Advisory Visit River Cober, Cornwall

Advisory Visit by Bruno Vincent [email protected] & Nick Lawrence [email protected]

Key Findings

• The outfall through the Loe Bar should be considered a total blocker to upstream migration of fish. Conveyance of water from the Loe Pool is essential for the safety of the town of Helston and improvements to fish passage are unlikely to come at the expense of flood protection.

• The minor streams that enter the lake appear less altered than the main Cober and offer good spawning potential.

• The lake is clearly proving supportive for the coarse fish populations with prime spawning habitat. Improving the salmonid habitat in the river above should be considered a priority to allow brown trout to compete successfully.

• The lower section of the Cober has the appearance of quality trout habitat; however, the reduced flow as it transitions into the lake limits its potential. Such benign neglect should be applied to the entire reach in the form of light touch management.

• The upper 500m of the Lower Cober inspected is canalised and overly managed. Remeandering and introducing woody material to the channel should be considered to reconnect the lake and the upper Cober with better salmonid habitat and spawning potential.

• Any increased flood risk that remeandering is thought to bring could be offset by better connecting the river to the flood plain.

• Gravel introduction to form artificial riffles may be needed to renaturalise the reach and augment spawning options.

Table of Contents

Key Findings ...... 2 Introduction ...... 4 Catchment / Fishery Overview ...... 7 Habitat Assessment ...... 7 Loe Bar Outfall ...... 8 Penrose Stream ...... 11 Lower Cober ...... 12 Recommendations ...... 24 Loe Bar ...... 24 Smaller streams ...... 24 Cober ...... 24 Channel Restoration ...... 25 Making it Happen ...... 27 Acknowledgement ...... 28 Disclaimer ...... 28 Appendix ...... 29

Introduction

This report is the output of a site visit undertaken by Bruno Vincent and Nick Lawrence of the Wild Trout Trust to the RiverCober and Loe pool at the request of Laura David, Area Ranger of the Penrose Estate, National Trust. After recent fish surveys identified issues with the trout population, WTT were asked to conduct an advisory visit. The report covers a visit to the Loe Bar outfall structure, approximately 800m of walkover of the River Cober within the Penrose Estate and an inspection of the Penrose Stream. Normal convention is applied with respect to bank identification, i.e. left bank (LHB) or right bank (RHB) whilst looking downstream. Upstream and downstream references are often abbreviated to u/s and d/s, respectively, for convenience. The Ordnance Survey National Grid Reference system is used for identifying locations.

Map 1 Overview of the Penrose Estate and surrounding area, including the town of Helston to the North East.

Map 2 The Loe, or Loe Pool with the Loe Bar at its South Western edge and the outfall structure shown at pin 1 with approximate route of the culvert (Black line). SW 64336 24267

Map 3 The Lower Cober. Red lines indicate approximate ownership. Pin 2 indicating the downstream start point of the walkover (Downstream of this point was not accessible due to high water levels). SW 65286 26214.

River River Cober

Waterbody Name Penrose Estate

Waterbody ID GB108048001172

Management Cober Catchment

River Basin District Loe Valley

Current Ecological Overall status of Moderate ecological potential based upon an overall Quality ecological potential of Moderate and overall chemical potential of Good

U/S Grid Ref SW 65217 26717 inspected

D/S Grid Ref SW 65286 26214 inspected

Length of river ~800m inspected

Table 1. Overview of the waterbody. Information sourced from: https://environment.data.gov.uk/catchment- planning/WaterBody/GB108048001172

Under the Water Framework Directive (WFD), the Environment Agency classify the Cober as Moderate with specific issues such as pollution from rural areas, pollution from wastewater and physical modifications preventing it from reaching Good status.

Catchment / Fishery Overview

The Loe valley comprises a number of small streams (Penrose, Chyvarloe, Degibna, Carminowe, Nansloe) and the main River Cober, that drain into Loe Pool, Cornwall’s largest natural lake. A shingle deposit separates the catchment from free flow into Mount’s Bay and the Atlantic, forming the lake. The most likely cause of the deposit is wave action moving shingle from drowned terraces of the former river as the sea level rose during the Holocene Period. The combination of historic mining spoil and agricultural run-off have reduced the porosity of the bar as deposits of finer sediments filled in the interstices within the shingle. As a result, a former tin mine adit (tunnel) at the northern end of the bar was utilised to convey overflow through to the sea, mitigating for the loss of porosity. Historically, the bar was also ‘cut’ (last occurring in 1984) to prevent flooding in the town of Helston, two miles upstream. Loe Pool, or The Loe, covers an area of approximately 50 hectares, with maximum depth of 6 metres. The main River Cober rises at Nine Maidens Down, winding across Porkellis Moor and skirting alongside Helston to reach Loe Pool. The lower reaches were canalised in 1946 affecting the Penrose Estate reach and Council owned water through the town of Helston. The Loe Pool, and River Cober within the Penrose Estate are a Site of Special Scientific interest (SSSI), citing scarce habitat not found elsewhere in Cornwall, with rare species of higher plants, bryophytes, and algae, together with many rare and local insect species.

Habitat Assessment

Due to high water on the day of the visit, access to certain areas was not possible and visibility was greatly reduced. The walkover was conducted in three parts: 1. Visit to the Loe Bar outfall structure 2. Inspection of small section of the Penrose stream 3. Walkover of the Lower Cober above Loe Pool

Loe Bar Outfall

The outfall through Loe Bar has had recent improvements made by the Environment Agency (EA) during 2019 (Figure 1). A second relief culvert has been buried in the Bar to be used in an exceptional high-water event or if the main culvert becomes fully or partially blocked. Previously, occasional overpumping of the Loe Bar was required to relieve water levels in the lake.

Figure 1 Installation of the second relief flood culvert in 2019.

Figure 2 The inlet to the Loe Bar outfall structure.

The Environment Agency has supplied the following information in answer to specific questions about the structure’s fish passability:

“There is a theoretical route of passage, under certain flow conditions, for sea trout although in reality passage under most flows would be difficult. Passage would only be possible if the control structures at the top of the culvert were fully open and Loe pool was at summer levels. The control structure would restrict passage when winter levels are maintained.” • There are no baffles or resting zones within the culvert. • There is no artificial lighting or “Light Chimneys” along the route. Though alterations have been made in the past, the outfall structure appears to be a complete blocker to upstream trout migration. The theoretical comment above has too many ideal conditions attached for fish passage to be anything other than improbable. Downstream smolt migration is very likely but without a returning run of adult sea trout, the catchment is unable to benefit from their u/s migration complete with their larger and greater amount of eggs. The volume of water entering the sea was considerable on the day of the visit (Figure 3). These velocities appeared to be far greater than salmonid swim speed though impoundment by high spring tides could reduce part of these velocities. Not having visited at lower flows, it is not possible to say whether this is generally a limiting factor to migration. Baffling to provide flow diversity and rest areas within the structure may make upstream passage more realistic in lower to medium flows. The structure is approximately 360m long. Though research suggests that the darkness of culverts does not present a challenge to migrating fish, the studies found did not look at a culvert of anywhere near this length (c.80 m). It is very possible, though not proved, that negotiating an artificial structure, such as this, in complete darkness may result in failures by sea trout attempting to run up. The introduction of daylight “chimneys” or artificial light sources may encourage fish to pass through, but as there is little evidence to suggest darkness is a behavioural barrier and other issues may well exist that form a greater challenge, this cannot be explored further. The outfall (Figure 4) is screened to prevent debris from lodging in the culvert and causing obstructions. This appears to present another barrier to migration but less so than the other issues mentioned. On the whole, it is fair to assume that no sea trout run exists so recruitment of resident, non-migratory trout should be the primary concern.

Figure 3 The outfall culvert entering the sea (red arrow). SW 63984 24332.

Figure 4 The trash screen at the inlet to the outfall structure. SW 64336 24267.

Penrose Stream

A brief inspection of the Penrose Stream showed a relatively unaltered stream with a good diversity of flows, gravels and habitat. Trout of any size could utilise this for spawning. The Penrose as well as Chyvarloe, Degibna, Carminowe and Nansloe should not be written off as they good sources for trout recruitment, as this was by far the best habitat noted for spawning within the visit. Careful management and light grazing can continue but should be monitored to prevent overgrazing.

Figure 5 Penrose Stream, looking downstream toward the Loe Pool. SW 64305 25719.

Figure 6 Bridge over Penrose Stream showing varied flows and gravel sizes.

Lower Cober

Due to the bank-full conditions, it was not possible to walk the lower section of the Cober before it enters Loe Pool. This section, below pin 2, is described as being set aside by the EA for benign neglect. Viewing downstream (Figure 7), there appeared to be plenty of wood in the stream and an unkempt appearance that suggests good trout habitat. On maps it looks suspiciously straight, however its current course predates the other straightening performed upstream (Figure 8) so this may be close to a natural route.

Figure 7 Looking downstream from pin 2. The EA requested this be left 'unmanaged'. This benign neglect appears to have created good trout habitat, with fallen and overhanging willows.

Upstream of pin 2, the channel is entirely straight for at least 500m with raised bunds, probably constructed from dredgings, perching the river significantly above the floodplain. The entire reach is without notable features, being trapezoidal and straight with heavily managed banks on the RHB. The LHB is somewhat more natural with lichen-covered willows lining most of the way. This still had the appearance of over management with little overhanging the channel and nothing lying within.

Figure 8 1894-1914 OS map overlaid with contemporary base map. Neglected section (below red dot) follows a similar path to the old channel, unlike the valley above.

The RHB has a number of erosion points from dogs (Figure 11) accessing the river. Some of these are deliberate and others evolved. On a near featureless channel, these are creating a little of the flow diversity needed but this is far from ideal and just serves to illustrate the barren nature of the reach. As the water was too high to view the bed, ideas about its composition come from previous recreational visits by the author. Due to the dredging, the bed is unnaturally flat and deep. No gravels of any size, required for spawning, are noticeable with sand and fine sediment covering the bottom. With no known barrier to bedload transit upstream, this would suggest that the stream energies are too low to wash the fine sediment through, thus burying any larger gravels that have made it downstream. The low energies

are due to the lack of gradient caused by the dredging and the primary channel’s excessive width (plus impoundment as it approaches the lake). Such alteration gives the reach the appearance of a navigation canal, far from how it should be functioning. Figure 9 shows a section of the Cober above the town of Helston. Though not entirely unaltered (a bund has been created on the RHB disconnecting the floodplain), this is a significantly more natural stretch of river, the rippling water’s surface showing the speed and diversity of flow that the river should exhibit, a stark comparison to that witnessed on the reach examined in this report.

Figure 9 A section of the less altered Cober, ½ a mile upstream of the Penrose Estate.

At two points on the RHB, sunken pipes with non-return valves have been set through the bund to recharge the wet woodland during bank-full conditions (Figure 10). Thanks to the conditions on the day, they were witnessed in action. The volumes they were able to convey was significantly less than the breaches caused by dog runs (Figure 11), one of which produced an energetic stream running perpendicular to the main channel down a footpath and into the wet woodland. The level of the pipes is also higher than the water level in the carr/wet woodland, negating the need for the non-return valve, and indicating a redesign is needed.

The river running sideways into the carr may also be symptomatic of a debris dam downstream that was not visible due to our limited access. As the river enters the lake and raises its height, the level in the carr should rise to a relatively similar height, albeit with a little delay. For the river to be so much higher than the surrounding carr would suggest that something within the bunded channel (below pin 2) is restricting the flow and backing up the Cober. If this is the case, it is currently working well to recharge the carr habitat, but obviously presents a greater risk to flooding and quite possibly restricts movements of fish. This should be investigated and cleared if found. If conditions are safe to work in, pulling smaller debris by hand from the blockage will start the flow which in turn will take more debris downstream. Benign neglect is generally preferable in terms of habitat but with the slower flows as the river enters the lake, it comes at a higher risk of creating impassable accumulations of debris. If this develops within an unnatural, bunded channel the water has nowhere to go but up. De-restricting the channel by reconnecting the full capacity of the floodplain allows the flows to circumvent the blockage, reducing upstream flood risk and isolation of fish populations. This is an assumption based on the flows witnessed on a single day, but investigation of the channel should be conducted when safe for the benefit of the local ecology and the town above.

Figure 10 One of the non-return valves supplying a recharge to the floodplain.

Figure 11 The river channel flows right to left. However, the dog run (red arrow) on the RHB above the bridge is drawing off significant flows into the wet woodland. Notice the footpath centre frame flowing into the flood plain. Although a footpath, it had the appearance of a healthy stream with riffles and glides unlike the impounded flows of the main river.

The purpose of a bund is to prevent high flows from leaving the channel and causing damage. In this instance, it has required pipes to re-connect the river to the floodplain, in high flows, to correct the damage caused to the carr as it dries out. Restricting the flow unnecessarily in this way, artificially raises the height of the river and increases flood risk upstream. The bund is not only managing to increase flood risk upstream, due to backing up, but also reduce the volume of water finding the floodplain. A more natural solution to reconnect the flood plain would serve the river and flood plain better than the current system, while reducing flood water from backing up into the town above. Reconnecting the floodplain would preserve the carr/wet woodland habitat (cited in the SSSI) and increase water storage capacity, negating the need for such a severe two stage channel and the associated periodic dredging costs. The maintenance of a useable footpath is a requirement of the National Trust to provide recreational access, placing it on the top of the bund currently provides a greater number of days of access throughout the year. Breaking large sections of the bund would improve recharge of the carr and reduce flood risk upstream. To mitigate for the loss of foot path, a boardwalk could be built over the top of the broken sections. The top part of the reach begins to morph into the Council-owned section above. A more obvious two stage channel (Figures 13 and 14) is present and was wetted but not full. This exhibits most of the same issues seen lower down, with little diversity other than erosion from dog runs. The over

management of the banks becomes more apparent at this point. Grasses are the only bank side vegetation other than some sparse willow whips that have managed to make a stand. This is possibly down to the worry that such vegetation may take up precious room in the second channel, thereby decreasing capacity and increasing flood risk upstream. Encouraging more than just grass has a multi-benefit of reducing the stream energy, causing fine sediment to drop out on the bank rather than the river. Such fine sediments would normally be carried into the lake below. As eutrophication from agricultural runoff has previously been identified as an issue to the lake (and river), allowing settlement on predominantly dry banks should be encouraged. Stopping the strimming of these banks will allow diverse bankside plants and saplings to develop. Reducing the unhindered access for dogs should also be looked at in places to allow the development of a more diverse bankside, using sections of temporary fencing.

Figure 12 Looking upstream of the footbridge at pin 2 the river is straightened and deepened. The revetted 'dog run' to the left was actually flowing off into the wet woodland carr as the bunded channel is perched above the floodplain.

A rougher, more diverse bankside is obviously good for biodiversity, but also reduces erosion risk as the root structure holds the bank together better than shallow rooting grass. Marginal plants and trees also provide

cover for trout, helping them evade predation from piscivorous birds and refuge from higher energy flows due to the hydraulic roughness that trailing branches and roots provide.

Figure 13 Looking upstream from where the two stage channel begins to appear.

Figure 14 Looking downstream from the top of the reach, the Two Stage channel is wetted but not full. This area could be an ideal place to introduce meanders by moving sections of the second stage to the opposite bank, adding flow diversity without reducing the channel capacity.

The LHB could be improved in a similar way by deliberately hinging branches and even whole trees into the channel. Such deliberate work begins to emulate the benign neglect exhibited downstream, building

refuge, flow diversity and the associated scour that forces fine sediment out of larger gravels to create areas suitable for spawning. Meandering the channel is important to allow the river to function more naturally and develop better salmonid habitat. Though not the most ideal solution, it is possible to move sections of the second stage of the two- stage channel to the opposite bank (Figure 15). This would not reduce the capacity of the channel at all but would further increase the diversity of flows needed for natural river function. As water flows round the outside of a meander, it increases in speed, washing away sediment and creating deeper sections. Conversely, the inside of a bend receives slower flows allowing sediment to drop out. This process sorts and grades sediment benefitting fish, invertebrates and aquatic plants.

Figure 15 This sequence illustrates how moving sections of the second stage of the two-stage channel to the opposite bank has no impact on channel capacity while introducing bends in the river.

Figure 16 The wet woodland carr that covers the floodplain either side of the Lower Cober. Paleochannels filled with standing water.

The wet woodland still has many of the original channels (paleochannels) existing from before straightening and restraining of the river occurred (Figure 16). This suggests there is scope for river restoration involving reconnecting through the wet woodland, which may provide significant benefits to habitats and flood risk reduction. Further investigations into the feasibility of such a project are required, including:

• Initial discussions with Natural England (NE) regarding benefits to/ constraints of the SSSI • Flood modelling and options appraisal • Environmental assessments, in particular regarding historic mining industry deposits.

Public access could meander through the woods along the new river.

Figure 17 National Trust volunteers have introduced these woven barriers to some of the eroded dog runs. Formalised dog runs should be encouraged in specific places with the aid of fencing and the establishment of better riparian plants and trees.

Figure 18 The straightened Lower Cober with minimal bankside cover. The willows on the LHB are ideal to introduce into the channel through hinging and wedging techniques (see appendix).

Figure 19 A paleochannel in the carr with some form of leaky dam, presumably to help reduce drying out. Could be an ideal place to re-route the main river.

Figure 20 The footpath was recently moved back from the river’s edge. Protecting this area with fencing would maximise the habitat value that is already being encouraged.

Recommendations

Loe Bar There is little that can be achieved with the outfall structure through the Bar: it is built for flood prevention and not fish passage. However, free passage should always be the aim and continued questioning and examination of the structure may help in the future. Smaller streams Maintain their quality as spawning areas by keeping or even partially reducing the existing management and livestock pressures. These streams have the potential to add valuable trout recruitment. The Riverfly surveys already in place, and other Citizen Science monitoring programmes, are an excellent way to keep an eye on their health. Cober Initially the primary goal should be to introduce some woody material in the channel. Building in lateral movement is more expensive and complicated so should be a secondary phase but ultimately still essential. Remeandering is a kick start to more natural river function. As discussed above, this could be developed in a number of ways: • Transferring sections of the second stage to the opposite bank would be an immediate solution but would require heavy machinery • Introducing man-made structures in the primary channel to pinch the flows and begin the process of erosion and deposition that will let the river carve its own course • Full restoration of paleochannels (discussed below). Meandering will require consultation and bespoke permitting with the EA regarding flood risk. Though this should be pursued as a priority, work can be done to improve habitat in the short term. Hinging select trees and branches from the LHB will begin to provide the refuge that trout need. As the Cober is classified as a main river and sits within the SSSI, this will also require a bespoke permit from the EA but will be less arduous than for rerouting the river. The stakeholders are numerous and local residents will undoubtably have their concerns about alterations to the river. Testing the water with some small-scale woody material may be a good first step to gain trust and involvement. Pinching the channel with sections of timber or small brash revetments can be constructed with a little guidance. Their impact on flood risk is small, but the effects on the habitat immediately visible. Partnering with the Council to complete similar works in the park upstream will have a positive impact on improvements made within the Estate.

Channel Restoration The wet woodland still has many of its original channels (paleochannels) existing from before straightening and restraining of the river occurred. Preservation of this unique habitat would be a difficult obstacle as would the modelling required to satisfy that it would not increase flooding problems in Helston. However, such rerouting would be a near complete river restoration, allowing the Cober to navigate, unconstrained, through its flood plain and utilising it for its greater flood capacity. Environmental assessments would need to be undertaken to establish the deposits that the local mining industry would have undoubtedly developed through history. The effect on the SSSI may also be a barrier to restoration and this will need to be discussed with Natural England. Licences such as a transfer licence may also be necessary to take water from the main channel to the new one. Despite these hurdles, such renovation would create better and more natural habitat for all native species, especially trout. LIDAR (Light Detection and Ranging) digital terrain models show that that some of the original course still exists and intersects the new, straightened channel. Figure 21 illustrates a number of old channels within the flood plain that could be reconnected, especially a section (highlighted within the red circle) that intersects the current channel about halfway up the reach and reconnects close to pin 2 just upstream of the neglected reach.

Figure 21 Lidar DTM showing the current straightened channel and a series of paleochannels that could be reinstated. The area within the red circle intersects the Cober at both ends and could be a starting point to trial re-routing.

With the proven capacity of the current channel as a backup, flows could be gradually introduced to old channel, letting it wash through and create

itself. As the new river establishes, the bund between the intersects could be moved back into the channel repairing the flood plain. Should such a pilot be successful, the other less visible paleochannels could be explored and restored on a modular basis thereafter. The National Trust’s own project Riverlands is designed to help make sure rivers and catchments are clean, healthy and rich in wildlife. Such improvements are exactly what could be achieved through channel restoration projects like this.

Making it Happen

The WTT may be able to offer further assistance:

• WTT Project Proposal - further to this report, the WTT can devise a more detailed project proposal report. This would usually detail the next steps to take and highlight specific areas for work, with the report forming part of an Environmental Permitting Regulations application.

• WTT Practical Visit - where recipients are in need of assistance to carry out the kind of improvements highlighted in an advisory visit report, there is the possibility of WTT staff conducting a practical visit. This would consist of 1-3 days’ work, with a WTT Conservation Officer teaming up with interested parties to demonstrate the habitat enhancement methods described above. The recipient would be asked to contribute only to reasonable travel and subsistence costs of the WTT Officer. This service is in high demand and so may not always be possible.

• WTT Fundraising advice - help and advice on how to raise funds for habitat improvement work can be found on the WTT website - www.wildtrout.org/content/project-funding

• In addition, the WTT website library has a wide range of free materials in video and PDF format on habitat management and improvement: o We have also produced a 70 minute DVD called ‘Rivers: Working for Wild Trout’ which graphically illustrates the challenges of managing river habitat for wild trout, with examples of good and poor habitat and practical demonstrations of habitat improvement. Additional sections of

film cover key topics in greater depth, such as woody debris, enhancing fish stocks and managing invasive species.

o The DVD is available to buy for £10.00 from our website shop or by calling the WTT office on 02392 570985.

Acknowledgement

The WTT would like to thank the Environment Agency for supporting the advisory and practical visit programme in England, through a partnership funded using rod licence income.

Disclaimer

This report is produced for guidance; no liability or responsibility for any loss or damage can be accepted by the Wild Trout Trust as a result of any other person, company or organisation acting, or refraining from acting, upon guidance made in this report.

Appendix

Figure 22 Hinged willow on the River Test. Hazel, small willows and small alders can be hinged into a river, creating diversity of flow and in-stream cover for fish. The trees are hinged in a similar manor to hedge laying, where the tree is partially cut through at the base and laid into the margins. Chestnut stakes and fencing wire can be used to secure the trees in place. Willow will survive perfectly well even with 70% of the branches submerged; however, hazel and alder should be laid to retain much of the structure above water level.

Figure 23 Another example of a tree successfully hinged into the margins of a river to improve habitat diversity.

Figure 24 Lodged woody material, the most natural of methods to mimic naturally fallen trees, wedged in another tree to secure it with no other materials required.

Figure 25 A tree kicker cabled to an existing tree stump on the River Yeo. Kicker tethers should be as short as can be realistically achieved: apart from too much metal cable being unsightly and unnatural, the risks of the trees being stranded on the banks in flow floods are significantly increased. Hiring or investing in a hand winch would allow the kickers to be winched back toward the stump, reducing the amount of cable needed, which in turn will reduce the likelihood of the kicker being stranded on the bank after high flows.