When designing a storm sewer network, the PEO shall review Chapter 8 (for pipe materials) and the list of acceptable pipe material (schedule pipe) in the Standard Specifications. Storm sewer pipe is subject to some use restrictions, which are detailed in Section 8-2.4.
Pipe flow capacity depends on the roughness coefficient, which is a function of pipe material and manufacturing method. Fortunately, most storm sewer pipes are 24-inch diameter or less and studies have shown that most common schedule pipe materials of this size range have a similar roughness coefficient. For calculations, the PEO shall use a roughness coefficient of 0.013 when all 24-inch-diameter schedule pipes and smaller are acceptable. For calculations during the preliminary design and when the pipe materials have not been determined, the PEO shall use a roughness coefficient of 0.013 for schedule pipes 24 inches in diameter or smaller. For larger-diameter pipes, the PEO shall calculate the required pipe size using the largest Manning’s roughness coefficient for all the acceptable schedule pipe values in Table 4-1. In the event that a single pipe alternative has been selected, the PEO shall design the required pipe size using the applicable Manning’s roughness coefficient for that material listed in Table 4-1.
In estimating the quantity of structural excavation for design purposes at any location where alternative pipes are involved, estimate the quantity of structural excavation based on concrete pipe because it has the largest outside diameter.
