Narrow Results

A set of governing equations for flows in the vegetated domain has been derived with the assumption that the vegetation is of straight and rigid cylinder. The effect of vegetation on flow motion is represented by the additional inertial and drag forces exerted by the vegetation on the ambient flow. The derived governing equations are incorporated into an existing 3D turbulent free surface flow model. The new model is validated by available experimental data for open channel flows passing through the vegetation with different vegetation size and density. The numerical results agree very well to the experiment data.

A biased coinflip Ansatz provides a stochastic regional scale surface climate model of minimum complexity, which represents physical and stochastic properties of the rainfall–runoff chain. The solution yields the Schreiber–Budyko relation as an equation of state describing land surface vegetation, river runoff and lake areas in terms of physical flux ratios, which are associated with three thresholds. Validation of consistency and predictability within a Global Climate Model (GCM) environment demonstrates the stochastic rainfall–runoff chain to be a viable surrogate model for regional climate state averages and variabilites. A terminal (closed) lake area ratio is introduced as a new climate state parameter, which quantifies lake overflow as a threshold in separating water from energy limited climate regimes. A climate change analysis based on the IPCC A1B scenario is included for completeness.

In semi-arid environments, vegetation is not homogeneous, but rather self-organized into spatial patterns. And spatial patterns of vegetation are a central feature of these semi-arid areas. Thus, in this paper, we give detailed analysis of a vegetation model in arid ecosystems. According to the dispersion relation formula, we discuss the changes of the wavelength, with respect to the rainfall and plant mortality rate. The obtained results show that, as rainfall being decreased, spotted, striped and "black-eye" patterns emerge successively.

This paper introduces a new technique in ecology to analyze spatial and temporal variability in environmental variables. By using simple statistics, we explore the relations between abiotic and biotic variables that influence animal distributions. However, spatial and temporal variability in rainfall, a key variable in ecological studies, can cause difficulties to any basic model including time evolution.

The study was of a landscape scale (three million square kilometers in eastern Australia), mainly over the period of 1998–2004. We simultaneously considered qualitative spatial (soil and habitat types) and quantitative temporal (rainfall) variables in a Geographical Information System environment. In addition to some techniques commonly used in ecology, we applied a new method, Functional Principal Component Analysis, which proved to be very suitable for this case, as it explained more than 97% of the total variance of the rainfall data, providing us with substitute variables that are easier to manage and are even able to explain rainfall patterns. The main variable came from a habitat classification that showed strong correlations with rainfall values and soil types.

A densely grown coastal vegetation belt of Pandanus odoratissimus for reducing the tsunami energy was quantitatively analyzed by an enhanced one-dimensional numerical model that included variations of topography and tsunami characteristics. The drag and inertia forces were assumed as the total resistance generated by the vegetation. It was found that a relatively small period tsunami wave was more destructive than a relatively large period tsunami wave of the same height, although densely grown vegetation effectively reduced the tsunami energy in the case of the small period tsunami wave. A very mild ground slope was also more vulnerable to thrashing by tsunami waves than a relatively steep ground slope. Moreover, densely growing coastal vegetation on very mild ground slope dissipated tsunami energy more efficiently than the same vegetation on relatively steep ground slope.

Vegetation along a coastline could offer significant protection of the adjoining land area against natural hazards such as storm surge and tsunami. In this context, the flexibility of the individual that stems within the green belt is understood to play an important role in the attenuation of momentum of the incoming waves. The physics of which, is yet to be understood completely. Difficulty in modeling the rigidity of the plantations, both numerically and experimentally, is the main cause for this lack of understanding. In the present work, a detailed laboratory study is taken up to examine the resistance characteristics of a group of model slender flexible cylinders. The individual cylinders of the group were fixed to the bed in a staggered configuration. The size, vegetation density and the elastic modulus of the individual stems were chosen such that the tests covered the practical ranges of vegetation in coastal forestry. The Manning's n for different flow conditions as well as for vegetative parameters was obtained from the physical tests in uniform steady current. The results clearly bring out the variation of flow resistance in terms of flow velocity, density of plantation, individual stem diameter and its elastic property. A new empirical relationship is proposed for estimation of Manning's n for staggered flexible emerging plantations which is valid for depths of flow greater than 0.8 times the undeflected plant height.

In order to design a vegetation structure to mitigate floods resulting from extreme events like tsunamis, vegetation density and thickness (width) are important parameters. Flow passing through vegetation faces great resistance, which results in a backwater rise on upstream (U/S) vegetation, increases the water slope inside the vegetation, and for some cases, forms a hydraulic jump downstream (D/S) of the vegetation, thus transforming a subcritical flow to supercritical [Pasha, G. A. and Tanaka, N. [2017] “Undular hydraulic jump formation and energy loss in a flow through emergent vegetation of varying thickness and density,” Ocean Eng.141, 308–325.]. Like the concepts of critical velocity and critical slope, this paper introduces the concept of “critical resistance of vegetation,” which is defined as “resistance offered by vegetation that transforms a subcritical flow to supercritical.” An analytical approach to find the water depths U/S, inside, and D/S of vegetation is introduced and validated well by laboratory experiments. Critical resistance was determined against vegetation of variable densities ($G∕d$, where $G=\mathrm{\text{spacing}}$ of each cylinder in the cross-stream direction, $d=\mathrm{\text{diameter}}$ of the cylinder), thicknesses (dn, where $d=\mathrm{\text{diameter}}$ of a cylinder and $n=\mathrm{\text{number}}$ of cylinders in a stream-wise direction per unit of cross-stream width), and the initial Froude number (Fr_o). A subcritical flow ($F{r}_o=\sim 0.55-0.75$, without vegetation) was transformed to a supercritical flow (D/S vegetation) with a range of Froude numbers of 1.6–1.9, 1.1–1.2, and 0.85–0.98 against $G∕d$ ratios of 0.25, 1.09, and 2.13, respectively, thus defining $G∕d=\sim 1.0$ as the critical resistance. However, altering vegetation thickness did not change the results.

The effectiveness of coastal vegetation as a barrier to mitigate a tsunami greatly depends on the magnitude of tsunami and vegetation structure. This paper summarizes a series of laboratory experiments that investigated the upstream flow structure and energy loss due to a hydraulic jump in a steady super-critical flow. The characteristics of the jump were determined against vegetation of variable density ($G∕d$, where $G=\mathrm{\text{spacing}}$ of each cylinder in cross-stream direction, $d=\mathrm{\text{diameter}}$ of cylinder), thickness (dn, where $d=\mathrm{\text{diameter}}$ of cylinder, $n=\mathrm{\text{number}}$ of cylinders in the stream-wise direction per unit of cross-stream width), and initial Froude number (Fr_o, where Froude number is obtained from a model without vegetation in the flume). In super-critical flow ($F{r}_o\text{-}1.67$–1.83), a weak hydraulic jump formed on upstream side of vegetation. The height of the jump, its location, and the resulting energy loss were increased by increasing both the vegetation density and thickness. Due to reduced reflection at vegetation front, the drag force against sparse vegetation ($G$/$d=2.13$) was higher compared to intermediate ($G$/$d=1.09$) and dense ($G$/$d=0.25$) vegetation. Under these conditions, the maximum energy reduction due to a weak hydraulic jump reached 9.4% for dense vegetation while it was 8.1% and 7.8% for intermediate and sparse vegetation, respectively.

The past tsunami events, despite inducing extensive damage to the coastal community, cognized coastal vegetation as a buffer to the inundating tsunami. This, as well as the endangerment of coastal forest as ecosystems, necessitates a deeper understanding of coastal vegetation’s potential for tsunami damage mitigation. The literature highlighted the energy reduction of up to 60% due to the presence of vegetation during tsunami inundation conditions. Thus, several studies in the past and recent times investigated the effect of vegetation on tsunami energy reduction. This chapter extensively reviews in detail the vegetation parameter and flow parameter influence on energy reduction with a review furthering on the hybrid defence system (combination of vegetation and hard structures). The chapter presents a clear idea for building a coastal forest, thereby helping identify the best combination of the parameters for availing the maximum advantage of the vegetation with the available land space.

A three-dimensional σ-coordinate LES model is proposed to simulate hydrodynamic behavior of free surface turbulent flows with vegetation. Vegetation is considered as an internal source of resistant force and a phenomenon model is employed to express the performance of vegetation in the flows. The model is modified from the σ-coordinate LES model developed by Lin and Li (2002, International Journal for Numerical Methods in Fluids, Vol 38, Page 1045-1068). The governing equations of the modified model include additional terms for drag force produced by vegetation and the modified model is distinctive in that not only compound channel flow can be solved but also wave motion through vegetation in coastal region can be taken into account. An operator splitting method, which splits the solution procedure into advection, diffusion and pressure correction steps, is employed so that different numerical schemes can be used for the solution of different physical processes. The model has firstly been applied to simulate the hydrodynamic behavior of turbulent flow in compound open channel with partly vegetated region. Then by using the model the attenuation of wave movement due to vegetation in coastal region is simulated. The results reveal that the present model has the capacity of describing three-dimensional structure of large eddy appearing in free surface turbulent flows with vegetation. It is believed that the model will become a useful tool to pursue further study of wave hydrodynamics in coastal region with the presence of aquatic vegetation.

In this paper, we discuss the applicability of a numerical wave channel CADMAS-SURF/3D with regard to hydrodynamics on a salt marsh by comparing numerical experimental results with previous studies for both flow and wave conditions. The present results indicate that the model is promising as long as appropriate values for numerical parameters such as the grid size are used. We present results for the bottom velocity and turbulence kinetic energy around a cylinder which are important understanding deposition in salt marsh vegetations. Furthermore, this paper shows that the drag coefficient and inertia coefficient for circular cylinders in waves depend on the density of the cylinders.