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Soft Engineering of Shorelines Based on a Binational Conference Sponsored by the Greater Detroit American Heritage River Initiative and Partners |
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Chapter 12 Soil Bioengineering for Streambank Protection and Fish Habitat Enhancement, Collingwood, Ontario (Rick Grillmayer, Nottawasaga Valley Conservation Authority) Introduction The goal of this project was to repair and stabilize an eroding streambank with the use of vegetation, to create in-stream cover by constructing a vegetated structure, and to create a demonstration site that displays the effective use of soil bioengineering technology. Project Description The Black Ash Creek Project was initiated in 1992
as a component of the Collingwood Harbour Remedial Action Plan. The overall
objectives of this watershed project were to reduce sediment loading from
the creek into the harbor and enhance fish and wildlife habitat. Black
Ash Creek was identified as contributing approximately 90% of the suspended
sediment load for Collingwood Harbour (Collingwood Harbour RAP Stage 2
Document 1992). Sources of this sediment include erosion induced by cattle
grazing on steep escarpment areas and eroding streambanks. The location of this project is the Thompson Property
on the 10th concession, Town of Collingwood. The reduction of channel
sinuosity and elimination of a functioning floodplain had created an unstable
reach of stream with significant erosion. The channel had been placed
in a roadside ditch and the shoulder of the road was eroding. A previous
attempt to stabilize this channel with field stone had failed because
the improperly sized and placed stone was being eroded by high stream
velocities. The stream gradient was steep (3.1%) and once the bed armor
was missing the streambed degraded, aggravating the eroding bank. The
bank slope on both sides was nearly vertical. The stream is intermittent,
with flows occurring only during snowmelt and storm events. The catchment
area upstream of the project site is approximately 10 km2. Bank stabilization and streambed armoring took place
during the fall of 1993. The confined nature of the channel prevented
excavating a flood plain or sloping the banks to a stable angle. The east
side of the channel was privately owned and the owner was not open to
any loss of property that would occur if regrading was used. It was decided
to construct a bioengineered cribwall. This structure would stabilize
a vertical bank and require little room. The streambed was armored to
prevent down cutting. No attempt was made at this time to stabilize the
road shoulder directly opposite the project site, however, the Town of
Collingwood did attempt to stabilize the road shoulder by constructing
a concrete wall during the early summer of 1995. By the late fall of 1999,
the concrete wall was beginning to show signs of failure. The soil bioengineered
cribwall was holding well. A soil bioengineered cribwall is a hollow, interlocking
arrangement of timbers constructed as a wall. This structure is filled
with suitable soil and a layer of live branch cuttings. Once the cuttings
have taken root and grown, they will eventually take over the structural
functions of the timbers. The end result is a stable, vegetated slope.
The cribwall was built into the bank so the face of
the cribwall would be at the same location as the original face of the
slope. This was done so the capacity of the channel would not be reduced.
A hi-hoe was used to excavate the cribwall site. The logs for the cribwall
were cut from a Nottawasaga Valley Conservation Authority jack pine plantation.
The wall itself was built by hand and measured 30 m long, 1 m high, and
2.2 m wide at the bottom. The wall was canted back so that the top brush
layers would not shade the bottom ones. Shrub willow cuttings were harvested from sites within the watershed and transported to the cribwall site. Care was taken to time the harvest so that only fresh material would be used. Species of willow used at this site were:
The cribwall was built by alternating layers of timbers,
soil, and cuttings. Once the cribwall was completed, unused soil was removed
from the site. Exposed soil was seeded with annual rye and oats. The soil
was then covered with anti-wash geojute to prevent surface erosion. Live
stakes (live rootable cuttings tamped into the ground) were placed at
random into the geojute. The streambed was protected from down-cutting
by placing 28 tons of rip-rap stone. Due to the absence of any horizontal sinuosity, stream energy had to be dissipated by vertical sinuosity. This was achieved by placing the stone in a series of steps, attempting to establish a step-pool formation common to high gradient streams. Regulatory Considerations Whenever a project may impact the natural ecosystem,
approvals and permits are needed. The project was approved by the local
Ministry of Natural Resources (Midhurst District) and a work permit was
issued under the Lakes and Rivers Improvement Act. A permit from the Nottawasaga
Valley Conservation Authority was required under the Fill, Construction,
and Alterations to Waterways Regulation. The stream at this site is intermittent. The fish community is predominantly cyprinids and catostomids, and is nonexistent through summer, fall, and winter. Fish are present, likely as migrants, during the spring. The cribwall was built during the fall, while the channel was dry. Sedimentation during construction was minimal. The addition of bed material would not have affected any fish or macroinvertebrates. Materials used were native and harvested from within the same sub-watershed as the cribwall. Cost Project costs are presented in Table 8. Costs do not include; tools, truck rental, fuel, office costs, indirect support for the crew, or permit fees. It would be impossible to separate these costs as they were used/required on more than one site.
Environment Canada Great Lakes 2000 Cleanup Fund;
Post Project Evaluation of Effectiveness 1995: Two years after completion, the streambank at
this site was completely vegetated (Figure 39). Any erosion was insignificant.
The soil bioengineered cribwall successfully weathered the spring flows,
which often saw the structure completely submerged. Growth from the cuttings
has been vigorous, with Salix eriocephala becoming the dominant willow. 2000: Seven years after completion, the streambank at this site is still doing very well. The concrete block wall built by the Town of Collingwood is beginning to show signs of failure (Figure 40). This failure is apparent in the form of undercutting, the slumping of several sections of wall, and widespread cracking and deterioration of the concrete. The soil bioengineered cribwall is more than adequately maintaining stability (Figure 41).
This project was an overwhelming success. Benefits of the Project Soil bioengineering was chosen as the preferred method of streambank stabilization for several reasons:
References Collingwood Harbour RAP Team. 1992. Collingwood Harbour Remedial Action Plan Stage 2 Report: a Strategy for Restoring the Collingwood Harbour Ecosystem and Delisting Collingwood Harbour as an Area of Concern (In consultation with the Public Advisory Committee). Dobbs, F., and Grillmayer, R. 1994. Black Ash Creek Rehabilitation Project Implementation Report. Collingwood Harbour Remedial Action Plan. ISBN 0-7778-2696-8. Ontario Ministry of the Environment, Toronto, Ontario, Canada. Sotir, R., and D.H. Gray. 1989. Fill Slope Repair Using Soil Bioengineering Systems. IN: Public Works, December 1989.
Contact Person Rick Grillmayer |
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Last updated:
July 9, 2008
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