(A) Detention policy. It is recognized that most streams and drainage channels serving the town do not have sufficient capacity to receive and convey stormwater runoff resulting from continued urbanization. Accordingly, except for situations provided in divisions (A)(1) and (4) below, the storage and controlled release of excess stormwater runoff shall be required for all developments and redevelopments (as defined in Appendix A) located within the town.
(1) General release rates. In general, the post-development release rates for developments up to and including the 10-year return period storm may not exceed 0.1 cfs per acre of development. The post-development release rate for developments for the 10 - 100-year return period storms shall not exceed 0.3 cfs per acre of development. For sites where the pre-developed area has more than 1 outlet, the release rate should be computed based on pre-developed discharge to each outlet point. The computed release rate for 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.
(2) Site-specific release rates for sites with depressional storage.
(a) 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 town 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 2-year pre-developed peak runoff rate for the post-developed 10-year storm and 10-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.
(b) Also note that for determining the post-developed peak runoff rate, the depressional storage must be assumed to be filled unless the Town Council and/or Town Manager and/or Building Commissioner can be assured, through dedicated easement, that the noted storage will be preserved in perpetuity.
(3) Management of off-site runoff.
(a) 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 outlet system or channel is incorporated for the safe passage of such flows, such as, not through the primary outlet of a detention facility. Unless the pond is being designed as a regional detention facility, the primary outlet structure shall be sized and the invert elevation of the emergency overflow weir determined according to the on-site runoff only. Once the size and location of primary outlet structure and the invert elevation of the emergency overflow weir are determined by considering on-site runoff, the 100-year pond elevation is determined by routing the entire inflow, on-site and off-site, through the pond.
(b) 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 Town Council and/or Town Manager and/or Building Commissioner when the ratio of off-site area to on-site area is larger than 5:1.
(4) Downstream restrictions.
(a) 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 Town Council and/or Town Manager and/or Building Commissioner, shall be required to store that portion of the runoff exceeding the capacity of the receiving sewers or watercourses. When such downstream restrictions are suspected, the Town Council and/or Town Manager and/or Building Commissioner may require additional analysis to determine the receiving system's limiting downstream capacity.
(b) 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.
(5) Exemptions for detention requirements detention will not be required for the following:
(a) Notwithstanding the requirements for an individual lot plot plan permit in this chapter, land alterations where the primary basis on which a stormwater drainage permit is required is the construction, enlargement, or location (on a permanent foundation) of a one-family dwelling, two-family dwelling, or accessory structure appurtenant to either a one- or two-family dwelling.
(b) Accepted fill areas or one-time addition to existing commercial buildings that do not increase the amount of impervious area on-site by more than a total of 0.5 acres, provided the existing runoff patterns and flow capacity within the property will not be altered by the filling operations.
(c) Not withstanding the provisions of division (A)(4) above, those site developments where the stormwater management system has been designed such that:
1. After combining flows from both the off-site and on-site drainage areas, there will be no increase in the total peak discharge from the developing site during the 2-, 10-, and 100-year storm events;
2. The volume of runoff for each project site outlet has not been increased for the 2-, 10-, and 100-year storm events; and
3. The flow width and velocity at the property boundary line for each sub-basin is less than or equal to that flow width and velocity which existed prior to the development for the 2-, 10-, and 100-year storm events.
(d) Where the direct release of runoff from the proposed development meets the conditions set forth in division (A)(6) below.
(6) Direct release provisions. It is the policy of the Town Council and/or Town Manager and/or Building Commissioner to allow the direct release (no detention) of runoff from a proposed development to an adjacent stream with more than 100 square miles of contributing drainage area at the direct release point. Therefore, direct release may be allowed for parcels adjacent to the following stream reaches in the town.
(B) Grading and building pad elevations.
(1) Maximum yard slopes are 3:1 where soil has been disturbed during construction processes. Finished floor elevation must be no less than 18 inches above finished grade and a minimum of 24 inches above an adjacent road elevation unless a written variance is granted by the Town Council and/or Town Manager and/or Building Commissioner.
(2) For all structures located in the Special Flood Hazards Area (SFHA) as shown on the FEMA maps, the lowest finished floor elevations of all residential, commercial, or industrial buildings, shall be such that lowest finished floor elevation, including basement, shall be at the flood protection grade and therefore have 2 feet of freeboard above the 100-year flood elevation.
(3) The lowest adjacent grade for residential, commercial, or industrial buildings outside a FEMA or IDNR designated floodplain shall have 2 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 2 feet of freeboard above the 100-year flood elevation under proposed conditions or be separated by a minimum distance of 50 feet from the proposed condition 100-year flood boundary.
(4) For areas outside a FEMA or IDNR designated floodplain, the lowest adjacent grade (including walkout basement floor elevation) for all residential, commercial, or industrial buildings adjacent to ponds shall be set a minimum of 2 feet above the 100-year pond elevation or 2 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 1 foot above the permanent water level (normal pool elevation).
(5) The 100-year flood flow paths throughout the development, whether shown on FEMA maps or not, must be shown as hatched area on the plans and 15 feet (measured perpendicularly to) each side of the centerline of the flow path shall 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 1 foot above the noted 100-year overflow path/ponding elevation.
(6) It shall be the property owners' responsibility to maintain the natural features on their lots and to take preventive measures against any and all erosion and/or deterioration of natural or manmade features on their lots.
(C) Acceptable outlet and adjoining property impacts 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 Town Council and/or Town Manager and/or Building Commissioner) 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 Town Council and/or Town Manager and/or Building Commissioner) 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.
(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. 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 – including the upstream property's tile system.
(3) If an adequate outlet is not located on site, then off-site drainage improvements may be required. Those improvements may include, but are not limited to, extending storm sewers, clearing, dredging and/or removal of obstructions to open drains or natural water courses, and the removal or replacement of undersized culvert pipes as required by the Town Council and/or Town Manager and/or Building Commissioner.
(D) No net loss floodplain storage policy.
(1) Floodplains exist adjacent to all natural and man-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 should be avoided. When the avoidance of floodplain disturbance is not practical, the natural functions of floodplain should be preserved to the extent possible.
(2) In an attempt to strike a balance between the legitimate need for economic development within town 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 Hancock County where drainage area of the stream is equal or larger than 1 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 town. The Town Council and/or Town Manager and/or Building Commissioner 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 should be located adjacent or opposite the placement of the fill and maintain an unimpeded connection to an adjoining floodplain area.
(4) Computations must show no net loss of floodplain storage for 2-year, 10-year, 50-year, and 100-year storm events. That is, the post-development 2-year floodplain storage along a stream shall be the same as 2-year pre-development floodplain storage along the stream within the property limits, the post-development 10-year floodplain storage along a stream shall be the same as 10-year pre-development floodplain storage along the stream within the property limits, and so on.
(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 should 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 2- through 100-year flood elevations noted on each cross section. The scale chosen should be large enough to show the intent of proposed grading. Cross-sections should 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 should be shown on the grading plan.
(Ord. 051805, passed 5-18-2005)