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Van der Wegen M.,UNESCO IHE | Jaffe B.E.,U.S. Geological Survey
Marine Geology | Year: 2013

Measured bathymetries on 30. year interval over the past 150. years show that San Pablo Bay experienced periods of considerable deposition followed by periods of net erosion. However, the main channel in San Pablo Bay has continuously narrowed. The underlying mechanisms and consequences of this tidal channel evolution are not well understood. The central question of this study is whether tidal channels evolve towards a geometry that leads to more efficient hydraulic conveyance and sediment throughput. We applied a hydrodynamic process-based, numerical model (Delft3D), which was run on 5 San Pablo Bay bathymetries measured between 1856 and 1983.Model results shows increasing energy dissipation levels for lower water flows leading to an approximately 15% lower efficiency in 1983 compared to 1856. During the same period the relative seaward sediment throughput through the San Pablo Bay main channel increased by 10%. A probable explanation is that San Pablo Bay is still affected by the excessive historic sediment supply. Sea level rise and Delta surface water area variations over 150. years have limited effect on the model results. With expected lower sediment concentrations in the watershed and less impact of wind waves due to erosion of the shallow flats, it is possible that energy dissipations levels will decrease again in future decades. Our study suggests that the morphodynamic adaptation time scale to excessive variations in sediment supply to estuaries may be on the order of centuries. © 2013 Elsevier B.V.

Crosato A.,UNESCO IHE | Crosato A.,Technical University of Delft | Saleh M.S.,UNESCO IHE
Earth Surface Processes and Landforms | Year: 2011

The effects of floodplain vegetation on river planform have been investigated for a medium-sized river using a 2D morphodynamic model with submodels for flow resistance and plant colonization. The flow resistance was divided into a resistance exerted by the soil and a resistance exerted by the plants. In this way it was possible to reproduce both the decrease in bed shear stress, reducing the sediment transport capacity of the flow within the plants, and the increase in hydraulic resistance, reducing the flow velocities. Colonization by plants was obtained by instantaneously assigning vegetation to the areas that became dry at low water stages. This colonization presents a step forward in the modelling of bank accretion. Bank erosion was related to bed degradation at adjacent wet cells. Bank advance and retreat were reproduced as drying and wetting of the computational cells at the channel margins. The model was applied to a hypothetical case with the same characteristics as the Allier River (France). The river was allowed to develop its own geometry starting from a straight, uniform, channel. Different vegetation densities produced different planforms. With bare floodplains, the river always developed a braided planform, even if the discharge was constant and below bankfull. With the highest vegetation density (grass) the flow concentrated in a single channel and formed incipient meanders. Lower vegetation density (pioneer vegetation) led to a transitional planform, with a low degree of braiding and distinguishable incipient meanders. The results comply with flume experiments and field observations reported in the literature. © 2010 John Wiley & Sons, Ltd.

Di Baldassarre G.,UNESCO IHE | Schumann G.,University of Bristol | Brandimarte L.,UNESCO IHE | Bates P.,University of Bristol
Surveys in Geophysics | Year: 2011

It is widely recognised that remote sensing can support flood monitoring, modelling and management. In particular, satellites carrying Synthetic Aperture Radar (SAR) sensors are valuable as radar wavelengths can penetrate cloud cover and are insensitive to daylight. However, given the strong inverse relationship between spatial resolution and revisit time, monitoring floods from space in near real time is currently only possible through low resolution (about 100 m pixel size) SAR imagery. For instance, ENVISAT-ASAR (Advanced Synthetic Aperture Radar) in WSM (wide swath mode) revisit times are of the order of 3 days and the data can be obtained within 24 h at no (or low) cost. Hence, this type of space-borne data can be used for monitoring major floods on medium-to-large rivers. This paper aims to discuss the potential for, and uncertainties of, coarse resolution SAR imagery to monitor floods and support hydraulic modelling. The paper first describes the potential of globally and freely available space-borne data to support flood inundation modelling in near real time. Then, the uncertainty of SAR-derived flood extent maps is discussed and the need to move from deterministic binary maps (wet/dry) of flood extent to uncertain flood inundation maps is highlighted. © 2011 Springer Science+Business Media B.V.

Giordano M.,SRI International | de Fraiture C.,UNESCO IHE
Agricultural Water Management | Year: 2014

Millions of smallholder farmers in sub-Saharan Africa and South Asia benefit from readily available and affordable irrigation technologies. The rapid uptake of small private irrigation in South Asia had a proven positive effect on poverty alleviation. In sub-Saharan Africa similar trends are emerging and several studies point to considerable upscaling potential. Achieving this potential would substantially boost smallholder incomes and food security. However, the spread of small private irrigation poses several challenges related to equity, efficiency, and sustainability. Women and resource poor farmers face challenges accessing affordable technologies; market inefficiencies and policy frameworks negatively affect farmer decision-making and technology access; and the unregulated spread of private irrigation may lead to over-abstraction, pollution, and conflicts. In this paper we argue that carefully designed intervention strategies and policy engagement are needed for two reasons. First, there is a need to address potential adverse effects of the ongoing, unregulated spread of small private irrigation while safeguarding its proven benefits on food security and poverty alleviation. Second, relatively straightforward measures can extend the benefits to a broader group of smallholders, including women and the poor, while at the same time ensuring sustainable use of the resource. Based on empirical evidence from case studies in six countries, we identified four elements of such an approach: (1) enhancing technology access; (2) catalyzing smallholder value chains; (3) fostering supportive policies; and (4) strengthening institutional capacity to manage potential trade-offs at the watershed scale. © 2013 Elsevier B.V.

De Fraiture C.,UNESCO IHE | Giordano M.,SRI International
Agricultural Water Management | Year: 2014

An increasing number of smallholder farmers engage in irrigation using their own resources. They buy or rent irrigation equipment and draw water from nearby sources without depending on or without interference from public agencies or water user associations. The individualization of Agricultural Water Management has been ongoing for several decades in South Asia where most irrigation now takes place from privately owned wells. Recently, small private irrigation is emerging also in sub Saharan Africa. It is farmer-driven, responds to a genuine demand from smallholders and has substantial potential for poverty alleviation and rural development. In many countries the area under privately managed and owned irrigation is larger than under public irrigation schemes. However, the individualization of irrigation and its spontaneous, unchecked spread pose challenges to equitable access to and sustainable management of water resources. Irrigation investments and research efforts have largely focused on the underperforming public irrigation sector, ignoring small private irrigation. This special issue describes and analyzes this thriving but overlooked sector, drawing from examples from five countries in sub-Saharan Africa and two states in India. The authors explore ways to enhance the potential of small private irrigation for all, without jeopardizing the sustainability of the available water resources. © 2013 Elsevier B.V.

Ranasinghe R.,UNESCO IHE | Ranasinghe R.,Technical University of Delft | Callaghan D.,University of Queensland | Stive M.J.F.,Technical University of Delft
Climatic Change | Year: 2012

Accelerated sea level rise (SLR) in the twenty-first century will result in unprecedented coastal recession, threatening billions of dollars worth of coastal developments and infrastructure. Therefore, we cannot continue to depend on the highly uncertain coastal recession estimates obtained via the simple, deterministic method (Bruun rule) that has been widely used over the last 50 years. Furthermore, the emergence of risk management style coastal planning frameworks is now requiring probabilistic (rather than deterministic, single value) estimates of coastal recession. This paper describes the development and application of a process based model (PCR model) which provides probabilistic estimates of SLR driven coastal recession. The PCR model is proposed as a more appropriate and defensible method for determining coastal recession due to SLR for planning purposes in the twenty-first century and beyond. © 2011 The Author(s).

Van der Wegen M.,UNESCO IHE | Jaffe B.E.,U.S. Geological Survey
Coastal Engineering | Year: 2013

Recent advances in the development of numerical, process-based models have led to remarkable performance in reproducing measured decadal morphodynamic developments. The advantage of this type of models is that they have a detailed output allowing for a close analysis of relevant processes. Drawback is that the output is associated with a high level of presumed uncertainty, because of the large number of processes involved and the high quality level of input data required.This study aims to explore possibilities to assess uncertainty levels associated with process-based morphodynamic modeling. In a probabilistic approach we consider the outcome of an ensemble of runs including variations of model input parameters and forcing schematizations. We propose to evaluate model performance by both a skill criterion (How well does the model reproduce observed patterns?), a confidence criterion (How sensitive are model results to uncertain input?) as well as a combination of these criteria. This methodology provides an objective assessment of the performance of process-based morphodynamic models. In addition, it can determine which input parameters cause largest uncertainty in the model outcome.The San Pablo Bay case study shows that 60% of the modeled volume meets the skill and confidence criteria for the depositional period (1856-1887) and 46% for the erosional period (1951-1983). Approximately 50% of the volume allocation meets the confidence criterion for a 30. year morphodynamic forecast (1983-2013). Model results are sensitive to model input variations only to a limited extend. We attribute this to the high impact of the San Pablo Bay plan form and bathymetry. The forecast shows continuous erosion of the channel and on the northern shoals and a continuous accretion of the channel slopes, albeit more concentrated in the western part of the channel than in preceding decades. © 2013 Elsevier B.V.

Brandimarte L.,UNESCO IHE | Woldeyes M.K.,UNESCO IHE
Hydrological Processes | Year: 2013

The presence of a bridge spanning over a river and its floodplain alters the condition of flow. In subcritical conditions, which are typical in alluvial rivers, this flow alteration results in the so-called backwater effect, that is, an increase of the water surface elevation upstream of the structure as a response to increased energy losses. The extension of the expected backwater affected area and the increase in water surface elevation are highly dependent on the river morphology, bridge geometry, flow and floodplain characteristics and may originate upstream flooding. Thus, in many countries, at the stage of design a new bridge (or renovation works), a specific investigation on the backwater effect has to be undertaken to analyse its effect on the flooding of its vicinity. Bridge waterway hydraulics has been widely investigated in terms of practical research and some standard guidelines have been suggested for modelling the backwater effect at bridge crossings. Although the scientific literature has widely proven that hydraulic modelling is affected by many sources of uncertainty, the hydraulic modelling of backwater effects is still undertaken within a deterministic approach, based on the outcomes of a calibrated hydraulic model. This article aims at approaching the prediction of backwater effects at bridge crossings by accounting for the main sources of uncertainty affecting the hydraulic modelling exercise. A bridge over a highly vegetated floodplain (Tallahala Creek, Mississippi, USA) was considered for this analysis, and uncertainty in the model parameters and input data was assessed to predict backwater flood design profiles. © 2012 John Wiley & Sons, Ltd.

Van Der Wegen M.,UNESCO IHE
Journal of Geophysical Research: Earth Surface | Year: 2013

The morphod ynamic adaptation of estuaries to sea level fluctuations has been subject of geological studies based on sediment core analysis and qualitative modeling efforts. Limited attention has been paid to understanding bathymetric evolution based on a detailed process level. The current study aims to explore governing morphodynamic processes and timescales by application of a 2D, process-based modeling approach. The starting point of the analysis is an 80 km long and 2.5 km wide basin. Starting from a sandy flat bed, stable channel-shoal patterns emerge within a century under semidiurnal tidal forcing. We impose a gradual rise in sea level (up to 0.67 m per century) and compare the results with a run excluding sea level rise (SLR). Model results show that SLR drowns the basin so that intertidal area disappears. This process generates tidal asymmetry reflected by an emerging M4 tidal constituent. The basin shifts from exporting to importing sediment reflected by shoal patterns migrating in the landward direction. The landward sediment transport remains too limited to compensate for the loss in intertidal area and to restore equilibrium within a millennial time scale. Further sensitivity tests on initial bathymetry, tidal amplitude forcing, and rate of SLR show that shallow basins with limited tidal forcing are most vulnerable to SLR. Key Points Sea level rise is imposed on a schematized tidal embayment over 1600 years Morphodynamic system shifts from a sediment exporting to an importing system Disappearing intertidal area generates tidal asymmetry ©2013. American Geophysical Union. All Rights Reserved.

Schumann G.,University of Bristol | Di Baldassarre G.,UNESCO IHE
Remote Sensing Letters | Year: 2010

A near-simultaneous satellite acquisition of a flood event on 12 December 2006 on theDee River in Wales (UK) is used to illustrate the potential to develop standalone space-borne flood risk mapping techniques. Both the ERS-2 and ENVISAT satellites recorded the event close to flood peak and only 28 min apart. This unique opportunity enables the creation of a very rare but extremely useful observed data set for flood inundation studies. This letter illustrates how this unique set of spaceborne radar images can be used for rapid flood risk mapping. An event-specific weighted hazard map was generated based on plausible flood area observations from an aggregation of widely applied image-processing techniques. This map can be further augmented to an event-specific fuzzy flood risk map by fusing the multialgorithm ensemble map with vulnerability-weighted land cover vector data in a geographic information system(GIS) environment. The technique presented is fairly flexible and leads to a potentially useful data set for direct use in flood management. © 2010 Taylor & Francis.

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