Thursday, 4 January 2018

Flood Alleviation Improvements to the Beaumont Sea Wall

Flood Alleviation Improvements to the Beaumont Sea Wall

Strong winds and high tides like those reported have frequently caused flooding in the Beaumont area. I remember last year, driving in and diverting up towards the Cannon as the water was washing along the road.

Since then work has gone head to what the media have referred to as “raising the wall”. This has been tested this morning. As BBC Radio Jersey reported, “New flood defences at Beaumont seemed to work this morning. It was the first test since the sea wall was raised in the summer”

But far from just "raising the wall", a lot of planning and engineering has gone into the project to look at the materials to use, the right height to prevent or minimise the risk of flooding, and the shape of the wall.

Here is the story of what was done, supplied to me by the Project Engineer at DoI, who has been extremely helpful in providing information which tells why was done, why it was done, and how it was done, and whom I would like to thank in this blog.

It has been taken from documents and responses given to me, but any significant omissions or errors in this précis are my mine.

Reason for the Plan

An increased frequency of severe weather events has led to a corresponding  increase in flooding at the Beaumont junction. This has in the past damaged a number of residential and commercial properties.

So the Department of Infrastructure investigated options to alleviate the issue. It came up with a scheme which was represents those measures determined to be the most suitable for flood alleviation and management of the site.

The main cause of the flooding is sea water overtopping the seawall between the Perquage car park and Gunsite Slip.It happens when a southerly storm coincides with a high tide waves overtop the seawall east of the Gunsite Slip. The seawall in this area is relatively low and has no wave return recurve. Flood water then drains along the promenade, or La Route De La Haule (A1), to the low point at the Beaumont junction.

The relatively low sea wall height in this area and a lack of a wave return recurve leads to overtopping of the wall when periods of inclement weather coincide with high tides. As sea levels rise and if the frequency and intensity of storm events increase due to climate change it is likely that the flooding at Beaumont will become more frequent.

Furthermore, when waves are thrown over the wall during storms and strong winds they also contain debris and seaweed that very quickly block up existing drains and make the flooding situation much worse.

There is also no separate surface water drainage system in the area. Any seawater reaching the Beaumont area must be pumped to the treatment plant. Not only is there a cost incurred in treating the seawater, the salinity damages microbiological treatment process which affects the effluent quality entering St Aubins Bay

The Plan

Engineering assessments showed that raising the sea wall can significantly reduce the amount of overtopping water getting on to the promenade and draining to Beaumont. The height of the wall was been carefully considered to provide maximum effect against storms, balanced with maintaining amenity and visibility for local residents and people using the promenade.

Community Involvement

Two public sessions took place on 14th September 2015 in the vicinity of the scheme. 20 to 30 members of the public attended the sessions and viewed posters which outlined the causes of flooding at Beaumont Junction, the advantages and disadvantages of the options considered, the details of the proposed scheme and the programme for submitting a planning application.

In addition, 100% of those that completed a questionnaire commenting on the scheme agreed that the issue of flooding needed addressing. 85% agreed that the scheme would improve the situation

Existing Wall

The seawall itself was built as a coastal Pansermauern (anti-tank wall) during the German Occupation and is now over 60 years old. The wall was originally constructed for military defence not as a flood prevention measure therefore does not currently have a wave return recurve.

The total length of the wall to be raised is from the Gunsite Café slipway heading east for 330m which will take the new wall beyond Le Perquage Car Park to the first part of the seawall with a higher construction

Proposed Improvements

The existing seawall was in need of repair. There are sections of spalled concrete along the face of the wall, and cracks can be seen along the back of it.

The quality and integrity of the existing German sea wall was investigated to ensure that the concrete can be used or recycled to take the additional height and wave pressures for the longer term. Concrete cores were been taken.

The engineer commented that: “Once we started work on site with the contractor Geomarine, the existing concrete was found to be generally in better condition than expected.

Recurve Wall

The key engineering strategy is to use a recurve wall. This, along with a raised wall, is so shaped to deflect the waves back into the sea thereby reducing the frequency and volume of overtopping onto the promenade. This is commonly called a recurve wall, although the shape of the wall on this occasion has been designed to match the existing shape found adjacent to the Gunsite slip. This is a common shape of sea wall found around the Island and is similar to modern sea wall construction throughout the world.

Increasing the wall height by 0.3m  provides better storm protection but will not stop the flooding completely from occurring at Beaumont during an extreme storm coinciding with a high tide. But it will reduce the amount of overtopping that occurs during storms, and so should reduce the frequency in which it occurs.

As per the illustration, the he top section of the wall and part of the face were removed. Stainless steel reinforcement bars were anchored into the remaining wall. Stainless steel was specified for its resistance to corrosion. A cost analysis showed that over the lifetime of the wall its initial higher cost will be offset by increased durability.

Formwork was be constructed to form the shape of the angular recurve. The new wall was be formed by pouring concrete in 14m sections one at a time. A ‘formliner’ (specialist formwork lines) was used in order to increase the durability of the new concrete surface. This has been used successfully on sections of the St Ouens Bay sea wall. It provides a fine matt textured surface with increased hardness and density. It also increases the concretes resistance to chloride and carbonation ingress further protecting the reinforcement.

Design and the Occupation Heritage

The design of the concrete ‘add-on’ shape to the top of the existing wall is also to resemble the angular shape of the German WWII top of wall at the Gunsite Café and at the top of the wall at St Ouen. This is rather a nice aesthetic touch in the design, so that it is pleasing to the eye as well as functional.

Future Proofing

Whilst not part of the work, an allowance has been made in the design of the proposed wall to allow it to be raised a further 0.3m in the future (to 9.0m AOD.) This was achieved by providing threaded reinforcement bars which can be connected to at a future date. The threaded bars were protected by wax and a plastic sleeve and cast into the concrete wall.

Engineering Analysis

Raising the sea wall can significantly reduce the amount of overtopping water getting on to the promenade and draining to Beaumont. The height of the wall has been carefully considered to provide maximum effect against storms, balanced with maintaining amenity and visibility for local residents and people using the promenade.

A LiDAR survey was used to determine the wall height and beach slope. LiDAR is a surveying method that measures distance to a target by illuminating that target with a pulsed laser light, and measuring the reflected pulses with a sensor. The overtopping calculations were carried out in accordance with the EurOtop Wave Overtopping of Sea Defences Assessment Manual for the existing wall height of 8.4m AOD, and for a proposed increase in wall height of 0.3m up to 8.7m AOD.

The engineer also noted this:

“A wave return recurve was added to the wall. Whilst its effectiveness is wind dependant, calculations showed it should reduce overtopping by 30% alone. By increasing the height of the wall and adding the recurve we estimate the effectiveness of the wall to overtopping has been increased by 70%. The angle of the wave return recurve was not critical to its effectiveness. To limit the walls visual appearance we chose to follow the profile of the existing wall at the Gunsite Café which had a return recurve.”

Building Materials

Grade C40 concrete was used to extend the wall. This was available locally and was chosen for its durability, and to match the strength of the existing wall on which it was placed. Cintec anchors ( were used to anchor the new construction to the existing. The top of these anchors are threaded which will allow the wall to be raised in the future if necessary. The design was carried out by Arup Rothwell and DfI Engineers.

As can be seen from the picture taken in the recent storm , the recurve is very effective at diverting waves and wave energy back out to sea.

It should be noted that final work is yet to be done. Part of the wall is still to be addressed, and this older section will be dealt with as part of phase 2, which will be constructed starting March 18. Given the success of the works so far, this has proven itself as real success story for the Department and there is no reason why phase 2 should not also be as successful as phase 1.

As the project engineer noted: "The construction was completed according to the program and on budget, it took approximately 12 weeks to complete."

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