Faculty of Engineering
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Publication Embargo Investigation of long-term river water quality variations using different urbanization indices and assessment of common scientific perspectives of urbanization on water quality(wiley, 2023-03) Karunatilaka, P. D.; Gomes, P. I. AThis study investigated the water quality variation spanning 30 years (1986–2017) in 16 catchments of Hong Kong against different urbanization indices, namely, built area fraction; population; and product of population and built area fraction. Pearson correlations of three different periods of time (1988–1990, 1998–2000, and 2015–2017) indicated that water quality trends were dependent on the urbanization index. Total solids, nitrite-nitrogen, total phosphorus, electrical conductivity, dissolved oxygen, and flow rate had significant deteriorative trends (Pearson r > 0.5 and p < 0.05) with population and product of built area and population. Results also interpreted that built area fraction and product of built area and population were the worst and best indices that represented urbanization and/or its impacts, respectively. Mann-Kendall test for the entire 30 year period showed that water quality had improved with time with respect to certain water quality parameters (e.g., dissolved oxygen, ammoniacal nitrogen and total suspended solids). The results portrayed that although the urbanization of catchments had increased with time, the river water quality with respect to many parameters showed signs of improvement and the legislative measures implemented seemed to be effective in controlling pollution.Publication Embargo Deriving optimal hydraulic, water quality and habitat quality criteria against a predefined reference state of urban canals via an analytical method: Implications on ecological rehabilitation(Elsevier, 2022-09) Gomes, P. I. A; Dehini, G. KThe aim of this study was to showcase derivation of numerical ranges of important environmental variables (hydraulics, water quality and habitat quality) for a predefined reference state of canals in an area where ecological rehabilitation is sought. The reference state was defined based on pollution tolerance index (PTI) of macroinvertebrates. From data collected for two years from three canals with varying levels of pollution for different seasons, detrended correspondence and redundancy analysis ordination plots revealed moderate to weak spatiotemporal gradients. Relationships were built via multiple linear regression (MLR) and by linear or quadratic bivariate models. MLR models managed to explain over 70% of the PTI variation and was significant at P < 0.1. Solving single parameter models with co-efficient of determination >0.3 and P < 0.1 with a targeted PTI of 11, gave season dependent feasible solutions and were mostly hydraulic and habitat quality variables. Out of 24 environmental variables, dry and wet seasons gave 15 and eight feasible solutions, respectively. This study validated the importance of certain environmental variables that are debatable in the context of a healthy stream (e.g., mesoscale physical habitats), showed instances where hydraulics became the defining factor of stream health, and also provided pros and cons of a widely discussed method in ecological rehabilitation.Publication Embargo Integrating vegetation indices and geo-environmental factors in GIS-based landslide-susceptibility mapping: using logistic regression(Springer, Cham, 2022-02) Abeysiriwardana, H. D; Gomes, P. I. AThis study aimed to assess the potential of in-situ measured soil and vegetation characteristics in landslide susceptibility analyses. First, data for eight independent variables, i.e., soil moisture content, soil organic content, compaction of soil (soil toughness), plant root strength, crop biomass, tree diameter at knee height, Shannon Wiener Index (SWI) for trees and herbs was assembled from field tests at two historic landslide locations: Aranayaka and Kurukudegama, Sri Lanka. An economical, finer resolution database was obtained as the field tests were not cost-prohibitive. The logistic regression (LR) analysis showed that soil moisture content, compaction of soil, SWI for trees and herbs were statistically significant at P < 0.05. The variance inflation factors (VIFs) were computed to test for multicollinearity. VIF values (< 2) confirmed the absence of multicollinearity between four independent variables in the LR model. Receiver Operating Characteristics (ROC) curve and Confusion Metrix (CM) methods were used to validate the model. In ROC analysis, areas under the curve of Success Rate Curve and Prediction Rate Curve were 84.5% and 96.6%, respectively, demonstrating the model’s excellent compatibility and predictability. According to the CM, the model demonstrated a 79.6% accuracy, 63.6% precision, 100% recall, and a F-measure of 77.8%. The model coefficients revealed that the vegetation cover has a more significant contribution to landslide susceptibility than soil characteristics. Finally, the susceptibility map, which was then classified as low, medium, and highly susceptible areas based on the natural breaks (Jenks) method, was generated using geographical information systems (GIS) techniques. All the historic landslide locations fell into the high susceptibility areas. Thus, validation of the model and inspection of the susceptibility map indicated that the in-situ soil and vegetation characteristics used in the model could be employed to demarcate historical landslide patches and identify landslide susceptible locations with high confidence.Publication Embargo Condominium price dynamics in Sri Lanka: correlation with inflation and periods of concern(Routledge, 2020-05-03) Gomes, P. I. A; Yathushan, VThis study conducted in Sri Lanka’s commercial capital – Colombo, investigated temporal variation of condominium prices from 1998 to 2018, to identify price – inflation relationships and periods of concern. The price increased non-linearly with time without abnormal trends. Unit root, cointegration and vector error correction method tests showed inflation can explain temporal variation of semi-luxury condominium’s price appreciation, and it showed a statistically significant (P < 0.01) positive impact in the long run. Price appreciation and inflation showed a weak negative, yet statistically significant impact for luxury condominiums. The impact of inflation on price appreciation in the short run was mostly a negative correlation and as per explanatory powers the impact was minor. It seemed that Sri Lankan condominium market would mostly be without any periods of concern considering the increasing presales and the decreasing price to rental ratios. However, there is a continuous increment of the ratio between the price and household annual income, and it was about 5.25 as at 2018 (price bubbles occurred when this ratio is over six). More studies are recommended specially on future periods of concern and studies that incorporate ultra-luxury apartments being developed by international developers.Publication Embargo Do sediments of ephemeral and perennial streams show different impacts on water quality when subjected to the same drying conditions?(Wiley Online Library, 2021) Gomes, P. I. A; Perera, M. D. DEmpirical evidence was studied to investigate whether ephemeral stream sediments have redeeming biological and physical attributes with respect to water quality, via a laboratory mesocosm study simulating stagnant pooled conditions in ephemeral and perennial streams. In addition to stream type, the effect of sediment quantity variation (sediment to water ratio) was also studied. From the water quality parameters tested (electrical conductivity, dissolved oxygen, pH, nitrogen species and phosphate), only electrical conductivity showed a significant difference between the two stream types irrespective of sediment to water ratio. However, the temporal water quality of a given stream type changed with sediment quantity in the mesocosm. Re-flooding of mesocosms after complete drying did not result in blackwater conditions, but a similar spike in nutrient concentration was observed in both stream types. The absence of blackwater was attributed to the lack of addition of new organic matter and litter, as well as the saturation of dissolved oxygen in mesocosms, therefore, indicating that aeration and litter control could be used as mitigation methods for blackwater events. It was evident that water quality variations in ephemeral streams are purely based on the flow regime (hydrological flow conditions) and organic loading. No evidence was found for any unique biological and physical properties of ephemeral sediment that redeem water quality.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 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.