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(a) Storm Sewers Criteria. The more important criteria to consider in designing storm sewer systems are listed below:

(1) All storm systems are to be designed using Manning’s Equation: Q = ((1.49 x R2/3 x S1/2 x A)) / (n)

(2) The sewer must be deep enough to receive the flow from all its sources within the watershed.

(3) The size of the sewer must be adequate for flowing full, based on the design storm.

(4) The gradient of the sewer must be sufficient to avoid deposition of solids.

(5) The storm sewer material will be concrete, or polyvinyl chloride (PVC). Other approved material may be used for special design. Polyethylene pipe, smooth interior may be used in backyard or side yard application only.

(6) The main pipe, if over twenty-four (24) inches, in a sewer system will be required to be separated from all curb inlets, unless a special design is submitted for approval.

(7) The flowline of pipes should be set such that the crown of pipes, at junctions, are at the same elevation; if the outlet elevation permits, the crown of the outlet pipe may be lower.

(8) The flowline elevations of sewers should be set to avoid using concrete encasement.

(9) Within the limits of an existing or proposed right-of-way, where minimum cover during construction or proposed cover over the outside top of the pipe is thirty (30) inches or less for pipe size six (6) to twenty-seven (27) inches in diameter inclusive, concrete encasement will be required.

(10) Inlets designed to capture stormwater runoff of streets are to be placed in the curb line and have a circular flat open grate that is twenty-four (24) inches in diameter or other casting approved by the Town Engineer (see concrete inlet detail from Design Standards).

(11) Manholes and inlets are to be constructed with sealed joints and adjusting rings and to seal the chimney externally.

(12) The location of the stormwater tap for each lot shall be marked on the curb in the color green with three (3) notches or cuts one-quarter (¼) inch wide, three (3) inches long and one (1) inch apart.

(b) Storm Sewers Information.

(1) Design Storm Frequency. Flowing full for five (5) year storm.

(2) Hydraulic Gradient.

a. Based on five (5) year storm, shall not exceed grate elevation for an inlet or catch basin.

b. Grade line based on Tailwater or 0.8 D at outlet or other critical points within the system.

(3) Design Flow. All design flow will be based on Rational Method Q = CiA or industry recognized computer programs.

Minimum time of concentration:

Curb inlet: ten (10) minutes.

Ditch C.B.: ten (10) minutes.

(4) Runoff Coefficient. Based on Table.

(5) Manning’s “n” Value. All storm sewers shall have an “n” value of 0.013 or less.

(6) Minimum Cover to Subgrade. Desirable thirty (30) inches to ground surface or pavement subgrade.

(7) Maximum Cover.

a. The supporting strength of the conduit, as installed, divided by a suitable factor of safety, must equal or exceed the loads imposed upon it by the weight of earth plus any superimposed loads.

b. The design procedure recommended for use in structural design of storm sewers is Design Manual concrete pipe, available from American Concrete Pipe Association.

(8) Velocity in Sewer for Design Flow.

a. 3 F.P.S. Minimum

b. 15 F.P.S. Maximum

(9) Maximum Length between Access Structures.

a. Pipes under thirty-six (36) inches: four hundred (400) feet.

b. Pipes thirty-six (36) inches and over: five hundred (500) feet.

(10) Curb Inlet Underdrains. The installation of perforated underdrains underneath the pavement for a length of between thirty (30) to forty (40) feet (to be determined by the Town Engineer) in each direction at each curb inlet or catch basin in all subdivisions shall be required.

(11) Rear Yard Inlets. Inlets or catch basins shall be placed along the rear yard property lines approximately every three hundred (300) feet or as approved by the Town Engineer.

(12) A storm sewer service line shall tie into a storm sewer unless a lot is adjacent to a ditch or detention/retention basin then its storm sewer service line can discharge into the ditch or detention/retention basin.

(Ord. No. 1707, T. 6, § 5, 7-22-20)