Tidal bedforms dynamics, Weser River, Germany

<p>The distribution, morphology and dynamics of tidal bedforms in the Weser estuary, Germany, between the tidal limit (river-km 0 at the tidal weir in Bremen) and the open North Sea (river-km 111 in the Outer Weser) has been analysed for a five-year period based on monthly bathymetric surveys carried out along the main waterway. For the years 2009 to 2014, bedforms were detected from gridded bathymetry data (2x2 m) and their geometric properties described. In particular, the presence and position of a slip face, here defined as the portion of the lee side steeper than 15°, was traced. This was shown to be a practical criterion for the presence of permanent flow separation and turbulent wake in the lee of bedforms. Here it is used as a simplified indicator of bedform roughness: if a bedform does feature a slip face, it is assumed to be an active roughness element. The results were related to river discharge, water levels, and flow velocities.</p><p>Bedforms were present along most of the river channel, apart from a large section between river-km 55 and 75. There, muddy cohesive sediment in the estuarine turbidity maximum zone hindered the formation of bedforms. Along the channel and throughout the years, bedform lengths varied between 20 and 60 m and heights between 0.3 and 1.6 m.</p><p>During times of high fluvial discharge, in winter and spring, ebb velocities were stronger than flood velocities. The bedforms then were small, long and ebb-oriented (i.e. the ebb lee side was shorter than the flood lee side) and many bedforms featured an ebb slip face but no flood slip face. This suggests that throughout the survey area, bedforms were active roughness elements during the ebb phase only.</p><p>In summer and autumn, when the discharge was low, bedforms in the upper reach (ca. river-km 15 to 30) gradually became flood-oriented and many bedforms there developed a flood slip face, implying that these bedforms were active roughness elements during the flood. Between km 30 and 55, bedforms were predominantly ebb-oriented, and many bedforms had an ebb slip face but only few had a flood slip face, so most bedforms were only active during the ebb phase.</p><p>The annual variations of bedform dimensions and shapes reveal an intricate feedback between river and tidal flows, channel morphology, sediment dynamics and bedforms. The results have implications for bedform research, river management and numerical modelling.</p>

Modelling Climate Change Impacts on the Seasonality of Water Resources in the Upper Ca River Watershed in Southeast Asia

The impact of climate change on the seasonality of water resources in the Upper Ca River Watershed in mainland Southeast Asia was assessed using downscaled global climate models coupled with the SWAT model. The results indicated that temperature and evapotranspiration will increase in all months of future years. The area could warm as much as 3.4°C in the 2090s, with an increase of annual evapotranspiration of up to 23% in the same period. We found an increase in the seasonality of precipitation (both an increase in the wet season and a decrease in the dry season). The greatest monthly increase of up to 29% and the greatest monthly decrease of up to 30% are expected in the 2090s. As a result, decreases in dry season discharge and increases in wet season discharge are expected, with a span of ±25% for the highest monthly changes in the 2090s. This is expected to exacerbate the problem of seasonally uneven distribution of water resources: a large volume of water in the wet season and a scarcity of water in the dry season, a pattern that indicates the possibility of more frequent floods in the wet season and droughts in the dry season.