A ‘belts and braces’ approach was taken to the conversion of an open infiltration sump in this established and high property value location in Perth, Western Australia. Peak flows from a 600mm diameter stormwater collector drain still needed to be stored at the sump location for a 1 in 100 rainfall event, however the council and local residents no longer wanted an unsightly 3-4 metre deep, eroding and overgrown hole in the ground, detracting from what is otherwise a very nice part of the world.
The sandy soils typical of the region initially allow rapid infiltration of the stormwater discharge, up to 6m/day, but over a number of rainfall events a cake forms on the surface of the sand as a result of dust, road grime, and building activity contamination in the catchment area being deposited in the storage via surface runoff. 
This change in infiltration capacity over time requires the sump storage volume to be designed larger than if silt and sludge could be effectively intercepted and managed. This is where EcoAID’s Catch-All rows can be employed to isolate and manage the Total Solids entering into the storage volume. Two Catch-All rows taking an equal inflow volume were installed. The large volumes within the Catch-All rows result in long detention times, therefore treatment of solids (through the process of sedimentation) down to very small particle sizes can be achieved - remember Stokes Law? The council also installed a number of soakwells, which remove larger particles from the sediment and other gross pollutants. The soakwells were installed at the start of each Catch-All row and provide surface access for servicing both the soak-pit and Catch-All row.
The advantages of using Catch-All rows over pit-style gross pollutant traps (GPTs) and sediment traps for stormwater treatment include:-
Larger device volume - resulting in longer detention times and more effective sedimentation across a large particle size range.
Longer maintenance return intervals due to the larger volume available for solids storage.
Cost - Catch-All rows add very little overall cost to an installation as storage volume and treatment functions are integrated.
Installation ease - no cranes or heavy lifting requirements.
100% of inflow is treated. Most pit style GPTs are designed to go into by-pass for larger storm events or flows after the first flush. This is not good enough for infiltration installations as silt and sludge will eventually form a cake on the surface of the in-situ soil, or at the interface between the storage and in-situ soil.
Finer soil fractions are treated, due to the longer detention time and shorter sedimentation distance (a function of row height), as governed by Stokes Law.
More contaminants, such as heavy metals, are removed due to improved sedimentation conditions as these contaminants are often adsorbed onto the surface of finer soil and clay particles, which are not pulled out by the shorter detention times and coarse screens of most GPTs.
The council’s Manager of Engineering Services stressed the importance of the system being capable of handling the peak inflow from the 600mm diameter collector, as some older installations experience issues with flow backing up during larger rainfall events. Many of the first generation of modular infiltration systems are not able to cope with large inflows, and lateral flow distribution or flow equalisation are not integral to system design, with header or equalisation pipes only possible at the start or end of rows, external to the system footprint. The EcoAID modular chamber system has lateral overflow pipe ports on every unit, so by applying conservation of flow principles, an extremely efficient design can be achieved, without increasing the footprint of the storage.
We look forward to seeing the site once landscaping and native planting is complete. Already there’s been a vast improvement.