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3.0 CHARACTERISTICS OF SAND PITS IN THE MINCH AREA

Introduction

3.1 A range of sites were visited in the study area during February 1996 to obtain a broad overview of the nature of the pits, to investigate the type and success of existing reinstatement and to identify opportunities and constraints to restoration. Both active and disused pits were visited, plus any features of the surrounding dune systems that might have some influence on reinstatement, for example, adjacent blowouts. Other dune restoration schemes were also examined to see if any of the methods used could be applicable to sand pit reinstatement.

3.2 A total of 19 sites were studied, as shown below, all of which possess sand pits at various stages of development.

Characteristics of sites

Introduction

3.4 All the pits visited are located in either machair grassland or fixed dunes, with the possible exception of Site 13 MOD Base South (Plate 36 and 37) which is located in semi-fixed dunes. In general, none of the sites receive fresh wind blown sand, although there may be local sand movement due to wind blow within pits.

3.5 Most of the pits visited lie within the lee of dune or machair ridges, but a few are directly exposed to the sea, such as Site 13 MOD Base South and the active pit at Horgabost, Site 17, which has a "hillside" location (Plate 42).

3.6 Many of the disused sites have either had some form of reinstatement or have revegetated naturally. It was often not possible to tell which process had taken place in the older pits. Few recently abandoned, but unrestored pits were visited.

Erosion

3.7 Because few unvegetated pits were visited, it was difficult to obtain a clear idea of the nature or extent of erosion that can occur in unrestored pits. What was easier to discern was continued erosional problems in partially restored pits.

3.8 From the literature, potential erosion features most pertinent to sand pits are wind scour surfaces and deflation surfaces. The wind scour surface can be any object which presents a barrier to the wind, impeding smooth air flow and resulting in eddying and scouring. At the study sites steep, unprofiled slopes and exposed fill were the most commonly found wind scour features.

3.9 Unprofiled faces are typically undercut at the top and slumped at the bottom, with turf falling onto the slumped slope, leading to further erosion of the machair surface. A typical example of this is seen at Site 4 Traigh Mhor, where the steep, unprofiled, western side of the pit is being eroded back towards an adjacent blowout (Plate 3). Sand is often blown beyond the upper slope onto the adjacent vegetation, as seen at Site 10, Daliburgh (Plate 13), and the active pit at Horgabost. Undercutting is prevalent at many of the sites, although it is generally fairly localised within the pits and, apart from exceptions like Traigh Mhor, it is not on its own causing large scale erosion problems at present. However, continuous erosion eating into the machair could cause future problems.

3.10 Slumping was also observed to be due to slope failure on steep slopes, with wind erosion then acting on the near vertical upper face. This is seen on the northern face of Site 10 Daliburgh and also at Site 19 Opinan (Plate 24). If wind erosion does not follow, if the slope is relatively sheltered, stability can occur even with a relatively steep slope angle.

3.11 Perhaps the more serious effect of undercutting is the funnelling effect the resultant steep slopes have on wind movement. This is especially a problem at rectangular or linear shaped pits. This is graphically seen at Site 1 Vatersay where an infilled pit has become re-exposed and the steep, unprofiled sides now funnel the wind through the pit with the pit floor acting as a deflation surface (Plate 2), creating an imminent blowout. Sand is being blown onto the machair at the far end of the pit.

3.12 Exposed fill, whether re-exposed or merely originally uncovered, was seen to cause wind scour at a number of sites, including Site 1 Vatersay (Plate 22), Site 13 MOD Base South (Plates 36 and 37) and Site 15 Culla (Plate 40).

3.13 Although in very broad terms, relative shelter of a pit gives some indication of its susceptibility to eroding winds, dominant wind direction and aspect of the pit or slope in question is not always a good indicator of potential erosion problems. It was apparent during the survey that whichever direction the wind blows from, it is either dissipated into different directions on hitting land or funnelled along surface features. This was particularly apparent at Site 4 Traigh Mhor and the active sand quarry at Site 17 Horgabost.

3.14 The greatest potential erosion problems seem to occur where adjacent erosion in the surrounding dunes or machair could be exacerbated by erosion in the pit, or conversely where external erosion threatens stability of a pit. The first situation is seen at Site 4 Traigh Mhor where breaching threatens to cause coalescence between the pit and an adjacent blowout (Plate 4), which could cause major erosion and sand blow. The second situation is seen at Site 2 Tangasdale and Site 13 MOD Base South, where both pits are infilled and relatively near to an eroding beach (Plates 26 and 36 respectively). Only measures to replenish the beach and reinforce the dune edge respectively have prevented re-exposure of the pits. Similarly, re-exposure of the infilled pit at Site 16 Horgabost by a migrating burn which has cut into machair has been prevented by reinforcement works. At Opinan, a burn has had to be diverted to prevent further undercutting of the old, steep, extraction face (Plate 44).

3.15 All these cases of remedial works to prevent or stabilise large scale and long term erosion may ultimately fail and the pits may yet be affected. In the case of Tangasdale, MOD Base South and Horgabost, this could result in breaching of the pit and exposure of the fill. This brings into question the prudence of infilling pits located very near to the sea, even in what are now apparently stable coastlines.

Slopes

3.16 The most stable slopes, whether graded or natural, are generally less than 45o with steeper slopes being generally unstable. At some of the active sand pits, e.g. Site 2 Tangasdale (Plate 26), Pit No. 3 at Site 10 Daliburgh, Site 16 Horgabost and Site 19 Opinan, quarrying has produced very steep sided slopes of between 60o and vertical despite severe wind blow. However, all are suffering from undercutting and slumping.

Water table

3.17 None of the pits surveyed appeared to have been quarried right down to the water table and most were therefore dry or slightly damp, with the exception of Site 10 Daliburgh which had water lying on part of the floor at time of survey, conferring stability on the sand surface. However, some of the pits will have been drier than normal because of the unusually dry winter and preceeding summer and it is likely, therefore, that a number of these are usually damp, including Traigh Mhor on Barra.

Substrate

3.18 Substrate was only examined in detail at the three case study sites but is similar at all the sites. The unquarried areas have a topsoil typical of machair, being a humic sand, overlying a fine to medium sand (e.g. Plate 6). Remnant indurated topsoil horizons are found within the profile at a number of sites. Where exposed these are either creating wind scour features, with horizontal surfaces forming above and below as at Traigh Mhor (Plate 5) and Opinan, or are forming deflation surfaces, as in the blowout at Traigh Mhor (Plate 7).

3.19 At the revegetated sites where infilling has not taken place, the profile is beginning to reform with the build up of organic matter in the upper horizons. At the infilled sites, the profile is of course totally altered, with the fill material, which at many sites comprises scrap metal and spoil, being capped by what appears to be a dark, humic, topsoil at a number of sites. This does appear to have stabilised the sand surface at a number of sites, including Traigh Mhor, Daliburgh (Plate 12) and Site 16 Horgabost. However, at others, it appears to have been eroded away, as in one area at Horgabost and possibly at Site 1 Vatersay.

Vegetation and grazing

3.20 The adjacent unquarried areas support typical machair vegetation at most of the pits, with fine-leaved turf and a high component of moss, which is dominant in some places. Marram is a frequent but scattered component of the sward, increasing on the dune ridges.

3.21 Some of the older revegetated pits support what appears to be naturally regenerated vegetation which comprises either machair type grassland or fixed dune type grassland which has a higher component of marram. However, some of the more recently reinstated pits, for example, Site 4 Traigh Mhor, Site 11 Drimsdale and Site 16 Horgabost, support areas of prolific, tall, woody ruderal species, or coarse grasses or both (e.g. Plates 1 and 20), neither of which are dominant in natural machair or dune grassland. In all cases this vegetation type appears to result from the use of a humic topsoil capping, which must be of greater fertility than machair topsoil.

3.22 Stock and rabbit grazing appears to be prevalent at most sites. It is not possible to say from the "snap-shot" nature of the survey how important grazing is in affecting erosion or reinstatement. However, rabbit burrowing activities are creating bare, eroded areas at many of the sites, which in some cases appears to be possibly preventing vegetation establishment and causing re-exposure of shallowly buried fill material (e.g. Plate 18). In other cases, rabbit burrowing and scraping activities, which provide a seed bed, may also be partly responsible for the growth of the ruderal species.

Visual impact

3.23 Few of the pits are very visible from vantage points or houses. This is mainly due to the location of many of the sites away from roads and settlements and their position within the rolling topography of the machair. One of the most visible pits is Site 17, the active pit at Horgabost, which is located on a hill-side (see Plate 22). Some of the smaller, revegetated pits were not that easy to find, demonstrating how unobtrusive they can become even in the absence of infilling.

3.24 The presence of bare sand at the sites was found to be much less obtrusive in the landscape than the presence of re-exposed fill, dumped material, stockpiled capping material, stands of ruderal species and other features associated with dereliction. For example, the pit at Site 4 Traigh Mhor would not be easily discernible from the adjacent road but for the presence of the derelict JCB. Furthermore, the small eroded sand "cliff" at this site hardly stands out against the large scale erosion of the adjacent blowouts. Several of the small pits at Site 11 Drimsdale would be hardly noticeable but for the uncovered scrap metal infill (e.g. Plate 34). Bare sand only had a significant impact on the long distance view at the active pit at Horgabost where large amounts of wind blown sand, as well as the extraction hole itself, is visible from some distance away. Features which emphasise the "artificialness" of reinstatement, such as straight, even slopes and dark capping materials, both of which are at present at Daliburgh (Plate 12), also have a visual impact.

Existing reinstatement

3.25 The most frequent form of reinstatement carried out at the sites, including pits and blowouts, is infilling, although in some cases it appears that landfill rather than reinstatement is the primary objective of infilling. The fill includes a wide range of material such as boulders, concrete blocks, builders rubble, scrapped cars, batteries, wire rope, plastic and wood. Capping has been undertaken at some sites, but not at others.

3.26 The success of infilling has been very variable. Some smaller pits, which are no more than hollows in the machair and which have been filled with scrap metal but no more, have apparently stabilised, but are visually unattractive (e.g. pits at Drimsdale in Plates 34 and 35). Covered fill has become re-exposed to varying degrees at a number of the sites, including Site 1 Vatersay, Site 13 MOD Base South 13 (Plate 36), Site 15 Culla (Plate 40), and Site 16 Horgabost. Initial exposure appears to be due to a variety of reasons, including settlement and possibly rabbit burrowing at Horgabost, settlement combined with lack of slope grading at Vatersay, and insufficient covering and location in exposed, fairly mobile dunes at MOD Base South. Once partly exposed, the material becomes very susceptible to wind scour and the fill is soon completely uncovered.

3.27 Infilling appears to be successful at Site 2 Tangasdale (Plates 25 and 26), Site 9 Garrynamonie (Plates 31 and 32) and Site 14 MOD Base North (Plate 38). At Tangasdale and Garrynamonie the pits are used as car and scrap metal dumps. However, in both cases the material is tightly crushed and covered with a thick capping layer soon after dumping, which according to Hector MacNeil, the operator at Tangasdale, is the key to stability. It is likely that this prevents the wind scour occurring on uncapped, exposed fill. The stock piles of capping material which occur at the sites look fairly unattractive, but serve an important function. At Garrynamonie and MOD Base North the surfaces of the filled areas are flush with the adjacent machair surface.

3.28 Slope grading and profiling has been carried out at a number of the sites. These include Site 4 Traigh Mhor, Site 6 Traigh Eais, Site 7 Eoligarry Jetty, Site 10 Daliburgh, Site 16 Horgabost and Site 19 Opinan. Angles of the graded slopes at these sites ranged from 30o to 45o, with local angles within the slopes extending to 65o. All these slopes have some form of vegetation cover, either turf grasses or, less frequently, marram grass, emphasising the success of the grading operations in stabilizing the sand. Lack of slope grading before infilling at Vatersay, a deep linear shaped pit, has caused problems with the pit acting as a wind funnel and exacerbating the erosion that is re-exposing the fill (Plate 22).

3.29 All slope grading has been carried out by mechanical plant. In some cases the final appearance of the pit is rather unnatural due to the uniformity of the graded slopes which run in straight lines and have very smooth, level surfaces, such as seen at Site 10 Daliburgh (Plate 10). Greater indentation of slopes and less attention to smoothing off surfaces would produce a more natural looking effect.

3.30 As discussed above, humic topsoil has been used as a thin capping at a number of sites. While apparently being successful in stabilising the sand at some sites, it has created ecological and visual impacts, including invasion by ruderal species, as discussed above.

3.31 Information is not available on revegetation techniques at the sites, but it is suspected that many are naturally colonised. At Site 2 Tangasdale the restored areas are seeded with a mix prescribed in the ESA scheme for dune and machair restoration. Turfs have been used on several sites to try and stabilize bare sand. This has only been carried out on a small scale using either turfs from areas of slumping, as at Site 4 Traigh Mhor, or cut from newly graded slopes, as at Daliburgh.

Opportunities and constraints to reinstatement

3.32 Perhaps the greatest constraints to reinstatement are the lack of suitable fill material and the difficulty of preventing fill from being exposed at a later date.

3.33 Another major constraint for most pits is the lack of wind blown sand which means that sand dune grasses cannot be grown and turf reseeding therefore has to be carried out, which can be difficult on bare sand.

3.34 A major constraint at certain pits is the presence of nearby erosion sites which may have to be tackled if reinstatement of the pit is to be successful.

3.35 The lack of topsoil storage is also a constraint, necessitating the importation to the site of capping or mulch material. While a sandy topsoil would be ideal as a thin cap, being the same material as the machair topsoil, suitable soil is only likely to come from existing machair. This is because topsoil taken from elsewhere may not support the appropriate range of machair species, particularly if too fertile. Extracting machair topsoil for restoration purposes is obviously not sensible unless it is to be removed anyway.

3.36 The best opportunity for reinstatement is the suitability of many sites for slope grading and contouring, which along with revegetation could in many cases create relatively unobtrusive, stable features. This is due in part to the rolling topography in which most pits are located and in part to the fact that few of the pits are very large or deep (compared to many inland sand quarries on the mainland).

3.37 The nearness of many of the pits to tracks and roads will allow the use of machinery with minimal damage to the surrounding machair.

3.38 The fact that most sites are located in machair grassland means that in many cases turfs, resulting from regrading operations, will be available for use. This will not be on a scale large enough for wholesale coverage, but scattered turfs can be used to "inoculate" a site with desired species.

3.39 Two mulch materials, peat and seaweed, are readily available in the Minch area, and both have been successfully used in machair restoration in the past. Both will provide the necessary stability, nutrient supply and moisture conservation for seeding on bare sand without resulting in an ingress of alien species.

3.40 It is possible that a high water table in some pits presents opportunities for the creation of dune slack communities, and furthermore, future extraction could be planned to reach the water table for just this purpose. (See Chapter 5 for further discussion of the conservation considerations of this point).

Summary

3.41 Generally, large scale erosion arising from sand pits does not appear to be a problem, possibly because many pits are in relatively sheltered locations. (It has to be remembered, though, that few, if any, totally unrestored pits were visited in the survey, so a true picture of the erosion problem is not possible from this study). However, one-off storm events, which are not predictable, could trigger significant erosion at any area of bare sand, and most sites are vulnerable in this respect.

3.42 Localised erosion is in evidence at many pits, with the potential for large scale erosion at a number of these. These include pits where poor reinstatement has exacerbated erosion, or where the pit is threatened by adjacent erosion features, such as blowouts, eroding beaches, dunes and machair "cliff", or by migrating burns. These latter situations are particularly serious if a pit has been infilled, highlighting the dangers of infilling in sensitive locations.

3.43 Infilling has been the most common form of reinstatement, but has only been successful at a few sites where attention has been paid to properly compacting and covering layers of fill to prevent settlement. Contouring has been a less frequently used technique, but has been successful where carried out. Where infilled pits have become re-exposed, ungraded slopes have exacerbated erosion.

3.44 The use of a humic topsoil capping layer has been successful in preventing sand blow, but has resulted in the ingress of ruderal species which cause an ecological impact.

3.45 Visually, the results of poor reinstatement are often more obtrusive than the bare sand of the unrestored pit.

3.46 The effects of grazing on reinstatement success could not be definitively ascertained from the field study, but heavy rabbit activity, particularly burrowing, did appear to be very detrimental.

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