A common misconception by designers of stormwater treatment devices is to use the physical drop between inlet and outlet pipes, Elevation Head (∆z), as the Total driving head instead of the hydraulic drop between upstream and downstream water surface elevations, Driving Head(∆h).
This relationship between Elevation Head (∆z) and Driving Head (∆h) can be further explained in Figure 1 below. The physical drop (Δz) between inlet and outlet pipe inverts is shown as 240mm, whereas the driving head (Δh) at maximum treatment peak flow is shown as 750mm between the outlet invert and the internal bypass weir invert. This configuration will result in a minimum 510mm (Δh-Δz) surcharging headwater condition upstream of the inlet pipe.
For a stormwater treatment device to hydraulically operate properly, sufficient driving head must be available upstream of the device to permit gravity flow of contaminated stormwater through the treatment mechanism (i.e. filter media) and to discharge treated water downstream.
Not allowing sufficient hydraulic head through the treatment device will reduce the ratio of captured design water quality flow and prematurely bypass untreated sediment laden stormwater. This can also introduce a backwater effect or headwater condition upstream of a treatment device’s inlet that will submerge the upstream reticulation, reduce upstream catchpit capacity and, as a worst case scenario, lead to localised upstream flooding.
Whereas, misinterpreting the Elevation head as Driving Head can lead to underestimating the actual available driving head in the upstream reticulation design. Hence the water surface level (HGL) may actually be at a higher reduced level, than originally calculated, and at a steeper hydraulic gradient (i). This will result in surcharging the upstream network (i.e. headwater) during low flows as a consequence of a higher flow depth. Additional head losses due to friction, and upstream structures entry/exit losses may also occur. Localised flooding of the upstream catchment and increased overland flows, where sump inlet capacity has been reduced, can consequently occur as worse case scenarios where this headwater condition has not been factored into the reticulation design.
It is recommended, if site constraints allow, to set the physical drop between inlet and outlet inverts at the same reduced level as the total hydraulic head in order to prevent unnecessary surcharge and headwater conditions as mentioned above. If this is not possible, it is recommended to design the upstream pipe network for the anticipated headwater condition.