Erodibility of hill peat

peer-reviewedThe project was funded by the European Union Structural Funds EAGGF distributed under the Department of Agriculture and Food Stimulus Fund.The energy necessary to entrain soil in water depends on the soil strength. Once entrained, the settling velocity of the eroded soil in water is of fundamental importance to the processes of sediment transport and deposition. In this paper, stream power theory and transport concepts coupled with the equation of continuity were used to derive a transport-limited peat concentration. The ratio of the log of the actual sediment concentration in surface run-off to the log of the transport-limited sediment concentration was the index of erosion used. The value of this index is a measure of the sensitivity of peat to erosion by sheet flow. Four peats were subjected to a range of overland flow rates under two slopes in a laboratory flume. The peats represented peat farmed in a sustainable manner (Leenane), overgrazed peat (Maam), peat undergoing erosion (Newport) and peat which had undergone weathering following exposure by a landslip (Croagh Patrick). Both in situ and surface damaged slabs were studied. The results indicate that shearing and remoulding of a wet peat surface (e.g., by animal treading) and weathering of exposed drained peat surfaces predispose peat to erosion. Defoliation by overgrazing is considered to be of secondary importance.Department of Agriculture, Food and the MarineEuropean Union Structural Funds EAGG

Scale-dependence of lithological control on topography: Bedrock channel geometry and catchment morphometry in western Scotland

We propose that a scale-dependent topographic signature of erodibility arises due to fluvial and glacial erosion acting on different parts of the landscape at different times. For 14 catchments in western Scotland, we define three levels of substrate erodibility in order of decreasing resistance: quartzite rocks, nonquartzite rocks, and zones of fault-related fracture. Then, using digital topographic and planimetric data coupled with field measurements, we identify regression based scaling relationships between substrate erodibility and morphometric parameters at two spatial scales. Catchment-scale morphometry shows a weak to variable relationship with substrate metrics overall. Erodibility can be inferred from catchment steepness indices (i.e., channel steepness index and relief ratio), but the existence of multiple exceptions could confound a more general application of this approach. Nonetheless, major valley troughs trace fault zones and nonquartzite rocks, leaving much of the higher and steeper ground formed in quartzite. At the reach scale, bedrock channel slope is far more sensitive to substrate erodibility than is channel width. Quartzite outcrops steepen bedrock channels by a factor of 1.5–6.0, and in terms of unit stream power, channels increase their erosional capacity by a factor of 2.7–3.5. Yet only 4%–13% of this increase is due to channel narrowing. Based on a large data set of bedrock channel width (n = 5825) from four rivers, we find that width scales with drainage area (in m<sup>2</sup>) as W = 0.01A<sup>0.28</sup>. Our results are consistent with the view that width-area scaling is similar in all single-thread rivers subject to transport-limited conditions but that for increasingly sediment supply limited settings, erosional thresholds at the channel boundary are the key determinants of bedrock channel width

Development of laboratory instrumentation for the study of soil erodibility

In order to carry out a study of the relative efficiency of various erodibility indices, and of the relative erodibility of soils developed in the Peak District of Derbyshire (England), three instruments were developed. These instruments were: a wet-sieve aggregate analyser of the Yoder pattern, a compact laboratory rainfall simulator using spray nozzles, a radiant drying unit using infra-red lamps. The efficiency of the instruments and the validity of the operating -techniques are critically evaluated and suggestions for improvement are advanced

A flume for studying the relative erodibility of soils and sediments

A flume has been built for studying the erodibility of soils and sediments by gullying. It consists of two boxes containing undisturbed soil samples. One box is set above the other and water from a stilling tank passes over the soil of the upper box and falls onto the soil of the lower box causing lip and channel scour and plunge-pool erosion. The sediment is collected and measured, and a measure of erodibility related to discharge, length of test and sediment yield is thus available

Effects of deposit-feeding bivalve (Macomona liliana) density on intertidal sediment stability

Effects of macrofaunal feeding and bioturbation on intertidal sediment stability (u*crit) were investigated by manipulating density (0-3 x ambient) of the facultative deposit-feeding wedge shell (Macomona liliana) on the Tuapiro sandflat in Tauranga Harbour, New Zealand. Sediment stability increased up to 200% with decreasing M. liliana density and this was correlated with greater sediment microalgal biomass and mucilage content. The change in stability occurred despite homogeneity of grain size amongst experimental treatments, highlighting the importance of macrofaunal-microbial relationships in determining estuarine sediment erodibility

Predicting soil erosion after land use changes for irrigating agriculture in a large reservoir of southern Portugal

The construction of the Alqueva reservoir in a semi-arid Mediterranean landscape brought new opportunities for irrigated farming. Land use changes and climate change may alter the risk of soil erosion that was not predicted in the initial development plans and decrease the lifetime of the investment. A comprehensive methodology that integrates the Revised Universal Soil Loss Equation (RUSLE) and geographic information system was adopted to study the effect on soil erosion of different land-uses of the Alqueva reservoir region. Analysing the soil erosion of each land-use it was obtained the following land use erosion vulnerability: Olive orchard>Vineyard>Montado>Alfalfa. The strong erosion variances that were observed in the study area show the importance of locating the 'hot spots' of soil erosion. Simulated scenarios for the entire area can be used as a basis for site-specific soil conservation plans, to promote sustainable land management practices and to facilitate localized erosion control practices and environmentally friendly farming. (C) 2015 The Authors. Published by Elsevier B.V

The Relationship Between Economically and Environmentally Marginal Land

Concerns have frequently been raised regarding the impact of federally-subsidized crop insurance and agricultural subsidy payments on land allocation and crop mix choices. If the reduction in production risk encourages farmers to plant on economically marginal land, it has often been asserted that this will lead to increases in environmental damage, including increases in soil erosion rates. This paper investigates the "conventional" wisdom that economically marginal land is also environmentally fragile, as defined by higher levels of inherent soil erodibility. We address this issue by looking at the distribution of crop yields for 4 major crops across National Resource Inventory (NRI) erodibility classes and by performing regression analysis. Our results indicate that land with higher levels of soil erodibility exhibit lower mean crop yields, a proxy for economic marginality, which lends support to the conventional wisdom.Environmental Economics and Policy,

Modeling meander morphodynamics over self-formed heterogeneous floodplains

This work addresses the signatures embedded in the planform geometry of meandering rivers consequent to the formation of floodplain heterogeneities as the river bends migrate. Two geomorphic features are specifically considered: scroll bars produced by lateral accretion of point bars at convex banks and oxbow lake fills consequent to neck cutoffs. The sedimentary architecture of these geomorphic units depends on the type and amount of sediment, and controls bank erodibility as the river impinges on them, favoring or contrasting the river migration. The geometry of numerically generated planforms obtained for different scenarios of floodplain heterogeneity is compared to that of natural meandering paths. Half meander metrics and spatial distribution of channel curvatures are used to disclose the complexity embedded in meandering geometry. Fourier Analysis, Principal Component Analysis, Singular Spectrum Analysis and Multivariate Singular Spectrum Analysis are used to emphasize the subtle but crucial differences which may emerge between apparently similar configurations. A closer similarity between observed and simulated planforms is attained when fully coupling flow and sediment dynamics (fully-coupled models) and when considering self-formed heterogeneities that are less erodible than the surrounding floodplain