Debris Flow Defensive Structures at Howe Sound

British Columbia, Canada

Howe Sound is a glacially carved fiord which extends from Horseshoe Bay (20 km northwest of Vancouver) to Squamish.  This region is particularly prone to debris flows because of the steep glacial-valley slopes.  The problem is exacerbated by a loose mantle of glacial sediments on those slopes, and by periodic heavy rains.  Human activities, including tree-harvesting on some of the steep slopes and the construction of a railroad and a major highway have added to the risk of slope failure.  Because of the steepness of the slopes, most of the small communities in this area have been built on the fans created by former debris flows - putting them in the direct path of future debris flows.

The steepness of the slopes of the Howe Sound area  can be seen from the topographic map.  Stream-bed slopes in this area are commonly in the 20 to 40°range, and are as high as 57° in some cases (Lister et al., 1984).  Slopes outside of stream beds tend to be even steeper.

The sequence of events that can lead to debris flows at Howe Sound is summarized on the diagram below.

To begin with heavy rains on slopes that are already covered with snow produce very large stream flows in the upper-slope areas.  The high flows create instability on the stream banks, causing loose overburden, snow and ice and even trees to slump into the flowing water.  Where the debris flood emerges from the incised stream valley onto the pre-existing fan area it spreads out, causing damage to buildings, bridges, roads and the railway.

Since 1921 there have been at least 48 deaths and millions of dollars of damage to private property and public infrastructure, related to debris flows in the area between Horseshoe Bay and Squamish (Evans and Savigny, 1994).

During the past two decades major efforts have been undertaken to reduce the potential for damage, injury and loss of life on this important transportation corridor.  Some examples of these are shown on the photographs below.  The photographs were taken in the areas of Harvey and Alberta Creeks at Lions Bay, and Charles Creek at Strachan Creek (as shown on the topographic map.)  

Charles Creek and the village of Strachan Creek. The upper parts of the drainage system is characterized by extreme slopes and sparse vegetation.  The houses are built on an alluvial fan from Charles Creek.  The British Columbia Railway (below) and the Sea to Sky Highway (99) are visible. The debris flow retention structure is situated immediately above the highway.

Overburden (alluvial and possibly glacial) exposed in the bed of a stream draining into Howe Sound.

Construction of a concrete culvert on one of the Howe Sound creeks.  (Location unknown)

Aerial view into the basin behind the retention structure at Charles Creek. Sediment and organic material from a debris flow will collect in this basin.  The basin can be cleared out after a major debris flow.

View from within the retention structure at Charles Creek.

Aerial view of the town of Lions Bay. The settlement has been built on the alluvial fans from Alberta Creek (left) and Harvey Creek (right).  The British Columbia Railway is close to the water's edge.  Highway 99 passes through the centre of the town.

Aerial view (looking north) of the 1983 debris flow on Alberta Creek in Lion's bay. Note damage to roads, the railway and to buildings.

Aerial view (looking south) of the 1983 debris flow on Alberta Creek.

Aerial view of Alberta Creek at Lions Bay with the concrete flume under construction.

Concrete flume under construction on  Alberta Creek in the upper part of Lions Bay. The flume is designed to allow water, mud and debris to pass unimpeded during major flooding events.

Concrete flume on Alberta Creek with road and highway crossings.

Completed concrete flume on Alberta Creek.

Concrete flume on Alberta Creek, and BC Railway crossing in the lower part of Lions Bay

Harvey Creek retention structure at Lions Bay.  Highway 99 is in the central part of the photograph.

Sediments from previous debris flows exposed during construction at Harvey Creek.

Retention structure at Harvey Creek.  The streambed has been cleared, straightened and deepened both above and below the structure. The Highway 99 crossing is at the bottom.

The bed of Harvey Creek below the retention structure.  Boulders have been set into the stream bed with concrete.

View to the west over Howe Sound across the upper part of the Harvey Creek retention structure. The large concrete riffles are designed to retain large debris (including boulders and logs) allowing water to pass through.

Construction of over-height bridges at Harvey Creek.  (note old debris-flow sediments exposed on the right)

Aerial view of the completed Harvey Creek retention structure.

John Price (right), principal of Ker Priestman Associates, and engineer of the debris flow retention structures and stream channelization installations on Charles, Alberta and Harvey creeks and nearby Magnesia Creek. The Association of Consulting Engineers of Canada awarded Ker Preistman Associates and Thurber Consulting  the Canadian Consulting Engineering Award for 1986 for this work.    The photos were provided by Lee Price.

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