AWARD OF EXCELLENCE: East Arrowwood Syphon Replacement, Alberta

Category: Water ResourcesUMA ENGINEERING

The East Arrowwood Syphon is an essential link in the Carseland-Bow River Headworks main canal, serving to convey water flow across the valley of East Arrowwood Creek, 31 kilometres downstream of Carseland, Alberta. The system provides water to 230,000 acres of irrigated farmland in the Bow River Irrigation District and the Siksika Nation, and supports sport and commercial fisheries on Lake McGregor and Travers Reservoirs.

By the 1990s the original wood-stave syphon had badly deteriorated. It was leaking and the flow capacity was insufficient to handle the projected demands from downstream, or the inflow from floods upstream. As well, creosote-treated wooden staves were contaminating adjacent land. In 1996 the Alberta government hired UMA Engineering’s Lethbridge office to provide design and construction management services for replacing the old syphon.

Replacement presented major challenges in both design and construction. These included:

steep, high, unstable slopes leading to the creek valley;

a high water table in the creek valley and heavy groundwater seepage on the west slope

shallow bedrock in the creek valley

risk of flood damage in the creek valley

environmental issues regarding fish passage, animal migration and preservation of the habitat

a requirement to maintain water flow in the canal during the irrigation season (the old structure and canal could not be disturbed during construction)

severely restricted access and area available for construction operations in the valley.

The designers examined several conveyance options and routes across the valley. After considering all factors and using value added and life-cycle analysis, they decided that the most cost effective solution was a single-barrel, reinforced concrete inverted syphon, buried 7 metres below the creek bed, and located 150 metres south of the existing syphon. One reason for this choice was the opportunity to reuse steel formwork from a previous syphon replacement project.

The replacement structure is a 437-metre long, 5 metres outer diameter, cast-in-place concrete barrel with 500 mm thick walls. It has a design flow capacity of 51-m3/s, a peak flood capacity of 60 m3/s, and a maximum operating head of 51 metres. The project involved building the syphon structure, 1,500 metres of tie-in canals, a major check drop structure and 3,800 metres of access roads.

The syphon is close to the Foothills seismic zone, so concerns for the structural stability of the barrel and the need for safe access to enable its inspection required cutting back the 45-metre high valley slopes from 30 to 35 degree grades, to 14 to 16 degree grades. This work involved excavating 132,000 cubic metres of material, including 16,000 cubic metres of bedrock. Much of the excavated material was used to build access road embankments.

The check drop structure accommodates a 1.8-metre drop upstream of the syphon inlet. The drop creates the head differential for the syphon to operate effectively. The check drop is equipped with automatically activated overshot gates that measure the flow and control the water level upstream. The gate system includes “intelligent” actuators that respond to commands from, and transmit gate level information to, a programmable logic controller via a communications protocol. All communication is delivered through one cable.

The tie-in canals include 1,100 metres of armoured canal at the inlet and 450 metres at the outlet. An upstream hairpin curve of the old canal had significant seepage and stability problems. One of the value-added features of the project was the use of 158,000 cubic metres of excavated materials from the inlet canal construction to cut off this troublesome reach. The strategy reduced the material handling costs, increased the hydraulic efficiency and reduced the long-term operation and maintenance requirements of the canal. Twelve hectares around the old canal are being reclaimed for wildlife habitat.

The project had to be executed within the time constraints of the irrigation and winter seasons. Almost all of the slope stabilization, inlet canal and syphon earthworks were completed within the first three months. The syphon barrels were built using the salvaged sliding forms. The crossing construction was done in three stages to maintain flow downstream. Operations included rescuing fish, mitigating siltation, and preserving sensitive riparian habitats.

UMA and Alberta Infrastructure worked closely to resolve protracted issues over land acquisition, water use and labour, and addressed the various concerns of the parties affected by the project. One of these, the Siksika First Nation, has territory bordering the north bank of the main canal and provided much of the labour and gravel armour requirements.

The syphon was commissioned to operate on schedule in May 2000. The final construction contract cost within 1% of the engineer’s estimate, and the total project cost, including ongoing site reclamation, was $12 million.CCE

Project name: East Arrowwood Syphon Replacement Project

Client: Alberta Infrastructure/Alberta Environment

Award-winning firm: UMA Engineering, Lethbridge, Alta.

Project team leaders: Dale Miller, P.Eng., Rod Bower, P.Eng., Dennis Miller, Nick Hernadi, P.Eng., Paul Kemp, P.Eng., Irv Martens, P.Eng., Dave Chalcroft, P.Eng., Steve Eberhardt, P.Eng., Roberto Rogue, P.Eng.

Other key players: Kiewit Management (prime contractor), Thurber Engineering (geotechnical)