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AIR TANAH (Ground Water)

AIR TANAH (Ground Water). Adhi Muhtadi Teknik Sipil UNIV NAROTAMA SBY. Air tanah :. Air hujan yg diserap permukaan tanah & meresap lewat lapisan2 di bawahnya sampai lap jenuh Air di dalam tanah ( subsurface water ) ada 2: Zone tdk jenuh air ( unsaturated zone )

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AIR TANAH (Ground Water)

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  1. AIR TANAH (Ground Water) Adhi Muhtadi Teknik Sipil UNIV NAROTAMA SBY

  2. Air tanah: • Air hujan yg diserap permukaan tanah & meresap lewat lapisan2 di bawahnya sampai lap jenuh Air di dalam tanah (subsurface water) ada 2: • Zone tdk jenuh air (unsaturated zone) • Zone jenuh air (saturated zone) Batas antara ke2 zone diatas = muka air tanah

  3. Gambar: penampang air dlmtanah moisture unsaturated zone tanah capillary zone MAT air kapiler saturated zone air tanah bebas

  4. Aquiferadl: • Lapisan tanah (formasi geologis) yg dpt menampung serta melepas air dlm juml yg cukup Macam2 aquifer: • Confined aquifer • Unconfined aquifer

  5. Confined aquifer • Aquifer yg dibatasi bag atas & bwhnya lap kedap air Unconfined aquifer: • Aquifer yg di bag bwhnya tdp lap kedap air & di atasnya muka air bebas Lihat: Gambar 7.2, Confined dan Unconfined aquifer, Sholeh (1984,85)

  6. Tabel: Tipetanah & kenaikankapiler Kesimpulan: smakin kasar tanah mk smakin kecil kenaikan kapilernya

  7. Istilah2 dlm air tanah • Aquiclude: formasi geologi yg dpt menampung air ttp tdk dpt melepas air dlm juml yg cukup • Aquifuge: formasi geologi yg tdk dpt menampung maupun melepas air dlm juml cukup • Specific yield: juml air yg dihslkan dr 1 unit vol tanah bila didrainase pengaruh gravitasi • Specific retention: juml air yg msh tertahan oleh 1 unit vol tanah setelah didrainase dgn gravitasi

  8. Specific storage: • Simpanan air dalam tanah • Bila recharge (air yg msk aquifer) = discharge (air yg keluar aquifer), mk simpanan air dlm aquifer seimbang

  9. Penyebab recharge: • Peresapan air hujan mel permuk tanah • Peresapan air mel dsr & tebing sungai, danau, laut dsb • Perembesan air dr aquifer lain Penyebab discharge: • Evapotranspirasi dr permuk tnh • Perembesan air aquifer menuju sungai, danau, laut, dsb • Perembesan ke aquifer lain • Aliran keluar sbg sumber air

  10. Aliran air tanah • Melalui media poreus sebesar 0,02-2 mm/dtk • Pengalirannya sgt lambat/aliran laminer

  11. Aliran air tanahdipengaruhioleh: • Kerapatan & viscositas air • Sifat tanah

  12. Kerapatan & viscositas air • Air tanah kebanyakan adl air tawar & kadang2 air asin • Kerapatan & viscositas air tdk banyak berubah meskipun tjd perubahan temperatur • Contoh: t=0oC; ρ=999,9 kg/m3; v=0,155 m2/hr t=40oC; ρ=992,2 kg/m3; v=0,057 m2/hr • Utk kerapatan air asin lihat Gbr 7.3, Sholeh (1984:87)

  13. Sifattanah Yg sgt mempengaruhi adl: • Porositas • Permeabilitas Porositas (n) = vol pori (Vi)/vol total (V) dinyatakan dlm % n = Vi/V = V – Vm/V = 1-(Vm/V) Vm = Vol butir • Makin merata butir tanahnya, makin besar porositasnya • Makin tegak lengkung hasil analisa ayakan makin besar porositasnya

  14. Permeabilitas tergantung: • Porositasnya • Struktur tanahnya • Besar kecilnya permeabilitas dinyatakan dlm koefisien permeabilitas (k) • Range permeabilitas dr tnh asli:

  15. Aliran Air Tanah (Ground Water Flow) • Hukum Darcy (1856) • Aliran dlm aquifer di bwh tekanan (confined aquifer) • Aliran dlm aquifer bebas (unconfined aquifer)

  16. Hukum Darcy Anggapan dlm Hkm Darcy: • Tanahnya homogen & isotropik • Tdk ada daerah kapilernya, seluruh jenuh • Aliran tetap (steady flow) • v = k.i (v=kecepatan, k=kons permeabilitas, i=kemiringan grs potensial) • Q=v.A = k.i.A • V = debit/luas penampang yg lolos air = Q/A.ne = A.V/A.ne = V/ne V = kecep rata2 sebenarnya; ne=efektif porositas

  17. Pemompaan air tanah Dimanfaatkan utk: • Irigasi • Air minum Bila dilakukan pemompaan, mk mat di skitar lobang pemompaan akan turun. • Lihat gbr 7.8 (h.93) utk confined aquifer dan 7.9 (h.94) utk unconfined aquifer

  18. SELESAI

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