Research Papers - Department of Civil Engineering
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Publication Embargo Impact of increased instream heterogeneity by deflectors on the removal of hydrogen sulfide of regulated urban waterways—A laboratory study(Wiley Online Library, 2021-03) Gomes, P. I. A; Samararatne, S.; Wai, O. W. H; Perera, M. D. DLaboratory experiments were conducted to test the hypothesis that increase in physical heterogeneity by deflectors would improve the water quality of urban regulated (straight and prismatic) waterways. Deflectors changed the near-uniform flow to a rapidly var-ied flow, as such the depth, velocity, and Froude number (Fr) variations were four, 10, and 14 times more than the without deflector scenario, respectively. Removal of hydro-gen sulfide (H2S), the main focus of the study, was significantly high when deflectors were placed in the laboratory urban waterway. Introduction of a sediment bed further improved H2S removal; however, in this case turbidity and color were significantly high too. These observations endorse the fact that attenuation induced by deflectors and assimilation promoted by the sediment bed aids the H2S removal. These facts were fur-ther strengthened by the significant strong negative correlations H2S made with DO and pH for all experiments. Further studies are recommended for different deflector orientations and modified sediment beds (e.g., mixture of sediment and gravel), identi-fication of localized water quality hot spots to capture spatial variation of water quality, and impact of increased heterogeneity on flood safetyPublication Open Access Vegetation dynamics of ephemeral and perennial streams in mountainous headwater catchments(Science Press, 2020-07) Gomes, P. I. A; Wai, O. W. H; Dehini, G. KEphemeral and perennial streams of mountainous catchments in Sabaragamuwa Province of Sri Lanka and Hong Kong of China were studied for two years on vegetation dynamics. Each year, sampling was conducted during a period when ephemeral streams had low surface flows. Sampling was realized contiguously using belt transects. The standing crop biomass (hereafter biomass) of herbaceous vegetation in ephemeral channels was comparatively lower than perennials and so was the herb diversity. Herb diversity showed a peak from 1.5 to 4.5 m from the centerline/thalweg of ephemeral and perennial streams. Out of 24 herbs, only three were common for both. A peak herb biomass zone was observed in perennials in the same region where diversity peaked. In ephemerals, herb biomass increased laterally up to ∼1.5 m, and was constant thereafter. Seedling experiment results tallied with the field diversity observations of both stream types, and suggested that seed dispersion was the main reason for herb colonization. Furthermore, it showed sapling emergence to be significantly higher in perennials than ephemerals. Return period of annual maximum monthly rainfall was a strong indicator of age of trees in ephemeral streams, and elucidated the possibility of hindcasting past flow episodes. Electrical conductivity was significantly high in ephemeral streams among all the water quality parameters. The contents of the water nutrients were approximately the same in both stream types. While recommending further studies on eco-hydrology of ephemerals, we recognize ephemeral streams to be valuable references in climate change studies due to their responsiveness and representativeness in long term hydrological changes.Publication Embargo Eco‐hydraulic evaluation of herbaceous ecosystems below headwater dams without a base flow: Observing below dam reaches as new stream sources(Wiley Online Library, 2017-01) Gomes, P. I. A; Wai, O. W. H; Yan, X. FThe roles of headwater streams are usually underestimated and inadequately managed compared with the larger downstream river systems. This two and half-year study covering several wet and dry seasons evaluated the impacts of small headwater dams without a base flow on downstream herbaceous dynamics. Results showed herb aerial cover and diversity below dams (new stream sources) are unique and different to original stream sources as well as to a comparable unregulated reach. The geomorphological habitats formed subsequent to damming did not show major influence on aerial cover of herbs; nevertheless, diversity showed a clear dependence. Downstream of dams were dominated by Acorus gramineus, which had >40% aerial cover, and this figure increased up to 60% within 2 years (in between a major flood event). Simultaneously, downstream herb diversity decreased by 33%. However, diversity and aerial cover of the unregulated reach and original sources remained more or less the same. Herb pulling tests (simulating an uprooting mechanism during a flood) showed A. gramineus can withstand an eightfold higher force than the other herbs, especially in concrete surfaces and rock/sand pools. The estimated shear force on bed during the major flood was several folds less than the observed. This suggests downstream of dams behave opposite to the conventional understanding that large floods are considered as an agent of restoration.Publication Embargo Ecohydrologic structure and function of stream networks with earthen upstream and concrete-lined downstream(Wiley Online Library, 2019-06) Gomes, P. I. A; Wai, O. W. HConcrete-lined streams are one of the most extreme forms of river regulation. This study investigated an urban stream network with concrete-lined (C) and earthen (E) sections. The C sections were positioned downstream of the E sections. This abrupt change of stream type due to concrete lining was compared with the predictions of river continuum concept and process domain concept. River continuum concepts predictions on the macroinvertebrate functional feeding group fractions against stream order and energy sources showed disagreements in this study. Furthermore, other than abundance of pollution intolerant species variation, most of the species responses did not support the process domain concept predictions. Correlations between explanatory variables (stream hydraulics or water quality) and response variables (macroinvertebrate composition and vegetation) observed to be dependent on stream lining and seasons. Stream discharge showed positive correlations with the pollution representative species in both section types but was more obvious in C sections. Water budget estimations showed that wastewater accounted for at least 5% of stream discharge of the C sections in the dry season. pH was the most important and significant water quality parameter of the C sections. Against expectations, nitrogen species and soluble reactive phosphorus did not have a major impact on species variables. Hydraulic variables that represented stream heterogeneity were more useful in explaining species variables of the E sections. Similar explanatory powers by the hydraulics and water quality variables of the C sections demonstrated the importance and interdependency of stream hydraulics and water quality in rehabilitating of this unique ecosystem.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 Embargo In‐stream physical heterogeneity, rainfall aided flushing, and discharge on stream water quality(Water Environment Federation, 2015-08) Gomes, P. I. A; Wai, O. W. HImplications of instream physical heterogeneity, rainfall-aided flushing, and stream discharge on water quality control have been investigated in a headwater stream of a climatic region that has contrasting dry and wet seasons. Dry (low flow) season's physical heterogeneity showed a positive correlation with good water quality. However, in the wet season, physical heterogeneity showed minor or no significance on water quality variations. Furthermore, physical heterogeneity appeared to be more complementary with good water quality subsequent to rainfall events. In many cases stream discharge was a reason for poor water quality. For the dry season, graywater inputs to the stream could be held responsible. In the wet season, it was probably the result of catchment level disturbances (e.g., regulation of ephemeral freshwater paths). Overall, this study revealed the importance of catchment-based approaches on water quality improvement in tandem with in-stream approaches framed on a temporal scale.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 Relationships among anthropogenic disturbances representative riparian and non-riparian herbaceous indicators (biomass and diversity), land use, and lotic water quality …(Springer International Publishing, 2014-09) Gomes, P. I. A; Wai, O. W. H; Kularatne, R. K. A; Priyankara, T. D. P; Anojika, K. G. M. S; Kumari, G. M. N. RThis study was carried out to evaluate the linkages among herbaceous plant biomass (i.e., aboveground and litter biomasses) and diversity (ShannonWiener index) in riparian and non-riparian areas, land use, and lotic water quality (which included first- and second-order natural streams, a canal, and a reach of a lowland river). Herb stands selected were free from anthropogenic disturbances such as farming and construction activities for a period of 3 years (this was the dominant and peak frequency of disturbances of the study area). The results suggested that herb indicators are good representatives of the land use. However, land use explanations for herb indicators were complex and not universal for all lotic waters. The correlations between herb indicators and water quality were strong for the low-order natural streams. In these streams, herb indicators explained >36 % of the total variation with several statistically significant herb indicators. However, the large river section showed weak correlations. Furthermore, the canal’s hydrology (connectivity to sea) seemed to be more influential in shaping its water quality. This study demonstrated that the rehabilitation works with a span of 3–4 years using herbs in riparian and/or non-riparian areas could significantly improve water quality of low-order streams with natural origin.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.