Increasing Storm Water Retention Capacity for Urban System

When the Lake water level is high it prevents discharge from the urban storm water system. Lake Victoria’s water level fluctuates with regional run-off inflows, and is affected by outlet constraints through its connection with the tidally affected Swan Bay. In addition the Lake is often hypersaline and it is desirable to prevent Lake water from migrating up into the urbanized area when Lake levels are high. The design solution is to use the Emily St sedimentation basin, established at the downstream end of the storm water system, to provide attenuation/storage of storm water during periods when Lake Victoria water levels are high. When Lake Victoria water levels are low the system is designed to operate in an unregulated fashion allowing free discharge of storm water through the Emily St sedimentation basin, into Lake Victoria.

Alluvium Consulting initiated the design concept and involved AWMA through the early stages of development. The partnership determined that a unique passive gate, operating on the basis of differential buoyancy forces, could be used in place of a traditional electro-mechanical gate solution. This saved considerable capital expenditure as well as providing a virtually maintenance free “set and forget” solution.

As the concept involved a gate design without local precedent, AWMA built and tested a to-scale prototype model. After successful prototype testing, AWMA proceeded with detailed design, manufacture, and dry factory commissioning to ensure the gates would perform as intended, in the field. Two 4m wide x 1.5m high LayFlat gate leaves were manufactured from marine grade aluminum and protected with sacrificial anodes to ensure a long design life in salt water conditions. The gate leaves were then hinged to automatically open (horizontal) or close (vertical) in response to upstream or downstream water levels, based on a combination of in-built buoyancy and self-weight. AWMA installed and wet commissioned the gates which included full functional testing, where the gates operated exactly as intended.The gates rise automatically when the downstream (saline) water levels rise above the crest of the weir, isolating the rising salt water. When upstream levels increase, the door opens allowing fresh water flows to pass into the lake.The gates are self-regulating and effectively maintenance free. They have an instantaneous response time, a guaranteed failsafe opening and no opportunity of failure from operator or equipment.