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Automating surface water detection for rivers : the estimation of the geometry of rivers based on optical earth observation sensors
This thesis studies the extent to which the geometry of rivers around the globe can be determined in an automated manner, based on openly available optical Earth Observational (EO) satellite sensors. Knowledge of the course of a river allows for the bathymetry to be estimated. The utilization of remote sensing technology is investigated to observe the dynamics of rivers at frequent time intervals. Google Earth Engine is adopted for the analysis, interpretation and manipulation of multispectral satellite data. Although the estimation of the geometry of rivers in a more or less automated manner was found to be achievable, its global applicability remains limited to a local scale. Currently, the biggest limitation is the fact that the amount of usable memory is capped within GEE. Computations were found to be too resource-demanding, limiting the maximum size of a river polygon that can be generated. Furthermore, the estimation of the geometry of a river is found to be limited to rivers that are at least three to four times wider than the corresponding satellite’s spatial resolution in order to obtain usable results. Rivers that are relatively narrow (<100 meters) were found to be difficult to identify using either Sentinel-2 or Landsat 8 multispectral satellite data.
Report on the Nexus Humera case study in Ethiopia
The Nexus approach aims to analyse the linkages between the water, energy and food sectors and especially the trade-offs (developing one sector at the expense of the other) and possible synergies (innovative win-wins) between the sectors. Especially in the light of rapid population growth -as is the case in Ethiopia- and uncertain climate change outcomes is it important to zoom in and interconnect the three fundamental assets of people’s livelihoods: water, energy and food. The approach was applied in Humera woreda in western Tigray in August 2017 to provide insights as to how a nexus approach could work, how possible nexus solutions could be integrated into government programmes and business strategies, and how a contribution could be made to sustainable development of the economy of Humera woreda. The focus of the case study in Humera was the emerging development of a large-scale Agro-industrial Park and its role in potentially boosting sesame production in the area. In the case study three different models were applied: the LEAP model for the energy sector, the RIBASIM model for the water sector, and the Water-limited yield model for the agricultural sector. The models were run on the basis of collected data, for different scenarios and based on assumptions that were tested with stakeholders.
Spatiotemporal patterns of extreme sea levels along the western North-Atlantic coasts
The western North-Atlantic coast experienced major coastal floods in recent years. Coastal floods are primarily composed of tides and storm surges due to tropical (TCs) and extra-tropical cyclones (ETCs). We present a reanalysis from 1988 to 2015 of extreme sea levels that explicitly include TCs for the western North-Atlantic coastline. Validation shows a good agreement between modeled and observed sea levels and demonstrates that the framework can capture large-scale variability in extreme sea levels. We apply the 28-year reanalysis to analyze spatiotemporal patterns. Along the US Atlantic coasts the contribution of tides can be significant, with the average contribution of tides during the 10 largest events up to 55% in some locations, whereas along the Mexican Southern Gulf coast, the average contribution of tides over the largest 10 events is generally below 25%. At the US Atlantic coast, ETCs are responsible for 8.5 out of the 10 largest extreme events, whereas at the Gulf Coast and Caribbean TCs dominate. During the TC season more TC-driven events exceed a 10-year return period. During winter, there is a peak in ETC-driven events. Future research directions include coupling the framework with synthetic tropical cyclone tracks and extension to the global scale.
Observations and modelling of shoreface nourishment behaviour
Shoreface nourishments are commonly applied for coastal maintenance, but their behaviour is not well understood. Bathymetric data of 19 shoreface nourishments located at alongshore uniform sections of the Dutch coast were therefore analyzed and used to validate an efficient method for predicting the erosion of shoreface nourishments. Data shows that considerable cross-shore profile change takes place at a shoreface nourishment, while an impact at the adjacent coast is hard to distinguish. The considered shoreface nourishments provide a long-term (3 to ~30 years) cross-shore supply of sediment to the beach, but with small impact on the local shoreline shape. An efficient modelling approach is presented using a lookup table filled with computed initial erosion–sedimentation rates for a range of potential environmental conditions at a single post-construction bathymetry. Cross-shore transport contributed the majority of the losses from the initial nourishment region. This transport was driven partly by water-level setup driven currents (e.g., rip currents) and increased velocity asymmetry of the waves due to the geometrical change at the shoreface nourishment. Most erosion of the nourishment takes place during energetic wave conditions as milder waves are propagated over the nourishment without breaking. A data-model comparison shows that this approach can be used to accurately assess the erosion rates of shoreface nourishments in the first years after construction.
Adapting to the sea : human habitation in the coastal area of the northern Netherlands before medieval dike building
Before medieval dike building, the coastal area of the northern Netherlands was a wide, regularly inundated salt-marsh area. Despite the dynamic natural conditions, the area was inhabited already in the Iron Age. The inhabitants adapted to this marine environment by living on artificial dwelling mounds, so-called terps. Terp habitation was a highly successful way of life for over 1500 years, and may be re-introduced as a useful strategy for present and future communities in low-lying coastal regions that are facing accelerated sea-level rise. This already has been recommended in several reports, but detailed knowledge of the technology of terp habitation is usually lacking. The aim of this paper is to present nearly two decades of archaeological research in the coastal region of the northern Netherlands, in order to inform the current debate on the possibilities of adapting to the effects of climate change in low-lying coastal areas. It presents the multi-disciplinary methods of this research and its results, supplying details of terp construction and other strategies such as the construction of low summer dikes that are still useful today. The results and discussion of the presented research also make it possible to describe the conditions that must be met to make terp habitation possible. Terp habitation could have continued, were it not for the considerable subsidence of inland areas due to peat reclamation. That made the entire coastal area increasingly vulnerable to the sea. In response to this threat, dike building began in the 11th or 12th century, but these increasingly higher dikes decreased the water storage capacity and caused impoundment of seawater during storm surges. Moreover, accretion through sedimentation was halted from then on. Unlike terp habitation, the construction of high dikes therefore cannot be considered a sustainable solution for living in lowlying coastal areas in the long term.
Commentary: The need for a high-accuracy, open-access global DEM
Schumann and Bates (2018) argue that there is a need for a global scale, high-accuracy, open-access Digital Elevation Model (DEM). They find current global DEM data unacceptable in accuracy and resolution. We support the statement that a DEM with the envisioned accuracy is useful. In fact, it is a general prerequisite to simulating flood behavior at large scales. However, at the level of detail suggested (urban zones), an accurate DEM alone is not enough if the objective of flood modeling goes beyond awareness raising or flood zoning. In this commentary, we argue that: (a) the “consortium effort” proposed by Schumann and Bates should focus on regions that will profit most: developing countries; (b) technically, at the local scale where the suggested accuracy (0.5m) with the suggested spatial resolution (~5m) becomes useful, other properties of the environment besides terrain become dominant in flood behavior and, consequently, in flood risk. These include the water infrastructure (bridges, channels, culverts, etc.) and its maintenance state. Such infrastructure information is key in establishing what makes a street, neighborhood or city more flood resilient, hence supporting communities in their decision-making for a sustainable future; and (c) that a bottom-up data collection approach for infrastructure and maintenance states will lead to less inequality in global data coverage and a reduction in dependency on outside capacities. This then leads to sustainable data collection and ownership in the regions where flood risk information is most needed.
A temperature-scaling approach for projecting changes in short duration rainfall extremes from GCM data
Current and future urban flooding is influenced by changes in short-duration rainfall intensities. Conventional approaches to projecting rainfall extremes are based on precipitation projections taken from General Circulation Models (GCM) or Regional Climate Models (RCM). However, these and more complex and reliable climate simulations are not yet available for many locations around the world. In this work, we test an approach that projects future rainfall extremes by scaling the empirical relation between dew-point temperature and hourly rainfall and projected changes in dew-point temperature from the EC-Earth GCM. These projections are developed for the RCP 8.5 scenario and are applied to a case study in the Netherlands. The shift in intensity-duration-frequency (IDF) curves shows that a 100-year (hourly) rainfall event today could become a 73-year event (GCM), but could become as frequent as a 30-year (temperature-scaling) in the period 2071–2100. While more advanced methods can help to further constrain future changes in rainfall extremes, the temperature-scaling approach can be of use in practical applications in urban flood risk and design studies for locations where no high-resolution precipitation projections are available.
Flooding in the Mekong Delta : impact of dyke systems on downstream hydrodynamics
Building high dykes is a common measure to cope with floods and plays an important role in agricultural management in the Vietnamese Mekong Delta. However, the construction of high dykes cause considerable changes in hydrodynamics of the Mekong River. Therefore, this paper aims to assess the impacts of the high dyke system on water level fluctuation and tidal propagation on the Mekong River branches using a modelling approach. In order to consider interaction 15 between rivers and seas, an unstructured modelling grid was generated, with 1D-2D coupling, covering the Mekong Delta and extending to the East (South China Sea) and West (Gulf of Thailand) seas. The model was manually calibrated for the flood season of the year 2000. To assess the role of floodplains, scenarios consisting of high dykes built in different regions of the Long Xuyen Quadrangle (LXQ), Plains of Reeds (PoR) and TransBassac were carried out. Results show that the percentage of river outflow at Dinh An sharply increases in the dry season in comparison to the flood season while the other Mekong 20 estuarine outflows rise slightly. In contrast, the lateral river flows of the Mekong River system to the seas by the Soai Rap mouth and the LXQ decrease somewhat in the dry season compared to the flood season due to overflow reduction at the Cambodia-Vietnam border. Additionally, the high dykes in the regions that are directly connected to a branch of the Mekong River, not only have an influence on the hydrodynamics in their own branch, but also on other branches because of the connecting channel of Vam Nao. Moreover, the high dykes built in the PoR, LXQ and TransBassac regions are the most 25 important factor for changing water levels at Tan Chau, Chau Doc and Can Tho, respectively. The LXQ high dykes result in an increase of daily mean water levels and a decrease of tidal amplitudes on the Song Tien (downstream of the connecting channel of Vam Nao). A similar interaction is also found for the the PoR high dykes and the Song Hau.
Benthic effects of offshore renewables : identification of knowledge gaps and urgently needed research
As the EU’s commitment to renewable energy is projected to grow to 20% of energy generation by 2020, the use of marine renewable energy from wind, wave and tidal resources is increasing. This literature review (i) summarizes knowledge on how marine renewable energy devices affect benthic environments, (ii) explains how these effects could alter ecosystem processes that support major ecosystem services and (iii) provides an approach to determine urgent research needs. Conceptual diagrams were set up to structure hypothesized cause-effect relationships (i.e. paths). Paths were scored for (i) temporal and spatial scale of the effect, (ii) benthic sensitivity to these effects, (iii) the effect consistency and iv) scoring confidence, and consecutively ranked. This approach identified prominent knowledge gaps and research needs about (a) hydrodynamic changes possibly resulting in altered primary production with potential consequences for filter feeders, (b) the introduction and range expansion of non-native species (through stepping stone effects) and, (c) noise and vibration effects on benthic organisms. Our results further provide evidence that benthic sensitivity to offshore renewable effects is higher than previously indicated. Knowledge on changes of ecological functioning through cascading effects is limited and requires distinct hypothesis-driven research combined with integrative ecological modelling.
Prospects for hydropower in Ethiopia : an energy-water nexus analysis
In this article we investigate the prospects for large-scale hydropower deployment in Ethiopia. With two distinct modelling approaches we find high projections for future hydropower generation: between 71 and 87 TWh/yr by 2050 in a stringent climate change control scenario in which Ethiopia contributes substantially to global efforts to reach the 2 C target of the Paris Agreement. This elevated level is obtained despite domestic water use and irrigated agriculture water demand expansions, and irrespective of hydrological effects from climate change in terms of a drop in average precipitation nationwide. This amount of hydro-electricity production matches the expected national hydropower potential. Yet, we encourage authorities to take due account of the large impact that climate change may have on rainfall during particular months or years at individual water reservoirs, which we think should be researched in greater detail. Our combined energy cost-minimisation and hydrological balance analysis shows that our models can be jointly used for the assessment of hydropower as climate change mitigation option, and can assist in the design of policies that integrate the energy and water sectors. Our case study did not yield direct reasons for the Ethiopian government to swiftly stop pursuing its current ambitious national hydropower development plan, but we encourage it to adequately internalise an extensive range of factors (including environmental, geopolitical and social) that may induce it to take a different course.