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Dewatering of Sensitive Riparian Area at Wolf Willow Ravine Work Site / Storm Water Outfall 13 City of Edmonton

Storm water Outfall Rehabilitation


Location: Storm water Outfall 13 at the mouth of Wolf Willow Creek Ravine, flowing into the North Saskatchewan River, City of Edmonton, Alberta

Client: City of Edmonton

Objective:  Dewatering of work site within sensitive riparian areas using coffer dams and sediment settling pond system.


In the fall of 2008 Terra Erosion Control, working in conjunction with Kitchton Contracting 1963 Ltd, armoured the banks of Wolf Willow Creek Ravine surrounding Outfall 13 using a riprap application.

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Storm Water Outfall Rehabilitation Outfall 13

In order to comply with the Department of Fisheries and Ocean’s (DFO) regulations prohibiting activities resulting in sediment laden water entering fish habitat, the work areas upstream and downstream of Outfall 13 needed to be dewatered.

The dewatering of Outfall 13 encompassed the following measures:

  • Coffer dams using nylon bags filled with clay material were built across the creek channel upstream of the work area and at the discharge of Outfall 13. 
  • A sediment settling pond lined with geotextile was located below the work area.
  • A sediment barrier was constructed using a combination of straw bales and silt fencing and located across the channel at the mouth.  


Initially, the water pooling below the outfall outlet was pumped directly into the sediment settling pond prior to proceeding with the rehabilitation work.  The water held behind these coffer dams was then drained by gravity through 4” plastic pipes into the sediment settling pond.  While excavation and riprap placement took place within the channel upstream from the outfall outlet, water was pumped from the upstream coffer dam into the coffer dam located within the outfall structure. 

The plastic pipes used for drainage were passed through a culvert, located within the access ramp (built to provide access to the channel).  The water discharged from the sediment settling pond was then filtered through a sediment barrier before entering the North Saskatchewan River.   All structures were removed after project completion.

This system has proven very effective at dewatering the work area and reducing sediment delivery into the North Saskatchewan River.  Although, as the work was carried out late in the year, problems associated with freezing of water in the plastic pipes was encountered toward the end of the project.

Wolf Willow Creek Ravine channel, October 2007.  Note pooling water below outfall outlet.


Creek channel downstream of outfall,
October 2007.


Coffer dam across channel, note plastic
drainage pipe, November 2008.

Drainage of accumulated water within
coffer dam across channel.

Coffer dam and drainage pipes
at outfall 13 outlet.

Coffer dam at outfall 13 outlet,
note water pooling below.

Pooled water pumped into sediment settling pond prior to implementation of project.

Dewatered channel, note plastic pipes
draining water to sediment settling pond.

Sediment settling pond.

Sediment settling pond discharge.

Pumping water out of upstream coffer dam into outfall coffer dam while excavation and riprap placement is taking place within the channel.

Plastic drainage pipes passing through
culvert under equipment access ramp.

Sediment settling pond downstream of
access ramp, November 2008.

Sediment barriers at mouth of Willow Creek Ravine filtering discharged water
from sediment settling pond into North Saskatchewan River.



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Sediment Control ~ Dewatering of Stormwater Outfall