River Crossings and Migratory Fish: Design Guidance

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PART 1

1 INTRODUCTION

1.1 Historically there has been little consideration given to fish passage in the design of in-river road structures. As a result fish which migrate in Scottish rivers and streams may be obstructed and unable to use habitat upstream or downstream of a culvert or bridge crossing.

1.2 A variety of interrelated factors affect the design of any in-river road structure including hydrology, topography, engineering and costs. Environmental issues including the requirements of fish must also be taken into account.

1.3 The purpose of this guidance is to identify good practice which will ensure that the free passage of fish in watercourses is not compromised by the construction, operation or maintenance of any public road. The potential to improve existing structures is covered as well as the design of new structures.

1.4 The guidance adopts an integrated approach to engineering, hydraulic and fish passage design. Wider environmental issues, such as aesthetic considerations and requirements of other fauna, are not covered. Appropriate guidance on these issues should be used in parallel with this guide.

1.5 The guidance concentrates on the requirements of species such as salmon and trout but also considers the implications of in-river structures for other migratory and non-migratory species.

1.6 The design of a variety of road structures including culverts, bridge aprons, fords and weirs is considered in this document. River diversion works are not included.

1.7 The guidance is intended for all those with interests in the design of in-river road structures including engineers, hydrologists and environmental consultants as well as local authorities and agencies such as SEPA and SNH, and other organisations including District Salmon Fishery Boards (DSFBs).

1.8 It is intended that this document should be used in conjunction with other relevant guidance including the recent CIRIA Culvert Design Guide (1997). The step-by-step approach which is adopted can be loosely related to the three stage process advocated in the Design Manual for Roads and Bridges (DMRB), Volume 5 and it is intended that the guidance should be used in the assessment and design processes. The guidance should also be used when existing in-river road structures are upgraded, for example during major maintenance or to accept heavier loads.

1.9 The guidance provides background information, as well as guidance on the assessment of requirements and design. The remainder of the document is set out as follows:

• Chapter 2 discusses Life Cycles and Migrations of relevant fish species;
• Chapter 3 reviews Common Problems for Fish Passage at River Crossings;

Part 2 - Assessment

• Chapter 4 introduces the Assessment Process which should be followed in the evolution of the road proposals to ensure that the requirements for fish passage are adequately addressed.

Part 3 - Design

• Chapter 5 introduces Design Requirements for Fish Passage.
• Chapter 6 provides Design Considerations.
• Chapter 7 describes options for Improvement at Existing Problem Sites.

1.10 The document is supported by the following:

• A Glossary of technical terms.
• Appendix A which provides details of District Salmon Fishery Boards.
• A Technical Report which is available from the Scottish Executive.

2 FISH LIFE CYCLES AND MIGRATIONS

2.1 Several species of fish living in Scottish rivers migrate between the sea and the upper reaches of rivers during their life cycle. Others make significant migrations within fresh water. River-crossing structures have potential to disrupt the life cycles of such species by interfering with both upstream and downstream movement. The overall production of fish stocks is limited by the area available for spawning and rearing. Optimum yield will depend upon access to as much of the catchment as possible. Often the best spawning and rearing areas are in small upland tributaries. In the case of salmon and sea trout, the fish return to spawn in the tributary and even the same part of the tributary where they themselves were spawned and reared. This has led to the formation of genetically distinct sub-stocks in larger catchments, which are likely to be adapted to conditions in that part of the catchment. It is therefore important that all spawning areas are accessible on a regular basis for the maintenance of the genetic integrity of the sub-stocks.

Atlantic Salmon

2.2 The Atlantic salmon (Salmo salar) is widespread throughout Scotland. The adult fish (typically 55 to 100 cm or more in length) may spawn in quite small headwater streams as well as in suitable areas in larger watercourses. The adult fish enter rivers from the sea at almost any time of year, but they migrate into smaller spawning streams on elevated flows following rainfall in the autumn (September - November). After spawning in October to December the adult fish return seawards over a period of up to several months.

2.3 The eggs are laid in areas of gravel where there is an adequate flow of water. After hatching, the young fish remain within the gravel for several weeks, eventually emerging in March to May. The fry disperse for distances of up to several hundred metres downstream. As the fish grow (known as parr at this stage) they redistribute themselves, generally downstream in direction. After 2 to 4 years the parr develop a silver colour and migrate seawards, usually in April to June. At this stage they are termed smolts. The survivors return to spawn after 1 to 3 years of feeding and growing in the sea.

Trout

2.4 There are two forms of trout (Salmo trutta) in Scottish waters: the sea trout, which migrates to the sea and the brown trout, which migrates only within the river catchment.

Sea Trout

2.5 The migratory sea trout has a life history very similar to that of the salmon. The main differences are that the fish may return to fresh water after only a few months at sea, and the adults are generally smaller than salmon (typically 25 to 60 cm). The adults may also enter smaller spawning tributaries earlier than salmon, often penetrating to the upper headwaters during the summer. A higher proportion survive to spawn again than is the case for salmon.

Brown Trout

2.6 Brown trout spend their whole life cycle in fresh water, but may nonetheless make extensive migrations between upper headwaters and the main river and lochs. The timing of these movements is similar to that of salmon, but the adult fish may be very much smaller, typically 15 to 50 cm in length.

Lamprey

2.7 Sea lamprey (Petromyzon marinus) and river lamprey (Lampetra fluviatilis) have similar life cycles. Eggs are laid in the gravel in streams and the young fish spend several years living in silt banks in fresh water before migrating to sea to feed and grow, returning to the river to spawn. Lamprey do not enter such small and rocky headwaters as salmonids, but nevertheless where they occur they can be affected by structures which do not permit access to their breeding grounds. Their swimming ability is limited, but they are able to ascend fairly rapid rocky reaches using their oral sucker to hold on between bursts of activity.

Eel

2.8 Eels (Anguilla anguilla) are also migratory but have quite a different life history, breeding in the sea and ascending the river as juveniles to feed and grow. They may migrate to the highest headwaters, but may take up to two years to do so. Eels may spend up to 40 years or more in fresh water before returning to sea to spawn. They are small when migrating upstream (typically 6 to 30 cm) with only limited swimming ability but are adept at exploiting slower moving water among rocks and in-stream vegetation.

Grayling

2.9 The European grayling (Thymallus thymallus) is a river dwelling fish which has been introduced to several river systems in south and east Scotland. They make limited migrations within the river systems and spawn in spring close to where they live the remainder of the year.

Summary

2.10 Some details of upstream and downstream migrant fish in small streams are summarised in Tables 2.1 and 2.2.

Table 2.1 Details of Upstream Migrant Fish in Small Streams ¹

¹ Temperature and flow conditions are relevant to the upstream migration because of their impact upon swimming performance and current speeds within culverts. These factors are not relevant to downstream migration.

Table 2.2 Details of Downstream Migrant Fish in Small Streams

Burst / Cruise Speed

2.11 The capacity of fish of various species and sizes to swim against fast currents and to leap at obstacles is clearly fundamental to consideration of the design criteria for in-river structures. Fish have two basic swimming modes: burst speed swimming and cruising speed swimming. Burst speed can be maintained for only a matter of seconds, and quickly leads to fatigue. The maximum velocity and the time for which it can be maintained are strongly influenced by water temperature. Fish can swim faster at higher temperatures but are unable to maintain swimming activity for as long as they can at lower temperatures. Cruising speed can be maintained for extended periods or even indefinitely. In Chapter 5 these observations are used to provide water velocity criteria for various sizes of fish for culverts of different lengths based upon the swimming ability of the fish. These involve minimising the requirement for fish to use burst speeds to attain passage.

2.12 Although salmonid fish in particular are renowned for their leaping behaviour, successful leaps to overcome obstacles are dependent upon certain conditions of water flow at the take-off point and depth and velocity at the landing point. At culvert outlets and inlets in particular these conditions may not occur. Further, salmon and trout only leap at obstructions when they have to, their preferred method of ascent being fast swimming wherever possible. The other species of interest do not leap at obstructions at all. Good design criteria for culverts should minimise the requirement for fish to leap and to use burst speeds.

3 COMMON PROBLEMS FOR FISH PASSAGE AT RIVER CROSSINGS

Introduction

3.1 Some river crossings may pose a problem for fish passage because some or all of the fish that attempt to pass the structure cannot do so, or can do so only with difficulty, under the environmental conditions prevailing at the time. This may be due to some hydraulic feature of the structure, such as high water velocity, inadequate water depth or extreme turbulence or due to very large level changes in the watercourse (falls) or as a result of the physical nature of the structure obstructing passage, such

as small pipes or narrow grids. While some river-crossing structures are effectively impassable to upstream migrants at all times, many others are passable only by certain species or sizes of fish under only part of the annual range of environmental conditions. Delay caused by this can be an important factor not only in terms of reaching spawning grounds in time but in making fish more vulnerable to predation and poaching.

3.2 Success of passage through such structures will be affected by many factors including:

• the species of fish: some fish are stronger swimmers or more adept "leapers" than others;
• the size of the fish: generally large fish can swim faster than smaller fish but larger fish require a greater depth of water to swim in;
• the condition of the fish: fish heavy with spawn are likely to be less agile than immature fish, and fish may be tired by their migration up to the crossing (particularly if migrating salmon and sea trout are a long way from the sea and have not fed since then);
• the stream discharge: the hydraulic conditions within and around the structure (e.g. velocity, depth and turbulence) may vary substantially with changes in discharge -which may vary 1000 fold or more between dry-weather flow and peak flow at some sites; and
• water temperature: generally speaking fish can swim faster at higher water temperatures.

3.3 Bridges generally result in fewer problems for fish passage than culverts because the original channel dimensions, gradient and stream bed tend to be retained. Bridge aprons or associated weirs can, however, cause problems.

3.4 Common problems at river crossings which have been encountered are described in the remainder of this chapter.

Inadequate Water Depth

3.5 Inadequate water depth for fish swimming can occur at low flows and/or in wide shallow culverts. This is also linked with the gradient and water velocity in a culvert where faster currents will result in shallower depths in a given culvert cross-section.

3.6 Bridge aprons which are flat in cross section may result in inadequate water depth for fish to swim effectively and safely. Similarly, fords may result in flows spread too thinly for fish passage.

Perched Structures

3.7 Perching is a term applied to a culvert outlet (the downstream end) which is set above the stream bed immediately downstream, so that there is a fall. This can occur when the culvert is installed too high, resulting in erosion of the downstream channel. While salmon and trout are capable of leaping at falls to surmount them, conditions at culvert outlets are frequently not conducive to successful jumps. The stream below the fall may be shallow and the water turbulent, representing poor conditions for "take off" for a leap. Water shooting from the culvert, rather than falling vertically, may confuse fish which may jump at the wrong point. Shallow, fast flowing water inside the culvert barrel presents difficult conditions for fish landing after a leap and the fish may be washed downstream out of the culvert.

3.8 Bridge aprons and weirs can also become perched in a similar way to culverts and can then cause difficulties for fish passage.

Culvert Inlets

3.9 Changes in the stream hydraulics at the culvert inlet resulting from the constriction of the flow into the culvert barrel may cause problems to fish passing upstream even though they have successfully negotiated the culvert itself.

Steep Culvert Gradient and Smooth Barrels

3.10 Excessive water velocity that impedes fish passage may occur when the gradient of a culvert is too great. The problem becomes more severe in periods of high flow and in installations with smooth walls, particularly if there are no resting places (e.g. behind baffles) for fish within the barrel.

Inadequate Culvert Diameter

3.11 Inadequate culvert diameter for fish to swim through can occur where a number of small pipes are provided rather than one large barrel. Even a pipe which is physically large enough for fish passage may deter the passage of fish if the fish is reluctant to enter a confined space.

Lack of Rest Places and Pools

3.12 If fish have to attempt passage through a culvert without the opportunity to rest immediately downstream, or have to continue strenuous swimming having just ascended a challenging culvert, they may become exhausted and be washed back downstream. A lack of rest areas and pools immediately upstream and downstream can thus render a difficult but theoretically passable culvert effectively impassable.

Debris Accumulation

3.13 Blockage by debris and the downstream movement of river bed material can occur in small-diameter culverts and where trash screens or farm animal barriers are installed. While total blockage is unlikely, collection of debris can effectively prevent passage of large upstream migrants. Any blockage may cause local increases in current speed which may defeat fish of all sizes. Downstream migrating fish may also become trapped in debris blockages.

Percolating Flow

3.14 Percolating flow through gabion mattresses and rip-rap can deplete the main flow or even cause the whole flow to pass through the gaps. This can prevent upstream passage but can be particularly harmful to downstream migrants, which may become stranded or trapped.

Impassable Weirs

3.15 Weirs are sometimes installed immediately downstream of bridges, either at the same time as the bridge was constructed or at a later date, to prevent or reduce erosion of the bridge footings. While this is likely to make fish passage through the bridge area straightforward, the weir itself can be an impediment to migration.

Construction Activities

3.16 Construction work and river diversions can cause temporary disruption of migration even if the permanent works are designed to allow fish passage.

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