Hydraulic habitat modelling for Natural Resources Wales


Lyn Padarn, a glacially formed lake in Snowdonia, North Wales, hosts a population of Arctic Charr (Salvelinus alpinus) fish. It is one of only a small number of native, as opposed to translocated, Welsh populations of the UK Biodiversity Action Plan (BAP) priority species.

The creation of the Dinorwig hydroelectric power (HEP) scheme in the 1970’s/’80’s, and the associated diversion of the Peris Brook and the River Hwch has prevented Arctic Charr accessing the historical, favoured spawning area. The Lake Padarn population is now principally being sustained by spawning in the gravel beds of the River Bala near the dammed out?ow from Lake Peris.

The aim of this project was to provide an unconstrained concept design for additional, robust Arctic Charr spawning habitat on the River Bala following these objectives:

  • Develop a detailed understanding of the hydrology of the system including the River Hwch, Peris brook, Lake Padarn and the hydropower plant.
  • Develop a detailed understanding of the hydromorphology of the system.
  • Undertake two-dimensional hydraulic modelling to build an understanding of the hydrodynamics of the River Bala and the implications for Arctic Charr spawning habitat.
  • Use the hydraulic model to develop and assess concept designs of three measures to improve the quantity and stability of Arctic Charr spawning habitat in the River Bala.


Building upon a thorough understanding of the hydrology and hydromorphology of the River Bala and its wider catchment, two-dimensional hydraulic modelling coupled with JBA’s in house habitat model (JHab) was used to prove the concept design for measures to improve the quantity and stability of Arctic Charr spawning habitat in the River Bala.

Arctic Charr spawning habitat is currently limited to a small area at the confluence between the River Bala and a channel which is used to discharge excess water from a hydropower storage lake upstream. The location of the existing spawning habitat was verified by both anecdotal evidence and results from the hydraulic model which took account of the interaction of low flows, flood flows and releases from the hydropower storage lake.

Using the combined hydraulic and habitat modelling, three measures were tested for their ability to improve the quality, quantity and stability of Arctic Charr spawning habitat in the River Bala under a range of conditions from low flow to flood flow.


The modelling analysis suggested that both lowering of the left-hand bank and the independent creation of a new channel on the left-hand floodplain, increased the quality, quantity and stability of Arctic Charr spawning habitat. For the proposed bank lowering the hydraulic conditions associated with an extreme (>100-years) flood event led to conditions appropriate to mobilise substrates suitable for Arctic Charr spawning habitat, whilst this substrate would be largely retained in the new channel. Retaining suitable substrate will improve the sustainability of the proposed scheme, by improving the stability of the spawning substrate and reducing the need to augment the gravels in the future.

Want to know more?

Email David Mould for more information on this and other fisheries projects. You can also find out more on our fisheries web page and in the Knowledge Hub on our web site.

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