Township of Mine Hill, NJ Stormwater Management

In order to duplicate as nearly as possible natural drainage conditions, the regulation and control of stormwater runoff and erosion for any land area to be developed shall be through on-site water detention and/or ground absorption systems, which include but are not limited to the following:

Detention areas, which may be depressions in parking areas, excavated basins, basins created through use of curbs, stabilized earth berms or dikes or any other form of grading which serves to temporarily impound and store water.

Rooftop storage through temporary impoundment and storage of stormwater on flat or slightly pitched building rooftops by use of drain outlets which restrict the stormwater runoff from the roof surface.

Dry wells or leaching basins which control stormwater runoff, ground absorption and temporary storage.

Porous asphalt pavement, which preserves the natural ground absorption capacity of a site and provides a subsurface reservoir temporary storage of stormwater.

Any system of porous media, such as gravel trenches drained or porous wall or perforated pipe, which temporarily stores and dissipates stormwater through ground absorption.

Any combination of the above-mentioned techniques which serves to limit stormwater runoff from a given site to what presently occurs.

Preservation of natural vegetation.

§ 310-258 Design standards for stormwater detention facilities.

Stormwater detention facilities shall be designed to contain an amount equal to the increase in volume of runoff which would result from development of any site during storm frequencies of one-, five-, ten-, twenty-five-, fifty- and one-hundred-year storm events. The volume of runoff shall be computed on the basis of the total rainfall which produced the flood of record for the area involved and shall be equivalent to the rainfall excess, as previously defined. The total rainfall which produced the flood of record shall be determined from accurate local records of the United States Department of Commerce, National Weather Service, or by calculations using accepted engineering design techniques.

Design of the detention basin must demonstrate that stormwater runoff from the site is so controlled that on-site and off-site erosion is neither caused nor worsened and that the potential of downstream flooding is not increased from the proposed development. In calculating the foregoing, volumes and rates for one-, five-, ten-, twenty-five-, fifty- and one-hundred-year storm frequencies having durations producing maximum runoff rates before and after the proposed development. The changes of runoff rates and volumes, to be caused by alterations in land use and time of concentration. The plans for disposition of stormwater, either by retention or detention on site, or by means of channeling detail, so as to protect downstream property. A description of potential flood damages, including a summary of flood stages from state and federal sources and an erosion and sedimentation control for both the construction phase and post-construction phase, including a review by the Morris County Soil Conservation Service, is required.

The runoff coefficients shall be determined for each site for both existing and proposed conditions, and the difference in the same shall be used to compute the volume of rainfall excess for design of stormwater detention facilities. The volume for design is equal to the depth of the rainfall excess multiplied by the area of the site. If any such facility will contain water under normal conditions, the amount so contained shall not be counted in calculating the capacity required for the detention of water for the design storm. Rainfall intensity shall be as recommended by the Board's engineer.

In the case of detention facilities utilizing porous media for ground absorption, such as dry wells, porous pavement or the like, the volume of the porous media shall be large enough to contain the total volume of rainfall excess within the voids. Ground, absorption systems shall be used only where the infiltration rate of the receiving soil is acceptable is determined by percolation tests and soil borings to determine the soil characteristics and groundwater table elevations or as determined by the Board's engineer. Provisions shall be made to contain overflow of such systems on site or to surface drain the overflow in such a way as not to adversely affect any other property. If detention facilities utilizing surface impoundment, such as detention basins or rooftop storage, are used, sufficient volume to fully contain the total volume of rainfall excess shall be provided. The outlets of such facilities shall be designed to limit the maximum discharge rate of stormwater runoff to what occurs at the site under existing conditions and shall discharge in such a way as not to adversely affect any other property. If rooftop storage is proposed, the weight of the impounded water on the roof shall be accounted for in the structural design of the building, and the roof shall be designed to provide maximum protection against leakage. If berms or dikes are used to create the impounding area, they shall be adequately stabilized and the slopes protected with vegetative cover paving or riprap to protect against failure or breaching.

If a combination of different stormwater detention techniques is used, combined volume of the systems shall be large enough to fully contain the total volume of rainfall excess.

Stormwater detention facilities shall be constantly maintained by the owner to ensure continual functioning of the systems at design capacity and to prevent the health hazards associated with debris buildup and stagnant water. In no case shall water be allowed to remain in any facility long enough to constitute a mosquito-breeding disease or any other type of health problem, unless approved as a multifunction facility to include water such as a pond. If the land or stormwater detention facility or facilities are proposed to be dedicated to the Township and said dedication is accepted by the Township Council, the procedures for the construction, dedication and acceptance and maintenance of such facilities set forth in this chapter, including but not limited to performance and maintenance bonds, inspections, etc., shall govern.

Detention and sediment and erosion control facilities shall be designed in conformance with the Standards for Soil Erosion and Sediment Control in New Jersey of the New Jersey Soil Conservation Commission as approved by the Board's engineer or the Morris Soil Conservation District under agreement, except where the Board's engineer has determined that conditions peculiar to a certain site warrant exception.

Sediment and erosion control measures shall be installed prior to any other site development, shall apply to all aspects of the proposed development and shall be in operation during all stages of development. Increased runoff and sediment resulting from modified soil and surface conditions caused by the proposed development shall be minimized and, where possible, retained on site.

§ 310-259 Design of detention facilities; hydrology.

This section presents the hydrologic method to be used by Mine Hill Township for developing inflow hydrographs and determining maximum release rates for proposed detention basins. It should be noted that, wherever possible, stream flow records should be utilized in determining design flows, and the hydrologic method presented in this section will only be used when such records are unavailable.

Mine Hill Township adopts the hydrologic methods of the Soil Conservation Service (SCS) as the basis for calculations, the SCS publication "Urban Hydrology for Small Watersheds," Technical Release No. 55, June 1986, as published by the United States Department of Agriculture, or as amended. Further, the SCS Type III twenty-four-hour design rainfall distribution as presented in TR-55 is to be utilized.

Briefly, the procedure for developing inflow and outflow hydrographs is as follows:

(1)

Determine tributary drainage area to proposed detention basin in square miles.

(2)

Determine the ground conditions and convert this to a runoff curve number (CN).

(3)

Obtain the twenty-four-hour design rainfall in inches for the given storm (see Table I, below).

(4)

Convert the twenty-four-hour rainfall into inches of runoff using the computed CN and the SCS runoff equation.

(5)

Multiply the drainage area in square miles from Step 1 by the inches of runoff computed in Step 4 to obtain a constant multiplier with units of square miles-inches.

(6)

Determine the time of concentration to the proposed basin utilizing TR-55 Chapter No. 3.

(7)

Construct the hydrograph by multiplying the Type III Tabular Hydrograph Coordinates (TR-55, Exhibit 5-III) for the time of concentration obtained in Step 6 by the multiplier in square mile-inches computed in Step 5 to obtain flows in cubic feet per second for the various times listed in the tabular coordinates.

(8)

The twenty-four-hour design rainfalls to be utilized by Mine Hill Township are as follows:

	Table I 24-Hour Principal Rainfalls
	Storm Frequency (years)	24-Hour Rainfall (inches)
	1	2.7
	2	3.3
	5	4.3
	10	5.2
	15	5.5
	25	6.0
	50	6.5
	100	7.5

(9)

Runoff curve numbers are parameters that reflect both the average soil type and land use prevalent in the drainage basin. All of the soil types in the municipality have been classified, labeled and mapped on aerial photographs by the National Cooperative Soil Survey of the Soil Conservation Service. Lists of these soils, by soil name and symbol, as well as the hydrologic soil group to which they belong, are contained in Exhibit A-1 of TR-55, June 1986. To account for varying land uses and soil covers within each soil group, these soil groups have been broken down into different land use categories as shown in Tables 2-2a, 2-2b and 2-2c of TR-55.

(10)

"Time of concentration" is defined as the time required for runoff to travel from the most hydraulically distant point in the drainage area to the point at which a hydrograph or peak flow rate is to be computed. It should be noted that separate times of concentration representing both undeveloped and developed conditions in a watershed are to be computed. Chapter 3 of TR-55 is to be utilized to calculate the time of concentration.

§ 310-260 Hydraulics.

The hydraulics of all proposed detention basins will be reviewed utilizing standard hydraulic formulas and parameters.

In order to prevent the erosion of channels and spillways, the maximum flow velocities at the outlet of proposed detention basins and erodible spillways are of particular concern. Minimum velocities are also of interest so as to prevent siltation and the subsequent loss of flow capacity. In light of the concerns, maximum allowable design velocities for unlined channels shall comply with the criteria contained in "Hydraulic Engineering Circular No. 15 - Design of Stable Channels with Flexible Linings," published by the Federal Highway Administration of the United States Department of Transportation. When the maximum allowable velocities cannot be maintained in an unlined suitable channel, channel lining shall be provided in accordance with "Hydraulic Engineering Circular No. 15."

All detention basin routings will be performed using acceptable routing methods. Selected time increments will be of short enough duration to allow reasonable approximation of the inflow hydrograph.

§ 310-261 Structural requirements.

The following list of general structural criteria shall be used when reviewing a proposed stormwater detention basin:

Principal outlets and structures.

(1)

To minimize the chance of clogging and to facilitate cleaning, outlet pipes should be at least 18 inches in diameter. All pipes are to be reinforced concrete pipe conforming to ASTM C 76 with rubber gaskets pursuant to ASTM C 443.

(2)

Eight-inch-thick anti-seep collars are to be installed along outlet pipes. Reinforcement steel shall be No. 5 at 12 inches both ways with two inches of cover on both faces.

(3)

All principal outlet structures shall be steel reinforced concrete cast in place. All construction joints are to be watertight. All pipes, stubs, and/or fittings are to be cast ethically in the walls.

(4)

Trash racks and anti-vortex devices will be required where necessary and should be designed to facilitate cleaning. A rise-type outlet structure which conveys flow over its top should have an anti-vortex cover constructed of reinforced concrete.

(5)

Suitable lining is to be placed upstream and downstream of principal outlets as necessary to prevent scour and erosion. Such lining shall conform to the criteria contained in "Hydraulic Engineering Circular No. 15 - Design of Stable Channels with Flexible Linings," or "Standards for Soil Erosion and Sediment Control in New Jersey," published by the State Soil Conservation Committee.

(6)

All concrete shall have a minimum twenty-eight-day compressive strength of 3000 PSI. Allowable extreme fiber stress in compression shall be 1200 PSI.

(7)

All reinforcing steel shall be intermediate grade, new deformed billet-steel conforming to ASTM A615 (latest edition). Grade 40 minimum. Allowable stress in tension shall be 20,000 PSI.

Emergency spillways.

(1)

Vegetated emergency spillways shall have side slopes not exceeding four horizontal to one vertical.

(2)

Emergency spillways not excavated from undisturbed soil shall be suitably lined and shall comply with criteria contained in "Hydraulic Engineering Circular No. 15" or "Standards for Soil Erosion and Sediment Control" mentioned above.

(3)

Maximum velocities in vegetated emergency spillways excavated from undisturbed soil shall be checked based on the velocity of the peak flow in the spillway resulting from the routed emergency spillway hydrograph.

Dams and embankments.

(1)

The minimum top widths of all dams and embankments are listed below. These values have been adopted from the "Standards for Soil Erosion and Sediment Control in New Jersey."

	Table II Minimum Top Widths
	Height (feet)	Top Width (feet)
	0 to 15	10
	15 to 20	12
	20 to 25	14

(2)

Maximum side slopes for all dams and embankments are four horizontal to one vertical.

(3)

All earth fill shall be free from brush, roots and other organic material subject to decomposition.

(4)

Cutoff trenches are to be excavated along the dam or embankment center line to impervious subsoil or bedrock.

(5)

An impervious central core is to be constructed in the dam/embankment consisting of compacted clay material.

(6)

Safety ledges shall be constructed on the side slopes of all detention basins having a permanent pool of water. The ledges shall be four feet to six feet in width and located approximately 2 1/2 feet to three feet below and one foot to 1 1/2 feet above the permanent water surface.

(7)

The upstream face of the dam and/or embankment of a permanent pool detention basin shall be stabilized to protect it against wave erosion.

(8)

The fill material in all earth dams and embankments shall be compacted to at least 95% of the maximum density obtained from compaction tests performed by the appropriate method in ASTM D698.

(9)

A detention basin with a permanent pool shall have a gravity drain to permit complete emptying of the basin when necessary.

§ 310-262 Water quality storage.

All site developments shall provide water quality storage in accordance with this article. The standard requires that a proposed dry stormwater detention basin must detain the runoff from a one-and-one-quarter-inch, two-hour rainfall or the one-year twenty-four-hour SCS Type III storm in such a way that no more than 90% of this runoff will be gradually drained from the basin at the end of:

(1)

Eighteen hours for residential development; and

(2)

Thirty-six hours for all other types of development.

The retention time shall be considered a brim-drawdown time, and therefore shall begin at the time of peak storage. In providing the above retention times, the required outlet diameter should not be less than three inches. For all projects, the required detention time shall be equal to the values given above or the time which results from the use of a three-inch-diameter orifice, whichever is less.

In permanent pond or wet basins, the water quality storage requirement shall be satisfied when the volume of permanent water is at least three times the volume of runoff produced by the water quality design storm.

All outlets shall be designed to function without manual, electric, or mechanical controls. The required orifice may be constructed from metal, concrete or appropriate material and placed over a larger opening in the basin's outlet structure and attached with noncorrosive, removable fittings where suitable. Gasket material should be placed behind the plate to insure a watertight fit where appropriate. A removable trash rack is required in front of the outlet to prevent clogging. All trash racks should be designed to limit flow velocities through them to a maximum of two feet per second.

For a proposed detention basin which has off-site as well as on-site areas tributary to it, the computation of the resultant runoff from the one-and-one-quarter-inch, two-hour water quality design storm should either include or exclude the off-site area depending upon the following conditions:

(1)

Include the off-site area if it is presently undeveloped or developed to a lesser extent than is proposed for the project site.

(2)

Exclude the off-site area if it is presently developed to an equal or greater extent than is proposed for the project site.

In addition, on-site areas not controlled by the detention facility should be kept to a minimum. Where the impact of an uncontrolled, on-site area is felt to be significant, special measures may be required, including, where possible, the storage of an equivalent amount of runoff from developed off-site areas normally excluded from the computations [see Subsection E(2) above]. Projects of this nature and all other projects with unusual site characteristics will be reviewed on an individual basis, and preliminary discussions with the Board's engineer. Both wet and dry basins can be utilized to comply with the requirements of the stormwater management regulations.

§ 310-263 General requirements.

A subsurface soils investigation is to be performed at all basin sites, the results of which are to appear on the plans.

The minimum bottom slope of the basin shall be 2% unless extraordinary site conditions prevent practical installation. In all cases, low-flow channels may be required to convey small inflows to the basin outlet.

The basin is to have sod or topsoil and seeded, including the bottom, side slopes and all earthen dams and embankments.

The design dimensions of the detention basin shall be maintained throughout construction unless it is to be used as a siltation basin during construction in the watershed. If so, it shall be immediately returned to design dimensions following the completion of such construction.

Water quality for the basin shall be provided by controlling the one-year storm. The minimum size of the orifice shall be three inches in diameter.