44 research outputs found
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Changing Sediment Transport Processes in Response to Anthropocene Forcing
The denudation of uplands and sediment deposition within lowlands comprises one of the most fundamental earth processes. Much like plate tectonics or isostasy, sediment transport runs through nearly every geologic sub-discipline. Its importance extends to other fields as well, such as soil and plant science, archaeology, and engineering. Research in the field of sedimentology remains current, with changes in global sediment transport invoked as a primary line of evidence for the arrival of the Anthropocene, and sedimentary archives frequently employed to evaluate current processes relative to the past. In this vein, my doctoral studies have centered on understanding some aspects of modern sediment transport in the Connecticut River watershed of the northeastern United States. Although each chapter varies with respect to time scale and specific location, a common thread of observing modern sediment transport and associated hydraulic or hydrologic conditions runs through the following chapters. Particular attention is given to how processes are changing or may change in the future given anthropogenic modifications of our climate and sea levels.
Hurricane Irene passed directly over the Connecticut River valley in late August, 2011. Intense precipitation and high antecedent soil moisture resulted in record flooding, mass wasting and fluvial erosion, allowing for observations of how these rare but significant extreme events affect a landscape still responding to Pleistocene glaciation and associated sediment emplacement. Clays and silts from upland glacial deposits, once suspended in the stream network, were routed directly to the mouth of the Connecticut River, resulting in record-breaking sediment loads fifteen-times greater than predicted from the pre-existing rating curve. Denudation was particularly extensive in mountainous areas. Sediment yield during the event from the Deerfield River, a steep tributary comprising 5% of the entire Connecticut River watershed, exceeded at minimum 10-40 years of routine sediment discharge and accounted for approximately 40% of the total event sediment discharge from the Connecticut River. Resultant sedimentation in Connecticut River floodplain cover was anomalously inorganic, fine grained, and enriched in elements commonly found in chemically immature glacial tills and glaciolacustrine material. These unique sedimentary characteristics document the crucial role played by extreme precipitation from tropical disturbances in denuding this landscape.
Lacustrine sediment archives spanning the last 100+ years indicate that flooding during Tropical Storm Irene caused the most severe erosion of any regional historic flood, surpassing that of events with greater precipitation and peak discharges. Compared to deposition from historic floods, Irene’s event layer was more massive and more enriched in unweathered upland sediments, indicating an anomalously high incidence of mass wasting and sediment entrainment. Precipitation records indicate that neither precipitation intensity nor total accumulation distinguished Irene from less erosive historic floods. However, cumulative precipitation prior to Irene exceeded the 95th percentile of all days in the record. When allowing for non-stationarity in 20th century background precipitation, we find a fourfold increase in the probability of Irene-like conditions, where impacts of extreme rainfall are enhanced by high antecedent precipitation. We conclude that irrespective of increases in extreme precipitation, the risk of highly erosive flooding in the region is increasing due to the influence of wetter baseline conditions associated with a changing climate.
Monthly sediment trap accumulation in an off-channel cove of the Connecticut River upper estuary showed that these settings comprise the main sites for sediment storage within high energy estuaries. At our study site, 20-25% of annual sediment trapping occurred during one month at seasonal low freshwater discharge and high estuarine salinity. Sediment deposited during these high accumulation times contained higher δ13C values, and lower 7Be activities, indicative of marine provenance. Therefore, this efficiently trapped sediment is not directly delivered from the watershed, but rather redistributed from within the estuary. Considerable seasonal variability in 7Be activity within the top 0.5 cm of cove sediments coupled with steady 7Be sourcing from the water column indicate that surface activity of 7Be can be used to reconstruct deposition rates at sub-seasonal time scales near the mouth of rivers. Water column observations as well as numerical modeling revealed that salt wedge dynamics play a critical role in resuspending estuarine sediment. The role of low discharge and high salinity in storing sediment suggests that sediment export from rivers is likely to decrease as sea level rises and warm season discharge decreases
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How Does Hydropeaking Alter the Hydrology of a River Reach? A Combined Water Budget, Modeling, and Field Observation Study. Deerfield River, Massachusetts
Hydroelectric releases on the Deerfield River in northwestern Massachusetts affect surface water-groundwater interactions there by daily reversing the head gradient between river and groundwater. Artificially elevated stage drives river water into the riparian aquifer. Water budget analysis indicates that roughly 10% of this bank-stored water is permanently lost from the river system in a 19.5 km reach, likely as a result of transpiration by bank vegetation.
Field observations as well as two-dimensional modeling results show that water losses are not uniform throughout the study reach. Riparian aquifer transmissivity in river sub-reaches largely determines the magnitude of surface water-groundwater exchange as well as net water loss from the river. These newly documented dam-induced losses from river systems inform decisions by river managers and hydroelectric operators of additional tradeoffs of oscillatory dam-release river management
Watershed Suspended Sediment Supply and Potential Impacts of Dam Removals for an Estuary
Observations and modeling are used to assess potential impacts of sediment releases due to dam removals on the Hudson River estuary. Watershed sediment loads are calculated based on sediment-discharge rating curves for gauges covering 80% of the watershed area. The annual average sediment load to the estuary is 1.2 Mt, of which about 0.6 Mt comes from side tributaries. Sediment yield varies inversely with watershed area, with regional trends that are consistent with substrate erodibility. Geophysical and sedimentological surveys in seven subwatersheds of the Lower Hudson were conducted to estimate the mass and composition of sediment trapped behind dams. Impoundments were classified as (1) active sediment traps, (2) run-of-river sites not actively trapping sediment, and (3) dammed natural lakes and spring-fed ponds. Based on this categorization and impoundment attributes from a dam inventory database, the total mass of impounded sediment in the Lower Hudson watershed is estimated as 4.9 ± 1.9 Mt. This represents about 4 years of annual watershed supply, which is small compared with some individual dam removals and is not practically available given current dam removal rates. More than half of dams impound drainage areas less than 1 km2, and play little role in downstream sediment supply. In modeling of a simulated dam removal, suspended sediment in the estuary increases modestly near the source during discharge events, but otherwise effects on suspended sediment are minimal. Fine-grained sediment deposits broadly along the estuary and coarser sediment deposits near the source, with transport distance inversely related to settling velocity
Turbidity hysteresis in an estuary and tidal river following an extreme discharge event
© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Ralston, D. K., Yellen, B., Woodruff, J. D., & Fernald, S. Turbidity hysteresis in an estuary and tidal river following an extreme discharge event. Geophysical Research Letters, 47(15), (2020): e2020GL088005, doi:10.1029/2020GL088005.Nonlinear turbidity‐discharge relationships are explored in the context of sediment sourcing and event‐driven hysteresis using long‐term (≥12‐year) turbidity observations from the tidal freshwater and saline estuary of the Hudson River. At four locations spanning 175 km, turbidity generally increased with discharge but did not follow a constant log‐log dependence, in part due to event‐driven adjustments in sediment availability. Following major sediment inputs from extreme precipitation and discharge events in 2011, turbidity in the tidal river increased by 20–50% for a given discharge. The coherent shifts in the turbidity‐discharge relationship along the tidal river over the subsequent 2 years suggest that the 2011 events increased sediment availability for resuspension. In the saline estuary, changes in the sediment‐discharge relationship were less apparent after the high discharge events, indicating that greater background turbidity due to internal sources make event‐driven inputs less important in the saline estuary at interannual time scales.This work was sponsored by the National Estuarine Research Reserve System Science Collaborative, funded by the National Oceanic and Atmospheric Administration and managed by the University of Michigan Water Center (NAI4NOS4190145), with additional support to Yellen and Woodruff from USGS Cooperative Agreement No. G19AC00091
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Turbidity Hysteresis in an Estuary and Tidal River Following an Extreme Discharge Event
Nonlinear turbidity‐discharge relationships are explored in the context of sediment sourcing and event‐driven hysteresis using long‐term (≥12‐year) turbidity observations from the tidal freshwater and saline estuary of the Hudson River. At four locations spanning 175 km, turbidity generally increased with discharge but did not follow a constant log‐log dependence, in part due to event‐driven adjustments in sediment availability. Following major sediment inputs from extreme precipitation and discharge events in 2011, turbidity in the tidal river increased by 20–50% for a given discharge. The coherent shifts in the turbidity‐discharge relationship along the tidal river over the subsequent 2 years suggest that the 2011 events increased sediment availability for resuspension. In the saline estuary, changes in the sediment‐discharge relationship were less apparent after the high discharge events, indicating that greater background turbidity due to internal sources make event‐driven inputs less important in the saline estuary at interannual time scales
Salt wedge dynamics lead to enhanced sediment trapping within side embayments in high-energy estuaries
Author Posting. © American Geophysical Union, 2017. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 122 (2017): 2226–2242, doi:10.1002/2016JC012595.Off-river coves and embayments provide accommodation space for sediment accumulation, particularly for sandy estuaries where high energy in the main channel prevents significant long-term storage of fine-grained material. Seasonal sediment inputs to Hamburg Cove in the Connecticut River estuary (USA) were monitored to understand the timing and mechanisms for sediment storage there. Unlike in freshwater tidal coves, sediment was primarily trapped here during periods of low discharge, when the salinity intrusion extended upriver to the cove entrance. During periods of low discharge and high sediment accumulation, deposited sediment displayed geochemical signatures consistent with a marine source. Numerical simulations reveal that low discharge conditions provide several important characteristics that maximize sediment trapping. First, these conditions allow the estuarine turbidity maximum (ETM) to be located in the vicinity of the cove entrance, which increases sediment concentrations during flood tide. Second, the saltier water in the main channel can enter the cove as a density current, enhancing near-bed velocities and resuspending sediment, providing an efficient delivery mechanism. Finally, higher salinity water accumulates in the deep basin of the cove, creating a stratified region that becomes decoupled from ebb currents, promoting retention of sediment in the cove. This process of estuarine-enhanced sediment accumulation in off-river coves will likely extend upriver during future sea level rise.NSF Grant Numbers: EAR-1148244 , OCE-09264272017-09-1
Sources, mechanisms, and timescales of sediment delivery to a New England salt marsh
© The Author(s), 2022. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Baranes, H., Woodruff, J., Geyer, W., Yellen, B., Richardson, J. & Griswold, F. Sources, mechanisms, and timescales of sediment delivery to a New England salt marsh. Journal of Geophysical Research: Earth Surface, 127, (2022): e2021JF006478, https://doi.org/10.1029/2021jf006478.he availability and delivery of an external clastic sediment source is a key factor in determining salt marsh resilience to future sea level rise. However, information on sources, mechanisms, and timescales of sediment delivery are lacking, particularly for wave-protected mesotidal estuaries. Here we show that marine sediment mobilized and delivered during coastal storms is a primary source to the North and South Rivers, a mesotidal bar-built estuary in a small river system impacted by frequent, moderate-intensity storms that is typical to New England (United States). On the marsh platform, deposition rates, clastic content, and dilution of fluvially-sourced contaminated sediment by marine material all increase down-estuary toward the inlet, consistent with a predominantly marine-derived sediment source. Marsh clastic deposition rates are also highest in the storm season. We observe that periods of elevated turbidity in channels and over the marsh are concurrent with storm surge and high wave activity offshore, rather than with high river discharge. Flood tide turbidity also exceeds ebb tide turbidity during storm events. Timescales of storm-driven marine sediment delivery range from 2.5 days to 2 weeks, depending on location within the estuary; therefore the phasing of storm surge and waves with the spring-neap cycle determines how effectively post-event suspended sediment is delivered to the marsh platform. This study reveals that sediment supply and the associated resilience of New England mesotidal salt marshes involves the interplay of coastal and estuarine processes, underscoring the importance of looking both up- and downstream to identify key drivers of environmental change.The project described in this publication was in part supported by Grant or Cooperative Agreement No. G20AC00071 from the U.S. Geological Survey and a Department of Interior Northeast Climate Adaptation Science Center graduate fellowship awarded to H.E.B (G12AC00001)
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Salt Marsh Response to Inlet Switch-Induced Increases in Tidal Inundation
There is widespread concern that rapidly rising sea levels may drown salt marshes by exceeding the rate at which these important ecosystems can build elevation. A significant fraction of marshes reside within backbarrier estuaries, yet little attention has been paid to how changes in inlet geometry influences estuarine tides and marshes. In 1898, a coastal storm eroded a new inlet through the barrier beach that fronts the North-South Rivers Estuary in Massachusetts, USA. The new inlet shortened the North River by 5.6 km and lengthened the South River channel by the same amount. Modern measurements of tidal attenuation suggest that channel shortening abruptly increased mean high tide along the North River by at least 30 cm. Foraminifera communities within North River marsh sediments indicated an environmental change from infrequent to frequent inundation at the time of the 1898 switch in inlet location, which supports this hypothesis. Increased mineral sediment deposition after the inlet switch played a dominant role in allowing marshes along the North River channel to adjust to greater inundation. Following the inlet switch, sediment accreted in North River marshes at 2–5 times the rate of sea level rise (SLR). The North River channel widened by an average of 18% relative to pre-1898 conditions to accommodate the increased tidal prism. The role of mineral sediment accretion in making this marsh resilient to an abrupt increase in inundation depth highlights the importance of maintaining adequate sediment supplies in coastal regions as SLR accelerates