2D numerical analysis of the influence of near-bank vegetation patches on the bed morphological adjustment

This paper investigates the influence of near-bank vegetation patches on the bed morphological adjustment in open channel flow systems. The 2D depth-averaged hydro-morphological model is adopted for this investigation, which is first validated by laboratory experimental data measured in an open channel with a single near-bank vegetation patch. The validated model is then applied for extensive numerical simulations, with the aim of conducting a systematic analysis of the influence of different geometric controlling parameters on the bed morphological evolution. The controlling parameters taken into account for numerical analysis include the angle of repose value (RAV) of sediment, vegetation density (VD), patch length (PL) and patch width (PW). The numerical results and analysis show that: (1) the RAV of sediment with slope-failure parametrization only influences the shape of the transverse bed topography in the junction region; (2) increase in VD, PL and PW that substantially enhances flow blockage effect encourages the growth of the pool adjacent to the patch in three dimensions; (3) increase in VD, PL and PW produces analogous retrogressive erosion (erosion toward the upstream) in the pool region, presumably due to the increase in flow resistance. Additional numerical experiments suggest that the staggered-order distribution of multiple patches might be an optimal choice for channel restoration and conservation since pools and riffles with larger scales can be produced.

     

Seasonal variation of air pollution index: Hong Kong case study

Air pollution is an important and popular topic in Hong Kong as concerns have been raised about the health impacts caused by vehicle exhausts in recent years. In Hong Kong, sulphur dioxide SO2, nitrogen dioxide (NO2), nitric oxide (NO), carbon monoxide (CO), and respirable suspended particulates (RSP) are major air pollutants caused by the dominant usage of diesel fuel by goods vehicles and buses. These major pollutants and the related secondary pollutant, e.g., ozone (O3), become and impose harmful impact on human health in Hong Kong area after the northern shifting of major industries to Mainland China. The air pollution index (API), a referential parameter describing air pollution levels, provides information to enhance the public awareness of air pollutions in time series since 1995. In this study, the varying trends of API and the levels of related air pollutants are analyzed based on the database monitored at a selected roadside air quality monitoring station, i.e., Causeway Bay, during 1999-2003. Firstly, the original measured pollutant data and the resultant APIs are analyzed statistically in different time series including daily, monthly, seasonal patterns. It is found that the daily mean APIs in seasonal period can be regarded as stationary time series. Secondly, the auto-regressive moving average (ARMA) method, implemented by Box-Jenkins model, is used to forecast the API time series in different seasonal specifications. The performance evaluations of the adopted models are also carried out and discussed according to Bayesian information criteria (BIC) and root mean square error (RMSE). The results indicate that the ARMA model can provide reliable, satisfactory predictions for the problem interested and is expecting to be an alternative tool for practical assessment and justification.     

Investigation of respirable suspended particulate trend and relevant environmental factors in Hong Kong downtown areas

As the addressing of high demand of good air quality in urban area, a study on air pollutant dispersion and distribution resulting from vehicular exhaust emission is strongly required. In particular, vehicular exhaust emission has become a major air pollution source in metropolitan city like Hong Kong, which is characterized with the heavy, dense traffic flow and the limited land resources. Respirable suspended particulate (RSP) is one of main pollutants resulted from vehicular exhaust emission in urban area. Hence, in this study, we focus on analyzing the variation of RSP levels including diurnal, monthly and annual patterns at selected roadsides in Hong Kong during the period of 1998--2005. Furthermore, the relationships between RSP level and the relevant meteorological factors such as temperature, rainfall and wind conditions in Hong Kong territory have been discussed as well.