Lower Hawkcombe Stream: Opportunities to establish a more natural course

Natasha Todd-Burley, Geomorphologist and Anissia Halwyn, Eco-geomorphologist, are attending CaBA Fest 2017, hosted by the Severn Rivers Trust, in Worcester on 12 October.

Catchment Based Approach (CaBA) is a community-led approach that engages people and groups from across society to help improve our precious water environments. CaBA Fest 2017 will be discussing relevant work that has taken place over the last year and will discuss plans for the future. Projects, volunteers and images that inspire the work being undertaken will be celebrated.

Natasha and Anissia are presenting a poster at CaBA Fest on the re-naturalisation of the lower reaches of Hawkcombe Stream, Porlock. This work forms one strand of the much wider multi-objective Porlock Marsh Vision Project that seeks to take a less intrusive approach to future catchment and coastal management in the area, with more focus on working with natural processes. The Vision sets out a long-term plan for the Marsh and its management and was developed in consultation with key landowners, farmers, the local community and other stakeholders. It is based on the principle of ‘letting nature take its course’ with one of its aims being to explore opportunities to enhance the natural environment in and around the Marsh.

Hawkcombe Stream

Hawkcombe Stream and its wider catchment, is a very dynamic system and is capable of transporting significant amounts of sediment. Upstream of Porlock the stream has a steep gradient, creating a rapid increase in flow in periods of high rainfall. Downstream of Porlock the watercourse occupies a wider and flatter area, which is likely to have been an old alluvial fan. The channel, in its present form in this area, is straightened, embanked and realigned and this has disrupted the natural local hydrological and morphological regime.

Significant incision has occurred immediately downstream of Porlock. However, shortly before the stream enters the marsh a perched channel has formed above the fields, where sediment has accumulated. The lower reaches flow over highly erodible material which is likely to change dramatically after a major flood event. This means that the course of the lower stretches of the stream are likely to change at some point in the future, even if nothing is done.

In 2014, a Vision for Porlock Marsh was developed with the two main landowners – Porlock Manor Estate and the National Trust, together with Porlock Parish Council, Exmoor National Park Authority and Natural England. The Vision sets out a long-term plan for the Marsh and its management, and was developed in consultation with key landowners, farmers, the local community and other stakeholders.

The Vision is based on the principle of ‘letting nature take its course’, and one of its aims is to explore opportunities to enhance the natural environment in and around the Marsh. This includes a proposal to restore the Hawkcombe Stream to a more natural course and forming a freshwater river delta. Consultation on the Vision indicated that there was significant support for this.

Re-naturalisation

In January 2017, we were commissioned to explore the options for restoring a more natural form of the Lower Hawkcombe Stream, where it meets the salt marsh. The focus of the study is located on the National Trust land adjoining the marsh. The restoration of a more natural course of the stream could create new wildlife habitat and may lead to opportunities to improve visitor access. It considers how the course of the stream might change and what effects this could have further upstream.

In order to address the aims of this study we have undertaken the following tasks:

  • A desk based review of all the relevant existing assessments and investigations which have previously been carried out on the Hawkcombe Stream by various organisations.
  • A detailed site walkover and appraisal of the Hawkcombe Stream by a geomorphologist and ecologist from Hawkcombe village down to the salt marsh.
  • Identification and exploration of options to re-naturalise the lower Hawkcombe Stream.
  • Hydraulic modelling to explore the impact of the re-naturalisation options on sediment erosion, transport and deposition dynamics.
  • Assess the impacts and/or benefits of the options on existing local habitats (including salt marsh development), including any enhancement opportunities.
  • Assess the impacts of the options on local access routes (including South West Coast Path), local heritage and any built assets/services, together with any construction and maintenance considerations.

Project options

A series of project options were short listed for assessment. Although a range of options along the reach were investigated, the main focus was on those which only use the National Trust field. These options included

  • ‘Do nothing’ scenario, where the channel continues to erode the incised reach and become more sinuous (depending on future maintenance commitments)
  • Embankment lowering and new channel excavation
  • Embankment breach and overspill within the National Trust field.

An unweighted Multi-Criteria Analysis (MCA) determined that breaching the embankment to create an overspill wetland is the best scoring option. However, the study is intended only to provide further information on the potential options and what the potential impacts and opportunities are. There is no commitment to take forward any specific option as this stage.

As the Porlock Marsh Vision project is drawing to a close, the results of the study and the subsequent public consultation will feed into any future work undertaken as part of the National Trust’s Riverlands initiative, working with the local community and landowners. Further modelling work (including erosion and deposition dynamics) and an in-depth site assessment (including ground investigation, flood risk, ecology and heritage) would be needed to explore a preferred option in more detail, in order to inform a subsequent formal design stage.

Want to know more?

Contact Anissia Halwyn to find out more about this project. You can also find out more about our geomorphology/hydromorphology work in our other project highlights.



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