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

OUTFALL 13

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 a 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 (see main project: City of Edmonton-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 out 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.  As the work was carried out late in the year, problems associated with water freezing in the plastic pipes was encountered toward the end of the project.