Improving flood hazard and vulnerability assessment based on social assessment in Bogowonto river

WIGATI, Mayasih, Dr. H.A. Sudibyakto, MS

2008 | Tesis | S2 Magister Geo Informasi dan Penataan Ruang dan M

Desa Bagelen kabupaten Purworejo berada di daerah rawan banjir dan telah memiliki peta bahaya banjir berdasarkan pendekatan geomorpologi skala 1:220.000 (PSBA, 2004). Peta tersebut masih perlu pendekatan sosial untuk memperbaiki kajian banjir. Tujuan dari penelitian ini adalah memperbaiki peta bahaya banjir dan kerentanan yang selama ini berdasarkan pendekatan geomorpologi dengan menambahkan pendekatan sosial. Pengetahuan masyarakat tentang banjir sangat penting karena mereka mengetahui situasi pada saat banjir. Pengumpulan data primer dengan menggunakan 3 metode: dengan mobile GPS, peneliti memperoleh 335 bangunan berdasarkan materi tembok (tembok, semi tembok dan non tembok). 60 responden diwawancarai untuk mengetahui kerusakan akibat banjir, kondisi sosial ekonomi, pengetahuan dan persepsi tentang banjir serta strategi mereka dalam menghadapi banjir. Dengan metode PGIS (Participatory Geographic Information System), peneliti menemui masyarakat dengan mambawa Ikonos, peta Bagelen dan mobile GPS untuk mendapatkan data tinggi, durasi, dan luasan banjir. Data tersebut kemudian diproses untuk membuat model bahaya banjir dengan cara Kriging Interpolation Gaussian Model. Informasi lainnya seperti curah hujan, kondisi geomorphologi dan penggunaan lahan juga ddigunakan untuk membahas banjir. Kerentanan fisik dan sosial diukur dan dipetakan. Tiga tipe bangunan diperoleh dan dibuat kurva tahap kerusakan (stage damage curve). Tipe 1 dengan kerentanan rendah, tipe 2 kerentanan sedang dan tipe 3 merupakan tipe yang paling rentan. Ketinggian fondasi rumah bervariasi dari 0-150 cm dan dibuat kategori rendah (0-40 cm) dan kategori tinggi (lebih dari 40 cm). Kerentanan fisik merupakan hasil kombinasi dari tipe bangunan, ketinggian fondasi dan model bahaya banjir dan menghasilkan nilai 0 (tidak rentan), 0.25 (kerentanan rendah), 0.50 (kerentanan agak sedang) dan 0.75 (kerenanan agak tinggi). Kerentanan sosial mengacu pada karakteristik masyarakat dengan fokus komposisi jenis kelamin dan umur, pendapatan rumah tangga dan kondisi sosial ekonomi. Strategi menghadapi banjir (coping strategy) telah dilakukan masyarakat untuk mengurangi resiko banjir berdasarkan pengalaman dan pengetahuan mereka (local knowledge).

The Bagelen sub district of Purworejo district is located in a flood prone area. The Purworejo district has flood hazard map based on geomorphological approach without considering social assessment. This map gives information where the area is low hazard, medium hazard or high hazard. The map was made in 2004 with scale 1:220.000 (PSBA, 2004). Community affected by flood should be involved in the flood assessment. The combination between flood assessment based on physical and social assessment is better to assess flood because people understand the condition better and this combination will help the community for quick assessment during and after flood. Primary data were collected using mobile GPS for a sample of 355 buildings, whose building materials were classified into 3 categories: tembok (wall), semi tembok (semi wall) and non tembok (pleated bamboo or wood); and 60 respondents were interviewed related to damage and social economic condition. The printed Ikonos image, Bagelen map and mobile GPS were used to make mental map/sketch map and flood depth points. Participatory Geographic Information System (PGIS) and flood marks measurement of the flood depths were processed by using interpolation in ILWIS. A new flood hazard model was made using Gaussian Model. Information such as rainfall data, geomorphological data, and land use were also important components. Physical and social vulnerability were measured and mapped. Three structural types of buildings were found and stage damage curves were made using the damage for each structural type of buildings: type 1 are less vulnerable houses, type 2 are moderate vulnerable houses, and type 3 are most vulnerable houses. The foundation height ranged 0-150 cm and was categorized: low (0-40 cm), and high (more than 40 cm). Physical vulnerability is the combination function of structural type of building, foundation height, and flood hazard model. There are 4 physical vulnerabilities: value is no vulnerability, value 0.25 is low vulnerability, value 0.5 is moderate low vulnerability, and value 0.75 is moderate high vulnerability. Social vulnerability in this thesis refers to the characteristic of the people with composition of sex and age in household, house hold income, head of household characteristics, and social economic condition. Coping mechanism applied in this area are based on the local knowledge and experience of flooding. People make efforts to reduce the effect of flood based on their knowledge.

Kata Kunci : Geomorfologi,Peta Bahaya Banjir,PGIS,flood hazard modelling, participatory geographic information system (PGIS), physical vulnerability, social vulnerability, damage curves, local knowledge, coping mechanism