Geology topics

P.S. Hill

Publications and source records attributed to P.S. Hill.

4 recordsLinked to original sources

USGS2009

Reply to the Comment on "Wave climate, sediment supply and the depth of the sand-mud transition: A global survey" by D.A. George and P.S. Hill [Marine Geology 254 (2008) 121-128]

An analysis of concepts presented by George and Hill [George, D.A., Hill, P.S., 2008. Wave climate, sediment supply and the depth of the sand-mud transition: A global survey. Marine Geology, 254, 121-128.] regarding the depth of the sand-mud transition (h SMT ) was performed by Guill??n and Jim??nez [Jorge Guill??n and Jos?? A. Jim??nez, Comment on "Wave climate, sediment supply and the depth of the sand-mud transition: A global survey" by D.A. George and P.S. Hill [Marine Geology 254 (2008) 121-128], Marine Geology, in press]. We are pleased that our proposed definition of the h SMT was confirmed to be appropriate. We are encouraged that the authors agree that wave period and wave height should both be used to determine h SMT as we demonstrated in our Eq. (1), which calculates the bed shear stress at h SMT . More in-depth research should focus on characterizing the role of sediment supply in determining h SMT . ?? 2009 Elsevier B.V. All rights reserved.

Marine Geology↗

USGS2008

Wave climate, sediment supply and the depth of the sand-mud transition: A global survey

The influences of wave climate and sediment supply on the depths of sand-mud transitions (hSMT) are investigated. Depths of sand-mud transitions (SMT) are based on published granulometric data from surface samples gathered from 14 sites in different wave-dominated coastal environments with fluvial input, including high energy (Columbia, Eel, Russian, San Lorenzo, Copper, and Nepean rivers), moderate energy (Ebro, Nile, Santa Clara, Tseng-wen and Kao-ping rivers), and low energy (Po, Pescara and Tronto rivers) regimes. Geometric mean diameter (GMD) and mud percent are compiled from samples along shore-normal transects, and significant correlation is found between these two textural descriptors. Nominally, the SMT is defined as the transition from GMD > 63????m to < 63????m. The correlation between mud percent and GMD permits an alternative, complementary definition of the SMT as the transition from < 25% mud to > 25% mud. This dual definition is applied to the 14 systems, and hSMT is tabulated for each system. Correlation is found between hSMT and the depth at which wave-induced bottom shear stress equals the critical erosion shear stress of the largest mud particles and also between hSMT and significant wave height. Lack of correlation between hSMT and sediment load of nearby rivers indicates either that the influence of sediment supply on depth of the sand-mud transition is small or is not adequately represented in this study. Shelf width and slope do not correlate with residuals from a formalized linear relationship between hSMT and significant wave height. The relationship between hSMT and wave climate is useful for calibration of numerical models of erosion and deposition in wave-dominated coastal environments, for prediction of seabed properties in remote or inaccessible areas, and for reconstruction of paleodepth based on facies changes from sand to mud in ancient rocks. ?? 2008.

Marine Geology↗

USGS2007

Flocculation, heavy metals (Cu, Pb, Zn) and the sand-mud transition on the Adriatic continental shelf, Italy

Across a limited depth range (5-10 m) on many continental shelves, the dominant sediment size changes from sand to mud. This important boundary, called the sand-mud transition (SMT), separates distinct benthic habitats, causes a significant change in acoustic backscatter, represents a key facies change, and delimits more surface-reactive mud from less surface-reactive sand. With the goal of improving dynamical understanding of the SMT, surficial sediments were characterized across two SMTs on the Adriatic continental shelf of Italy. Geometric mean diameter, specific surface area (SSA), mud fraction (<63 ??m) and heavy metal concentrations were all measured. The SMT related to the Tronto River is identified between 15 and 20 m water depth while the SMT associated with the Pescara River varies between 15 and 25 m water depth. The sediment properties correlate with a new, process-based sedimentological parameter that quantifies the fraction of the sediment in the seabed that was delivered as flocs. These correlations suggest that floc dynamics exert strong influence over sediment textural properties and metal concentrations. Relative constancy in the depth of the SMT along this portion of the margin and its lack of evolution over a period during which sediment input to the margin has dramatically decreased suggest that on the Adriatic continental shelf energy is the dominant control on the depth of the SMT. ?? 2006 Elsevier Ltd. All rights reserved.

Continental Shelf Research↗

USGS2007

The impact of floods and storms on the acoustic reflectivity of the inner continental shelf: A modeling assessment

Flood deposition and storm reworking of sediments on the inner shelf can change the mixture of grain sizes on the seabed and thus its porosity, bulk density, bulk compressional velocity and reflectivity. Whether these changes are significant enough to be detectable by repeat sub-bottom sonar surveys, however, is uncertain. Here the question is addressed through numerical modeling. Episodic flooding of a large versus small river over the course of a century are modeled with HYDROTREND using the drainage basin characteristics of the Po and Pescara Rivers (respectively). A similarly long stochastic record of storms offshore of both rivers is simulated from the statistics of a long-term mooring recording of waves in the western Adriatic Sea. These time series are then input to the stratigraphic model SEDFLUX2D , which simulates flood deposition and storm reworking on the inner shelf beyond the river mouths. Finally, annual changes in seabed reflectivity across these shelf regions are computed from bulk densities output by SEDFLUX2D and compressional sound speeds computed from mean seafloor grain size using the analytical model of Buckingham [1997. Theory of acoustic attenuation, dispersion, and pulse propagation in unconsolidated granular materials including marine sediments. Journal of the Acoustical Society of America 102, 2579–2596; 1998. Theory of compressional and shear waves in fluidlike marine sediments. Journal of the Acoustical Society of America 103, 288–299; 2000. Wave propagation, stress relaxation, and grain-tograin shearing in saturated, unconsolidated marine sediments. Journal of the Acoustical Society of America 108, 2796–2815]. The modeling predicts reflectivities that change from <12 dB for sands on the innermost shelf to >9 dB for muds farther offshore, values that agree with reflectivity measurements for these sediment types. On local scales of ∼100 m, however, maximum changes in reflectivity are <0.5 dB. So are most annual changes in reflectivity over all water depths modeled (i.e., 0–35 m). Given that signal differences need to be ⩾2–3 dB to be resolved, the results suggest that grain-size induced changes in reflectivity caused by floods and storms will rarely be detectable by most current sub-bottom sonars.

Continental Shelf Research↗