The purpose of this chapter is to establish procedures to control the flow of stormwater from developing areas so as to maintain the rate of the flow in natural or manmade channels equal to the rate of flow from those areas in their undeveloped state, so as to provide for the safety, health, and well-being of those living within the developing area as well as those downstream who will be affected by its development.

   The provisions of this chapter are applicable to:

   1.   All new residential, commercial, and industrial developments one acre or larger.

   2.   Any new development, less than one acre, where the percentage of the impervious area of the lot is 50 percent or greater.

   3.   Any new development, less than one acre, which, in the opinion of the City Engineer, lacks an adequate outlet for the passage of stormwaters.

   When used in this chapter, unless the context clearly indicates otherwise, the following words and phrases shall have the meanings ascribed to them in this section.

   1.   “By-pass channel” means a channel formed in the topography of the earth’s surface to carry stormwater runoff through a specific area.

   2.   “Control structure” means a structure designed to control the flow of stormwater runoff that passes through it during a specific length of time.

   3.   “Development” means the improvement of the land from its natural state to one providing for residential, industrial, or commercial use.

   4.   “Dry bottom stormwater storage area” means a facility designed to be normally dry and contain water only when excess stormwater runoff occurs.

   5.   “Excess stormwater” means that portion of stormwater runoff which exceeds the transportation capacity of storm sewers or natural drainage channels serving a specific watershed.

   6.   “Natural drainage” means channels formed by the existing surface topography prior to changes made by unnatural causes.

   7.   “Safe storm drainage capacity” means the flow of stormwater runoff that can be transported by a channel or conduit without causing a rise of the water surface over the conduit or adjacent to the channel.

   8.   “Stormwater runoff” means the flow of water resulting from precipitation which is not absorbed by the soil or plant material.

   9.   “Stormwater runoff release rate” means the rate at which stormwater runoff is released from dominant to subservient land.

   10.   “Stormwater storage areas” means areas designed to store excess stormwater.

   11.   “Tributary watershed” means all of the area that contributes stormwater runoff to a given point.

   12.   “Wet bottom stormwater storage area” means a facility designed to be maintained as a pond or free water surface and which has the capacity to contain excess stormwater runoff.

   13.   “‘X’-year storm” means the average recurrence intervals within which a rainfall of given intensity and duration will be equaled or exceeded only once. A 100-year storm would have an intensity of rainfall which would, on the average, be equaled or exceeded only once in 100 years. This does not imply that it will occur in 100 years, or having occurred, will not happen again for 100 years.

   1.   Preparation and Certification. The design of stormwater control or detention facilities shall be prepared and certified by a Registered Engineer, Architect, or Landscape Architect familiar with stormwater control or detention. Design calculations shall be made available to the City Engineer.

   2.   Release Rate. The release rate of stormwater from any detention basin required under this chapter shall not exceed the stormwater runoff rate from the drainage area as a totally grassed site during a five-year frequency storm. Applicants may claim a higher natural rate of runoff if documented by detailed computations to show that higher capacity exists in the natural outlets serving the area. However, only the “safe storm drainage capacity” of the conduit or channel may be included in these calculations. Design of the floodway system shall also take into consideration control of stormwater velocity to prevent erosion or other damage to the facility which will restrict its primary use. Depths of flow shall be consistent with the “safe storm drainage capacity” of the facility and detention or channel configurations shall be totally under City control.

   3.   Detention Requirements. The required volume of stormwater detention shall be that necessary to handle the runoff of a 100-year rainfall, for any and all durations from the drainage area tributary to the stormwater storage area based on full development of said tributary area, less the volume discharge during the same duration at the approved release rate. The stormwater release rate shall be considered when calculating the stormwater storage capacity and the control structure shall be designed to maintain a relatively uniform rate regardless of the depth of stormwater in the storm area. Thus, the “Required Detention Storage” (RDS) will be that found to be the most critical resulting from the inflow from the runoff of a fully developed tributary area from a 100-year storm and outflow of the five-year storm with the same area in its urbanized or natural state, totally grassed. Also, see Section 146.04(2), Release Rate. Detention storage may be provided as a “dry bottom” or “wet bottom” storm area.

      A.   Dry bottom stormwater storage areas shall be designed to serve a secondary purpose for recreation, open space, or other types of uses that will not be adversely affected by intermittent flooding.

         (1)   A method of carrying the low flow through these areas shall be provided in addition to a system of drains to prevent soggy areas. Both shall be provided with an outlet to a natural channel or storm sewer with adequate capacity as described in Section 146.04(4), By-Pass Channel. Dry bottom stormwater storage areas should be designed to drain completely within 24 hours after a storm.

         (2)   Outlet control structures shall be designed as simply as possible and shall require little or no attention for proper operation. Each stormwater storage area shall be provided with a method of emergency overflow in the event that a storm in excess of the 100-year frequency storm occurs. This emergency overflow facility shall be designed to function without attention and shall become part of the “natural” or surface channel system described in Section 146.04(4). Hydraulic calculations shall be submitted to substantiate all design features.

         (3)   Both outlet control structure and emergency overflow facilities shall be designed and constructed to fully protect the public health, safety, and welfare. Existing downstream hazards (garages, houses) must be considered. Stormwater runoff velocities shall be kept at a minimum and turbulent conditions at an outlet control structure will not be permitted without complete protection for the public safety. The use of fences shall be kept to a minimum and used only as a last resort when no other method is feasible. All impounding structures within the City designed to be over 10 feet in height or designed to store more than five ac-ft. must be approved by the Department of Natural Resources (DNR). Complete engineering plans must be submitted to DNR for their review.

         (4)   Paved surfaces that are to serve as stormwater storage areas and rooftop storage shall be designed and permanent-type control inlets and retaining or parapet walls to contain runoff on the surface. Emergency overflow areas shall be provided.

      B.   Wet bottom stormwater storage areas shall be designed with all of the items required for dry bottom stormwater storage areas, except that the provisions of Section 146.04(3)(A)(1) shall not be required. However, the following additional conditions shall be complied with:

         (1)   Water surface area shall not exceed 1/15 of the tributary drainage area.

         (2)   Facilities shall be provided to lower the pond elevation by gravity flow for cleaning purposes and shoreline maintenance. Shoreline protection shall be provided to prevent erosion from wave action.

         (3)   Minimum normal water depth shall be four feet. If fish are to be maintained, some portion of the pond area should be a minimum of 10 feet deep.

         (4)   Control structures for stormwater release shall be designed to operate at full capacity with only a minor increase in water surface level. Hydraulic calculations shall be submitted to the City Engineer to substantiate all design features.

         (5)   Only that portion of the detention area above the normal water level shall be used in calculating the storage capacity. Wet bottom stormwater storage areas shall be designed to provide a storage duration not exceeding 24 hours after a storm.

   4.   By-Pass Channel. A “natural” or surface channel system shall be designed with adequate capacity to convey through the development the stormwater runoff from all tributary upstream areas. This “by-pass” channel shall be designed to carry the peak rate of runoff from a 100-year storm, assuming all storm sewers are blocked and the upstream areas fully developed.

   1.   Where development of a property presents the threat of flooding or damage by flash runoff to downstream residents, the facilities for stormwater runoff control shall be construed as a part of the first phase of construction of that project.

   2.   The construction of the stormwater control system shall be accomplished as part of the cost of land development. If the amount of storage capacity can be increased to provide benefit to the City, negotiations for public participation in the cost of development shall be initiated.

   3.   All flood control items such as earthen embankments, conduits, outlet structures, flood control structures, spillways, by-pass channels, etc., shall be built as permanent facilities and all materials and their manner of construction shall be assembled to accomplish as much permanency as is possible.

   4.   Detention ponds shall be designed and landscaped to enhance the overall site design whenever possible. Underground detention may be required on sites that are being intensely developed or have unique site constraints. Large ditches or ponds in the front of the site shall be discouraged and approved only as a last resort. All detention facilities shall be shown on the site plan.

   All plans submitted for stormwater detention systems shall describe an adequate procedure of normal maintenance for the detention system. Any failure of the stormwater detention system, due to inadequate normal or capital maintenance, shall be the responsibility of the owner of the property on which the detention system is located. It shall also be the property owner’s responsibility to remedy any negligence in maintenance that resulted in the failure of the system. The submittal of plans for such a system or the purchase of property on which such system is located shall be deemed as acceptance of responsibility for normal and capital maintenance of the system.

   Drainage easements shall be provided for all conduits and those by-pass channels where the 100-year runoff exceeds one cubic foot per second.

   1.   Plans, specifications, and all calculations for stormwater runoff control shall be submitted for review and approval prior to the approval of a final plat (in the case of a subdivision or planned unit development) or prior to approval of a site plan (in case of commercial or industrial construction).

   2.   No Certificate of Occupancy for any building in the development will be issued until the storm detention facilities are constructed, inspected, and approved.