User's Guide: This article contains specific standards relating to the design and construction of stormwater management systems.
A. 
The storm drainage system will not adversely affect neighboring properties, downstream water quality, or cause soil erosion. Whenever possible, on-site absorption of runoff waters shall be utilized to minimize discharges from the site.
B. 
Surface water runoff shall be minimized and detained on site if possible or practicable.
(1) 
If it is not possible to detain water on site, downstream improvements to the channel may be required of the developer to prevent flooding caused by the project.
(2) 
The natural state of watercourses, swales, floodways or rights-of-way shall be maintained as nearly as possible.
(3) 
The design basis shall be a fifty-year storm.
A. 
Adequate provision shall be made for disposal of all stormwater generated within the development and any drained groundwater through a management system of swales, culverts, underdrains, and watercourses.
B. 
The stormwater management system shall be designed to conduct stormwater flows to existing watercourses.
C. 
All components of the stormwater management system shall be designed to meet the criteria of a twenty-five-year storm based on rainfall data for the closest reporting station to Islesboro, Maine.
D. 
The minimum pipe size for any storm drainage pipe shall be 12 inches.
(1) 
Maximum trench width at the pipe crown shall be the outside diameter of the pipe plus two feet.
(2) 
Pipe shall be bedded in a fine granular material containing no stones larger than three inches or containing lumps of clay or organic matter within a minimum of six inches below the bottom of the pipe extending to six inches above the top of the pipe.
E. 
Catch basins shall be installed where necessary and located at the outermost edge of the travel way of the road.
F. 
Inlets and outlets shall be stabilized against soil erosion by stone riprap or other suitable materials to reduce stormwater velocity.
G. 
The stormwater management system shall be designed to accommodate upstream drainage, taking into account existing conditions and approved or planned developments not yet built, and shall include a surplus design capacity factor of 25% for potential increases in upstream runoff.
H. 
Downstream drainage requirements shall be studied to determine the effect of the proposed development.
(1) 
The storm drainage shall not overload existing or future planned storm drainage systems downstream from the development.
(2) 
The developer shall be responsible for financing any improvements to existing drainage systems required to handle the increased storm flows.
I. 
Where soils require a subsurface drainage system, the drains shall be installed and maintained separately from the stormwater drainage system.
A. 
Reinforced concrete pipe shall meet the requirements of ASTM Designation C-76 (AASHTO M 170).
(1) 
Pipe classes shall be required to meet the soil and traffic loads with a safety factor of 1.2 on the one-hundredth-inch crack strength with a Class B bedding.
(2) 
Joints shall be of the rubber gasket type meeting ASTM Designation C 443-70, or of an approved preformed plastic jointing material such as "Ramnek."
(3) 
Perforated concrete pipe shall conform to the requirements of AASHTO M 175 for the appropriate diameters.
B. 
Corrugated metal pipe shall be bituminous coated meeting the requirements of AASHTO Designation M 190 Type C for iron or steel pipe or AASHTO Designation M 196 for aluminum alloy pipe for sectional dimensions and type of bituminous coating. Pipe gauge shall be as required to meet the soil and traffic loads with a deflection of not more than 5%.
C. 
ABS (Acrylonitrile-butadiene-styrene) composite pipe and fittings shall conform to the requirements of AASHTO M 264 and AASHTO M 265. Perforated pipe shall conform to the requirements of AASHTO M 36, Type III.
D. 
Corrugated plastic pipe shall conform to the requirements of AASHTO 252.
E. 
Manholes and catch basins:
(1) 
Manholes and catch basins shall be constructed of precast concrete sections or portland cement concrete blocks or a combination of both. An eight-inch prepared concrete base be used for block and a prepared earth foundation for precast concrete sections.
(2) 
Blocks shall be machine-made, solid segments not less than eight inches in width.
[Amended 5-10-2014]
(3) 
Cement concrete blocks shall be laid on the prepared concrete base by a mason and in a workmanlike manner.
(4) 
Prior to laying, the blocks shall be wet with water.
(5) 
All joints shall be completely filled with mortar, and no joint shall be greater than 1/2 inch in thickness.
(6) 
Joints on the inside of the structure shall be neatly tooled.
(7) 
Mortar composition shall be two parts sand to one part portland cement.
(8) 
Precast portland cement catch basins and manholes shall be in conformance with dimensions and specifications described in MDOT standards.
(a) 
Concrete blocks or equivalent shall be used for the course or layers around inlet and outlet pipes.
(b) 
Concrete block may be used for the remaining upper section of the structure.
F. 
Metal frames and traps.
(1) 
Manholes. Metal frames and traps shall be set in a full mortar bed, and tops shall conform to the requirements of AASHTO M 103 for carbon steel castings, AASSHTO M 105, Class 30 for gray iron castings or AAHTO M 183 (ASTM A 283, Grade B or better) for structural steel.
(2) 
Catch basins. Metal frames and traps shall be set in a full mortar bed and tops shall conform to the requirements of AASHTO M 183 (ASTM A 283 Grade B or better) for structural steel.
(3) 
Gratings. Castings shall be sized for the particular inlet condition with the gratings set perpendicular to the curbline or travel portion of the road.
G. 
Drain inlet alignment shall be straight in both horizontal and vertical alignment unless specific approval of a curvilinear drain is obtained in writing from the Board, after consultation with the Town's consulting engineer.
H. 
Manholes shall be provided at all changes in vertical or horizontal alignment and at all junctions. On straight runs, manholes shall be placed at a maximum of four-hundred-foot intervals.
I. 
Upon completion, each catch basin or manhole shall be cleaned of all accumulation of silt, debris, or foreign matter. Maintenance of catch basins and manholes shall be the responsibility of the developer until such time maintenance becomes the responsibility of a homeowners' association or until final acceptance of the road by the Town.