(A) Applicability and exemptions. The storage and controlled release rate of excess stormwater runoff shall be required for all new business, commercial, and industrial developments; residential subdivisions; planned development; rural estate subdivisions; and any redevelopment or other new construction located within the city. The city, after thorough investigation and evaluation, may waive the requirement of controlled runoff for minor subdivisions and parcelization.
(B) Policy on stormwater quantity management.
(1) Detention policy. It is recognized that most streams and drainage channels serving the city do not have sufficient capacity to receive and convey stormwater runoff resulting from continued urbanization. Accordingly, the storage and controlled release of excess stormwater runoff shall be required for all developments and redevelopments (as defined in § 54.24) located within the city.
(a) General release rates.
1. In general, the post- development release rates for developments up to and including the ten-year return period storm may not exceed 0.1 cfs per acre of development. The post-development release rate for developments for the 11- to 100-year return period storms shall not exceed 0.3 cfs per acre of development. The above fixed general release rates may be set at a lower value by the city for certain watersheds if more detailed data becomes available as a result of comprehensive watershed studies conducted or formally approved and adopted by the City Council. The applicant shall confirm the applicable release rates with the city prior to initiating the design calculations to determine whether a basin-specific rate has been established for the watershed.
2. The release rate for pre- developed areas that have more than one outlet must be computed based on pre-developed drainage area tributary to each outlet point. The computed release rate at each outlet point shall not be exceeded at the respective outlet point, even if the post-developed conditions would involve a different arrangement of outlet points. Also, if an outlet control structure includes an orifice to restrict the flow rate, such orifice shall be no less than four inches in diameter, even if the four-inch diameter orifice results in a discharge that exceeds the above-noted general release rates.
(b) Site-specific release rates for sites with depressional storage.
1. For sites where depressional storage exists, the general release rates provided above may have to be further reduced. If depressional storage exists at the site, site-specific release rates must be calculated according to methodology described in the city’s “Stormwater Technical Standards Manual”, accounting for the depressional storage by modeling it as a pond whose outlet is a weir at an elevation that stormwater can currently overflow the depressional storage area. Post developed release rate for sites with depressional storage shall be the two-year pre-developed peak runoff rate for the post-developed ten-year storm and ten-year pre-developed peak runoff rate for the post-developed 100-year storm. In no case shall the calculated site-specific release rates be larger than general release rates provided above.
2. Also note that, for determining the post-developed peak runoff rate, the depressional storage must be assumed to be filled unless the city can be assured, through dedicated easement, that the noted storage will be preserved in perpetuity.
(c) Management of off-site runoff.
1. Runoff from all upstream tributary areas (off-site land areas) may be bypassed around the detention/retention facility without attenuation. Such runoff may also be bypassed through the detention/retention facility without attenuation, provided that a separate secondary outlet system is incorporated for the safe passage of such flows, i.e., not through the primary outlet of a detention facility. Unless the pond is being designed as a regional detention facility and therefore all off-site runoff to the pond detained, the primary outlet structure shall be sized and the invert elevation of the secondary outlet for bypassing off-site runoff determined according to the on-site runoff only. To accomplish this, the 100-year on-site runoff must be determined by temporarily ignoring the off-site runoff and routed through the pond and through the primary outlet pipe. The resulting pond elevation would be the invert elevation of the secondary outlet. Once the size and location of primary outlet structure and the invert elevation of the secondary outlet for bypassing off-site runoff are determined by considering on-site runoff only, the size of the secondary outlet and the 100-year pond elevation is determined by routing the entire inflow, on-site and off-site, through the pond. Once the 100-year pond elevation is determined in this manner, the crest elevation of the open emergency weir is set at that elevation.
2. Note that the efficiency of the detention/retention facility in controlling the on-site runoff may be severely affected if the off-site area is considerably larger than the on-site area. As a general guidance, on-line detention may not be effective in controlling on-site runoff where the ratio of off-site area to on-site area is larger than 5:1. Additional detention (above and beyond that required for on-site area) may be required by the city when the ratio of off-site area to on-site area is larger than 5:1.
(d) Downstream restrictions.
1. In the event the downstream receiving channel or storm sewer system is inadequate to accommodate the post-developed release rate provided above, then the allowable release rate shall be reduced to that rate permitted by the capacity of the receiving downstream channel or storm sewer system. Additional detention, as determined by the city, shall be required to store that portion of the runoff exceeding the capacity of the receiving storm sewers or watercourses. When such downstream restrictions are suspected, the city may require additional analysis to determine the receiving system’s limiting downstream capacity.
2. If the proposed development makes up only a portion of the undeveloped watershed upstream of the limiting restriction, the allowable release rate for the development shall be in direct proportion to the ratio of its drainage area to the drainage area of the entire watershed upstream of the restriction.
(C) Grading and building pad elevations.
(1) Maximum yard slopes are 3:1 where soil has been disturbed during construction processes. Finished floor elevation or the lowest building entry elevation must be no less than six inches above finished grade around the building. The building’s lowest entry elevation that is adjacent to and facing a road should be a minimum of 15 inches above the road elevation or top of curb.
(2) All buildings shall have a minimum flood protection grade shown on the secondary plat. Minimum flood protection grade of all structures fronting a pond or open ditch shall be no less than two feet above any adjacent 100-year local or regional flood elevations, whichever is greater, for all windows, doors, unsealed pipe entrances, window well rim elevations, and any other structure member where floodwaters can enter a building.
(3) The lowest floor elevation of all residential, commercial, or industrial structures, located within the special flood hazards area (SFHA), as shown on the FEMA maps, shall be at the flood protection grade and therefore have two feet of freeboard above the 100-year flood elevation.
(4) The lowest adjacent grade for residential, commercial, or industrial buildings outside a FEMA or IDNR designated floodplain shall have two feet of freeboard above the flooding source’s 100-year flood elevation under proposed conditions, unless the flooding source is a rear-yard swale. When the flooding source is a rear-yard swale, the lowest adjacent grade for residential, commercial, or industrial buildings shall have two feet of freeboard above the 100-year flood elevation under proposed conditions.
(5) The lowest adjacent grade to ponds for all residential, commercial, or industrial buildings (including walkout basement floor elevation) shall be set a minimum of two feet above the 100-year pond elevation or two feet above the emergency overflow weir elevation, whichever is higher. In addition to the lowest adjacent grade requirements, any basement floor must be at least a foot above the permanent water level (normal pool elevation).
(6) The 100-year overflow paths throughout the development, whether shown on FEMA maps or not, must be shown as hatched area on the plans. A minimum of 20 feet along the centerline of the 100- year overflow path must be contained within permanent drainage easements. A statement shall be added to the plat that would refer the viewer to the construction plans to see the entire extent of overflow path as hatched areas. No fences or landscaping can be constructed within the easement areas that may impede the free flow of stormwater. These areas are to be maintained by the property owners or be designated as common areas that are to be maintained by the homeowners association. The lowest adjacent grade for all residential, commercial, or industrial buildings shall be set a minimum of one foot above the noted overflow path/ponding elevation, calculated based on all contributing drainage areas, on-site and off-site, in their proposed or reasonably anticipated land use and with storm pipe system assumed completely plugged.
(7) It shall be the property owners’ responsibility to maintain the natural features and the natural flow of drainage on their lots and to take preventive measures against any and all erosion or deterioration of natural or human-made features on their lots.
(D) Acceptable outlet and adjoining property impact policies.
(1) Design and construction of the stormwater facility shall provide for the discharge of the stormwater runoff from off-site land areas as well as the stormwater from the area being developed (on-site land areas) to an acceptable outlet(s) (as determined by the city) having capacity to receive upstream (off-site) and on-site drainage. The flow path from the development outfall(s) to a regulated drain or natural watercourse (as determined by the city) shall be provided on an exhibit that includes topographic information. Any existing field tile encountered during the construction shall also be incorporated into the proposed stormwater drainage system or tied to an acceptable outlet. In addition, no activities conducted as part of the development shall be allowed to obstruct the free flow of flood waters from an upstream property.
(2) Where the outfall from the stormwater drainage system of any development flows through real estate owned by others prior to reaching a regulated drain or watercourse, no acceptance shall be granted for such drainage system until all owners of real estate and/or tenants crossed by the outfall consent in writing to the use of their real estate through a recorded easement.
(3) If an adequate outlet is not located on-site, then further reduction in allowable release rates or off-site drainage improvements may be required. Those improvements may include, but are not limited to, extending storm sewers, clearing, dredging, or removal of obstructions to open drains or natural water courses, and the removal or replacement of undersized culvert pipes as required by the city.
(E) No net loss floodplain storage policy.
(1) Floodplains exist adjacent to all natural and human-made streams, regardless of contributing drainage area or whether they have been previously identified or mapped. Due to potential impacts of floodplain loss on peak flows in streams and on the environment, disturbance to floodplains must be avoided. When the avoidance of floodplain disturbance is not practical, the natural functions of floodplain must be preserved to the extent possible.
(2) In an attempt to strike a balance between the legitimate need for economic development within the city and the need to preserve the natural functions of floodplains to the extent possible, compensatory excavation equivalent to the floodplain storage lost shall be required for all activities within floodplain of streams located in the city where drainage area of the stream is equal or larger than one square mile. This requirement shall be considered to be above and beyond the minimum requirements provided in the applicable flood hazard areas ordinance currently in effect in the city. The city may alter the compensation ratio, based on extenuating circumstances, for a specific project.
(3) Note that, by definition, COMPENSATORY STORAGE is the replacement of the existing floodplain and, in rare exceptions, the floodway storage lost due to fill. Compensatory storage is required when a portion of the floodplain is filled, occupied by a structure, or when as a result of a project a change in the channel hydraulics occurs that reduces the existing available floodplain storage. The compensatory storage must be located adjacent or opposite the placement of the fill and maintain an unimpeded connection to an adjoining floodplain area.
(4) Computations must show the provision of compensatory floodplain storage for 10-year, 50-year, and 100-year storm events. That is, the post-development 10-year floodplain storage along a stream shall be equivalent to the 10-year pre-development floodplain storage along the stream within the property limits, the post-development 50-year floodplain storage along a stream shall be equivalent to the 50-year pre-development floodplain storage along the stream within the property limits, and the post-development 100-year floodplain storage along a stream shall be equivalent to the 100-year pre-development floodplain storage along the stream within the property limits.
(5) Calculations for floodplain volume shall be submitted in tabular form showing calculations by cross-section. The volume of floodplain storage under the without-project conditions and the with-project conditions must be determined using the average-end-area method with plotted cross-sections at a horizontal to vertical ratio of between 5:1 and 10:1, with 10- through 100-year flood elevations noted on each cross-section. The scale chosen must be large enough to show the intent of proposed grading. Cross-sections must reflect both the existing and proposed conditions on the same plot. The location and extent of the compensatory storage area as well as the location and orientation of cross-sections must be shown on the grading plan.
(F) Calculations and design standards and specifications. The calculation methods as well as the type, sizing, and placement of all stormwater facilities shall meet the design criteria, standards, and specifications outlined in the city’s “Stormwater Technical Standards Manual”. The methods and procedures in the “Stormwater Technical Standards Manual” are consistent with the policy stated above.
(G) Drainage easement requirements. There shall be no trees or shrubs planted, nor any structures or fences erected, in any drainage easement, unless otherwise allowed by the city.
(1) All new channels, drain tiles equal to or greater than 12 inches in diameter, inlet and outlet structures of detention and retention ponds, and appurtenances thereto as required by this section, that are installed in subdivisions requiring a stormwater management permit from the city shall be contained within a minimum of 20 feet of drainage easement (ten feet from centerline on each side) and shown on the recorded plat. New drain tiles refer to all sub-surface stormwater piping, tubing, tiles, manholes, inlets, catch basins, risers, and the like.
(2) A minimum of 25 feet from top of the bank on each side of a new channel shall be designated on the recorded plat as a drainage easement. If the top of the bank is not vegetated according to the development’s landscape plan, a minimum 25-foot width of filter strip shall be installed within the drainage easement.
(3) Rear-yard swales and emergency overflow paths associated with detention ponds shall be contained within a minimum of 20 feet width (ten feet from centerline on each side) of drainage easement.
(4) A minimum of 20 feet beyond the actual footprint (top of the bank) of stormwater detention facilities shall be designated as drainage easement. A minimum of 20-foot width easement shall also be required as access easement from a public right-of-way to the facility, unless the pond is immediately next to a public right-of-way.
(H) Placement of utilities. No utility company may disturb existing storm management facilities without the consent of the city. All existing drainage facilities shall have senior rights and damage to said facilities shall result in penalties, as prescribed in § 54.33.
(I) Inspection, maintenance, record keeping, and reporting.
(1) After the approval of the stormwater management permit by the city and the commencement of construction activities, the city has the authority to conduct inspections of the work being done to ensure full compliance with the provisions of this section, the “Stormwater Technical Standards Manual”, and the terms and conditions of the approved permit.
(2) The city also has the authority to perform long-term, post-construction inspection of all public or privately owned stormwater quantity facilities. The inspection will cover physical conditions, available storage capacity, and the operational condition of key facility elements. Stormwater quantity facilities shall be maintained in good condition, in accordance with the terms and conditions of the approved stormwater management permit, and shall not be subsequently altered, revised, or replaced except in accordance with the approved stormwater permit, or in accordance with approved amendments or revisions to the permit. If deficiencies are found during the inspection, the owner of the facility will be notified by the city and will be required to take all necessary measures to correct such deficiencies. If the owner fails to correct the deficiencies within the allowed time period, as specified in the notification letter, the city will undertake the work and collect from the owner using lien rights if necessary.
(Ord. 2006-13, passed 10-26-2006)