Storm Water Outfall Rehabilitation

OUTFALL 13

Location: Lower reach of Wolf Willow Creek Ravine, which flows into the North Saskatchewan River in Edmonton, Alberta

Client: City of Edmonton

Objective: To protect the infrastructure of Outfall 13 by stabilizing the banks of Wolf Willow Creek Ravine using a riprap application. 

In the summer of 2008 the City of Edmonton contacted Terra Erosion Control Ltd (TEC) regarding creek bank erosion problems at storm water Outfall # 13 (City of Edmonton Infrastructure), which discharges into Wolf Willow Creek Ravine and then into the North Saskatchewan River. The creek bank erosion (in the form of shallow slope failures) appeared to have been caused by fluctuating water levels over the years.  During spring flood periods, water from the North Saskatchewan River backs up into Wolf Willow Creek Ravine. 

Based on a rehabilitation plan developed by Associated Engineering Alberta Ltd the City of Edmonton Asset Management and Public Works Department sent out a Request for Proposals to implement the rehabilitation of storm water Outfall # 13.  This plan consisted of excavation, re-grading and placement of riprap class 1 & 2 over geotextile fabric.  The plan also included the construction of a narrow access path of approximately 400 m, within a sensitive riparian area, to Wolf Willow Creek Ravine.  The construction also included the building of a temporary access ramp in order to reach the channel bed.  Additional placement of root wads within the riprap at the channel level was suggested by TEC in order to enhance fish habitat.

The proposal underwent the competitive bidding process and was successfully won by Terra Erosion Control Ltd in conjunction with Kichton Contracting 1963 Ltd.  Kichton provided the rocks, cobbles and various materials for the project.

The work was implemented in November and December of 2008.  The access path layout was revised and carefully constructed using a walking excavator (Spyder Hoe) to minimize cut height and soil disturbances.  Excess fill material was transported out of the ravine and rocks were transported in using two Komatsu tracked dump trucks.  Re-contouring and grading of the creek bank around the outfall, followed by geotextile and rock placement, was done using the Spyder Hoe.  All machinery involved in the construction and transportation of materials for the project used Panolin HLP Synth hydraulic oil, allowing the machinery to work in stream and within sensitive riparian areas. During construction, three root wads were set into the riprap along the creek channel to provide fish habitat during high water.

The work area within Wolf Willow Creek Ravine had to be de-watered prior to commencing work.  The dewatering was accomplished using two coffer dams.  These were constructed with sandbags filled with clay material in conjunction with 4” plastic pipes placed within the lower portion of the dam to drain the water into a settling pond located below the work area.  Water was also pumped into the settling pond using a gas powered pump set-up on a platform lined with petroleum absorbent pads.  Sediment barriers made of a combination of straw bales and silt fencing were also constructed down stream from the settling pond, across the creek channel, to trap and reduce sediment delivery to the North Saskatchewan River.