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(A) The 100-year discharge elevation must be checked for all locations to avoid flood damage to adjacent structures. Manning’s Equation is recommended to calculate pipe flow and velocity. The storm sewer hydraulic grade line for the 100-year event shall be contained within the storm sewer system.
(1) Hydraulic capacity.
(a) The hydraulic capacity of storm sewers shall be determined using Manning’s Equation:
Q = 1.486 R 2/3S1/2A
Q = volumetric flowrate (cfs)
R = the hydraulic radius in feet
S = the slope of the energy grade line in feet per foot
n = roughness coefficient (for reinforced concrete pipe, n = 0.012)
A = cross-sectional area (Ft2)
(b) Roughness coefficient (n) values for other sewer materials can be found in standard hydraulics texts and references.
(2) Minimum size. To minimize the potential for pipes to become clogged, the minimum size of all storm sewers shall be 12 inches. This does not pertain to outlet structures for detention/retention basins. (The rate of release for detention storage shall be controlled by an orifice plate of other devices, subject to approval of the Board, where the 12-inch pipe will not limit rate of release as required.)
(3) Grade. Sewer grade shall be such that, in general, a minimum of 18 inches of cover will be maintained over the top of the pipe when reinforced concrete pipe is used. Pipes with less cover than the minimum may be used only upon approval of the Board. Uniform slopes shall be maintained between inlets, manholes and inlets to manholes. Final grade shall be set with full consideration of the capacity required, sedimentation problems and other design parameters. Minimum and maximum allowable slopes shall be those capable of producing velocities of two and 15 feet per second, respectively, when the sewer is flowing full. National Transportation Product Evaluation Program (NTPEP) and can be found at www.ntpep.org. Joints shall be an integral bell and spigot with rubberize gasket conforming to ASTM F477 unless otherwise noted. Alternatively, plain-end pipe connected with accompanying split-coupler can be used for driveway and culverts.
(4) Pipe bedding, backfill and surface restoration.
(a) All concrete pipes must be bedded (according to the detail in the Appendix, § 4 to the ordinance from which this section is derived, which is on file in the office of the County Clerk), but not limited to the following.
1. Where pipe is installed in earth areas, not immediately adjacent to a street or road, place Indiana No. 9 crushed stone to the spring line for RCP. The remainder of the trench must be backfilled with selected earth materials, humped over the trench to allow for settling.
2. Where pipe is installed in a graveled area, the remainder of the trench must be backfilled with Indiana No. 9 to a point eight inches below original grade.
3. Where pipe is installed in an asphalt street, driveway or parking area, the remainder of the trench must be backfilled with Indiana No. 9 to the subgrade. The trench must then be trimmed back six inches on each side and filled with 3000-psi concrete. After all construction is completed, the trench must be cleaned, primed and paved with a one inch compacted thickness of INDOT HAC Surface to be flush with the surrounding area. All patch seams can only be saw cut, cut smooth, straight and tarred.
4. Where pipe is installed in a concreted area, the remainder of the trench must be backfilled with DGA to a point nine inches below original grade. The trench must then be trimmed back six inches along each side and filled with 3000-psi concrete flush with original grade. All patch seams must be saw cut only, smooth and straight.
5. All cutting of trenches in existing asphalt or concrete pavements must be done with a saw only to provide a straight, smooth joint when new paving is done.
6. In areas where rock is encountered, all rock shall be removed from the trench. Voids created by such removal shall be refilled with Indiana No. 9 crushed stone.
7. Before placing of the aggregate base course, the upper six inches of all subgrade and subbase shall be compacted to a minimum of 95% of maximum dry density as determined in accordance with AASHTO T99, as modified in 203.24.
(b) All HDPE pipe shall be installed and bedded according to the manufacture’s specifications. Installation of each product shall conform to approved county installation specifications and details. When necessary, additional guidelines shall conform to either:
1. INDOT specifications and standard drawings;
2. ASTM D2321, standard installation for thermoplastic pipe; or
3. Published manufacturer’s installation guidelines.
(c) For all pipe bedding details, refer to the detail in the Appendix, § 4 to the ordinance from which this section is derived, which is on file in the office of the County Clerk.
(Ord. 2010-11, passed 7-22-2010)