Terry Lee Wager and Mary M: FYI. I wrote the following on Feb 5, 2014 and asked the technical commitee to follow up on several comments and questions. I. The configuration (dimensions/width/length etc.) of the concrete core wall for the lower dam seems to be unknown with certainty. The engineering drawings to date show the concrete core wall as being assumed to extend from the location where the peninsula begins right up to the edge of the bridge over the spillway. Further the following are some examples of why there appears to be confusion, uncertainty and conflicts as to the actual dimensions: (In particular note Item e. below) a. From Note 4 on page 10 of the Golder 2003 DSR: “It is not known where the information on core wall thickness came from” b. From the Golder 2003 DSR: “Lower Chase River dam is an earthfill structure some 77 m in length...” c. From the Golder 2004 MSM: A concrete wall provides the impervious barrier, and also forms the front face of the dam at normal pool level in the reservoir. This wall is 0.3m thick at the top and extends at this thickness to 0.6m below the crest. It then thickens to a reported 1.2m (EBA, 1992), and a horizontal construction joint is evident. It is unclear whether the wall thickens further with depth. d. From the 2010 EBA Seismic Analysis: The dam is approximately 24 m high and has a crest length and width of 77 and 10 m respectively. e. From the Golder 1978 Geotechnical Investigation Report: “The dam is 24.0 m high with a crest width of 10.0 m and a crest length of 50.0 m.......” Questions: 1. Is it possible that the dam concrete core wall length is much less than 77 meters? 2. Was the length of the dam crest actually extended during the 1980 remedial work by extending the earth embankment and abutments only? 3. If the wall is actually 50 meters or even less then would this not greatly affect the modeling and calculations done by previous engineering studies to determine strengths and possible flow rates during breaches?(Example the following statement is taken from AE’s 2012 Inundation Study: “Notably, the Lower Dam has a much larger potential “gate” than does the Middle Dam, since it has a greater span and depth”) 4. Can the actual dimensions be determined with certainty? 5. If the actual dam is only 50 meters or less then is it possible that any overtopping erosion protection need only cover an area which reflects the reduced dam length? II. There also appears to be some doubt, confusion or uncertainty as to the capacity of the lower dam spillway. The following examples support this statement: a. From the Dam Safety Program Summary Report: “The hydrological aspects of the Chase River System were assessed and reported in the 1978 Storm Drainage Study. The inflow to the Lower Chase River reservoir was estimated to be 57.8 m3/s, for a 100 year storm in the 1978 study. The spillway capacity with a 0.9 m discharge depth (determined by the top of the concrete wall) was estimated to be 53.8 m3/sec, therefore the spillway may not be adequate to transmit the flow resulting from a 100 year storm.” b. From the 2002 WMC Hydrology Study: “Previous studies noted in EBA, 1992b indicated that the maximum capacity was 55 cubic m/s” c. From the 2002 WMC Hydrology Study: Questions: 1. Are the spillway capacity calculations done by WMC which put the lower spillway capacity at 25 cubes accurate? 2. Did WMC use the correct method and formula to calculate this capacity? Was the effect of having the concrete divider at the mouth of the spillway considered and factored into any calculations? 3. Did WMC consider the effect of the drop to the water stream level below in their calculations or was this overlooked? Why did they stop at the top of the free drop for measurement purposes? Is the drop not actually an extension of the spillway? 4. Could the lower spillway capacity not be easily increased by one or all of the following methods? a. Increase the width of the spill way by extending in the direction of the dam. b. Increase the height of the spillway walls to match the height of the crest. c. Remove the concrete divider. d. Install a middle outlet to supplement the existing spillway. III. Many have said that the previous engineering reports and studies were in large part based on extremely conservative probabilities, assumptions and personal judgments and may have in effect been exaggerated leading to an extreme classification. Is it not much more likely that any breach caused by a seismic event would lead to water leaving the reservoirs in a much more controlled and less immediate and bursting manner? Let’s suppose for the sake of argument that the lower dam would take approximately one hour to completely drain due to a breach. This leads me to ask a simple question? Then why are we going to increase the capacity of the spillways at great cost to accommodate any more than 30 cubes per sec????? After all at that rate the spillways would be capable of completely draining the lower lake in one hour!
Posted on: Wed, 16 Jul 2014 19:26:14 +0000
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