Andres Payo

Cross-shore suspended sand and bed load transport on beaches

Simple formulas are developed to predict the time-averaged rates of cross-shore suspended sand and bed load transport. The net suspended sand transport rate is expressed as the product of the depth-averaged current and the suspended sediment volume per unit bottom area with a reduction factor that accounts for the correlation between the time-varying fluid velocity and sediment concentration. The net bed load transport rate under nonlinear waves is assumed to be onshore and proportional to ?U3 where ?U is the standard deviation of the horizontal velocity. The probabilities of sediment movement and suspension are introduced to account for the initiation of sediment movement and suspension. Simple functions are proposed to account for the effects of a steep bottom slope on the bed load and suspended sediment transport rates. The proposed formulas are found to be in agreement with three data sets within a factor of about 2. The proposed formulas are shown to be consistent with existing simple formulas. The formulas are incorporated into a time-averaged wave model and the continuity equation of bottom sediment to predict the beach profile evolution. The numerical model is compared with seven small-scale tests including berm erosion tests and seven large-scale tests including dune erosion tests. The numerical model predicts the overall beach profile evolution including the berm and dune erosion but does not always predict the fairly subtle profile changes including bar migration accurately.

Feedback structure of cliff and shore platform morphodynamics

It has been suggested that studies of geomorphological systems should identify potential system feedbacks, determine their direction of influence, and assess their relative importance. In this paper we show how a core set of processes and feedback loops can be distilled from existing literature on rock coast morphodynamics. The structure has been represented using Causal Loop Diagrams and a methodology to estimate the strength of a single feedback loop is presented. The backwearing erosion rate (cliff horizontal erosion) has been found to be controlled by at least four feedback loops; three balancing (cliff toe wave energy depletion, ground-water pore pressure diminution and cliff deposit protection) and one positive loop (abrasion enhancement). The downwearing erosion rate (vertical erosion) has been found to be controlled by at least three balancing feedback loops (weathering limited, shear depletion, cover-protection). Mean sea level directly influences the downwearing rate, through the water depth relative to the wave base, and indirectly influences the backwearing erosion rate through the wave energy dissipation that determines the amount of energy reaching the cliff toe. The offshore wave non-linearity parameter is proposed to capture the complex interaction between waves and shore platform geometries. The strength of the cliff toe energy depletion loop is assessed by reasoning on its causal pathway and found to be O(−10−10 to −10−4) for poorly lithified rock coasts. By understanding how the individual and overall feedback strengths are influenced by different future environmental and human intervention scenarios we could provide better assessment at the time scales needed for coastal management.

Modeling daily soil salinity dynamics in response to agricultural and environmental changes in coastal Bangladesh

Understanding the dynamics of salt movement in the soil is a prerequisite for devising appropriate management strategies for land productivity of coastal regions, especially low-lying delta regions, which support many millions of farmers around the world. At present, there are no numerical models able to resolve soil salinity at regional scale and at daily time steps. In this research, we develop a novel holistic approach to simulate soil salinization comprising an emulator-based soil salt and water balance calculated at daily time steps. The method is demonstrated for the agriculture areas of coastal Bangladesh (∼20,000 km2). This shows that we can reproduce the dynamics of soil salinity under multiple land uses, including rice crops, combined shrimp and rice farming, as well as non-rice crops. The model also reproduced well the observed spatial soil salinity for the year 2009. Using this approach, we have projected the soil salinity for three different climate ensembles, including relative sea-level rise for the year 2050. Projected soil salinity changes are significantly smaller than other reported projections. The results suggest that inter-season weather variability is a key driver of salinization of agriculture soils at coastal Bangladesh.

Uncertainty assessment: Application to the shoreline

It is impossible to know beforehand the planforms of a stretch of beach without being first aware of the maritime climate affecting it. This article describes a procedure for objectively calculating the uncertainty associated with the prediction of the evolution of a stretch of beach in terms of probability. On the basis of oceanographic data records as well as empirical orthogonal functions (EOF), we propose a procedure for the simulation of possible sequences of storm events. Such sequences were then entered as input for a morphodynamic model with a view to the subsequent generation of possible planforms. EOF methodology was then used to estimate the probability of each of the planforms thus generated. The case study presented here is that of the evolution of an initially straight sand beach where a rectangular tapered fill had been constructed. The beach is located upshore of a groin perpendicular to the coastline, and had blocked all longshore sediment transport. For this analysis we used a one-line model with time-dependent boundary conditions and a non-homogeneous diffusion coefficient.

Experiential Lock-In: Characterizing Avoidable Maladaptation in Infrastructure Systems

Facing the combined challenges of environmental, social, and technological change, long-lived infrastructure systems run the risk of getting locked into unsustainable, maladapted pathways. This is particularly challenging in the context of climate change, given projected climate impacts are characterized by high degrees of uncertainty (Hallegatte 2009). Lock-in is a concept developed by economic historians to describe how economies get tied into using inefficient technologies, and it is linked to the concept of path dependence (Arthur 1983; David 1985), which refers to the fact that infrastructure systems follow specific trajectories that are difficult and costly to change. As shown in Arthur (1989), these trajectories depend on historical circumstances, timing, and strategy as much as on optimality. In the 1990s, some investigations highlighted the need to approach the analysis of technological changes through coevolutionary approaches that recognize the technological systems influences and are influenced by the social, economic, and cultural setting in which they develop (Rip and Kemp 1998). Liebowitz and Margolis (1994) argued that the role of some elements of the system, such as network externalities, remains contested. Of particular interest is the extent to which favoring incumbent infrastructure systems limits the development capacity of socioeconomic groups such as communities, industries, or countries. While exploring the whole phase space of possible fundamental influences is impractical, the authors argue that it is still possible to avoid some lock-in by effectively utilizing existing anticipatory capacity. The paper elaborates on three ideas, firmly rooted in the scholarly literature and recent studies, which characterize one type of avoidable lock-in: (1) the observed dominance of experiential versus analytical anticipatory capacity of communities, industries, and countries in the governance of sociotechnical systems; (2) the existence of formal approaches to quantify the limits to adaptation in such systems; and (3) limitations of the impact and capacity approach to adaptation. The elements of an avoidable lock-in are then summarized and illustrated by an example. Finally, some conclusions are given on the implications of this type of avoidable lock-in and how it might increasingly affect policy decisions that have long-term implications, such as those related to long-lasting infrastructure systems and spatial planning.

Discussion of Ford, M.R.; Becker, J.M., and Merrifield, M.A. 2013. Reef Flat Wave Processes and Excavation Pits: Observations and Implications for Majuro Atoll, Marshall Islands. Journal of Coastal Research, 29(3), 545–554.

ABSTRACT Payo, A. and Muñoz-Perez, J.J., 2013. Discussion of Ford, M.R.; Becker, J.M., and Merrifield, M.A., 2013. Reef flat wave processes and excavation pits: Observations and implications for Majuro Atoll, Marshall Islands. Ford, Becker, and Merrifield observed reef flat wave conditions during two deployments over a 41 day period to investigate the impact of reef flat excavation pits on wave processes at Majuro Atoll. They noticed that the shoreline with the excavation pit received wave heights slightly less (∼8%) than those recorded at the nearby unmodified cross section. They suggested that this net decrease was the net product of a slight increase in sea and swell (SS) wave energy due to a bottom roughness reduction and a decrease in infragravity (IG) wave energy due to the disruption of the cross-shore quasi-standing modes caused by the excavation pit. We argue that, for this particular experiment, the coupling between the SS and IG energy waves may provide an alternative explanation of the observations, and we suggest that further investigations are needed. Although the coupling between SS and IG waves may be important for assessing the impact of excavation pits on IG-dominated shorelines, we show that these excavation pits in SS-dominated surf zones can lead to events such as the observed destruction of the Cadiz (SW Spain) seawall in 1792.

Integrative Analysis Applying the Delta Dynamic Integrated Emulator Model in South-West Coastal Bangladesh

A flexible meta-model, the Delta Dynamic Integrated Emulator Model (ΔDIEM), is developed to capture the socio-biophysical system of coastal Bangladesh as simply and efficiently as possible. Operating at the local scale, calculations occur efficiently using a variety of methods, including linear statistical emulators, which capture the behaviour of more complex models, internal process-based models and statistical associations. All components are tightly coupled, tested and validated, and their behaviour is explored with sensitivity tests. Using input data, the integrated model approximates the spatial and temporal change in ecosystem services and a number of livelihood, well-being, poverty and health indicators of archetypal households. Through the use of climate, socio-economic and governance scenarios plausible trajectories and futures of coastal Bangladesh can be explored.

Improving decadal coastal geomorphic predictions: an overview of the iCOASST project

Coastal areas are already at high risk from a range of geohazards. The cumulative effect of human intervention on soft coastlines has frequently left them far from equilibrium under today’s conditions, especially in densely populated areas. Future changes in marine forcing due to climate change reinforce the need to understand and predict processes of change in shoreline position and configuration at management (decadal) scales. The UK-based iCOASST project is developing new and improved methods to predict decadal geomorphic evolution, linked to coastal erosion and flood risk management. This is based on a framework that links several components to develop a system-level understanding of this change. The framework includes: (1) new methods for system-level analysis and mapping of coast, estuary and inner shelf landform behaviour; (2) well validated ‘bottom-up’ hydrodynamic and sediment transport shelf models which can be applied at shelf scales to investigate inner shelf-coastal interactions; and (3) model compositions formed of existing or new ‘reduced complexity models’ of selected coastal landforms and processes that are suitable for multiple decadal length simulations. This will ultimately allow multiple simulations of coastal evolution which can explore uncertainties in future decadal-scale coastal response, including the effects of climate change and management choices. This paper outlines the current state of progress in the iCOASST Project.

Dynamics of the Sundarbans Mangroves in Bangladesh Under Climate Change

Mangroves provide a range of ecosystem services ranging from mitigation of global climate change by carbon capture to the sustenance of local communities whose livelihood depends upon mangrove forest products. A hybrid model is used to analyse and project the spatial distribution of mangrove species assemblages with results showing that the distribution will alter substantially within a hundred years associated with a significant decrease in the forest cover area. These changes are reflected in the amount of carbon sequestered in the mangroves in the form of biomass with a decrease anticipated over this century. Simulations of relative sea-level rise also indicate a loss in mangrove area mainly driven by erosion rather than inundation. Re-afforestation is one option of management strategy which could compensate for these changes.

An Integrated Approach Providing Scientific and Policy-Relevant Insights for South-West Bangladesh

Bangladesh is identified as an impact hotspot for sea-level rise in multiple studies. However, a range of other factors must be considered including catchment management, socio-economic development and governance quality, as well as delta plain biophysical processes. Taking an integrated assessment approach highlights that to 2050 future changes are more sensitive to human choice/policy intervention than climate change, ecosystem services diminish as a proportion of the economy with time, continuing historic trends and significant poverty persists for some households. Hence under favourable policy decisions, development could transform Bangladesh by 2050 making it less vulnerable to longer-term climate change and subsidence. Beyond 2050, the threats of climate change are much larger, requiring strategic adaptation responses and policy changes that must be initiated now.