Virginia Ruiz-Villanueva

Recent advances quantifying the large wood dynamics in river basins: New methods and remaining challenges

Large wood is an important physical component of woodland rivers and significantly influences river morphology. It is also a key component of stream ecosystems. However, large wood is also a source of risk for human activities as it may damage infrastructure, block river channels, and induce flooding. Therefore, the analysis and quantification of large wood and its mobility are crucial for understanding and managing wood in rivers. As the amount of large-wood-related studies by researchers, river managers, and stakeholders increases, documentation of commonly used and newly available techniques and their effectiveness has also become increasingly relevant as well. Important data and knowledge have been obtained from the application of very different approaches and have generated a significant body of valuable information representative of different environments. This review brings a comprehensive qualitative and quantitative summary of recent advances regarding the different processes involved in large wood dynamics in fluvial systems including wood budgeting and wood mechanics. First, some key definitions and concepts are introduced. Second, advances in quantifying large wood dynamics are reviewed; in particular, how measurements and modeling can be combined to integrate our understanding of how large wood moves through and is retained within river systems. Throughout, we present a quantitative and integrated meta-analysis compiled from different studies and geographical regions. Finally, we conclude by highlighting areas of particular research importance and their likely future trajectories, and we consider a particularly underresearched area so as to stress the future challenges for large wood research.

Floods at the northern foothills of the Tatra Mountains — A Polish-Swiss research project

The present paper introduces the topical area of the Polish-Swiss research project FLORIST (Flood risk on the northern foothills of the Tatra Mountains), informs on its objectives, and reports on initial results. The Tatra Mountains are the area of the highest precipitation in Poland and largely contribute to flood generation. The project is focused around four competence clusters: observation-based climatology, model-based climate change projections and impact assessment, dendrogeomorphology, and impact of large wood debris on fluvial processes. The knowledge generated in the FLORIST project is likely to have impact on understanding and interpretation of flood risk on the northern foothills of the Tatra Mountains, in the past, present, and future. It can help solving important practical problems related to flood risk reduction strategies and flood preparedness.

Decadal variability of floods in the northern foreland of the Tatra Mountains

Floods in the northern foreland of the Tatra Mountains considerably contribute to the total flood damage in Poland. Therefore, the question whether the magnitude and frequency of floods have changed in this region is of high interest. This study aims at investigating the inter-decadal variability of magnitude, frequency and seasonality of floods since the mid-twentieth century, to better understand regional changes. The analysis was accomplished in a multi-temporal approach whereby trends are fitted to every possible combination of start and end years in a record. Detected trends were explained by estimating correlations between the investigated flood parameters and different large-scale climate indices for the northern hemisphere, and by trends found in intense precipitation indices, number of days with snow cover, cyclonic circulation types, temperature and moisture conditions. Catchment and channel changes that occurred in the region over the past decades were also considered. Results show that rivers in the area exhibit considerable inter-decadal variability of flows. The magnitude and direction of short-term trends are heavily influenced by this inter-decadal variability; however, certain patterns are apparent. More extreme, although perhaps less frequent floods are now likely to occur, with a shift in the seasonality, decreasing flood magnitudes in winter and increasing during autumn and spring. The identification of the factors contributing to the occurrence of flood events and their potential changes is valuable to enhance the flood management in the region and to improve the resilience of the population in this mountainous area.

Recent catastrophic landslide lake outburst floods in the Himalayan mountain range

Among the more complex and devastating interactions between climate and hydromorphological processes in mountain environments are landslide lake outburst floods (LLOFs), resulting from mass movements temporarily blocking a drainage system. This work reviews these processes in the Himalayas and highlights the high frequency of this type of phenomenon in the region. In addition, we analyse two recent catastrophic trans-national LLOFs occurring in the Sutlej river basin during 2000 and 2005. Based on high resolution satellite images, Tropical Rainfall Measuring Mission (TRMM), Moderate-Resolution Imaging Spectroradiometer (MODIS) derived evolution of snowline elevation and discharge data we reconstruct the timing and hydrometeorological conditions related to the formation and failure of landslide dams. Results showed that the 2005 flood, originating from the outburst of the Parchu Lake, was not related to heavy precipitation, but was likely enhanced by the rapid and late snowmelt of an unusually deep and widespread snowpack. The flood in 2000 was triggered by the outburst of an unnamed lake located on the Tibetan plateau, identified here for the first time. In this case, the outburst followed intense precipitation in the lake watershed, which raised the level of the lake and thus caused the breaching of the dam. As stream gauges were damaged during the events detailed discharge data is not available, but we estimated the peak discharges ranging between 1100 m3 s−1 and 2000 m3 s−1 in 2005, and 1024 m3 s−1 and 1800 m3 s−1 in 2000. These events caused significant geomorphic changes along the river valleys, with observed changes in channel width exceeding 200 m. Results also demonstrate that remotely-sensed data enables valuable large-scale monitoring of lake development and related hydrometeorological conditions, and may thereby inform early warning strategies, and provide a basis for flood risk reduction measures that focus on disaster preparedness and response strategies.

Large wood clogging during floods in a gravel-bed river: the Długopole bridge in the Czarny Dunajec River, Poland

During floods, large quantities of wood can be mobilized and transported downstream. At critical sections, such as bridges, the transported wood might be entrapped and a quick succession of backwater effects can occur as a result of the reduction of the cross-sectional area. The aim of this work is to explore large wood-related hazards during floods in the gravel-bed river Czarny Dunajec (Polish Carpathians), where the river flows through the village of Dlugopole. This work is based on the numerical modelling of large wood transport together with flow dynamics in which inlet and boundary conditions were designed based on field observations. The exploratory approach developed in this study uses multiple scenarios (193) to analyse the factors controlling bridge clogging: wood size, wood supply, flow conditions, morphology and obstacles in the riverbed. Results highlighted the strong control of log length (stronger than that of log diameter) on potential blockage probability; however, according to our results the main factor controlling the bridge clogging was the flood discharge. River morphology and wood supply play an important role as well. Especially the river morphology may reduce bridge blockage, as it influences flow velocity and depth, and creates natural retention zones for wood. In addition, the impacts of the bridge blockage were analysed in terms of afflux depth and length, and flooded area. Results showed that the bridge blockage may result in a significant increase in the water depth (up to 0.7 m) and flooded area (up to 33% more), therefore increasing flood risk in the village. This article is protected by copyright. All rights reserved.

Glacial lake inventory and lake outburst potential in Uzbekistan

Climate change has been shown to increase the number of mountain lakes across various mountain ranges in the World. In Central Asia, and in particular on the territory of Uzbekistan, a detailed assessment of glacier lakes and their evolution over time is, however lacking. For this reason we created the first detailed inventory of mountain lakes of Uzbekistan based on recent (2002-2014) satellite observations using WorldView-2, SPOT5, and IKONOS imagery with a spatial resolution from 2 to 10m. This record was complemented with data from field studies of the last 50years. The previous data were mostly in the form of inventories of lakes, available in Soviet archives, and primarily included localized in-situ data. The inventory of mountain lakes presented here, by contrast, includes an overview of all lakes of the territory of Uzbekistan. Lakes were considered if they were located at altitudes above 1500m and if lakes had an area exceeding 100m2. As in other mountain regions of the World, the ongoing increase of air temperatures has led to an increase in lake number and area. Moreover, the frequency and overall number of lake outburst events have been on the rise as well. Therefore, we also present the first outburst assessment with an updated version of well-known approaches considering local climate features and event histories. As a result, out of the 242 lakes identified on the territory of Uzbekistan, 15% are considered prone to outburst, 10% of these lakes have been assigned low outburst potential and the remainder of the lakes have an average level of outburst potential. We conclude that the distribution of lakes by elevation shows a significant influence on lake area and hazard potential. No significant differences, by contrast, exist between the distribution of lake area, outburst potential, and lake location with respect to glaciers by regions.

Flood Generation Mechanisms and Changes in Principal Drivers

Mechanisms generating floods are reviewed and next discussed with regard to the Upper Vistula Basin. Here, floods typically result from (i) moderate-intensity rain that lasts a few days over a large area and drives large-scale flooding, or (ii) high-intensity, short-lasting convective rain causing local flash floods. Outside the mountain part of the basin, especially in the San River catchment, floods are also caused by intensive snowmelt. Interpretation of climate track in flood generation is presented, based on the analysis of observation records from the last six decades and projections for the future. Catchment and river changes affecting the conditions of flood generation are next considered for the last 130 years. They comprise changes regulating flood runoff (catchment reforestation and dam reservoirs construction), changes reducing floodwater storage and accelerating flood runoff (channel regulation, flood embankments, river incision, and permanent impoundment of the Upper Vistula for navigation purposes), as well as the expansion of riparian forests increasing large wood recruitment to channels during floods.

Methods to Assess Large Wood Dynamics and the Associated Flood Hazard in Polish Carpathian Watercourses of Different Size

Applicability, advantages and limitations of a range of methods applied to determine large wood dynamics in Kamienica Stream and the Czarny Dunajec River, Polish Carpathians, are discussed. Results of a 6-year-long monitoring suggest an increased rate of wood recruitment to Kamienica Stream caused by recent bark beetle infestation of the spruce forests in the valley. However, both monitoring of wood transport and wood inventories indicate that the mobility of large wood in the stream is low and can increase only during major floods. Thus flood hazard to downstream valley reaches potentially resulting from the considerable amounts of large wood stored in the upper stream reach is limited. In the Czarny Dunajec, wood inventories, a tracking experiment with logs tagged with radio transmitters, and numerical modelling indicated high potential for wood transport in the narrow river reaches formed by channelization or channel incision, and high potential for wood deposition in the wide, multi-thread channel. Vegetative regeneration of living willow wood considerably reduces its remobilization by subsequent floods. Efficient transport of large wood along narrow river reaches implicates that during floods substantial amounts of wood may be delivered from distant sources to the channel sections located downstream of the narrow reaches. Wide, multi-thread reaches operate as natural wood traps, considerably limiting further transfer of wood to vulnerable sites/reaches.

Modelling Hydraulic Parameters of Flood Flows for a Polish Carpathian River Subjected to Variable Human Impacts

Channelization and channel incision have considerably modified channel morphology of the Czarny Dunajec River, and now it varies from a single-thread, incised or regulated channel to an unmanaged, multi-thread channel. Effects of these distinct channel morphologies on the conditions for flood flows were investigated in a study of 25 cross-sections from the middle river course. Cross-sectional morphology, channel slope and roughness were used as input data for the 1D steady-flow hydraulic modelling performed for discharges with recurrence interval between 1.5 and 50 years. Adjustment of roughness coefficients to obtain the agreement between simulated and observed peak levels of the 2014 flood allowed calibration of the model for particular cross-sections. As a result of differences in flow widths, cross-sectional flow areas and channel slope, flood flows in the three river reaches differ in unit stream power and bed shear stress, with the highest values of the parameters recorded in the incised reach, intermediate values in the channelized reach and the lowest values in the multi-thread reach. The recognised differences in the flow power and in tractive forces exerted on the flow boundary underlie and explain different evolutionary tendencies of particular river reaches during the past decades. Stabilization of river banks in channelized reaches induces a progressive increase in floodplain elevation; sedimentation in the analysed channelized cross-section of the Czarny Dunajec have reduced its initial flow conveyance by half and elevated water stages at given flood discharges by about 0.7 m during 30 years since the river channelization.

Floods in Mountain Basins

This chapter provides a general introduction to recent research on floods in mountain catchments and reviews state-of-the-art contemporary knowledge on the topic in Poland and Switzerland. The selection of the areas illustrated in this chapter is motivated by the fact that the Swiss Agency for Development and Cooperation (SDC) had funded a research project on floods in the Polish Tatra Mountains and their forelands, to which this book is also dedicated.