§ 62-3756. Conflicting regulations.

In the case of direct conflict between any provision of this division, or the stormwater management criteria established hereunder, and any part or provision of any other applicable federal, state, or the county regulation, the more restrictive shall apply.

(Ord. No. 93-22, § 1(14-90), 8-23-93)

EXHIBIT A TO DIVISION 6. (STORMWATER MANAGEMENT CRITERIA)

PURPOSE AND AUTHORITY

The purpose of these standards is to establish the minimum criteria for the design, construction, operation and maintenance of all stormwater management systems and any activity which affects a stormwater management system within the county. Any land alteration activity, modification of existing stormwater management system, or modification of existing site other than subdivisions, site plans, or single family homes which results in stormwater quality degradation or flooding, must be approved by the surface water improvement division. Subdivisions and site plans must be coordinated with surface water improvement division and approved by the engineering department director or his designee (known hereafter as the reviewer).

These criteria do not apply to agricultural zoned properties which are under or entering into a soil conservation service conservation plan or a St. Johns River Water Management District (SJRWMD) agricultural discharge permit.

WAIVERS

1.

General. Where the board finds that undue hardship or unreasonable practical difficulty may result from strict compliance with this exhibit, the board may approve a waiver to the requirements of this exhibit if the waiver serves the public interest.

2.

Conditions. An applicant seeking a waiver will submit a written request to the board stating the reasons for the waiver and the facts which support such waiver. The board shall not approve a waiver unless it determines as follows:

(a)

The particular physical conditions, shape, or topography of the specific property involved causes an undue hardship to the applicant if the strict letter of ordinance [this exhibit] is carried out.

(b)

The granting of the waiver will not be injurious to the other adjacent property or water body.

(c)

The conditions, upon which a request for waiver are based, are peculiar to the property for which the waiver is sought and are not generally applicable to other property and do not result from actions of the applicant; and,

(d)

The waiver is consistent with the intent and purpose of the county zoning ordinance, the county comprehensive plan, and the requirements of this exhibit.

If the board approves a waiver, the board may attach such conditions to the waiver as will assure that the waiver will comply with the intent and purpose of this exhibit.

GENERAL

1.0 Background. The problems associated with stormwater runoff in rapidly urbanizing areas have become well known. These problems relate to both the quantity and quality of stormwater runoff. Major problems include the increased runoff and flooding magnitude and frequency, accelerated erosion of the land and stream channels, and water quality degradation, wetland mismanagement, and under utilization of prime aquifer recharge areas.

The basic underlying cause of these problems is not difficult to understand. The hydrologic systems which have reached a natural equilibrium over centuries simply cannot adjust gracefully to the sudden impact of urban development. Flooding occurs because the increased volume and peak rate of runoff exceeds the natural carrying and/or storage capacity of natural or manmade water bodies. Erosion is accelerated by the decrease of natural vegetation and increase in the volume of runoff which increases flow velocities and flooding frequency. The water quality itself is degraded by the pollutants of urbanization, decrease in natural vegetation, and increase in erosion of soil which is washed off the land surfaces into the streams, lagoon and lakes. In order to minimize the future degradation of the environment, the designer will be required to work within the guidelines of the following general criteria:

(a)

The drainage system for each phase of a development shall be capable of standing on its own if subsequent areas planned for development are not developed.

(b)

The stormwater management system for new development shall have the capacity to carry existing upstream runoff through or around the development.

(c)

The storage and controlled release or retention on site and infiltration into the ground of excess stormwater runoff rate from any new commercial, industrial, and residential developments will be required so that runoff rate therefrom will not be greater than it was prior to such development as specified in section 4.1.

(d)

No site alteration shall allow water to become a health hazard.

(e)

Data shall be required from the applicant to prove the adequacy of the stormwater system.

(f)

While the following design and construction criteria are given, it is not the intention to discourage innovative designs or construction techniques. Such innovations shall be submitted to the reviewer prior to plan preparation to determine their applicability on a case by case basis.

(g)

In the event the applicant chooses to discharge off-site, the applicant shall provide the necessary facilities to drain site run-off to positive outfalls. Those outfalls shall be public drainage systems with adequate capacity, direct discharge to natural creeks, rivers or major wetland systems, or positive outfalls that can be legally maintained in permanent use. If the applicant elects to drain the site into a public drainage system, the applicant must have the consent of the governmental entity which exercises control over the public drainage system. See section 2.2C for more details on easement requirements and section 4.1 for properties with no outfalls.

1.1 Water quantity. All site alteration activities shall provide for such storm-water control structures as may be necessary to insure that the post-development runoff rates will not be greater than the pre-development runoff rates for designated storm events shown in section 4.

No site alteration shall adversely affect the existing surface water flow pattern. Watershed boundaries shall be maintained in order to conform with existing conditions where practical.

1.2 Water quality. Site alterations shall minimize siltation and pollution of Class 1, 2, or 3 or Outstanding Florida Waters (OFW) waters of the state and shall optimize the natural retention and filtering capabilities of wetlands. Surface water discharges of chemicals, hazardous materials, concentrated pollutants, or other substances deemed improper by chapter 14 of this Code shall be prohibited.

1.3 Recharge. Drainage facilities shall be designed to promote the infiltration of natural rainfall into the soil and to minimize direct overland runoff into adjoining streets and watercourses. Storm-water runoff from roofs and other impervious surfaces should be diverted into vegetated areas whenever possible.

Certain areas of the county have soils which are considered to be aquifer recharge zones and have additional drainage requirements. Consult the county office of natural resources management division for specific details of the aquifer recharge ordinance.

1.4 Wetlands. The use of existing wetlands for stormwater treatment rather than destruction of the wetlands is encouraged.

1.5 Flood prone areas. Development within the ten, 25, and 100 year floodplains has further stormwater requirements. Consult the comprehensive plan and floodplain protection ordinance for specific details.

For subdivision and site plans altering or amending the 100 year floodplain, a conditional letter of map revision or amendment must be initiated with FEMA prior to final plan approval.

1.6 Off-Site Improvements. Developments may require off-site drainage improvements in order to insure the proper functioning of the on-site system.

2.0 Easements:

2.1 Purpose. Easements for storm water facilities are needed for maintenance purposes. Conveyance facilities such as pipes, inlets, ditches, and weirs need to be kept open and functional to prevent flooding. Water treatment facilities such as ponds need to be maintained to their design standards to insure the stormwater is cleaned before leaving the system.

Generally, for county maintained systems, i.e. public roads and ditches, the county will maintain the conveyance facilities to prevent flooding. These conveyance facilities include the roads, ditches, inlets, pipes, and outfall structures that carry water into and out of a pond.

The county generally will not maintain the treatment systems because they are not necessary to prevent flooding within the subdivision. It is up to the property owners to maintain the pond or other treatment facilities.

2.2 Requirements. Easements for stormwater facilities shall be provided as follows.

(a)

Retention/detention ponds in subdivisions.

1.

A drainage easement over the pond will be dedicated to the maintenance entity who will be responsible for maintaining the pond to meet the water quality and quantity design standards of the approved plans.

2.

Deed restrictions shall be written making the maintenance entity responsible for maintaining the pond to meet the water quality and quantity design standards of the approved plans. See section 7 for maintenance entity requirements.

3.

A drivable stabilized access easement a minimum of 12 feet wide shall be provided for emergency access from a road to the pond control structure.

4.

Fences shall not be constructed in drainage easements if the fence interferes with the function of the drainage pipes, swales, or ditches in the easement. If the county removes fences in drainage easements in order to maintain drainage facilities, the county shall not be required to replace or pay for said fences.

(b)

Easement dimensions.

1.

Drainage easements shall be given over any portion of a stormwater system not within a right-of-way and necessary for the functioning of the system. The easement shall include the facility plus any required access area to be used for maintenance.

2.

The facility width for open systems shall be measured from top of bank to top of bank. The following criteria establishes the required minimum width of drainage easement.

Drainage System (Other than swales)	Minimum Stabilized Maintenance Access Easement
Open Drainage Systems
Top width 20' or less	15' on one side
Top width 20' to 40'	15' on both sides
Top width greater than 40'	20' on both sides
Pipe systems	Diameter + 4 feet + 2D D=depth from finished grade to pipe invert, in feet. Minimum width = 15 feet
For county maintained ponds
With fencing	25' around pond perimeter
Without fencing	20' around pond perimeter
Other ponds	None required

 

The maintenance easement shall have a cross slope no steeper than 10:1.

The minimum allowable width of drainage easements or right-of-way may be increased. Any administrative waiver to the above easement width must be approved by the reviewer.

(c)

Other drainage easements.

1.

Private development. The property owner is responsible for maintaining the stormwater system to meet the water quality and quantity standards of the approved plans.

Public drainage facilities which are located within a private development shall be located in a public drainage easement.

2.

Unplatted land. Developments may contain drainage systems which traverse property outside the project limits. These may be adjacent lands which were not platted, future phases of the development to be platted at a later date, part of an overall master plan, as in a planned unit development or site plan, or other circumstances. A drainage easement for the system per section 2.2(b) and its maintenance must be provided to the entity responsible for maintenance.

2.3 Computer software.Computer programs used for drainage calculations shall meet the approval of the reviewer. Stormwater systems with more than one pond should use multiple pond routing software. The reviewer shall maintain a list of approved software which shall be kept at the office of engineering design and review division and may be viewed during normal business hours. Software not on this list shall be submitted to the reviewer along with any documentation and manuals prior to plan submittal. The software will be evaluated against approved software to determine its applicability.

3.0 Plan preparation.

(a)

Master drainage map. The applicant shall include in the construction plans a master drainage map showing all existing and proposed features. The map is to be prepared on a 24-inch by 36-inch sheet on a scale not less than one-inch equaling 100 feet. Size and scale may be adjusted for the larger development if approved by the reviewer. The review of said plans shall be made by or under the direct supervision of a professional engineer. Said drainage map shall include the following features:

(1)

Delineation of all areas draining to the proposed subdivision, all areas tributary to existing structures, and all areas tributary to proposed structures. The pervious, impervious, and total acreage of each area must be stated.

(2)

Topographical information with sufficient elevations to verify the location of all ridges, depressions, conveyances and other water resources. Direction of flow shall be clearly labeled.

(3)

Existing and high water data on existing structures upstream and downstream from the subdivision.

(4)

Notes indicating sources and date of high water data.

(5)

Notes pertaining to existing standing water, areas of heavy seepage, or springs.

(6)

Existing and proposed drainage features such as ditches, swales, roadways, and ponds. Ponds shall have dimensions and be tied to property lines for construction staking purposes.

(b)

Plans. In addition to the drainage map the following information shall be provided:

(1)

Cross sections and details of proposed control structures in ponds shall be drawn to determine feasibility of construction.

(2)

Plan and profiles shall be drawn for all roads with storm sewers at a maximum scale of one-inch to fifty feet. The plan view and profile view for a road shall be drawn on the same sheet of paper. Profiles of all storm sewers in the road shall be shown. Other existing or proposed utilities shall also be shown in such a manner as to easily check for conflicts and minimum cover.

(c)

Drainage calculations. Drainage calculations shall be attached providing the following information:

(1)

Pre-development and post development hydrographs, storage available and required, and minimum and maximum water elevations, stage-discharge calculations, and aquifer recharge calculations.

(2)

Culvert calculations.

(3)

Storm sewer calculations.

(4)

A subsoil investigation report following ASTM standards and performed by a geotechnical engineer shall be submitted with the construction plans and shall include:

a.

Test borings at 500-foot intervals along roadways to a depth sufficient to locate the ground water table or to a depth five feet below the proposed centerline grade, whichever is greater. Additional bores may be required in muck areas to determine the limits of muck areas.

b.

Test bores in all stormwater ponds to determine seasonal high groundwater levels to the satisfaction of the reviewer. If dry ponds are proposed, the test bore should extend to ten feet below existing ground to determine the presence of hardpan or impermeable layers. Permeability tests are required where infiltration calculations are used.

Generally all of the above information is necessary to perform an effective review. Submittals without the above information will not be reviewed until such information is provided.

(d)

Record drawings. After construction of the required improvements, one copy of record drawings signed and sealed by a professional land surveyor shall be provided to the surface water improvement division by the land development division for subdivisions and site plans. Said record drawings shall provide elevations, dimensions, and sizes of the constructed stormwater facilities.

4.0 Design criteria:

4.1 Design storm (minimum).

Facility	Design Storm
Retention/detention ponds (w/positive outfall)	25 year, 24 hour
Retention/detention ponds (landlocked w/no positive outfall)	25 year, 96 hour total retention
Roadside swales, swales, channels	10 year, 24 hour
Arterial and collector street pipe systems	10 year, hydraulic gradient line 1.0 feet below gutter line
Local street pipe systems	10 year, hydraulic gradient line 0.5 feet below gutter line
Canals, outfall ditch or outfall channels	25 year, 24 hour
Bridges	100 year, 24 hour

 

Streets shall be designed so that the lowest crown elevation is at or above the 25-year peak flood stage.

Rainfall amounts shall be as follows:

10-year 24-hour storm = 7.9"

25-year 24-hour storm = 9.0"

100-year 24-hour storm = 11.0"

25-year 96-hour storm = 12.5"

In order to protect upstream or downstream properties, the retention/detention requirements may be increased if site specific drainage problems are known by the county.

4.2 Retention volume. Stormwater facilities shall meet SJRWMD criteria for retention volumes, treatment methods, and pond design criteria unless otherwise noted.

4.3 Pond flow calculation methods.

1.

The following methods shall be used to design retention/detention facilities:

(a)

Santa Barbara urban hydrograph.

(b)

SCS synthetic unit hydrograph — Use peaking factors as recommended by SJRWMD.

(c)

Overland flow model — Appropriate for flat, rural areas.

(d)

TR 55 — Appropriate for sites ten acres or less. Use the modified tables shown in "appendix A."

(e)

Other approved hydrograph methods.

4.3.1 Calculation variables.

(a)

Rainfall distribution. For hydrograph methods, use the SCS Type II Modified rainfall distribution.

(b)

The soil survey of the county, published by the U.S. Department of Agriculture, shall be the document to determine soil classifications. Curve numbers used in hydrograph calculations shall be determined using exhibit 4-2.

(c)

Soil storage. Post development curve numbers may be calculated by either of the following methods:

1.

Curve numbers from the developed urban section of exhibit 4-2.

2.

Under post development conditions, the pervious areas generally will not percolate water as well as in predevelopment conditions due to compacted yard fill, topsoil additions, sodded grass, and other conditions. Therefore, when calculating a composite post development CN, the pervious areas shall use a CN value based on a 25 percent reduction in soil storage capability. This is accomplished using the following formula:

Soil storage = 1000 / CN - 10

3.

Wet ponds shall be considered impervious areas and dry ponds shall be considered pervious areas.

(d)

Time of concentration. The time of concentration should be divided into various components of flow conditions, i.e. sheet flow, channelized flow, gutter flow, pipe flow, etc. The individualized components will be summed for a total time of concentration.

The following equations from TR-55 shall be used for time of concentration calculations:

1.

Sheet flow for a maximum of 300 feet use the kinematic wave equation.

Tt =	.007(nL) 0.8
	(P 2 ) 0.5 s 0.4

 

where: Tt = travel time in hours
n = Mannings coefficient from exhibit 4-5
L = flow length
P ₂ = rainfall depth for a two-year storm (five inches)
s = land slope in ft./ft.

2.

Shallow concentrated flow in small swales and gutters.

Unpaved V = 16.13 (s) ^0.5

Paved V = 20.33 (s) ^0.5

Tc = L / Vx60

where: Tc = travel time in minutes
V = velocity in f/s
s = land slope in ft./ft.

3.

Other conveyance systems shall use the velocity calculated from Manning's equation.

4.4 Retention—Detention facilities.

(a)

Design criteria.

1.

Peak attenuation. Detention facilities shall be designed so that the peak discharge computed for the post development conditions, resulting from the design storm shown in section 4.1, shall not exceed the predevelopment peak discharge resulting from the design storm.

(b)

Outfall. Retention/detention ponds shall be required to have an outfall structure. The location of the structure and the shape of the pond shall be designed such that no "short-circuiting" of flow occurs and that maximum retention of suspended solids is achieved.

Unless otherwise approved by the reviewer outfall structures shall be as simple as possible and shall employ fixed control elevations (i.e., no valves, removable weirs, etc.). The outfall structure shall have a skimmer blade to filter floating trash and oils. The gap between the skimmer and structure will be designed as a free flow, non restricting orifice to meet peak flow rates. It should extend six inches above the design highwater elevation and six inches below the weir or orifice.

(c)

Recovery rates. The retention volume recovery systems should be designed to comply with St. Johns River Water Management District (SJRWMD) criteria from F.A.C. 40C-42. Land locked systems shall infiltrate one-inch of runoff in 72 hours and the remaining volume in 14 days. All other systems shall recover the required retention volume in 72 hours. See "exhibit 4-1" for typical underdrain calculations. A safety factor of two shall be used in the design of infiltration or underdrain systems for retention volume recovery.

(d)

Groundwater table. The seasonal high groundwater table may be lowered to the normal groundwater table depth with the following exceptions:

1.

When a lowering of the groundwater table results in adverse impacts to the hydrology and beneficial functions of adjacent wetlands.

2.

Barrier islands.

3.

When facilities which would negatively impact an aquifer are located in an aquifer recharge zone or when lowering of the groundwater table impacts users of the surficial aquifer as a drinking water supply.

4.

When the tailwater will not allow positive outfall for retention recovery or detention discharges.

5.

The county's wetlands and aquifer recharge ordinances shall be used as a guide in determining the above exceptions.

(e)

Detention facility outfall weir design. In the design of detention facilities, where direct discharge is allowed, that discharge may be controlled by the use of a weir structure. In the event that this control device is utilized, the following weir flow formulas shall apply:

1.

Free discharge: Q=CLH ^1.5

Discharge over the crest of a weir structure

where:
Q = discharge, cfs.
^*  C = 3.16 for sharpcrested weirs
^*  C = 2.7 for broadcrested weirs
L = effective weir length, feet
H = head on weir, feet-head

*Other weir coefficients may be used if the engineer can justify their use to the reviewer.

2.

Submerged discharge. When the tailwater rises above the weir crest elevation the actual discharge over the weir is inhibited by the backwater conditions. The above calculated "free" discharge value is multiplied by the following reduction factor to account for the submergence effect:

Q _s = Q _f [[(1-H ₂ )] ^1.5 /H ₁ ] ^0.385

where, Q _s = submerged flow, cfs.
Q _f = free flow, cfs.
H ₁ = upstream head above crest, feet-head.
H ₂ = downstream head above crest, feet-head.

3.

Narrow weirs. For conditions of high head and/or short weir length, end contractions can restrict flow. To account for this, use the equation:

Leff = L (1-.1nH1)
Leff = effective weir length
L = actual weir length
n = number of end contractions
H1 = head on weir

(f)

Detention facility bleeder design. Bleed down devices may be used in wet detention systems. Such bleed down devices include, but are not limited to, V-notch weirs, slots, under flow gates, circular orifices, etc. All design of the above devices shall be in accordance with recognized formulas and constants, for the bleed down configuration used. A minimum size for all orifices shall be three inches diameter.

(g)

Retention outfall filters. The stormwater retention volume must be treated prior to drawdown to the discharge system. Percolation is encouraged for retention systems to utilize the natural filtration of the existing site soils.

(h)

Side slopes. Wet ponds shall have side slopes no steeper than 5:1 to two feet below normal water surface and 2:1 to 12-foot maximum depth and five-foot minimum depth. For non-wet detention systems, as defined by SJRWMD the five-foot minimum depth can be waived if littoral vegetation is planted in the areas less than five feet deep. The littoral plant types shall follow SJRWMD criteria for littoral zones.

Dry ponds shall have side slopes no steeper than 3:1.

(i)

Slope protection. The disturbed areas in and around the ponds shall be revegetated as follows:

Side slopes and berms	5 to 1 and up = seed & mulch 4 to 1 and steeper = sod
Bottom of dry pond	Grass (seeded and mulched)

 

(j)

Pond location. It is strongly recommended that centralized retention/detention ponds be utilized rather than numerous small ponds or individual ponds on individual lots. Swales along side lot lines should be used for conveyance only, not for retention areas.

(k)

Dry ponds. Dry ponds may be used if the bottom of the pond is at least one-foot above the seasonal high ground water level. In type I and II aquifer recharge areas, as defined by the county comprehensive plan, the bottom of a retention pond shall be at least two feet above the seasonal high water table. The reviewer may require a ground water mounding analysis where appropriate. This may occur for soils with low permeability rates, perched groundwater, high pond storage depths, or other reasons. If the bottom of a dry pond is less than one-foot above the seasonal high ground water level due to physical constraints of the site, or with the approval of the reviewer, the bottom of the pond shall be planted with the wetland vegetation to control cattail growth as follows:

COMMON NAME	SCIENTIFIC NAME
Cordgrass	Spartina bakeri
Maidencane	Panicum hemitomon
Sedges	Cyperus spp.
Wax myrtle	Myrica cerifera
Red bay	Persea borbonia

 

Other wetland plants may be used with reviewer approval.

4.5 Roadway (pavement) drainage design.

(a)

General. Good pavement drainage design consists of the proper selection of grades, cross slopes, curb types, inlet location, etc., to remove the design storm runoff from the pavement in a cost effective manner while preserving the safety, traffic capacity and integrity of the highway and street system. These factors are generally considered to be satisfied when excessive spreads of the water are removed from the vehicular travel way and when siltation at pavement low points is not allowed to occur. The standards included herein are intended to accomplish these objectives.

(b)

Roadway grade. The minimum allowable centerline grade for all streets with curb and gutter shall be 0.30 percent and 0.40 percent for curb radii grades.

(c)

Cross-slope. The minimum allowable cross-slope for all streets shall be ¼-inch per foot.

(d)

Drainage structures. All drainage structures, unless specifically detailed in these guidelines, shall at a minimum conform to the latest edition of the FDOT roadway and traffic design standards.

Any drainage structure not detailed in the FDOT design standards shall be designed in conformance with good engineering practices and shall require approval by the reviewer.

Where deemed necessary, the reviewer may require a drainage structure design differing from FDOT design standards.

(e)

Swales.

1.

The use of roadside swales is encouraged to pro mote groundwater recharge and provide stormwater treatment.

2.

Roadside swales shall have maximum front slope of 5:1 seeded or 4:1 sodded and maximum back slope of 3:1 sodded. The minimum flow line grade shall be 0.10 percent when used for conveyance and 0.00 percent when used for retention.

3.

Runoff may be accumulated and carried in the swales up to, but not above, the point where flooding of the shoulders or adjacent property would occur. Ditch blocks in roadside swales shall have a maximum height of 12 inches and shall not be constructed of earth.

4.

Roadside swales can be used for retention if the flow line is a minimum of one-foot above the SHGWT with demonstration by approved groundwater mounding or flow net analysis that the retention volume is recovered within 24 hours.

(f)

Curbs and gutters.

1.

Curb and gutter sections shall have a maximum run of 400 feet between on grade inlets. Distances greater than 400 feet must be substantiated with calculations.

(g)

Runoff determination. The peak rates of runoff for the pavement drainage system shall be determined by the rational method.

FDOT methodology, forms, and intensity curves shall be used. Calculation of junction losses will permit the hydraulic grade line to be raised to the gutter elevation. The minimum time of concentration will be ten minutes.

Exhibit 4-3 shall be used to determine runoff coefficients (c) for drainage areas.

(h)

Stormwater spread into traveled lane. Inlets shall be spaced at all low points, intersections and along continuous grades so as to prevent the spread of water from exceeding tolerable limits. The acceptable tolerable limits for multilane arterial and collector roadways is defined as one traveled lane width. The acceptable tolerable limit for interior subdivision roadways is defined as the seven feet from the face of curb if curb and gutter is used.

(i)

Inlet types. The curb inlet types and capacities to be used shall be the latest version of the state department of transportation inlets.

(j)

Low point inlets. All inlets at low points (sumps) shall be designed to intercept 100 percent of the design flow without exceeding the allowable spread of water onto the traveled lanes as defined above.

(k)

Inlet capacities. Inlet capacities shall be determined using FDOT criteria and tables.

4.6 Storm sewer and culvert design.

(a)

Minimum pipe size. The minimum diameter pipe is 18 inches or equivalent elliptical pipe. The minimum size box culverts shall be three feet by three feet.

(b)

Pipe grade. All storm sewers shall be designed and constructed to produce a minimum velocity of two fps when flowing full. No storm sewer system or portion thereof will be designed to produce velocities in excess of 12 fps for reinforced concrete pipe or eight fps for metal pipe. For other pipe materials the maximum velocity shall be per the manufacturer's recommendations.

The maximum velocity at the outlet pipe shall be in accordance with section 4.7(e) unless velocity control devices are used.

(c)

Pipe cover. Unless otherwise authorized by the reviewer, the minimum cover for all storm pipes shall be:

Description	Cover
From finished grade to outside crown of pipe	1.0 feet

 

(d)

Pipe materials. All pipes within public rights-of-way and saltwater or brackish outfalls shall be reinforced concrete pipe or PVC meeting DOT approval. Other pipe materials may be used at other locations if soil conditions allow and with the approval of the reviewer. All steel pipe shall be completely bituminous coated. Round concrete pipe shall use "O" ring joints.

(e)

Conflict manholes. Conflict manholes shall be used only when there is no reasonable alternative design. Where it is necessary to allow a sanitary line or other utility to pass through a manhole, inlet or junction box, the utility shall be ductile iron.

Where utility lines pass through manholes, the utility shall be placed in such a manner as to provide a minimum of one-foot clearance between the bottom of the manhole and the bottom of the shell of the utility pipe.

Conflict manholes shall be over-sized to accommodate the decreased maneuverability inside the structure and flow retardance. All holes in concrete boxes must be sealed and watertight.

(f)

Maximum lengths of pipe. The following maximum runs of pipe shall be used when spacing access structures of any type:

Pipe Size	Maximum
18 inches	300 feet
24 to 36 inches	400 feet
42 inches and larger	500 feet
Box culverts	500 feet

 

(g)

Design tailwater for pipe systems. The design tailwater level in a pond can be assumed to be the ten-year pond level corresponding to the time at which peak inflow occurs from the storm hydrograph into the pond. In lieu of the above analysis, an optional design tailwater estimate can be obtained by averaging the established 25-year design high water elevation and the normal water surface elevation of the pond. Culverts shall be designed taking into account the tail-water of the receiving facility during design storm conditions.

(h)

Hydraulic gradient line computations. The rational method shall be used for pipe flow calculations. The hydraulic gradient line for the storm sewer system may be computed taking into consideration the design tail-water on the system and the energy losses associated with entrance into and exit from the system, friction through the system, and turbulence in the individual manholes/catch basins/junctions within the system.

The following junction losses shall be used for all inlets and manholes:

H L =	K(V 1 )
	(2g)

 

where:

H _L = head loss in feet
V ₁ = highest velocity into or out of junction
K = energy loss coefficient from exhibit 4-4.

In order to minimize head losses and clogging, storm drain pipes shall not turn angles greater than 90 degrees when going through inlets and junction boxes without the approval of the reviewer.

(i)

Headwalls. All pipe outlets shall have headwalls or mitered end sections.

(j)

Inverted siphons. Inverted siphons shall not be permitted.

(k)

Outfall sumps. Storm drain pipes with outfall sumps shall not be permitted without the prior approval of the reviewer.

(1)

The required culvert size shall be determined using FDOT design nomographs and procedures.

4.7 Open channel design.

General:

Grass swales and open channels may be used in lieu of a closed conduit system to convey stormwater runoff outside of dedicated roadway and street rights-of-way when sufficient drainage easements or rights-of-way are available. Open conveyance systems are often desirable to assist in the mitigation of pollution problems.

The flow line of roadside and lot line swales shall be at least one-foot above the seasonal high groundwater elevation. It is preferred that other channels also have flow lines one-foot above the seasonal high groundwater level for ease of maintenance and cattail control.

(a)

[ Design discharges. ] Design discharges shall be calculated per section 4.1.

(b)

Design tailwater. All open channel systems shall be designed taking into consideration the tailwater of the receiving facility or body of water. The tailwater must be determined by calculations based upon the design criteria and frequencies contained in section 4.1.

(c)

Design formula. Design of all open channel systems shall be based upon Manning's equation.

(d)

Roughness coefficients for use in Manning's equation.

Channel Lining	Description	Design "n"
Bare earth, fairly uniform	Clean, recently completed	0.022
Bare earth, fairly uniform	Short grass and some weeds	0.028
Dragline excavated	No vegetation	0.030
Dragline excavated	Light brush	0.040
Channels not maintained	Dense weeds to flow depth	0.10
Channels not maintained	Clean bottom, brush sides	0.08
Maintained grass or sodded ditches	Good stand, well maintained	0.035
Maintained grass or sodded ditches	Fair stand, length 12 — 24"	0.20 *
Concrete paved	No finish	0.016
Concrete paved	Finished	0.015
Rip-rap	Fairly uniform	0.03

 

^* Decrease 30 percent for flows greater than 0.7 feet depth.

(e)

Maximum allowable velocities for unlined open channels on bare soils.

Soil Type	Allowable Velocity (F.P.S.)
Silt or fine sand	1.50
Sandy loam	1.75
Silt loam	2.00
Firm loam	2.50
Stiff clay	3.75
Hardpans	6.00

 

(f)

Maximum allowable velocities for lined open channels.

Type	Allowable Velocity (F.P.S.)
Sod	4.0
Concrete ditch paving	10.0

 

(g)

Open channel geometry. Open channels may be designed as either a trapezoidal or "V" cross section.

(h)

Minimum longitudinal grade. For open channels the minimum grade allowable shall be 0.10 percent.

(i)

Channel curvature. Channel protection shall be provided when curvature produces erosive velocities as shown above.

(j)

Minimum freeboard. A minimum freeboard of one-foot shall be maintained between peak water surfaces designed from section 4.1 and the top of slope for all open channels.

(k)

[ Preferred geometry. ] The use of serpentine channel geometry rather than straight channels is encouraged.

(l)

Erosion protection. All open channels shall be seeded or sodded for erosion and silt control. If side slopes are steeper than 5:1 they must be sodded. Side slopes steeper than 3:1 may only be used with reviewer approval.

4.8 Exfiltration trenches. Exfiltration trenches may only be used with the approval of the reviewer. The feasibility of long term maintenance and effectiveness must be demonstrated to the satisfaction of the reviewer.

5.0 Material specifications:

5.1 Pipe. All pipe materials shall conform to the latest edition of the FDOT standard specifications for road and bridge construction.

All pipes not employing a water tight band at the joints shall have all joints wrapped with filter fabric.

5.2 Inlets, manholes and junction boxes. All materials used in the construction of inlets, manholes and junction boxes shall conform to the latest editions of the FDOT roadway and traffic design standards and the FDOT standard specifications for road and bridge construction. Inlets shall have poured inverts.

5.3 Underdrains/exfiltration systems. The following is a list of underdrain materials acceptable for use in the county.

(a)

Perforated corrugated tubing. Corrugated, polyethylene tubing perforated throughout and meeting the requirements of AASHTO M-252.

(b)

Perforated PVC pipe. Polyvinyl-chloride pipe conforming to the requirements of ASTM D-3033. The perforations shall meet the requirements of ASTM C-508.

(c)

Exfiltration pipe. The following is a list of pipe materials acceptable for use in ex-filtration systems:

1.

Aluminum pipe perforated 360 degrees, meeting the requirements of AASHTO M-196.

2.

Perforated class III reinforced concrete pipe with perforations meeting the requirements of ASTM C-444.

3.

Polyvinyl-chloride pipe perforated 360 degrees, meeting the requirements of ASTM D-3033.

(d)

Fine aggregate. Sand filter media shall be of a quality sufficient to satisfy the following requirements:

Washed (less than one percent silt, clay and organic matter)
Uniformity coefficient: 1.5 or greater
Effective grain size: 0.2—0.55 mm

(e)

Coarse aggregate. Clean noncalcarious stone containing no friable materials and a gradation equivalent to FDOT size number 56 or 57.

(f)

Filter fabric envelope (sock). Strong, porous polyester knitted fabric. The envelope shall be a continuous one-piece material that fits over the tubing like a sleeve. It shall be knitted of continuous 100—200 denier yarn and shall be free from chemical treatment or coating that might significantly reduce porosity and permeability. The fabric envelope shall conform to the following minimum properties:

Equivalent Opening Size (U.S. Std. Sieve)	100 (0.150 mm)
Tensile strength	50
Bursting strength (PSI)	90—125

 

(g)

Filter fabric. Pervious sheet of monofilament yarn woven, knitted or bonded to form a fabric with the following minimum properties:

Equivalent Opening Size (U.S. Std. Sieve)	100 (0.150 mm)
Tensile strength (lbs)	50
Bursting strength (PSI)	200
Puncture strength (lbs)	32

 

(h)

Impermeable liner. Eight mil. Visqueen or approved equal.

5.4 Drainage structures. All materials used in the construction of drainage structures shall conform to the latest editions of the FDOT roadway and traffic design standards and the FDOT standard specifications for road and bridge construction.

Sand cement is not an acceptable material for drainage structures, but can be used for erosion control.

5.5 Fencing. Unless otherwise approved by the reviewer, all fencing around ponds shall be six-foot chain link fence with a minimum 15-foot wide double gate opening conforming to the FDOT specifications.

5.6 Sod, seed and mulch. All sod, seed and mulch materials and installation shall conform to the latest edition of the FDOT standard specifications for road and bridge construction.

5.7 Skimmer blades. Skimmer blades shall be made of aluminum, fiberglass, or other material approved by the reviewer.

6.0 Erosion and sediment control. Construction activities can result in the generation of significant amounts of pollutants which may reach surface or ground waters. One of the primary pollutants of surface waters is sediment due to erosion. Excessive quantities of sediment which reach water bodies of floodplains have been shown to adversely affect their physical, biological and chemical properties. Transported sediment can obstruct stream channels, reduce the hydraulic capacity of water bodies of floodplains, reduce the design capacity of culverts and other works, and eliminate benthic invertebrates and fish spawning substrates by siltation. Excessive suspended sediments reduce light penetration and, therefore, reduce primary productivity.

Therefore, the following minimum standards shall apply to any construction or maintenance activities within the county.

6.1 Minimum standards.

1.

Sediment basins and traps, perimeter dikes, sediment barriers and other measures intended to trap sediment shall be constructed as a first step in any land-disturbing activity and shall be made functional before upslope land disturbance takes place.

2.

All sediment control measures are to be adjusted to meet field conditions at the time of construction and be constructed prior to any grading or disturbance of existing surface material on balance of site. Perimeter sediment barriers shall be constructed to prevent sediment or trash from flowing or floating on to adjacent properties.

3.

Permanent or temporary soil stabilization shall be applied to denuded areas within seven days after final grade is reached on any portion of the site. Temporary soil stabilization shall be applied within seven days to denuded areas that may not be at final grade but will remain undisturbed for longer than 30 days. Permanent stabilization shall be applied to areas that are to be left undisturbed for more than one-year.

4.

During construction of the project, soil stock piles shall be stabilized or protected with sediment trapping measures. The applicant is responsible for the temporary protection and permanent stabilization of all soil stockpiles on site as well as soil intentionally transported from the project site. Chapter 62 [section 62-2101 of this Code] should be consulted for protection of stockpiles from wind.

5.

A permanent vegetative cover shall be established on denuded areas not otherwise permanently stabilized. Permanent vegetation shall not be considered established until a ground cover is achieved that, in the opinion of the reviewer, is uniform, mature enough to survive and will inhibit erosion.

6.

Stabilization measures shall be applied to earthen structures such as dams, dikes and diversions immediately after installation.

7.

Surface runoff from disturbed areas that is comprised of flow from drainage areas greater than or equal to three acres shall be controlled by a sediment basin. The sediment basin shall be designed and constructed to accommodate the anticipated sediment loading from the land-disturbing activity. The outfall device or system design shall take into account the total drainage area flowing through the disturbed area to be served by the basin.

8.

After any significant rainfall, sediment control structures will be inspected for integrity. Any damaged devices shall be corrected immediately.

9.

Concentrated runoff shall not flow down cut or fill slopes unless contained within an adequate temporary or permanent channel, flume or slope drain structure.

10.

Whenever water seeps from a slope face, adequate drainage or other protection shall be provided.

11.

Sediment will be prevented from entering any storm drain system, ditch, or channel. All storm sewer inlets that are made operable during construction shall be protected so that sediment-laden water cannot enter the conveyance system without first being filtered or otherwise treated to remove sediment.

12.

Before temporary or newly constructed stormwater conveyance channels are made operational, adequate outlet protection and any required temporary or permanent channel lining shall be installed in both the conveyance channel and receiving channel.

13.

When work in a live watercourse is performed, precautions shall be taken to minimize encroachment, control sediment transport and stabilize the work area to the greatest extent possible during construction. Nonerodible material shall be used for the construction of causeways and cofferdams. Earthen fill may be used for these structures if armored by nonerodible cover materials.

14.

When a live watercourse must be crossed by construction vehicles, a temporary stream crossing constructed of nonerodible material shall be provided.

15.

The bed and banks of a watercourse shall be stabilized immediately after work in the watercourse is completed.

16.

Periodic inspection and maintenance of all sediment control structures must be provided to ensure intended purpose is accomplished. The developer, owner, and/or contractor shall be continually responsible for all sediment leaving the property. Sediment control measures shall be in working condition at the end of each working day.

17.

Underground utility lines shall be installed in accordance with the following standards in addition to other applicable criteria:

(a)

No more than 500 linear feet of trench may be opened at one time.

(b)

Excavated material shall be placed on the uphill side of trenches.

(c)

Effluent from dewatering operations shall be filtered or passed through an approved sediment trapping device, or both, and discharged in a manner that does not adversely affect flowing streams or off-site property.

(d)

Restabilization shall be accomplished in accordance with these regulations.

18.

Where construction vehicle access routes intersect paved public roads, provisions shall be made to minimize the transport of sediment by tracking onto the paved surface. Where sediment is transported onto a public road surface with curbs and gutters, the road shall be cleaned thoroughly at the end of each day. Sediment shall be removed from the roads by shoveling or sweeping and transported to a sediment control disposal area. Street washing shall be allowed only after sediment is removed in this manner. This provision shall apply to individual subdivision lots as well as to larger land-disturbing activities.

19.

All temporary erosion and sediment control measures shall be removed within 30 days after final site stabilization or after the temporary measures are no longer needed, in the opinion of the reviewer. Disturbed soil areas resulting from the disposition of temporary measures shall be permanently stabilized to prevent further erosion and sedimentation.

20.

Properties and waterways downstream from construction sites shall be protected from sediment deposition and erosion.

21.

Phased projects should be cleared in conjunction with construction of each phase.

22.

Erosion control design and construction shall follow the requirements in Index Nos. 101, 102, and 103 of FDOT roadway and traffic design standards.

23.

The reviewer may approve modifications or alternate plans to these erosion control criteria due to site specific conditions.

7. Maintenance entity. A maintenance entity must be established to maintain the stormwater system. The maintenance entity shall meet the qualifications of the St. Johns River Water Management District rules (F.A.C. ch. 40C-42 as may be occasionally amended) regardless of whether a St. Johns River Water Management District permit is required or not.

The maintenance entity shall maintain the stormwater management system so that it functions according to the original design intent. Stormwater management systems which are improperly maintained as determined by the surface water improvement division may be subject to having their stormwater utility mitigation credits revoked and the maintenance entity may be subject to the penalties set forth in this exhibit.

Proper maintenance of a stormwater treatment system shall include at a minimum the following items.

1.

Pond side slopes shall be vegetated for erosion control.

2.

Pond storage volume and geometry shall be maintained to the dimensions shown on the approved plans.

3.

Littoral zone coverage with wetland plants shall be maintained to SJRWMD standards.

4.

Vegetation shall be mowed frequently enough to provide access to the ponds for inspections and maintenance.

5.

Weirs and orifices shall be kept clear of debris to allow their proper functioning.

6.

Skimmer blades shall be maintained to minimize floating debris and oils from leaving the ponds and allow unrestricted flow through the control structure.

7.

The dimensions and elevations of control structures shall be preserved.

8.

Underdrains shall be properly functioning.

9.

Channel dimensions and geometry shall be maintained to approved designs.

10.

Vegetation in dry ponds and channels shall be kept to a minimum so as to maintain flow and storage capacities.

GLOSSARY

Aerial support means structural supports used to suspend utility lines above the ground.

Arterial streets means streets and highways which are used primarily for arterial traffic and defined as such by state statutes.

Bridge means a traversing work for vehicular traffic which maintains the basic cross-section of the waterway below it.

Storm sewer system means a stormwater collection and transmission system consisting primarily of inlets and storm sewers.

Construction means any activity including land clearing, earthmoving or the erection of structures which will result in the creation or alteration of a stormwater management system or the existing ground.

Design high water means the elevation of the water surface as determined by the flow conditions of the design storm.

Design storm means a selected rainfall pattern of specified amount, intensity, duration and frequency that is used as a basis for design.

Detention means the collection and temporary storage of stormwater with subsequent release, at a specified rate, into a downstream system.

Drawdown means lowering the water surface, water table or piezometric surface resulting from a withdrawal of water.

Effective grain size means the diameter of filter sand or other aggregate that corresponds to the ten percentile finer by dry weight on the grain size distribution curve.

Engineer means a professional engineer registered in the state, or other person exempted pursuant to the provisions of F.S. ch. 471.

Exfiltration means a stormwater management procedure which stores runoff in a subsurface collection system and dispose of it by percolation into the surrounding soil.

Filtration means the selective removal of suspended matter from stormwater by passing the water through at least two feet of suitable fine textured granular media such as porous soil, uniformly graded sand and gravel or other natural or artificial aggregate, which may be used in conjunction with filter fabric and underdrain pipe.

Freeboard means a vertical distance between elevation of the design highwater and the lowest elevation of the top of the bank, levee or berm.

Impervious means land surfaces which do not allow, or minimally allow, the penetration of water; included as examples are building roofs, normal concrete and asphalt pavements and some fine grained soils such as clays.

Legal positive outfall means the point at which a stormwater system discharges off the project site; which must be lower than the upstream pipes, ditches, or waterbodies. The discharge point must be in, or flow to, a county or city maintained ditch, pipe, or waterbody.

Maintenance means routine custodial maintenance needed to ensure the functioning of a stormwater management system to meet original design criteria.

Nonstructural means a method or methods of stormwater management which attempts, to the greatest degree possible, to employ natural and self-maintaining systems and limit the use of man-made, maintenance intensive structures.

Open drainage system means a stormwater collection and transmission system consisting primarily of swales, ditches, and canals.

Outfall means the point, location or structure where stormwater runoff discharges from a surface water management system to a receiving body of water or other system.

Outfall structure means the structure or structures which control the discharge of a water management system into another system or receiving water body.

Peak discharge means the maximum instantaneous flow from a given storm condition at a specific location.

Permanent soil stabilization means the use of sodding, seed and mulch, rip rap, or other approved methods to prevent erosion during and after construction activities.

Project area means the area being modified or altered in conjunction with a proposed activity.

Recharge means replenishment of ground water reservoirs by infiltration and transmission through permeable soils.

Retention means the prevention of discharge of a given volume of stormwater runoff by complete on-site storage with subsequent release through accepted water treatment facilities or underdrains.

Riprap means man-made or natural materials placed against an embankment or other work for protection against the action of water. Materials may include sand-cement bags, concrete block, rubble, or formed concrete.

Roadway means a designated travel pathway, either public or private, which is designed for vehicular traffic and is not used primarily as a driveway access to a property.

Seasonal high water table means the highest level of the saturated zone in the soil in a year with normal rainfall.

Short-circuiting means flow characteristics of a detention pond in which a direct flow path exists between the inflow and outflow points, thus diminishing the velocity reduction and settling capability of the pond.

Stabilized maintenance berm means a maintenance pathway stabilized to a LBR of 15.

Stormwater means the flow of water which results from, and which occurs immediately following, a rainfall event.

Stormwater management system means the designed features of the property which collect, convey, channel, hold, inhibit or divert the movement of stormwater.

Sub-basin means a physical division of a larger watershed associated with one reach of the storm drainage system.

Tailwater depth means the depth of flow immediately downstream from the discharge structure, or at the point of discharge.

Temporary soil stabilization means the use of seeding, mulching, netting, blankets, or other approved methods to prevent erosion during construction activities.

Total land area means land holdings under common ownership which are contiguous or land holdings which are served by common surface water management facilities.

Traversing work means any artificial structure or construction that is placed in or across a stream, or other watercourse, or an impoundment.

Underdrain means a system of pipes, gravel, sand, and filtercloth used to recover retention volumes from stormwater ponds or lower the water table under roads or stormwater ponds.

Uniformity coefficient means the number representing the degree of homogeneity in the distribution of particle sizes of filter sand or other granular material. The coefficient is calculated by determining the D60/D10 ratio where D10 and D60 refer to the particle diameter corresponding to the ten and 60 percentile of the material which is finer by dry weight.

Watershed means a geographical area or region that is so sloped either by man or nature that surface runoff is carried away by a single drainage system by gravity to a common outlet or outlets. Also referred to as a drainage basin or drainage area.

Water table means the upper surface of the free ground water in a zone of saturation; locus of points in the soil water at which hydraulic pressure is equal to atmospheric pressure.

EXHIBIT 4-1

UNDER DRAIN DESIGN:
(Pond _______)

Necessary retention volume to meet county criteria water quality standards _______ AF

Le = Average effective seepage length

K = Permeability coefficient. The filter is multilayered; Kgravel - Ksand. The actual permeability coefficient of the sand is estimated to be _______ ft/day. With a safety factor of two use K = _______ ft/day.

H = Head difference between water level and invert of underdrain pipe

i = H/Le (hydraulic gradient)
NOTE: For a conservative approach, assume L = 1.0 when L < 1.0

A = Surface area per linear foot at given elevation

Qn = Discharge per day through sand layer per linear foot at given elevation

Qn = KiA

Elev. (ft)	Le (ft) *	H (ft)	A (ft )	Q (CFD/LF)

 

^* Measured from scaled drawing       Q _AVE. = _______ CFD/LF

For 3 day drawdown period: Volume Dissipated = _______ CFD/LF x 3 day = _______ Ft /LF

Length of underdrain required:

Lud =

( _______ AF) (43560 ft ) _______ft /LF

= _______ LF

 

Pipe diameter:

Pipe must pass peak flow of

CDF/LF x  ft 86400 Sec/Day

= _______CFS

 

Q = _______CFS, N = .015 , S = 0.001FT/FT

Use _______ Linear Feet of _______ In. Dia. fully perforated underdrain pipe.

EXHIBIT 4-2

Runoff Curve Numbers for Selected Land Uses
COVER DESCRIPTION		CURVE NUMBERS FOR HYDROLOGIC SOIL GROUP
COVER TYPE AND HYDROLOGIC CONDITION	AVERAGE PERCENT IMPERVIOUS AREA	A	B	C	D
Fully developed urban areas (vegetation established)
Open space (lawns, parks, golf courses, cemeteries, etc.) :
Poor condition (grass cover < 50%)	.....	68	79	86	89
Fair condition (grass cover 50% to 75%)	.....	49	69	79	84
Good condition (grass cover > 75%)	.....	38	61	74	80
Impervious areas:
Paved parking lots, roofs, driveways, etc. (excluding right-of-way)	.....	98	98	98	98
Streets and roads:
Paved; curbs and storm sewers (excluding right-of-way)	.....	98	98	98	98
Paved; open ditches (including right-of-way)	.....	83	89	92	93
Gravel (including right-of-way)	.....	76	85	89	91
Dirt (including right-of-way)	.....	72	82	87	89
Urban districts:
Commercial and business	..... 85	89	92	94	95
Industrial	..... 72	81	88	91	93
Residential districts by average lot size:
1/8 acre or less (town houses)	..... 65	77	85	90	92
1/4 acre	..... 38	61	75	83	87
1/3 acre	..... 30	57	72	81	86
1/2 acre	..... 25	54	70	80	85
1/2 acre	..... 20	51	68	79	84
2 acres	..... 12	46	65	77	82
Developing urban areas
Newly graded areas (pervious areas only, no vegetation)	.....	77	86	91	94
Idle lands (CN's are determined using cover types similar to those below)
Pasture, grassland, or range:
Poor (< 50% ground cover or heavily grazed)	.....	68	79	86	89
Fair (50% to 75% ground cover and not heavily grazed)	.....	49	69	79	84
Good (> 75% ground cover and lightly or only occasionally grazed)	.....	39	61	74	80
Meadow — continuous grass protected from grazing and generally mowed for hay:	.....	30	58	71	78
Brush-brush-weed-grass mixture with brush the major element:
Poor (&lt 50% ground cover)	.....	48	67	77	83
Fair (50% to 75% ground cover)	.....	35	56	70	77
Good (&gt 75% ground cover)	.....	30	48	65	73
Woods — grass combination with 50% woods and 50% grass (pasture) cover. Other combinations of conditions may be computed from the CN's for woods and pasture:
Poor	.....	57	73	82	86
Fair	.....	43	65	76	82
Good	.....	32	58	72	79
Woods:
Poor (Forest litter, small trees, and brush are destroyed by heavy grazing or regular burning)	.....	45	66	77	83
Fair (Woods are grazed but not burned and sore forest litter covers the soil	.....	36	60	73	79
Good (Woods are protected from grazing and litter and brush adequately cover the soil)	.....	30	55	70	77

 

Average runoff condition, and I _a = 0.2s.

The average percent impervious area shown was used .to develop the composite CN's. Other assumptions are as follows: impervious areas are directly connected to the drainage system, impervious areas have a of 98, and pervious areas are considered equivalent to open space in good hydrologic condition. CN's for other combinations of conditions must be computed.

CN's shown are equivalent to those of pasture. Composite CN's may be computed for other combinations of open space cover type.

(Source: USDA. Soil Conservation Service, TR-55, Second Ed., June 1986)

EXHIBIT 4-3

RUNOFF COEFFICIENTS ^a FOR A DESIGN STORM RETURN PERIOD OF 10 YEARS OR LESS

		Sandy Soils		Clay Soils
Slope	Land Use	Min.	Max.	Min.	Max.
Flat (0—2%)	Woodlands	0.10	0.15	0.15	0.20
	Pasture, grass, and farmland b	0.15	0.20	0.20	0.25
	Rooftops and pavement	0.95	0.95	0.95	0.95
	Pervious pavements c	0.75	0.95	0.90	0.95
	SFR: ½-acre lots and larger	0.30	0.35	0.35	0.45
	Smaller lots	0.35	0.45	0.40	0.50
	Duplexes	0.35	0.45	0.40	0.50
	MFR: Apartments, townhouses, and condominiums	0.45	0.60	0.50	0.70
	Commercial and industrial	0.50	0.95	0.50	0.95
Rolling (2—7%)	Woodlands	0.15	0.20	0.20	0.25
	Pasture, grass, and farmland b	0.20	0.25	0.25	0.30
	Rooftops and pavement	0.95	0.95	0.95	0.95
	Pervious pavements c	0.80	0.95	0.90	0.95
	SFR: ½-acre lots and larger	0.35	0.50	0.40	0.55
	Smaller lots	0.40	0.55	0.45	0.60
	Duplexes	0.40	0.55	0.45	0.60
	MFR: Apartments, townhouses, and condominiums	0.50	0.70	0.60	0.80
	Commercial and industrial	0.50	0.95	0.60	0.95
Steep (7%+)	Woodlands	0.20	0.25	0.25	0.30
	Pasture, grass, and farmland b	0.25	0.35	0.30	0.40
	Rooftops and pavement	0.95	0.95	0.95	0.95
	Pervious pavements c	0.85	0.95	0.90	0.95
	SFR: ½-acre lots and larger	0.40	0.55	0.50	0.65
	Smaller lots	0.45	0.60	0.55	0.70
	Duplexes	0.45	0.60	0.55	0.70
	MFR: Apartments, townhouses, and condominiums	0.60	0.75	0.65	0.85
	Commercial and industrial	0.60	0.95	0.65	0.95

 

^a Weighted coefficient based on percentage of impervious surfaces and green areas must be selected for each site.

^b Coefficients assume good ground cover and conservation treatment.

^c Depends on depth and degree of permeability of underlying strata.

Note:

SFR = Single Family Residential
MFR = Multi-Family Residential

EXHIBIT 4-4

ESTIMATING ENERGY LOSSES THROUGH MANHOLES/JUNCTIONS

SINGLE PIPE JUNCTIONS
TYPE OF MANHOLE/JUNCTION		HEAD LOSS COEFFICIENT (K)
Trunkline only with no bend at junction		0.5
Trunkline only with 45º bend at junction		0.6
Trunkline only with 90º bend at junction		0.8

 

MULTIPLE PIPE JUNCTIONS
TYPE OF MANHOLE/JUNCTION		HEAD LOSS COEFFICIENT (K)
Trunkline with one small lateral		0.6
Trunkline with one large lateral		0.7
2 Roughly equivalent entrance lines with angle of < 90º between lines		0.8
2 Roughly equivalent entrance lines with angle of > 90º between lines		0.9
3 or more entrance lines		1.0

 

NOTE: Above values of K are to be used to estimate energy or head losses through surcharged junctions/manholes in pressure flow portions of a storm sewer system. The energy loss equation is:

h j (ft) =

K [V(ft/sec)]     64.4

 

with V = larger velocity. in main entrance. or exit line of junction/manhole.

EXHIBIT 4-5

OVERLAND FLOW MANNING'S n VALUES

	Recommended Value	Range of Values
Concrete	.011	.01 — .013
Asphalt	.012	.01 — .015
Bare sand a	.010	.010 — .016
Graveled surface a	.012	.012 — .030
Bare clay-loam (eroded) a	.012	.012 — .033
Fallow (no residue) b	.05	.006 — .16
Chisel plow (< 1/4 tons/acre residue)	.07	.006 — .17
Chisel plow (1/4 — 1 tons/acre residue)	.18	.07 — .34
Chisel plow (1 — 3 tons/acre residue)	.30	.19 — .47
Chisel plow (> 3 tons/acre residue)	.40	.34 — .46
Disk/harrow (< 1/4 tons/acre residue)	.08	.008 — .41
Disk/harrow (1/4 — 1 tons/acre residue)	.16	.10 — .25
Disk/harrow (1 — 3 tons/acre residue)	.25	.14 — .53
Disk/harrow (> 3 tons/acre residue)	.30	— —
No till (< 1/4 tons/acre residue)	.04	.03 — .07
No till (1/4 — 1 tons/acre residue)	.07	.01 — .13
No till (1 — 3 tons/acre residue)	.30	.16 — .47
Plow (Fall)	.06	.02 — .10
Coulter	.10	.05 — .13
Range (natural)	.13	.01 — .32
Range (clipped)	.08	.02 — .24
Grass (bluegrass sod)	.45	.39 — .63
Short grass prairie a	.15	.10 — .20
Dense grass c	.24	.17 — .30
Bermuda grass c	.41	.30 — .48
Woods	.45	— —

 

All values are from Engman (1983), unless noted otherwise.

^a Woolhiser (1975).

^b Fallow has been idle for one year and is fairly smooth.

^c Palmer (1946). Weeping lovegrass., bluegrass, buffalo grass, blue gramma grass, native grass mix (OK), alfalfa, lespedeza.

Note: These values were determined specifically for overland flow conditions and are not appropriate for conventional open channel flow calculations. See Chapter 7 of this volume for open channel flow procedures.

EXHIBIT 4-5

RATIONAL FORMULA
STORM SEWER TABULATION FORM

SS-I

SHEET NO.

DATE _______ PROJECT NO. _______ ROAD _______ COUNTY _______ BY _______  _______ of _______

FROM STRUCTURE NO.

TO STRUCTURE NO.

TYPE OF STRUCTURE

TYPE OF LINE *

LENGTH (FT.)

DRAINAGE AREA (ACRES)

TIME OF CONCENTRATION (MIN.)

TIME OF FLOW IN SECTION (MIN.)

INTENSITY (I)

TOTAL (C.A.)

TOTAL RUNOFF (C.F.S.)—(C.I.A.)

INLET ELEVATION (FT.)

ELEV. OF H.G.

DIAMETER (IN.)

SLOPE (%)

VELOCITY (F.P.S.)

CAPACITY (C.F.S.)

NOTES

CROWN ELEV.

ZONE       FREQUENCY       YR.

C:

SUB-TOTAL (C.A.)

FLOW LINE ELEV.

C:

UPPER END

LOWER END

FALL (FT.)

C:

INCREMENT

SUB TOT.

REMARKS

Kc:

Kc:

Kc:

Kc:

Kc:

Kc:

Kc:

Kc:

Kc:

Kc:

Kc:

Kc:

 

^*  M — Main Line
  S — Stub Out/Secondary Line

APPENDIX A TO EXHIBIT A OF DIVISION 6. (STORMWATER MANAGEMENT CRITERIA)

(Ord. No. 93-22, § 1, 8-23-93)