(A) Detention policy. It is recognized that most streams and drainage channels serving Boone County 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 § 53.007) located within Boone County.
(1) General release rates.
(a) In general, the post-development release rates for developments up to and including the ten-year return period storm may not exceed 0.3 cfs per acre of development. The post-development release rate for developments for the 11 to 100-year return period storms shall not exceed 0.5 cfs per acre of development. The above fixed general release rates may be set at a lower value by the Boone County Drainage Board and/or Boone County Surveyor for certain watersheds if more detailed data becomes available as a result of comprehensive watershed studies conducted and/or formally approved and adopted by the Boone County Drainage Board. The applicant shall confirm the applicable release rates with the Boone County Surveyor's Office prior to initiating the design calculations to determine whether a basin-specific rate has been established for the watershed.
(b) For sites where the pre-developed area has more than one 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. For above-ground facilities, if an outlet control structure includes an orifice to restrict the flow rate, such orifice shall be no less than six inches in diameter, even if the six-inch diameter orifice results in a discharge that exceeds the above-noted general release rates. However, note that the detention pond size would still need to be based on the actual calculated release rate.
(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 Boone County 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.
(b) Also note that for determining the post-developed peak runoff rate, the depressional storage must be assumed to be filled unless the Boone County Drainage Board and/or Boone County Surveyor 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, that is 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 emergency overflow weir 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 emergency overflow weir. 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 only, 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 Boone County Drainage Board and/or Boone County Surveyor 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 Boone County Drainage Board and/or Boone County Surveyor, 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 Boone County Drainage Board and/or Boone County Surveyor 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.
(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 six inches above finished grade around the building. Also, the building's lowest entry elevation that is adjacent to and facing a road shall be a minimum of 15 inches above the road centerline elevation unless a written variance is granted by the Boone County Drainage Board and/or Boone County Surveyor.
(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) For all structures located in the special flood hazards area (SFHA) as shown on the FEMA maps, the lowest floor elevations of all residential, commercial, or industrial buildings shall be such that lowest floor elevation, including basement, 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) 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 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 centerline of the 100-year flow paths throughout the development must be shown by a distinctive line symbol on the plans and 30 feet along this centerline designated as permanent drainage easements. No fences or landscaping can be constructed within the easement areas. These areas are easements that 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 of 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) The emergency overflow route from the pond to the receiving system must be sized to accommodate the design flow of the pond's emergency overflow weir and shown on the plans. Thirty feet along the centerline of this emergency overflow route shall be designated as permanent drainage easement. No fences or landscaping can be constructed within the easement areas. The lowest adjacent grade of all residential, commercial, or industrial buildings along this emergency overflow route shall be set a minimum of two feet above the flood elevation along the route, calculated based on the pond's emergency overflow weir design discharge.
(8) 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 man-made 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 Boone County Drainage Board and/or Boone County Surveyor) having capacity to receive upstream (off-site) and on-site drainage. A roadside ditch is generally not considered an adequate outlet. The flow path from the development outfall(s) to a regulated drain or natural watercourse (as determined or approved by the Boone County Drainage Board and/or Boone County Surveyor) 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 approval shall be granted for such drainage system until all owners of real estate and/or tenants crossed by the outfall either consent in writing to the use of their real estate or are notified of a hearing relevant to the proposed use. Notification of the time and place of the hearing shall be made in person or by certified mail at least five to ten days prior to the hearing. Proof of notice to each landowner shall be filed by affidavit with the Boone County Drainage Board and/or Boone County Surveyor prior to the hearing. 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.
(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 Boone County Drainage Board and/or Boone County Surveyor.
(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 Boone County 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 Boone County 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 Boone County. The Boone County Drainage Board and/or Boone County Surveyor 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 two-year, ten-year, 50-year, and 100-year storm events. That is, the post-development two-year floodplain storage along a stream shall be the same as two-year pre-development floodplain storage along the stream within the property limits, the post-development ten-year floodplain storage along a stream shall be the same as ten-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 two- 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. 2004-20, passed 12-13-2004)