Geology topics

David Hart

Publications and source records attributed to David Hart.

5 recordsLinked to original sources

USGS2026

Biophysical controls on sediment erodibility in shallow estuarine embayments

The erodibility of cohesive sediment is known to vary both spatially and temporally but the factors governing its variation are not well understood. We conducted a field investigation of the influence of hydrodynamic forcing, sediment properties, and benthic infauna on erodibility in the muddy shallows of San Pablo and Grizzly Bays in northern San Francisco Bay in summer 2019 and winter 2020. An erosion rate parameter M c was determined from regressions between near-bed vertical turbulent sediment flux, as a proxy for erosion, and bed shear stress due to currents. During each 2-month study period, we measured benthic infauna abundance and dry bulk density, particle size distribution, percent organic carbon, chlorophyll a , pheophytin a , and carbohydrates carbon concentrations of surficial bed sediments five or six times. M c increased with bed shear stress due to waves in both embayments. In San Pablo Bay, erodibility was approximately 50% lower during the winter than the summer deployment, whereas in Grizzly Bay, there was no significant difference. The factor most strongly related to the decrease in M c in San Pablo Bay was increased abundance of the amphipod Ampelisca abdita . The observed reduction in erodibility may occur in many muddy estuaries because A. abdita is broadly distributed in the coastal waters of North America. Erodibility was also directly related to biomass of the invasive clam Potamocorbula amurensis . Erodibility did not depend on dry bulk density: bulk density did not vary seasonally in San Pablo Bay and was lower in winter than summer in Grizzly Bay.

California↗

USGS2024

Sand supply to San Francisco Bay from the Sacramento and San Joaquin Rivers of the Central Valley, California

Sediment from the Central Valley via the Sacramento-San Joaquin Delta (Delta) and Suisun Bay is a primary source of sand to San Francisco Bay, California. Sand is mined from San Francisco Bay for commercial purposes, such as for use in concrete for construction. To better understand the supply of sand to Suisun Bay and San Francisco Bay, the U.S. Geological Survey (USGS), in cooperation with the San Francisco Bay Estuary Institute (SFEI) and the San Francisco Bay Conservation Development Commission (BCDC), initiated this study to compile and synthesize historical data and estimate the total sediment and sand portion of sediment exiting the Delta to Suisun Bay for a 20-year period between water years 2001 and 2020. Sediment exiting the Delta is a combination of suspended sediment and bedload sediment. Seaward bedload transport was estimated using bedload transport equations and available hydraulic data at the two downstream-most streamgages in the Delta (where velocity is measured). Those two streamgages are about 25 kilometers upstream from the “exit” of the Delta at Mallard Island. The combined average annual net (seaward) bedload at these two streamgages was estimated to be 0.102 million cubic meters per year (Mm 3 /yr) for the study period. This volume of bedload is equivalent to 0.155 million metric tons per year (Mt/yr), assuming a bulk density of 1.517 metric tons per cubic meter (t/m 3 ). The bedload composition was estimated to be 88 percent sand. Between the two streamgages and Mallard Island, an annual average of 0.076 Mm 3 /yr of material was removed through mining during the study period, of which 97.5 percent was sand. In addition, 0.053 Mm 3 /yr was removed through dredging to support shipping and navigation, of which 76 percent was sand. The total volume of mined and dredged sediment material was approximately 0.128 Mm 3 /yr, equivalent to 0.194 Mt/yr, assuming a bulk density of 1.517 t/m 3 . Assuming the estimated bedload reaching Mallard Island was reduced by mining and dredging, a mean bedload flux of −0.009 Mm 3 /yr was computed (using a bulk density of 1.517 t/m 3 ), suggesting a deficit or landward transport of bedload. However, the total suspended-sediment and suspended-sand flux was in the seaward direction. The average total suspended flux of sediment to Suisun Bay through the cross section at the Mallard Island streamgage was estimated to be 0.482 million metric tons per year (Mt/yr; 0.015 Mt/yr sand) in the seaward direction. The results indicate a net flux out of the Delta of 0.469 Mt/yr of total sediment and 0.003 Mt/yr of sand. The primary limitation of the study was the lack of physical bedload measurements to validate the bedload estimates. To better refine the estimates of bedload, physical measurements of bedload or repeat bathymetry would be necessary for a range of flow conditions. Such measurements could be used to calibrate transport equations and quantify the uncertainty in such estimates.

California↗

USGS2020

Coordinated river infrastructure decisions improve net social-ecological benefits

We explore the social, ecological, economic, and technical dimensions of sustainable river infrastructure development and the potential benefits of coordinating decisions such as dam removal and stream crossing improvement. Dam removal is common practice for restoring river habitat connectivity and ecosystem health. However, stream crossings such as culverts are often 15 times more abundant than dams and may pose similar ecological impacts. Using multi-objective optimization for a model system of 6100 dams and culverts in Maine, USA, we demonstrate substantial benefit-cost improvements provided by coordinating habitat connectivity decisions. Benefit-cost efficiency improves by two orders of magnitude when coordinating more decisions across wider areas, but this approach may cause inequitable resource distribution. Culvert upgrades improve roadway safety and habitat connectivity, creating cost-effective opportunities for coordinating and cost-sharing projects between conservationists and safety managers. Benefit-cost trends indicate significant overlaps in habitat and safety goals, encouraging flexible stakeholder collaborations and cost-sharing strategies.

Maine↗

USGS2019

A simple method for simulating groundwater interactions with fens to forecast development effects

Protection of fens–wetlands dependent on groundwater discharge–requires characterization of groundwater sources and stresses. Because instrumentation and numerical modeling of fens is labor intensive, easy-to-apply methods that model fen distribution and their vulnerability to development are desirable. Here we demonstrate that fen areas can be simulated using existing steady-state MODFLOW models when the unsaturated zone flow (UZF) package is included. In cells where the water table is near land surface, the UZF package calculates a head difference and scaled conductance at these “seepage drain” cells to generate average rates of vertical seepage to the land. This formulation, which represents an alternative to blanketing the MODFLOW domain with drains, requires very little input from the user because unsaturated flow-routing is inactive and results are primarily driven by easily obtained topographic information. Like the drain approach, it has the advantage that the distribution of seepage areas is not predetermined by the modeler, but rather emerges from simulated heads. Beyond the drain approach, it takes account of intracell land surface variation to explicitly quantify multiple surficial flows corresponding to infiltration, rejected recharge, recharge and land-surface seepage. Application of the method to a basin in southeastern Wisconsin demonstrates how it can be used as a decision-support tool to first, reproduce fen distribution and, second, forecast drawdown and reduced seepage at fens in response to shallow pumping.

Wisconsin↗

USGS2018

A multiscale approach to balance trade-offs among dam infrastructure, river restoration, and cost

Aging infrastructure and growing interests in river restoration have led to a substantial rise in dam removals in the U.S. However, the decision to remove a dam involves many complex trade-offs. The benefits of dam removal for hazard reduction and ecological restoration are potentially offset by the loss of hydroelectricity production, water supply, and other important services. We use a multi-objective approach to examine a wide array of trade-offs and synergies involved with strategic dam removal at three spatial scales in New England. We find that increasing the scale of decision-making improves the efficiency of trade-offs among ecosystem services, river safety, and economic costs resulting from dam removal, but this may lead to heterogeneous and less equitable local-scale outcomes. Our model may help facilitate multilateral funding, policy, and stakeholder agreements by analyzing the trade-offs of coordinated dam decisions, including net benefit alternatives to dam removal, at scales that satisfy these agreements.

New England↗