WESTWOOD LAKE DAM INUNDATION STUDY 2004 — SOME COMPARISONS WITH THE CHASE RIVER DAMS SITUATION
Lawrence Rieper: March 28, 2013 (See also a Colliery Dam examination from Mr. Rieper published here on Dec. 1, 2012)
I am not going to challenge the technical engineering data, nor am I prepared to believe in computer modeling at the expense Of my own senses. I am going to throw some simple facts and figures around and make some observations.
I recently read the Westwood Lake Dam Inundation Study of 2004 by Water Management Consultants. This was not available to me when I wrote “Some Observations on the Decision to Remove Colliery Park Dams”. It got me thinking.
I was immediately struck by the relationship between the surface area of Westwood Lake and the flooded area downstream, compared with that between the two Colliery Park dams and the huge area of inundation below, shown in “Chase River Dams Breach Flood Inundation Study of 2012 by Associated Engineering”.
Westwood Lake is about 130 acres. According to the Dam Safety Branch in 2013, it holds 2,300,000 cubic metres of water, However, the 2002 report has a table showing at dam crest a volume of 4,688,000 cubic metres and up to 5,000,000 cubic meters at overflow. These two volumes should not be achieved as long as the spillway at the other end of the lake functions properly (unblocked by debris). Dam Safety states that the zoned earthfill embankment and saddle dam is12 metres high and 100 metres wide. It was built in 1906. Its Dam Failure Consequence Classification is ‘High —Significant’, indicating less than 10 people may die (1999 CDA Dan Safety Guidelines).
Again, according to the Dam Safety Branch, the Middle and Lower Colliery Dams were built in 1910 of earthfill concrete wall construction. The former is 12.5 metres high and 50 metres wide. The latter is 23.3 metres high and 77 metres wide. The Middle Dam holds 93,000 cubic metres and the Lower Dam holds 173,000 square metres, for a total of 266,000 cubic metres. However the 2012 Inundation Study gives, respectively, the capacities as 110,000 cubic metres and 112,000 cubic metres, for a total of 222,000 cubic metres. These totals average out at 244,000 cubic metres. Their Dam Failure Consequence Classification is Extreme — Risk Level 1′. Before 2012, it too was ‘High’.
The surface areas of each Colliery dam is about 8 acres or 16 acres combined. Together they represent about 1/8th of the area of Westwood Lake. The Colliery Dams have about 119th or 1/10th of the normal volume of Westwood Lake, although Westwood. Lake could theoretically hold more than twenty times the capacity of the Colliery Dams combined. All of the lakes are roughly of similar general depth (up to around 10 metres), although at the dam, Lower Colliery may be deeper,
For a Dam Breach with or without a Probable Maximum Flood in the maps of the 2004 Westwood Dam Study the main flooded areas are about three times the area of the lake, suggesting up to about 3 metres of coverage (although deeper in the river courses).
The corresponding maps for the 2012 Chase River study show an area 12 to 15 times the combined area of the two dams, with a peak depth of 6 metres at the river course, but much less further away. This seems to be inconsistent with the previously quoted maps.
In the 2012 Chase River Inundation Study, within 30 minutes some 130 buildings would be flooded. From 110 to 341 structures could be damaged. Between 35 to 40 people (during the day) or between 45 and 60 persons (if at night) could die. Estimated damage between $13.8-million and $46.9-million could occur. Overall fatalities could reach between 81) and 150 by the time the flood waters reached the sea. This is the basis for the Extreme classification (more than 100 deaths). The population in the flooded area is considered to be between 1810 and 1883. I do not understand from the report how the fatality figures are arrived at. I presume that there is an explainable formula.
As I had done in Harewood some months previously, I drove and walked the potential flood area for a breach at Westwood Dam. I noted the low areas along the Millstone River, by Maxey Road and through Buttertubs Marsh (now about 100 acres, including the Conservation Area, West Marsh and Valley Oak Park). I also noticed places where the river was only about 2 or 3 meters below. The following locations appeared at risk: Riverside Drive/Girvin Avenue — 12 houses (in winter the river almost reaches the road); Fuller Street — 20 homes (including 9 new ones constructed since 2009 on flood plain); Bartlett Street — 24 homes + 7 units (Bowen Terrace Apartments at ground level)Adams Avenue — 15 homes + 4 houses; Buttertubs Drive – 10 units (Rebekah Villa) 32 units (Millstream Acres) 83 units (George Pearkes) + the community centre; Buttertubs Place — 32 units (Twelve Oaks); Maxey Road — 22 houses; Durnin Road — 8 houses; Westwood Road — 6 (near bridge); Jingle Pot Road — 6 houses Bird Sanctuary Drive — 9 houses; Valley Oak Estates — 72 units (mobile homes); Caledonia Avenue — German Cultural Centre + area opposite zoned for multiple housing.
For a total of at least 362 dwellings at 1.2 or 2.2 persons per household (a formula used in the 2012 Study) would give 434 for daytime and 796 at night. How many dead could there be? – Perhaps more than 10. There were no damage or fatality comments in the 2004 Westwood Lake Inundation Study.
There are also businesses (Quarterway pub, a liquor store, a fitness centre, farms with barns, crops, cattle, horses, equipment. With a dam breach, a surge might take out the intersection of Jingle Pot Road and. Westwood Road, the Parkway over the Millstone River, as well as Quarterway Bridge, How many millions of dollars in damage? And more homes are being built or planned below the dam.
The 2004 report suggests that a flood would be dissipated into Buttertubs Marsh and slowed due to the spillway (if remains unblocked) and McNeil Creek. However a dam breach would suggest a surge down Darrough Creek, possibly excavating the sides of the valley and damaging properties backing on to it. It would reach the Millstone River within a few minutes but slowed thereafter. In spite of the flat area below (Buttertubs March etc.) the water still wants to complete its descent to the sea along the Millstone River. Once the water got low enough to avoid the spillway it would all go through the dam break. Besides, whichever way it goes, it still ends up in the Millstone River. For a flood event, rain will be coming down locally and into the Millstone River from Brannen Lake and the rest of its watershed, so the flooding may be even greater.
Hazards (from the City of Nanaimo website) quotes “For example, during the early summer of 2008, the city seismically stabilized Westwood Lake Dam — a 100 year old structure. Should a large magnitude earthquake occur, the dam is expected to slump, but the upgrade would slow the release of water to a manageable level”.
Bill Sims, City of Nanaimo Water Manager (6th December 2012, Colliery Dams meeting), “Westwood. Lake is a different situation, the downstream consequence is High since there is less potential for loss of life”. “In 2003, Westwood Lake was the highest risk dam and a higher priority than Colliery Dam.”
The 2012 Inundation. Study for Chase River included 100-year and 1000-year return period flood events as well as PMF and dam breaches. The Westwood study did not include the former two items.
The 2004 Westwood Dam study used only MIKE 11, a 1-dimentionial river modeling program. The 2012 Chase River study used MIKE 11, as well as MIKE URBAN (also l dimensional and MIKE 21 (a 2-dimentional programs). Article 4.1 states that use of 1 dimentional programs for hydrologic analysis of river channels is limited to flows below the top of the bank. These models are unable to account for lateral outflows and alternate flow paths above the top of the cross section. Clearly, a different dimension was added.
Westwood Lake was given a partial fix for about $500,000 in 2008. The “Middle & Lower Chase River Dams Spillway Hydrology Study of 2002 by Water Management Consultants”, offered a $1,000,000 emergency spillway through rock from the Middle Dam, by-passing the Lower Dam. One wonders how much that translates into today. Certainly less than $7,000,000.
It is apparent from the proceeding statements that two different standards are being applied to the Westwood and Colliery Dams. Partial fixes aren’t allowed at the Colliery Dams, only definitive ones. Isn’t it time that the residents and businesses that live in the shadow of Westwood Dam be offered the same degree of safety as is being imposed on Harewood residents and businesses. There should be a new inundation study for Westwood using more advanced tools. Why should their lives remain at such high risk?
South Forks Dam, built 1931, height 25,6m, width 50.6m, volume 2,000,000 cubic metres. Consequence Classification ‘Very High’ (between 10 and 100 people might die). South Forks Dam is half the width, contains slightly less water than Westwood Dam and would flood into a rural – semi wilderness river valley. Westwood Dam would flood right into the city, yet has a lower Consequence Classification. What incongruences!