Proposals for management works on the saltmarsh between Felixstowe Ferry and Kings Fleet on the River Deben, Suffolk.
This is a large area of fringing saltmarsh that, although at present in a relatively healthy state, is showing signs of stress that if not addressed within the very near future, will inevitably lead to its break down.
The pressures this site is subject to are:
Sea level rise is a significant contributor to all of the symptoms for change identified within this paper. There is evidence that the saltmarsh is not accumulating enough sediment to keep up with incremental sea level rise, with the effect that some mid range vegetation such as saltmarsh grass, sea lavender and sea purslane are giving way to pioneer species such as Cord Grass, Samphire and Sea Aster.
The marsh is separated from the river defence wall by a continuous borrow]dyke channel. The remaining link between the saltmarsh and the flood defence wall has disappeared within the last 50 years, allowing the tide to flow through uninterrupted. As a consequence the saltmarsh has become substantially eroded along the entire fringe of this channel, increasing the volume of water passing between the flood defence wall and the saltmarsh on the tide and therefore exacerbating the effects of scour. The erosion is greater towards the outfall from Kings Fleet in the northern section, where the saltmarsh has become dissected by enlarged and converging channels to such an extent that in parts it has diminished to a vestigial form of freestanding banks and isolated islands. In recent years the outfall for Kings Fleet has been converted from a sluice to a pumped system with the consequence that water is evacuated at a higher velocity than previously, promoting further scour and enlargement of the channel complex in the immediate vicinity, drawdown of the saltmarsh and deepening the drains. This in turn promotes greater tidal ingress and incrementally increases erosion. The sheet pile shuttering that directs the water from the outfall into the channel that drains into the river is collapsing and the adjacent saltmarsh is washing out.
Although the system is still operable, this is to the detriment of the stability of the surrounding saltmarsh and if left alone will cause erosion back to the base of the flood defence wall. Due to increased flow within the site, all of the channels draining the marsh towards the river in the northern section above the location of the original land bridge have enlarged at their heads. As a consequence they have increased in depth and are progressively enlarging along their entire lengths. In this area there are two are two lagoons marked on the current Ordnance Survey Map:
these are no longer standing water but drain separately towards the river and as a consequence are subject to regular tidal incursion via their drainage channels. These relict lagoons are lower than the surrounding marsh and have reverted to a pioneer vegetation stage.
The entire frontage of the marsh is subject to erosion, many of the banks are in a state of collapse and the mouths of the drain channels are progressively widening. The leading edge of the cliff top of the marsh has lost a great deal of vegetation, which could be the result of tidal scour, wave action or boatwash.
This is most evident where the cliff height between the foreshore and the crest is at its greatest (+]1.5 m) in the northern part of the saltmarsh. Although the same pattern persists along the entire frontage, this is not so extreme towards the Felixstowe Ferry end because the foreshore is higher with a distance to the crest of +]0.5m due to the protection afforded by the Horse Sand Shoal.
The lower region of the marsh near Felixstowe Ferry is in a healthier state with a more consistent range of saltmarsh vegetation and has a slightly higher elevation than the upper end. However there is an increasing amount of cord grass growing in lower lagoon sections that could be an indicator of die]back of middle range saltmarsh vegetation and general failure to keep up with sea level rise.
There is now only one small sluice emptying into the lower creek from the golf course. As a consequence the main cause of erosion at the lower end appears to be tidal water emptying from the enlarged channel systems within the marsh.
The main flow through the site on the flood is north]south and the ebb south-north due to the deeper channel system at the upper end of the marsh and the shoaling channel between the saltmarsh and Horse Sand. There is a natural watershed point for the inside channel 30m south of the derelict wooden structure placed where the final piece of marsh formed a bridge across to the main body.
The sediment budget for the site is very small and there is little scope for sediment recharge within the immediate vicinity, therefore sediment
conservation and capture must be key to any management strategy. The area immediately inside the entrance is flood dominant. The incoming tide passes on the Bawdsey side, dropping its heavier tidal]borne sediments on Horse Sand Shoal before passing the into the marsh area as a less dynamic counter current.
The lighter sediments on the foreshore of the saltmarsh could be tidally borne, however, there is also a strong possibility that this could be the product of silt washed out of the marsh.
Any effort to capture tidally borne sediment must depend upon an understanding of the currents that bring it in and decisions over the retention of
material carried out on the ebb must safeguard against the risk of it being lost to the system.
It is our opinion that the outfall from Kings Fleet has had such a detrimental effect upon the saltmarsh that any management project must include a strategy for mitigating the impact of frequent and prolonged introduction of fresh water into the system at a velocity beyond that normally expected from a sluice outfall.
The installation is jointly managed by the IDB and EA. We suggest that it is essential to the success of a project on the site that these two organisations accept responsibility for managing the impact of the outfall by either altering the installation or attenuating the effects of increased flow at higher velocity.
Since this is a fringing saltmarsh, it does not have the capability to roll back. As a consequence, its loss will not only be a net loss of habitat, but will also have a direct effect upon the resilience and sustainability of the flood defence asset.
If no management strategy is put in place, the effect of increased frequency of pumping operations and associated incremental enlargement of the tidal channels will erode the remaining areas of isolated saltmarsh back to mudflat across entire northern half of the site.
The inside channel will enlarge and erosion into the body of the marsh from the back will eventually join up with the heads of the drain system, when the eventuality of tidal ingress from front to back as well as north-south between the defence wall and the marsh will cause further dissection of the marsh.
The drain channel system through the entire marsh will enlarge and join up with the lagoons turning them into tidal features and lead to further loss of sediment and reduce the range of saltmarsh vegetation. This in turn will lead to further enlargement of the drains widening and deepening their channels with the result of greater velocity of water movement and further collapse of the banks.
Since the source of new sediments in the system is limited, accelerated loss of material from the marsh will increase the amount of flow through the site and therefore the likelihood of it being borne out on the tide and lost to the system.
The certain disintegration of this fringing saltmarsh will be a significant loss of intertidal habitat plus it will expose the flood defences to increased tidal pressure where currently it forms a first line of defence.
This is therefore not a recommended option.
Manage the impact of the pumped outfall system:
This could be more than one option depending on whether it is considered economically justifiable to alter the configuration of the pipework. At present the proposed engineering solutions to the collapsing sheetpile gulley address the efficiency of managing the outfall but not its impact upon the fabric of the saltmarsh. Accepting the unlikely event of a radical reconfiguration of the engineering, it will be necessary to consider mitigation through the use of a soft engineering solution both in the immediate vicinity and within the channel that carries the freshwater out to the river.
The channels that feed and empty the saltmarsh should have permeable barriers of willow bundles, coir and hurdles to their crest level to reduce the dynamic of tidal exchange into highly eroded area. The frontage of the saltmarsh with the greatest potential for scour must be lined with similar material to reduce impact and reduce loss of sediment. There is a single substantial channel feeding laterally into the drain through which it will be necessary to reduce flow. This could be affected by placing a sill across the entrance to retain silt and possibly further structures to interrupt flow within the body of the channel.
We would like to consider the possibility of a short training wall leading from the downstream corner of the marsh at the end of the channel where it meets the tidal estuary. The intention would be to alleviate the pressure on the edge of the marsh at this point and the frontage further downstream.
We consider the negative impact of the outfall in its current configuration to be a responsibility that must be acknowledged by the IDB and EA and expect logistical and practical support to provide mitigation. This measure would only manage the impact of the outfall upon the intertidal habitat. It would have an immediate effect upon the flow of the tide through the marsh, but considering the size of the tidal prism, as a stand]alone solution it will cause further channels to develop and eventual failure in the system. As a consequence, our conclusion is that this cannot be considered in isolation from a wider suite of solutions across the entire saltmarsh. But it is an essential precondition to a successful management strategy for the site.
Manage tidal flow through the site:
The most immediate cause of loss of saltmarsh is the north-south tidal flow through the borrow dyke.
- There is a watershed point approximately midway along the channel where the depth of water at high tide is at its shallowest and where the last of the ebb flows both north and south. This is approximately 30metres south of the point where the saltmarsh had once joined the land. Since pattern of erosion of the marsh is not complex at the point of the original link, it would be an appropriate location for a permeable barrier of willow bundles, coir and straw bales up to and above the crest level by approximately 30cm and extending laterally over the bank and the marsh to limit the potential for scour around the edges.
- A similar but more resilient structure should be placed at the northern end adjacent to the sheet pile shuttering for the outfall to close off what has become a dynamic channel system. This should extend in front of the area of marsh most immediately vulnerable to the effects of the high velocity of flow during pumping operations and should continue along the drain channel as described in the section upon managing the pump outfall.
- A further barrier at the southern end of the dyke: this need not necessarily be to the height of the crest of the saltmarsh but needs to be above the height of the bed of the dyke where it is at its shallowest at the centre. As such it would form a permeable sill structure where the intention is not so much to manage tidal ingress but more to retain sediment.
Many of the channels that drain towards the river have become enlarged and some also extend close to the eroded areas at the back of the site. This is more pronounced towards the northern end where the heads of the channels have become enlarged to the extent that they hold a large volume of water and promote erosion on the ebb. In order to limit the threat that the drain system might join up to the eroded area, it will be necessary to establish a strategy to control the amount of tidal exchange in the most extensive channel systems and to retain sediment.
- Place a sill of willow fascines and coir inside the mouth of the particularly enlarged channels.
- Place further sills at the points where the channels split.
- Where there is an enlarged cavity at the head of a channel, reduce its capacity by filling it with straw bales or placing a bund across the channel at a point before it widens.
Address surface flow of the tide across the site where feasible, particularly where tidal lagoons have developed channels and are now draining on every tide.
- Observe the direction of flow of the tide across the site and fix lines of straw bales to impede further loss of sediment upon the ebb.
Manage erosion due to tidal and wave energy across the front of the marsh:
Although this saltmarsh possibly owes its survival to the protection afforded by Horse Sand Shoal and the diversion of both the ebb and flood towards the Bawdsey shore, there has nonetheless been steady loss along the entire frontage as evidenced through comparison of OS material from 1926 to present day, Google Earth imagery from 2007 and contemporary GPS and LIDAR surveys. A system to attenuate tidal and wave energy may be best placed at the northern end of the site where the cliff height is at its greatest and where it does not benefit from the shelter of Horse Sand Shoal. In the first instance a geogrid and brushwood fence structure extending from the downstream corner of the outfall channel could serve as a training wall to mitigate the combined impact of the pumping operation and the ebb tide and create suitable conditions for accretion of sediment at that point.
If the intention is to retain the saltmarsh, do nothing is not an option, since this will lead directly to its disintegration from the combination of the unmitigated impact of the outfall for Kings Fleet, increased tidal flow through the site, the failure to keep up with sea level rise and incremental loss of material across the front of the site through tide and wave action.
Kings Fleet Outfall:
For any management scheme to succeed the destructive effect of the pumped outfall must be contained. If it is not considered justifiable to find an alternative engineering solution for this it should be recognised that there is a responsibility to alleviate the damage caused to the saltmarsh by direct impact and by drawdown. We would seek collaboration with EA to agree a management strategy for the saltmarsh in the immediate vicinity of the outfall and its main drain channel. Containment of the negative impact of the outfall must be a priority in the overall management of the site.
This should be considered a necessary measure combined with the management of the impact of the outfall. The amount of tidal energy in the channel systems will ensure the failure of any attempt to contain the effects of the outfall and vice versa.
This proposal would certainly include the management of the borrow-dyke and those drainage channels that threaten to break through or have enlarged to such an extent to threaten the integrity of the surrounding marsh.
Management of surface flow across the marsh would benefit from further research, however where there is evidence of slumping and loss of material and vegetation it might be advantageous to break up flow and prevent loss of sediment through direct run]off. I suggest a simple experiment with lines of straw bales in some of the most affected areas.
Tidal and wave energy:
Certainly there is strong evidence of loss of saltmarsh material through tide and wave action across the entire front of he site. At the northern end where both cliff height and exposure to wave action are greatest, it would seem advantageous to use a wave screen to deflect both tidal flow and wave energy. However since erosion is consistent along the entire frontage an independent scheme to build up the foreshore using willow fascines would be most desirable but possibly too expensive and in the first instance too much of a diversion from the main aim to control the effects of tidal movement through the site. There could be benefit derived from experimentally managing some of the foreshore in this manner where the impacts are obviously greatest.
We consider it essential that a scheme to mitigate the impact of the outfall should be carried out in tandem with management of tidal flow through
- We propose the installation of a wave energy attenuation screen at the top corner of the marsh to manage tidal and wave energy and to act as a training wall for the outfall channel.
- We would like to explore the potential of controlling surface movement of the tide in areas that are at risk of further depletion through loss of vegetation and sediment through a low key intervention by staking in lines of straw bales. This will need further discussion and research.
- We would like to explore further the use of willow fascines to reinforce and stabilise the foreshore and alleviate the effects of erosion across the frontage of the site.
- We need to clarify the movement of sediment into, through and from the site in order to satisfactorily to establish a balance between encouraging tidally borne sediments to accrete, whilst discouraging further loss of material from the site through erosion.