The hydraulic properties and response of colluvium-filled bedrock depressions, Wellington, New Zealand

Abstract

This thesis investigates the hydraulic properties and monitors the hydrologic response of a CBD to climatic variables. CBDs are a distinctive element of the Wellington landscape and are characterised by their U- to V- shaped bedrock morphology that is mantled by coarse angular, poorly sorted material. An intensive hydrometric study has shown that; 1. Soil moisture displays a high degree of spatial and temporal variation. 2. Topography has a major impact on soil moisture content. 3. Soil moisture not only converges along the depression axes of hollows, but remains elevated at such locations for longer periods than at other locations within the fossil gully. 4. Steeper sites drain more quickly than gentle ones, and unless saturation occurs over a wide area, generally have a lower soil moisture content. 5. Temporal soil moisture variation is related to both depth and location. 6. Soil physical properties influence soil moisture content. For example, as bulk density increase with depth there is less potential for the soil to hold moisture and as a result moisture contents are lower and variations less. 7. Piezometric response is determined by effective precipitation. 8. Sites lower down the slope have significantly longer lag times between precipitation and piezometric response. Such sites also tend to show greater response in piezometric levels to increments of rainfall. 9. SWIM proved useful in simulating soil moisture conditions onsite. 10. Recorded lag times between precipitation and piezometric response compare favourably with those modelled using SWIM. 11. Antecedent soil moisture has significant effects on the piezometric response to precipitation. As a result of this study coinciding with a period of below average rainfall, the hydraulic data were used in SWIM to model the response of the CBD to an extreme event. A good fit was achieved between the soil moisture response recorded and that produced by SWIM. The control of the CBD on soil moisture variation and piezometric response was confirmed by the model, which in turn supports the concept that CBDs are preferential sites for slope instability. Further work is required to qualify the relationship between slope stability and precipitation indices.