Llanrhystud Ford detailed design

Challenge

In 2015, Natural Resources Wales commissioned us to produce a detailed design and business case for works to deliver the following objectives at a failing ford on the Afon Wyre on the west coast of Wales:

  • Stabilise the structure and repair scour
  • Increase minimum depth over the structure at low flows to enable manual gauging and calibration of a NRW flow gauge immediately upstream, permanent submergence of the inlet to the gauge stilling chamber and ensure sufficient depth over the ford to accommodate large migratory salmonids
  • Improve the porosity of the structure from a fish passage perspective.

The site is spatially constrained by adjacent properties with thresholds only approximately 2.5m higher than Wyre bed level. There is an immediately adjacent B-road, a number of overhead and buried services crossing the Wyre at the ford, including a high pressure Welsh Water main. The Afon Carrog also joins the Wyre immediately downstream of the ford on the right bank. All of these spatial constraints combined with key hydrometric and fish passage design criteria, make this a particularly challenging site.

Solution

Our team of in-house technical specialists worked through the options appraisal process and undertook the following key work:

  • Influenced the client to install a temporary level gauge immediately downstream of the site – to provide stage information for use in accurate design of the elevation of the fish pass entrance, and calibration of the hydraulic model.
  • Developed a 1D hydraulic model of the Wyre and Carrog to assist in the design of mitigation measures – to reduce the negative impacts on flood risk associated with delivery against key hydrometric and fish passage design criteria.
  • Specified, procured and supervised ground penetrating radar and intrusive investigation services with the objective of:
    • Determining the presence of other buried services adjacent to the site
    • Verifying the depth of the Welsh Water main to inform detailed structural design
    • Determining the construction of the ford and underlying foundations
    • Determining the presence of asbestos in the inlet pipe to the gauge stilling chamber
    • Determining the presence of contaminants in the sediments accumulated upstream of the ford. This work involved managing the consideration of the presence of Japanese Knotweed in close proximity to the intrusive works, in production of the Environmental Management Plan and method statement for the works.
  • In the optioneering process, we considered passage facilities specified in British Standard ISO 26906:2009 Hydrometry – Fish Passes at Flow Measurement Structures, as well as best available fish pass technology including:
    • Emerging baffle technologies that are the subject of published research on their use at flat-crested crump weirs
    • Research on their use at flat v crump weirs.

Benefit

Discharging our duties as appointed Principal Designer under the Construction (Design and Management) Regulations 2015. We ensured the named Principal Designer on this project was fully integrated into the design process. Design decisions are being made in consideration not only of technical performance and design criteria but also buildability, duration and scale of construction and operational phase health and safety.

Want to know more?

Find out more about the work our fisheries team does in our environment section. You can also contact Jon Whitmore for more information on this specific project.



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