Katinka Koll

Aquatic interfaces: a hydrodynamic and ecological perspective

ABSTRACT Ecologically-appropriate management of natural and constructed surface water bodies has become increasingly important given the growing anthropogenic pressures, statutory regulations, and climate-change impacts on environmental quality. The development of management strategies requires that a number of knowledge gaps be addressed through interdisciplinary research efforts particularly focusing on the water-biota and water-sediment interfaces where most critical biophysical processes occur. This paper discusses the current state of affairs in this field and highlights potential paths to resolve critical issues, such as hydrodynamically-driven mass transport processes at interfaces and associated responses of organisms through the development of traits. The roles of experimental methods, theoretical modelling, statistical tools, and conceptual upscaling methods in future research are discussed from both engineering and ecological perspectives. The aim is to attract the attention of experienced and emerging hydraulic and environmental researchers to this research area, which is likely to bring new and exciting discoveries at the discipline borders.

The terms of turbulent kinetic energy budget within random arrays of emergent cylinders

This article is aimed at quantifying and discussing the relative magnitude of key terms of the equation of conservation of turbulent kinetic energy (TKE) in the inter-stem space of a flow within arrays of vertical cylinders simulating plant stems of emergent and rigid vegetation. The spatial distribution of turbulent quantities and mean flow variables are influenced by two fundamental space scales, the diameter of the stems and the local stem areal number-density. Both may vary considerably since the areal distribution of plant stems in natural systems is generally not homogeneous; they are often arranged in alternating sparse and dense patches. The magnitude of the terms of the budget of TKE in the inter-stem space has seldom been quantified experimentally and is currently not well known. This work addresses this research need. New databases, consisting of three-component LDA velocity series and two-component PIV velocity maps, obtained in carefully controlled laboratory conditions, were used to calculate the terms of the TKE budget. The physical system comprises random arrays of rigid and emergent cylinders with longitudinally varying areal number-density. It is verified that the main source of TKE is vortex shedding from individual cylinders. The rates of production and dissipation are not in equilibrium. Regions with negative production, a previously unreported feature, are identified. Turbulent transport is particularly important along the von Karman vortex street. Convective rate of change of TKE and pressure diffusion are most relevant in the vicinity of the cylinders.

Vertical variations of coral reef drag forces

Modeling flow in a coral reef requires a closure model that links the local drag force to the local mean velocity. However, the spatial flow variations make it difficult to predict the distribution of the local drag. Here we report on vertical profiles of measured drag and velocity in a laboratory reef that was made of 81 Pocillopora Meandrina colony skeletons, densely arranged along a tilted flume. Two corals were CT-scanned, sliced horizontally, and printed using a 3-D printer. Drag was measured as a function of height above the bottom by connecting the slices to drag sensors. Profiles of velocity were measured in-between the coral branches and above the reef. Measured drag of whole colonies shows an excellent agreement with previous field and laboratory studies; however, these studies never showed how drag varies vertically. The vertical distribution of drag is reported as a function of flow rate and water level. When the water level is the same as the reef height, Reynolds stresses are negligible and the drag force per unit fluid mass is nearly constant. However, when the water depth is larger, Reynolds stress gradients become significant and drag increases with height. An excellent agreement was found between the drag calculated by a momentum budget and the measured drag of the individual printed slices. Finally, we propose a modified formulation of the drag coefficient that includes the normal dispersive stress term and results in reduced variations of the drag coefficient at the cost of introducing an additional coefficient.

Experimental Investigation of Hydraulically Different Surface Roughnesses

Surface roughnesses present hydraulic differences that can be characterised investigating time averaged velocity profiles, turbulence, Reynolds and form induced stresses. The latter, unlike the other flow characteristics, have not been considered in detail in previous studies. An experimental study of the turbulent flow over five different surface roughnesses is presented. They have been constructed with natural and artificial materials, and with different geometries. The experiments were carried out in a laboratory flume and the 3D flow field has been measured with a Nortek Vectrino Profiler. The double averaged Navier-Stokes equations methodology has been applied to study the spatial heterogeneity of the time averaged flow. The results confirm the studies made in previous works with different surface roughnesses and the form induced stresses reveal the hydraulic differences and similarities for the different investigated roughnesses.

Sensitivity of the Flow to the Inclination of a Single Submerged Groyne in a Curved Flume

In order to investigate the effect on the flow field due to small changes in the inclination of a single submerged groyne, laboratory tests have been conducted. A double curved S-flume with a length of 26 m, width of 2.4 m and depth of 0.4 m was used. The investigations were done at a reference inclination of 60° for a single submerged groyne. The inclination was then varied by ±5° and ±10°. The groyne width and height as well as the hydraulic conditions (discharge of 130.6 l/s and water depth of 10 cm) were kept constant to ensure that the effect is only due to the change in the inclination. The groyne was installed in the first cross-section of the first curve. The 3D flow field was measured using a Nortek Vectrino Plus in nine cross-sections with seven vertical profiles each. The total flow measurement points were 325 in each run. The results of the experiments have shown no significant changes in the flow field due to the change of inclination up to ±10° given that the projected length and the hydraulic boundary conditions were kept constant. However, localized effects at cross-section no 1 are evident and the inclination of 60° showed the lowest stream-wise velocity. Comparison of the flow field with and without groyne revealed decreasing velocities at the outer bank and increasing velocities at the inner bank supporting the purpose of the structure which is bank protection.

Assessment of Morphodynamical Reactions on Redynamisation Measures - Case Study Upper Rhine

The paper reports on the methodology applied to assess the morphodynamical development of a river reach which is planned to be re-dynamised, according to the “INTERREG IVa - Redynamisation of the Old Rhine” project (2009-2012). The river reach is located at the Upper Rhine parallel to the Grand Canal d’Alsace. Most of the discharge is directed through the canal and only a minimum discharge is flowing through the River Rhine. In the ca. 40 km long section morphological features are nowadays absent due to limited sediment supply and transport. A dynamic river bed shall be supported by means of floodplain lowering, sediment deposition and removement of bank protection. The applied methodology combines 3D-numerical modelling of the flow field and stability approaches to assess morphological changes with particular reference to the sediment dynamics and stability of the main channel and the floodplain areas.