© Author(s) 2016. CC Attribution 4.0 License. ISSN 2047 - 0371 3.5.3 The use of erosion pins in geomorphology John Boardman1,2 and David Favis-Mortlock1 1Environmental Change Institute, Oxford University Centre for the Environment, Oxford, UK ([email protected]) 2Department of Environmental and Geographical Science, University of Cape Town, South Africa ABSTRACT: Erosion pins have been widely used in geomorphology since the 1950s to estimate rates of change (erosion and – less commonly – accumulation) in land surfaces. They may be used for short- and long-term surveys and are quick and easy to install and measure. Erosion pins are particularly suited to bare, undisturbed environments such as badlands and sand dunes. Our recommendations for their use follow those of Haigh (1977) and Lawler (1993) but we also discuss the need for researchers to be aware of issues which arise from of measurement error, particularly for short-term studies and analytical methods which rely on few pin measurements. There is also a not inconsiderable challenge involved in extrapolating results derived from erosion pin measurements to larger areas. KEYWORDS: badlands, erosion pins, erosion rates, measurement errors erosion pins is not necessary: useful data can Introduction be collected with occasional visits. The basic idea behind the use of erosion pins However, this simplicity is deceptive. to quantify land-surface change is very Considerations of pin siting, measurement straightforward. A rod is firmly fixed into the error, and interpretation of results rapidly ground (or other substrate), and a note made introduce additional complexity. Thus, any of the length of rod which remains exposed. geomorphological study that uses erosion pins After some time, this exposed length is again – initially driven, perhaps, by the seductive measured. Increased exposure is assumed to apparent simplicity of the technique – will have indicate erosion, and decreased exposure is to grapple with issues which may well not have assumed to indicate accumulation. Results been obvious at the outset, leading to possible from pin measurements may then be difficulties and confusion when answering the correlated with other measurements. These question “What do these results tell us?”, and might be measurements of some even (at worst) disillusionment regarding the hypothesized driver of land-surface change, usefulness of the technique. such as rainfall. But in this case forewarned is, to a large Thus, erosion pins are a simple, robust, and extent, forearmed. In this review of the use of relatively cheap approach to small-scale erosion pins, which is an up-dating of those by measurement of erosion rates, and one in Haigh (1977) and Lawler (1993), we aim to which the design can be easily adapted to the cover these complicating factors, and present aim of the project. The advantages of erosion some mitigating suggestions. pins are still more notable when compared with the expense and field/laboratory effort Historical background to the required by techniques such as 137Cs and technique 210Pb tracers, installation of flumes and sediment traps, laser scanning, etc. Perhaps As Haigh (1977) points out, the use of erosion most attractive of all, continuous monitoring of pins originated in studies of badlands in the USA by Colbert (1956) and Schumm (1956). British Society for Geomorphology Geomorphological Techniques, Chap. 3, Sec. 5.3 (2016) The use of erosion pins in geomorphology 2 The design of erosion pins has varied little. perennial geomorphological considerations The earliest used wood (Schumm, 1956; regarding choice of study area location are Ranwell, 1964) but it quickly became clear that beyond the scope of this review but they are metal, and preferably non-rusting metal (either fundamental to any erosion pin study that aims non-ferrous, or treated so as to prevent to extrapolate results outside the confines of rusting) is to be preferred (Emmett, 1964; the pin grid, either spatially or temporally (see Clayton and Tinker, 1971). Brass or steel below). welding rods have been commonly used An additional important consideration in the (Evans, 1967) but even plastic knitting needles selection of pin sites in proximity to locations have proved successful (Lawler, 1993). A at which other kinds of environmental data are loose-fitting washer has been used as an gathered, principally precipitation and indicator of exposure in some studies (Kirkby temperature. Ideally, pin sites should be near and Kirkby, 1974) but not in others (Keay- a weather station but in many cases this is not Bright and Boardman, 2009; Boardman et al., feasible. In remote areas where medium- or 2015). The pros and cons of using washers long-term weather records are rare this may are discussed by Haigh (1977). be a problem, and some alternative must be A major use of erosion pins has been in found. Boardman et al. (2015) use informal badland environments (Figure 1). Nadal- daily precipitation records from nearby farms, Romero et al. (2011, Figure 6) compare the arguing that these data are generally reliable techniques used to measure sediment yield and have been gathered within 5 km of all 10 from Mediterranean badland areas. Erosion erosion pin sites but nonetheless, micro- pins are the second most-used approach, climate differences between sites are behind only gauging stations, with 25 study inevitable. sites out of 105 using pins. Having decided on the locations where pin measurements will be made, the next step is to decide on the placement of individual pins. The aim in the placement of a single erosion pin is that the resultant measurements will adequately represent erosion, and possibly deposition, on an area around the pin. There is no general agreement on the size or indeed the limits of this area: however, assumptions regarding this area affect attempts (either overt or implicit) to estimate average rates of erosion or deposition on larger areas surrounding the pin (see below). Neither is there general agreement on the Figure 1: Erosion pin site Karoo, South Africa number of replicate erosion pins that should Many more recent papers refer to the be used, or on the spatial configuration of advantages and disadvantages of using multiple pins. Almost all researchers have erosion pins, one such example is that by used multiple pins. Most researchers have Hancock and Lowry (2015). tended to cluster pins fairly closely but the numbers have varied. For example, Higuchi et Spatial and temporal issues in al. (2013) place 10 pins to represent a badland erosion pin placement and area: compare this with 301 pins in Evans’ measurement (1977) study and 250 pins in Boardman et al. (2015). Pins have generally been laid out in Before driving in a single pin, placement in the regular grids or in lines (usually straight). In wider landscape context must be very both cases, a factor is ease of relocation for carefully considered (Lawler 1993 p. 798). measurement after a considerable time Does the study aim to quantify rates of ground- period. If using erosion pins in a grid, a surface change over the whole landscape? If spacing of 1 m is frequently used e.g. Sirvent so, then some form of random or stratified- et al. (1997). random siting of pin grids may be necessary. The length of time during which erosion pin Or does the study aim to capture rates on a measurements are conducted and the time particular land usage or soil type? Such British Society for Geomorphology Geomorphological Techniques, Chap. 3, Sec. 5.3 (2016) 3 Boardman and Favis-Mortlock interval between pin measurements also studies have accepted that trampling is an require serious consideration. A major factor erosional factor (Fanning, 1994). here is the aims of the study. Table 1: The varied uses of erosion pins If the aim is to obtain information regarding Environment Reference long-term average rates of erosion, then measurements should be collected over Salt marsh Ranwell (1964) several years in order to adequately capture Waste tips Haigh (1977); the effects of year-on-year variations in Schumm (1956) weather including relatively uncommon River banks Lawler 1978, 1991, extreme events. However, most erosion pin 1992, 1993 experiments have been of rather short Peat moorlands Birnie (1993); Tallis duration, c. 2-3 years. Longer-term studies are (1981 p. 76) rare: cf those of Clarke and Rendell (2006) for Footpaths/trails/tracks Streeter (1975); 6 years, Hancock and Lowry (2015) for 9 Summer (1986) years, Lazaro et al. (2008) for 11 years, Badlands Clarke and Rendell Boardman et al. (2015) for 13 years, and (2006); Nadal- Godfrey’s studies of creep and surface Romero et al. lowering on Mancos Shale badlands for 40 (2011); Hancock years (Godfrey 1997; Godfrey et al., 2008). and Lowry (2015); Boardman et al. If, however, the aim is to determine seasonal (2015) variations in erosion and deposition then Sand dunes Jungerius et al. short-duration pin experiments with a frequent (1981); Jungerius measurement interval are suitable, e.g. Evans and van der (1977): a 2-year experiment with a 4-weekly Meulen (1989); sampling interval. In all cases, sampling Wiggs et al. (1995); intervals must take into account rates of Livingstone (2003) erosion and therefore measurement error (see Bare, contaminated Bridges and below). soils Harding (1971) Issues with particular Bare soils Slaymaker (1972) environments Gullies Harvey (1974) Erosion pins have been used to measure Examples of the variety of environments changes in the surface of dunes in the where erosion pins have been used are shown Netherlands and desert dunes, e.g. in in Table 1. Erosion pins are particularly suited Namibia. Jungerius et al. (1981) recorded to measurement of change on bare peat changes at weekly intervals over a 2-year surfaces where erosion rates are high, e.g. period. In a later study, Jungerius and van der because of trampling by animals and related Meulen (1989) took measurements every six disturbance of vegetation.