soil conservation service method. Either may be used for sites up to six acres in size. The soil conservation method must be used for all sites six acres or larger.
(b) Provide a drainage area map (or maps) for the pre-developed and post developed sites. Perform field surveying as necessary to insure that this drawing is an accurate representation of actual field conditions. The scale of this drawing must be sufficient to clearly present the following information:
(1) Existing and proposed elevation contours, at an interval of two feet or less. Contour lines shall be labeled frequently enough to be easily read and interpreted.
(2) Hydrologic boundaries of watersheds, including areas outside the proposed development that flow into the project area.
(3) Points of analysis, or the location where the drainage is being evaluated, for each watershed.
(4) The path used to calculate the time of concentration for each watershed. Indicate types of flow and provide calculations.
(5) Delineate and label the types of land use, surface features, ground cover, soil types, etc. used to determine the runoff coefficients or curve numbers for the project areas.
(6) Show the proposed development layout.
(7) Provide a USGS Topographic Map copy indicating the proposed development area and 1,000 feet beyond the proposed development limits in each direction.
(c) Calculations.
(1) Provide routing calculations for each drainage area and drainage structure and their corresponding inflow and outflow hydrographs for the 2-year storm up through the 100-year storm. Prepare a summary table of results of the routing calculations. The table shall include, at a minimum, peak inflow, peak outflow, water surface elevations, and storage volume requirements.
(2) Provide pond volume -vs- elevation summaries.
(3) Provide calculations for the inlet/outlet works including weirs, orifices, culverts, spillways, grates, etc. Show their capacities and stage-discharge calculations, including tailwater assumptions.
(4) Verity capacities of receiving drainage features such as ditches, storm sewers, ponds, streams, etc. to verify sufficient capacity to accommodate the runoff of the development. The receiving drainage feature must be approved by the Planning and Zoning Department. Check all storms with a return frequency of 100-years or less.
(5) Provide runoff coefficients or curve number calculations for each watershed or sub-area, including impervious calculations.
(6) Provide time of concentration calculations for each watershed or sub-area.
(7) Calculate hydraulic grade line for outlet structures with pipes.
(8) Determine velocities of discharged waters and show details to control erosion.
(9) Provide narratives, as needed, to describe the methods, assumptions, formulas, and intent of the calculations.
(d) Detention basins.
(1) The minimum length of width ratio of the basin shall be two-to-one.
(2) The grading of an open detention basin shall be such that it reflects the surrounding topography. The embankment slopes for the detention basin should be four feet horizontal to one foot vertical preferred or a maximum of two feet horizontal to one foot vertical.
(3) The maximum water depth shall not exceed ten feet.
(4) The minimum top width of the side embankments shall be eight feet for non-vehicular traffic and 12 feet for vehicular traffic. The embankment shall be set at an elevation of at least 12 inches above the emergency spillway.
(5) Principal outlet (or outlets in a multi-stage configuration) shall have the capacity to pass the 100-year design storm flow.
(6) Outlet pipes shall have a minimum diameter of six inches. Orifice plates must be used for restrictions of smaller diameter. Consideration should be given to removable trash racks or other means to prevent clogging.
(7) Access to entire outlet structure for maintenance and inspection shall be provided and shall follow current OSHA Standards.
(8) An emergency spillway shall be provided, emergency spillways shall be designed to pass the 100-year storm frequency flow without the principal outlet capacity (in the event the principal outlet becomes plugged). The location of the emergency spillway shall be such that its overflow can be directed to an acceptable location. Permanent erosion control measures at the emergency spillway may be required by the city based on potential erosion at the site.
(9) Dry type detention structures shall be graded to drain to the outlet structure. The minimum grade in an earthen pond shall be 1%.
(10) All pipes through the embankment shall have anti-seep collars.
(11) Soil borings and testing may be requested by the city, such testing shall be performed by an approved soils testing laboratory. Submit a report certifying suitability of the soils on-site for embankment and basin construction.
(e) In certain applications, alternative detention facilities may be permitted and encouraged. These methods may be especially useful on small sites or where the increase in runoff is small. They may also be useful in conjunction with these methods described above. Requests for alternative detention facilities shall be submitted to the Planning and Zoning Department within the appropriate design standards.
(1) Infiltration basin, dry wells and infiltration ditches:
A. Soil borings and testing shall be performed by an approved soils testing laboratory. Submit a report certifying the suitability of the soils on-site for infiltration.
B. Infiltration measures must make provisions for overflow to a suitable area.
(2) Underground detention tanks or pipes:
A. May be used for commercial, institutional or industrial sites.
B. Must provide access for inspection and maintenance.
C. Must have sufficient load bearing capacity for proposed surface uses.
(3) Detention areas in parking lots using catch basins may be used providing the following criteria can be met:
A. A maximum water depth of 12 inches in parking areas may be used.
B. Maximum water depth shall not exceed six inches below the finished floor elevations of the existing or proposed structures.
C. Slopes for parking lots used for this purpose shall be a minimum of 0.75% and a maximum of 10%.
(4) Retention basins: soils report will be required to determine suitability.
(f) Maintenance of storm water management practices.
(1) The city shall approve an inspection and maintenance agreement binding on all subsequent owners of lands served by the planned storm water management practices before the city accepts the final plat or plan of the proposed project.
(2) All inspection and maintenance agreements shall do the following:
A. Designate the party responsible for maintenance of structural and nonstructural storm water management practices including, mowing and ensuring outlet structures are clear and in good repair. Unless otherwise approved by the city, this shall be an entity of common ownership within the proposed subdivision (such as an association) or the owner on an industrial or commercial site.
B. Prohibit unauthorized alterations of structural and nonstructural storm water management practices.
C. Provide access to storm water management practices for inspection by the city to document the condition of the practices. Authorize the city to make corrections to the facility, if deemed necessary, and assess the affected property owners.
(3) The location, dimensions, and bearings (including the depth and capacity) of all storm water management practices shall be incorporated on the final plat or plan, prior to approval by the city, and reference thereon shall be made to the entity or individual(s) responsible for maintenance.
(g) Inspection of storm water management practices.
(1) The city may inspect storm water management practices periodically.
(2) Upon finding a malfunction or other need for maintenance, the city shall notify the responsible party of the need for maintenance.
(3) Upon notification, the responsible party shall have 15 workings days, or other time stipulated by the city, to make repairs.
(4) Should repairs not be made within the time, or a plan approved by the city for these repairs not be in place, the city may undertake necessary repairs and assess the repair cost to the responsible party.
(Ord. 15-O-2820, passed 5-19-2015)