River Crossings and Migratory Fish: Design Guidance

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EXECUTIVE SUMMARY

Introduction

Fish passage at stream crossings is an important consideration in the planning, design, installation and upgrading of river-crossing structures. The purpose of this publication is to give design guidance to the engineer and others with an interest in river crossings that will ensure free fish passage can be achieved.

For many fish, migration is essential to the survival of the species. Salmon, for example, travel from the sea up river to spawn, and both juveniles and spawned adults migrate downstream to the sea. Other fish such as trout migrate upstream and downstream during their life cycle seeking a variety of aquatic habitats for spawning, rearing young and hiding. Although these migrations may only be over a few kilometres, they can be important for the long-term survival of the species and maintenance of fish production in a watercourse.

Improperly planned and designed culverts and other river-crossing structures can be barriers to such migration. Common problems at river crossings are mainly associated with culverts, but can also be present at bridges, fords and weirs, and include:

• inadequate water depth during periods of migration;
• excessive water velocities; and
• vertical barriers to fish passage (e.g. perching).

The Road Planning Process

Avoiding or minimising obstruction to fish passage requires consultation by the engineer with SEPA, SNH, SERAD and DSFBs throughout the design process to ensure that all relevant requirements are identified (see Figure 1). The resolution of engineering, biological, hydraulic and cost factors requires a balanced and interactive approach across these disciplines so that structures do not present a barrier to fish migration. The first three steps indicated can be equated to the three stage DMRB assessment process used in the development of a road project. The process is also relevant to minor improvement and major maintenance projects although these are not subject to the three stage process.

Design Requirements for Fish Passage

In general, fish will be able to travel through a river crossing structure provided that the conditions in the structure are not substantially different from those in the existing watercourse. The design parameters detailed in Table 1 have been developed to ensure the free passage of most migratory fish species through river-crossing structures (specific advice relating to individual fish species is given in Table 5.1 of this document). These parameters must be incorporated into the design of new or altered structures to ensure that they do not present a barrier to migrations.

It is not necessary to design for fish passage at all flow conditions. A suggested "passage design flow range" for checking for excessive velocity at high flows and inadequate water depth at low flows is the 10 percentile and 90 percentile of normal flows respectively (the flows which are exceeded for 10% and 90% of the time).

The preferred form of structure is one in which the characteristics and dimensions of the original watercourse are maintained. This may be achieved by using a bridge or a culvert. If using a culvert, the following alternatives should be considered in turn until a successful solution for fish passage is determined:

Table 1 Design Criteria for Salmonids

⁽¹⁾ also relevant in bridge design

Existing Problem Sites

There are two approaches to retro-fitting existing culverts and bridge aprons to allow for fish passage:

(a) raising the tailwater level by installing a weir which backs up water; and

(b) installing baffles.

Design details of these measures are contained in Chapter 7 of this document. Both are effective in deepening water levels and reducing excessive velocities. However, they result in a reduction of the structure's flow capacity which may have significant effects on upstream flooding during storm flow conditions.

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