4.1 Three sites were selected for detailed case study to produce recommendations for reinstatement. The sites are Site 4 Traigh Mhor, a partly restored sand pit on Barra, Site 10 Askernish/ Daliburgh, a partly restored pit on South Uist and Site 16 Horgabost, a partly restored pit on Harris. They were chosen on the basis of their relative ease of access, and their diversity of character and location.
4.2 Fieldwork was carried out in the same week as the general site survey. Geomorphological, visual and ecological character of the pits and environs were described and the constraints and opportunities to reinstatement identified.
4.3 The following basic field survey methods were employed:
- tape survey
- slope angle survey
- slope stability survey
- trial pit and sampling survey
- ground water table survey
- selected cross sections of exposure
- walk over of adjacent landforms
- visual impact of landform from all accessible points
- basic survey to note main vegetation types
- assessment of man-made reinstatement (if any already carried out)
4.4 The success of the reinstatement operations to date were judged against four restoration aims, namely the creation of:-
- a stable, low erosion environment
- a visually unobtrusive site
- area that is safe for people and animals
- as ecologically natural an end result as possible
4.5 The site is situated adjacent to the Airport to Eoligarry Jetty Road at Grid Ref. NF 698 062, about 400 metres north west of the airport terminal building. The site lies approximately in the centre of the Eoligarry Site of Special Scientific Interest (SSSI), which is designated for both its biological and geomorphological interest.
4.6 The sand pit, shown in Plate One and drawing Fig. Two, appears to have been worked from a small fixed dune at the southern end of the dune system which formed on the centre of the Eoligarry Isthmus after the destruction of the main dune ridge on the Atlantic beach. The dune is orientated north-south, with sand extraction having removed much of its east facing slope, leaving a sand "cliff" which forms the western slope of the pit. On the other side of the dune is a large blowout, also orientated north-south (Plate Two).
4.7 The rest of the pit comprises a relatively shallow saucer-shaped shallow hollow. The eastern and northern sides of the pit comprise gently graded slopes, while the low southwestern slope, which curves away from the sand "cliff", appears to be either original dune surface or long restored ground. The dimensions of the "pit" measure some 67m long (north-south) by 32m wide (east-west).
4.8 The site is relatively exposed, being open to Traigh Mhor beach on its east side. A study undertaken for SNH (Hansom and Combor 1995) states that the quarry has contributed considerable volumes of sand onto adjacent surfaces and that no regrading of the slopes had been attempted by 1994. However, all but the "cliff" slope now appear to be table. Erosion on the "cliff" comprises undercutting of the turf cap with turves falling on to the slumped slope below (see Plate Three). The northern end of the slope is more stable, with no undercutting and vegetation is starting to stabilize the bare sand.
4.9 On the other side of the ridge in the blowout the dune slope is being undercut in an identical fashion, eroding eastwards towards the pit. A flat, turfed ridge separates the pit from the blowout, the southern end of which is just a few metres wide. At this point a breach is starting to form (see Plate Four) and is at present 8.70m long, 7m to 8m wide and 3.70m deep. Extraction appears to have been halted just before the breach was reached, leaving the original dune face intact below the gap. However, sand is being blown from the breach onto this slope.
4.10 Buried topsoil horizons can be seen in the pit in the "cliff" (Plates One and Three) and in the sides of the blowout (Plate Five). These often form hard wind scour surfaces, with vertical faces forming above and below them.
4.11 The prevailing south westerly winds on Eoligarry result in net sand transport from west to east from the Traigh Eais dune system and blowouts, but with secondary flows from the flat expanse of Traigh Mhor to the east (Hansom and Combor 1995). The effects of multi-directional wind erosion are clearly seen at the site. Wind appears to be funnelled south to north through the adjacent blowout, while the breach through to the pit is being formed from the west. In contrast, the undercutting of the sand "cliff" in the pit is clearly caused by easterly winds.
4.12 In general the "cliff" ranges from vertical at the undercut top, down to 45° to 60° for the slumped slope. In some places additional vertical faces are forming above and below the hard layer of remnant topsoil, as shown in Plate Two.
4.13 The other slopes in the pit have been graded to a maximum of 45o and have very rounded profiles. The quarry floor is generally flat, with an obvious break of slope between it and the face of the dune. The floor and graded slopes are stable.
4.14 There was no evidence of flooding in either the pit or blowout. Two trial pits were dug to gauge the depth of the water table. (In most cases with small sand quarry developments, operation becomes uneconomic once the water table has been reached, or nearly reached, and extraction cease).
4.15 The water table was encountered at 0.45m in the sand quarry and at 0.50m in the adjacent blowout. After some twenty minutes the water entry had stabilised at some 2cm higher than the original strike in the pit. However, the usual state of the pit floor is reported to be damp (J.Love, pers. comm. 1996), the dry winter and preceding summer creating drier than normal conditions.
4.16 Representative samples of "topsoil" and subsurface sand were taken from the sand "cliff" in the pit. The sand consists of a fine and medium grained deposit with very occasional shell fragments. The surface topsoil, as seen in Plate Six, is an organic sand and supports generally a 6/7cm "turf", but with scattered roots penetrating deeper.
4.17 Throughout the upper vertical face lie individual organic horizons up to 1cm thick marking some form of topsoil development before sand burial. The main buried horizon which is seen throughout the western edge of the quarry and across the neighbouring dunes, is a remnant topsoil about 10cm thick. Over time this has been compressed by the weight of sand above, resulting in a hard indurated layer. As mentioned above, erosion occurs in places above and below this horizon. To the west of the sand pit the remnant topsoil is exposed as a hard surface where the upper sand has been totally stripped off (Plate Seven). This may represent the original machair surface prior to re-deposition of the sand forming the "inland" dunes.
4.18 The remainder of the pit, including the lower slope of the "cliff", has been capped by a dark grey brown material, which has been described as glacial till, but which appears to be very sandy and in part quite humic. It appears to have little silt or clay content and is obviously derived from a sandy till, with the coarse end of the grading fraction represented by numerous cobbles and occasional boulders which are strewn across the quarry floor. It has been spread in a layer which ranges from 10cm to 15cm thick at the trial pit locations, but is much less thick in other places.
4.19 Much of the steep western "cliff" comprises bare sand with turves lying on the surface which have fallen from the ridge above. The sand supports scattered Stonecrop and Daisy, while the turves and the ridge support typical fixed dune grassland with fine-leaved grasses and moss, as does the southwestern slope of the pit. This latter slope is being inundated with wind blown sand from the breach, causing the turf to dieback in places.
4.20 The floor and graded sides of the pit support sparse, naturally regenerating vegetation, including English Stonecrop, Daisy and Fescue grass. Regeneration appears to have progressed more rapidly in some areas than others, possibly due to either greater thickness or greater fertility of the capping. Two areas in particular, the eastern slope adjacent to the entrance and the floor midway along the base of the "cliff", support a relatively high density of Ragwort and coarse grasses. Ragwort is a component of machair and fixed dune grassland, but does not usually occur so prolifically, indicating that the capping material is more fertile than machair topsoil.
4.21 Rabbit grazing occurs on the site and appears to be heavy on the turf ridge and on the grassland on the south west slope below the breach. Rabbit holes and scrapes, creating areas of loose sand which provide a seed bed for the Ragwort, are found over much of the site. The coarse grasses in the ragwort areas do not appear to be much grazed. Cattle grazing is reported to occur on the site, which is thought to also encourage the ragwort (J.Love, pers.comm. 1996).
4.22 The site is visible from quite a long stretch of the Eoligarry to Airport road and from the airport terminal, a number of houses and the school house. However, only the sand "cliff" is readily visible and this looks as if it were part of the adjacent blowout system rather than a sandpit. It is only at close range that the derelict JCB and the dark capping material indicate that quarrying has taken place. In summer the flowering Ragwort and any other woody ruderal species will probably exacerbate the derelict look.
4.23 Although apparently not regraded in May 1994, the northern and eastern sides and floor of the site now appear to have been re-profiled and then capped with the sandy till. The graded slopes consist of sand with the 10 to 15cm deep cap. Repose angles of up to 45° have been formed, creating generally stable landforms which do not appear to be affected by wind erosion. Stability has been increased by the vegetation which has started to naturally regenerate.
4.24 Unfortunately the capping material also contains cobbles and boulders which litter the surface of the sand pit. These, in addition to the rusting JCB, now detract from the visual aspect of the reinstatement, as possibly do the woody ruderal species, when in flower, that have developed on the thickest areas of cap material. These also impact on the ecological interest of the machair, which is particularly pertinent as the site is within a biological SSSI.
4.25 It is difficult to establish what measures have been carried out on the "cliff", apart from the humic capping on the lower slope. It is possible that turves may have been placed on the upper slope immediately adjacent to the breach.
4.26 Reinstatement on all but the "cliff" has been successful in terms of stabilization, but the thick capping which has successfully prevented sand blow has created both visual and ecological impacts, making the restoration unsatisfactory. Visually, slope regrading has also been successful where carried out. The visual impact of the dark capping layer, which presently stands out, will decrease as vegetation cover increases, as will that of the stones and boulders, although probably not completely. The visual impact of the ruderal species is likely to remain. Ecologically, the reinstatement measures have not been very successful because of the use of inappropriate capping material and ingress of undesirable species.
4.27 Reinstatement of the "cliff" has not been successful (if attempted at all), with undercutting still occurring. Without remedial action, the turf ridge will eventually be eroded away, with additional breaching and possible coalescence with the adjacent blowout. The existing breach will continue to erode, there being very little at present, except for the placed turves, to prevent it from expanding sideways into the "cliff" face of the pit.
4.28 The following considerations are made in reviewing need for further reinstatement:
4.29 Visually the site is relatively unobtrusive except for the presence of a few factors that could be easily remedied (e.g. JCB, dark capping material, etc). Reinstatement of the sand "cliff" is not considered necessary in visual terms. Further reinstatement for visual reasons, beyond remedial measures, is not therefore considered necessary. However, remedial measures to reduce the ecological impact of the capping material are considered to be necessary.
4.30 Further stabilizing measures on the quarry floor and southern, eastern and northern sides are not considered necessary. However, without some form of remedial treatment to the "cliff", there is a danger of eventual coalescence with the adjacent blowout, with resultant large scale erosion. If the quarried dune was completely breached, there would be a high risk of sand blow east across the road and adjacent machair. In the worst case scenario, breaching may eventually lead to the complete destruction of the dune, causing large scale sand blow before the blowout stabilized itself.
4.31 Hansom and Combor (1994) state that while it is important to maintain the supply of fresh sand for the machair from the blowouts on Eoligarry, the intervention of manmade factors has probably worsened the erosion. This is particularly true at the blowout adjacent to the pit where the risk of eastward breaching might not have existed prior to quarrying. Furthermore, a water pipe has been laid at some time in the past on the present deflation surface created by the remnant topsoil horizon in the blowout (Plate Eight). This pipe was originally buried but is now exposed. (Attempts have been made to stabilize the sand around the pipe by dumping peat on it, as shown in Plate Nine). It is possible that pipe laying reactivated the blowout, or may have started it in the first place. Additionally, further attempts to cover the pipe may increase the funnelling effect of the wind in the blow out. Clearly, the blowout process at this particular site is affected by factors other than natural ones.
4.32 The final factor to consider is the extent of erosion and sand blow that might occur should the dune be breached. There appears to be net sand removal from the blowout at present, but a permanent deflation surface appears to be forming on a remnant topsoil layer, suggesting that further downwards erosion might be limited. Eastward erosion of the blowout might also be limited once it lowers the height of the ridge, which could significantly reduce wind funnelling. A finite period of eastwards sand blow might be acceptable if the road did not become impassable. (Whether fresh sand deposition onto the adjacent machair is acceptable from an agricultural and ecological point of view would have to be considered). Conversely, erosion on the west side of the blowout may continue which could result in long term sand movement eastwards, which would be much less acceptable.
4.33 Further study to accurately assess future erosion and its acceptability in the absence of remedial action is needed. It is possible that the amount of work that would have to be carried out to stabilise the breach would not be worthwhile if a limited degree of eastwards sand blow was acceptable. It is not therefore possible to give a judgement on the desirability of reinstatement in this report.
4.34 The recommendations given below, for this and the other two case study sites, should be considered as guidelines rather than specifications.
4.35 There are two aspects to reinstatement at this site. The first comprises remedial work to improve the existing restoration of the floor and northern and eastern slopes. The second is the treatment of the steep western sand "cliff".
Floor and northern and eastern slopes
4.36 Treatment of this area should comprise:-
- removal of the derelict JCB
- stone picking to remove the largest stones and boulders
- either complete removal of the topsoil cap or scraping off down to 2-3cm depth and incorporation into the sand surface
- seeding the slopes and floor with an appropriate fine-leaved turf mix
4.37 Because the site is located in a SSSI, the capping should either be completely removed or reduced to a depth that will not sustain the tall ruderals. This should only be carried out where the tall ruderals or coarse grasses are growing, as elsewhere the capping appears to be too thin to support vigorous species. The natural regeneration on these thinner areas is acceptable and is creating stability and a survey will have to be carried out in the growing season to accurately map their extent. Where the capping is completely removed from the ruderal/coarse grass areas, an alternative mulch such as peat, seaweed or machair topsoil should be re-laid. If retained, the capping should be reduced to 2-3cm depth and incorporated into the sand.
4.38 Although the present natural regeneration indicates relative stability, this could be due to the capping layer and the scraped areas should therefore be reseeded. This should be by hand to minimise trafficking in the pit generally. Likewise, hand scraping to remove or reduce capping thickness would be preferable, if the areas involved were not prohibitively large. The unscraped areas should be left untreated to allow the existing natural regeneration to continue. However, care will have to be taken to minimise impacts on these areas during operations, and remedial re-seeding may be necessary where disturbance occurs.
4.39 Because the pit is within the SSSI, the seed mix should be as natural as possible in composition to machair grassland. The ideal would be to use green hay, taken from nearby machair, as a seed inoculum, spreading it on the site. If this is not possible, either the CCS recommended seed mix (see CCS 1987) or that given in Appendix Two should be used. Use of a sand binder is probably not necessary on the floor and graded sides, unless conditions drastically change (e.g. the breach enlarges).
4.40 It is desirable to keep fertilizer application to a minimum because the site is in an SSSI. With the stability of this part of the site, rapid establishment is not such a priority as it is on the "cliff". However, the sward should be monitored and fertilizer applied as a top dressing at a low rate if growth is very retarded (but allowance should be made for the fact that growth will be naturally slower and the sward much less vigorous than in agricultural or horticultural reseeding operations).
4.41 Scraping and seeding operations should not be carried out until remedial works have been carried out on the sand "cliff" to prevent machinery
trafficking and sand blowing over the seed bed. Disturbance to the pit floor should be kept to a minimum while work is carried out on the "cliff".
Western sand "cliff"
4.42 At present repose angles of the slumped part of the slope are currently standing at 45° - 60° which appear to be stable. However, there is some 1.40 m/1.60m of upper face still suffering from undercutting, which is likely to continue as long as the face is vertical. The solution will be to grade the upper vertical section of the slope to create a smooth profile up to the turf. It will be necessary to protect the newly graded slope from further undercutting by covering with brashing.
4.43 While this operation will be fairly straight forward at the northern end of the slope, it will be very difficult at the southern end where the turf ridge is only a few metres wide. However, to leave any upstanding turf "cap" in place in this area will merely result in further undercutting until the "cap" is destroyed.
4.44 The solution will be to take off all of the turf "cap" and grade the vertical slopes on either side down to 30° to 35°. Further north, where the ridge is very much wider, the turf need only be cut back as far as necessary to allow grading. There should be no interruptions in slope at any point as these will catch the wind and restart the undercutting process, and grading should create an 's' profile. The breach should be graded as well.
4.45 It will not be possible to use machinery on the narrowest part of the ridge, and cutting and grading will therefore have to be carried out by hand. A tracked machine with a long boom, based on the floor of the pit and blowout could work the faces. A JCB could be used to cut and grade the northern end of the slope working from the top.
4.46 Because of the difficulties involved in grading the ridge, an alternative, but more long term, approach would be to peg in rolled up, seaweed filled geotextile into the undercut areas, allowing the sand to fall onto and fill the rolls. This will eventually produce a naturally graded face that is suitable for seeding, with plant growth enhanced by the seaweed. This technique may be particularly needed if no brashing is available to protect the newly graded face. A combination of under-pegging and grading might also be possible, with the former undertaken at the southern, narrow stretch of the ridge, and the latter on the wider stretch.
4.47 The grass turves, both those fallen and cut, should be placed along the bottom and middle of the slope in a grid pattern to aid stabilization and to act as an inoculum for turf species (see Chapter Two for technique). Turves should not be placed at the top of the slope as these will form wind scour surfaces. The bare sand should be hand seeded and raked in. Mechanical seeding will not be possible because of the intervening turves and because of the instability of much of the slope.
4.48 A mulch, either seaweed, peat or a top dressing (no more than 5cm deep) of machair topsoil should be applied. If peat is used, this should be applied prior to seeding. (See Chapter Two for detailed methods on use of these three mulches). Alternatively, a complete cap (rather than a dressing) of machair topsoil could be used to a minimum thickness of 10cm to create a rooting zone above the sand. However, it should be noted that this latter material should only be used if being excavated from elsewhere.
4.49 The turf seed mix should again be appropriate for use in the SSSI, as discussed above for the northern and eastern areas of the pit, but should also contain some marram. Although marram does not do well in the absence of wind blown sand, there is likely to be some local sand movement in the early months after seeding and the species may thrive well enough to aid stabilization. After seeding it may be necessary to spray the surface with a sand binder, using one of the products listed in Chapter 2 as being suitable for seeding machair. Ideally, trials should be carried out to establish the efficacy of the use of binder with a mulch.
4.50 It is possible that even with slope grading, turf cannot establish on the blowout side because of heavy erosion and,or sand deposition. In this latter case, marram should be planted instead, supplemented by the use of other sand trapping techniques.
4.51 Although, again, it is desirable to keep fertilizer application to a minimum, this should not be to the detriment of quick establishment, which is needed on the unstable conditions of the "cliff", particularly near to the breach. Chemical fertilizer should therefore be applied at time of sowing (before application of the binder), in addition to the mulch, and at regular intervals through the season, which may have to be determined by monitoring, but with one last application before winter.
4.52 The whole site should be fenced from grazing stock and rabbits. If rabbit fencing is not feasible for the whole site, localised fencing of the worst eroded areas may have to suffice. Rabbits will also have to be cleared from the burrows inside the fenced area and continuous control measures may have to undertaken throughout the establishment period, rabbit fencing being rarely totally effective. Fencing should avoid long straight runs to prevent sand build up and also to discourage stock damage.
4.53 It is imperative that the site is monitored, possibly for up to five years, until complete revegetation has taken place. Remedial treatment should be carried out as necessary, and which may take the form of regrading, reseeding, further mulch or fertilizer applications, rabbit control or fence maintenance.
4.54 It is possible that treating a localised area of the blowout in this way will not be successful. If this is the case, a completely new strategy may have to be sought. This could include attempting to stabilize the whole blowout, or to completely grade down the sandpit dune, removing the wind funnelling effect that the topography currently creates. Before any work is carried out on the pit, a study of the blowout needs to be carried out to establish whether partial or complete restoration in the blowout is possible. It is possible that reinstatement could be tied in with work to rebury the water pipe, possibly requiring collaboration with the local council.
4.55 The ecological and geomorphological desirability of partial or complete reinstatement in the blowout should also be considered in view of the importance of the blowouts on Eoligarry as conduits for sand movement. Hansom and Combor (1995) recommend that partial work in the blowouts might be acceptable as human influences have exacerbated their erosional capacity.
4.56 This partially restored sand pit, located in the Eoligarry SSSI, has been quarried from the lee side of a dune, but is open to the wide expanse of Traigh Mhor beach to the east. The only area of active erosion in the pit now occurs on the steep dune "cliff" on the west of the pit, where undercutting and slumping is occurring. The pit floor and other sides have been graded and covered with a capping material which appears to have stabilised the sand and which is naturally regenerating. However, areas of tall ruderals and coarse grasses indicate that the capping is too fertile. Attempts have possibly been made to stabilise the "cliff" by placing fallen turves on the bare upper face, but these have largely been unsuccessful. A blowout is eroding the western side of the dune back towards the pit and a breach has developed between the pit and the blowout where the turf ridge separating the two is just metres wide. Sand is already starting to blow into the pit through the breach.
4.57 The reinstatement measures to date are acceptable in terms of stabilization. Some remedial work is necessary to improve visual appearance, including removal of the JCB and stone picking to remove the rocks and boulders in the capping layer. Remedial work is also necessary to reduce ecological impact by either removing or reducing depth of the capping layer in the areas of tall ruderals and coarse grasses, followed by reseeding to maintain stability.
4.58 Further reinstatement measures, in the form of grading and reseeding, is necessary to stabilise the western sand "cliff" to prevent coalescence of the pit with the adjacent blowout. However, if reinstatement is to be anything more than short term, it has to include the slope on the opposite side of the dune in the blowout. This is because grading will remove the turf cap at the southern end of the ridge, which in turn will result in the need to grade and possibly seed or plant the blowout side to prevent further undercutting and sand blow into the pit. However, partial, or even complete reinstatement of the blowout may not be feasible and further study is needed to determine this. Site 10 Askernish/Daliburgh, South Uist
4.59 The site is situated 4_km west north west of the village of Daliburgh and some 600m west north west of the cemetery. The grid coordinates for the centre of the site are approximately NF 731 221. It is located in fixed dune and dry machair grassland between the Atlantic to the west and the Loch Hallan SSSI to the east, which is designated mainly for its wetland machair, including lochs, and coastal marsh and grassland.
4.60 The site comprises a complex of three pits, two disused and one still worked. The case study pit (Pit Two, Fig. Three) comprises one of the disused pits which has been recently partially restored. It is located on the boundary between the fixed dune ridge and flat machair grassland.
4.61 The pit is roughly an oblong shape and its dimensions are 75m long, 45m wide and 1.10m to 2.00m depth. Extraction has mainly been into machair, with only the northern end of the pit eating into the dune ridge (see Plate Ten).
The north west "corner" of the pit encroaches onto an adjacent fixed dune (see Plate Eleven). Extraction faces have been formed on the eastern, northern and northwestern sides of the pit only, the southern and much of the western sides being level with the adjacent machair surface.
4.62 The eastern slope and a small length of the northern slope of the pit have been recently graded and covered with a capping layer and appear stable (Plate Twelve). The dune slope in the north west corner comprises mobile bare sand which does not appear to have been reinstated. This is probably because the dune is currently being investigated by an archaeological "dig" which has delayed restoration in that area.
4.63 The pit appears to be relatively sheltered, lying in the lee of the dune ridges and surrounded nearly on all sides by higher topography. Erosion in the pit at present appears to be limited to the unrestored section of the northern face and the "archaeological" dune. The top part of the latter slope is being undercut by wind erosion, with turves falling onto the slumped face below. The sand appears to be fairly mobile and is blowing onto the adjacent machair. The turf ridge dividing the pit from the adjacent Pit One has been eroded at one end and re-laid (temporarily?) at the other over the archaeological dig in an attempt to stabilize the sand. The top of the ridge is being actively eroded and a small breach between the two pits has started to form (Plate Thirteen).
4.64 Much of the northern face of the pit also suffers undercutting and slumping, with turves falling on to bare sand on the slumped slope below. This is probably due to both wind scour and slope failure resulting from the steep slope angle. Much of the slope appears to have recently been covered with turves, probably in an attempt to stabilise it.
4.65 The pit can be divided along a SW-NE axis between graded and ungraded slopes, as shown in Fig. Three. To the west of this line the slopes of the "archaeological" dune have repose angles of 20° to 35,° although these may have been recently modified by the trampling of the "dig" team. The upper undercut part of the slopes are near vertical in places. Much of the northern face is near vertical to about 70°.
4.66 To the east of the "dividing" line the slopes have been re-profiled and battered to an angle of 30° to 45°and range from 1.10m to 1.65m in height. These appear to be more stable than the untreated faces. The slopes of the adjacent disused sand pit (no. 1) have repose angles of 30° to 40° and are mainly unvegetated.
4.67 As seen in the Plates, there is a distinct visual division of the site, due to the partial use of "capping" material. The eastern slope and all but the north west part of the floor have been capped with a dark brown/ black material which consists of a very humic sandy reworked topsoil, the origin of which is unknown. It may have come from a dump site as it contains extraneous material such as slate, brick, wood, plastic, rope, stone, metal, household rubbish, clay pipe, concrete, and the occasional remains of fishing net. However, the capping is exclusively a surface layer rather than a fill, with little evidence of the foreign objects at depth.
4.68 The subsoil beneath the "cap" and in the north west part of the pit is a fine to medium sand with occasional organic traces and is similar to the sand of the exposed dune faces.
4.69 Three trial holes were excavated in the pit and are shown in section in Fig. Three. The water table was quite shallow and was recorded from 0.15m in TP2 to 0.30m in TP's 1 & 3. Some areas of the pit are waterlogged (see Plates Eleven and Twelve), indicating that the water table was reached during extraction, probably bringing the operation to a close.
4.70 The pit floor and slopes are devoid of vegetation except for the turved northern slope and the southern face of the "archaeology dune". The adjacent machair and dune grassland is dominated by moss, with abundant fine-leaved grasses and a fairly high frequency of marram grass. Patches of the machair have been cultivated or are being prepared for cultivation with seaweed top dressings, as seen in centre fore and middle ground and distant right in Plate One.
4.71 The site is open to rabbit, sheep and cattle grazing. Rabbit grazing is very heavy in the adjacent grassland and scrapes and burrows are abundant in the dune area.
4.72 Due to the rolling topography and relatively isolated location of the site, the pit is only visible from the adjacent track and machair immediately south of the site. The eroded "archaeological" dune, but not the rest of the pit, is visible from the adjacent cemetery. The visual impact of the pit itself on long distance views from any direction is therefore virtually non existent.
4.73 However, the pit is obtrusive from medium and short range due to several factors. These are the dark colour of the capping material, which is much more obtrusive than the bare sand, the straight, even graded eastern slopes, and the dumped objects in the capping material. The impact of the capping and dumped objects will greatly decrease with revegetation, although the latter could have adverse impacts on wildlife. The impact of the straight slopes, particularly the sharp break of slope with the machair surface, will diminish somewhat, but will still be very noticeable.
4.74 Reinstatement works appear to have been recently carried out. The base of the quarry has been levelled and about two thirds of the area has been capped with the humic sandy topsoil. The thickness of this layer, which varies between 5cm to 10cm, is determined by wheel rutting. The eastern slopes have also been covered with a thick layer of this capping after being graded back to 30° to 45°. There is no sign of obvious slumping or instability of these faces, but the break of slope with the machair surface is very sharp. However, it is too soon after reinstatement to tell if this will lead to undercutting. As yet there is no vegetation development.
4.75 Without knowing the source of the topsoil and with no previous season's growth, it is impossible to gauge the fertility of the 'cap'. If it is relatively high, ruderal species and coarse grasses could invade where the capping is more than a few cms deep.
4.76 The steep northern slope has not been graded, but an attempt has been made to stabilise it by laying turves on the face. This appears to have worked on the eastern end of the slope, but the central and western ends show signs of undercutting and slumping. No reinstatement has been carried out on the western dune slope.
4.77 Without some form of reinstatement, there is the risk that the northern face of the pit and the dune slope could further erode, particularly in the latter case where enlargement of the breach between the two pits could result in large scale erosion. The floor and eastern regraded sides are stable and therefore only require to be revegetated if recontouring is not carried out.
4.78 Since the visual impact of the pit is low from a distance, the need for further reinstatement for visual reasons is low. However, if this site is selected for the demonstration project, the "ideal" reinstatement may be preferred, which would include recontouring.
4.79 If the capping material is too fertile, there is a high probability of invasion by non-machair species, creating an ecological impact. Whether remedial measures are required to remove or reduce capping depth will depend on the policy adopted by the Minch Project. From a pragmatic point of view, the non-designated nature of the site may possibly not warrant such a measure, although any area of machair can be considered as a valuable habitat. Remediation of the capping layer may also be considered necessary if the site is selected for the demonstration project. (Before any such project could proceed, close collaboration with the archaeologists would be necessary to ensure that all site investigation excavation had finished).
"Archaeology dune"
4.80 Reinstatement of the "archaeology" dune slopes needs to encompass the whole dune to ensure long term stability. This will entail regrading both sides in the two pits, necessitating removal of the remaining turf ridge, an operation which will have to be carried out by hand. Turf removal and grading elsewhere on the dune could be carried out by machinery based on the adjacent fixed dune ridge. Grading should create an 's' shaped profile and slopes of no more than 35-40o. The graded faces should be protected from further undercutting with brashings while the newly sown sward establishes.
4.81 Because of the difficulties involved in grading the dune ridge, an alternative, but more long term, approach would be to peg in rolled up, seaweed filled, geotextile into the undercut areas, allowing the sand to fall onto the rolls, filling them in. This will eventually produce a naturally graded face that is suitable for seeding, with plant growth enhanced by the seaweed. This technique may be particularly needed if no brashing is available to protect the newly graded face. A combination of under-pegging and grading might also be possible.
4.82 Grass turves, either fallen or cut, should be placed along the bottom and middle of the slopes in a grid pattern to aid stabilization and to act as an inoculum for turf species (see Chapter Two for technique). Turves should not be placed at the top of the slope as these will form wind scour surfaces. The bare sand should be hand seeded and raked in. Mechanical seeding will not be possible because of the intervening turves and because of the instability of much of the slope.
4.83 A mulch, either seaweed, peat or a top dressing (no more than 5cm deep) of machair topsoil should be applied. If seaweed is used, this should be applied after seeding and raking in. (See Chapter Two for methods on use of these three mulches). On the very exposed part of the dune ridge, a complete cap of machair topsoil could be used to a minimum thickness of 10cm instead, but only if it has to be anyway excavated from elsewhere.
4.84 The slopes should be seeded, using either the CCS recommended seed mix (see CCS 1987) or that given in Appendix Two. Although conditions are not ideal for the species, marram should be included in the mix as it is fairly common in the sward on the vegetated parts of the dune and will aid stabilization. Seeding should be carried out by hand. It may be necessary to apply a sand binder in this rather exposed area of the pit, but ideally, trials should be carried out to establish this need.
4.85 It is desirable to keep fertilizer application to a minimum, but this should not be to the detriment of quick establishment which is needed with the unstable conditions. A minimum level should therefore be applied at seeding (before the binder, if applied) and at regular intervals thereafter. The sward should be closely monitored through the season and fertilizer levels adjusted as appropriate.
Northern face
4.86 The west and central parts of the northern face should be regraded to an angle of 30 - 40o and to an 's' shaped profile. Grading will need to be tied in with the dune contouring as the two slopes are contiguous. The existing laid turves should be removed and re-used, as should cut turves. Any established vegetation on the lower slope should be retained if slope angles permit. Under-pegging may also have to be used as an alternative to grading, a discussed for the dune slope. Turf placing, mulch, seed mix composition and fertilizer application requirements will also be the same as those recommended for the dune, but sand binder is less likely to be necessary. It should be possible to operate grading machinery from the top and bottom of the slope, but seeding should be carried out by hand. Under-pegging may also have to be used as an alternative to grading, as discussed for the dune slope.
Eastern slopes
4.87 Two options are possible for the eastern slopes, depending on whether "ideal" reinstatement measures are to be implemented or not. If purely functional measures are undertaken, grading would be limited to reducing the angle of the break of slope with the machair surface, which is presently a potential wind scour feature. The cut turves could be placed on the base and middle of the face to provide an species inoculum. Before seeding is carried out, any litter or rubbish in the capping layer should be removed by hand picking. Seeding can be carried out directly on to the capping between the turves. It is possible that the capping layer is rather too fertile and that non machair species may invade. However, since the site does not lie in a designated area, remedial measures may not be worthwhile if functional reinstatement is carried out.
4.88 If "ideal" reinstatement is to be undertaken, the slopes should be graded to a more natural appearance, contouring their straight lines, creating an 's' shaped profile and reducing the upper break of slope. The capping should be removed if it is too fertile (this can be determined either by observing which species appear next season, or by analysing the material). Regrading will probably result in its burial or at least incorporation into the sand, doing away with the need to scrape it off. However, the litter and foreign objects will need to be hand picked from the capping before regrading. The slopes should be mulched with either peat, seaweed, or machair topsoil. (There may not be a need to mulch should the present capping have been incorporated into no more than the top 5-10cm of sand).
4.89 In both situations, hydroseeding will have to be employed as the slopes will be unsuitable for drilling and the area too large for hand seeding. The seed mix should be as recommended for the dune area, including the low proportion of marram, which is a scattered component of the adjacent machair. A low level of fertilizer should be included in the hydroseed mix to encourage establishment and growth should be monitored to assess the need for future applications. However, the need for rapid establishment is not as imperative as on the eroded dune and northern slopes.
Pit floor
4.90 Litter and rubbish should be removed from the surface before any reseeding operations are carried out. This should be carried out by hand to keep trafficking to a minimum.
4.91 Seeding is likely to be difficult on the waterlogged areas of the pit floor without using a specialised seed mix for the wet conditions. Furthermore, the seeding operation will be difficult. A better solution will be to allow natural regeneration in those areas, scraping off or incorporating the capping (assuming it to be too fertile) into the top 10cm of sand to prevent ruderal species invading.
4.92 The drier areas of the floor, some of which appear not to have been capped, should be mulched and seeded. A sand binder is probably unnecessary as the base of the pit appears to be relatively sheltered. Seeding can be carried out by drilling, but ensuring that the minimum amount of trafficking occurs. The seed mixes recommended above should be used. Fertilizer in either area should not be applied as the need for quick establishment is absent. Growth should be monitored, however, and top dressings applied if establishment is very poor or stability does appears to be threatened.
4.93 The whole site should be fenced from grazing stock and rabbits. If rabbit fencing is not feasible for the whole site, localised fencing of the worst eroded areas may have to suffice. Rabbits will also have to be cleared from the burrows inside the fenced area and continuous control measures may have to undertaken throughout the establishment period, rabbit fencing being rarely totally effective. Fencing should avoid long straight runs to prevent sand build up and also to discourage stock damage.
4.94 It is imperative that the site is monitored, possibly for up to five years, until complete revegetation has taken place. Remedial treatment should be carried out as necessary, and which may take the form of regrading, reseeding, further mulch or fertilizer applications, rabbit control or fence maintenance.
4.95 The pit comprises a large oblong shaped quarry located mainly in machair, but with some extraction into a dune ridge at the northern end. Much of the pit floor is near to the water table and is damp. Erosion is limited to the northern extraction face and the north western slopes which make up part of an eroded dune. Both suffer from undercutting on the upper face and slumping on the lower slope. A small breach has appeared in the turf ridge on top of the latter dune which separates the study pit from an adjacent disused pit to the west. Excavations for an archaeological dig in the dune have exacerbated this erosion. The pit is generally invisible in the landscape, due to the rolling topography and lack of nearby vantage points.
4.96 Reinstatement to date comprises grading of the eastern slopes and covering with what appears to be a dark, humic topsoil, which has also been spread over much of the pit floor. Although these measures have been successful in terms of creating stability, the sharp break of slope with the machair may create later problems of undercutting. In visual terms the straight, even landforms produced by grading look very artificial, and dumped objects in the capping layer are unsightly. It is likely that the topsoil is too fertile for a machair site, creating an ecological impact.
4.97 Remedial work to re-contour the eastern slopes to a more natural shape, remove the objects in the capping and remove or incorporate the topsoil into the sand should be carried out if "ideal" reinstatement is to be undertaken. If minimum measures are undertaken instead, remedial work can be limited to reducing the break of slope of the eastern sides and removing the objects from the capping. In both cases the slopes and drier areas of the floor should be seeded while the wet areas can be left to naturally regenerate. Mulch need only be applied if the capping is totally removed and fertilizer need only be applied on the slopes.
4.98 Further reinstatement work is needed to stabilise the northern and north west slopes. Much, if not all, of the steep northern face requires regrading to reduce slope angle, followed by mulching and hand seeding. The dune between the two pits will need to be stabilized if long term treatment of the north western slope is to be successful. The slopes on both sides of the dune should be graded, necessitating turf removal and grading on the intervening ridge, followed by mulching, hand seeding, fertilizing and possible application of a sand binder. These measures should only be undertaken once the archaeological "dig" has finished.
4.99 The site is situated north of the A859 at Horgabost, immediately adjacent to the coast, and lies within a National Scenic Area. The grid co-ordinates for the centre of the site are NG 046 968. The pit is located in machair grassland with some dune features, and lies within the Luskentyre Banks and Saltings SSSI, designated for its biological and geomorphological interest. Small "islands" of remnant machair, and associated accreting sand shelter the site from the open sea. To the east of the pit a burn flows north towards the sea, but turns sharply westwards to flow in front of the pit on reaching the beach.
4.100 The pit is shown in Fig. Four and Plates Fourteen, Fifteen and Sixteen. Quarrying has left a large hollow measuring approximately 72m long north to south, 50m wide west to east and between 5m and 7m deep in places. A barrier has been constructed between the pit and the back beach. Within the quarry, some of the extraction faces have been re-profiled and several areas have been infilled. Piles of rubble, rock and topsoil from road and building works have been dumped in the northern end of the pit, whilst a large mound of road salt has been dumped at the entrance.
4.101 The erosional history of the pit's environs is vital to understanding past and present processes within the quarry. It appears that the burn to the east of the pit changed course quite dramatically and possibly fairly recently. This looks to be due to the deposition of sand on the upper beach which blocked the mouth of the burn. With the westwards change of course, the burn cut through what would have been a small headland of machair, creating remnant islands. Sand is now accreting around the bases of these islands, adding to their sheltering effect.
4.102 Prior to breaching, the pit would have been some distance from the beach (c.20-30m) and it is possible that the northern barrier was built in response to this breaching event. The barrier comprises solid material rather than sand and is stable at present, but the adjoining dune slopes are quite badly eroded, particularly on the seaward side, where machair "cliffs" have formed. These may have been subsequently profiled when the barrier was built (indicated by the contiguous nature of the barrier with the "cliffs").
4.103 Wind scour appears to be the main erosion process on the "cliffs", indicated by the undercutting of the turf cap, and probably exacerbated by wind deflection on the hard surface of the barrier. Although it is likely that high tide reaches the back beach, there is no evidence of wave attack, probably because of the sheltering effect of the machair "islands". However, the base of the eastern dune is being eroded by the burn.
4.104 Erosion inside the pit is limited and in general there is about an 80% vegetation cover. The south eastern sides are being undercut on the steep upper faces, with slumping on the lower slopes which appear to support naturally regenerated vegetation and which are stable. Undercutting also appears to be starting on the north western slopes at the sharp break of slope with the machair surface (see Plate Fourteen).
4.105 Slope angles in the pit vary from a minimum of 25 to 30° for the grass slopes of the south western side to a maximum of 60° on the north western side of the pit. The northern barrier generally has angles of 35° to 45° for both its faces. The slopes of the adjacent dunes to the west and east of the pit have repose angles of 40° to 45° and 35 to 40° respectively.
4.106 From initial survey, the surface ponding and low base level of the pit seemed to indicate a high water table. However, a trial pit excavated at the lowest point of the sand pit proved to be dry at 1.00m depth, and it is possible that waterlogging is probably due instead to surface compaction caused by machinery during infilling and dumping operations. That this is occurring suggests that the surface material must, in places, be either of a silty or clayey nature.
4.107 The barrier at the northern end of the pit comprises hard fill material rather than sand, although its exact composition is not known. It has been capped with topsoil.
4.108 The undercut and slumped areas of the eastern slopes of the pit appear to have revegetated naturally on bare sand, while the remaining slopes, probably representing areas of fill, have been covered with topsoil. Part of the north western slope is covered with stones.
4.109 Much of the floor and lower slopes of the pit have been infilled, although the base of the quarry at the northern end appears to be natural. The trial pit in this area (see Fig. Four for position) consisted of a fine to medium sand with occasional shell fragments which was uniform for the entire depth (see Plate Seventeen). A small cover of topsoil is seen at the ground surface extending to approximately 2 - 5cm in depth. The trial pit in the north eastern slope comprised layers of fine to medium sand, interlain with remnant topsoil horizons.
4.110 The fill on the site can be divided into "historical" and "recent". The historical fill consists of a sandy topsoil with cobbles, boulders and occasional concrete blocks. Also included in the historical fill are crushed and buried cars mainly in the south-western corner of the site, pieces of which are exposed on the surface (Plate Eighteen).
4.111 The recent fill material is dumped on the floor of the pit and in general appears to be "inert" builder's rubble, sandy topsoil, peat, cobbles, boulders and rock, with few foreign objects, such as metal, wood, plastic etc, suggesting that dumping has been selective and controlled. No attempt has been made to level this fill and the individual lorry loads can still be seen where they were dumped. A large salt mound, presumably road salt, has been dumped at the pit entrance.
4.112 The vegetation on the adjacent original land surfaces comprises short turf dominated by fine-leaved grasses and moss and is grazed by sheep and cattle.
4.113 Much of the pit is vegetated, with only some bare areas on the floor and on localised areas of erosion on the slopes. Vegetation which has apparently naturally regenerated on bare sand on the eastern slopes is fairly similar to that of the adjacent machair grassland. However, areas which have been spread with topsoil, most noticeably slopes in the south west and on the landward face of the barrier, have a high component of woody ruderals in the sward. It is not known whether these latter areas have been reseeded or have colonised naturally.
4.114 Rabbit grazing is heavy in the pit, and scrapes and burrows are numerous. Sheep grazing also appears to be heavy.
4.115 The pit is visible from a short length of the main road (i.e. long to medium range), but would be much less so without the dumped spoil and, particularly, the salt pile. The erosion on the upper faces of some of the slopes is noticeable but is not obtrusive because of the presence of naturally eroded areas in nearby dunes. The reinstated barrier looks rather artificial because of the sharp break of slope and flat top, but is not too obtrusive at either long or medium range.
4.116 At medium and close range, the woody ruderal species on the reinstated slopes of the pit probably give the site a rather disturbed, derelict appearance during spring and summer, as they do to a lesser extent in winter.
4.117 Unlike the other two case study pits, partial reinstatement of the pit appears to have taken place a number of years ago, judging by the stage of vegetation development. The most critical part of this has been the construction of the northern barrier which has been instrumental to the stability of the pit as a whole and appears to have been successful to date with as yet no undercutting either by wind or the adjacent burn.
4.118 It appears that some remedial work has been carried out on the seaward faces of the dunes which adjoin the barrier. The smooth contiguous nature of the barrier and dunes suggests that the dune faces were profiled when the barrier was constructed. Topsoil seems to have been placed on the dune faces, but has largely eroded away, as shown by Plates Nineteen and Twenty in which baler twine is being exposed from under a fallen turve and a ruderal species, possibly Dock, is growing from the remnant pieces of topsoil. Erosion of the topsoil may well be due to slippage on the steep slope, as well as to wind erosion.
4.119 The contouring and infilling that has been undertaken within the pit has been reasonably successful to date with only localised areas of erosion where grading appears not to have been carried out. However, future problems could arise with the infilled areas. Inert material (rubble, boulders etc) and scrap-metal (including crushed cars) have been used to form both floor and slopes. These areas have been sealed and capped with sandy topsoil and have revegetated. In some areas surface metal can be seen where the capping has not been effective, and furthermore, settling of fill has resulted in holes appearing in the ground surface. Apart from the fact that both features present quite a safety hazard for people/animals wandering into the pit, they could also initiate wind scour.
4.120 Revegetation has been fairly successful in terms of cover, but has been less so in terms of species composition. As discussed above, the areas spread with topsoil support more prolific ruderal species than would normally be expected in a machair grassland. This indicates that the fertility of the topsoil used was inappropriately high.
4.121 There are three features which need attention in further reinstatement work. The first is the further profiling of the northern barrier, the second is the complete burial of the exposed metal in the infilled areas, and the third is the re-profiling of the slopes on which undercutting is occurring, or starting to occur. All three measures would reduce the risk of erosion which could potentially be very serious at a site so close to the beach.
4.122 The ideal original reinstatement for the pit would have been just regrading and revegetation. The beach locality and the nearness of the burn makes the site very vulnerable to long term erosional processes and it is therefore not an ideal location for infilling. Should the barrier or adjoining dunes be breached, there would a real risk of initiating a blowout with re-exposure of the buried material. Although the barrier appears to be stable at present, the eastern adjoining dune is being eroded by the burn and future changes in this watercourse could undermine the barrier as well. In the long term, gross changes to the shoreline and course of the burn could result in encroachment of the pit by the sea, with exposure of the infill material.
4.123 A further factor to consider is the possibility that the barrier will be used as a footpath should the proposal go ahead to site a caravan park to the west of the pit and toilet block to the east. In this case, the barrier represents the shortest route between the two locations and would probably be used as an access route which could threaten its stability. Widening and forming the barrier to encourage people to leaward side would help prevent erosion arising from the effects of trampling.
4.124 Further study and a risk assessment should be made of long term erosional processes on that part of the coast, including future movement of the burn. From the initial survey, the situation appears to be relatively complex with both erosional features, such as machair "cliff" and the remnant islands, and depositional features, such as the accreting sand, in juxtaposition with each other. If the risk of future large scale erosion proves to be low, further reinstatement, along the suggested guidelines given below, could be carried out. If there is more than a low risk of such erosion occurring, removal of the fill might have to be considered in the long term.
4.125 Further investigation to confirm that the northern end of the pit has not been filled would be also be useful. The risk of exposure of infill material will be reduced if the southern end only has been filled.
4.126 Of lesser importance are the visual improvements which would result from re-profiling the barrier to a more natural shape, repairing the eroded areas and covering the exposed fill. The latter measure will also improve safety in the pit, an important point in view of the proposed caravan park.
Northern barrier and adjoining dunes
4.127 The slope of the barrier should be decreased and its height and width increased, while the adjoining dunes should also be graded to decrease their slope and remove the undercut faces. Slope angles for both barrier and dunes should be no more than 35o. A reduction in slope will facilitate wind movement over the barrier, reducing deflection on to the adjoining dunes. It is possible that grading may not be feasible on the exposed seaward slope if protective brashing is not available. In this case, under-pegging would have to be considered as an option.
4.128 The landward slope of the barrier should be graded to 25 to 30o. All graded faces should be 's' shaped to avoid sharp breaks of slope which could initiate undercutting. The top of the barrier should be formed, if possible, to encourage people to walk on the landward side, so preventing erosion on the more vulnerable seaward slope. It should be possible to carry out the grading operations by machine.
4.129 The dumped fill material can be used to build up the barrier, but should be capped with a thick layer of sand or infertile sandy topsoil (minimum of 15cm) to prevent the ingress of inappropriate species. Large boulders should be removed before landforming to prevent future slumping as the fill settles. Stones and boulders should be removed from the surface to prevent exposure through the capping layer.
4.130 The grass turves, both those fallen and cut, should be placed along the bottom and middle of the graded faces in a grid pattern to aid stabilization and to act as an inoculum for turf species (see Chapter Two for technique). Turves should not be placed at the top of the slope as these will form wind scour surfaces. The bare sand should be hand seeded and raked in. Mechanical seeding will not be possible because of the intervening turves and because of the instability of much of the slope. Hydroseeding will not be appropriate because of the sandy topsoil.
4.131 Because the pit is within an SSSI, the seed mix should be as natural as possible in composition to machair grassland. The ideal would be to use green hay, taken from nearby machair, as a seed inoculum, spreading it on the site. If this is not possible, either the CCS recommended seed mix (see CCS 1987) or that given in Appendix Two should be used.
4.132 A mulch, either seaweed, peat or a top dressing (no more than 5cm deep) of infertile sandy topsoil should be applied. If peat is used, this should be applied prior to seeding. (See Chapter Two for detailed methods on use of these three mulches). After application of the mulch, the surface may possibly have to be sprayed with a suitable sand binder. Ideally, trials should be carried out to determine the efficacy and need for a binder with a mulch.
4.133 Although the pit is located in an SSSI, fertilizer should be applied because of the need for rapid establishment. Growth should be monitored and further applications made if necessary through the season, taking care to apply a minimum effective rate because of the ecologically sensitive nature of the locality. Repeat fertilizer application may be less necessary on the landward side which is more sheltered.
Dumped car area
4.134 The area with exposed dumped cars should be covered with ideally 20 to 25cm depth of the fill material (assuming there to be enough remaining after the barrier works). Prior to covering, the area should be tracked over to further consolidate the fill and crush the protruding objects. The covering material should also be well consolidated once spread to prevent further settling and large boulders should be removed. It should then be overlain with at least 20cm of either sand or a sandy topsoil. The existing slope profiles and angles should be retained. The seed mix should only be drilled in if it is possible to carry out the operation without mixing the sandy capping with the fill material, otherwise the area will have to be hand sown. (Seed mix considerations are the same as discussed in Section 4.131). After raking in (if hand sown), a mulch and, possibly, sand binder should be applied, as outlined above for the barrier and dunes. Fertilizer should be applied at time of sowing, and growth monitored to establish whether further applications are necessary.
Eroded slopes within pit
4.135 The undercut upper faces on both the eastern and north western sides of the pit should be cut back and graded to about 30 to 35o, and the stones on the latter slopes removed. Regrading may have to be carried out by hand on some of the eastern slopes where machine access might be difficult. The cut turves should be laid on the graded faces (but not at the upper break of slope) and the intervening areas of bare sand hand seeded. (Seed mix considerations as listed in Section 4.131 will apply). A mulch should be applied. Fertilizer should be applied at time of sowing, and growth monitored to establish if further applications are required. Sand binder application is likely to be less necessary within the pit than on the seaward face of the barrier, but the situation should be monitored.
4.136 The whole site should be fenced from grazing stock and rabbits. If rabbit fencing is not feasible for the whole site, localised fencing of the treated areas only may have to suffice. (It may be very difficult, if not impossible, to fence the seaward face of the barrier). Rabbits will also have to be cleared from the burrows inside the fenced area and continuous control measures may have to undertaken throughout the establishment period, rabbit fencing being rarely totally effective. Fencing should avoid long straight runs to prevent sand build up and also to discourage stock damage.
4.137 It is imperative that the site is monitored, possibly for up to five years, until complete revegetation has taken place. Remedial treatment should be carried out as necessary, and which may take the form of regrading, reseeding, further mulch or fertilizer applications, rabbit control or fence maintenance.
4.138 The pit is situated in machair very close to the back beach of Traigh Nisabost. It forms a rectangular hollow with relatively steep eastern and western sides, and less steep south western sides which have been infilled. The pit is about 80% vegetated, with erosion mainly limited to undercutting on the eastern slopes. A burn runs westwards immediately north of the pit along the back beach, which is only separated from the pit by a solid barrier. This barrier forms the northern end of the pit and was possibly constructed after the burn changed course from north to west, cutting into the machair north of the pit.
4.139 The southern end and sides of the pit have been infilled with scrap metal and rubble and covered, and have largely revegetated. However, settlement and wind scour is resulting in re-exposure of some of the fill material. Woody ruderal species grow prolifically on the reinstated areas, indicating that the covering used was probably too fertile. Heaps of spoil have recently been dumped on the pit floor.
4.140 Reinstatement to date has been reasonably successful in preventing a blowout occurring by blocking the northern "gap" through to the beach, and in stabilizing most of the slopes within the pit. However, infilling has not been totally successful because of the exposed dumped material and settling of the fill.
4.141 Problems could occur, however, in the long term as this is not an ideal site in which to carry out infilling. Being very close to the beach it is at risk from large scale erosion should the pit be located in eroding, rather than static or developing, machair. The wider erosional processes of the locality need to be further studied to determine this point, as does the adjacent burn to determine if it too would be a future threat to site stability. If large scale erosion, either by wind or water, did occur, there would be a real danger of re-exposure of the fill material. Natural erosion on this scale is often not worth trying to prevent, and removal of the infilled material may have to be a long term option.
4.142 If future large scale erosion of the site is thought unlikely, remedial measures to reduce existing and potential erosion on the barrier and adjoining dunes should be carried out. These include grading to reduce slope angles of the barrier and adjoining slopes, and increasing the width of the barrier. Remedial measures that should be carried out within the pit to further increase stability, and also improve safety, include grading the undercut slopes, and compressing and further covering the exposed areas of landfill.
4.143 All three pits are only partially restored and are in need of some form of further treatment, but reinstatement to date has been reasonably successful in achieving localised slope stability, mainly by grading and capping operations. Erosion has continued where slope angles have not been sufficiently reduced, with both slope failure and undercutting by wind occurring, particularly at sharp breaks of slope at the upper face. The one example of infilling, at Horgabost, has been less successful because of settling and insufficient covering of fill material, which is initiating wind scour.
4.144 The most visually successful landforming operations have been those in which slopes have been contoured to a rounded, irregular profile, as seen at Traigh Mhor and Horgabost. The straight, hard lines at Daliburgh are fairly obtrusive, although the impact will decrease once the site is vegetated.
4.145 Most visually obtrusive, however, is the dark capping material used at two sites, Traigh Mhor and Daliburgh, and also the features associated with dereliction (e.g. spoil material and derelict machinery). All of these can be considered temporary impacts which can be mitigated by remedial measures. In most cases, bare sand and erosion features were the least obtrusive features of the pits.
4.146 Also of visual impact is the growth of woody ruderal species, such as Ragwort and Dock, which give the sites a disturbed, "wasteground" appearance, as seen at Traigh Mhor and Horgabost where relatively fertile topsoil has been used as thick capping. This also causes an ecological impact, these species being much less prolific in true machair grassland. Where topsoil has not been used, re-established vegetation is more natural, as seen in places at Horgabost. The topsoil capping does, however, appear to have prevented sand blow on newly graded slopes.
4.147 It is not possible to comment on the success of seeding techniques because it is not known whether reseeding was carried out at Horgabost (the only pit to have an established vegetation cover).
4.148 The main cause for concern at all three pits is that reinstatement could fail in the long term without consideration of geomorphological processes beyond the pit boundaries. This is most pertinent at Traigh Mhor where remedial measures proposed for the breach between the pit and adjacent blowout are dependant on stabilisation of at least part, if not all, of the blowout. At Horgabost large scale erosion of the machair coastline could eventually open up the pit and expose the landfill, although the probability of such erosion occurring is less clear. The situation at Daliburgh is less serious, as treatment of the breach between the pit and the adjacent disused pit should be relatively straight forward.
4.149 For the reasons discussed in 4.148, it is imperative that further studies to establish wider geomorphological processes at Traigh Mhor and Horgabost are undertaken before further treatment is carried out. (Work could commence at Daliburgh without further study, but only if no further investigations are planned for the "archaeological" dune, the stability of which is crucial to the success of further reinstatement).
4.150 In addition to these further studies, trials on the various reinstatement techniques are strongly recommended (A.Scott, pers. comm. 1996) at whichever site is selected for treatment. These include studying different methods of slope stabilization, i.e. digging/ grading and under-pegging, the use of mulches and binders, and revegetation techniques.
4.151 Investigation into under-pegging will be necessary if a supply of brashing is not available, as it is possible that newly graded upper slopes will suffer undercutting without a protective cover. It will be difficult to definitively predict which sites will be sheltered enough to allow a lack of protection.
4.152 Trials into the use of binders are necessary because of the confusion in the literature as to their efficacy when used in conjunction with mulches. Not enough is known to be able to make definitive recommendations on this aspect of reinstatement. As discussed in Chapter Two, there is confusion in the literature on the subject and trials at Oldshorebeag were inconclusive because of adverse weather conditions. The evidence seems to suggest that there are problems with binders, particularly when used in conjunctiion with some mulches, but there may be situations in which the problems are outweighed, such as with extreme exposure or on very steep slopes.
4.153 Revegetation trials need to cover the use of natural regeneration as a technique, as, again, it is not possible to definitively predict which sites are sheltered enough to allow this treatment (unless they are permanently damp). The use of a green hay crop for reseeding purposes on areas of very high conservation value also needs to be studied, there being little in the literature on this subject for the sand dune/machair environment.
4.154 Because trials have to be set up several years ahead of full treatment, they may not be practicable for this project because of funding constraints. If this proves to be the case, it may be worth integrating trials into the reinstatement itself, by carrying out two parallel treatments, for example, mulch used with and without a binder within a discrete treatment area. To avoid complicating the annotated plans, one treatment only is usually recommended, but reference should be made to the text for discussion of options.
4.155 Monitoring will be imperative, even in the absence of trials or parallel treatments. As discussed in Chapter Two, one of the most crucial elements to the success of reinstatement programmes is preventative post-treatment measures. Funds therefore have to be available for monitoring and maintenance.
