Environmental Implication of Urban River Reach Morphology (The Case of Big Akaki River in Addis Ababa)
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Date
2020-06
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Addis Ababa University
Abstract
Environmental degradation in urban rivers are becoming common problems, even worsening due to the increased anthropogenic causes led by urbanization. Urban river reach‟s morphology is highly sensitive for alteration by the human settlement and municipal demands. Flooding during the rainy seasons and zero environmental flows with poor water quality are the current guise of small rivers at residential areas. This study is conducted on the environmental implication of river morphology on the big Akaki and Kebena Rivers with combined application of quantifying the land use/cover and morphological characteristics change with remote sensing and GIS techniques and with selected water quality nutrient loads parameters estimated using stream flow simulation. The land use/cover and morphological assessment was analyzed from high resolution satellite images of SPOT and the Google earth using the Arc-GIS 10.4 for the study period. The stream flow has been simulated by employing the HEC-HMS hydrologic model. The water quality parameters collected by various researchers and organizations were compiled to understand the changes for certain physico-chemical parameters. The nutrient loads of phosphate and nitrate simulated with LOADEST model using the flow output of the hydrologic model. The result shows the land use/cover has undergone a dynamic change, in particular the crop land reduced by 11 % and the urban area shows 7.33 % increase. The morphometric characteristics of sinuosity (SI) and River Network Change Index (RCIM) shows slight change over the decade with -0.008 and +0.039 values for Kebena respectively. While the big Akaki morphometric characteristics of SI and RNCI has been found to be +0.027 and 0.00 respectively over the decade. On the other hand, the water quality parameters show complex and undefined pattern of the pollution status. However, all the assessed parameters were below the water quality standards of both the international and national organizations. The sedimentation problem observed to be higher especially when the slope becomes gentle and the velocity decrease. Moreover, the average Nitrate and phosphate loads where larger than the standard limits with high level of seasonal variation for both Kebena and Akaki Rivers between 2006-2016 year. The link between the percent of urban area and the two morphometric parameters were found to be significant with a p-value of 9.08*10-8. From this study, high level of correlation between the morphometric characteristics, the water quality parameters were perceived. The linkage between RNCI and the nine physico-chemical and nutrient water quality parameters where highly correlated. While the SI found to be insignificant on the pH and nutrient parameters determination. Based on the results obtained, the recommended Nature Based and contextualized channel rehabilitation measures for the two river reaches can be a best management practice to mitigate the environmental pollution.
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Keywords
Big Akaki, Satellite image, Morphology, Nutrients, HEC-HMS, LOADEST