River Crossings and Migratory Fish: Design Guidance
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PART 2 ASSESSMENT
4 ASSESSMENT PROCESS
4.1 The requirements for fish passage should be considered throughout the
development of the road project. A step-by-step approach is provided in Figure
4.1 which shows the progression of proposals through a staged process leading
to the conceptual design of the in-river structure. The procedure for integrating
fish passage requirements within the development of the conceptual design is
described in Chapter 6.
Stage 1
4.2 As part of the initial assessment of the overall road improvement strategy
the location of river and burn crossings should be considered and included in
the review of options for positioning the road alignment to achieve a balance
between broad engineering, cost and environmental considerations. The need for
crossings should be confirmed through examination of broad alternatives for
the route of the road. If possible, crossings of important watercourses should
be avoided. This step should be considered as part of the constraints analysis
in the Stage 1 environmental assessment process outlined in the DMRB.
4.3 Early consultation with SEPA, SNH and SERAD(1) is essential.
Information obtained from consultation and initial site visits should be taken
into account in considering the siting and justification for a stream crossing
as part of the Stage 1 assessment. In assessing the requirement for fish passage
facilities at a river crossing, it is important to consider any existing obstructions
to fish passage nearby. There is little point in providing good conditions for
migrating fish passage if the stream is impassable downstream or a short distance
upstream due to a natural obstruction.
Stage 2
4.4 In this stage further relevant data on the watercourse should be collated
in order to identify the future requirements for the design of the structure.
The hydrological data for the catchment are collated for later use in calculation
purposes in Stage 3 and also to identify any significant flooding risks. Other
considerations at this stage include identification of potential ground conditions
for foundation design, investigating the potential to temporarily divert the
watercourse so that the structure can be built "in the dry" and any specific
access and maintenance requirements (e.g. for cattle, farm traffic etc). Consultations
with relevant bodies (including the relevant DSFB (see Appendix
A)) and, if appropriate, site surveys, should be undertaken to establish
the need to accommodate particular fish species, in conjunction with other wildlife
needs (for example, allowance for otter passage).
4.5 This step concludes with the comparison of options which have been identified
and consideration of their relative costs in relation to the overall costs of
the road improvement. These options could, for example, include a short culvert
with a realigned watercourse, a long culvert on original line, a bridge, etc
(see Figure 4.2). This review of options should contribute to the Stage 2 DMRB
route option environmental assessment.
Stage 3
4.6 The development of the conceptual design should be undertaken as part
of the Stage 3 environmental assessment of the preferred option. For culverts
this stage closely relates to that presented in the CIRIA Culvert Design Guide
(1997) and firms up on the type and size of the in-river structure. The sizing
is derived from the hydrological parameters gathered in Stage 2 by calculating
the acceptable maximum headwater depth for flood flows. In terms of fish passage,
the flow velocity and depth under typical conditions when the fish are running
needs to be identified (see Section 5.5). Consultations
should continue with relevant organisations to ensure the developing design
takes adequate account of the requirements for fish passage.
Scheme Procurement
4.7 With Design and Construct schemes it is necessary to complete sufficient
development of the conceptual design to ensure that robust requirements can
be included in the contract documentation. Development of the detailed design
will be the responsibility of the appointed contractor.
4.8 For road projects which are not formally assessed using the 3-stage
DMRB process, such as maintenance and minor improvement schemes, the principles
of the staged process described above should be followed to ensure early consideration
of fish passage requirements.
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