System hydraulics refers to how water flows through a stormwater filtration system. The following three steps should be part of the hydraulics evaluation process.
1. Evaluate the hydraulic grade line (HGL) at the design flow rate. Typically, a backwater calculation from the point of downstream control should be performed to ensure the system can convey the peak water quality design flow rate. The downstream control can sometime be the filters or an orifice control downstream of the filter to throttle the flow. This analysis should include head loss through porous media (filter head loss) and pipe entrance, exit, and barrel losses.
2. Check scour velocities in tanks and pipes. Velocities should be evaluated with particular reference to where sediments are deposited or where high-energy flows can dislodge or scour the filtration media. For example, the velocity at which the inlet pipe discharges into the filter bay determines if there is sufficient energy dissipation to minimize scour. Beware of super critical flows and a sudden enlargement or grade change will evoke a hydraulic jump. Energy dissipation associated with the jump will cause scour of deposited sediments.
3. Ensure high flow bypassing. For extreme flows one should consider that the filters are occluded and 100% of the flow is in bypass. There is both online bypassing and offline bypassing. All filters will retain deposits of sediment, organic matter, trash, and debris. As flow through a system increases, the turbulent energy will reach a point that causes re-suspension of these materials. Therefore, it is critical to ensure that high-energy flows be kept away from the filter bed(s). Typically, areas with intense rainfall such as Type II and Type III storms require offline bypassing.