Rencana Pengendalian Banjir Sungai Lengkong Kabupaten Tuban dengan ALternatif Floodway Tunnel System

Sungai Lengkong secara administratif berada di Kecamatan Jenu, Kabupaten Tuban, Provinsi Jawa Timur. Sungai yang memiliki panjang ± 7 Km tersebut merupakan bagian hilir dari Sungai Sumberlangsih yang memiliki total panjang sungai secara keseluruhan yakni 20 Km menjadi daerah rawan banjir. Banjir ini kerap terjadi meskipun di wilayah hulu sungai tidak terjadi. Hal itu disebabkan oleh luas daerah aliran sungai (DAS) yang besar yaitu 109.77 Km2, topografi Kabupaten Tuban yang berbukit-bukit dan datar pada daerah hilir sungai serta kemiringan dasar sungai yang landai sehingga ketika curah hujan tinggi dan juga adanya pengaruh pasang surut air laut Jawa menjadikan debit banjir tidak mampu ditampung seluruhnya oleh kapasitas sungai di bagian hilir Sumberlangsih sehingga diperlukan adanya upaya dalam pengendalian banjir di sungai Lengkong. Dari analisa hidrologi untuk perhitungan debit banjir rencana Q25th dihasilkan sebesar 274,68 m3/dt dan tinggi muka air laut rata-rata (Mean Sea Level) setinggi 0,65 meter dengan pasang tertinggi (HWS) setinggi 1,40 meter. Sedangkan pada analisa hidraulika dengan menggunakan program HEC-RAS 5.0.5 dihasilkan kapasitas tampung Sungai Lengkong eksisting maksimum sebesar 50 m3/dt dengan kondisi dari hulu sungai hingga muara hampir seluruhnya mengalami banjir. Sehingga direncanakan normalisasi pada daerah hilir sungai Lengkong tepatnya pada STA. 4+250 sampai STA. 0+000. Setelah dilakukan normalisasi, kapasitas tampung Sungai lengkong diperbesar menjadi 150 m3/dt. Maka selisih debit banjir yang ada sebesar 124,68 m3/dt akan dialirkan melewati pelimpah samping (side weir) lalu melewati pipa tunnel untuk dibuang menuju laut yang dinamakan Floodway Tunnel System. Dari hasil perencanaan Floodway Tunnel System didapatkan dimensi pelimpah samping dengan lebar pelimpah sebesar 35 meter, panjang kolam tampung (ponds) sepanjang 250 meter dengan lebar 25 meter dan dimensi pipa tunnel berdiameter 3,3 m dengan Panjang pipa 4.750 meter. Dengan perencanaan Floodway Tunnel System tersebut debit banjir rencana Q25th dapat teratasi seluruhnya. =============================================================================================== Lengkong River is administratively located in Jenu District, Tuban Regency, East Java Province. The river which is ± 7 Km long is the downstream part of the Sumberlangsih River which has a total length of the river as a whole which is 20 km into a flood-prone area. This flood often occurs even though the upstream area does not occur. This is due to the large watershed area of 109.77 Km2, topography of Tuban Regency which is hilly and flat on the downstream area of the river and the slope of the sloping riverbed so that when rainfall is high and also the influence of tidal waters of Java making the flood discharge unable to be accommodated entirely by the capacity of the river in the lower reaches of Sumberlangsih so that efforts are needed in controlling floods in the Lengkong river. From the hydrological analysis for the calculation of the resulting Q25th flood discharge amounting to 274.68 m3/sec and the mean sea level as high as 0.65 meters with the highest tide (HWS) as high as 1.40 meters. Whereas the hydraulic analysis using the HEC-RAS 5.0.5 program produced a maximum capacity of the existing Lengkong River capacity of 50 m3/sec with conditions from upstream to estuary almost entirely experiencing flooding. So that it is planned to normalize the downstream area of the Lengkong river precisely at the STA. 4,250 to STA. 0,000. After normalization, the capacity of the sticky river is enlarged to 150 m3/sec. Then the difference in the existing flood discharge of 124.68 m3/sec will flow through the side weir then pass through the tunnel pipe to be discharged towards the sea called the Floodway Tunnel System. From the results of the Floodway Tunnel System planning obtained side spill dimensions with an overflow width of 35 meters, ponds with a length of 250 meters with a width of 25 meters and dimensions of tunnel pipes with a diameter of 3.3 m with a pipe length of 4,750 meters. With the Floodway Tunnel System planning, the flood discharge of the Q25th plan can be completely overcome