The erodibility of upland soils and the design of preafforestation drainage networks in the United Kingdom

Abstract

Hydraulic thresholds for erosion of fourteen upland mineral and organic soils were determined in a hydraulic¯ume. These soils are from areas to be aorested in the United Kingdom. Some of the group are erosion resistant but others are susceptible to erosion once denuded of vegetation; for example, by preaorestation ploughing. These threshold data were required to calibrate a hydraulic model for eective design of preaorestation drainage networks on a variety of soils. However, simple ®eld measures of soil properties indicative of erosion potential would be of value to the forestry industry for management purposes. Consequently, hydraulic threshold data were related by linear regression methods to basic soil properties, including organic content, grain size, bulk density, compression strength and penetration resistance.The investigation concluded that four peat soils are not eroded by clear water velocities up to 5Á7 m s À1 , although a mineral bedload might induce erosion at lesser current speeds. Penetration resistance is a good ®eld indicator of the degree of humi®cation of the peat soils. Although selected physical parameters contribute resistance to water erosion, an increased organic content is pre-eminent in reducing erosion susceptibility in both organic and mineral soils. Although compressive strength was not indicative of soil erodibility, ®eld measurements of penetration resistance on a variety of soils could be related to hydraulic thresholds of erosion; albeit through the construction of discriminant functions interpolated by eye. Consequently, organic content (laboratory) or penetration resistance (®eld) might form the basis of classifying upland soils in terms of erodibility.Mineral soils dier widely in terms of their erodibility, so that subject to further consideration, the use of ploughing for forestry cultivation might be appropriate in wider circumstances than presently recommended by the Forests and Water Guidelines. Ploughing should be acceptable on deep peat providing the underlying mineral soil is not exposed in the bottom of the furrow, and furrows are not led from mineral soils on to deep peat.

Spatial and temporal patterns in erosion from forest roads

“…Other authors [e.g. Carling et al, 1997] have seen similar behavior with respect to soil texture, and the general trend seems to be reflected in erodibility estimates published in soil surveys. Because the ditch is commonly set in the native soil, we expect that road segment sediment production would initially be greater on silty soils than on sandy soils.…”

Effect of soil properties on peat erosion and suspended sediment delivery in drained peatlands

“…The first observations of formation of a fibrous peat mat that protects peat against erosion were made by Carling et al . []. It can be hypothesized that erosion and SS transport from moderately decomposed, bare peat surfaces decreases after easily erodible parts of the peat have been detached and transported by rain splash impact and runoff.…”

“…Carling et al . [] demonstrated that intact peat is highly resistant to erosion and that a water velocity of 5.7 m s −1 (in a hydraulic flume) is required for continuous erosion of peat material ranging from amorphous to fibrous. Wind erosion and peat properties in abandoned milled peatlands were studied by Campbell et al .…”

Effects of rainfall, overland flow and their interactions on peatland interrill erosion processes

“…The observed difference in erosion between the Rainfall and Inflow treatments primarily resulted from the effects of raindrops. Under the low flow velocity conditions, the impact of sheet flow without the impact of rainfall has limited effect on peat erosion as peat is fibre-rich and highly resistant to water erosion, requiring a high flow velocity before continuous erosion of peat material occurs (Carling et al, 1997). The peak corresponded to the period when peat aggregates previously weathered by processes such as freezethaw and desiccation (Francis 1990;Labadz et al, 1991;Shuttleworth et al, 2017) were detached and splashed by raindrop impact, and the peat soil shear strength decreased with saturation.…”