Lowthorpe Beck 2018

Channel Modifications

The removal of the Lowthorpe Mill Sluice in 2015, although very successful, created an unnaturally steep bed gradient.  Over the following two seasons this section was monitored to see what natural geomorphological  influences would affect the flows across this section of the Beck.

It was believed that the stream bed was clay lined as was the practice on many water mills in this area. The clay could be seen on the stream bed and it appeared that it was breaking up.  As the survey proceeded upstream this broken clay disappeared and was replaced by a thin gravel layer. To ascertain the levels of this material a study was undertaken during the winter of 2017 where several small sections were excavated to enable the makeup of the material to be acertained.

It was found that the gravel layer was only seen on the surface of the stream bed and below this consisted of very hard boulder clay. The maximum depth of the excavation was 0.75 metre with no change in the material found.

The original plan to import gravel was dropped as it was thought unlikely that this material could be stabilised and would be quickly washed downstream.

The final project was decided on with the approval of the owners & fishing interests. It was agreed that two meanders would be installed at this site to slow the flow and create deeper areas which would increase fish cover and flow diversity.

Photograph showing the over wide and straight section during a low flow period.  No cover and compacted gravel bed.

Aerial shot of the same section. Flows slightly higher but still poor cover, straight, featureless, even bed depth and very sparse compacted gravels.

New meander at the top of the project site.  The photograph is taken at a higher view point to enable the full length of the meander to be shown.  The stream course is about 12 metres longer compared to its original course.

The stream on the ‘outside’ (left bank) of the bend is 0.7 metres deeper than the right bank. It is accepted that the river will change over time depending on winter flow velocities.It is not thought that this deeper ‘manmade’ feature will change too much as this would naturally be the deeper part of a natural river bed.  

Prior to excavation any gravel presesnt at the site was stock piled at the upstream end of the site. Once the new meander had been completed the gravel was re-distributed and raked over by hand.

During excavation the material being taken from the left bank margin was all organic sediment and roots of the glyceria on which this bank was formed. This material was placed within the new coir log margin. This will form the basis of a strong marginal growth during next spring.

A coir geotextile was pinned to the leading edge to prevent erosion in the event of very high winter flows. 

An aerial shot of the second smaller meander site.

Second meander completed.  The material on this site was very hard clay and needed no bank reinforcement of coir protection.  The deeper water can be seen on the outside of the bend. Shown as a darker colour on this photograph.Old timber piles were found under the trees showing that the stream is being returned to part of its original course.

View from bottom of site looking upstream across both meanders.  As the material on the left bank of this meander is hard clay the bank profile was left vertical to create suitable habitat for Kingfishers.

Photograph of ‘sedimats’ installed across the bottom of the site to intercept any sediment caused by the ‘in river works’.