What is a debris control structure: As the name implies it is a structure that collects and controls debris during flood events. There are 4 typical debris control structures. These are 1) Debris Deflectors: A series of vertical or horizontal poles that are 45° to the inlet of drainage structures and divert medium to large debris to collection areas; 2) Debris Racks: A series of large vertical poles concreted into the stream at 90° to capture large debris such as trees and cars; 3) Trash Racks or Trash Screens: A series of metal or wooden bars enclosed in a frame that are placed at 90° to the inlet or outlet of drainage structures to collect smaller debris or trash; 4) Debris Sumps or Debris Dams: A pit designed to slow the speed of water and allow for the deposition of heavy debris such as sediment and rock.
Although these structures do not manage floods, they are important in reducing the blockage or failure of drainage structures in streams with high debris loads.
Not applicable. Why? Debris structures are designed purely to reduce debris impacting and blocking critical structures. As a result they cannot improve community access and recreational use.
Debris control structures generally do not disadvantage individual members of the community. Why? Debris control structures are usually unobtrusive and located on public property within the watercourse.
Debris control structures provide safety to the community during flooding. Why? It can reduce debris impact and build-up that can cause bridges and culverts to block and re-divert flow, or fail.
Debris control structures generally do not raise community awareness and understanding of the local flood risk. Why? Debris control structures are usually perceived as water quality improvement measures and not as flood reminders.
Debris control structures may threaten local plants and animals and their habitat particularly in the short term. Why? Construction typically involving excavation is required at the site which can cause initial habitat destruction. If not well designed debris control structures can affect the free movement and migration of local aquatic and terrestrial animals. Ongoing access is also required to the site to clear debris and/or pollutants. However, debris control structures may also act like natural log jams or riffle ponds and if completed with a riparian vegetation management scheme can create new habitat opportunities for local plants and animals.
Debris control structures in general do not cause water quality impacts. Why? They act like natural log jams or riffle ponds and do not significantly alter the flow regime. In some instances they actually remove trash and pollutants improving water quality downstream.
Debris control structures have moderate to major initial costs to council. Why? Although they are relatively simple to construct, feasibility studies and detailed design studies are vital to insure their placement and function does not cause adverse impacts both upstream and downstream. The costs for debris control structures for reducing medium to large debris as required in flood events typically range from $50,000 to $200,000 for deflectors and large racks.
Debris control structures have moderate ongoing maintenance costs to council. Why? Maintenance generally involves removing and dumping debris, and routine inspections for defects or vandalism.
Debris control structures can reduce annual average flood related damages to the community if there are high debris loads and blockage is a significant issue. Why? As stated previously debris control structures can reduce potential debris impact and build-up that can cause bridges and culverts to block and re-divert flow, or fail.
Debris control structures have the potential to cause adverse flood impacts to other areas. Why? Debris control structures have the potential to block 100% which then turns them into a weir or dam. As a result feasibility and detailed design studies for a full range of flood events (from regular to extremely rare floods) are required to insure their placement and function does not cause adverse impacts both upstream and/or downstream.