The soils and tunnel-gully erosion of a small catchment in the Wither Hills, Blenheim

Abstract

The primary aim of this research is to study in detail the soil pattern and erosion characteristics of a small representative catchment in the Wither Hills, and to determine the interrelationship between soil properties, both field and laboratory, on the initiation of tunnel erosion and its sequential development into open gullying. The basic steps of this approach to the problem of tunnel-gully erosion are briefly outlined as follows. 1. A survey of the soils of the representative catchment in order to delimit the problem. It was noticed from cursory examination of the Wither Hills that tunnel-gully erosion was not prevalent over every section of the landscape. The lower spurs and ridges seemed to be relatively free of tunnel gullies, although minor ones were apparent in isolated localities. The main ridges and higher slopes of the hills were definitely devoid of this erosion form although scars of former slips were evident. 2. A detailed study of a toposequence on those soils where the tunnel-gully problem was most acute. This involved intensive field and laboratory determinations in an effort to relate pedological, chemical and physical properties to the incidence and severity of erosion. In addition this detailed study was used to characterise these soils as typical of the dry-subhygrous, central yellow grey earth subgroup, on the basis of their morphology, chemistry, mineralogy and physical properties. 3. To fully describe the process of tunnel-gully formation as it occurs in the field. 4. To compare the tunnel-gullied soils of the Wither Hills with: (a) soils from other regions showing similar erosion forms, (b) with soils in similar environments but not liable to tunnel-gully erosion. It was thought that such a comparison using morphological properties and selected laboratory determinations would help explain the susceptibility of some soils to tunnel-gullying and also the actual process of tunnel formation. Considerable time was devoted to the soil survey to gain as much information as possible on the geomorphology and pedology of the representative catchment so as to relate slope development and soil formation which were seen to be interdependent. Hence, an attempt to understand the genesis of these soils is an essential part of the work as not all yellow grey earths have exactly the same pedogenic history. The ultimate aim of the whole study was to elucidate the tunnel-gully erosion pathway on the Wither Hills, and to provide such data that practical methods could eventually be devised to control existing tunnel-gullies.