The brief is to manufacture two filtration curtains to prevent solids entering drinking water reservoir. These custom silt curtains face an inlet into a dammed reservoir and are put in place to reduce forest debris in the system. By reducing the debris, there would be reduced water processes for the Silvan drinking water treatment plant. The dam has very high flow rates and a quality boom is required to withstand the stresses placed on it.
Date: March, 2009
Location: Silvan Dam, Yarra Valley, Victoria, Australia
The inlet where the silt curtain is to be implemented has one of the highest flows into Melbourne Water’s Silvan Dam reservoir. A canal provided passage for all the rainwater that falls in the surrounding catchment area. Sticks, leaves, bark, forest debris etc all collect in the canal and are washed into the reservoir. It’s common for bushfires to occur in the forests in and around the catchment area, after rain a lot of ash is washed into the reservoir as well. All need to be lowered to provide the catchment with low solid content.
Drinking water catchments need to be reasonably clear of debris to decrease further treatment processes. For this reason, Chatoyer opted for a system of two curtains; the first to dissipate the water load and the second to filter out fine particles. The secondary curtain has two skirts – one to filter out fines (filtration media) and the other to provide structural strength. The material in the first curtain is an effective filtration media but has low strength. It was designed to be replaced once clogged. It can be removed from the curtain to avoid replacing the whole silt curtain. Seven two metre wide flaps hang from the bottom of the skirt and block particles escaping through the submerged canal.
Submerged Canal Solution:
As each curtain was installed over the submerged 4.5m wide canal there was a sudden drop off where potential fines could escape underneath the curtain. Geomembrane flaps on each curtain drop down into the submerged canal stopping any of these particles. A 4.5m wide, 1.2m deep rectangle wasn’t used because the curtain will naturally move back and forth and side to side to some extent this will cause the one big flap to catch along the edge of the canal potentially ripping open the material. It would also leave a big gap underneath where the particles can enter the reservoir. To solve this problem 1m wide, 2m long flaps were used, if the curtain moved side to side then the flaps will rise up out of the canal and the adjacent flaps will keep the canal mostly blocked from particles. Each flap has its own ballast chain to weigh it down.
The proposed job included two arching silt curtains. Both anchored from the same anchoring point on each bank. The first one takes the brunt of the water force exiting through the 1.2m deep canal as much of the water will stay centred due to the fact that the canal keeps running along the floor of the reservoir. By taking the brunt of the water force in the centre it won’t do a very efficient job of filtering out fine particles. Its main goal is to dissipate the water load so it hits the second curtain much more uniformly allowing effective filtration. The primary curtain has a single skirt and no filtration media but will stop some floating debris and suspended solids. This design layout can be seen in the below drawing.
XR5 1,000gsm High Performance Reinforced Geomembrane
150mm x 150mm Closed Cell Foam
|Skirt material: (double skirt design)|
Woven 450gsm Geotexile
Non Woven 340gsm Geotextile
13mm galvanised chain
Heavy duty aluminium connectors with Dee ring and chain supports
The customer agreed that their short term and long term goals were met.
- The customer received silt curtains designed that are as reliable and effective as they expected.
- We designed the requested set up where an effective amount of forest debris and particles would be filtered out from the canal leading to clearer and cleaner reservoir water.
- The curtains achieved performance for heavy rainfall events that increased flow rates from the inlet.
- The curtains retained their position and continued to filter out particles during heavy flows.
- The materials, components and structure of the silt curtains were expected to last 3 years.
- All timelines were met during the course of the project.
Overall the team at Melbourne Water were very happy with the hard work and dedication that Chatoyer invested to complete the job. They were impressed with our simple and effective designs and the quality of the silt curtains.