The influence of instream formations on hydraulic characteristics in natural lowland rivers

Abstract

Midchannel islands are ubiquitous geomorphic units in lowland rivers running through forest biomes. These self-forming natural obstacles exposed to flow regulate the river morphodynamics and bring about ecological productivity. The trilateral interactions between flow, sediment, and vegetation render the key dynamics behind the emergence of midchannel islands. Islands and bars can be erosion-based or accretion-based formations. Erosion-based islands are detached from a continuous floodplain. On the other hand, depositional islands form due to the aggradation of sediment and woody debris where the circumstance and flow regime are suitable for development of the pioneer entities. As a common form of the midchannel island, this study focuses on the latter case. During the high flow period, riparian vegetation can be uprooted and carried by flow. Once this vegetation is situated downstream, the root system resists erosion, and fluvial sediments start accumulating. The interaction between resisting factors and the consequent flow disturbance leads to island stabilization and further expansion. The emergence of midchannel islands in river corridors modifies the flow structures. As a result, the secondary flow patterns influence the river eco-geomorphology. These in-stream landforms were conventionally considered the source of unpleasant hydraulic resistance. However, island removal could cause negative environmental consequences and channel degradation. The purpose of this thesis was to unveil the key eco-geomorphological outcomes of midchannel islands by evaluating the flow structures around them. Accordingly, the different analyses were conducted on the individual scale, reach scale, and basin scale. The synthesis of the achieved results and the pertinent literature revealed valuable outcomes of island presence in lowland rivers. Obtained knowledge from this study can be useful in river restoration projects where deforestation, urbanization, and simplifying the natural channels brought about the degradation of the riverine environment. For the first part of the study, three major rivers in Turkey were chosen. The GIS tools helped to identify midchannel islands in these rivers. The basin-scale analysis showed the comparability of the midchannel islands due to the similarity in the basin characteristics. Since the development of the depositional islands experiences the same procedure in different rivers, a characteristic one was arbitrarily chosen among the islands of the three examined rivers. Afterward, its geometry was utilized for numerical models. The individual-scale and reach-scale analyses were carried out based on the Reynolds-averaged Navier–Stokes model. In the numerical flume, the planform of the characteristic midchannel island was scaled up and scaled down to mimic the island expansion from the pioneer to the mature one. The simulations included two types of channels with rectangular and trapezoidal cross-sections. The results showed that the presence of a midchannel island alters the flow patterns in the channel. This modification leads to channel adjustment which was observed by Landsat imagery. The significant outcome of the island presence is channel incision in its downstream zone. The channel incision in the river bottom induces a deeper and narrower cross-section, which provides the channel stability. In addition, the velocity fluctuations in the side channels and large-scale counter-rotating streamwise vortices in the wake zone make remarkable morphological changes. The asymmetrical distribution of flow patterns due to the asymmetry of island geometry triggers the meandering process, which is very advantageous for the river. Moreover, the water depth increase in the upstream side of the island enhances the base flow. All these positive outcomes demonstrate that preserving midchannel islands is vital in river corridors. Investigating the flow structures around a midchannel island was conducted from another perspective in the second part of the numerical studies. The self-streamlining mechanism of a natural obstacle against the flow alters the geometrical characteristics of the body over time. To perform the precise analysis of the streamlining effect, the circular cylinder was modeled first due to the availability of the experimental dataset for model validation. Subsequently, the circular planform was modified to a streamlined island which is a well-known hydrodynamic form. By keeping the same initial and boundary conditions, the planform of the characteristic island extracted from satellite views was imposed on the model. Since the realistic geometry of the midchannel island is not two-dimensional like the above-mentioned obstacles, the three-dimensional island by adding the slopes was modeled as the last scenario. This comparative analysis between four scenarios helped to understand the role of the island's three-dimensionality in the generated flow patterns. The comparison between flow patterns in the wake region of different obstacles shows that the realistic midchannel island gains more hydrodynamic form towards the channel bed. Therefore, the three-dimensional obstacle has highly variable flow structures along the depth of the body. Furthermore, the lee-wake vortices have a disorganized pattern due to the asymmetrical geometry of the natural body. Besides, the streamlining process weakens the strength of secondary flows, which leads to their gradual recovery in the downstream area. This condition brings about ecological heterogeneity and benefits aquatic life. Providing the dissolved oxygen for microorganisms of the hyporheic zone through vertical exchange of water, fulfilling the fish spawning criteria, enhancing the swimming speed of the fish species by turbulence, providing the wide range of flow velocities that favor the invertebrates, and many other ecological services by midchannel islands cannot be overlooked. The basin-scale analysis was conducted to find the relationship between midchannel islands and the river basin characteristics. The occurrence and distribution of midchannel islands in different rivers of the world were investigated utilizing GIS tools. The studied rivers belong to specific ecoregions, in which the vegetated islands are abundant in their major rivers. The morphometric and hydrological characteristics of the river basins were determined for the aim of this study. Finally, the morphometric analysis of the individual midchannel islands and their temporal evolution were investigated. Midchannel islands are abundant in the ecoregions with high vegetation cover. Accordingly, the major rivers from tropical/subtropical forests, temperate broadleaf/grassland, and boreal forests were examined. The results revealed the relationship between basin elevation and the distribution of midchannel islands. As the relief thus the mean slope is lower in a basin, the midchannel islands start appearing earlier in the river. Furthermore, the midchannel islands' density has a relationship with sediment yield. The basins with higher sediment yield have more midchannel islands in their existing length.