Case study:Glaisdale Beck diversion scheme
Project overview
| Status | Complete |
|---|---|
| Project web site | |
| Themes | Environmental flows and water resources, Fisheries, Habitat and biodiversity, Hydromorphology, Land use management - agriculture, Monitoring, Water quality |
| Country | England |
| Main contact forename | Matthew |
| Main contact surname | Perks |
| Main contact user ID | User:Perksieuk |
| Contact organisation | Newcastle University |
| Contact organisation web site | |
| Partner organisations | |
| Parent multi-site project | |
| This is a parent project encompassing the following projects |
No |
Project summary
The Esk is a river of both ecological and economic importance at a national scale. It is the principle river in Yorkshire for Atlantic salmon and sea trout and is one of only two rivers on the east coast of England to have known populations of the freshwater pearl mussel, one of the most critically endangered bi-valves in the world. However, siltation and excessive suspended sediment concentrations (SSCs) have been attributed to causing their
decline. This has led to local conservation and restoration efforts being driven by the National Park over the last 20 years.Previous research has highlighted the Glaisdale subcatchment as a key contributor to fine sediment fluxes in the Esk catchment. Through local surveys, a critical source area of fine sediment supply to the beck was identified. This was a section of exposed, near-vertical, ∼ 3 m high channel banks ∼ 100 m in length consisting of unconsolidated sediments and overlain by shallow surface vegetation, which is regularly accessed by livestock. The availability of accessible material is also exacerbated by progressive movement of a large hillslope failure complex which supplies large quantities of easily eroded sediment directly to the river channel. Such failures are well documented in the North York Moors. It was deemed that this combination of factors limited the potential for success of traditional channel margin stabilization approaches. Following consultation and the presentation of available options, the competent agencies decided the most appropriate course of action was to divert the existing channel away from the toe of the large hillslope landslide,
and re-establish the stream course further to the north. The impact of this management was considered from a biotic and geomorphic viewpoint through the collection of data between 2007 and 2014.
Works included: Diversion of the existing channel away from the toe of the large hillslope landslide.
Monitoring surveys and results
From the analysis of over 2 years of river flow and in-stream sediment concentration data prior to, and following the diversion of Glaisdale Beck, it is clear that the sediment transfer regime has become more restrictive. This is evidenced by
- Reductions in median suspended sediment concentrations from 35.19 to 18.98 mg/L,
– 5 % reduction in flow-weighted mean sediment concentrations.
– Negative trend in sediment concentrations.
- Dampened response of sediment concentration during periods of high flow.
Although direct monitoring of the hydrology and sediment dynamics at Glaisdale beck was concluded in 2009, 2 years after the channel diversion, the longer term development of the site was observed through site visits up until 2014. During this period, continued erosion in the form of a headward migrating knick point (visible as a step in the river bed) has resulted in a progressive wave of channel instability that has migrated upstream. This is the response to over-steepening of the channel gradient in the vicinity of the original channel diversion.
Due to a lack of appropriately engineered grade control (drop) structures in the engineered reach this has resulted in channel bed lowering, bank undercutting and lateral bank failures upstream. During this time extensive bank erosion and channel widening occurred. At this particular site, erosion was evident only 2 weeks after the initial diversion with the knickpoint migrating through the reach, lowering the bed elevation. In response, the banks started to slump. However, due to increased channel width the final phase of bank collapse resulted in a soil wedge at the base of the bank which appears to have protected the toe of the bank preventing further lateral expansion.
Lessons learnt
Channel reconfiguration can be effective in mitigating fine sediment flux in headwater streams but the full value of this may take many years to achieve whilst the fluvial system slowly readjusts.
The channel is continuing to adjust to the diversion with evidence of continuing local instability. It is therefore recommended that this approach to reducing the fine sediment flux of upland rivers should not be adopted as standard practice.
However, where significant modifications to upland channels are made, comprehensive in-stream monitoring and geomorphological assessments should be regularly conducted to evaluate the response of the river to the new conditions.
This research has highlighted the importance of ensuring appropriate controls on sediment release during in-stream works and effective installation and maintenance of grade control (drop) structures. If these measures had been rigorously applied the overall goal of reducing fine sediment flux through the fluvial system could have been achieved in a more timely fashion.
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