The New Receptor Sites:
The two identified receptor sites for spoil are essentially dissimilar and although the materials have so far remained consistent across the whole project, each site imposes its own conditions upon the strategy for containing sediment.
1. Loompit Lake Site
Once reclaimed farmland, this is now an area of mudflat and vestigial saltmarsh of approximately 5 hectares bounded to the east by Loompit Lake, an artificial lake created behind the river defence wall, to the west by the yacht harbour and retained to the south by the derelict rock revetment of the failed defence wall. The site has a natural gradient, roughly on a northwest southeast axis; to the north it remains close to the level it was when it was farmed and rises to the wooded slope of the coastal plateau. To the west and adjacent to the marina there is an interesting sand bar formation that is evidently the fall out of material from the time that the spoil was discharged directly into a holding pen on the immediate foreshore and when heavier material was drawn down and deposited by the tide.
At the start of the project, the character of the mudflat was largely undifferentiated with channels forming to reflect the gradient of the site and exiting at two key locations: towards the centre of the redundant floodwall and at the furthest south east corner of the site where the fresh water overflow from Loompit Lake is located and where this combines with a freshwater spring from the north east corner of the site.
The principle vegetation on the site ranges from sea purslane halimione portulacoides and shrubby sea blight sueda fruticosa in the sheltered area between the sand bar and the marina to cord grass spartina anglica, sea aster aster tripolium and some purslane colonizing the higher levels of the old pasture beneath the woods. Where there is a mixture of salt and fresh drainage at the higher levels, there are stands of common reed phragmites australis.
1a. Response to the site:
This was based upon its perceived characteristics, which are the gradient and natural drainage of the site coupled with a need to prevent excessive loss of material via the two weak points in the derelict wall, towards its centre and to the southeast corner.
From the start it was realized that the project must remain sensitive to how the flow of the sediment responds to intervention, and to continue to act in an adaptive way over consecutive years. Since every year a considerable amount of material is deposited, it is predictable that some structures will become buried and new configurations of flow across the site will form. This in turn has prompted a conversation with the behaviour of the site on a year-by-year basis.
- Coir logs 300mm/3m
- Coir blanket
- Brushwood bundles 300mm/2m
- 1.2m softwood stakes
Build up the lowest drainage points in the derelict perimeter wall to approximate the average wall height.
Position the discharge point at the highest point of the site at its northwest corner. Stake down a coir blanket to prevent excessive scour due to the impact of high velocity discharge of spoil.
Construct a low holding pen of brushwood to contain heavy spoil and attenuate flow of liquid slurry across the site.
Manage the existing drainage channels using coir logs to hold sediment, divert flow and promote an equal distribution of material across the site.
1d. Responding to change:
In parallel with the incremental accumulation of sediment through the duration of the project, it has become vital to keep an adaptive approach. The use of materials has remained consistent and the principle of exploiting gradient and interrupting flow through the use of coir logs has so far worked well to distribute material evenly.
When, within the first two years, the original brushwood holding pen structure became overwhelmed with sediment, we raised its level to retain more. Over the following years we have raised the discharge point and moved it further into the site as a response to the accumulation of solid material above the crest level of the adjacent salt marsh.
Correspondingly, we have created a new brushwood holding pen further out in the site and concentric to the original one that had since become buried. Since the relative heights of the existing marsh and the bed level of the mudflat was no more than 300mm, we agreed that there would be nothing to gain by putting higher structures in place and that we should concentrate upon incrementally raising levels across the whole site to the point where pioneer growth might establish.
When we embarked on this project, we agreed that it would be long term and that there may be little appreciable change within its early years. However this is its fourth year and it is clear that the Loompit site has successfully retained sediment and that the strategy of making minor adjustments each year to facilitate an even accumulation of material has been successful.
Since the beginning of the project there are some new stands of cord grass on the higher levels close to the sand bank and a more considerable accumulation close to the discharge point. There is also some colonization by samphire salicornia perennis in the area downstream from the holding pen. Although the bed level of the mudflat has become close to the older colonized area, this is insufficient to promote colonization so far.
This site is a feeding ground for brent geese and other wildfowl such as teal and wigeon. It is both feeding ground and high roost area for familiar wading birds such as redshank, curlew and black tailed godwit. It is also worthwhile noting that little ringed plover have nested on the sandy/gravelly areas above and around the sandbank at the western boundary of the site.
1f. Future action:
For this site, so long as the sediments continue to accumulate and a point is reached where colonization by saltmarsh vegetation becomes more comprehensive, we recommend continuing with the current strategy. It is clear that the build up of sediments in some parts of the site will dictate the need to shift attention to other areas and also to shift the position of the discharge. This is in the nature of the project and is evidence that we are not working with “once and for all” solutions and that to enjoy any success it needs to be an adaptive and iterative process.
2. Levington Creek site
This differs substantially from the Loompit site: it is an open area of fringing saltmarsh backed by a sea defence wall and exposed to the effects of tidal scour, fetch and wash. This is an old and established saltmarsh site that has become severely degraded, particularly around its outer perimeter where it is severely eroded and suffers from loss of vegetation. Where it is most exposed it has become deprived of any resilience to wave and tidal action.
The system of the marsh is fundamentally consistent with that of any fringing saltmarsh: it has a typically dendritic pattern of channels radiating from large drains running into the body of the marsh. However, these channels have become complex and multiple where they are constrained by the flood defence wall; this is possibly due to amount of water carried into the site by the tide and the corresponding evolution of complex capillary structures to contain and dissipate tidal energy. This has most likely contributed to the incremental breakdown of the saltmarsh from the inside.
At first sight this is a well-vegetated marsh, but it is predominantly a monoculture of cord grass on both the outer and inner marsh with occasional sea aster. Closer to the wall where drainage is good, there are stands of purslane and standard saltmarsh succession clings where the marsh rises to the sea wall. Where there is low tidal energy the creeks have become well colonized by Samphire.
2a. Response to the site:
Given that much of the stress upon the site is caused from the inside by the sheer volume of the tidal exchange and the expansion of the channel system to contain it, we decided to start work from the inside outwards. It was agreed that the delivery point for the discharge should be via an existing drain in the defence wall just before the lagoon areas managed by Suffolk Wildlife Trust. This does not quite sit at an intermediate point between two major creek systems with the effect that drainage is inclined to follow the gradient to the south.
As at the Loompit site we needed to ensure that pumped sediments should be captured within the channel system to not only raise the overall level of the site, but also to fill the extensive capillaries of the creek system.
In a similar strategy to that applied to the Loompit site, we agreed to operate at a responsive level, preparing the site in the first instance to train the entrapment and flow of sediments and by adapting to its evolution over the following years.
Consistent with the Loompit site, we used a combination of brushwood bundles 300mm/2m and coir logs 300mm/3m, all put in place using 1.2m stakes. The brushwood bundles were only put in place as a holding pen in 2016/2017.
To absorb the impact of the discharge we fixed an 8ft/4ft ply board beneath the outfall.
Prepare the creek system with coir logs to entrap and train sediments before they reach the main drain channels.
Construct a holding pen of brushwood around the discharge point, to capture heavy sediments and to attenuate the flow of liquid slurry across the site for 2016/2017 and 2017/2018 seasons.
2d. Responding to change:
As with the Loompit site it is certain that some of the system will fill up with sediment whilst other channels may receive very little. This may be caused by nuances in gradient or simply that the flow develops a trend that favours one direction over the other. However the site has behaved as a result of each year’s dredge gives clues for how to prepare for the next year, whether it is favourable to install structures further out in the system or to manage those areas that the flow of sediment may have by-passed. In all events the aim is to work outwards from the holding pen to raise levels through the site as far as the main drain channels and eventually to lower the tidal exchange overall.
As the site has accumulated more sediment, it has become necessary to extend the holding pen and this year, 2017/2018, to raise it by a further layer of brushwood. During the campaign of 2016/2017, the level of sediment around the outfall rose by more than a metre. The result of this is that although the muds remained very soft most of original drain channels within about 40metres of the holding pen have become overwhelmed.