Geohydrologic conditions on a steep forested slope : modeling transient piezometric response to precipitation

Graduation date: 1997Presentation date: 1996-12-06Two hilislope sites in the central Oregon Coast Range were instrumented and monitored for winter precipitation and saturated and unsaturated subsurface conditions. The study sites were near-ridge depressions typically known as headwalls. Based on results of the monitoring, two existing mathematical models\ud were adapted to predict piezometric levels in headwalls during storms. The first is a\ud statistically based model, using an Antecedent Precipitation Index (API) as an\ud independent variable in a regression model. The second is a mass balance model based on the kinematic assumption that the hydraulic gradient is equal to the slope of the impermeable base of the control volume. Several extreme storm events recorded during the monitoring period were used to calibrate the models. Precipitation data from a subsequent extreme event was then used in verification runs of both the API and the kinematic storage models. Soil from one site was sampled for index properties, strength parameters, and hydraulic properties. The soil is a non-plastic, sand-silt mixture derived from sandstone. A relatively undisturbed sample tested in a consolidated-undrained triaxial test had a φ' value of 32.5° and a c' value of 5.2 kPa. Laboratory testing suggests that the soil is hydraulically similar to other soils in similar geographic and topographic locations. An estimate of the "effective" saturated hydraulic conductivity, considering both macropore and matrix flow, is approximately 10⁻² cm/s. In general, the API models for individual storms were capable of reproducing\ud observed piezometric hydrographs. However, the use of API was limited by the high degree of variability in API values and antecedent hydraulic head conditions from storm to storm. A multi-storm API model was developed to overcome these limitations, and produced reasonably good results. The kinematic storage model also performed well for this site, for two of three methods of determining drainable porosity of the soil. Hillslope discharge measurements made on one occasion suggested that approximately 70% of flow at the outlet was occurring in pores larger than 2.5 centimeters in diameter. Macropore flow would seem to be an important feature of the subsurface flow regime under certain precipitation and antecedent soil moisture conditions