THE ERODIBILITY OF UPLAND SOILS AND THE DESIGN OF PREAFFORESTATION DRAINAGE NETWORKS IN THE UNITED-KINGDOM

Hydraulic thresholds for erosion of fourteen upland mineral and organi c soils were determined in a hydraulic flume. These soils are from are as to be afforested in the United Kingdom. Some of the group are erosi on resistant but others are susceptible to erosion once denuded of veg etation; for example, by preafforestation ploughing. These threshold d ata were required to calibrate a hydraulic model for effective design of preafforestation drainage networks on a variety of soils. However, simple field measures of soil properties indicative of erosion potenti al would be of value to the forestry industry for management purposes. Consequently, hydraulic threshold data were related by linear regress ion 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 cle ar water velocities up to 5.7 m s(-1), although a mineral bedload migh t induce erosion at lesser current speeds. Penetration resistance is a good field indicator of the degree of humification of the peat soils. Although selected physical parameters contribute resistance to water erosion, an increased organic content is pre-eminent in reducing erosi on susceptibility in both organic and mineral soils. Although compress ive strength was not indicative of soil erodibility, field measurement s of penetration resistance on a variety of soils could be related to hydraulic thresholds of erosion; albeit through the construction of di scriminant functions interpolated by eye. Consequently, organic conten t (laboratory) or penetration resistance (field) might form the basis of classifying upland soils in terms of erodibility. Mineral soils dif fer widely in terms of their erodibility, so that subject to further c onsideration, the use of ploughing for forestry cultivation might be a ppropriate in wider circumstances than presently recommended by the Fo rests and Water Guidelines. Ploughing should be acceptable on deep pea t providing the underlying mineral soil is not exposed in the bottom o f the furrow, and furrows are not led from mineral soils on to deep pe at. (C) 1997 John Wiley & Sons, Ltd.