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The Contribution of Bedrock Groundwater Flow to Storm Runoff and High Pore Pressure Development in Hollows

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Many analyses of runoff generation, pore pressure development, and slope stability in colluvium-man tled hollows assume that the bedrock-colluvium boundary forms a major hydrologie barrier during periods of intense precipitation. Here we present detailed field observations from one year of intensive monitoring at our 16,000 m2 grassland basin in Mt. Tamalpais State Park, Marin County, California, U.S.A. These data demonstrate that significant large- and small-scale interactions between storm flow in the bedrock and the colluvium occur at our site. The basin-scale, topographically driven, flow system terminates at the channel head where a low-permeabili ty block of bedrock forces groundwater to the surface with an average exfiltration gradient of 0.5 m/m and excess pressure head of 1 m within the basal colluvium. This low- permeability block also forces the water table to remain close to the ground surface along the hollow axes, enabling several storms per year to generate significant saturation overland flow. Surface erosion by saturation overland flow may be the dominant process causing channel incision in our relatively low-gradient hollow. Local bedrock permeability heterogeneities cause fluctuations in the position of the bedrock water table and result in unexpected midslope high pore pressures and a persistent discontinuous zone of saturation overland flow. Presently, exfiltration along the hollow prevents build up of pore pressures in the bedrock and colluvium sufficient to cause landsliding, but on steep hillslopes local upwelling of bedrock storm flow may strongly influence soil instability.
... The GW fluctuation and soil moisture variation and its response according to rainfall events typically vary by location. Wilson and Dietrich (1987) assessed a zero-order basin in California, and found a significance influence of underlaying weathered bedrock on hydrological processes at the hillslope-scale. Kosugi et al. (2008), Katsuyama et al. (2005), and Onda et al. (2001) have verified the importance of bedrock GW in a granitic catchment of Central Japan, finding that transient saturation within the soil bedrock interface was related to the rise and fall of bedrock GW. ...
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Soil and bedrock characteristics play important roles in groundwater (GW) and soil moisture dynamics along hillslopes. Compared to temperate climate regions, runoff in the humid tropics remains poorly understood, being broadly characterized by deeply weathered bedrock and thick soils with rich clay content. To better understand subsurface runoff processes in humid tropics, GW and soil moisture were monitored in two adjacent hillslopes with different underlying soil depths and land cover (forest and oil palm). The monitoring results showed that the average depths and temporal variations of GW varied substantially between the two sites. At the forest site, where the topography is comparatively steeper and covered with a shallower soil layer, the GW at the foot of the slope was more responsive to rainfall. Alternatively, the comparatively gentle slope and deeper soil layer of the palm oil site produced GW patterns that responded more slowly to rainfall. To elucidate the predominant controlling factors, a physically based hydrologic model was employed whose parameters were estimated from the field observations, and calibrated further to represent the observed patterns. Subsequently, a numerical experiment was conducted by varying the model parameters. The findings indicated that soil depth and saturated hydraulic conductivity have important roles in the dynamic response of GW; whereas soil water retention curves were also prominent determinants of surface soil moisture. The results also supported the importance of lateral saturated subsurface flow in soil layers, leading to the rapid responses of GW at the forest site, while such dynamic patterns did not appear in thicker soil layers, indicating different subsurface flow mechanisms, even at adjacent hillslopes.
... A hollow is defined as a shallow eroded landform that evolves during bedrock weathering (Hack, 1965;Dietrich et al., 1982). Topographic convergence of hollows facilitates the accumulation and concentration of colluviums, as well as shallow surface runoff (Wilson and Dietrich, 1987). In this context, it has been found that a thick colluvium deposit (CD) may gather in the center of a hollow because of numerous slope processes, including (but not limited to) soil creeps and mass movements (Zhang et al., 2019). ...
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Soil mass failure disasters are among the most destructive disasters in mountainous areas worldwide, and they continue to receive significant attention in the literature. However, factors that influence soil mass failure in hollows have not been explored via experimental approaches. In this study, we conducted a series of indoor hollow-flume experiments to exhibit how the three factors, namely, initial moisture content, slope angle, and clay content, influence the collapse of soil mass under surface runoff conditions. Our findings revealed that the major processes causing soil mass failure were internal seepage erosion resulting from subsurface runoff, migration of fine particles, development of tension cracks, increase in pore water pressure, and decrease in cohesion. Fluctuations in pore water pressure were characterized by two main trends: (1) an abrupt rise under static liquefaction and (2) a decrease before failure due to dilatancy. The time required for soil mass failure first decreased and then increased with the increase in moisture content. The soil mass with an initial moisture content of ~12.5% was more critical to the failure. In addition, the time required for soil mass failure decreased with increasing slope angle, revealing a slope angle of 40 • to be more prone to failure. The buildup of pore water pressure was found to increase with clay content; soil with a clay content of ~7.5% required the shortest amount of time to initiate slides. Thus, our results provide a thorough insight into the processes and factors involved in the initiation and development of soil mass failure in hollow areas, which can be used by public agencies to improve monitoring, early warning, and forecasting systems. Notably, our study can help identify risk-prone source areas, and sensors can be installed to monitor potential mass failure locations.
... A hollow is defined as a shallow eroded landform that evolves during bedrock weathering (Hack, 1965;Dietrich et al., 1982). Topographic convergence of hollows facilitates the accumulation and concentration of colluviums, as well as shallow surface runoff (Wilson and Dietrich, 1987). In this context, it has been found that a thick colluvium deposit (CD) may gather in the center of a hollow because of numerous slope processes, including (but not limited to) soil creeps and mass movements (Zhang et al., 2019). ...
Article
Full-text available
Soil mass failure disasters are among the most destructive disasters in mountainous areas worldwide, and they continue to receive significant attention in the literature. However, factors that influence soil mass failure in hollows have not been explored via experimental approaches. In this study, we conducted a series of indoor hollow–flume experiments to exhibit how the three factors, namely, initial moisture content, slope angle, and clay content, influence the collapse of soil mass under surface runoff conditions. Our findings revealed that the major processes causing soil mass failure were internal seepage erosion resulting from subsurface runoff, migration of fine particles, development of tension cracks, increase in pore water pressure, and decrease in cohesion. Fluctuations in pore water pressure were characterized by two main trends: (1) an abrupt rise under static liquefaction and (2) a decrease before failure due to dilatancy. The time required for soil mass failure first decreased and then increased with the increase in moisture content. The soil mass with an initial moisture content of ~12.5 % was more critical to the failure. In addition, the time required for soil mass failure decreased with increasing slope angle, revealing a slope angle of 40 degree to be more prone to failure. The buildup of pore water pressure was found to increase with clay content; soil with a clay content of ~7.5 % required the shortest amount of time to initiate slides. Thus, our results provide a thorough insight into the processes and factors involved in the initiation and development of soil mass failure in hollow areas, which can be used by public agencies to improve monitoring, early warning, and forecasting systems. Notably, our study can help identify risk-prone source areas, and sensors can be installed to monitor potential mass failure locations.
... Factors such as localized reduction in down slope saturated conductivity can force shallow groundwater to the surface, with significant exfiltration head gradients (e.g., Wilson & Dietrich, 1987;Wilson et al., 1989). Exfiltration gradients can also occur when groundwater is forced to the surface due to downslope boundary conditions (e.g., Iverson & Reid, 1992;Tóth, 1963). ...
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Aeolis Mons (informally, Mount Sharp) exhibits a number of canyons, including Gediz and Sakarya Valles. Poorly sorted debris deposits are evident on both canyon floors and connect with debris extending down the walls for canyon segments that cut through sulfate-bearing strata. On the floor of Gediz Vallis, debris overfills a central channel and merges with a massive debris ridge located at the canyon terminus. One wall-based debris ridge is evident. In comparison, the floor of Sakarya Vallis exhibits a complex array of debris deposits. Debris deposits on wall segments within Sakarya Vallis are mainly contained within chutes that extend downhill from scarps. Lateral debris ridges are also evident on chute margins. We interpret the debris deposits in the two canyons to be a consequence of one or more late-stage hydrogeomorphic events that increased the probability of landslides, assembled and channelized debris on the canyon floors, and moved materials down-canyon. The highly soluble nature of the sulfate-bearing rocks likely contributed to enhanced debris generation by concurrent aqueous weathering to produce blocky regolith for transport downslope by fluvial activity and landslides, including some landslides that became debris flows. Subsequent wind erosion in Gediz Vallis removed most of the debris deposits within that canyon and partially eroded the deposits within Sakarya Vallis. The enhanced wind erosion within Gediz Vallis was a consequence of the canyon’s alignment with prevailing slope winds.
... Surface topography, the topography of the failure surface (usually the bedrockesoil interface) and spatial variability in the permeability of the hillslope materials and their substrate have a strong influence on the spatial relationship and timing between rainfall and groundwater responses. For example, the confluence of water along the bedrockesoil interface of bedrock hollows or fossil gully features (colluvium-filled bedrock depressions) compounds the saturation of materials in these hollows, and consequently these are common hillslope failure sites (Crozier et al., 1990;Wilson and Dietrich, 1987). Whilst identifying and modelling the surface topography of a hillslope is relatively easy, the topography of the bedrockesoil interface is more difficult to identify. ...
Chapter
Hillslope (in)stability is governed by the balance of stability factors. If stability is lost, gradually or instantly, slope failure ensues. Assessing the causes of instability is useful for hazard analysis and mitigation, and for considering the role of landslides in landscape systems and evolution. Geological and geomorphological conditions (e.g. material type, strength and structure and hillslope geometry) predispose slopes to failure; knowledge of these conditions can help to predict the location, types and volumes of potential failures. The timing of failure, often by a specific trigger, can be anticipated by detecting and assessing movement patterns, establishing triggering thresholds or using probabilistic methods. However, predicting timing remains challenging due to the difficulty of measuring material strength degradation which can lead to failure with no readily observable trigger. This chapter describes concepts of stability and explores some of the major causes and triggers of hillslope failure and opportunities for further research.
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Water age and flow pathways should be related; however, it is still generally unclear how integrated catchment runoff generation mechanisms result in streamflow age distributions at the outlet. Here, we combine field observations of runoff generation at the Dry Creek catchment with StorAge Selection (SAS) age models to explore the relationship between stream water age and runoff pathways. Dry Creek is a 3.5 km² catchment in the Northern California Coast Ranges with a Mediterranean climate, and, despite an average rainfall of ≈1,800 mm/yr, is an oak savannah due to the limited hillslope water storage capacity. Runoff lag to peak—after initial seasonal wet‐up—is rapid (∼1–2 hr), and total annual streamflow consists predominantly of saturation overland flow, based on field mapping of saturated extents and an inferred runoff threshold for the expansion of saturation extent beyond the geomorphic channel. SAS modeling based on daily isotope sampling reveals that streamflow is typically older than 1 day. Since streamflow primarily consists of overland flow, a significant portion of overland flow must not be event‐rain but instead derive from older, nonevent groundwater returning to the surface, consistent with field observations of exfiltrating head gradients, return flow through macropores, and extensive saturation days after storm events. We conclude that even in a watershed fed primarily by overland flow, runoff is primarily not composed of event water. Our findings have implications for the interpretation of stream chemistry and the assumptions built into widely used hydrograph separation inferences, namely, the assumption that overland flow consists of new (event) water.
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Streamwater transit time distributions display a variable proportion of old waters (≥1 year). We hypothesize that the corresponding long transit times result from groundwater contributions to the stream and that seasonal streamwater transit time variations result from (a) the variable contributions of different flowpaths (overland flow, seepage flow and baseflow) and (b) the stratification of groundwater residence times. We develop a parsimonious model to capture the groundwater contribution to the stream discharge and its effect on transient transit times. Infiltration is partitioned according to the aquifer saturation between Boussinesq groundwater flow and overland flow. Time‐variable transit time distributions are obtained with a new 2D particle tracking algorithm. Hydraulic conductivity, total and drainable porosities are calibrated by using discharge and CFC tracer data on a crystalline catchment located in Brittany (France). The calibrated models succeed in reproducing CFCs concentrations and discharge dynamics. The groundwater flow contribution to the stream is controlled by the aquifer hydraulic conductivity, while its age is controlled by the drainable and total porosities. Old groundwater (≥1 year) is the source for approximately 75% of the streamflow with strong seasonal variations (between 40% and 95%). Mean transit times are approximately 13 years, varying between 6 and 20 years, proportional to the groundwater contribution. These seasonal variations are driven by the groundwater versus overland flow partitioning. The stratification of groundwater residence times in the aquifer plays a minor role in the streamwater transit times but is key for the transit time dynamics of the groundwater contribution to the stream.
Chapter
This chapter discusses the major mechanisms for strain localization in geomaterials (i.e. soil and rock) and their possible implications for slope instability. In certain geological environments and slopes subjected to external forces, soil or rock does not completely fail, but deformation zones are created due to intense strain localization, resulting in morphological and geotechnical changes. Strain localization is a feature of elastoplastic materials where shear bands are formed as a result of inhomogeneous material deformation leading to permanent expressions of intense strain zones. Strain localization can be considered an instability in material constitutive behavior. Localized elevated pore pressures can drive the growth of a slip-failure surface in a manner similar to that observed during earthquake nucleation. The material within a deformation band is thought to strain harden as a result of the deforming mechanism. In porous geomaterials such as sandstone, deformation bands are the most common strain localization feature. The localized strain bands can occur in a shear or compaction form. Particle grains within deformation bands tend to be smaller, more compact, possess stronger preferred orientations, and have more elongate shapes than particles outside the band. To illustrate, the Oso Landslide that struck Snohomish County, Washington on Saturday, March 22, 2014, resulting in 43 fatalities, several injuries, and significant destruction of property is discussed. Observed sand boils and other signs of confined elevated water pressure reaching or exceeding total overburden pressure point to liquefaction at depth in Zones E and F during the Oso landslide. Strain localization likely occurred during Stage 2 of the failure, triggering the 300 m length shearing of the failure surface.
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Slope failure occurs due to an increase in the saturation level and a subsequent decrease in matric suction in unsaturated soil. This paper presents the results of a series of centrifuge experiments and numerical analyses on a 55° inclined unsaturated sandy slope with less permeable, stronger silty sand layer inclusion within it. It is observed that a less permeable, stronger silty sand layer in an otherwise homogeneous sandy soil slope hinders the infiltration of water. The water content of the slope just above the stronger layer increases significantly, compared to elsewhere. No shear band is found to initiate in a homogeneous sandy soil slope, whereas for a non-homogeneous slope, they initiate just above the less pervious, stronger layer. A discontinuity of the shear zone is also observed for the case of a non-homogeneous soil slope. The factor of safety of a non-homogeneous, unsaturated soil slope decreases because of the less permeable, stronger layer. It decreases significantly if this less permeable, stronger soil layer is located near the toe of the slope.
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Insight for understanding the effect of groundwater flow on the potential for hillslope failure and liquefaction is provided by a novel limit-equilibrium analysis of infinite slopes with steady, uniform Darcian seepage of arbitrary magnitude and direction. Normalization of the limit-equilibrium solution shows that three dimensionless parameters govern completely the Coulomb failure potential of saturated, cohesionless, infinite homogeneous hillslopes: (1) the ratio of seepage force magnitude to gravitational body force magnitude; (2) the angle θ − Φ, where θ is the surface slope angle and Φ is the angle of internal friction of the soil; and (3) the angle λ + Φ, where λ is the angle of the seepage vector measured with respect to an outward-directed surface-normal vector. An additional dimensionless parameter affects the solution if soil cohesion is included in the analysis. Representation of the normalized solution as a single family of curves shows that minimum slope stability universally occurs when the seepage direction is given by λ = 90° − Φ. It also shows that for some upward seepage conditions, slope stability is limited by static liquefaction rather than by Coulomb failure. Close association between these liquefaction conditions and certain Coulomb failure conditions indicates that slope failure in such instances could be responsible for nearly spontaneous mobilization of destructive flowing soil masses on hillslopes.
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The topography of hillslopes or whole catchments is analyzed numerically to calculate local geometric and drainage attributes that can be combined to test for the expectation of soil waterlogging. At locations where accumulated drainage flux from upslope exceeds the product of soil transmissivity and the local slope, saturation to the soil surface occurs. Results of the analysis of specific landscapes are presented as a location dependent function. The function may be mapped as isolines to define successive boundaries of zones of soil saturation, depending on the wetness state of the landscape as a whole. The analysis is applied to two catchments, to predict the growth or contraction of zones of waterlogging for a range of drainage fluxes and to simulate the effects of transpiration changes in part or all of the catchment. In the second application, the predicted boundaries of saturated zones are used to calculate the minimum proportion of a catchment's area that produces rapid surface runoff. This proportion is shown to depend on the value of a normalized wetness parameter. Storm runoff data support the predicted form of the relationship.
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Thesis (Ph. D.)--Johns Hopkins University, 1969. Vita. Typescript (carbon copy). Bibliography: l. 244-248.