Geology topics

Navid H. Jafari

Publications and source records attributed to Navid H. Jafari.

7 recordsLinked to original sources

USGS2025

How high? Identifying elevation thresholds to guide coastal marsh restoration

Introduction Coastal marshes are highly valuable ecosystems facing threats from rising sea levels and intensifying storm events. To elevate marsh surfaces and prevent loss of ecosystem services, the beneficial use of dredged material (BUDM) is increasingly being implemented across the United States. Objectives The objective of this study was to aid decision-makers and restoration practitioners by identifying elevation thresholds that control the optimal function of Spartina patens -dominated marshes along the Texas Coast. Methods This study was conducted in the coastal marshes of the J.D. Murphree Wildlife Management Area (Texas). We collected elevation and vegetation cover data along ecological transition zones in marshes nourished with BUDM to identify elevation thresholds that define zones of optimal plant survivorship and growth. Results We identified lower (−0.05 m mean high water, MHW) and upper (0.26 m MHW) elevation thresholds that defined transitions to open water and unvegetated bare ground, respectively. Elevation targets ( T ) and elevation target zones, which defined areas with the greatest vegetation cover, were determined for S. patens ( T : 0.11 m, target zone: −0.06 to 0.26 m MHW), S. alterniflora ( T : 0.00 m, target zone: −0.12 to 0.10 m MHW), and Distichlis spicata ( T : 0.17 m, target zone: 0.07–0.25 m MHW). Conclusions Our analyses provide species-specific elevation targets for coastal marsh restoration through BUDM, which can improve restoration outcomes for coastal wetlands.

Texas↗

USGS2024

Short-term sediment dispersal on a large retreating coastal river delta via 234Th and 7Be sediment geochronology: The Mississippi River Delta Front

Many Mississippi River Delta studies have shown recent declines in fluvial sediment load from the river and associated land loss. In contrast, recent sedimentary processes on the subaqueous delta are less documented. To help address this knowledge gap, multicores were collected offshore from the three main river outlets at water depths of 25–280 m in June 2017 just after the peak river discharge period, with locations selected based on 2017 U.S. Geological Survey seabed mapping. The coring locations included the undisturbed upper foreset, mudflow lobes, gullies, and the undisturbed prodelta. Nine multicores were analyzed for Beryllium-7 activity, and four cores were analyzed for excess Thorium-234 activity via gamma spectrometry, granulometry and X-radiography. Our results indicate a general trend of declining 7 Be and 234 Th activities and inventories with increasing distance from sources and in deeper water. The core X-radiographs are graded from the predominantly physically stratified nearshore to the more bioturbated offshore, consistent with the sedimentation patterns. Sediment focusing assessed via the 7 Be and 234 Th sediment inventories shows preferential sedimentation in gully and lobe environments, whereas the upper foreset and prodelta focusing factors are relatively depleted. Overall, short-term sediment deposition from the main fluvial source remains active offshore from all three major river outlets, despite the overall declining river load.

Louisiana↗

USGS2024

Sedimentary processes and instability on the Mississippi River Delta Front near the shipwreck of the SS Virginia

Sediment cores were collected from a mudflow lobe (80 m water depth) offshore of the Mississippi River’s Southwest Pass in 2017 to better understand the sedimentology near the lobe entraining the SS Virginia shipwreck (sunk by a German U-boat in 1942) and surrounding Mississippi River delta front. Core analyses included 210 Pb/ 137 Cs geochronology, granulometry, and X-radiography. Sediment accumulation rates (SAR) calculated from excess 210 Pb activity in multicores are 0.22–0.29 cm/y at seabed depths less than 20 cm and 0.29–0.51 cm/y at depths greater than 20 cm. Accumulation rates for 137 Cs have been ~0.15 to ~0.37 cm/y since 1954 and 1963, respectively. Sediment accumulation rates from 210 Pb, 137 Cs geochronology and indicators of relative sedimentation and bioturbation from X-radiographs suggest that rates of sediment accumulation near the Virginia have declined since the mid-20th century. This may be explained by the multi-decade downslope mass transport of the mudflow lobe in which the shipwreck is embedded and decreases in sediment supply delivered offshore from the Mississippi river. Mass transport calculations of the Virginia lobe derived from core properties and published lobe advection rates suggest downslope mass transport is far higher than sediment resupply from the Mississippi river, consistent with recent studies of delta retreat.

Mississippi↗

USGS2023

Towards a unified drag coefficient formula for quantifying wave energy reduction by salt marshes

Coastal regions are susceptible to increasing flood risks amid climate change. Coastal wetlands play an important role in mitigating coastal hazards. Vegetation exerts a drag force to the flow and dampens storm surges and wind waves. The prediction of wave attenuation by vegetation typically relies on a pre-determined drag coefficient C D "> C D . Existing C D "> C D formulas are subject to vegetation biomechanical properties, especially the flexibility. Accounting for vegetation flexibility through the effective plant height (EPH), we propose and validate a species-independent relationship between C D "> C D and the Reynolds number R e "> Re based on three independent datasets that cover a wide range of hydrodynamic conditions and vegetation traits. The proposed C D − R e "> C D − Re relationship, used together with EPH, allows for predicting wave attenuation in salt marshes with high accuracy. Furthermore, a total of 308,000 numerical experiments with diverse wave conditions are conducted using the proposed C D − R e "> C D − Re relationship and EPH to quantify the wave attenuation capacity of two typical salt mash species: Elymus athericus (highly flexible) and Spartina alterniflora (relatively rigid). It is found that wave attenuation is controlled by wave height to water depth ratio and EPH to water depth ratio. When swaying in large waves in shallow to intermediate water depth, a 50-m-long Elymus athericus field may lose up to 30% capacity for wave attenuation. As wave height increases, highly flexible vegetation causes reduced wave attenuation, whereas relatively rigid vegetation induces increased wave attenuation. The leaf contribution to wave attenuation is highly dependent on the leaf rigidity. It is recommended that leaf properties, especially its Young’s modulus be collected in future field experiments.

Coastal Engineering↗

USGS2023

The concept of land bridge marshes in the Mississippi River Delta and implications for coastal restoration

Louisiana has high coastal wetland loss rates due to natural processes such as subsidence and anthropogenic activities such as construction of river levees and dams, pervasive alteration of surface hydrology by local industries such as oil and gas, and navigation. With the exception of the Atchafalaya River discharge area, most of Louisiana's marsh coastline is retreating and coastal marshes are degrading. In the inactive degrading delta regions, there exists a previously uncharacterized landform referred to colloquially as coastal ‘land bridge’ marshes. Land bridge marshes are saline or brackish marshes fronting large estuarine bays or lakes with sufficient fetch and wave energy to supply high levels of resuspended sediments to the marsh surface. They are generally linear features that are oriented parallel to the coast and the shoreline front retreats landward due to erosion from wave energy. These marshes persist over time vertically due to input of resuspended sediments but are experiencing rapid edge erosion due to wave attack. Comparison of data from Louisiana's Coastal Reference Monitoring System (CRMS) sites show that land bridge marshes have a greater frequency of higher soil surface elevation and higher soil bulk density than non-land bridge marshes. Because land bridges are vertically stable relative to other coastal wetlands, identification of measures to sustain these landscape features is important. Simulations using MarshMorpho2D, a process-based reduced-complexity morphology model, suggest that protection barriers installed on the seaward side of land bridge marshes will attenuate wave energy and, thus, edge erosion. Shoreline protection that can reduce wave energy but still allow sediment input to marshes include living shorelines, rock barriers, and/or breakwaters. Periodic thin layer nourishment of the marsh surface may be necessary to help sustain vertical growth. Further, marsh creation projects directly landward of land bridge marshes may benefit from their protection from waves and as a source of sediment. Consideration of land bridge marshes as distinct marsh types in the State Master Plan and integrated modeling could help to identify measures to sustain these landscape features.

Louisiana↗

USGS2022

Characterization of vegetated and ponded wetlands with implications towards coastal wetland marsh collapse

Coastal wetlands provide numerous ecosystem services; yet these ecosystems are increasingly vulnerable to climate change stressors, especially excessive flooding from sea-level rise and storm events. This study highlights the important contribution of vegetation belowground biomass to marsh stability and identifies loss of vegetation as a critical driver of marsh collapse. We investigated the shear strength of salt marshes and unvegetated interior ponds using a modified cone penetrometer along a chronosequence of wetland marsh collapse (0 to 21 + years following pond formation) to characterize changes in the structural integrity of the marsh soil. Following conversion from vegetated marsh to open water pond, the surficial soils experienced a dramatic loss in shear strength resulting from the loss of vegetation and compaction of soil pore space. The Cone Penetrometer Testing (CPT) data indicate that higher shear strength in the surficial layers of the vegetated marsh sites were never recovered, up to 21 + years following marsh collapse. Coupled with significant elevation loss from marsh collapse, additional sea-level rise, deep subsidence, and reduced sedimentation may contribute to conditions that can exceed critical flooding thresholds, making recovery from marsh collapse difficult or impossible. Therefore, characterizing mechanisms and thresholds of marsh collapse are critical for identifying those coastal marshes that are vulnerable to collapse before conversion from vegetated marsh to open water occurs.

Louisiana↗

USGS2018

Basin-scale model for predicting marsh edge erosion

Recent attempts to relate marsh edge retreat rate to wave power have met varying levels of success. Schwimmer (2001) correlated wave power to marsh boundary retreat rates over a five-year period along sites within Rehoboth Bay, Delaware, USA. Marani et al. (2011) derived a linear relationship between volumetric retreat rate and mean wave power density using Buckingham’s theorem of dimensional analysis. Leonardi and Fagherazzi (2015) added an exponential function to the Schwimmer (2001) equation to account for variability in soil resistance and mean wave height. These equations factor in soil type, water elevation, vegetation, and macrofauna through field-calibrated empirical constants, i.e., they are not explicitly considered. Consequently, the existing capability of predicting marsh edge erosion rate as a function of wave power and soil and vegetation properties is rather limited for engineering applications. For instance, Allison et al. (2017) show that without taking the marsh platform, soil, and vegetation into account, the relationships between marsh edge erosion rates and wave power on a basin or coastal-wide scale are not strong enough statistically to serve as a useful predictive model. The objective of this study is to develop a more robust marsh edge erosion model by characterizing the shear strength, wave power, and retreat rates in Terrebonne Bay, Louisiana.

Louisiana↗