Badronnisa Yusuf

Flow and sediment transport in vegetated waterways: a review

One of the most important waterway components is vegetation, which play a pivotal role in the flow and sediment transport. Vegetation environment and characteristics, including vegetation porosity or density, shape, flexibility and vegetation height, are significantly affected in vegetated channels. Various vegetation positions and arrangements such as patches significantly affect the sediment deposition rate and flow turbulence. This paper reviews recent works conducted on vegetated open channels, which include the effect of different vegetation arrangements and vegetation characteristics on mass transport and turbulence structure. Studies based on laboratory, field works, and modeling, have been reviewed based on previous methods used by different researchers. Methods used in vegetation porosity evaluation, rate of flow and sediment transport properties are presented. In short, flow and transport depend on the vegetation properties and flow structure conditions.

Object-based classification of QuickBird image and low point density LIDAR for tropical trees and shrubs mapping

Abstract This paper assessed the performance of object-based supervised support vector machine (SVM) and rule-based techniques in classifying tropical vegetated floodplain using 0.6m QuickBird image and LIDAR dataset of 1.4 points per square meter (PPSM). This is particularly significant in hydraulic modelling in which vegetation roughness is a big uncertainty and largely relies on land cover classification. The supervised classification resulted in 79.40% overall accuracy whilst the results improved by 8% with rule-based classification. 40 sample plots of trees and shrubs were measured to be compared to obtain the best classification results. The results showed a linear relationship between tree diameters and NDVI with a high Pearson correlation of 0.76 and coefficient of determination (r2) of 0.58. The canopy areas of shrubs were found to be representative spatially with an even higher Pearson correlation of 0.98 and r2 of 0.95. The study concluded that the fusion of QuickBird image and low point density LIDAR in rule-based classification together with field data were useful in quantifying tropical trees and shrubs.

Clear-water scour at long skewed bridge piers

Abstract The results of an experimental study on clear-water scour at long skewed rectangular piers under steady flows at threshold velocity are presented. The scour mechanisms due to skewness effects with a wide range of angle of attack, α, and two sizes of uniform cohesionless bed sediment (d50 = 0.23 and d50 = 0.80 mm) have been investigated in a 50-m-long, 1.5-m-wide, and 2.0-m-deep flume. Empirical relation to demonstrate the effects of skewness in terms of dimensionless variables, such as aspect ratio (L/b), the angle of attack (α), flow shallowness (y/Bα), sediment coarseness (Bα/d50), the sediment gradation coefficient (σg), on scour depth is presented. The empirical relation is focused on predicting the angle of attack factor, Kα. The experimental data obtained in this study and data available from the literature were used to validate the predictions of existing methods and the accuracy of the proposed method. The proposed method gives reasonable scour depth predictions and was verified with statistical methods where the mean error was reduced from 53.6 to 18.1%.

Permeable Channel Cross Section for Maximizing Stormwater Infiltration and Seepage Rates

AbstractMaximization of infiltration and seepage rates is important to better control the quantity of stormwater to foster sustainable urban stormwater management. In this study, the effect of the ...

Effects of depth-varying vegetation roughness in two-dimensional hydrodynamic modelling

ABSTRACT For detailed hydrodynamic modelling of vegetated floodplains, the ability to quantify vegetation is advantageous as vegetation significantly influences the flow mechanism. Although it is widely known that roughness changes with depths, many two-dimensional (2D) models assign constant or generic roughness and the values are typically adjusted for calibration. This practice is likely to lead to the misinterpretation of the flow mechanism. This paper assesses the effects of depth-varying vegetation roughness in 2D hydrodynamic modelling based on vegetation density derived from a remotely sensed regression analysis. The simulated flood extents, depths and velocities of a historical flood event were compared between the constant and depth-varying vegetation roughness coefficients and verified against historical data and literature. A minimum value of 0.03 was found for vegetation with the lowest density of 0.01 m−1 at 0.2 m depth and a maximum value of 0.20 for vegetation with the highest density of 0.20 m−1 at 2 m flow depth, resulting in the maximum differences in flood depths and velocities of 0.40 m and 0.25 m/s, respectively. This study presented a bridge between the theoretical and practical applications which can potentially be used for evaluating vegetation restoration and removal.

Effects of Cross-Section on Infiltration and Seepage in Permeable Stormwater Channels

Factors affecting the infiltration rate have been studied fairly well by many researches; however, the effects of the cross-section of a permeable stormwater channel on the surface water depth reduction due to infiltration and seepage have largely been neglected. In the present study, towards improving the efficiency of permeable channels, the effects of the three components of a trapezoidal section, namely, the water depth, side slope, and base width, on the infiltration and unsteady seepage rates were investigated. Laboratory studies using models of the channel with unsaturated soil were performed under ponding condition using various initial water levels, base widths, and side slopes for two soil textures, namely, sandy loam and loamy sand. The results showed that the rate of surface water depth reduction by infiltration and seepage increases with increasing water level irrespective of the base width and side slope. In addition, an increase of the side slope increases the infiltration rate, with the effect becoming more significant with increasing initial water level, while the effect of varying the base width is insignificant.