Environment
How will the scheme effect the management of Pennywort?
Floating pennywort is a fast growing invasive species of freshwater plant. It is well established in the south and east of England and is widespread in the channels that form the Lower Mole.
Floating pennywort grows in the margins of slow flowing watercourses and drains, forming dense mats of vegetation. These dense mats grow rapidly (up to 20cm per day) and can grow up to 15m out from the bank in one season. Due to the rapid growth of floating pennywort, it can quickly dominate a watercourse, restricting flows and pushing out native plant species. Oxygen levels in the water often become reduced and this can result in fish deaths. The plant also limits the movement of animals and boats, restricting the recreational use of the river.
The Lower Mole Flood Alleviation Scheme directs the majority of flood flows through the engineered channel. This is the river channel where Viaduct, Island Barn and Molember are located, called the River Ember.
We have a limited budget within our Environment Agency Area (Kent, South London and East Sussex) that is set aside for ongoing maintenance works, including the removal of pennywort. We prioritise this maintenance spend on areas at greatest flood risk. As the River Ember channel is essential for managing flood risk, our main focus on pennywort management is the Ember channel.
We work alongside the riparian owners along the River Mole and Ember Loop to raise awareness of and manage pennywort. During the past year a number of riparian owners along both of these areas of river have carried out work to remove pennywort. We will continue to engage with and work alongside riparian owners in the future to carry on the work that has been started to manage pennywort.
We have sprayed the pennywort in the past, though we favour its physical removal by hand pulling and offsite disposal as this is an effective way to reduce the strength of the plant. We have experimented with lowering the retained water levels within the river during frosty weather to expose a greater area of the pennywort plant to frost, which has been successful and we consider using this technique again in the future if necessary.
Floating pennywort thrives on ponds and lakes being a coloniser of still or very slow flowing wetlands and watercourses. Where flows are increased and more naturalised, we would expect the dominance of floating pennywort to reduce as it is out competed by species that favour a more natural riverine environment.
What does naturalisation mean?
In terms of this project, it is looking for where sections of the river could be restored to a more natural state. This could include allowing the river to naturally meander within the bed of the larger channel, planting areas of aquatic vegetation, and also changing its appearance from a hard-engineered concrete channel to a more natural-looking one where possible. With regards to options that propose the removal of sluice gates, this opens the opportunity to allow the river channel to adapt into a more natural state, with greater variability in water levels.
Will the scheme bring about opportunities for naturalisation and natural flood management?
The Lower Mole Flood Alleviation Scheme was primarily designed to protect property from flooding.
Natural flood management is typically implemented either in upper river catchments where water is retained in wet woodlands, bogs, in rural/parkland, or is encouraged to flow onto the floodplain to make full use of this area. Almost all of the floodplain around the Lower Mole is urbanised meaning there is very limited option for this here.
The current river channel could be considered as a slow flowing ‘canal’ of deep-water with limited diversity in flora and fauna. A naturalised meandering channel without sluice gates would vary in flow, depth and width throughout due to its generally shallower nature. A naturally flowing river with pool, riffle and glide habitats would offer a more diverse habitat for river species and an increase in fish populations and variety.
If the option to naturalise the area is selected, what maintenance would be needed, if any?
The options that propose the removal of sluice gates to offer more natural flows would likely involve a significant reduction in maintenance. However, we would expect ongoing maintenance to include cutting the grass on the channel banks, infrequent cutting of vegetation within the channel as well as inspection/maintenance of the sheet pile and concrete walls. This is required to ensure that the channel’s capacity is not reduced which could increase flood risk. For the options that propose the installation of fixed weirs or rock ramps, these would require a higher level of maintenance than if no gates were present, though this is expected to be less then what is currently required. The longer-term maintenance of the scheme will continue to be assessed as part of the overall appraisal process for the project.
Will groups such as the Wildlife Trusts, Greenpeace and Friends of the Earth be involved to advise on the most appropriate ways to provide naturalisation and wildlife enhancements?
We have been speaking to a number of wildlife groups and have shared our initial design concepts with them as part of the early engagement process for this project. We will continue to work with these groups as the project progresses and will seek their views and feedback. The ecologists and geomorphologists within the project team will use information provided by these wildlife groups to help when considering the potential naturalisation and wildlife aspects of any future proposal for the scheme.
If fish passage to the River Thames is being improved, will this result in a loss of fish from the River Mole to the River Thames?
As part of updating the scheme, we hope to open up over 20km of habitat to improve passage for fish. This will also improve fish populations and the general wildlife and biodiversity within the River Mole. At present, the structures along the river restricts the free movement of fish throughout the Lower Mole and also into the River Thames. This project offers an opportunity to investigate options for improving fish movement.
Improved connectivity will allow fish in the River Mole and River Thames to mix and create stronger, larger and more diverse fish populations which will benefit the fish, the fisheries and the wildlife throughout both catchments.
If the sluice gates are removed will invasive species from the Thames enter the Mole and impact on the river ecology?
Where we are opening fish passage there is a risk that invasive fish species could enter the Mole from the River Thames. However, the design for the options that propose the removal of sluice gates, is that the target flow conditions and habitats should be more suitable for native fish species.
It is unlikely that the presence of invasive fish species will significantly offset the benefits of improving fish passage along the Lower Mole.
Have you accounted for climate change?
Climate change is expected to make flood risk worse in the future. We include the impact of climate change in our appraisal. We do this by incorporating percentage increases in river flow into our flood risk modelling, mapping and other calculations. These percentage increases are taken from our national guidance which utilises the latest climate change projections.
Are there plans to control mink?
We usually control invasive species that potentially damage our assets and undermine our ability to carry out our regulatory duties.
Whilst we have general duties to protect and enhance biodiversity, we do not have specific duties for water fowl and would not be looking to manage the local mink population.
A long term, sustained and concentrated effort, involving a wide range of landowners and partners would be the only effective way to control mink along the Lower Mole.
What is the foam that we keep seeing in the river?
We are recording all reports of foam and are continuing to investigate its source along the Lower Mole.
Large amounts of foam can accumulate in certain areas of the river, especially downwind and within eddies, or just downstream of turbulent water, for instance below weirs. This foam is usually harmless; in fact only about 1% of the foam is made up of the foaming agent which is usually a naturally occurring fatty acid. Most of the foam is simply air and water.
Foam seen in a river can be a natural event with no adverse environmental implications, or it can be a sign of pollution. Most foam in rivers is natural and is produced when molecules such as fatty acids act as ‘surfactants’ interfering with the surface tension of water and allowing air and water to mix more easily.
Surfactants are molecules that act in a way which interferes with the surface tension of water. There are many natural and synthetic surfactant molecules.
These surfactant molecules often float on the surface of the water as a thin film.
Turbulence from waves, currents and wind cause the surfactant molecules to trap small bubbles that make up the foam.
Naturally produced surfactants, usually a fatty acid, are released from decaying organic material, but are also released in small amounts by living organisms.
Not all foam is natural, certain manmade products such as detergents can cause foam. Although it is difficult to know for sure, foam from various sources can have different characteristics. Natural foam may start off as white but often becomes light tan or brown in colour as it collects sediment and organic matter, and has a natural, earthy, fishy or fresh cut grass smell. Manmade foam appears white in colour, and as a fragrant, perfumed or soapy smell.
How does the Environment Agency measure turbidity within the river?
The Environment Agency measures turbidity (Nephlometric Turbidity Units - ntu) at a water quality sampling point on the Lower Mole. The instrument used for measuring it is called nephelometer or turbidimeter, which measures the intensity of light scattered at 90 degrees as a beam of light passes through a water sample. Our data shows there has been no significant change in turbidity since 2009.
What impacts will the scheme have on fish stocks and wildlife?
All the proposed options include the installation of fish passes, which will allow more fish species to migrate up and downstream. Currently, the river channel has a series of impounded areas of water that support specific fish populations. By removing the sluice gates and installing fish passes, the diversity of the fish populations would be enhanced.
A naturally flowing river with pool, riffle and glide habitats would offer a more diverse habitat for river species and an increase in fish populations and variety. The existing fish populations that prefer those slow, still waters would be able to find suitable places to live, either in the Thames, or in sections of the River Mole upstream of the Flood Alleviation Scheme.
We carried out fish and invertebrate surveys to understand what the current baseline conditions are and to give us up to date information. The survey findings will be reported in a Preliminary Environmental Information Report (PEIR) which we will make available when the Outline Business Case (OBC) is completed.
We will use the results of the latest fish and invertebrate surveys to help inform the options design.
Will the scheme encourage establishment of marginal aquatics, reed beds, digging offline ponds, and wet woodland?
The current slow flowing ‘canal’ which is the Ember in its present state has very limited marginal aquatic vegetation. A naturalised meandering channel would have a greater variety of habitat types including further stretches of marginal aquatics and backwaters.
Will there be improvements of shelter and access for wildlife, such as kingfishers?
There would be potential additional space to design and engineer features to benefit wildlife such as banks for sand martins and kingfishers.
Will there be improvements to assist the passage of fish and eels?
Fish will be able to move in both directions, into and out of, and between the River Thames and the River Mole. This is perfectly natural for all sorts of species of fish, many of which will travel for several kilometres in a day to find food, habitat or refuge. The expectation is that with improved habitat and free passage, fish populations in the Thames and Mole will improve and there will be more fish available for anglers to catch.
Will there be multi-species fish passes installed on the structures, ideally as bypasses rather than just attached to the main structures?
If correctly designed, installed, and maintained fish passes provide an effective mean for fish to migrate up and down stream. All fish pass designs are agreed internally by Environment Agency Fisheries Scientists. All fish passes will require regular maintenance to fully work, and this is likely to be a legal requirement if used by Salmonid fish species.
Would removing the gates affect the safety of the trees along the river bank, as they depend on the water level to keep their roots wet and embedded in the river bank?
A tree survey will be carried out to assess the impacts to trees as part of the options development process.