105 Environmental Monitoring and Changes in the Watershed Area Analysis

ABSTRACT

As one of the top three countries that suffers from threats of debris flow, watershed conservation and disaster mitigation has been the ultimate goal that the government tries to achieve. Concerning aspects of watershed management, erosion control, slopeland conservation and sustainable development, Soil and Water Conservation Bureau has employed various engineering practices, such as check dam, submerged dam, river regulation, revetment, levee and water drainage work in torrent erosion, gully erosion and landslide. In recent years, the concepts ,such as ecological engineering methods, energy conservation and carbon reduction, have been designated as highly valued and promoted. However, it is crucial to track and assess the efficiency of every management practice while considering the landscape variation and disaster susceptibility analysis of each management watershed.

To perceive results of engineering works and disaster susceptibility history of critical management watersheds, this project aims to employ Formosat-2 satellite imagery database (processed through procedures of coregistration, orthorectification, geometric registration, radiance normalization, spectral summation intensity modulation, edge sharpening and adaptive enhancement) in landscape variation analysis. Then overlaying data of engineering sites on Formosat-2 imagery to distinguish management watersheds, sub-watersheds and slopelands and outline the areas that might be influenced by disasters according to land use map and data of protected objects. Furthermore, this study applies three quantitative methods. Fist, use NDVI, Greenness index and gradient to delineate vegetation areas. Second, employ pre and after disaster Formosat-2 imagery with DEM to delineate landslide area. Third, apply river course map to delineate variation of river course. The interpretation of satellite images to map landslides and river courses will be the resources for the quantitative analysis of landslides on the river courses. Moreover, a more detailed analysis will be processed on the outcomes of each engineering work and disaster susceptibility as the reference of future management works and priority management.

In the first stage, this project establishes a database and proceeds a thorough analysis of 87 sub-watersheds. Landscape variations are interpreted for those 24 updated sub-watersheds of southern Taiwan, 20 of them are new ones, with intention to analyze the current status of sub-watersheds and critical management watersheds. 107 river watersheds were chosen for detailed survey to construct watershed profiles. 1572 maps, scale of 1:5000, of 24 new watersheds are undertaken for landscape character assessment, which are the materials to analyze watersheds and acquire information about critical management areas. This year, 6 sub-watersheds are selected for UAV photo shooting and onsite investigation. The most updated and high-spatial-resolution aerial photos of the important engineering works and critical management watersheds are successfully acquired. The critical management watersheds of Daxixi in I-Lan County and Dongxi keng creek in Chiayi County, were also selected to establish before- and after-event DEMs of high-spatial-resolution from the UAV aerial photos. The changes in the critical management watershed can be quantitatively evaluated by comparing the before- and after-event DEMs, which would significantly benefit the detailed analysis of the results of engineering works.

From 2013 to 2016, there had been 107 watersheds chosen for detail survey to generate 7,613 maps which scale of 1:5000. 19 watersheds, 1,440 hectares, were selected to take aerial photographs using UAVs. 31 land management report videos about the results of the detail survey , engineering methods and aerial photographs are made. The project reports are provided for several engineering management, including He-Zhong creek engineering management of Hualien County, Daxixi 2nd phase of engineering management of I-Lan County and Dongxi-Keng creek 5th phase of engineering management of Chiayi County, to win Public Construction Golden Award. Through integrating basic data and efficiency assessment of detail survey of years as Kmz (Keyhole Markup Language) files, they are published on the GEE platform of SWCB. To promote and educate the public how does a management practice influence their lives.

Keywords:Formosat-2, Critical Management Watershed, Management Efficiency, Variation Analysis.