Resistance and reconfiguration of natural flexible submerged vegetation in hydrodynamic river modelling

In-stream submerged macrophytes have a complex morphology and several species are not rigid, but are flexible and reconfigure along with the major flow direction to avoid potential damage at high stream velocities. However, in numerical hydrodynamic models, they are often simplified to rigid sticks. In this study hydraulic resistance of vegetation is represented by an adapted bottom friction coefficient and is calculated using an existing two layer formulation for which the input parameters were adjusted to account for (i) the temporary reconfiguration based on an empirical relationship between deflected vegetation height and upstream depth-averaged velocity, and (ii) the complex morphology of natural, flexible, submerged macrophytes. The main advantage of this approach is that it removes the need for calibration of the vegetation resistance coefficient. The calculated hydraulic roughness is an input of the hydrodynamic model Telemac 2D, this model simulates depth-averaged stream velocities in and around individual vegetation patches. Firstly, the model was successfully validated against observed data of a laboratory flume experiment with three macrophyte species at three discharges. Secondly, the effect of reconfiguration was tested by modelling an in situ field flume experiment with, and without, the inclusion of macrophyte reconfiguration. The inclusion of reconfiguration decreased the calculated hydraulic roughness which resulted in smaller spatial variations of simulated stream velocities, as compared to the model scenario without macrophyte reconfiguration. We discuss that including macrophyte reconfiguration in numerical models input, can have significant and extensive effects on the model results of hydrodynamic variables and associated ecological and geomorphological parameters

Road Safety Management at Work Zones : Final report

Accidents nearby work zones are a persistent road safety problem in many European countries. The Conference of European Directors of Roads (CEDR) has initiated and finances the IRIS project (Incursion Reduction to Increase Safety in road work zones) with the aim to collect and share information about best practices in temporary traffic management at road works. An analysis of work zone accidents and a review of best practices were made. Psychological issues to improve safety at work zones were studied by a literature review. Interviews with stakeholders were carried out in eight European countries to gather information on guidelines, standards and procedures in temporary traffic management. Best practice findings cover organizational/management issues, work zone safety reviews, establishment/de-establishment of a road work zone, informing/warning and guiding road users through work zone areas, speed management, protecting devices for road workers’ and road users’ safety and incursion warning systems

2D modeling of tidal wave propagation through a large vegetated marsh

Water Qualit

Mechanisms of Pond Expansion in a Rapidly Submerging Marsh

The development and expansion of ponds within otherwise vegetated coastal marshes is a primary driver of marsh loss throughout the world. Previous studies propose that large ponds expand through a wind wave-driven positive feedback, where pond edge erosion rates increase with pond size, whereas biochemical processes control the formation and expansion of smaller ponds. However, it remains unclear which mechanisms dominate at a given scale, and thus how, and how fast, ponds increase their size. Here, we use historical photographs and field measurements in a rapidly submerging microtidal marsh to quantify pond development and identify the processes involved. We find that as small ponds emerge on the marsh platform, they quickly coalesce and merge, increasing the number of larger ponds. Pond expansion rates are maximized for intermediate size ponds and decrease for larger ponds, where the contribution of wave-driven erosion is negligible. Vegetation biomass, soil shear strength, and porewater biogeochemical indices of marsh health are higher in marshes adjacent to stable ponds than in those adjacent to unstable ponds, suggesting that pond growth rates are negatively related to the health of the surrounding marsh. We find that the model of Vinent et al. (2021) correctly predicts measured pond growth rates and size distribution, which suggest the different mechanisms driving pond growth are a result of marsh drowning due to sea level rise (SLR) and can be estimated by simplified physical models. Finally, we show that all relevant processes increasing pond size can be summarized by an empirical power-law equation for pond growth which predicts the temporal change of the maximum pond size as a lower bound for the total pond area in the system. This gives a timescale for the growth of ponds by merging and thus the critical time window for interventions to prevent the irreversible pond expansion associated with large scale pond merging

Coastal Marsh Degradation Into Ponds Induces Irreversible Elevation Loss Relative to Sea Level in a Microtidal System

Coastal marshes and their valuable ecosystem services are feared to be lost by sea level rise, yet the mechanisms of marsh degradation into ponds and potential recovery are poorly understood. We quantified and analyzed elevations of marsh surfaces and pond bottoms along a marsh loss gradient (Blackwater River, Maryland, USA). Our analyses show that ponds deepen with increasing tidal channel width connecting the ponds to the river, indicating a new feedback mechanism where channels lead to enhanced tidal export of pond bottom material. Pond elevations also decrease with increasing pond size, consistent with previous work identifying a positive feedback between wind wave erosion and pond size. These two positive feedbacks, combined with bimodal elevation distributions and sharp topographic boundaries between interior ponds and the marsh platform, indicate alternative elevation states and imply that marsh loss by pond formation is nearly irreversible once pond deepening exceeds a critical level

Role of delta-front erosion in sustaining salt marshes under sea-level rise and fluvial sediment decline

Accelerating sea-level rise and decreasing riverine sediment supply are widely considered to lead to global losses of deltaic marshes and their valuable ecosystem services. However, little is known about the degree to which the related erosion of the seaward delta front can provide sediments to sustain salt marshes. Here, we present dataf rom the mesomacrotidal Yangtze Delta demonstrating that marshes have continued to accrete vertically and laterally, despite rapid relative sea-level rise (approx.10 mm yr−1) and a \u3e 70% decrease in the Yangtze River sediment supply. Marsh progradation has decelerated at a lower rate than fluvial sediment reduction, suggesting an additional source of sediment. We find that under favorable conditions (e.g., a mesomacrotidal range, strong tidal flow,flood dominance, sedimentary settling lag/scour lag effects, and increasing high-tide level), delta-front erosion can actually supply sediment to marshes, thereby maintaining marsh accretion rates in balance with relative sea-level rise.Comparison of global deltas illustrates that the ability of sediment remobilization to sustain marshes depends on coastal processes and varies by more than an order of magnitude among the world’s major delta

Flow measurements around a submerged macrophyte patch in an in-situ flume setup

In most aquatic ecosystems, hydrodynamic conditions are a key abiotic factor determining species distribution and aquatic plant abundance. Recently, local differences in hydrodynamic conditions have been shown to be an explanatory mechanism for the patchy pattern of Callitriche platycarpa Kütz. vegetation in lowland rivers. Those patches are often subject to strong hydrodynamic forces as they act as a resistance against the current. A plant‟s ability to tolerate water movement without suffering mechanical damage often relies on minimizing the hydrodynamic forces by avoiding stress. In this paper, we have quantified the behaviour and influence of a C. platycarpa patch in an in situ flume, manipulating the incoming discharge on a single patch. The knowledge obtained helps to understand the plant-flow-sediment interactions that form the basis of the explanatory mechanism for the patchy vegetation pattern

HISTORICAL EVOLOTION OF MUD DEPOSITION AND EROSION IN INTERTIDAL AREAS OF THE SCHELDT ESTUARY (BELGIUM AND SW NETHERLANDS)

ABSTRACT The mud dynamics in an estuary are recognized as an important element of estuarine functioning, because increasing suspended sediment concentrations may be both harmful for ecological functions (e.g., biomass production by phytoplankton) and deteriorative for human functions (e.g., by siltation of shipping channels). Considering the potential risk of increase in suspended sediment concentration in the Schelde estuary, this study aims to quantify the mud deposition/erosion in different time periods since 1930 to present, different intertidal ecotope types, and different zones along the Schelde estuary, including the Westerschelde and Zeeschelde. We analyzed the height change, volume change, eroded or deposited mud mass, and the overall mud balance. Our results suggested that net mud deposition occurred in intertidal areas in both the Westerschelde and Zeeschelde in almost all time periods. The mud deposition in stable marshes plays an important role. A large amount of mud deposition is also observed in stable intertidal flats and areas that shifted from intertidal flat to marshes or from subtidal zone to intertidal flat. Over 90% of mud erosion is observed in areas that shifted from intertidal flat to subtidal zone. Mud erosion is also observed in areas that shifted from marsh to intertidal flat