Remotely sensed mid-channel bar dynamics in downstream of the Three Gorges Dam, China

The downstream reach of the Three Gorges Dam (TGD) along the Yangtze River (1560 km) hosts numerous mid-channel bars (MCBs). MCBs dynamics are crucial to the river’s hydrological processes and local ecological function. However, a systematic understanding of such dynamics and their linkage to TGD remains largely unknown. Using Landsat-image-extracted MCBs and several spatial-temporal analysis methods, this study presents a comprehensive understanding of MCB dynamics in terms of number, area, and shape, over downstream of TGD during the period 1985−2018. On average, a total of 140 MCBs were detected and grouped into four types representing small ( 2 km2), middle (2 km2 − 7 km2), large (7 km2 − 33 km2) and extra-large size (>33 km2) MCBs, respectively. MCBs number decreased after TGD closure but most of these happened in the lower reach. The area of total MCBs experienced an increasing trend (2.77 km2/yr, p-value 0.01) over the last three decades. The extra-large MCBs gained the largest area increasing rate than the other sizes of MCBs. Small MCBs tended to become relatively round, whereas the others became elongate in shape after TGD operation. Impacts of TGD operation generally diminished in the longitudinal direction from TGD to Hankou and from TGD to Jiujiang for shape and area dynamics, respectively. The quantified longitudinal and temporal dynamics of MCBs across the entire Yangtze River downstream of TGD provides a crucial monitoring basis for continuous investigation of the changing mechanisms affecting the morphology of the Yangtze River system

Remote sensing applications for the assessment of the geomorphic response of fluvial systems to the Holocene Climate Changes

The general goal of this thesis is the identification and description of the geomorphological responses of the fluvial system to the Holocene Climate Changes, proposing a multi-sensor remote sensing approach. In particular, the specific aim of this work is the improvement of the present knowledge on the Holocene and historical morphodynamics of the Lower Mesopotamian waterscape, especially on the paleo-hydrology of the ancient Tigris-Euphrates fluvial system, focusing on the specific process in the dynamics of the waterscapes which plays a key role in the drainage network evolution in lowland areas. Crevasse splays represent significant geomorphological features for understanding the fluvial morphodynamics in lowland areas where avulsion processes prevail. The southern Mesopotamian Plain is the area where the ancient State of Lagash developed between the prehistoric Ubaid Period (c. 5200 - c. 3500 BC) and the late Parthian era (247 BC - AD 244), representing an ideal case study, where the Italian Archaeological Mission has been recently carried on extensive field-works at Tell Zurghul archaeological site. Here, an interdisciplinary approach, combining field surveys and geomorphological mapping through remote sensing techniques, has been applied for analyzing the function and role of the waterscape on the early civilization. Indeed, the geomorphological analysis through a remote sensing approach and the archaeological surveys are both essential for the reconstruction of a complex environmental system, where landforms due to different morphogenetic processes occur, related to the presence of a wide fluvial-deltaic paleo-system and early human societies. The main aim of the focus on this archaeological site is to contribute to the reconstruction of the surrounding waterscape and know more about waterscape-human interactions during the Holocene. The question of human-waterscape relationship worldwide has been and still is a central topic in geomorphological, environmental, and archaeological research. During the Holocene, the Tigris-Euphrates river system, in the lower sector of the Mesopotamian Plain (Iraq), has been characterized by complex morphodynamics in response to both climate fluctuations and extensive construction of artificial canals, dug since the first human settlements belonging to the Early River Valley Civilizations. The Lower Mesopotamian Plain (LMP) coincides with the southern Tigris and Euphrates deltaic plain, developed starting since the mid Holocene. During the early Holocene, the sea-level rise caused a general and rapid northward shifting of the Persian Gulf shoreline: the maximum marine ingression reached the area where the present towns of Nasiriyah and Al-Amara are located about 6000 yrs BP; after which the widespread progradation of the Tigris and Euphrates delta system accounted for the southward shoreline regression up to the present position. The development of a typical bird-foot delta guaranteed an amount of water indispensable for agriculture, cattle, settlements, and transport. Indeed, the high mobility of the channels and the frequent occurrence of avulsion processes (i.e., levees break and related crevasse splays formation) are the main features typically connected to a multi-channel system, guarantying the water supply through seasonal floods. In the area, the water management during the mid Holocene, digging an extensive network of canals and building several dams, can either improve the socio-economic conditions of a settlement or cause the end of another one. Within a wide floodplain characterized by very low elevation ranges such as the LMP, a remote sensing, multi-sensor approach is a suitable method for identifying the main geomorphological features related to the fluvial avulsion processes, describing the associated morphogenetic processes. Optical and multispectral Landsat 8 satellite images have been processed for computing NDVI and Clay Ratio indices, as well as to extract the Regions of Interest (ROIs) focused on the main features that made up a crevasse splay (i.e., crevasse channel, crevasse levee and crevasse deposit). The spectral signatures from active and abandoned crevasse splays have been extracted and compared among them, adopting four different methods of Supervised Classification. The analysis of the crevasse splays has been integrated with the investigation of the micro-topography leading to recognize the crevasse channels and levees, the upward convexity of the crevasse deposits and the distal or proximal position of the parent channel; the re-classification of different DEM sources, such as the optical AW3D30 and GDEM2 datasets with ground resolution of 1 arcsec (i.e., 30 m cell-1), leads to highlighting the “above-floodplain” topographic configuration of these landforms. The analysis here performed leads to investigating the entire Lower Mesopotamian Plain through both large and medium scale geomorphological investigation, identifying active and abandoned channels, discerning between active and abandoned avulsion processes and distinguishing crevasse channels, levees, and deposits. In like manner, human features are recognized, allowing the evaluation of human-environmental interactions

Contemporary and future stresses on estuaries: examples from the Yellow River Delta, the Vietnamese Mekong Delta, and the German Bight

Formed where the water from rivers meets the sea, processes in estuaries are driven by the mixing between freshwater and seawater. Being home to diverse plant and animal communities, which have adapted to this unique environment, estuaries are one of the most productive ecosystems in the world. Providing a multitude of ecosystem services, estuaries are also of high economic value and contribute to human well-being. Besides providing habitats to aquatic species, estuaries are a source of food and raw materials while also cycling nutrients and contributing to coastal protection by damping the damaging effects of extreme events (e.g., storm surges). In addition, estuaries ensure safe navigation to and from ports and are used for recreational activities. However, with many of the world’s largest cities located on estuaries, they are directly exposed to impacts from human activity, such as overexploitation of resources or pollution. Being located in low-lying coastal areas, estuaries are also vulnerable to sea-level rise while simultaneously being impacted by climate change-induced alterations in hydrology. The combination of human-driven and climate-induced changes may lead to the degradation or loss of estuarine ecosystems and the services they provide. In order to minimize negative impacts and to promote a sustainable management of estuaries, it is thus important to investigate how estuarine environments respond to drivers of contemporary and future changes. Since no two estuaries are alike, examples from the Yellow River Delta (China), the Mekong Delta (Vietnam), and the German Bight are presented in this thesis. Major drivers, which impact these focus regions, include: sand mining/dredging, damming, climate change-induced alterations in hydrology, and sea-level rise. The aim of this thesis is to improve the understanding of how the selected estuaries are impacted by predominant contemporary and projected future drivers. This is accomplished by addressing different research questions with a focus on: (i) improving methods for assessing the present-day impact of selected drivers, (ii) improving projections by addressing less visible impacts and by integrating recently identified relevant processes, and (iii) applying different scenarios of plausible future developments in estuarine environments. (i) In a first study focusing on the Vietnamese Mekong Delta, improved methods were used to gain new insights into the intensity of regional sand mining activity. It was shown that the regional extraction of sand from the Mekong riverbed, which is driven by socio-economic developments in the region, is significantly higher than the river’s natural supply of sand. These findings have strong implications for the stability of the Vietnamese Mekong Delta, which is already subject to riverbank and coastal erosion under present-day conditions. (ii) Projections of future developments in estuaries were improved by addressing less visible impacts in numerical models and by integrating previously unaddressed processes. A second study concentrating on the Vietnamese Mekong Delta was used to project, for the first time, the morphodynamic evolution of the Mekong in response to a combination of major drivers, including sand mining, damming, climate change-induced alterations in hydrology, and sea-level rise. In a third study, which focuses on the North Sea, it could also be shown that the morphological evolution of intertidal flats in the Wadden Sea has a significant impact on the tidal dynamics in the region when considering future sea-level rise. (iii) By applying numerous plausible scenarios of future developments, the second study concentrating on the Mekong was able to identify the operation of hydropower dams as the major driver for future morphodynamic changes in the region, followed by sand extraction. Furthermore, this approach enabled to investigate the local interactions between different drivers. By combining different rates of sea-level rise with various rates of vertical accretion in the intertidal flats of the Wadden Sea, several plausible scenarios were also addressed in the study focusing on North Sea tides. If no vertical accretion is assumed in the intertidal flats, sea-level rise will lead to enhanced tidal asymmetry in the German estuaries Elbe, Weser, and Ems, potentially leading to increased sediment import. In contrast, tidal asymmetries resemble present-day conditions if intertidal flats are able to keep up with sea-level rise

The Application of Hydraulic and Sediment Transport Models in Fluvial Geomorphology

After publishing the famous “Fluvial Processes in Geomorphology” in the early 1960s, the work of Luna Leopold, Gordon Wolman, and John Miller became a key for opening the door to understanding rivers and streams. They first illustrated the problem to geomorphologists and geographers. Later, Chang, in his “Fluvial Processes in River Engineering”, provided a basis for engineers, showing this group of professionals how to deal with rivers and how to understand them. Since then, more informative studies have been published. Many of the authors started to combine fluvial geomorphology knowledge and river engineering needs, such as “Tools in Fluvial Geomorphology” by G. Mathias Kondolf and Hervé Piégay, or focused more on river engineering tasks, such as “Stream Restoration in Dynamic Fluvial Systems: Scientific Approaches” by Andrew Simon, Sean Bennett, and Janine Castro. Finally, Luna Leopold summarized river and stream morphologies in the beautiful “A view of the river”. It appears that we continue to explore this subject in the right direction. We better understand rivers and streams, and as engineers and fluvial geomorphologists, we can establish tools to help bring rivers alive. However, there is still a hunger for more scientific tools that we could use to further understand rivers and to support the development of healthy streams and rivers with high biodiversity in the present world, which has started to face water scarcity