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Samphire Hoe (Channel Tunnel)




The six ha created platform
Location
County: Kent
Contact details
Website: Samphire Hoe website
Project description


The following article was originally published in D.R. Helliwell (Ed) Case Studies in Vegetation Change, Habitat Transference and Habitat Creation. Published by Reading Agricultural Consultants © 1996.

Introduction
Construction of the Channel Tunnel produced almost 4 million cubic metres of Chalk Marl on the British side of the Channel. This was placed at the foot of the cliffs, about 3 km west of Dover, behind a concrete sea-wall, and was used as a working area during construction and fitting out of the Tunnel.

There was already a platform of about 6 ha which had been created by blasting of the cliffs when the railway from Folkestone to Dover was constructed (1843), and which was used by a coal mine in the early 1900s. This platform had a surface at about 16 m AOD, retained behind a sea wall of variable height from about 6 to 8 m AOD. The seaward edge consisted mainly of waste shale from the colliery at an angle of about 40°. This had been colonised by a similar range of plant species to that on the cliffs. The main differences were a reduced abundance of some grasses (Elymus pycnanthus and Brachypodium pinnatum) and absence of some others (Arrhenatherum elatius and Dactylis glomeratus) and woodland species (Euphorbia amygdaloides and Teucrium scorodonia). The main similarities were in the abundance of maritime species such as Crithmum, Beta, Atriplex, Brassica, Daucus, Limonium, Plantago coronopus, Rubis peregrina and Spergularia marina.

When the construction of the tunnel was completed, the surface of the spoil was formed into a series of hollows, ridges and mounds, to create a natural appearance and to provide a wide variety of ecological conditions, including brackish and non-brackish water, and sheltered and exposed land. The majority of the new land surface is designed to drain into the wetland areas, which have outlets to the sea to allow any surplus water to be discharged. These potential wetland areas have been left to be colonise naturally, as there is no risk of serious erosion there, and wetlands usually become colonised relatively rapidly.

The total area of the enlarged platform is about 36 ha, and includes cooling and ventilation facilities for the Tunnel, car parking, a concrete apron on the seaward edge, wetlands and about 20 ha of sown vegetation. Eventual habitat types are expected to range from "cliff crevice" vegetation near to the sea, through "cliff grassland" and "chalk grassland" to scrub.

The option of leaving the bare spoil to become colonised naturally was considered, but was rejected due to the fact that colonisation was likely to be very slow, during which time there would be a considerable amount of erosion. It was regarded by the Local Planning Authority as unacceptable to have 25 ha of waste material without a cover of vegetation for an indefinite period. There would also be a perceived problem of marine pollution if large amounts of chalk marl were washed into the sea (although, in fact, the input of chalk to the sea is a frequent natural occurrence in a location such as this).

It was therefore decided that some sort of vegetation should be established in order to reduce the time during which the spoil was bare and vulnerable to erosion.
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Chalk marl
The tunnel was bored mainly through a single stratum in the Lower Chalk, consisting of chalk marl, which forms a more fertile soil than Middle or Upper Chalk (Burnham, Green and Buckley, 1987), having a greater amount of clay. Typical samples contain around 40% insoluble material. The marl also contains variable amounts of glauconite, which is rich in potassium.

Analysis of samples of chalk marl from the Tunnel showed a high pH (around 9.3), very little available phosphorus (zero on the ADAS scale (Ministry of Agriculture, etc., 1973)), moderate amounts of potassium (ADAS index 3), large amounts of magnesium (ADAS index 7), and virtually no nitrogen.

Plant communities
The vegetation of the cliffs to the west of Dover had been described by Malloch (1986). In 1987 the Institute of Terrestrial Ecology (ITE) sae out 64 permanently marked quadrats in locations to the east and west of the proposed spoil platform, as a basis for monitoring any effects of the enlarged platform. ITE found seven vegetation types (following the National Vegetation Classification):

MC4A - Brassica oleracea maritime cliff edge community: Beta vulgaris ssp maritima subcommunity
MC4B - Brassica oleracea maritime cliff edge community: Ononis repens subcommunity

CG4 - Brachypodium pinnatum grassland
MC11 - Festuca rubra - Daucus carota maritime grassland
W21 - Crataegus monogyna - Hedera helix scrub
MC1 - Crithmum maritimum - Spergularia rupicola maritime rock crevice community
SM24 - Elymus pycnanthus saltmarsh

As a matter of interest it should be noted that the Crataegus monogyna - Hedera helix community did not actually contain any Crataegus.

The choice of nurse species
Earlier examination of vegetation on highway verges had indicated that, on verges of moderately high fertility, initial sowing of a mixture including 53% Lolium perenne and 18% Festuca rubra resulted in a sward of tall grasses such as Arrhenatherum elatius and Dactylis glomerata within a few years. On verges of low fertility a sward dominated by Festuca rubra developed. In neither case did Lolium persist as a major component of the vegetation. Gilbert (1991) also reports that nineteen years after the sowing of verges on the M1 motorway, Lolium perenne was scarce.

Lolium perenne seed is readily available, has the merit of rapid and reliable germination, and under favourable conditions forms, within a few days, sufficient cover to reduce the risk of erosion. Moreover, even though it is a generally abundant species, it was not recorded in this locality. Any plants of this species which were sown would almost certainly disappear within a few years. The Nature Conservancy Council were agreeable to its use, as it presented no threat of persisting or invading surrounding vegetation.

One possible alternative to Lolium perenne might have been Lolium multiflorum, as recommended by Wells et al. (1981) for use on soils of low fertility. However, it was thought that Lolium multiflorum would probably be even more susceptible to death from salt spray than Lolium perenne, and it was felt that a short-lived perennial would be preferable to an annual species on this exposed site. The type of Lolium perenne which was used ("Hermes") was claimed to have some small degree of tolerance to salt.

Initial experiments
In 1989, 540 m³ of tunnel spoil were placed in a layer 0.9 m deep in a field near to Wye College, after removal of the existing topsoil. This was sown with four seed mixtures and three additives - gypsum, inorganic fertiliser and sewage compost. The results after two growing seasons were reported by Mitchley, Howell and McRae (in press).

The spoil was not as saline as had been expected, with initial electrical conductivity of 3.1 mScm¯ ¹ declining to around 1 mScm¯ ¹ over the first two growing seasons, and the gypsum therefore had little effect.

Growth was good either with inorganic fertilizer or sewage compost, and poor without.
The seed mixtures used in this experiment were:

Lolium perenne "Hermes", (a sports turf cultivar with some claimed salt tolerance);
a commercial "coastal" mixture of 7 grasses, including 25% of the same cultivar of Lolium;
Festuca rubra "Hawk" (a salt tolerant cultivar) with sainfoin;
Brassica and Echium were very prominent in the first two years.

At the same time, an area of about 500 m² was covered in spoil, on a 30° slope at the eastern edge of the old platform, just above the old sea wall. This slope was used as a trial for the same four seed mixtures in combination with a range of different surface treatments, including perforated concrete blocks, plastic mesh, jute fabric and straw matting. This work is reported in Howell, Mitchley and Buckley (1991) and Kershaw et al. (1995).

The Festuca/sainfoin mixture was very successful, at both sites, in establishing a dense sward which maintained its density without further addition of fertilizer, and it was moderately resistant to the effects of salt-spray in severe storms at the coastal site.

However, as the spoil was to be placed adjacent to cliffs carrying vegetation of interest and within a Site of Special Scientific Interest, it was considered to be undesirable to use seed of plants which were not of local origin, if the plants which resulted were likely to persist.

Additionally, it was evident that if a sward dominated by Festuca rubra of local provenance were to be established it could be a relatively long time before this became suitable for colonisation by other species.

A small amount of Festuca rubra of local origin was included in the final seed mixtures. Germination and growth is slower than that of Lolium perenne, which was an additional reason for relying on Lolium to provide rapid cover and create stable conditions for the growth of other species.

Additional experiments
In May 1990, one of the replicate treatments at Wye was replaced by another experiment. The top 150 mm of spoil was removed, 60 g of inorganic fertilizer (15:15:15) was added per m², and three new seed mixtures, based on locally collected seeds from different plant communities, were sown. These represented "cliff crevice", "cliff grassland", and "chalk grassland" mixtures, which contained selected elements of the vegetation types listed previously.

Composition of seed mixtures sown in trials and experiments, May 1990:

Cliff crevice
mixture:
Tripleuospermum
maritimum
Inula crithmoides
Spergula marina
Beta vulgaris ssp maritima
Crithmum maritimum
Spergula media
Limonium binervosum
Brassica oleracea
Atriplex hastata
Total
Species Seeds
per m²

125
100
85
50
40
40
25
25
10
500


Cliff grassland
mixture:
Festuca rubra
Daucus carota
Ononis repens
Plantago coronopus
Anthyllis vulneraria
Festuca arundinacea
Silene nutans
Hypochoeris radicata
Tragopogon pratensis
Total


Species Seeds
per m²
125
100
75
75
50
25
25
20
5
500


Chalk grassland
mixture:
Festuca rubra
Brachypodium pinnatum
Briza media
Avenula pratensis
Leontodon hispidus
Lotus corniculatus
Koeleria macrantha
Plantago coronopus
Helianthemum
nummularium
Sanguisorba mino
Scabiosa columbaria
Plantago media
Galium mollugo
Primula veris
Total
Species Seeds
per m²
100
80
75
50
50
25
25
25

20
20
10
10
10
10
500


These mixtures were sown with three densities of ryegrass (2, 5 and 10 g per m²). Two plots of each treatment were included in each replicate, and one of each had an addition of salt-water by spraying on several occasions. The amount of salt was based on recording (by staff from Wye College) of salt deposition on coastal sites. However, it had no detectable effect, and it is probable that the main effect of wind-borne salt on this vegetation is related to the large amounts of salt deposited during very severe storms, perhaps once every year or more, rather than the smaller amounts deposited during minor storms.

1990 was a dry year, and germination of Lolium (sown in May) was slow, and the amount which grew was much less than expected. Many of the other species were even slower, and some were not seen until 1991. However, after two years, a satisfactory mixture of species was obtained. This illustrates some of the problems of seasonal variation!

The most prominent species in the Cliff Crevice mixture was Beta. In the Cliff Grassland mixture Ononis gave the greatest cover, and in the Chalk Grassland mixture Lotus was prominent, but was not as dominant as the Ononis. Most of the sown species were present in each mixture, even if not very abundant, and gave confidence that the basic approach was suitable.

In October 1990 a trial area of about 600 m² was hydoseeded on spoil on the site near Dover, using Lolium at a rate of 5 g per m². This area was approximately 100 m from the sea and was partly sheltered by a mound of spoil. It was on a slope which was constructed to vary from 0° to 30°, in 5° increments. Various surface protection materials were used, including an organic soil stabiliser and cellulose fibre mulch at different rates, and a straw matting mulch.

The Lolium grew well, with or without added soil stabiliser and fibre mulch.

An adjacent area of similar size, at a constant slope of 20° was sown with Lolium in October 1991, including Cliff Grassland and Chalk Grassland seed mixtures, and different rates of fertilizer and dates of application. At the same time, part of the 1990 trial was oversown with these mixtures. Some small pot-grown plants of Ononis, Lotus and Sanguisorba minor were also planted into the 12 month old Lolium sward.

These two trials were terminated in April 1993, when the area was re-shaped to its final form. Survival of other species in the Lolium sown in 1991 was less than expected, but the area which was oversown at the same time contained numerous vigorous plants of several species, including Daucus, Galium mollugo and Anthyllis.

The planted Ononis had grown well, with several plants covering about 0.25 m². Planted Lotus had grown moderately well, but most of the planted Sanguisorba had died, grown very little, or been eaten by rabbits.

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Amount and type of fertiliser
In the October 1990 trial, superphosphate at 50 g per m² was applied before sowing, and two types of slow release organic fertilizer at the time of sowing. (These were 9:3:10 at 17.5 g per m² or 6:17:12 at 50 g per m².) Additional fertilizer was also applied to some areas 12 months after sowing.

The October 1991 sowing included inorganic NPK (15:15:15) at 50 g per m² on part of the area and slow release organic fertilizer (6:17:12) at 25 g per m² over the whole area. The inorganic fertilizer was added either at the time of sowing, 6 weeks later, or 5.5 months later.

The results of these trials were monitored, under contract, by staff from Wye College, and are reported by Howell and Mitchley (1992) and Mitchley (1993).

From this work it was deduced that the optimal growth of Lolium and other sown species would probably be obtained by using 50 g per m² of the slow release (6:17:12) organic fertilizer at the time of sowing. (The inorganic fertilizer, with a relatively large amount of N, tended to promote the growth of Lolium at the expense of the other species.)

In September 1992 just over 1 hectare of the site was sown with seed mixtures based on those shown above. Experimental plots with different levels of N and P were set up. These confirmed that 50 g per m² of the slow release organic fertilizer gave the best overall results.

Based on areas where salt-laden winds in November 1992 had killed most of the newly germinated ryegrass, it was concluded that the Cliff Crevice mixture, which contains several species which germinate quickly, might function better without any ryegrass. It was also concluded that, in areas where chopped straw had been applied as a mulch over the seeds, the amount of ryegrass seed could be reduced below the 5 g per m² which had been used. These results were used to modify the specification for the sowing of the major part of the site in 1993.
About half of the site has been sown with ryegrass only, and a few sheltered areas around wetlands have been left completely unsown, in the expectation that other species will arrive naturally in due course.

The development of the vegetation has been monitored by staff from Wye College. There are some unexplained variations from place to place, but most of the vegetation has developed as expected. Lolium has declined fairly rapidly, except in trial areas where additional fertilizer has been added. The western end of the platform has suffered more severely than expected from the effects of storms, and trials are still taking place on an area of about 1 ha to obtain a vegetative cover. Additional species are being tested, including Elymus pycnanthus and Frankenia laevis, and the planting of pot-grown plants of these and other species through jute netting, in an attempt to obtain better establishment before the next severe storm comes along.

Summary/conclusions
Lolium perenne can be used as a means of obtaining rapid cover without smothering all other species, if it is sown at a relatively low rate and if the fertility of the soil is low. However, ruderal species (for example, Triplospermum maritimum), which require open conditions for successful growth, tend to be suppressed. In most cases this will be an advantage but, if such species are required, the rate of sowing of Lolium needs to be very low, or zero. A balance needs to be struck between initial rapidity of cover and the long term development of the vegetation. Lolium is very vulnerable to the effects of salt spray, and any rapid cover which it may provide in highly vulnerable areas is likely to be short-lived, although even that may be useful, and the dead material may give some protection for a useful period.

Elsewhere, some degree of control can be exercised by further additions of fertilizer if the growth of Lolium is slowing down too much before other species are adequately developed.

References cited:
Burnham, C.P., Green, B.H. and Buckley, G.P. 1987. The establishment of vegetation on Lower Chalk spoil material. Channel Link Studies. Report No.1. Wye College, Ashford, Kent.
Gilbert, O. 1991. Grassland diversification. Landscape Design. May 1991, 15-16.

Howell, R. and Mitchley, J. 1992. Vegetation establishment on Channel Tunnel spoil. Hydroseeding trial 1990-1991. TML report No.WYEA 0006.

Howell, R., Mitchley, J. and Buckley, GP 1991. Shakespeare Cliff slope protection trial. Report No. WYEA 0003 to Transmanche-Link Joint Venture. Wye College, University of London.

Kershaw, K.R., Mitchley, J., Buckley, GP and Helliwell, DR 1995. Slope protection and establishment of vegetation on Channel Tunnel spoil in an environmentally sensitive coastal site. In: Vegetation and Slopes (ed. D.H. Barker). Proceedings of conference, Sept 1994. Institution of Civil Engineering. pp 115-126. Thomas Telford, London.

Malloch, A.J.C. 1986. Implications of the developments associated with the Channel Tunnel on the vegetation of the sea-cliffs between Folkestone and Dover. Report prepared for the Nature Conservancy Council, Wye, Kent.

Ministry of Agriculture, Fisheries and Food. 1973. The analysis of agricultural materials. Technical Bulletin 27. HMSO, London.

Mitchley, J. 1993. Shakespeare Cliff. Grass and wild flower hydroseeding trial 1991-1993. Final report. TML Report No. WYEA 0016.

Mitchley, J., Howell, R. and McRae, S. (in press). Vegetation establishment on Channel Tunnel spoil: an experimental approach and implications for habitat creation at Shakespeare Cliff, Dover, UK. in Habitat Creation and Wildlife Conservation in Post-Industrial Environments. (ed). Packard, Chichester.

Wells, T.C.E., Bell, S.A. And Frost, A. 1981. Creating attractive grasslands using native plant species. Nature Conservancy Council. Peterborough.

In, Case Studies in Vegetation Change, Habitat Transference and Habitat Creation

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Page last modified on Monday 10 of December, 2012 13:25:09 GMT