Faculty of Engineering

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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. A
This 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.
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. K
The 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.
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. A
This 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.
Open Access
Comparison of Cell-In-Series and Meso-Scale Physical Habitat Sampling for the Interpretation of Spatiotemporal Variation of Stream Water Quality
(IAHR-HK Student Research Forum, 2012-11-17) Gomes, P. I. A; WAI, O. W. H
Although scale dependence of ecological patterns is conceptually recognised, the studies involving quantitative assessments are rare and rudimentary. Here we evaluate spatiotemporal variation of water quality using two sampling scales (approaches): cell-in-series (CIS) and meso-scale physical habitats (MPH). CIS has its origins in probabilistic sampling and relatively simple. It also reported to be suitable for streams with advective transport. MPH approach is relatively novel for water quality assessments and it considers medium scale morphological units such as pools, riffles, glides, etc. for sampling. Sampling was carried out in the short and steep Tseng Lan Shue stream, during Spring and Summer of 2012. The stream is subject to regulation and various anthropogenic inputs, but with irregular occurrence. For each season, observations were carried out during periods with no influence of severe weather events (typical state) as well as after a rainfall (flushed state). The response variables including water chlorophyll, turbidity, dissolved oxygen, nitrate, nitrite, ammoniacal nitrogen and soluble reactive phosphorous were checked against a set of hydro-environmental variables. These included: stream velocity, width, depth, and slope, bankfull dimensions, and substrate conditions. Relationships among variables were evidenced using redundancy analysis. In general, the water quality parameters showed an irregular variation in the longitudinal direction of the stream. Response and hydro-environmental variables based on two best axes showed a 41% of variance in spring response data in the MPH approach. For CIS it was around 44%. But in flashed floods these were observed to be 60% and 35 % for MPH and CIS, respectively. Similar trend was observed in summer where explanatory power based on CIS was higher for typical state but otherwise for the flashed state. Furthermore, significant environmental variable(s) for respective cases changed with the scales being used: substrate conditions for CIS and stream width and slope for MPS. This study shows that MPH approach is more suitable than CIS as a modelling tool when the stream has less anthropogenic loads. We conclude that the explanatory powers of the MPH and CIS scales (approaches) could be useful in providing a quantitative definition on identifying a “pristine stream”.
Embargo
Condominium price dynamics in Sri Lanka: correlation with inflation and periods of concern
(Routledge, 2020-05-03) Gomes, P. I. A; Yathushan, V
This 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.
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. D
Empirical 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.
Embargo
Deflectors to introduce heterogeneity to canals
(2017 6th National Conference on Technology and Management (NCTM, SLIIT, 2017-01-27) Randima, W. A. H; Wijethilaka, M. A. D. K; Kumara, G. H. V. S; Gomes, P. I. A
Canals are manmade lotic water systems constructed for flood control, navigation, irrigation, etc. In general, canals are eco-hydrologically homogeneous. As such, a canal might have the same prismatic cross section and an even bed profile along its course. Such characteristics do not support rich biota, something a sustainable society expects. Several past studies have attributed heterogeneity to be an important factor for a healthy stream. In this study the deflectors were used to introduce the heterogeneity in canals. In tandem studying flow complexity by deflectors, their role with respect to sediment (sand) and seed transport, and fisheries were also studied. The model canal's (laboratory flume) homogenous velocity of 0.4 m/s changed to a range of values from 0.1 to 0.7 m/s after placing deflectors. Similarly, water depth changed from a uniform 30 mm to a range from 20 mm to 70 mm. This resulted in formation of flow patches similar to flow patches formed by meso-scale physical habitats such as pools and riffles. Sediment deposition was patchy and seemed to be stable as only a minor bedload movement was observed for prolonged durations. This meant it is possible to achieve a quasi-equilibrium state in field canals where upstream brings sediments. Seed trapping was weak for floating type seeds, whereas non-floating seeds managed to get deposited with sediment. Fishes quickly migrated to the pool like areas that resulted from deflectors. The results suggested the role of deflectors in transforming a homogenous channel to heterogeneous channel that has the potential to have rich biota which is essential in ecological rehabilitation works.
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. D
Laboratory 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 safety
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. K
Ephemeral 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.
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. F
The 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.

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