EVALUASI DAMPAK BANJIR AKIBAT PERUBAHAN ALUR SUNGAI CITANDUY HULU DI DESA TANJUNGKERTA, TASIKMALAYA-JAWA BARAT

Abstract

Since 1996 there have been frequent floods in the upstream area of the Citanduy River, especially during the rainy season. The flood also inundated part area of the Suryalaya Islamic Boarding School located in Tanjungkerta Village, Pagerageung District, Kabupaten Tasikmalaya. In this case, the community took the initiative with self-subsident activities to reduce the impact of flood by diverting the Citanduy River channel without any communication with river authority. This analysis purposed to evaluate the cross-section capacity due to narrowing the upper Citanduy River. Hydrological analysis was initiated to determine the characteristics of catchment area in study area. 1D and 2D hydraulic analysis using HEC-RAS application with unsteady flow analysis to determine water level, flood distribution, and river flow velocity. Using the annual Q25 plan discharge taking into existing capacity and geometric characteristics. Validation of modeling results by observing directly the water level at locations that affected by flooding condition due to high rainfall intensity. Conclusions are based on the results of hydraulic hydrological analysis applied to the research object. Shortening of river channel due to siltation causes a decrease in the area of the river channel in bank conditions. Actually condition of the Citanduy River after draining discharge over a 2-year return period, there was a flood in the river cross-section of Sta. 400 – 1035 rivers overflow with variations in depth of ±40 – 80 cm when model with a discharge period of 25-years.  The recommendation for further research is to assess the river capacity to analyze erosion and sedimentation that occurs in the watershed. Conducting a more comprehensive study, involving the three core components, that is Hydraulics, Ecology, and Morphology, so that proposed alternative solutions have a good impact on all aspects, not only solving one problem but causing other problems.

Keywords : Flood modeling, river shortcut, Upper Citanduy River

 

Abstrak

Sejak tahun 1996 sering terjadi banjir terutama saat musim hujan di  wilayah hulu Sungai Citanduy. Banjir tersebut juga menggenangi sebagian kawasan Pondok Pesantren Suryalaya berlokasi di Desa Tanjungkerta, Kecamatan Pagerageung, Kabupaten Tasikmalaya. Dalam hal ini masyarakat berinisiatif melaksanakan kegiatan swadaya untuk mengurangi dampak banjir tersebut dengan melakukan pengalihan alur Sungai Citanduy namun dilaksanakan tanpa sepengetahuan pihak pengelola sungai. Adapun analisis ini bertujuan mengevaluasi kapasitas penampang akibat penyudetan Sungai Citanduy Hulu. Analisis hidrologi dimulai untuk menentukan karakteristik daerah tangkapan air di lokasi pengukuran. Dilanjutkan analisis hidraulika 1D dan 2D dengan menggunakan unsteady flow analysis aplikasi HEC-RAS untuk mengetahui tinggi muka air, sebaran banjir dan kecepatan aliran sungai. Digunakan debit rencana Q25 tahunan mempertimbangkan kapasitas & karakteristik geometri eksisting. Validasi hasil pemodelan dengan meninjau langsung tinggi muka air pada lokasi terdampak banjir karena intensitas hujan tinggi. Ditarik kesimpulan berdasarkan hasil analisis hidrologi hidraulika yang diterapkan pada objek studi, bahwa pemendekan alur sungai akibat penyodetan, menyebabkan pengurangan luas alur sungai kondisi bankfull. Kondisi Sungai Citanduy setelah penyodetan debit diatas periode ulang 2 tahun, terjadi banjir pada penampang sungai Sta. 400 – 1035 sungai yang mengalami overflow dengan variasi kedalaman ±40 – 80 cm ketika dimodelkan dengan debit periode ulang 25 tahun. Adapun rekomendasi studi selanjutnya agar dilaksanakan peninjauan terhadap kapasitas sungai untuk menganalisis erosi dan sedimentasi yang terjadi pada Daerah Aliran Sungai. Melakukan kajian yang lebih komprehensif, melibatkan ketiga komponen inti yakni Hidraulik, Ekologi, dan Morfologi sehingga didapatkan hasil usulan alternatif solusi pengaruh baik terhadap seluruh aspek tidak hanya menyelesaikan satu masalah namun menimbulkan masalah lainnya.

Kata kunci : Pemodelan banjir, sodetan, Sungai Citanduy