Research Papers - Department of Civil Engineering
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Publication Open Access Concrete lined urban streams and macroinvertebrates: a Hong Kong case study(Springer US, 2020-02) Gomes, P. I. A; Wai, O. W. HIn this study an urban stream network with earthen and concreted sections were studied for different seasons (wet and dry) to investigate the macroinvertebrate composition and the governing factors. The factors considered were water quality (nitrogen species, soluble reactive phosphorus etc.) and stream eco-hydraulics (velocity, wetted depth and width, vegetative indicators, number of mesoscale physical habitats, etc.). In contrast to common perception, results showed that concrete lined sites are not ecologically dead. Even though low, concrete line section had viable populations of macroinvertebrates and importantly a few native species. Interestingly, some macroinvertebrate indices of the concrete lined sites did not show a significant difference with the earthen sites. About 60% of the macroinvertebrates were grazers and filterers, and these two group populations always showed an inverse relationship. Earthen and concrete lined sites had similar diversities, but for different reasons. In general earthen sites diversity and pollution tolerance index of macroinvertebrates (PTI) was positively correlated, but no uniform correlation was observed in concrete lined sites. Some concrete lined sites with high diversity showed low PTIs (i.e. high diversity as a result of many types of pollution tolerant species) whereas in some the high diversity associated with high PTIs. Macroinvertebrate composition and the influencing factors were to a greater degree section dependent and to a lesser extent were dependent on the season. Physical heterogeneity played an important role in the macroinvertebrate responses in earthen sites. Water quality and flow rate explained comprehensively, the variations in the concrete lined sites. Results proved that nutrient levels need to be limited and instream heterogeneity needs to be improved to enhance diversity and populations of pollution intolerant species. Also, controlled vegetation harvesting is recommended in contrast to the current practice of complete removal for flow conveyance.Publication Open Access Investigation of Long-Term River Water Quality Trends in Hong Kong to Identify Role of Urbanization, Seasons and Pollution Sources(Springer International Publishing, 2020-07) Gomes, P. I. A; Wai, O. W. HThis study investigated the long-term stream water quality trends of nine catchments in Hong Kong with different levels of urbanization using monthly water quality data for a 30-year period at annual and seasonal (wet and dry) scales. Raw data were modeled using redundancy analysis and Mann–Kendall test. Only one river showed a clear difference of water quality responses between the upstream and downstream monitoring stations. Nevertheless, in general, water quality of monitoring stations that had built areas less than 40% showed improving trends, whereas their downstream counterparts with built areas more than 70% showed deterioration trends for some parameters. Majority of water quality trends were season-independent. Out of the variables that were indicative of a long-term deterioration trend, total solids, total suspended solids, turbidity and electrical conductivity (all surrogates of sediment load of the river) were prominent. Nitrate concentration demonstrated an increasing trend for most streams, whereas phosphates a decreasing trend. This study concluded that the main source of pollution could be the surface runoff (nonpoint sources), not the wastewater inputs (point sources). Stream discharge was increasing and decreasing in the downstream and upstream stations, respectively. This could be attributed to the increase in imperviousness in the downstream and water extraction in the upstream. The downstream discharge increment with time would also support the fact that contamination was due to surface runoff. This study provides evidence that the Hong Kong legislative control actions on point source pollution work well, but not on nonpoint source pollution.Publication Embargo Sampling at mesoscale physical habitats to explain headwater stream water quality variations: Its comparison to equal-spaced sampling under seasonal and rainfall aided flushing …(Elsevier, 2014-11-27) Gomes, P. I. A; Wai, O. W. HWe investigated the applicability of sampling at mesoscale physical habitats (MPH) for water quality variation of headwater streams. Sampling was carried out for several hydro-environmental and water quality variables in a headwater stream subjected to moderate levels of anthropogenic disturbances. To compare results with the much popular conventional methods, sampling was also carried out at equal size stream sections (cell-in-series (CIS)). Data collection covered dry (low discharge) and wet (high discharge) seasons. Each season sampling was repeated for typical and flushed (less polluted) states of the stream. Relationships were evidenced primarily by redundancy analysis. Correlations between hydro-environmental and water quality variables were practically same for both approaches. Also, ordination diagrams showed a tendency of first order tributaries to separate from other samples or to form clusters. This was more obvious for the MPH approach. The ordination lengths between adjacent sampling locations in flushed states were much closer than the corresponding typical state for both approaches. The most influential (i.e., with the ability to explain high variance and/or significance) hydro-environmental variables varied on the approach. Habitat (sampling unit) dimensions (width and depth) had high influence in MPH approach; it was the substrate condition (fine particle fraction) for CIS approach. The total variance explained (i.e., explanatory power) by the respective approach seemed to depend on season and state. MPH approach showed highest explanatory power in dry season’s flushed state (53%) which is greater than the CIS approach. Explanatory powers of CIS approach observed to be about 50% or more for all cases, with the exception being the flushed state in dry season (36%). This study revealed some important empirical and theoretical frameworks for MPH based assessments. Firstly, MPHs are more complementary to low flows and less polluted streams. Low flow makes MPHs assimilation and attenuation optimal, making them hotspots for water quality studies. More complementary to less polluted waters elucidate that there is a capacity a MPH can work effectively on pollutant removal. Secondly, the correlation between levels of pollution expressed as a flushing signature and sampling approach elucidate that the explanatory powers of MPH and CIS approaches could be used to differentiate polluted streams against pristine or unpolluted streams. Nevertheless, these empirical and theoretical frameworks proposed need further validation. Therefore, we recommend carrying out similar studies in different geo-environmental conditions with more replications under natural as well as controlled experimental setups.