§ 10-4.303. Stormwater management design standards.

The design and construction of stormwater management systems shall be consistent with the following minimum standards:

(1)

Treatment prior to discharge. Stormwater shall be treated in accordance with the provisions of subsection 10-4.301(1) before it is discharged off-site, unless the stormwater is approved for discharge into a permitted regional stormwater treatment facility. All treatment facilities shall be designed and constructed so as to remain completely functional during the 25-year flood.

(2)

Additional pretreatment for intensive land uses. Any post-development land use involving as a principal activity: The sale or handling of petroleum products; the repair, maintenance, or cleaning of motor vehicles; the outside storage of fertilizers; or the production, distribution or use of any hazardous material regulated by any federal, state, or local government agency; shall include a separate and distinct stormwater management system to treat the runoff from those areas where such activities are conducted, so as to provide intensive pre-treatment and removal of such products or pollutants from the runoff prior to discharge into the stormwater treatment system serving the other areas of the site. Routine maintenance must be specified and a maintenance plan must be provided. The maintenance plan must address removal and approved disposal of products and pollutants to be performed at specific maintenance intervals. A separate operating permit is required if the project is part of a stormwater facility serving more than one parcel.

(3)

Design provisions for maintenance. The design and construction of all facilities shall include adequate provisions, including easements and rights-of-way, to accommodate maintenance activities for the proper operation of the facility.

(4)

Design standards and design life. Stormwater facilities, including pipelines, culverts, retaining walls, and channel linings, shall be designed and constructed to have a minimum 50-year useful life to the maximum extent feasible and in accordance with specifications adopted by the governmental entity.

(5)

Design capacity. Design of stormwater management facilities shall control the rate of flow of stormwater runoff for all intensities, durations and return frequencies up to and including the 25-year storm except that underground pipelines may be designed for lesser magnitude storms, in accordance with the local government policies, if an overland conveyance meeting the requirements of this article is provided for the flow in excess of pipeline capacity. Where use of existing regional or off-site facilities is proposed, documentation must be presented which verifies that adequate capacity is available within such regional or off-site facility.

(6)

Maximum design depth. Design of a development shall ensure that the depth of water in vehicular and pedestrian use areas does not exceed six inches for the critical duration storm, up to and including the 25-year storm.

(7)

Overflow systems. All stormwater detention and retention facilities shall include overflow design features as necessary to minimize the risk of any significant adverse impacts caused by overtopping during storms up to and including 100-year storm events. There shall be at least one foot of freeboard above the peak stage of the 100-year critical storm event and below the top of the pond.

(8)

Runoff buffers. For sites which will contain less than 10,000 square feet or ten percent of impervious surface, whichever is less, alternatives to stormwater detention or retention facilities that utilize landscaped buffers or natural areas as a means of attenuating rate of runoff may be allowed and are encouraged, provided the applicant can demonstrate, to the satisfaction of the county administrator or designee, that adequate water quality treatment can be provided and no significant adverse impacts will result and that the purposes and standards of this article are met by the proposed alternative. Runoff buffers are not allowed for treatment of roadway runoff.

(9)

Interbasin transfer restrictions. Transfer of stormwater runoff from one drainage basin to another, including stormwater runoff from closed basins, shall only be allowed where an assessment of impacts has been made, to the satisfaction of the county administrator or designee, indicating minimal negative impacts to the receiving watershed relative to water quality, quantity and rate of discharge. The assessment of impacts to the receiving watershed shall be in the form of a detailed study which addresses not only the specific impacts of the immediate development activity proposed, but also the potential cumulative impact on the receiving watershed and water bodies which would result from continued interbasin transfers within the watershed. The proposed interbasin transfer shall meet either [subsection] a. or b. below:

a.

The interbasin transfer is necessary for a public sector project, or a private/public joint venture, either of which must benefit a broad segment of the community; or

b.

The interbasin transfer mitigates an existing stormwater problem.

(10)

Slope requirements.

a.

Pond slopes. The side slopes of detention and retention facilities shall conform to design guidelines appropriate to the functional type of the facility but, except as provided for below, shall not be steeper than a 4:1 slope. Grades steeper than 4:1 may be allowed, provided that:

1.

A demonstration is made to, and approved by, the county administrator or designee that there is no reasonable alternative;

2.

The area is surrounded by fencing or another appropriate physical barrier at least four feet in height, the fencing must be green, brown or black, and composed of steel, aluminum, vinyl-clad or other material approved by the county administrator or designee, and is landscaped as required by subsection 10-4.350(a); and

3.

All or part of the detention or retention facility is at or above natural grade, in which case retaining walls may be utilized for sides adjacent to those portions; and

4.

The sides are stabilized with appropriate material having a suitable design life.

(11)

Cut or fill slopes. Cut slopes greater than 2:1, fill slopes greater than 3:1, and any retaining wall higher than two feet, shall require design and certification by a registered professional engineer retained by the applicant, and post-construction certification by the designer that such structure is built consistently with the approved plans and that the required slope compaction and elevation have been achieved. No slopes greater than 1:1 will be allowed unless the county administrator or designee specifically approves the slope stabilization technique to be utilized based on sufficient proof by the applicant that the purposes and standards of this article will be met.

(12)

Landscape credit availability. Credit can be given toward the landscape area requirements of section 10-4.344, if the holding pond meets the criteria of sections 10-4.350 and 10-4.351(e)(6).

(13)

Ditches. Ditches shall conform to the following standards:

a.

Ditch side slopes shall be no steeper than 3:1, unless paved, in which case a 2:1 maximum side slope will be allowed for depths greater than one foot and 1:1 for depths up to one foot.

b.

Ditches shall be grassed and mulched if the peak velocity of stormwater is from zero to two and one-half feet per second, sodded if the velocity is two and one-half to four feet per second, and paved if the velocity is greater than four feet per second. Sodding may be allowed for velocities from four to five and one-half feet per second if sod is lapstaked and joint staggered. Alternate stabilization techniques may be allowed by the county administrator or designee if sufficient additional information is provided by the applicant to assure that ditch integrity will be maintained. Peak stormwater velocities shall be calculated based upon the critical duration of a 25-year return period storm.

(14)

Recovery for rate and volume retention. All facilities that provide retention for rate and volume control shall:

a.

Recovery shall be in accordance with subsection 10-4.301(3)(b). More restrictive recovery times may exist for particular standards, including, but not limited to the Bradfordville Study Area and the Northwest Florida Water Management District Environmental Resource Permitting;

b.

Provide soil borings to a depth of three and one-half times the depth of the pond extended below the proposed bottom of pond elevation (one boring if pond is less than one-half acre, two borings if pond is greater than one-half acre but less than one acre and one additional boring for each additional acre of pond);

c.

Substantiate percolation rates by providing stabilized double ring percolation tests, or other tests approved by the county administrator or designee, located one foot below the proposed pond bottom and in each soil strata indicating low permeability (one test if pond is less than one-half acre, two tests if pond is greater than one-half acre but less than one acre and one additional test for each additional acre of pond).

d.

Substantiate recovery: A geotechnical report, prepared and certified by a licensed professional engineer qualified to do such a report, pursuant to F.S. ch. 471, shall demonstrate recovery by addressing the following:

1.

The saturated soil infiltration rate below the retention facility;

2.

Whether confining layers exist below the proposed retention facility that will restrict infiltration;

3.

Whether groundwater mounding is likely to occur in the retention facility;

4.

The recommended design infiltration rate to be used for the design of the proposed retention facility. The recommended design infiltration rate shall consider impacts from construction operations, groundwater mounding and long term operation of the retention facility to the satisfaction of the county administrator or designee.

e.

Site specific conditions may dictate stormwater facility testing, as determined by the county administrator or designee, to demonstrate that the design recovery rate is being met, prior to allowing development activity to proceed beyond the construction of the stormwater management facility.

(15)

Trash screen. The discharge orifice or weir in the stormwater pond shall be protected from clogging by a submerged trash screen and shall be designed for self cleaning.

(16)

Minimum access stabilization requirements and permitting. It is the intent of this subsection to provide stabilization of all new roadway accesses to prevent erosion, sedimentation and water quality problems in surface waters.

a.

All new roadways and ingress/egress easements that serve three or more single-family residential parcels must be stabilized to prevent erosion, sedimentation and water quality problems in surface waters. This stabilization requirement applies as long as there are no conflicts with more stringent standards in the Code. If a conflict exists, the more stringent standard shall apply.

b.

A short form permit will be required provided that the roadway or ingress/egress easement:

1.

Has an exposed sandy soil footprint of less than 5,000 square feet, or an impervious area of less than 8,000 square feet, or a graveled area of less than 10,000 square feet.

2.

Has a width of ten feet or less and constructed on grade.

3.

Has a parallel downstream swale at least six feet in width and between 12 and 15 inches in depth, ditch blocked as necessary to retain at least 1.125 inches of runoff from the roadway, and directed to properly sized and stabilized conveyances if the swale provides a concentrated discharge.

4.

Provides no anticipated adverse impact on downstream property.

c.

Roadways and ingress/egress easements meeting the requirements in subsections (16)a. and b. above shall not be required to obtain an operating permit and property owner's association as required in section 10-4.209.

(17)

Pervious pavement for pre-existing unstabilized roads. Pre-existing unstabilized roads which are proposed for stabilization with pervious surfacing treatments such as hard gravel, porous concrete, or open grade cold mix asphalt shall be eligible for a permit if all of the following conditions are demonstrated by the applicant to the satisfaction of the county administrator or designee:

a.

The road was in existence and had active use prior to February 20, 2001.

b.

The road is available without having to obtain the permission of any property owner or group of property owners to access or use the road.

c.

The road is a source of sediment or turbidity that discharges to water courses, water bodies, wetlands, or other offsite areas and such discharge results in environmental degradation.

d.

There is insufficient property or right-of-way associated with the road to construct water quality treatment facilities as required by code or it will not be possible to obtain such property or right-of-way.

e.

Engineering specifications and typical cross-sections for the proposed pervious surfacing treatment must be submitted and approved by the division of environmental compliance.

f.

The applicant provides assurance in an operating permit that periodic maintenance to retain the pervious nature of the surface treatment will be performed.

(18)

Additional requirements for the Bradfordville Study Area. For those properties within the Bradfordville Study Area, these provisions shall be supplemented by the more stringent standards set forth in section 10-4.382.

(19)

Required disclaimers. Any project for which stormwater management is provided by a system or facility not maintained by the city or the county, shall contain the following statement on the plans: "Leon County (or where applicable: The city) is not responsible for the maintenance, upkeep or improvement of any stormwater management facility utilized by the land described herein. Title to this property carries with it the requirement that the current and all subsequent owners or their authorized agent obtain a stormwater management facility operating permit from the county (or where applicable: City). The owner of this property shall be legally responsible, jointly with other owners using the facility and based on pro rata share, for compliance with all stormwater management facility operating permit maintenance and operation requirements, as well as all other permit conditions.

(Ord. No. 07-20, § 2, 7-10-07; Ord. No. 09-23, § 8, 7-14-09; Ord. No. 12-07, § 6, 5-8-12; Ord. No. 14-10, § 10, 6-10-14)