River Isbourne works

River Isbourne works

            For a number of years, GWSR’s civil engineering department has been watching the River Isbourne as it passes under the railway to the east of Winchcombe Station. Over the years, it has meandered on either side of the large culvert through the embankment.  Although we call this a culvert, technically it is a bridge being a brick arch of 16ft span, 13ft high and 128ft long.

Upstream on the south side, it has been cutting into the embankment to the west of the culvert whereas downstream the bend of the river has been getting more acute and threatening our neighbour’s garden. In addition, the river has narrowed downstream as it has deposited silt on both sides. 

Earlier this year, David Symonds Associates were commissioned to prepare a report on the state of the river and its implications for the railway. The first part of this was a detailed survey on both sides of the railway including the depth of the water. This revealed that on the upstream side, the river had eroded so much ground that the stability of the carriage siding above was in doubt. In addition the neighbour downstream was getting increasingly anxious about losing his garden. Therefore David Symonds Associates were commissioned to prepare a scheme to safeguard the railway and our neighbours.

Upstream side with erosion on left and deposition on right of river.

Downstream side showing meander in front of neighbour’s fence.

            In an ideal world, the river would enter the culvert on a straight line and leave it on a straight line avoiding any bends that might lead to erosion. However that was not how the river was when the railway was built and it is not possible for such a course today without destroying our neighbour’s garden and possibly the back of his house! Accordingly the adopted plan is to return the river to its original course and strengthen the embankments to prevent meandering in the future. Designing such a scheme requires the cooperation and approval of a number of parties including Tewkesbury Borough Council as the river authority for the Isbourne. A wildlife survey was done which revealed that there were no bats in the culvert and whilst there might be crayfish in the river, this would not be an issue and the wildlife consultant would visit during the works to check. Unfortunately Himalayan Balsam was present on both sides of the railway. This is an invasive species and there are regulations for removing it which was done as an enabling works package by the contractor. Negotiations were also required with our neighbour, Winchcombe Reclamation, to the south east of the site to allow the contractor access. They have very kindly given permission for this access. After putting the scheme out to tender, Walsh Construction were awarded the contract and a start date agreed.

            All this process takes time with the result that instead of starting in late summer, it was late October before all the permissions and contracts were in place. The approved scheme utilises gabion baskets and mattresses, which are wire cages filled with pieces of hard rock. A membrane is placed behind and under the gabions to stop water eroding the soil underneath. Above the gabions the existing soil banks are being reinforced with a mesh membrane. Work started well with two large steel pipes being placed in the river on the downstream side to contain most of the flow and leave the sides as reasonably still water.

Flume pipes being installed with sandbag dams to encourage flow through pipes.

            This worked well for a time and then it rained. And rained. The river flow became too much for the flumes and Walsh went to Plan B: using sheet piles to isolate a section of the bank. The work resumed but then it really rained and the site was overwhelmed.

Sheet piles virtually submerged and our neighbour’s garden in peril.

Upstream the river was encroaching on the reclamation yard as well as the bank on our side.

At this stage, there was nothing to do but wait until the river level dropped and then seize each opportunity.

 

Completing the sheet piles on the NE corner

Once sheltered by the sheet piles, construction went well despite the very sticky clay and the interruptions of yet more rain.

Getting the stone cross the river.

Transferring the stone for the gabions became a game of pass the parcel with one excavator picking up the stone from the stockpile and handing it to the other excavator to place in the gabions. Watching from the top it was like a delicate ballet exercise! Perseverance paid off and the NE corner was completed.

NE corner complete apart from final landscaping.

Operations then moved to the NW side and the sheet piles were installed for this phase. This time excavation and construction was easier as there was no river in the way. Also the weather was a little kinder in that the rainfall was not as intense and the river level fell fairly quickly after each storm. By Christmas, the NW side was nearing completion as the site was secured for the holiday and the contractors took take a well-earned break after an extremely wet and muddy six weeks. Hopefully the weather will be better in the New Year

Work completed on the NE corner and underway on NW side.

Good progress on NW side and site secured for Christmas.