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At least 667 records · Page 37Linked to original sources
USGS2008

Calculating wave-generated bottom orbital velocities from surface-wave parameters

Near-bed wave orbital velocities and shear stresses are important parameters in many sediment-transport and hydrodynamic models of the coastal ocean, estuaries, and lakes. Simple methods for estimating bottom orbital velocities from surface-wave statistics such as significant wave height and peak period often are inaccurate except in very shallow water. This paper briefly reviews approaches for estimating wave-generated bottom orbital velocities from near-bed velocity data, surface-wave spectra, and surface-wave parameters; MATLAB code for each approach is provided. Aspects of this problem have been discussed elsewhere. We add to this work by providing a method for using a general form of the parametric surface-wave spectrum to estimate bottom orbital velocity from significant wave height and peak period, investigating effects of spectral shape on bottom orbital velocity, comparing methods for calculating bottom orbital velocity against values determined from near-bed velocity measurements at two sites on the US east and west coasts, and considering the optimal representation of bottom orbital velocity for calculations of near-bed processes. Bottom orbital velocities calculated using near-bed velocity data, measured wave spectra, and parametric spectra for a site on the northern California shelf and one in the mid-Atlantic Bight compare quite well and are relatively insensitive to spectral shape except when bimodal waves are present with maximum energy at the higher-frequency peak. These conditions, which are most likely to occur at times when bottom orbital velocities are small, can be identified with our method as cases where the measured wave statistics are inconsistent with Donelan's modified form of the Joint North Sea Wave Project (JONSWAP) spectrum. We define the 'effective' forcing for wave-driven, near-bed processes as the product of the magnitude of forcing times its probability of occurrence, and conclude that different bottom orbital velocity statistics may be appropriate for different problems. ?? 2008 Elsevier Ltd.

Computers & Geosciences↗
USGS2023

Monitoring and modeling dispersal of a submerged nearshore berm at the mouth of the Columbia River, USA

A submerged, low-relief nearshore berm was constructed in the Pacific Ocean near the mouth of the Columbia River, USA, using 216,000 m 3 of sediment dredged from the adjacent navigation channel. The material dredged from the navigation channel was placed on the northern flank of the ebb-tidal delta in water depths between 12 and 15 m and created a distinct feature that could be tracked over time. Field measurements and numerical modeling were used to evaluate the transport pathways, time scales, and physical processes responsible for dispersal of the berm and evaluate the suitability of the location for operational placement of dredged material to enhance the sediment supply to eroding beaches onshore of the placement site. Repeated multibeam bathymetric surveys characterized the initial berm morphology and dispersion of the berm between September 22, 2020, and March 10, 2021. During this time, the volume of sediment within the berm decreased by about 40%to 127,000 m 3 , the maximum height decreased by almost 60%, and the center of the deposit shifted onshore over 200 m. Observations of berm morphology were compared with predictions from a three-dimensional hydrodynamic and sediment transport model application to refine poorly constrained model input parameters including sediment transport coefficients, bed schematization, and grain size. The calibrated sediment transport model was used to predict the amount, timing, and direction of transport outside of the observed survey area. Model simulations predicted that tidal currents were weak in the vicinity of the berm and wave processes including enhanced bottom stresses and asymmetric bottom orbital velocities resulted in dominant onshore movement of sediment from the berm toward the coastline. Roughly 50% of the berm volume was predicted to disperse away from the initial placement site during the 169 day hindcast. Between 9 and 17% of the initial volume of the berm was predicted to accumulate along the shoreface of a shoreline reach experiencing chronic erosion directly onshore of the placement site. Scenarios exploring alternate placement locations suggested that the berm was relatively effective in enhancing the sediment supply along the eroding coastline north of the inlet. The transferable monitoring and modeling framework developed in this study can be used to inform implementation of strategic nearshore placements and regional sediment management in complex, high-energy coastal environments elsewhere.

Oregon, Washington↗
USGS2020

Anthropogenic pollutants and biomarkers for the identification of 2011 Tohoku-oki tsunami deposits (Japan)

Organic geochemistry is commonly used in environmental studies. In tsunami research, however, its applications are in their infancy and it is still rarely used. We present results for two types of organic geochemical markers, biomarkers and anthropogenic markers, present in deposits left by 2011 Tohoku-oki tsunami on the Sendai Plain, Japan. As the tsunami inundated the coastal lowland up to 4.85 km inland, sediments from various sources were eroded, transported and deposited. This led to the distribution of biomarkers from different sources across the Sendai Plain creating a unique geochemical signature in the tsunami deposits. The tsunami also caused destruction along the Sendai coastline, leading to the release of large quantities of environmental pollutants (e.g., fossil fuels, tarmac, pesticides, plastics, etc.) that were distributed across the inundated area. These anthropogenic markers, represented by three main compound groups (polycyclic aromatic hydrocarbons, pesticides, and halogenated compounds), were preserved in tsunami deposits (at least until 2013, prior to land clearing). Their concentrations differed significantly from the pre- and post-tsunami background contamination levels. Organic proxy concentrations can differ for sand and mud deposits due to various factors (e.g., preservation, dilution, microbial alteration). However, it can be concluded that anthropogenic markers and biomarkers have the potential to be a valuable proxy for future studies of recent tsunami deposits because of their high source specificity and relatively good preservation potential providing information about sediment sources and transport pathways (e.g., marine source, evidence of backwash).

Sendai↗
USGS2007

Ground water stratification and delivery of nitrate to an incised stream under varying flow conditions

Ground water processes affecting seasonal variations of surface water nitrate concentrations were investigated in an incised first-order stream in an agricultural watershed with a riparian forest in the coastal plain of Maryland. Aquifer characteristics including sediment stratigraphy, geochemistry, and hydraulic properties were examined in combination with chemical and isotopic analyses of ground water, macropore discharge, and stream water. The ground water flow system exhibits vertical stratification of hydraulic properties and redox conditions, with sub-horizontal boundaries that extend beneath the field and adjacent riparian forest. Below the minimum water table position, ground water age gradients indicate low recharge rates (2-5 cm yr-1) and long residence times (years to decades), whereas the transient ground water wedge between the maximum and minimum water table positions has a relatively short residence time (months to years), partly because of an upward increase in hydraulic conductivity. Oxygen reduction and denitrification in recharging ground waters are coupled with pyrite oxidation near the minimum water table elevation in a mottled weathering zone in Tertiary marine glauconitic sediments. The incised stream had high nitrate concentrations during high flow conditions when much of the ground water was transmitted rapidly across the riparian zone in a shallow oxic aquifer wedge with abundant outflow macropores, and low nitrate concentrations during low flow conditions when the oxic wedge was smaller and stream discharge was dominated by upwelling from the deeper denitrified parts of the aquifer. Results from this and similar studies illustrate the importance of near-stream geomorphology and subsurface geology as controls of riparian zone function and delivery of nitrate to streams in agricultural watersheds.

Maryland↗
USGS2012

Effects of urban development on stream ecosystems in nine metropolitan study areas across the United States

Urban development is an important agent of environmental change in the United States. The urban footprint on the American landscape has expanded during a century and a half of almost continuous development. Eighty percent of Americans now live in metropolitan areas, and the advantages and challenges of living in these developed areas—convenience, congestion, employment, pollution—are part of the day-to-day realities of most Americans. Nowhere are the environmental changes associated with urban development more evident than in urban streams. Contaminants, habitat destruction, and increasing streamflow flashiness resulting from urban development have been associated with the disruption of biological communities, particularly the loss of sensitive aquatic species. Every stream is connected downstream to larger water bodies, including rivers, reservoirs, and ultimately coastal waters. Inputs of chemical contaminants or sediments at any point along the stream can cause degradation downstream with adverse effects on biological communities and on economically valuable resources, such as fisheries and tourism. In response to general concerns about the degradation of urban streams, the U.S. Geological Survey (USGS) conducted a national-scale, scientific investigation of the effects of urban development on stream ecosystems. Nine metropolitan study areas of the United States were selected—Portland, Oregon; Salt Lake City, Utah; Birmingham, Alabama; Atlanta, Georgia; Raleigh, North Carolina; Boston, Massachusetts; Denver, Colorado; Dallas, Texas; and Milwaukee, Wisconsin. The studies were conducted in Salt Lake City, Birmingham, and Boston in 1999–2000; in Atlanta, Raleigh, and Denver in 2002–2003; and in Portland, Dallas, and Milwaukee in 2003–2004. The comprehensive investigation of all nine studies focused on three broad questions of interest to decision makers: What are the primary effects of urban development on stream ecosystems? How do the effects of urban development on stream ecosystems vary regionally across the country? Which urban-related stressors are most closely linked to biological community degradation, and how can multiple stressors be managed to protect stream health as a watershed becomes increasingly urbanized?

Alabama;Colorado;Georgia;Massachusetts;North Carol↗
USGS2008

Documentation of the U.S. Geological Survey Oceanographic time-series measurement database

The U.S. Geological Survey (USGS) Oceanographic Time-Series Measurement Database contains oceanographic observations made as part of studies designed to increase understanding of sediment transport processes and associated dynamics. Analysis of these data has contributed to more accurate prediction of the movement and fate of sediments and other suspended materials in the coastal ocean. The measurements were collected primarily by investigators at the USGS Woods Hole Coastal and Marine Science Center (WHCMSC) and colleagues, beginning in 1975. Most of the field experiments were carried out on the U.S. continental shelf and slope. This report describes the instrumentation and platforms used to make the measurements; the methods used to process, apply quality-control criteria, and archive the data; the data storage format, and how the data are released and distributed. The report also includes instructions on how to access the data from the online database at http://stellwagen.er.usgs.gov/ . As of 2016, the database contains about 5,000 files, which may include observations of current velocity, wave statistics, ocean temperature, conductivity, pressure, and light transmission at one or more depths over some duration of time.

Open-File Report↗
USGS2011

Suspended sediment and organic contaminants in the San Lorenzo River, California, water years 2009-2010

This report presents analyses of suspended sediment and organic contaminants measured during a two-year study of the San Lorenzo River, central California, which discharges into the Pacific Ocean within the Monterey Bay National Marine Sanctuary. Most suspended-sediment transport occurred during flooding caused by winter storms; 55 percent of the sediment load was transported by the river during a three-day flood in January 2010. Concentrations of polyaromatic hydrocarbons can exceed regulatory criteria during high-flow events in the San Lorenzo River. These results highlight the importance of episodic sediment and contaminant transport in steep, mountainous, coastal watersheds and emphasize the importance of understanding physical processes and quantifying chemical constituents in discharge from coastal watersheds on event-scale terms.

Open-File Report↗
USGS2001

Assessment of water-quality conditions in the J.B. Converse Lake watershed, Mobile County, Alabama, 1990-98

J.B. Converse (Converse) Lake is a 3,600-acre, tributary-storage reservoir in Mobile County, southwestern Alabama. The lake serves as the primary drinking-water supply for the city of Mobile. The Converse Lake watershed lies within the Coastal Plain Physiographic Province. Semiconsolidated to unconsolidated sediments of sand, silt, gravel, and clay underlie the watershed, and are covered by acidic soils. Land use in the watershed is mainly forest (64 percent) and agriculture (31 percent). Residential and commercial development account for only 1 percent of the total land use in the watershed. Converse Lake receives inflow from seven major tributaries. The greatest inflows are from Big Creek, Crooked Creek, and Hamilton Creek that had mean annual streamflows of 72.2, 19.4, and 25.0 cubic feet per second, respectively, for the period 1990 to 1998, which represents about 72 percent of the total annual streamflow to the lake. The total mean annual inflow to the lake is estimated to be about 163 cubic feet per second. In general, water quality in Converse Lake and its tributaries meets the criteria established by the Alabama Department of Environmental Management (ADEM) for drinking-water supplies, whole-body contact, and aquatic life. The exceptions include acidic pH levels, iron and manganese levels above secondary or aesthetic criteria, and fecal bacterial levels in some tributaries above whole-body contact (swimmable) criteria. The pH levels throughout the watershed were commonly below the criteria level of 6.0, but this appears to have been a naturally occurring phenomenon caused by poorly buffered soil types, resistant sediments, and forested land use. Median iron and manganese levels were above aesthetic criteria levels of 300 and 50 micrograms per liter, respectively, in some tributaries. All tributary sites in the Converse Lake watershed had median and minimum dissolved-oxygen concentrations above the ADEM criteria level of 5 milligrams per liter except for Boggy Branch, which had a minimum dissolved-oxygen concentration of 3.7 milligrams per liter. The degree to which nutrient contributions from tributaries were causing nutrient enrichment and eutrophication in Converse Lake was assessed. Trend analysis detected little or no change in nutrient concentrations at the tributary and lake sites in the Converse Lake watershed from the 1991 to 1998 water years. Nutrient concentrations at most tributary sites exhibited a significant, positive relation with streamflow that indicated the dominant source of nutrient input to the watershed is from nonpoint contributions. From 1990 to 1998, computed mean annual loads of 75,400 kilograms of total nitrogen, 36,950 kilograms of total Kjeldahl nitrogen, 28,870 kilograms of total inorganic nitrogen, and 3,480 kilograms of total phosphorus were contributed to the lake by Big Creek, Hamilton Creek, and Crooked Creek combined. These mean annual loads of nutrients corresponded to borderline eutrophic/mesotrophic conditions in the lake. Of the combined loads, 62 percent of the total nitrogen, 70 percent of the total Kjeldahl nitrogen, 54 percent of the total inorganic nitrogen, and 47 percent of the total phosphorus originated from the forested subbasin of Big Creek. The more residential and agricultural subbasins of Crooked Creek and Hamilton Creek, however, yielded over twice the total phosphorus load per hectare of land use. Crooked and Hamilton Creek subbasins also had higher yields of the more bioavailable total inorganic nitrogen. A simplistic empirical model could not explain the relation between year-to-year nutrient contributions to Converse Lake from the tributaries and the lake's ability to assimilate those contributions. The potential presence of pathogens in the lake and its tributaries was assessed based on fecal bacterial concentrations. Fecal bacterial concentrations at some tributary sites were above existing criteria for swimmable uses. Contributions of fecal bacte

Water-Resources Investigations Report↗
USGS2022

Turbidity and estimated phosphorus retention in a reconnected Lake Erie coastal wetland

Nearly all of the wetlands in the coastal zone of Lake Erie have been degraded or destroyed since the 1860s, and most of those that remain are separated from their watersheds by earthen dikes. Hydrologic isolation of these wetlands disrupts ecosystem benefits typical to Great Lakes coastal wetlands, particularly the ability to trap sediments and retain nutrients when inundated by runoff and lake water. High-frequency measurements of turbidity and discharge were taken in 2013 and 2014 to observe turbidity and water flow dynamics to estimate total phosphorus flux of a hydrologically reconnected diked wetland pool in the Crane Creek-Lake Erie wetland complex. Modeled estimates suggest the reconnected pool retained 8% of the total phosphorus loading in 2013 and 10% in 2014, which included short periods of phosphorus export to Lake Erie. Water flowing out of the wetland generally had lower turbidity than inflowing water, but flux in and out of the pool varied seasonally and was linked to changes in lake-levels, seiche dynamics, and weather conditions. More frequent storms, higher winds, and stronger seiches in the spring and fall created turbidity patterns that suggest more phosphorus retention than in summer or winter. Estimates suggest that phosphorus was released during the summer when higher lake levels and the absence of frequent storms, larger short-term seiche oscillations, and potentially soil oxygen availability were driving flux dynamics. This study demonstrated that reestablishing lake hydrology through reconnection of wetland pools can reduce loading and alter timing of delivery of total phosphorus to Lake Erie.

Ohio↗
USGS2014

Maximizing the social and ecological value of Cape Romain National Wildlife Refuge, South Carolina as the effects of global change processes increase.

Coastal ecosystems in the eastern U.S. have been severely altered by processes associated with human development, including drainage of coastal wetlands, changes in hydrology that alter sediment and freshwater delivery to the coast, land clearing, agricultural and forestry activity, and the construction of seawalls and other structures that “harden” the coast. Sea-level rise and the changing frequency of extreme events associated with climate change are now further degrading the capacity of those ecological and social systems to remain resilient in the face of disturbance, largely through the degradation and loss of land and habitat. The goal was to develop a problem framing that would broadly reflect the type, scale and scope of SLR-adaptation decisions faced by refuges in general, and that could be used by individual refuges to help understand how their specific problems fit into a larger context of SLR planning and implementation. We worked with refuge managers in developing a common frame of reference for some shared SLR-related management problems, while also facilitating communication about these problems with other refuges and with surrounding jurisdictions.

South Carolina↗
USGS2018

Remote sensing and modeling to fill the “gap” in missing natural capital

This chapter reviews recent advances in remote sensing and environmental modeling that address the first step in ecosystem accounting: biophysical quantification of ecosystem services. The chapter focuses on those ecosystem services in which the most rapid advances are likely, including crop pollination, sediment regulation, carbon sequestration and storage, and coastal flood regulation. The discussion highlights data sources and modeling approaches that can support wealth accounting, next steps for mapping and biophysical modeling of ecosystem services, and considerations for integrating biophysical modeling and monetary valuation. These approaches could make the inclusion of some ecosystem services increasingly feasible in future versions of wealth accounts.

Book chapter↗
USGS2011

Rapid wetland expansion during European settlement and its implication for marsh survival under modern sediment delivery rates

Fluctuations in sea-level rise rates are thought to dominate the formation and evolution of coastal wetlands. Here we demonstrate a contrasting scenario in which land-use–related changes in sediment delivery rates drive the formation of expansive marshland, and vegetation feedbacks maintain their morphology despite recent sediment supply reduction. Stratigraphic analysis and radiocarbon dating in the Plum Island Estuary (Massachusetts, United States) suggest that salt marshes expanded rapidly during the eighteenth and nineteenth centuries due to increased rates of sediment delivery following deforestation associated with European settlement. Numerical modeling coupled with the stratigraphic observations suggests that existing marshland could survive, but not form under the low suspended sediment concentrations observed in the estuary today. These results suggest that many of the expansive marshes that characterize the modern North American coast are metastable relicts of high nineteenth century sediment delivery rates, and that recent observations of degradation may represent a slow return to pre-settlement marsh extent. In contrast to ecosystem management practices in which restoring pre-anthropogenic conditions is seen as a way to increase ecosystem services, our results suggest that widespread efforts to restore valuable coastal wetlands actually prevent some systems from returning to a natural state.

Massachusetts↗
USGS2014

Segmentation of plate coupling, fate of subduction fluids, and modes of arc magmatism in Cascadia, inferred from magnetotelluric resistivity

Five magnetotelluric (MT) profiles have been acquired across the Cascadia subduction system and transformed using 2-D and 3-D nonlinear inversion to yield electrical resistivity cross sections to depths of ∼200 km. Distinct changes in plate coupling, subduction fluid evolution, and modes of arc magmatism along the length of Cascadia are clearly expressed in the resistivity structure. Relatively high resistivities under the coasts of northern and southern Cascadia correlate with elevated degrees of inferred plate locking, and suggest fluid- and sediment-deficient conditions. In contrast, the north-central Oregon coastal structure is quite conductive from the plate interface to shallow depths offshore, correlating with poor plate locking and the possible presence of subducted sediments. Low-resistivity fluidized zones develop at slab depths of 35–40 km starting ∼100 km west of the arc on all profiles, and are interpreted to represent prograde metamorphic fluid release from the subducting slab. The fluids rise to forearc Moho levels, and sometimes shallower, as the arc is approached. The zones begin close to clusters of low-frequency earthquakes, suggesting fluid controls on the transition to steady sliding. Under the northern and southern Cascadia arc segments, low upper mantle resistivities are consistent with flux melting above the slab plus possible deep convective backarc upwelling toward the arc. In central Cascadia, extensional deformation is interpreted to segregate upper mantle melts leading to underplating and low resistivities at Moho to lower crustal levels below the arc and nearby backarc. The low- to high-temperature mantle wedge transition lies slightly trenchward of the arc.

Geochemistry, Geophysics, Geosystems↗
USGS2009

Coastal change along the shore of northeastern South Carolina: The South Carolina Coastal Erosion Study

The U.S. Geological Survey, in cooperation with the South Carolina Sea Grant Consortium, conducted a 7-year, multi-disciplinary study of coastal erosion in northeastern South Carolina. The main objective was to understand the geologic and oceanographic processes that control sediment movement along the region's shoreline and thereby improve projections of coastal change. The study used high-resolution remote sensing and sampling techniques to define the geologic framework and assess historic shoreline change. Based on these findings, oceanographic-process studies and numerical modeling were carried out to determine the rates and directions of sediment transport along South Carolina's Grand Strand.

South Carolina↗
USGS2020

Spatially referenced models of streamflow and nitrogen, phosphorus, and suspended-sediment loads in streams of the Pacific region of the United States

Although spatial information describing the supply and quality of surface water is critical for managing water resources for human uses and for ecological health, monitoring is expensive and cannot typically be done over large scales or in all streams or waterbodies. To address the need for such data, the U.S. Geological Survey developed SPAtially Referenced Regression On Watershed attributes (SPARROW) for the Pacific region of the U.S. for streamflow and three water-quality constituents–total nitrogen, total phosphorus, and suspended sediment, based on a decadal time frame centered on the year 2012. The domain for these models included the Columbia River basin, the Puget Sound, the coastal drainages of Washington, Oregon, and California, and the Central Valley of California. Landscape runoff (represented by the difference between precipitation and evapotranspiration) was the largest source of streamflow, wastewater discharge, and atmospheric deposition were the largest contributors to total nitrogen yield from the Pacific region, wastewater discharge was the largest contributor to total phosphorus yield, and forest land was the largest contributor to suspended-sediment yield. Watersheds with relatively high water yields also generally had relatively high yields of total nitrogen, total phosphorous, and suspended sediment–except where there were large contributions from developed land and wastewater discharge. The data used in this study, including many that improved upon existing national data or were compiled specifically for the Pacific region, characterized the complex hydrologic and water-quality conditions in the region more completely than previous models. By using these new datasets, this investigation was able to account for the complex network of water diversions and transfers, quantify the contribution of nutrients from different sources of livestock manure, discern a signal from unpaved logging roads in the suspended-sediment yields from forested coastal watersheds, show how recent wildfire disturbance influences phosphorus and sediment delivery to streams, and how sediment delivery to streams is also sensitive to the intensity of cattle grazing. The results from this study could complement research and inform water-quality management activities in the Pacific region. Examples might include identifying potentially impaired waterbodies and guiding remediation efforts where impairment has been documented, explaining the spatial patterns in harmful algal blooms, and providing estimates of sediment and nutrient loadings to Pacific coast estuaries where such data are scarce or non-existent.

Pacific Region↗
USGS1996

Channel response to sediment wave propagation and movement, Redwood Creek, California, USA

Redwood Creek, north coastal California, USA, has experienced dramatic changes in channel configuration since the 1950s. A series of large floods (in 1955, 1964, 1972 and 1975) combined with the advent of widespread commercial timber harvest and road building resulted in extensive erosion in the basin and contributed high sediment loads to Redwood Creek. Since 1975, no peak flows have exceeded a 5 year recurrence interval. Twenty years of cross-sectional survey data document the downstream movement of a ‘sediment wave’ in the lower 26 km of this gravel-bedded river at a rate of 800 to 1600 m a −1 during this period of moderately low flows. Higher transit rates are associated with reaches of higher unit stream power. The wave was initially deposited at a site with an abrupt decrease in channel gradient and increase in channel width. The amplitude of the wave has attenuated more than 1 m as it moved downstream, and the duration of the wave increased from eight years upstream to more than 20 years downstream. Channel aggradation and subsequent degradation have been accommodated across the entire channel bed. Channel width has not decreased significantly after initial channel widening from large (>25 year recurrence interval) floods. Three sets of longitudinal surveys of the streambed showed the highest increase in pool depths and frequency in a degrading reach, but even the aggrading reach exhibited some pool development through time. The aggraded channel bed switched from functioning as a sediment sink to a significant sediment source as the channel adjusted to high sediment loads. From 1980 to 1990, sediment eroded from temporary channel storage represented about 25 per cent of the total sediment load and 95 per cent of the bedload exported from the basin.

California↗
USGS2006

Temporal and spatial variability in the flow and dispersal of suspended-sediment on a fringing reef flat, Molokai, Hawaii

A multi-year study was conducted on a shallow fringing reef flat on Molokai, Hawaii to determine the temporal and spatial dispersal patterns of terrigenous suspended sediment. During this study, trade-wind conditions existed for the majority of the year on the reef flat. The trade-wind conditions produced strong currents and resuspended moderate amounts of sediment on the reef flat on a daily basis during the year of study, resulting in an overwhelming contribution to the total sediment flux. The magnitude and direction of the trade winds relative to the orientation of the coastline, the shallow-relief and broad morphology, and tidal elevation, provided the primary control of the physical processes that resuspended and transported sediment on the reef flat over the period of record. Spatial data indicate that much of the terrigenous sediment resuspended on the reef flat is transported predominantly alongshore and is confined to the inner- to mid-reef flat. Evidence for the limited across-shore mixing and transport is provided by the dominantly alongshore wind-driven currents during trade-wind conditions and the well-defined across-shore gradient in percentage calcium carbonate of the suspended sediment. Regions of slightly offshore suspended-sediment transport along the reef flat can be attributed to the circulation pattern set up by the interaction between the trade winds, coastal morphology, and anthropogenic coastal structures (i.e., fish ponds and wharf). The regions in which sediment were seen to move offshore provide the strongest link between the sediment dynamics on reef flat and fore reef, and qualitatively appears to be correlated with low coral coverage on the fore reef. ?? 2005 Elsevier Ltd. All rights reserved.

Estuarine, Coastal and Shelf Science↗
USGS2009

Sources of land-derived runoff to a coral reef-fringed embayment identified using geochemical tracers in nearshore sediment traps

Geochemical tracers, including Ba, Co, Th, 7 Be, 137 Cs and 210 Pb, and magnetic properties were used to characterize terrestrial runoff collected in nearshore time-series sediment traps in Hanalei Bay, Kauai, during flood and dry conditions in summer 2006, and to fingerprint possible runoff sources in the lower watershed. In combination, the tracers indicate that runoff during a flood in August could have come from cultivated taro fields bordering the lower reach of the river. Land-based runoff associated with summer floods may have a greater impact on coral reef communities in Hanalei Bay than in winter because sediment persists for several months. During dry periods, sediment carried by the Hanalei River appears to have been mobilized primarily by undercutting of low 7 Be, low 137 Cs riverbanks composed of soil weathered from tholeiitic basalt with low Ba and Co concentrations. Following a moderate rainfall event in September, high 7 Be sediment carried by the Hanalei River was probably mobilized by overland flow in the upper watershed. Ba-desorption in low-salinity coastal water limited its use to a qualitative runoff tracer in nearshore sediment. 210 Pb had limited usefulness as a terrestrial tracer in the nearshore due to a large dissolved oceanic source and scavenging onto resuspended bottom sediment. 210 Pb-scavenging does, however, illustrate the role resuspension could play in the accumulation of particle-reactive contaminants in nearshore sediment. Co and 137 Cs were not affected by desorption or geochemical scavenging and showed the greatest potential as quantitative sediment provenance indicators in material collected in nearshore sediment traps.

Estuarine, Coastal and Shelf Science↗