Eco-hydraulic Signature of Colombo Dutch Canals and Implications for Rehabilitation

Abstract

Dutch canals (DC) in Colombo, the commercial hub of Sri Lanka, playa significant role in surface drainage. Although flood control is the contemporary ise of urban waterways such as DC, their importance in provision of ecological services is started to value and is the case with the changing socio-economic and environmental requirements. Once the basic economic development is achieved and ur an dwellers are satisfied, need for a better environment (also referred as green nvironment) emerges - this is one important socio-economic need legislative authorities of developed countries have to work on. Even though yet to be realised, Sri Lankan government agencies tinie to time have shown commitment 01 ecological rehabilitation of DC. Ecological rehabilitation can be defined as -ehabilitation strategies that make the canal self-sustainable, whilst providing ecosystems services similar to a pristine stream/river. One major constraint faced in canal ecological rehabilitation is a lack of a reference state. Reference conditions would define a feasible status quo of the rehabilitated canal with estimations of thresholds of governing variables. Identification of optimum conditions in engineered systems or laboratory are not new, yet, ecosystems, especially those threatened by anthropogenic activities are rather unfound or limited to developed countries, which is particularly t'ie case with running water (Iotic) ecosystems. Those countries have straightened and concrete lined urban waterways, in many cases with fast moving waters whic 1 are close to super critical conditions. Ecological rehabilitation studies dealing subcritical urban waterways are rare, and this study's focus is on such a lotic system. DC is subject to rather somewhat basic maintenance - such activities only consider f1 od safety and have largely decreased the natural variability of the flow regime and channel ....ite.~~ morphology. However, no acbons taken so far have introduced co-hydraulic functions that would make canals self-viable and provide ecosyst m amenities expect from a natural stream/river. The aim of this study was to investigate the eco hydraulic signature of C to identify optimum eco-hydraulic conditions that correspond to an ecologically healthy canal. The specific objectives were to investigate eco hydraulic data against varying spatial and temporal scales to identify the impact of space and time on macroinvertebrate responses, model eco-hydraulic data with macroinvertebrate responses to derive optimum eco-hydraulic conditions and compile guidelines based on tho e optimum conditions that can be used in ecological rehabilitation works. Three canals including DC within urban, peri-urban (Talangama cana : TC) and rural-urban fringe (Heen canal: HC) catchments were studied for a two year period against different spatiotemporal scales. The methodology involved a comprehensive set of field observations and data analysis. Targeted optimum conditio 1S for ecohydraulic variables (hydraulic, habitat quality and water quality parameters) were derived according to the response of macroinvertebrate (diversity, and pollution tolerance index (PTI)) data. Hydraulic variables included velocity, depth. discharge, Froude number etc.; and habitat quality variables included a set of varia les such as epifaunal substrate, pool substrate characterization, channel alteration etc. that can be observed in situ. Water quality variables included dissolved oxyge , electrical conductivity (EC), pH, turbidity etc. Relationships among eco-hydraulic variables and macroinvertebrate responses were built firstly via multiple linear regression followed by bivariate models using curve estimation regression. Statistically acceptable (co-efficient of determination (R2) > 0.3 and P < 0.1) relationships were solved for a targeted PTI of 11. PTI was decided as the m st suitable macroinvertebrate index, amongst macroinvertebrate indices, as it was more generic and representative. HC showed the greatest spread of diversity, PTI and richness; an indication that macroinvertebrate composition was dynamic than TC or DC. A majority of macroinvertebrates in DC belonged to the most pollution tolerant category, while they were mostly under intolerant category in HC. TC had a mixed composition of all four categories of pollution t5ferance. Habitat quality of DC was always lower than TC and HC; therefore, belonged to poor, marginal and sub optima categories, respectively. Based on water quality parameters, HC had better water uality than TC and DC, where in many cases with a statistical significance at P < 0.05. Twodimensional variation of EC and turbidity of three canals suggested th dominance of molecular diffusion in longitudinal as well as lateral directions. Patchy distribution of turbidity ofHC indicated that attenuation also playing a major role 01 the fate of pollutants. According to the grey water footprint analysis, the urban and sem -urban canals have already exceeded their natural assimilation capacities and the s tuation was critical in DC.The study revealed that hydraulic and habitat quality parameters dominated over water quality in explaining the response of macroinvertebrates. Some of tile ecohydraulic variables showed a significant relationship with diversity, but only a few parameters from that showed a significant relationship with PTI and most of them were from He. Having a positive correlation only with diversity suggested that the pollution tolerant species had contributed to the diversity more than the p lIution intolerant species. Furthermore, .numerous significant relationships were observed in dry season than that of wet season. Eco-hydraulic variable response curves that gave significant relationships either in linear or quadratic forms were solved with the targeted PTI of 11 and solutions were suggested as target conditions that can be incorporated in ecological rehabilitation. Out of 24 eco-hydraulic variables, dry season gave 15 feasible sol utions, whereas wet season gave on ly six. Most of the feasible solutions were from hydraulic and habitat quality parameters. The study gave optimum ranges for velocity, standard deviation of velocity, Froude rumber, depth, number of mesoscale physical habitats, diversity and percentage areal coverage of transverse vegetation, score of epifaunal substrate, pool substrate characterization, bank vegetative protection, riparian vegetative zon width, electrical conductivity, dissolved oxygen, ammoniacal nitrogen and pH. Iso, the study validated some generic understanding of interrelationships within lotic ecosystems such as the fact that increased mesoscale physical habitats p sitively correlate with a healthy canal. The study gave quantifiable target conditions that could be vital in rehabi I tation of DC. Results would be applicable oot>zOnlyfor DC, but also similar canals ithin the same climatic conditions in Sri Lanka and elsewhere. It is recommended to liaise with government agencies in pilot studies, and in tandem making knowledge and data transfer with similar research groups. As future studies, it is recommended to validate the models and/or solutions obtained in this study with more lotic waters within the same geo-climatic conditions, and develop similar target conditions for different geo-climatic zones.