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A collection of poems.

Predicting sediment transport at sandy beaches is a significant challenge in civil engineering owing to the variability in hydrodynamic, morphological, and geotechnical properties within a site and across multiple sites. Additionally, there are difficulties in measuring in-situ properties, and challenges in identifying and quantifying the different relevant driving and resisting forces. These challenges are further exacerbated in the intertidal zone where the addition of infiltration-exfiltration, wave run-up and run-down, bore collapse, cyclic emergence and submergence of sediments, interactions between standing waves and incident bores, and other processes must be considered. Among these many processes, pore pressure gradients within sandy beach sediments affect sediment transport by reducing the sediment's effective stress to zero (this process is called liquefaction). Despite the known importance of these pressure gradients with respect to sediment transport, there has been little field evidence of the role that these pore pressure gradients have on sediment transport, how they relate to the hydrodynamic properties, and their inclusion into predictive sediment transport equations. This study is based on field measurements of hydrodynamic and geotechnical properties, as well as pore pressure gradients during storm and non-storm conditions at sandy beaches in the intertidal zone. From the analysis of these field measurements, it was found that (1) liquefying pressure gradients are likely to develop in sediments that are rapidly inundated during storm conditions; (2) the magnitude of pore pressure gradients is related to the asymmetry of the pressure gradient and can occur with shoreward-directed near bed velocities; and (3) during non-storm conditions, pressure gradients that often do not exceed liquefaction criteria occurred more (less) frequently during a time period where erosion occurred in large (small) quantities, indicating that small non-liquefying pore pressure gradients may facilitate sediment transport. The results of this study demonstrate that current methods of scour calculations must include effects of pore pressure gradients to reduce error. Additionally, from this work it was found that sediment transport can be directed shoreward under momentary liquefaction. Finally, the results of this study show that sediment pore pressure gradients are related to wave skewness, spatial group steepness, and temporal group steepness which may aid modelling of pore pressure gradients. / Doctor of Philosophy / The transport of sediment particles (in this case, sand grains at beaches) is difficult to predict because of the many different governing processes that can be hard to measure, may be hard to relate to erosion or sediment accumulation specifically, and the variability in sediment and flow properties (grain size, fluid velocity, and others) at a specific location and across different locations. Storms, like hurricanes, tropical storms, and tsunamis, can drastically change the expected water properties (like water depth, wave height, and wave period), and the effects of water pressure within the sand bed. When a wave moves across the sand it causes a change in the water pressure that is within the sand. This water pressure is not the same throughout the sand with depth. When the gradient, or the difference between the water pressure at two different vertical locations, is large enough, the sand behaves like a fluid (like quicksand) and becomes easier to move, this process is called liquefaction. Even though previous work has shown that these pressure gradients (and the resulting liquefaction) is important for sediment transport, there have been few field measurements demonstrating their impact on sediment transport and how these gradients (and the resulting liquefaction) relate to wave and sand properties. This study presents field measurements of pressure gradients, wave and sediment properties, and sediment transport events during both storm and non-storm conditions. From these field measurements, it was shown that (1) during an extreme storm event, pressure gradients that liquefy the sediment are likely to occur on sediments that are not normally subjected to waves; (2) liquefying pressure gradients can occur when waves arrive at the beach, which may cause sediment to be moved shoreward; and (3) during non-storm conditions, pressure gradients that do not liquefy the sand occurred frequently during a sediment transport event, suggesting that these smaller pressure gradients may contribute to sediment transport by reducing the effective weight of the sediment. This work can be used to further understand the behavior of sediment pore pressure gradients, their relation to hydrodynamic properties, and how they influence sediment transport allowing for better predictions of sediment transport, beach nourishment calculations, and the design of coastal structures.

Shallow groundwater flow in the critical zone of steep headwater mountain catchments is often assumed to mimic surface topography. However, groundwater flow is influenced by other variables, such as the elevation of the water table and subsurface hydraulic conductivity, which can result in temporal variations in both magnitude and direction of flow. In this study, I investigated the temporal variability of groundwater flow in the soil zone (solum) within the critical zone of a headwater catchment at the Hubbard Brook Experimental Forest in North Woodstock, NH. Groundwater levels were continuously monitored throughout several seasons (March 2019 to Jan 2020) in a network of wells comprising three hillslope transects within the upper hillslopes of the catchment. Five clusters of three wells per cluster were screened from 0.18 – 1.1 m depth at the base of the solum. Water levels were also monitored in five deeper wells, screened from 2.4 - 6.9 m depth within glacial sediments of the C horizon. I conducted 47 slug tests across the well network to determine hydraulic properties of the aquifer materials surrounding each well. In addition, our team conducted a large-scale auger investigation mapping soil horizon depths and thicknesses. Results show that the magnitude of hydraulic gradients and subsurface hydrologic fluxes varied at each site with respect to changing water-table elevation, having a maximum range of 0.12 m/m and 9.19 x 10-6 m/s, respectively. The direction of groundwater flow had an arithmetic mean deviating from surface topography by 2-10 degrees, and a total range that deviated from surface topography by as much as 51 degrees. During lower water table regimes, groundwater flow direction deviated from the ground surface, but under higher water table regimes, in response to recharge events, flow direction mimicked surface topography. Within most of the well clusters, there is an observable connection between the slope direction of the top of the C horizon and the direction of groundwater flow during lower water table regimes. Slug test results show the interquartile range of saturated hydraulic conductivity (Ksat) within the C horizon (1.5×10-7 to 9.8×10-7 m/s) is two orders of magnitude lower than the interquartile range of Ksat values within the solum (2.9×10-5 to 5.2×10-5 m/s). Thus, the C horizon is on average a confining unit relative to the solum that may constrict groundwater flow below the solum. Additionally, results from the larger scale auger investigation suggest that deviations in subsurface topography of the C horizon may be generalizable at the larger hillslope scale. Overall, these results indicate that 1) shallow groundwater flow direction and magnitude within this headwater catchment are dynamic and can deviate from surface topography, and 2) the subsurface topography of the C horizon can influence groundwater flow direction. These results imply that temporal dynamics of groundwater flow direction and magnitude should be considered when characterizing subsurface flow in critical zone studies. Additionally, knowledge of subsurface topography of confining units may provide constraints on the temporal variability of groundwater flow direction. / M.S. / Streams that originate at higher elevations (defined as headwater streams) are important drinking water sources and deliver water and nutrients to maintain freshwater ecosystems. Groundwater is a major source of water to these streams, but little is known about how groundwater flows in these areas. Scientists delineate watersheds (areas of land that drain water to the same point) using surface topography. This approach works well for surface water, but not as well for groundwater, as groundwater may not flow in the same direction as surface water. Thus, assuming that the ground-watershed is the same as the surface watershed can lead to errors in hydrologic studies. To obtain more accurate information about groundwater flow in headwater areas, I continuously measured groundwater levels in forest soils at the Hubbard Brook Experimental Forest in North Woodstock, NH. My main objective was to determine if there is variability in the direction and amount of groundwater flow. I also measured the characteristics of the soils to identify the thicknesses of soil units and the permeability of those units. I used these data to evaluate the relationship between groundwater flow direction, surface topography, and the permeability of soil units. Overall, I found that groundwater flow direction can differ significantly from surface topography, and groundwater flow direction was influenced by the groundwater levels. When groundwater levels were high (closer to the land surface), groundwater flow was generally in the same direction as surface topography. However, when groundwater levels were lower, flow direction typically followed the slope of the lowest permeability soil unit. These results suggest that scientists should not assume that groundwater flow follows the land surface topography and should directly measure groundwater levels to determine flow direction. In addition, results from this study show that characterizing soil permeability can help scientists make more accurate measurements of groundwater flow.