Low-tech Process Based Restoration Workshop

Sam Austin, Science & Technical Officer

This recent British Society for Geomorphology (BSG) organised Cumbrian workshop, led by Prof. Joe Wheaton from the University of Utah, gave an excellent insight into practical aspects of restoring natural riverscapes. The two day event held at the Rheged Centre, near Penrith and on-site at a National Trust owned plantation near Bassenthwaite, provided an opportunity for river managers, trusts, regulators and geomorphologists to learn more about using beaver dam analogues (BDAs) and post assisted log structures (PALs) as tools to improve in-channel complexity and promote floodplain connectivity. We were able to put learning into practice by creating a series of ‘flow obstructions’ in the form of various wood structures in manmade plantation drainage ditches. The main goal being to improve water retention within the plantation to alleviate downstream flooding of settlements and farm pasture.

Based on initial site walk overs, groups selected homogenous stream regions, with narrow incised, steep banks to test our low-cost restoration techniques. Full channel PALs were constructed to promote flow back up and sediment deposition by layering natural woody deposits of varying sizes, held in place by thicker diagonal sticks pushed into the banks and bed. The messier the better, as increasing roughness slows velocities and encourages deposition, a great way to raise the stream bed level. Blow out of these small structures during high flows, will provide material for the creation of new entirely natural structures downstream. Bank attached PALs were used in areas where wood had fallen into the river, enhancing the erosional processes already started on the opposite bank.

BDAs, generally created in areas where beaver activity is already present, were employed to enhance naturally ponded stream sections. Their construction, albeit on a micro scale in this scenario, worked well to slow and in some cases stop flows. Vertical wood ‘stakes’ driven into the stream bed, provided the base for weaving suitable length sticks which were densely packed together with leaf litter and nearby silty bed sediment. Bed level, woven stick ‘mattresses’, parallel to the flow, created extra structural integrity for the pyramidal shaped dams and the level dam top would provide opportunity for downstream scour pools to be created in high flow events. Building multiple small scale structures within the system will not only provide material to the system for natural processes to take over from human intervention but also reduces the importance of any one structure. Due to the low flows experienced during construction, it will be really interesting to discover how the structures will move and change during high flows!

PALs and natural log jams

BDA in a naturally ponded stream reach

The manual of Low Tech Process Based Restoration (LTPBR) gives some great insight into practical river restoration for river systems where the main processes that drive system change are wood accumulation and beaver activity, more often found in the USA than the UK at present. The approach champions using local materials, stream power and bio-engineers (or mimicking bio-engineer activity) to reinstate natural processes to modified rivers. For process-based, UK specific restoration case studies the RRC’s Manual of River Restoration Techniques (MoT) and the recently published NFM manual are great resources. This manual attempts to provide low cost solutions and wood structure implementation detail which will initiate self-sustaining recovery of river systems. It builds on previous process-based river restoration knowledge, some dating back to the 1900’s. Whilst the manual is based on work carried out in the USA, the principles for creating healthy riverscapes, are relevant universally and echoed in the UK manuals;

  1. Rivers need space and water – rivers are dynamic systems and healthy rivers will change given space and time
  2. Structure (bedrock, trees, dams etc.) forces complexity, allowing habitat diversity and building system resilience
  3. Inefficient conveyance of water is healthy - creating a range of water residence times is key to mitigating extreme flood events

Process-based restoration principles summarised below provide a baseline for carrying out restoration work to re-create healthy riverscapes where wood is (or historically was) the main process driving change:

  1. Follow nature - messy is best and enhance natural features
  2. Quantity of structure - larger number of smaller structures is better than one large structure
  3. Use natural, local building materials
  4. Rely on natural processes
  5. Deferring decisions to the system – allowing change
  6. Self-sustaining systems are the solution

The RRC’s Manual of River Restoration Techniques (MoT) provides alternative, reinstatement of natural processes for river systems without wood being the predominant driver for change.

For me, the general take home messages I left the course with were:

  • Trial low-cost restoration processes on rivers in need! With permissions and considering your specific river. Particularly in first order streams, taking calculated risks (using best practice, advice and sound geomorphological and hydrological theory) can lead to greater water retention and the creation of new wet moorland/forested habitats in a relatively short time-frame.
  • Consider the processes you are trying to promote and choose structural elements that drive them. By focusing on and promoting the processes, the actual structures and their construction/location become less significant over time and a self-sustaining river is the likely outcome.
  • Use existing tree falls and natural wood build-up in streams to enhance flow impoundment.
  • Install small, frequent structures with limited lifespans for noticeable responses (particularly in small streams). This builds redundancy to the system – wood will inevitably move or rot away so expect to maintain your structures as indeed a beaver would. In the US, wood dams are given a 1yr lifespan in current legislation.
  • Mimicking beavers by incorporating ‘wood mattresses’, parallel to flow, will improve dam stability and allow biota to move between structures during low-flows.
  • Public response to natural eco-engineers (e.g. beavers) carrying out ‘river restoration’, is less contentious than human-led intervention in rivers. Maybe its time to revisit legislation on beavers in the UK to encourage the further roll out of their reintroduction to suitable landscapes.
  • Include local people in your restoration projects as much as you can – this encourages ownership and agency for the scheme and improves connected.

Note: ‘Riverscape’, I find, is a useful term to describe the landscape areas that concern rivers, including source inputs and a rivers ‘zone of influence’ namely their floodplain, or potential floodplain. As we are more acutely aware in this age of climate consciousness, a rivers floodplain is vital to improving rivers natural form and function, not least for attenuating flood events. The importance of floodplains has historically been overlooked during river engineering works, carried out for various purposes (navigation, infrastructure, land reclamation etc.) that ultimately contained rivers within channels to suit anthropogenic needs. Recognising the need to allow rivers to behave naturally, in a dynamic way is so important to improving the condition of rivers around the world today. This is no easy task and requires nuance and negotiation with stakeholders, changes in government policy and also education so people can understand the wide range of benefits that naturally functioning rivers can provide for humans and other species.     



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