Appendix 5.5.0. HYDRAULIC GRADIENT  

In storm drain systems flowing full, all losses of energy are a function of resistance of flow in pipes and by interference with flow patterns at bends and structures. These losses are cumulative and must be accounted for along the entire system from the outlet to the most upstream inlet. The purpose of determining headlosses is to include these values in a progressive calculation of the hydraulic gradient along the storm drain system. The hydraulic gradient line shall be computed and plotted for all sections of a storm drain system flowing full or under pressure flow. The determination of friction loss and minor loss are important for these calculations.

When computing the hydraulic grade line, the tailwater elevation of the system shall be established after careful consideration of the flow elevation and timing in the receiving stream. When detailed hydrologic and hydraulic studies are available for the receiving stream, the modeling results from these studies must be used to estimate the tailwater elevation. If detailed studies are not available, the design engineer must use his professional judgment in calculating the appropriate tailwater elevation. The use of coincident frequencies is not encouraged, and will only be considered for use on a case-by-case basis when an appropriate means for estimating or calculating the tailwater elevation is not feasible. In no case shall the tailwater elevation of the system be considered to be below the top of pipe (overt elevation).

Storm drain profile drawings submitted for review and final profile drawings for construction shall show directly on the drawing the 25-year and 100-year hydraulic gradeline, flow rates and flow velocities for each segment of the storm drain system.