River Crossings and Migratory Fish: Design Guidance

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7 IMPROVEMENT AT EXISTING PROBLEM SITES

Introduction

7.1 Many existing river crossings present significant or even total obstacles to fish migration, restricting the overall productivity of the river system. This chapter examines the process of evaluating, planning and executing improvements.

Assessment of the Extent of the Existing Problem

7.2 A complete bar to migration is more serious than an obstruction that lets some fish through on occasions. Visual inspection by an appropriate expert will immediately indicate that some crossings are probably a complete barrier while others are likely to represent no hindrance to all. It is important that the level of barrier represented by those that fall between these two extremes is carefully evaluated. Obstructions which appear severe, and indeed may represent a major delay or hindrance to migration under some conditions, may not warrant remedial action if surveys show that stocks of juvenile salmon and trout are routinely present upstream.

Assessment of Responsibilities

7.3 The approval of the Roads Authority is required for proposed works to any ditch, watercourse, bridge, culvert, tunnel or pipe constructed, laid or erected by the Roads Authority for the purpose of draining a road.

Options for Amelioration

7.4 Options for introducing measures to assist fish passage should be identified based on local topographical constraints, hydraulic considerations and cost effectiveness. Relatively low cost measures can be effective in many cases. Complete rebuild, using sound design principles to ensure that the new installation provides good fish passage conditions (see Chapters 5 and 6) may be the only viable option for extreme problems.

Culverts

7.5 There are two approaches to tackling the problem of excessive water velocity and both can help ameliorate the problem of water being too shallow for effective fish passage:

(i) reduce the velocity throughout the section of the flow by increasing the depth of water through the installation of one or more additional structures;

(ii) introduce some roughness to the bed of the culvert causing local lowering of current speed and thus leading to some increase in water depth.

7.6 In evaluating these options consideration should be given to the effective reduction in flow capacity of the culvert and the significance of increased water levels upstream.

Weirs at Culvert Outlets

7.7 An effective approach to deepening the water and slowing the flow is to raise the tailwater level by installing a weir downstream of the culvert outlet, backing up the water through the culvert itself. Installation and maintenance are easier in the open, and there is no requirement to attach any structure to the fabric of the culvert. This approach can also ameliorate any perching problem.

7.8 Several North American reports present guidelines for the design of low stone weir installations intended to raise tailwater height. It is recommended that weirs should be a minimum of 6m apart and have a maximum fall of 30 cm between successive weir crests, and that the most downstream structure should have its crest level with the stream bed to act as an erosion control mechanism (see Figure 7.1).

Baffles

7.9 The shape and orientation of baffles depends on the culvert barrel shape and its gradient. The recommended solutions, based on recent research are shown in Figures 7.2 and 7.3.

Increasing Bed Roughness

7.10 A simple low-cost approach to increasing bed roughness has been applied in a number of streams in Montana, USA. This involves installation of a pre-fabricated steel frame rather like a ladder which lies along the bed of the culvert. It can be constructed in 6 metre long sections for transport to site, and the cross-bars or "rungs" are fixed at 1200mm intervals (see Figure 7.4).

7.11 After installation an average of three "large" rocks are placed against, and are therefore anchored by each cross-member. The rocks thus provide the turbulence and depth to allow fish to migrate.

Bridges

7.12 Bridges generally represent much more benign conditions for fish passage than culverts. The main potential for problems is with bridge aprons or sills, installed either when the bridge was constructed or retro-fitted to correct erosion that potentially threatens the bridge structure. These can be perched in the same manner as culverts, making it difficult or impossible for fish to ascend at certain flows. Flat bridge aprons can also present problems for fish passage at low flows as the available water is spread very thinly over the width of the channel.

7.13 The most straightforward solution to perching at bridge aprons is the installation of low stone weirs as described in Sections 7.8 and 7.9. This approach may also be appropriate to overcome inadequate water depth problems on the apron itself. An alternative approach would be to re-cast the apron into a v-shaped channel so that the flow is concentrated in the centre. This creates shallow slow-moving water at the edges which will ease the passage of smaller fish, while providing a deeper zone for larger fish in the centre. Alternatively the lip of aprons can be broken down or baffles or boulders used on the apron to channel low flows.

Fords

7.14 The simplest solution to the inadequate water depth problem at fords is to install a pipe or culvert to carry low flows beneath the roadway, allowing the high flows to pass over the roadway. The culvert can be much smaller than would be required if flood flows had to be conveyed, but it should not be below 30 cm diameter for trout passage or 45 cm diameter for salmon passage. Such small diameter pipes are vulnerable to blockage by debris so regular inspection and maintenance may be required. Pipes installed beneath natural bed level will also be subject to blockage through siltation.

Weirs

7.15 Weirs have often been installed immediately downstream of road bridges to prevent erosion of the bridge foundations. Whilst making the bridge itself readily passable to fish the weir may represent a major or even total obstruction. There are two approaches to alleviating this problem. The first is to raise the tailwater level of the weir using a series of further stone weirs (as described in Sections 7.8 and 7.9). The second is to install a fish pass in the weir. Design of fish passes should be made with reference to the guidelines published by SOAFD (1995).

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