River Crossings and Migratory Fish: Design Guidance

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PART 3 DESIGN

5 DESIGN REQUIREMENTS FOR FISH PASSAGE

Introduction

5.1 Consideration of the life history, migratory behaviour and swimming ability of various types and sizes of fish, together with a review of where fish passage problems have arisen, indicates that the following factors must be acceptable for successful fish passage:

• adequate depth of water at the time of passage;
• appropriate water velocity;
• adequate resting places above and below the structure; and
• no physical obstructions to passage.

Discussion

5.2 Attention to the above design factors is only required where a culvert installation modifies the stream profile. If the preferred option is for a bridge or a large culvert with effective retention or re-creation of the stream channel features these considerations need not apply (see Chapter 6).

5.3 It is not necessary for installations to be passable to fish at all times. Most upstream migrations take place at flows well above the dry-weather (Q95) flow, and well below the peak discharges. Peaks of discharge are generally short-lived events in smaller watercourses and a short delay in passage periods need not be critical.

5.4 In planning fish passage facilities it is important to consider the range of flows over which conditions for fish passage are to be optimised - the "passage design flow range". Good passage conditions should be provided during the period when fish are migrating in a particular area.

5.5 The appropriate DSFB ⁽²⁾ or other fisheries experts can provide advice on the passage design flow range, as it will vary between sites and fish species. Generally, there will be a shorter window of time over which fish passage can occur in small streams than in larger ones. A suggested "passage design flow range" for checking for excessive velocity at high flows and inadequate water depth at low flows is that lying between the 10 percentile and 90 percentile of normal flows respectively (the flows which are exceeded 10% and 90% of the time).

Design Criteria

5.6 The design criteria listed in Table 5.1 for river-crossing structures to enable successful salmonid passage have been determined from a combination of biological data and other guideline publications. Key references are included in a Technical Report which is available to accompany this guidance document.

5.7 Small fish are able to exploit boundary layers close to culvert walls, where the water velocity is significantly lower than that in the central section of the water flow. For this reason the criteria presented in Table 5.1 for brown trout are likely to allow passage for much of the time of smaller fish and weaker swimmers such as eel and lamprey. In most situations no special consideration of other species is necessary. However, where other species, especially smaller fish and weaker swimmers, are of specific conservation interest, more stringent design criteria may be justified. Such cases are beyond the scope of this document, and specialist advice should be sought.

(2) Contact addresses for each DSFB are provided in Appendix A and a summary of the role of the DSFBs is included.

Table 5.1 Design Criteria for Salmonids

Notes
a) Mean velocity of cross-section (there will be areas of lower and higher velocity).
b) The velocities for the shorter culverts approximate to the burst speed achievable by each species at 5ºC, and the velocities for culverts > 30m approximate to the cruising speed.
c) These velocities should not be exceeded at any flow within the passage design flow range.
d) Minimum depth acceptable at the lower end of the passage design flow range.
e) Maximum drop at either intake or outlet.
f) The minimum gap a fish can pass through will depend upon the size of the fish - these gaps are for typical large adults. Trash screens should be avoided whenever possible but if this is not possible a grid of sufficient size to allow fish passage should be used.

5.8 Resting areas immediately downstream of and upstream of the culvert are desirable with the requirements being:

• an area of water of adequate depth (at least 30cm for trout, 45cm for salmon);
• an area of deeper water with adequate cover for resting;
• rocks or overhanging vegetation; and
• moderate flow conditions (well within the cruising speed of the fish ⁽³⁾).

5.9 Resting pools within the culvert are not recommended as they are likely to collect silt and debris and would be difficult to maintain.

(3) See Table 5.1 (Note b).

6 DESIGN CONSIDERATIONS

Introduction

6.1 This chapter introduces the steps for ensuring that fish passage considerations are integrated into the design process for in-river structures.

Culvert Conceptual Design

6.2 It is recommended that the conceptual design of a culvert is carried out using the process described in the Culvert Design Guide (CIRIA, 1997). The guide identifies that the conceptual design takes into account the relevant importance of hydraulic, environmental, operational and economic performance criteria. If the Culvert Design Guide is used with the guidance set out below it will be possible to ensure that issues of fish passage are adequately addressed in the culvert design process.

6.3 During the earlier stages of the road project design, initial data will have been collated such as the fish species which may be affected, the passage design flow range, the design flood return period etc (see Chapter 4). At the initial stage of the conceptual design, calculations of trial sizes for the culvert barrel size will have been undertaken and various data collated including the design flow rate, gradient, barrel roughness and culvert length.

6.4 This is the stage when the designer must check that the predicted hydraulic conditions in the structure at critical times are appropriate for any fish species which will migrate through it (see Section 5.5).

6.5 The main parameters to be considered in the calculation of flow velocity and water depth are culvert slope, length and the Manning n value (roughness) for the culvert invert material being considered. These parameters together with the culvert size can be adjusted through iteration to develop an economical solution which allows both for fish passage and flood flow conditions.

6.6 The criteria which will allow the successful passage of fish are provided in Chapter 5.

6.7 The suggested approach is to review a number of alternative culvert designs and check which will ensure the free passage of fish.

6.8 The alternatives should be considered in the order listed below. If the basic culvert in Case 1 is found to be unsatisfactory for fish passage then the designer considers Case 2, etc until a satisfactory design is achieved.

(i) Case 1: Culvert barrel with dimensions derived for the passage of flood flows.

(ii) Case 2: Culvert with a depressed invert to allow the inclusion of stream bed material within the barrel. For circular and pipe arch culverts this will result in the specification of an increased diameter for the barrel. Similarly for rectangular shaped culverts the height of the box section will need to be increased to accommodate both flood flows and the bed material.

(iii) Case 3: Provision of a bottomless arch culvert to retain the natural stream bed.

(iv) Case 4: Provision of a low flow channel within the culvert invert, resulting in increased structural dimensions as identified in Case 2.

(v) Case 5: Provision of baffles within the culvert (particularly appropriate for steeply sloping culverts), resulting in increased structural dimensions as identified in Case 2.

6.9 In Chapter 3 common problems which affect the passage of fish have been described. Other factors which will help avoid these problems and should be considered during conceptual culvert design include the following:

(i) Matching culvert gradient with the stream gradient where possible to minimise changes in stream hydraulics which may affect fish passage.

(ii) The provision of outlet pools as a means of raising tailwater levels and avoiding extensive erosion protection works at the outlet. These pools provide resting places for migratory fish prior to negotiating the culvert. Inlet resting pools may also be required in certain situations.

(iii) When a multi-pipe structure is used ensure at least one barrel is set at sufficiently low level to ensure an adequate depth of water for fish passage under low flow conditions.

(iv) Designing inverts to allow for both the free passage of fish and the passage of other animals (otters etc).

(v) Maintaining appropriate conditions for fish passage through long culverts. (Long culverts do not in themselves represent an increased obstruction to fish as long as appropriate conditions for fish passage are maintained throughout.) Lack of light in a culvert does not appear to influence fish passage.

(vi) The design of a trash screen (if this cannot be avoided) should ensure fish passage is not impeded.

(vii) Approach conditions should be within the cruising ability of the fish in the watercourse.

Bridges

6.10 The design of river bridges and their foundations should accord with BA 59/94 in Volume 2 of the DMRB. This gives advice in the consideration of scour protection of in-river piers and abutments. If the foundations are designed at an adequate depth to avoid scouring then it is unlikely that bridge aprons and weirs will be required and issues of fish passage will not be of concern. Where a bridge apron is unavoidable it should be designed to ensure an adequate depth of water to ensure fish passage is facilitated through the passage design flow range and to avoid the need for a control structure such as a weir downstream. In situations where a weir is unavoidable it is important to ensure that there is an adequate depth of water to facilitate fish passage in low flow conditions over at least part of it. Baffles may be necessary to reduce water velocity through any fish passage measure.

General Design Principles to Minimise Construction Impacts to Fish

6.11 The following principles should be followed in the design process and contract preparation to ensure that construction impacts on fish are minimised.

(i) Avoid in-river structures where possible to prevent scour and reduce the impacts of temporary works during construction which could affect fish passage.

(ii) Where appropriate use a bridge in preference to a culvert to minimise impacts to free fish passage.

(iii) Design all new in-river structures to avoid the necessity for retrofit measures.

(iv) Wherever possible, schedule in-river works to minimise impacts to fish.

(v) Ensure the detailed design is complete and construction process adequately planned before starting construction to reduce impacts on fish and other wildlife.

(vi) Inspect culverts regularly during construction to ensure inlets and outlets are kept free from debris which could prevent fish passage.

(vii) Explore the potential to restore the natural river banks at the end of construction and avoid the use of gabions and rip rap where possible to ensure that shelter at the bankside is retained for fish.

(viii) Use of rip rap or boulders for erosion protection is preferred to gabion baskets which may harm fish if they become damaged (and also results in lower maintenance liabilities).

(ix) Remove all redundant in-stream works where these affect fish passage unless such action would cause other significant environmental damage or where cost would be prohibitive.

(ix) If a stream requires realignment as part of the works, consideration should be given to provision of straight lengths upstream and downstream of the structure to reduce the need for extensive erosion protection works which may be detrimental to fish passage.

Other Issues

6.12 As well as guidance contained in this report other general best practice guidance, such as SEPA Best Management Practices, should also be implemented during construction.

6.13 The risk of disturbance to and pollution of watercourses should be minimised during the construction process by careful control of site run-off, chemicals and fuels.

6.14 Maintenance issues should be considered at an early stage in collating contract requirements. Requirements for timing and form of maintenance should be discussed with SNH and DSFBs during consultations. For example, it may be necessary to include a maintenance requirement to clean trash screens prior to fish runs to that ensure that these do not obstruct fish passage.

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