[HISTORY: Adopted by the Permanent Drainage Committee of the Town of Norwell; amended 12-11-1968; approved as current November 1970. Subsequent amendments noted where applicable.]
The following rules and regulations are set forth by the Permanent Drainage Committee of Norwell, hereinafter referred to as the "Committee." They are to be used in the design of drainage facilities (i.e., pipes, culverts, drainage structures and appurtenances) and are the basis under which approval for any drainage plan will be granted by the Committee.
Recognizing its responsibility to the protection of the public, the Committee sets forth the following, with the intent of establishing reasonable rules and regulations that are neither highly restrictive to the orderly development of raw vacant land into properly drained livable home sites nor so loose, vague and weak in their intent that costly redesign and revisions to drainage systems generally paid for by public funds are required.
It is further the intent of these rules and regulations that accepted design practices, proven construction methods, materials and procedures be followed. These are more particularly described in the current edition of the Commonwealth of Massachusetts, Department of Public Works, Standard Specifications for Highways, Bridges, and Waterways, including amendments and addenda, and in the same Department's Construction Manual, Part 3, Constructions Standards. Description of various drainage structures contained herein are not intended to supersede the Commonwealth's standards.
In accordance with Chapter 584 of the Acts of 1958, as amended, the Committee will require that designs submitted for the Committee's consideration will be prepared by a registered professional engineer, duly registered under the provisions of the above-named law. The design necessarily rests with the professional engineer. It is expected the quality of his design and analysis will meet the standards implied by his professional engineer's stamp.
Editor's Note: See MGL c. 112, § 81J.
Each drainage plan submitted for approval shall be accompanied by a design analysis prepared on a design computation form available from the Town Clerk. The drainage plan shall show profiles, typical sections and details, pipe sizes and slopes, elevations, grades and other pertinent data necessary to the support of the design analysis and the analysis shall clearly indicate all the computations for the drain, including determination of pipe size and strength, and a statement concerning the disposition of flow. If the flow is discharged to the ground surface on land not belonging to the applicant, then a flowage easement over the ground subject to flow shall be obtained by the applicant and a statement to that effect shall be included with the design analysis.
A rainfall having a frequency of occurrence of once in 10 years has been selected for design computations. This storm amounts to 1.8 total inches of rain in one hour. Intensities for small areas correspond to the time of concentration for the area.
Runoff to each inlet on the drainage system or to a culvert shall be based upon the Rational Method and shall be computed for the entire tributary area. Computations shall be based upon full development of the tributary area. Runoff coefficients shall be as follows:
The proper drain size may be calculated by using Manning's Formula with a Kutter's "n" value of .013 for concrete pipe and 0.024 for corrugated metal pipe. Standard charts for deriving drain sizes using these figures may be obtained upon request from the office of the Town Clerk. For culverts less than 30 feet in length and all drains, the minimum size of pipe shall be 12 inches in diameter. Drains shall be large enough to pass the design storm without surcharge.
Culverts shall be sized in accordance with accepted culvert design formulas taking into account headwater and tailwater conditions. Culvert design computation shall indicate the headwater and tailwater conditions for which the culvert is designed.
All pipes shall be laid on a slope so that the minimum velocity with the pipe flowing full shall be 2.5 feet per second. Consideration will be given to flatter slopes if adequate provisions are made for cleaning the pipes. All plans having drains with slopes which will produce pipe velocities less than 2.5 feet per second, flowing full, shall be accompanied by a letter stating the reason for the flat slope. The letter shall have a space for approval by the Committee and the drain shall not be constructed until the letter has been approved.
All storm drains shall be reinforced concrete of adequate strength except that in off-street locations bituminous coated, galvanized, corrugated metal pipe or pipe arch may be used, if approved by the Committee. Concrete pipe shall conform to the State of Massachusetts Standard Specifications for Highways, Bridges and Waterways as amended.
Inlets shall have an adequate waterway opening to pass the design runoff with not more than 0.2 foot of surcharge. Grates and frames shall be cast iron suitable for the loads which can occur either during the construction or afterward. Inlets shall be constructed either of brick and mortar with eight-inch-thick walls, precast segmental concrete blocks not less than six inches thick mortared in place, or of precast pipe sections. Inlets shall be set on a base of either poured concrete eight inches in thickness or precast segmental base blocks not less than four inches in thickness. Inlets shall be used in off-street locations and the grate frame shall be mortared in position with the top 0.2 foot below the grade of the finished ground surface. Side openings may be used in lieu of a grate if the quantity of runoff exceeds the capacity of a grate of reasonable size as approved by the Committee. Inlet shall be 4.0 feet inside diameter below the corbeling and shall not be used on drains greater than 30 inches in diameter. A shaped invert is not required but the bottom of the inlet shall be finished at the same grade as the lowest pipe invert. At inlets where the outlet pipe is larger than the inlet pipe, the crown of the outlet pipe shall be at the same elevation or lower than the crown of the inlet pipe.
Drain inlets located in streets shall be catch basins with a sump 24 inches or more lower than the invert of the outlet pipe. The grate shall be cast iron designed for the same loading as the street. Catch basins shall be constructed of one of the materials specified herein under inlets and shall be 4.0 feet in diameter below the corbeling. Catch basins shall be located as required to collect the runoff but shall not be spaced more than 200 feet apart on each side of the road unless otherwise approved by the Committee. The top of the grate shall be set 0.1 foot lower than the finished road surface, and the road surface shall be shaped to the grate in a smooth, uniform transition. The crown of the outlet pipe shall be at or below the crown of the inlet pipe.
Manholes shall be constructed similar to an inlet, except that a solid cast-iron cover of adequate strength shall be provided in lieu of a grate. The top of the cover shall be at the same elevation or above the adjacent ground or street surface as directed. A manhole shall be used at every change in pipe size or direction, vertical or horizontal. Manholes shall be spaced not over 300 feet apart. Manholes only shall be constructed on the drain with all inlets and catch basins discharging into the drain directly through a manhole. Manhole covers shall have the letter "D" cast into the cover at least three inches in size.
The trench for the pipe shall be excavated to the required line and grade, including earth boulders and ledge. Trenches for storm drains shall be wider than the outside diameter of the pipe plus 16 inches for pipes through 18 inches nominal diameter, and the outside diameter plus 24 inches for pipes larger than 18 inches. This trench width shall apply from the top of the pipe to the bottom of the trench. Above the top of the pipe, the trench may be as necessary to properly install the pipe. Trenches with side slopes steeper than the natural angle of repose of the soil shall be sheeted in an approved manner, as necessary, to avoid cave-ins and sloughing. All excavations shall be properly barricaded and lighted at night where they are near pedestrian or vehicular traffic. Before any pipe is placed in a newly constructed fill, the contractor shall, as indicated, place the filling two feet above the top of the pipe after which the pipe trench may be excavated. If any cross pipes, conduits, drains or other unforeseen obstacles are encountered in the excavation, which cannot be relocated, the drain shall be redesigned to avoid the obstruction in a manner suitable to the Committee. Possible obstructions to the line shall be investigated prior to the construction of the drain in its immediate vicinity.
Trenches may be excavated with a flat bottom, but the full length of the pipe, except the bell, must rest upon undisturbed soil, except as hereinafter specified. Where trenches have been overexcavated, a selected earth or gravel foundation, thoroughly compacted, shall be provided for proper pipe bedding. Soil which is considered to be unstable by the Committee shall be removed to a depth of not less than two feet below the bottom of the pipe and replaced with compacted sand and gravel to the bottom of the pipe. Unstable soil or other unsuitable material shall be disposed of off site, as directed by the Committee.
Pipe shall be laid starting with the downstream end. Grade boards or other approved devices shall be provided to ensure that the pipe is laid true to line and grade. Reference bench marks shall be clearly marked to enable the inspector to quickly check the grade and invert elevations. The joints of all pipes shall be filled with mortar composed of one part portland cement to three parts clean sharp sand. Lime may be added up to 25% of the cement and enough water to make a workable mix. The downstream pipe shall be laid with groove or bell and facing upstream in the proper position, and a dab of mortar shall be placed in the bell or groove, such that the inverts match and the peripheral space shall be filled with stiff mortar. All mortar squeezed out on the inside of the pipe shall be removed before it sets.
After the pipe has been laid and inspected by a representative of the Committee, the trench shall be backfilled. The space under the pipe haunches shall be carefully filled with selected material, free from stones or frozen earth, and compacted carefully to prevent the pipe from moving. The layer of backfill, up to 12 inches over the top of the pipe, shall also be of selected material free from stones and frozen earth, well compacted. The remainder of the trench shall be backfilled in twelve-inch layers, except as noted below, and each layer shall be full compacted in an approved manner. Under roads or other traffic areas the trench shall be backfilled in six-inch layers with each layer compacted to the density of the surrounding soil. Pavement and base course materials removed during the excavation process shall be replaced with pavement and base course to match those removed. When, in the opinion of the Committee, the excavation is deep enough to warrant it, temporary pavement shall be provided as directed. Trenches not in pavement shall be left in a mounded condition as directed by the Committee.
Debris grates at the entrance to culverts or open pipe drains as directed may be required. The grate shall be constructed of steel bars not less than 1/2 inch in diameter, welded together to provide a grate not smaller than the pipe opening. The vertical bars shall be placed with two-inch clear openings between them, and the horizontal bars shall be 12 inches on center. The grate shall be installed not closer than one pipe diameter upstream from the entrance in approved manner. A suitable sketch of the grate and method of installation shall be submitted for approval with the plans for the drains and appurtenances.
Concrete or rubble masonry headwalls shall be provided at both ends of culverts and discharge ends of storm drains. They shall conform in all respects to standards of the Commonwealth of Massachusetts, Department of Public Works, referred to in § 308-2.
The discharge ends of all drains with flowing full velocities of 10 feet per second or more shall be protected with a riprap apron of a width not less than three times the nominal diameter of the pipe. The riprap apron shall extend for a distance of not less than 10 times the nominal pipe diameter from the end of the discharge pipe. The riprap for exit velocities of 10 feet per second or less shall be composed of a layer of stones 12 inches in thickness or more, placed upon a bed of sand and gravel six inches in thickness. The stones shall be sized so that not less than 60% shall have one dimension 12 inches or more. The stones, after being laid, shall be carefully chinked by hand to make a reasonably smooth and shaped surface. Where exit velocities are greater than 10 feet per second, the thickness of stones and the dimensions of the individual pieces shall be sized to prevent displacement by the flow. In this case, details shall be submitted to the Committee for approval.