Improving Pointe du Bois20 August 2012
Early works are underway for spillway replacement at Pointe du Bois dam, in Manitoba, reports Patrick Reynolds
Initial site works began early this year for a major project to replace the Pointe du Bois spillway in Manitoba to improve safety both at the structure and for the public as well as provide safer working conditions for Manitoba Hydro staff.
The oldest hydropower plant in the province, the 78MW (nameplate) dam site is situated on the Winnipeg River in eastern Manitoba. Improvement works include: construction of a new primary spillway; a new secondary spillway; new concrete and earthfill dams; stability measures for the existing gravity dam structures; and, decommissioning of the existing spillway.
The new spillways are to be operational by 2015 and 2016, respectively.
Constructed by Winnipeg Power, the Pointe du Bois plant is 150km north east of Winnipeg and began operations just over a century ago.
The unit initially had three turbines installed and more were added to take the total to 16 over 1914-1926. The head at the run-of-river site is 14m, discharge capacity at full gate is 712m3/sec, and, in total, the plant has 15 double horizontal shaft Francis units and a Straflo turbine. One of the turbines, Unit 11, was re-runnered and upgraded in 1995, and the Straflo replaced Unit 1. The changes increased the plant capacity by 6.5% from nameplate to 83MW.
The facility was acquired by Manitoba Hydro, in 2002, as part of its purchase of Winnipeg Hydro. The station generates approximately 600GWh of electricity per year.
Key infrastructure at the complex is arranged in an angular S-shape on the river, and in addition to the powerhouse includes east and west gravity dams, 92 spillway bays grouped in sections, a five-bay gated sluiceway, a rockfill dam and access bridges. The base of the S-shape is held by the powerhouse while the rest to the S form, hugging the head of the rapids, comprises the spillway bays and the sluiceway.
The drainage area of the catchment at Point du Bois is approximately 126,000km2, and according to the Environmental Impact Statement (EIS) maximum safe flood capacity is 2850m3/sec, or the 1 in 90 year event. The new spillway arrangements will have total discharge capacity of 4800m3/sec at full supply level (FSL) and 5430m3/sec at inflow design flood (IDF) level.
During the first 40 years of operation the scheme’s infrastructure suffered deterioration mainly due to durability and performance of early concrete mixes. Improveent options were studied in the 1950s, 1970s and more recently. Extensive repairs and upgrades of the infrastructure have been undertaken at different times, such as anchoring at the gravity dams in the 1980s and later.
Concrete gravity blocks were placed downstream of the east dam in 2002, and in the same period Geo-Foundations Contractors installed anchors at a forebay wall and spillway bays 121-133. Previously, in 2000, the firm had installed anchors and also placed mass concrete at the the sub-apron chambers of the powerhouse structure.
But the bigger challenge is that Pointe du Bois doesn’t meet current Canadian Dam Association guidelines for passing extreme flood events, and a significant upgrade is needed for safety reasons.
While options for the upgrade began to be investigated, in 2006, about four years after Manitoba Hydro acquired the asset, a dam safety operations support group was formed to manage risks and increase the reliability of existing spill in the interim period.
A failure modes and effects analysis was performed in 2007, and projects completed under the advisement of the support group included new gates and hoists for some spillway bays, a new steaming system to improve reliability in winter gate operations, and development of a spill operation sequence to improve efficiency of spill mobilisation.
A number of options were considered to improve the infrastructure at the facility. Studies performed after 2007 established the Rebuild Alternative, which included construction of a new 120MW powerhouse and dams as well as the spillway, and other Renovate, Repair and Decommission options.
However, by 2009 the Rebuild plan were diluted into a phased strategy, focused first on the spillway improvement only, due to both rising construction costs as well as wider economic weakness in the global markets.
A modernisation alternative for the existing spillway was considered before the replacement solution was elected. In terms of replacement, two alternatives were assessed – a central or east side spillway, the latter being elected.
The new, east-side option is to construct a five-bay, 86m wide ogee type spillway and chute at the top of the S-shape, at the opposite side of the rapids from the powerhouse. Discharge capacity of the primary spillway is 3670m3/sec at FSL, or 4030m3/sec for the IDF level which is 0.8m higher.
The design solution at Pointe du Bois also calls for a secondary new structure, and the existing spillways are to be decommissioned with new dams built in front and running parallel to much of the original S-shape layout.
Key factors considered for the layout of the spillway design included: fisheries protection; discharge capacity of the system being unaffected during construction to maintaining dam safety; limiting changes to water levels and flow; cost; constructability; provision for possible future construction of a replacement powerhouse.
According to the EIS, the configuration was significantly influenced by the need to maintain a healthy lake sturgeon spawning habitat downstream of the spillway shelf. Location and orientation are similar to the existing spillway, and the discharge elevation is above the normla tailrace level for the power plant, which preservices the plunging charactersistics while also avoiding cofferdam construction in the fish habitat.
The secondary spillway, located more centrally on the river, will offer extra spill capacity of 1130m3/sec at FSL and 1400m3/sec at IDF level. A seven-bay, 84m long ogee type overflow structure, the spillway could also help improve fisheries during spawning seasons should the primary structure not do so sufficienctly well.
Environmental assessment consultation and communication for the project development work included contact with Aboriginal groups, local residents, government departments and agencies, municipalities, the public, and other stakeholders
As seen in late 2010, schedule as antiicpated in the EIS, in early 2011, was for a late 2011 start to construction, the primary spillway becoming operational by late 2014, the secondary spillway by late 2015 and completion of the scheme by mid-2016.
With regulatory approval, construction commenced at the beginning of 2012 and the budget total is Can$400M (US$390M). The revised completion schedule for the primary and secondary spillways are March 2015 and October 2016, respectively. Completion of the entire scheme is expected in the third quarter of 2017.
alstom was recently awarded a contract to design and supply gates and hoists for both new spillways. The company said that commissioning of the equipment for the primary and secondary spillways is scheduled for November 2014 and April 2016, respectively.