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تعداد صفحات این فایل: ۲۹ صفحه

بخشی از مقاله انگلیسیعنوان انگلیسی:Evaluation of erosion and surface roughness in peatland forest ditches using pin meter measurements and terrestrial laser scanning~~en~~

Abstract

Anthropogenic activities on peatlands, such as drainage, can increase sediment transport and deposition downstream resulting in harmful ecological impacts. The objective of this study was to quantify changes in erosion/deposition quantities and surface roughness in peatland forest ditches by measuring changes in ditch cross-sections and surface microtopography with two alternative methods: manual pin meter and terrestrial laser scanning (TSL). The methods were applied to a peat ditch and a ditch with a thin peat layer overlaying erosion sensitive mineral soil within a period of two years following ditch cleaning. The results showed that erosion was greater in the ditch with exposed mineral soil than in the peat ditch. The two methods revealed rather similar estimates of erosion and deposition for the ditch with the thin peat layer where cross-sectional changes were large, whereas the results for smaller scale erosion and deposition at the peat ditch differed. The TLS-based erosion and deposition quantities depended on the size of the sampling window used in the estimations. Surface roughness was smaller when calculated from the pin meter data than from the TLS data. Both methods indicated that roughness increased in the banks of the ditch with a thin peat layer. TLS data showed increased roughness also in the peat ditch. The increase in surface roughness was attributed to erosion and growth of vegetation. Both methods were suitable for the measurements of surface roughness and microtopography at the ditch crosssection scale, but the applicability, rigour, and ease of acquisition of TLS data were more evident. The main disadvantage of the TLS instrument (Leica ScanStation 2) compared with pin meter was that even a shallow layer of humic (dark brown) water prevented detection of the ditch bed. The geomorphological potential of the methods was shown to be limited to detection of surface elevation changes >~0.1 m.

۱ Introduction

Boreal peatlands cover 80% of the world’s peatland area (Wieder et al., 2006). In regions with abundant peatland cover, such as Finland, Sweden, and Russia, peatlands are widely used for forestry (Paavilainen and Pivnen, 1995). In Finland, more than 50% of the 9 Mha of peatlands have been drained with open ditches (Finnish Forest Research Institute, 2014), with the peak years of drainage occurring in the 1960s and 1970s. At present, pristine peatlands are not drained for forestry, but the focus has shifted to the maintenance of existing ditch networks. The maintenance includes the cleaning of old ditches and, to some extent, excavation of new complementary ditches in the drainage area. Despite its importance in forest growth, cleaning of the ditch network has adverse effects on surface waters due to increased sediment loads that lead to reduced water quality (Joensuu et al., 2002; Marttila and Klve, 2010a, 2010b; Nieminen et al., 2010), and to the disturbance of aquatic habitats and biota (Bilotta and Brazier, 2008). Thus, water protection methods have been developed and they are operationally applied to reduce the harmful impacts of ditch cleaning (Marttila et al., 2010). However, their implementation requires a better understanding of erosion processes in the source areas. Stenberg et al. (2015a, 2015b) raised the importance of assessing and quantifying the processes at the headwater source areas as the eroded soil available for transport within the ditch network is large compared with the observed sediment load at the drainage network outlet. In peatlands, ditch erosion processes differ according to, for instance, variations in peat and underlying mineral soil type. Examples of easily erodible underlying soils in areas of thin peat layer are: silt, sand, and other Quaternary deposits. To advance the studies of erosion processes a reliable method for detecting small changes in ditch topography needs to be developed. Erosion can be measured from changes in topographical reliefs with erosion pins (Lawler, 1993), pin meter (Kornecki et al., 2008), photogrammetry (Rieke-Zapp and Nearing, 2005), and terrestrial (Day et al., 2013) or airborne (Thoma et al., 2005) laser scanning. Laser scanning produces large sets of point cloud data without disturbing the observed area, as it is based on the travel times of laser pulses from the scanner to the surface elements. The accuracy and spatial density of the points depend on the distance of the scanned target. Thus, terrestrial laser scanning (TLS) is preferred for high-resolution and more accurate data acquisition whereas airborne laser scanning (ALS) is suitable for scanning larger areas, e.g. in forestry (Nsset et al., 2004). TLS has been widely used to assess the topography and erosion of various land surfaces (Resop and Hession, 2010; Day et al., 2013; Vinci et al., 2015) and for determining geometric data of hydraulic models (Milan, 2009) and parameters of vegetated channels, such as blockage factor (Jalonen et al., 2014). Although there are uncertainties caused by factors such as sunlight, rainfall or fog (Reshetyuk, 2006) and wet surfaces (Day et al., 2013) when applying TLS, it is regarded as being superior to the other methods since it quickly and accurately covers large areas without disturbing the surface. There are only a few studies where TLS has been used to assess the topography of peatlands. Grayson et al. (2012) produced TLS-based estimates of blanket bog erosion in an area of 7.6 ha during one winter season in the North Pennines, UK, and achieved different results with TLS and erosion pin measurements: a net increase of 2.5 mm in peat surface height was measured with TLS, while a net surface lowering of 38 mm was measured with erosion pins. They suggested that both results should be treated with caution. Ballhorn et al. (2009) used ALS to determine burn scar depths over large areas (27 900 km2 ) of Indonesian peatlands and considered laser scanning capable of providing sufficiently accurate results in the inaccessible peatland terrain. In the current study, TLS was preferred to ALS for the detection of ditch topography because of better resolution and accuracy. However, it is not well known how TLS performs in ditches or channels excavated into peat where dark colour and wet surfaces adsorb light and may affect the accuracy of the measurement. Therefore, it remains to be demonstrated in what way TLS can estimate erosion after the cleaning of forest ditches. Surface roughness is another important environmental parameter to consider. It affects channel flow hydraulics by changing flow velocity and turbulence. Increased roughness slows down the flow velocity, thus also affecting erosion and sediment transport capacity. Characterization of surface roughness has been successfully conducted in many studies with TLS (Haubrock et al., 2009; Brasington et al., 2012; Rychkov et al., 2012; Mills and Fotopoulos, 2013) and pin meter (Gilley and Kottwitz, 1995; Garca Moreno et al., 2008a). Random roughness is a measure of the spatial variation in surface heights calculated from soil microtopography and is typically used as an index for surface roughness (Cremers et al., 1996). Before ditch cleaning, surface roughness in ditches is high after decades of degradation processes, such as the collapse of ditch banks and vegetation growth. After cleaning, the surface of the ditch is assumed to be rather smooth. Thereafter, erosion and deposition processes change surface roughness conditions and can notably affect the hydraulic properties of the ditch. However, surface roughness and its changes have not been documented after the cleaning of ditches with varying peat thickness. The main objective of this study is to quantify changes in topography and surface roughness in newly cleaned peatland forest ditches. Based on these changes, the aim is to subsequently illustrate the ways in which erosion and deposition processes occur in a ditch with a thick peat cover, and in another where erosion sensitive mineral soil (stony till) under the thin peat layer is exposed. To assess the reliability of the results a secondary aim is to compare the results of two different methods used to measure surface topography: the pin meter and TLS. These methods have rarely been compared, especially in peatland dominated catchments. The methods are used to quantify both small-scale spatial distribution and aggregated cross-sectional quantity of erosion, deposition, and surface roughness. The geomorphological potential of the methods is assessed by evaluating the statistically significant changes obtained with the pin meter and the TLS data following the method presented by Lane et al. (2003). The elevation changes in the excavated part of the ditch are assumed to represent the occurrence of erosion and deposition. In addition, the effect of vegetation growth on the estimated erosion/deposition quantities in the ditches is discussed. Since the environmental impacts of ditch cleaning are most visible soon after excavation, we determine the changes occurring during the first two years after the operation.

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