River Ecology and Restoration - River Dynamics and Integrated River Management - page 693

Deflectors— Perhaps the most widely used structures for habitat improvement are current deflectors.
These function either to direct flow and eliminate accumulated sediment, or to narrow a channel, thereby
locally increasing the velocity and creating a scour pool with a corresponding riffle downstream. Other
effects include assisting the development of a meandering thalweg within a straightened channel,
protecting stream banks from erosion, and encouraging the establishment of riparian vegetation through
the formation of bars of sediment.
Deflectors are commonly angled in a downstream direction at approximately 45 degrees from the
current (Wesche, 1985), although several different angles have been used depending on the local
circumstances (Cooper and Wesche, 1976). Double wing deflectors, which consist of two current
deflectors placed opposite each other at the same point in a reach, have also been used (Seehorn, 1985).
Shape is another consideration and varies from single peninsular wing to a triangular wing (White and
Brynildson, 1967). The latter shape has been successful in certain circumstances in regulating the
tendency for erosion of the bed and bank behind the structure during high flows. The elevation of the
water surface at low flow generally determines the structure height and it has been found that to avoid
excessive damage to the structure itself during high flows the structure should not extend more than 0.15
to 0.30 m above the low flow elevation (Seehorn, 1985; Wesche, 1985). The distance that the deflector
protrudes into the channel will vary from site to site and depends on the specific results desired. For
example, in streams in the southeastern U.S., Seehorn (1985) found that to have any effect the channel
would need to be narrowed to a width approximating the natural width. This width can be determined
from adjacent or neighboring natural reaches with a similar slope, flow regime, and bed and bank
materials.
Deflectors are very effective at manipulating flow and creating the diversity of habitat required for fish
and other biota. Figure 10.48 shows a spur dike extension on the Lijiang River, to promote the formation
of low velocity habitats. Patterns of scour and deposition are evident. Such structures can be built in series,
alternating from one bank to the other, to create a meandering thalweg. Several authors have recommended
a spacing of between five and seven channel widths, corresponding to the natural pool-riffle spacing of
some natural streams (White, 1975; Everhart et al., 1975). This helps to formalize the low flow sinuous
channel within a wide flood channel (Brookes, 1995). Many of these structures demonstrated marked
improvement in fish population and the benthic macroinvertebrates. However, rarely have studies adopted a
more integrated approach of objectively assessing the potential hydraulic and geomorphologic impacts at
the outset, and most have been aimed at the restoration of single species.
An understanding of how deflectors typically function to provide habitat may aid in the selection of
building material and design. Whilst deflectors by design, create and maintain pool-riffle sequences, they
also provide zones of higher velocities. These areas are critical to some species of fish and macro
invertebrates. Swiftly moving water transports food and oxygen into a river reach. To exploit these
resources fish must either swim against the current at a rather high cost of energy to maintain their
position, or they must find sheltered areas close enough to the fast water to derive the benefits of higher
dissolved oxygen and food availability. Although many organisms typical of high velocity reaches are
physically adapted to maintain position or spatial orientation, most species require ambush locations out
of the direct flow of water. In unaltered streams, organisms use velocity shelters provided by undercut
banks, boulders or large woody debris, but if these natural habitats are absent in degraded reaches,
current deflectors may provide them. Studies in large rivers and small streams have shown that stone
deflectors (groins, spur dikes) used for erosion control provide aquatic habitat superior to stone blanket
revetment, which is not as effective in creating the juxtaposition of high and low velocity zones (Shields
et al., 1995)