Maintaining Navigation

A possible plan in relation to dredging

December 2008

Introduction

The public meeting at the end of September 2008 resulted in two actions

  • An investigation into the possibilities of dredging the shallows at the Webley turn and at Bryn Du should be conducted
  • Consideration should be given to minor works on the spit

The minor works on the spit are likely to be

  • To remove the recurve as this is likely to contribute to the silting of the channel to the pontoon
  • To regrade the tip – mainly to give safe access to the recurve

These spit activities are currently being drawn up in a separate paper for consideration by CCW in the first place and then to be consolidated into the business plan for the next 5 years for consideration by the Fairway Committee

This section is concerned with the dredging investigation, and with the business case resulting. The section does not cover the regulatory and legal issues likely to be encountered as these will be have to be covered separately and to a different timescale after the Fairway Committee have given their approval to the investment in principle via the business plan.

The section is divided into the following articles

  • Definition of the problem
  • Cost issues
  • Trials so far
  • Potential methods of meeting the problem

This is an intermediate note on dredging. There is no immediate conclusion at this stage in relation to the viability of dredging either in terms of the practicality or the business case, but the paper does outline the scale of the problem, and identifies one possible solution (Jet Pump) to resolve the problem.

To complete the business plan will require the following articles to be produced

  • Views of CCW on the proposed methods
  • Risks
  • Financial mechanisms for investment
  • Recommended way ahead

Trials have been organised to determine the capability of the jet pump solution, following which detailed discussions will need to be conducted with Jet Pump to determine the financial implications and risks of the different options before a final paper is produced recommending the way forward to the Fairway Committee

The problem

This section is divided into the following

  • Areas of interest in terms of dredging and the potential volumes to be removed
  • Possible sites to deposit the arisings
  • Financial constraints

The areas of interest

Areas requiring improved navigation depths are indicated on the LIDAR survey below in blue with other features in black

The need to keep the pontoon clear for fishing boat access can be met by normal shore based methods as the area is dry at low water.

The difficult areas are the Webley turn and the Bryn Du shallows where over a length of about 170 metres the water depth needs to be increased by about 0.8 metres. These can only be accessed from a floating unit

The quantitiy of sand to be removed will vary with the condition of the sand bar. One estimate concludes we might need to move 10,000 cubic metres, a minimum estimate is about 4,000 cubic metres on the basis of a channel 20 metres wide (allowing for a low angle of ripose of the sand)

The total volume to be removed at each site therefore estimated to be between 4000 and 10,000 cubic metres or 10,000 tonnes to 25,000 tonnes of sand each or a total for both sites of 20,000 to 50,000 tonnes

See annex A for the results of channel survey

Possible sites for the arisings

Four sites for the arisings can be envisaged

  • The foreshore between Nant y ferwig and Bry y mor.
  • Replenishment behind the low dunes protecting the Webley salt marsh
  • Along the beach infront of the sailing club
  • In the middle of the sandbank beween the two shallow areas

The distance to these sites is approximately

  • Webley turn to Nant y ferwig 700 metres
  • Webley turn to Dunes 500 metres
  • Bryn du to Nant y ferwig 600metres
  • Bryn du to Dunes 1200 metres
  • Both sites to the sand bank about 200 metres

The distance to the beach infront of the sailing club varies within the parameters above

Looking at each site in turn.

Nant y ferwig foreshore – This is composed of clay and mud with a gravel and grass overlay. Introducing sand to the area may be controversial as 20,000tonnes of sand will disrupt the eco system substantially. It is however a good site from the point of view of the pumping distance (almost equidistant from both sites) and from the retention of the spoil.

The practical difficulty of using this foreshore from the Webley turn are

  • The disruption to navigation that the floating pipes will create unless they are taken around the outside of the bend in which case the pie length will increase to something of the order of 1000 metres. Sinking the pipes might be an option
  • The mooring of the pipes in the strong cross current where they cross the main channel
  • The vulnerability of the pipes to damage from trees etc in flood conditions

Dunes -The dune system is currently severely eroded (see picture below taken in a high tide + storm condition). Potentially it needs building up to better protect the salt marsh. The deposition of spoil in that area might stand a better chance of getting the necessay approvals whist putting the sand in a place where sand is naturally occurring and by putting it inland from the current dunes it might stand a better chance of not being immediately restored to the sand banks.

The pipe length is long but there would be little disruption to navigation and the pipes would be generally aligned to the flow of the river thus reducing the mooring problem

Along the beach infront of the sailing club – This was the site of the material excavated to make the pontoon useable in the first installation. The sand very quickly migrated into the channel to the detriment of navigation

This site could be used if there was agreement to restrain the movement of sand by creating a tourist feature to the area using railway sleepers or similar on edge to create an undulating shoreline that gave shelter from the wind and encouraged the development of natural dune system. In the past such an agreement has not been forthcoming

Centre of the sand bank – This is a least good alternative. The sand will probably be quickly spread out by the tide and returned to the channel – possibly somewhere where it causes more problems than the shallows the dredging is designed to solve

The most likely place to put the arisings in terms of approvals looks like the dune system. The specification for any dredging system is therefore to

  • Remove up between 10,000 and 25,000 tonnes of material from each site
  • and deposit it at a distance of at least 600 metres from the dredge site

Any consideration of investing in dredging should take account of the risk that it will be

  • Abortive due to the large flows of sand in the estuary
  • Have unforeseen consequences leading to legal problems

HR Wallingford have produced a summary of the issues involved (annex B) and these should be a major factor in the decision as to whether to risk a considerable sum of money in any operation to reduce the shallow spots.

Cost issues

Noting the comments above in relation to risk, two sorts of funding have to be considered in judging actions on the river

  • Capital improvements
  • Revenue related activities

One off capital improvements have the potential to attract considerable grants (vide the £2.6m just spent on the river) and given the nature of the river, its special status as an SAC and the number of SSSI surrounding it together with the perceived economic deprivation within the Teifi valley there will be considerable potential for obtaining grants given a suitable case.

Indeed it could be argued that the £2.6m will not result in the improved economics expected unless there is an improvement to the navigation on the river

Whilst there may be large grants available for one off activities of a capital nature any repetitive actions to improve the river has to take account of the ability of the community to finance them since grants for such activities are not available. There is no possiblility that a one off dredging operation will permanently improve navigation.

A possible source of funding for such revenue related repetitive actions such as dredging may be the existing users above. Given that current users could agree on any such action (very difficult) the maximum surcharge that might be applied might be a 50% increase on current mooring fees, plus a contribution from the Boating club. At best this might bring in of the order of £15,000 to £20,000 pounds.

Slipways could be a source of income but the problems of economically collecting the income when the useage is essentially quite low has not so far brought forward a proposal that does not cost more to collect than the income gained

Pontoon useage might bring in some additional revenue but the 2008 season has not been a good one for judging potential incomes as the weather has been poor throughout. Perhaps a few thousand pounds say - at best say £5,000

In considering future work on the estuary therefore capital activities of up to £1m might be a reasonably be considered, but revenue activities should bear in mind that at the very best the limit will be of the order of £15,000 to £25,000 per year

Early trials in relation to dredging

ATFL have conducted a trial into what might be called trawl dredging reported in reference A which was done using a variety of trawl equipment from scallop dredges to chains. The result was unstatisfactory in that although the trawl deepened the channel by about 0.5 metres the deposition of spoil downstream of the trawl area effectively only moved the shallow part downstream. There was no nett benefit

The trawl initially brought up trees, sections of old boats, bundles of rope, and all manner of small items including high heeled shoes. At the end of the period the scallop dredge was bringing in large lumps of coal probably indicating that the 19th century river bed of had been reached.

The trial indicated that trawling could increase the depth of channel, but that the suspended sediment would very quickly settle downstream. Note that sand in still water settles 1m in between 5sec for coarse sand and 5mins for very fine sand. 1 knot is about 0.5m/sec. Therefore in a 1 knot current fine sand would sink a metre in 150m and coarse sand in about 2 to 3m.

It may be that the method might work with equipment designed for the job but only if the planned deposition point is relatively close to the trawl area

Potential Methods

Two methods have been looked at

  • A variation on the trawl method using purpose designed equipment
  • Sucker dredging with deposition by pipe to a nominated position

Wyre Marine have designed a rig that is portable and uses a shallow draft tug towing a trawl dredge designed for the task. The cost of a weeks hire of the equipment is over £6,000 with no guarantee that the system would help. The recent hydrographic survey failed to find any obvious deep points which such a trawl could deposit the arisings. It might just be possible to envisage the system for the Webley turn with the arisings being deposited on the seaward side of the spit, but this is more in hope than any real belief that the system would work

This leaves the sucker dredge as the only option so far likely to work. At least one UK company (Jet Pump) has developed a relatively low cost machine which uses the venturi effect to entrain sand into a discharge tube. There are no moving parts in the water, and as result it is about as benign as such a vigorous process can be (live fish have been through the pipes successfully!)

Jet Pumps have suggested a pump capable of pumping 50 tonnes per hour of sand. This would require an intermediate pumping station. Ten thousand tonnes of sand would require 200 hours of operation which allowing for the tidal restrictions say 6 hours per tide might require something like 30 days work per site. There will be concerns that at this relatively slow rate of removal as fast as the sand is pumped out it will refill but only trials will tell if this will happen

The total cost of the components to do the task has been estimated by Jet Pump to be

  • Jet pump dredge£5,000
  • Pump to power same (100 HP) £8,000 secondhand
  • Repeater pump to extend delivery£8,000 ditto
  • 700 metre of pipe £20,000 the minimum conceivable length

Jet Pumps are in negotiation with the Royal Edinburgh yacht club to provide a similar rig to maintain their channel, and if successful have negotiated a lease back of the rig to Jet Pump, so it might be possible to mount a full scale trial of the Edinburghequipment at a rented cost

The pipe cost and repeater pumps would have to be covered somehow as the Edinburghsituation does not require a long discharge pipe.

Not in the above costs are the pontoons to mount the rig on plus all the gear for mooring of pipes etc The Edinburgh rig is of the order of £80,000 to which must be added the pipes and the repeater pumps.

If we assume that the total investment by Jet pump for the basic rig is of the order of £80,000, plus pipe of at least £20,000 plus a minimum of one repeater pump at £8,000 plus gear to moor and maintain position say in round figures at least £120,000 and even at £200,000 iswell within the grant structure envisaged at the outset

Such a rig is likely to require a considerable maintenance load in such an abrasive environment and a number of properly paid crew to operate the rig both at the suction site and at the deposition site to move the discharge pipe.

Assuming some form of capital grant, the revenue costs are the more sensitive ones these need to properly assessed

Accordingly ATFL arranged to hire the rig for one week at a cost of £1500 to see how it performs in materiel near the pontoon. The trial was with a 50 tonne pump with a short length of discharge pipe discharging onto the sand bank to the south of the pontoon.

The trial results are appended and were generally successful. Before contemplating the purchase of the equipment a large scale trial needs to be mounted to better assess the true costs of the equipment (noting that with dredging the ancillaries such as moorings can be significant) and to determine the longevity of any dredging within the natural estuary dynamics.

The full scale trial would be best conducted at the Webley turn as the most likely discharge point (the Dunes) are nearer reducing the pipe length needed. To make a significant impact the rig would need to move something like 5,000 tonnes

The cost of such a trial is likely to be

TRIAL COSTS - ROUGH CUT
Costs per hour - rough cut / Diesel usage / 0.279 / litres/kw/hour
2 pumps each of / 100 / hp / 75 / kw
Fuel / £1/litre) / £42
Insurance / £2
Labour 2 men at £10/hr / £20 / Man barge anchors etc
Maintenance pipes seals etc / £1
Mechanical overhauls / £3
Pump wear / £4
Hull mainteance / £1
Total cost/hr / £72
Costs per week / 40 / hour week / £2,894 / per week
Hire costs / £3,000 / per week
Total weekly cost / £5,894
Amount to remove in trial / 5000 / tonnes
Net rate of removal allowing for dead time etc
35 / tonnes per hour
Total time on station / 142.9 / hours
3.6 / weeks
Total cost / 21,050 /
Plus mobilisation / 1,000 / £23,050
Plus demobilisation / 1,000
Not included is the cost of the long pipe - this might be
20,000
Also not included is the cost of a JCB to control sand deposition on spit
Cost per tonne ignoring pipe costs etc
£4.61 / per tonne
Note for reference - Cost of JCB / 1 / per tonne

A meeting with Jetpump focused on a trial at the Webley turn concluded that the cost of such a trial would be of the order of £50,000. ATFL are to draw up a spec for such a trial so that Jetpump can offer a quote and suggest options to reduce costs

ATFL are to look at funding sources for this sort of major trial

Annex A

ANNEX B

Assessment of the present situation

  • Low-water channels in estuaries meander and change unpredictably;
  • This meandering is probably essential to the long-term health of the saltmarshes and other marine plants and animals. If prevented, the nature of the estuary’s habitats would change and its biodiversity would suffer;
  • Long-term movement of coastal sediment is generally into estuaries, and in many cases they gradually infill over long-time scales, i.e. centuries, even when there is a large river discharging into them;
  • Such changes may be accelerated by reclamations or flood control measures that reduce peak flows down rivers;
  • Changes seen in the Teifi are typical of many estuaries across southern Britain;
  • There is little sediment along the coastline either side of the Teifi estuary that can travel into it, and probably very little sediment comes into the Outer Estuary from offshore. Recently, however, it appears there has been some transfer of sediment from the Outer Estuary through the narrows at the end of Pen yr Ergyd spit and into the Inner Estuary;
  • The Pen yr Ergyd spit serves as a breakwater to protect the moorings along the northern side of the Inner Estuary, as well as restricting the width of the low water channel between the Inner and Outer Estuary. If this spit were to disappear, there would be increased wave disturbance at the moorings and potentially a wider but shallower channel. Both consequences would have an adverse effect on boat users in the estuary.

Discussion of possible intervention

  • The effects of dredging in estuaries can be unpredictable, and there is always a danger that such operations may trigger, or perhaps more importantly be the cause of a sudden and significant change in the route of tidal channels;
  • Detailed pre-dredging investigations are necessary to ensuredredged spoil does not return quickly to the excavated area, especially when the methods used seek to redistribute sediment within the estuary rather than removing it entirely;
  • It is not considered economically viable to manage the tidal flows and hence the levels of the seabed in the estuary by means of structures such as training walls;
  • It may be possible, as well as environmentally acceptable to increase the inter-tidal area and volume of the Upper Estuary, i.e. upstream of the road bridges, although it seems unlikely that such changes will radically alter the behaviour of the Outer Estuary or indeed the Inner Estuary downstream of Nant-y-Ferwig.
  • Where erosion of some part of the coastline of the estuary is a concern, then it is likely that the best way of managing this would be by means of local shoreline protection, for example by installing a rock revetment, and/ or short rock groynes, along the affected frontage, after taking professional advice to ensure that these measures are designed to minimise any changes further afield;
  • In view of the conservation interests in the Teifi Estuary, it is likely that an Environmental Impact Assessment study, together with an Appropriate Assessment, will be necessary before any major dredging or other intervention works could be carried out;
  • While predicting future changes in the estuary, whether natural or as a result of interventions, is always going to be subject to considerable uncertainty, improving the current knowledge of the tidal regime will be helpful. It will be useful to monitor the extent of changes in the bed levels, e.g. by LIDAR, and the coastlines along the estuary, and to record dates and times when significant changes occurred
  • In addition, it would be useful to obtain information on tidal currents and levels and on river discharges. This would help in the calibration of a computational model of the estuary that could reproduce its main hydrodynamic characteristics and then be used to gauge the changes that might occur as a result of interventions such as excavating spoil from the Upper Estuary. It seems likely that the most cost-effective way of developing such a model would be in cooperation with a university in a similar manner to a recent study of the estuary of the Dovey.

Annex C