South Fork Dam


Description: An aerial wintertime view of the concrete arch South Fork Dam - the intake and head pond for the City of Nanaimo's drinking water since 1931.

Description: Une vue aérienne hivernale de l'arche en béton du barrage South Fork - la prise d'eau et le bassin de tête de l'eau potable de la ville de Nanaimo depuis 1931.

Submitted By: Bill Sims

City: Nanaimo

Region: British Columbia

Benefits to society [Translate]

The dam, which is very remote from Nanaimo - especially when it was built - has provided gravity flow to the City of Nanaimo for 90 years. South Fork Dam was built at the precise elevation to ensure that the City did not need to use pump stations. Even today, the hydraulics afforded by the dam only require less than 20% of the City's area be fed by booster pumps. The saving in electricity and maintenance cost for this benefit alone are astronomical.

Economic benefits [Translate]

Planning for the dam had been ongoing since the early 1900's, when the existing water system serving Nanaimo's 8300 residents was found to be maximized, leading to intermittent flow during summer months. In the 1920's, a number of "aldermanic engineers" proposed that the dam be built, but this was not well received by the Council of the day. Finally, in January 1930 "a forceful and unanimous Council came into office and determined to improve the water supply and alleviate the seasonal shortage."

Thus began the saga of the South Fork Dam - and the beginning of economic growth and healthy prosperity for Nanaimo.

The dam (along with a newer storage reservoir further upstream) have permitted growth and stability for Nanaimo for over 90 years. Nanaimo's population growth, now over 100,000, is one of the top five growth rates in Canada. None of this would have occurred without the dam.

Technical merits [Translate]

The dam is a variable radius concrete arch dam, built in a rock canyon, to replace an existing lower dam and diesel pump. It is founded in a rock canyon consisting almost entirely of gneissic andesite from the Triassic, making it "hard and strong".
The dimensions of the dam, highlighting the geometry are:
Height of dam (invert of river to crest): 84 feet (25.6m);
Width of crest between abutments: 166 feet (50.6 m)
Thickness at base: 10.5 feet (3.2 m)
Thickness just below crest: 3 feet (0.9 m)
Thickness at crest: 5 feet (1.5 m)
Wall lean downstream: 31.5 feet (9.5 m) - yes the top is over 30 feet further downstream than the base.
Radius at invert of canyon: 65 feet (19.8 m)
Radius at crest: 97.5 feet (29.7 m)

The technological advances - the main feature of the dam is the grouting pocket, patented as "Vogt Patent Process of Differential Pressure Grouting." The process allows for "the equalization of arch stresses caused by correcting for distortion of water loads; cantilever stresses are equalized and tension eliminated in the cantilever elements; it compensates for shrinkage and compensates for the lame effect in thick arches." A number of unique features of the dam - especially given the era in which it was built include: No reinforcing steel, yet designed such that it is very stable; The crest is the spillway - no auxiliary spillway required and designed to allow floating debris to flow over.

Historical background [Translate]

South Fork Dam was constructed for about $115,000 between 1930 and 1931, being ceremoniously handed over to Nanaimo Council on October 14 of that year.

The contractor mobilized on August 2, 1930 and faced many challenges, including forest fire destroying access bridges and major floods in the fall, which washed away bypass flumes and coffer dams. In December, the contractor was in financial difficulty, but was able to arrange financing to finish the work in 1931.

One hundred and fifty men lived in built log cabins on the site, given its remoteness. In this writer' early career, the remnants of the cabins were reasonably intact, but have now been reclaimed by the forest.

Aggregate for the concrete was hauled by inclined rail from the bed of the river, washed and stockpiled above the site. Bagged cement and the aggregate were mixed just below the aggregate bunkers and transferred by a tracked truck to the dam site.

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