Habitat Improvements
Cabled Log Jam
Description
Just like the name implies,
the cabled log jam resembles a natural log jam. It is well anchored to the bed
of the river and does not disrupt the flow of water when constructed properly.
It is built as a skeleton of logs secured with aircraft cable and steel posts.
The internal pockets are filled with woody debris and lashed or nailed into
place. These structures are best suited for the outside bends of the river or
along the banks of deep runs. Each year they will continue to collect more woody
debris as it washes in from upstream under higher flows.
Purpose
The structure is designed
to provide overhead cover for aquatic creatures in the diverse form typical
of large woody debris. Log jams tend to attract turtles, frogs and mink which
use them for basking in the sun or foraging for food. It is an excellent habitat
improvement technique that ties in well with the application of woody debris
management. The surface and exposed river edge is rough providing numerous nooks,
crannies and back eddies. The form follows the lines of the river current without
impeding flow.
Cabled log jams can also
reduce erosion at the toe of an eroding bank. The structure, depending on the
size of river and degree of anchoring, can usually withstand spring floods and
ice movement. In behind the log jam, the eroding bank can be re-graded to a
stable slope and planted with native grasses and willow shrubs.
Application
Cabled log jams work well
as a means of re-positioning material salvaged from woody debris obstructions
in the channel. The main consideration is building the structure without obstructing
the bankfull channel’s capacity to move water, sediment and debris. The
alignment of the log jam should follow the curvature of the meander bend.
A knowledge of channel
slope, entrenchment, sinuosity and substrate will help you select the A, B,
and C channel forms which accommodate cabled log jams. In general, these streams
and rivers are known for their ability to sustain large woody debris based on
the stable nature of the channel and limited sediment supply. Ideal placement
is in pools on the outside of the bend or along the bank of a deep run. Caution
should be exercised in gravel, sand and silt based C type streams where bank
placed large woody debris can aggravate erosion on the opposite bank.
The channel width provides
the direction for determining the size of woody material which can be considered
stable in the stream and the manner in which it can be positioned in relation
to the flow. Refer to the previous section on woody debris management. Largest
diameter (greater than 30 cm) and longest logs are to be placed on the river
edge to absorb the erosive energy of the current, deflect debris and ice. These
logs are positioned in an arc or straight line parallel with the bank. Similar
sized logs are also used for imbedded cross braces into the banks. Cross braces
are positioned into the bankfull current in order to absorb the impact of higher
flows against the log face.
Small streams less than
7 metres in width can support a larger amount of woody debris with a variety
of diameters and lengths. Logs less than 30 cm in diameter can be used in the
construction of the skeleton structure. Brush, branches and short logs can be
used to fill the log jam. Streams and rivers, greater than 7 metres in width,
typically transport debris smaller than 10 metres in length to areas of accumulation
such as log jams. Keep this in mind when you are building a log jam on this
size of river. You will need larger sized debris to fill it.
Construction Guidelines
This is a summer construction
project giving workers a chance to cool off in the river when a sweat breaks
out. It is also the best time to see the river at low flow stage which makes
the construction much easier. The choice sites for building log jams are on
the outside bend of a pool or along the bank of a straight run. Some bank erosion
may be evident. Look for depths of 0.3 to 1.2 metres as ideal sites. They can
also be constructed at the confluence with an old river channel (oxbow). Watch
for existing debris accumulation, anchored logs or stumps in pools and runs
and consider this as fixed material which can be a part of the overall skeleton
structure. The length, width and height is dependent on site conditions and
the size of the river. Typically they are from 5 to 30 metres in length.
The width of the log jam
should draw to the edge of the current/eddy interface without extending into
water greater than 1.2 metres in depth. The deeper the water, the more difficult
it is to anchor the logs. By no means should the structure stretch out more
than 25% of the low flow channel width.
The height of the log jam
should not impede higher flows. The front edge should be less than 25 cm out
of the water and gradually taper up to the bankfull height on the slope. Branches
and limbs should not extend beyond the front of the log jam in order to reduce
the risk of snagging debris in drift.
The backbone of the structure
is the imbedded braces and the reinforced front edge. Several 1.5 to 2.0 metre
long trenches into the bank are required for installing “deadmen”
braces. The trenches should be aligned such that they face into the bankfull
current and are spaced 2.5 metres apart. The deadmen are logs at least 4 to
5 metres long and 30 cm in diameter which are cabled and spiked into placed
with “T” bar posts. These logs should extend out to the front face
of the structure since they form a vital component of the overall anchoring
system. Log cross members can be used to secure braces together which provides
added stability.
The front edge of the log
jam is built from the largest and longest logs which are anchored into the river
bed with “T” bar posts and aircraft cable. Each end of the front edge
should gradually taper into the shoreline with a good portion of the log imbedded
into the bank and secured. These logs are also anchored by cable and spikes
into the deadmen which protrude from the bank. The upstream end of the log jam
should be reinforced with river stone where it meets the shoreline. This helps
reduce the risk of structural failure.
With the skeleton in place,
the next step is to fill the internal open pockets with woody debris. With large
log jams, use the largest debris possible and make sure everything is either
nailed or cabled into place. Smaller streams are less likely to be affected
by flooding and ice, so you can get away with using smaller logs and brush to
fill the spaces. Old Christmas trees work great.
If you are working beside
an eroding bank, this is a great technique to stabilize the slope. Once the
log jam is in place, you can regrade the slope and try some bioengineering later
in the fall. In the meantime, plant it with native grasses to encourage quick
soil stability.
Materials
You will need the following materials and equipment for constructing a cabled log jam:
Cost and Maintenance
Needs
Depending on the size of
the watercourse and the availability of woody debris, cabled log jams can be
constructed in a matter of a few hours to a couple of days. A crew of six can
build a twenty metre long log jam in about one day. The main cost associated
with the construction, besides labour, is the “T” bar posts, aircraft
cable and crimps which, in most cases, can be purchased for less than $800.00
in total. Frequent monitoring is recommended following construction to ensure
the log jam remains well secured. Annual maintenance is also suggested as a
means of ensuring continued stability following the spring freshet.
Integration
Usually this type of structure
is a product of activity related to instream woody debris management. Cabled
log jams can incorporate a number of other habitat improvement structures including:
Bioengineering is recommended
on the slope in behind the log jam. Following slope reduction, fascines, brush
layering and live staking work equally well in stabilizing an eroding slope.
Demonstrations
This type of habitat structure
has been applied in the following demonstration projects:
For More Information
Please refer to the following
authors and their respective publications located in the bibliography:
O.M.N.R. 1984
November 5, 1998