Browsing by Author "Dehini, G. K"

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Open Access
Assessment of pollution sources, fate of pollutants, and potential instream interventions to mitigate pollution of earthen canals of urban to rural-urban fringe
(Springer International Publishing, 2019-11) Gomes, P. I. A; Fernando, B. A. V. W; Dehini, G. K
Three representative earthen canals from urban, peri-urban, and rural-urban fringe of Sri Lanka were studied for a 2-year period against different seasons to capture insights important in ecological rehabilitation. Only the canal from rural-urban fringe showed a better water quality in wet season; elucidating, the impact of contaminated catchment runoff in the other canals. At a given sampling session, one or two peaks (relative maxima) were observed in urban and peri-urban canals for pollution representative parameters such as nitrate nitrogen and soluble reactive phosphorus. Those peaks were highly localised, an indication of poor advection. In general, two-dimensional variations of electrical conductivity and turbidity in dry season were uniform in urban and peri-urban canals, an indication of dominant molecular diffusion. This was further evidenced via physical models for different flow stages (low, high, and bankfull). Therefore, fate of contaminants had to be mainly governed by assimilation via sediments. However, grey water footprint analyses showed urban and peri-urban canals have over utilised the natural assimilation capacity of many water quality parameters by several folds. This study proved the importance of inducing attenuation by instream physical heterogeneity similar to natural streams or naturalised canals such as the canal from the rural-urban fringe of this study.
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.
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.