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. 2 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 3 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. 4 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. 5 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. 6 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 7 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. 8 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