Case study:Ddu Restoration Project

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Location: 52° 57' 37", -4° 8' 29"
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Project overview

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Status In progress
Project web site
Themes Fisheries, Habitat and biodiversity, Hydromorphology, Land use management - agriculture, Monitoring
Country Wales
Main contact forename Huw
Main contact surname Jones
Main contact user ID User:Huw Jones
Contact organisation Natural Resources Wales
Contact organisation web site
Partner organisations Snowdonia National Park Authority, Welsh Government
Parent multi-site project
This is a parent project
encompassing the following
Restoration of the river

Project summary

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This river was badly damaged by dredging / drainage operations in 1995-6. Investigation work led to the discovery of a previously unknown population of Margaritifera margaritifera - the population was badly damaged by this work. Work to restore some sections of the Afon Ddu has been carried out by legacy Environment Agency Wales and Countryside Council for Wales and partner organisation Snowdonia National Park Authority since 2005. The bulk of this work concentrated on fencing out the river to reduce livestock access, but a short section of the most damaged section was restored in 2006. The river was designated as a pearl mussel Site of Special Scientific Interest in 2010/11. Since the formation of Natural Resources Wales, further work has been carried out with the aim of restoring in channel geomorphology, through the replacement of boulders and gravel seeding with an objective of improving the habitat for salmonids and Margaritifera margaritifera. The small catchment size means that close co-operation with landowners is likely to be more effective than in larger, dispersed, catchments in terms of achieving water quality objectives. Approx. £90,000 has been spent on fencing and habitat works on this river since 2005 by the partner organisations. This river is an UK Biodiversity Plan Priority River for Margaritifera margaritifera, along with two other Welsh rivers.

Monitoring surveys and results

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Long term monitoring of Margaritifera margaritifera numbers in this river indicate a continuing decline in adult numbers, in common with other Welsh rivers. In the Afon Ddu, this has been mostly linked to unsuitable river habitat, both due to physical modifications and some issues with water quality. Monitoring of the success of the restoration work will in future be linked to NRWs use of the Redox potential measuring technique to determine the quality and cleanliness of the interstitial gravel which the juvenile stage of Margaritifera margaritifera rely upon. Monitoring indicates that the encystment stage of the Margaritifera margaritifera is occurring normally in this river, and that juvenile salmonid host numbers are good.

Lessons learnt

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Landowner buy in and suitable catchment management is essential, and there is a good uptake of the Welsh Government's Glasdir Agri-Environment scheme in this small catchment. Successful captive rearing of juvenile mussels at the NRW Aquaculture Unit at Cynrig is considered to be an important future development to secure the future of the species. Due to the sensitive species present in this river, a spread out one step at a time approach to restoration has been appropriate.

Image gallery

2009Restored section.JPG

Catchment and subcatchment


Name Afon & Llyn Du
WFD water body codes GB110065053690
WFD (national) typology
WFD water body name Cwmystradllyn
Pre-project morphology Over deepened
Reference morphology Pool-riffle
Desired post project morphology Pool-riffle
Heavily modified water body No
National/international site designation UK - Site of Special Scientific Interest
Local/regional site designations
Protected species present Yes
Invasive species present No
Species of interest Freshwater pearl mussel (Margaritifera margaritifera)
Dominant hydrology Perennial
Dominant substrate Gravel
River corridor land use Non-intensive agriculture (grazing)
Average bankfull channel width category Less than 2 m
Average bankfull channel width (m)
Average bankfull channel depth category Less than 0.5 m
Average bankfull channel depth (m)
Mean discharge category Less than 0.1 m³/s
Mean annual discharge (m3/s)
Average channel gradient category
Average channel gradient
Average unit stream power (W/m2)

Project background

Reach length directly affected (m)
Project started 2005/06/01
Works started
Works completed
Project completed
Total cost category
Total cost (k€)
Benefit to cost ratio
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Cost for project phases

Phase cost category cost exact (k€) Lead organisation Contact forename Contact surname
Investigation and design
Stakeholder engagement and communication
Works and works supervision
Post-project management and maintenance

Reasons for river restoration

Mitigation of a pressure
Other reasons for the project


Structural measures
Bank/bed modifications Boulder restoration, Addition of substrate and in channel features
Floodplain / River corridor Fencing of riparian zone
Planform / Channel pattern Creation of fish passes
Other Deculverting
Non-structural measures
Management interventions Agricultural business changed its way of working
Social measures (incl. engagement)


Hydromorphological quality elements

Element When monitored Type of monitoring Control site used Result
Before measures After measures Qualitative Quantitative

Biological quality elements

Element When monitored Type of monitoring Control site used Result
Before measures After measures Qualitative Quantitative

Physico-chemical quality elements

Element When monitored Type of monitoring Control site used Result
Before measures After measures Qualitative Quantitative

Any other monitoring, e.g. social, economic

Element When monitored Type of monitoring Control site used Result
Before measures After measures Qualitative Quantitative

Monitoring documents

Additional documents and videos

Additional links and references

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Supplementary Information

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