Long-term changes in stream bank soil pipes and the effects of afforestation

Natural soil pipes are now recognized as potentially significant elements in hillslope drainage systems, sometimes developing into open channel tributaries or contributing often substantial volumes of quick flow to streams. However, there has been no detailed, long-term monitoring study of the evolution of pipe networks to indicate how permanent they are or how readily they may develop into open channels. This paper reports a resurvey of a section of stream bank in the English Peak District and compares it with the original survey 35 years previous. Comparison of the distribution, size, and shape of pipes on both banks of a 250 m stretch of the stream reveals significant changes. There were no cases of roof collapse forming new open channels. However, there has been a significant change in land use within the basin, with afforestation of the east bank. The resurvey shows a marked reduction in the number and size of pipes on the forested bank, but no significant change on the opposite bank that has remained moorland. The number of pipe outlets on the afforested bank halved over the period, and their mean diameter has reduced by 30%. In combination with the reduced number the smaller size resulted in a 71% reduction in the total area of stream bank occupied by pipe outlets on the forested bank. It is postulated that the change is primarily due to a change in the amount of throughflow beneath the forest caused by an increase in evapotranspiration.