(A) 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, except for situations provided in divisions (A)(2) and (A)(3), the storage and controlled release of excess stormwater runoff shall be required for all developments and redevelopments located within the city.
(1) General release rates. In general, the release rates of stormwater from developments shall be:
Post-developed peak rate | Must not exceed | Pre-developed peak rate |
10-year frequency storm | 10-year frequency storm | |
100-year frequency storm | 25-year frequency storm |
(a) That is, the release rates for developments up to and including the ten-year return period storm may not exceed the pre-developed peak ten-year return period stormwater runoff rate. The release rate for developments for the 11 to 100 year return period storms shall not exceed the pre-developed peak 25-year return period rate. For sites where the predeveloped area has more than one outlet, the release rate should be computed based on pre-developed discharge to each outlet point. Methodology and computer models used for the analyses of pre- and post-developed conditions shall be the same.
(b) In cases where adequate infrastructure exists, and downstream properties would not be affected adversely, storm events greater than 4% AEP may be released undetained. In cases where adequate infrastructure does not exist and/or downstream properties would be affected adversely by releasing storm events greater than the 4% AEP storm, the facility shall control the release rate from the 2% and the 1% AEP storm events to the undeveloped flow rates. Downstream analyses shall extend to a point as directed by the city to assure adequate investigation into the impact of the development on existing facilities, properties, and structures.
(2) 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, i.e., 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 onsite runoff may be severely affected if the off-site area is considerably larger than the onsite area. As a general guidance, on-line detention may not be effective in controlling onsite 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.
(3) 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 or other measures, as determined by the city, shall be required to store that portion of the runoff exceeding the capacity of the receiving sewers or watercourses.
(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 unless site or other conditions favor adjusting the ratio up or down.
(c) Sandy Branch is a watershed that is recognized as having downstream restrictions where additional controls may be necessary.
(d) Von Fange Legal Drain is a watershed that is recognized as having downstream restrictions where additional controls may be necessary.
(4) Exemptions for detention requirements. Detention will not be required under the following circumstances with written justification submitted by the designer for city review and approval:
(a) 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) Approved 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 of the property will not be altered by the filling operations.
(c) Not withstanding the provisions of division (A)(2), 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 two-, ten-, or 100-year storm events; and
2. The volume of runoff for each project site outlet has not been increased for two-, ten-, or 100-year storm events; and
3. The flow width and velocity at the property boundary line for each subbasin is less than or equal to that flow width and velocity which existed prior to the development for the two-, ten-, or 100-year storm events.
(d) Where the direct release of runoff from the proposed development meets the conditions set forth in division (A)(5).
(e) In certain circumstances, the design of a system that holds storage volumes other than the minimum amounts set out in this chapter may be appropriate. Instances in which variance from these standards may occur include, but are not limited to:
1. Locations adjacent to a river or large stream, or properties in the 1% AEP flood plain;
2. Where runoff storage is deemed by the city as not beneficial to the proposed improvement and surrounding areas; and
3. Areas in which the circumstances and conditions of the existing system require higher levels of detention than are defined in these standards.
(5) Direct release provisions.
(a) It is the policy of the city to allow the direct release (no detention) of runoff from a proposed development to an adjacent stream where the stormwater runoff from the development would be negligible to the receiving channel if supported by studies from the developer's engineer and agreed to in writing by the city.
(b) Due to unknowns regarding the future development patterns and the associated proposed stormwater management systems within a watershed, it is the policy of the city to discourage direct release. However, in circumstances, where a comprehensive watershed-wide hydrologic study or watershed plan of a major stream adopted by the city substantiates the benefits of (or allows for) direct release for a proposed development located adjacent to a major stream, the detention requirements set in (A)(4)(c) may be waived.
(c) In substantiating the potential benefits of direct release, the watershed-wide hydrologic study provided by the applicant must demonstrate that the peak discharge associated with two-year, ten-year, and 100-year precipitation events would not increase along the receiving stream. At a minimum, the stream reach to be examined needs to extend from the direct release point to a point downstream with a drainage area at least ten times the drainage area of the proposed development and its off-site contributing drainage area. The required analyses must be done both for the existing land use and future potential land use (developed conditions) in the watersheds involved.
(d) To be applicable to the development site, the sub-basin sizes for the watershed-wide analyses of the major stream (including the sub-basin area containing the proposed development and its off-site contributing areas) must be generally uniform (between 0.5 and two times the average sub-basin size). Furthermore, the maximum size of the sub-basin area containing the proposed development and its off-site contributing areas should not exceed five times the area of the proposed development.
(B) Grading for building pads and minimum floor elevations.
(1) Residential, commercial or industrial structures located within a FEMA Special Flood Hazards Area or within the Floodplain of an area with a drainage basin greater than or equal to one square mile shall have a lowest floor elevation, including basement, greater than two feet above the Q100 Base Flood Elevation (BFE). (See Appendix A)
(2) Residential, commercial or industrial structures located adjacent to regulated drains or detention facilities shall have the lowest adjacent grade greater than two feet above the Q100 BFE or emergency spillway high water elevation elevation, whichever is higher. In addition if the structure is located within 50 feet of the thalweg of a regulated drain or normal pool of a detention basin, the basement floor elevation shall be one foot above the thalweg or normal pool. (See Appendix B)
(3) Residential, commercial or industrial structures located adjacent to rear yard swales shall have the lowest adjacent grade greater than two feet above the Q100 BFE. (See Appendix C)
(4) Residential, commercial or industrial structures located adjacent to Drainage System Overflow shall have the lowest adjacent grade elevation greater than one foot above the Q100 overflow path/ponding elevation.
(5) Residential, commercial or industrial structures shall have a first floor elevation no less than 6 inches above the adjacent grade.
(6) Residential, commercial or industrial structures shall have a first floor elevation no less than 15 inches above the adjacent street gutter elevation. (See Appendices A through D)
(7) Residential, commercial or industrial structures shall have maximum yard slopes less than or equal to 3:1 and greater than or equal to 2% where ground has been disturbed during construction. (See Appendices A through D)
(8) The 100-year flow paths throughout the development, whether shown on FEMA maps or not, must be shown as hatched area on the plans and 30 feet along the centerline of the flow path contained within permanent drainage easements. 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 homeowners association or property owner. 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 or be separated by a minimum distance of 50 feet from the overflow path/ponding boundary.
(9) It shall be the property owner's responsibility to maintain the natural features on their lots and to take preventive measures against erosion and/or deterioration of natural or manmade features on their lots.
(C) Adjoining property impacts policy.
(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.
(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 natural 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 city 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, the developer will be responsible to determine if the project requires County Drainage Board approval and show proof to the city prior to obtaining City Stormwater Plan approval.
(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 city.
(D) Calculations, design standards and specifications. The calculation methods as well as the type, sizing, and placement of all stormwater facilities shall meet the adopted design criteria, standards, and specifications outlined in the Seymour Design Manual and the Jackson County Stormwater Technical Standards Manual as identified in § 158.31(B). The methods and procedures in the manuals are consistent with the policy stated above.
(E) Gasoline and diesel fuel regulations. New retail gasoline or diesel fuel outlets, new municipal, state, federal, or institutional gasoline or diesel refueling areas, or new privately owned gasoline or diesel refueling areas, or existing gasoline or diesel outlets and refueling areas that replace their existing tanks or install additional new tanks must install appropriate practices to reduce lead, copper, zinc, and hydrocarbons in stormwater runoff. The treatment system installed must be capable of removing 80% of floatables, sediments, and oils from the on-site stormwater system before discharging to a City of Seymour storm sewer, Jackson County Legal Drain, or other natural or artificial drainage way.
(Ord. 20, 2007, passed 12-11-2006; Am. Ord. 13, 2019, passed 8-12-2019)