Options as described by Lorne on May 15. CDPPS has never supported the spillway option. GSIs overtopping proposal is 3m for both dams. Further to my summary email below sent back in March in regards to the remediation options (please reread) find below some further considerations: Spillway Option: 8.1million - Requires substantial work in the water way (expensive coffer dam required). Possible fishery and contamination concerns. Time sensitive. - Low Level Outlet is only a partial LLO (10’ down) and has anyone given consideration to lowering the water level of the dam in the middle of summer at the max recreational time (or at any time when the inflow has stopped), any thought to how it would go around the edges (mud, sloughing, habitat etc). A single 12” outlet would conservatively drain the top 10’ of the lake at an average rate of approx. 1” per hour (ie in less than 5 days the lake would be 10’ lower and would remain that way until an inflow (rain) occurred) - New spillway capacity has a finite limit and after that the water will overtop the dam - New price makes this option expensive Overtopping Option: 7.2million - unlimited overflow capability (relative to the spillway) - If LLO is gaining support (if some how the public would accept lowering the water level) then a permanent siphon could equally be installed for this option. - In terms of the reshaping of land to the NW and possible loss of habitat what is the sensitivity to not doing anything to that area and allowing the water to flow to the SE (current spillway and possibly to Harewood Ck)? The dam has been safeguarded (by hardening) from failure by erosion so would it matter if the low frequency flood water made its path of least resistance rather than forcing it all back over the dam wall? The logic here is that we permanently change the shape of our land and remove trees for events that are unlikely to occur. What is there has worked for 100+yrs…. - Maintenance and/or repairs are only likely for very low frequency events. To be confirmed. From Lorne March 05 Option 3: Labyrinth. Novel idea but expensive and invasive. Means losing a substantial amount of the peninsula. Enlarged spillway means a wide deep trench. 5m deep (spillway is already 2m deep) 5m is 17’ deep. This is a daily public safety issue and ugly. Furthermore there would have to be a new bridge and the end result is a very different looking lower Colliery Dam. One that can’t be pictured until the work is done. It is receiving some support because it is more traditional in design and offers a partial lower level outlet option (which SFN indicated interest in). Average cost of $2.86M or a range of $2.29M - $4.29M NOW AT $5+M!! Option 4: Overtopping protection is cost effective (even using the most expensive technique of soil mixing and asphalt topping) and minimally invasive. One potential drawback is some possible regrading/reshaping of the area between the end of the wall and the washroom to assist with the unlikely event of water over the crest – this is not well detailed and should have further investigation before seeing this as a con – might not even be needed. The other drawback is the contaminants in the coal fill – it has been assumed that this can just be soil mixed and left in place but further investigation is required. It has been there for 100 years…. Furthermore if option 3 is going to gain support because of a lower level outlet then a similar approach via permanent siphon is equally possible. Average cost $1.65M or a range of $1.32M - $2.475M Option 3 was nearly twice the money of Option 4 and changes the park in a way that we can not picture. We know what we have with Option 4 – we know what the end result will look like because it likely won’t look any different to what it now looks like.
Posted on: Fri, 01 Aug 2014 00:23:08 +0000
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