South East Rivers Trust

Hogsmill River Restorers 2019


Back in September we recruited a small team of River Restorers to help us design and deliver a small scale river restoration project which would help re-naturalise a stretch of the Hogsmill in Ewell – no heavy machinery involved, just people power!

But what does a natural river look like?

We started off with a workshop to learn all about Natural River Processes, how rivers have been modified over the years, and what techniques we can use to kick start re-naturalisation.

Our interactive EM-River model was a big hit! It shows the natural movement of rivers across their floodplain sped up to show 100’s of years of erosion and deposition in just a few minutes.

With all this in mind the team headed out along the degraded stretch we wanted to restore and saw the following problems:

  • Very straight channel – the channel was artificially straightened to alleviate flood risk; a technique now known to be flawed. This left the channel very uniform and straight with unvaried flow patterns and habitats.
  • Over-wide channel – the channel here was far too wide for the volume of water flowing along it. So the water was very shallow and slow moving.
  • Silty river bed – the low energy water meant that all the river’s natural gravels were clogged with silt. This is bad news for invertebrates living there and fish trying to spawn, because the habitat they need has been smothered.

We wanted to come up with a solution to these problems so developed a restoration plan together. We wanted to narrow the channel to a more natural width and introduce more varied flow, which will hopefully increase the stream’s power to help clean up the gravel bed and improve habitat quality for wildlife.

The techniques included in our volunteer’s designs included introduction of Large Woody Material (LWM), narrowing the channel by installing berms, adding gravel, and planting natural marginal vegetation – all great ideas!

Recording Change

In our second workshop we covered the importance of recording and monitoring changes we make. We learnt the MoRPh method of looking at the physical characteristics of the river channel, banks and surrounding land use which can be used to give an indication of habitat quality.

We used this method to survey 70 m of the channel we hoped to restore.

As you can see from the screenshot below, the results showed the habitat complexity of the channel along this stretch to be very low. We hope to see an improvement in this over time now that we’ve completed our project. You can see the full results of the surveys by visiting modularriversurvey.org/ and zooming in on the Hogsmill in Ewell.

Getting stuck in!

As with any in-river project we deliver, we applied for and were granted a Flood Risk Activity Permit. This gave us permission from the Environment Agency to go ahead with the work we planned as they were happy it would have no adverse affects on flood risk to properties in the area.

Now, with that in place, it was time to don some waders and get stuck in to delivering our restoration plan. We spent 3 chilly (but thankfully dry!) November days installing brash berms to narrow the channel and introducing pieces of LWM to create different flow patterns and hopefully encourage some deeper pools to form.

The results of everyone’s hard work and dedication can be seen below.

Day 1 – we narrowed the channel and stabilised some silt that had been deposited during the low flows the Hogsmill experienced in recent months. We hope that our brash berms will encourage more deposition of silt in this area and naturally vegetate so that the berm becomes a seamless extension of the natural bank.

The low flow channel created means that we have a faster, more natural, flow of water which will hopefully see the gravels along this stretch become less silty over time.

Day 2 and 3- we moved further upstream to where our volunteers had previously delivered some restoration work in 2016 (you can read all about that project HERE). Some of the berms and LWM installed then were working well, whereas other areas the channel needed further narrowing to create the stream power needed for natural processes to be reinstated.

Along this stretch we worked to extend the existing berms and introduced some nice pieces of LWM .

Just two weeks later when revisiting the site, we can already see the effects of our work taking shape; with a nice narrow meandering channel, with gravels that are starting to become cleaner already.

A MASSIVE thank you to our Hogsmill River Restorers Team, I look forward to seeing how this stretch continues to improve over the coming years thanks to your hard work.

Thank you also to Epsom and Ewell Borough Council for your help with facilitating the project, to the Lower Mole Project for assisting with the delivery and to Thames Water for funding this through our wider Water Catchments for Communities project.

If you’re interested in taking part in out next River Restorers Projects we have one on the Wandle and one of the Beverley Brook taking place in early 2020. Full details can be found on our Events Page.

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