Download - HUJAN (PRESIPITASI)
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Adi Prawito
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Hujan Siklonik Hujan Konvektif Hujan Orografik
Hujan Siklonik: berasal dr naiknya udara yg dipusatkan di daerah dgn tek rendah
Hujan Konvektif: berasal dr naiknya udara ke rempat yg lbh dingin
Hujan Orografik: berasal dr naiknya udara krn adanya rintangan pegunungan
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Curah hujan Waktu hujan Intensitas hujan Frekuensi hujan
Ch: tinggi hujan dlm 1 hari, bulan atau thn dgn satuan mm, cm, inch. Misal:124 mm/hr, 462 mm/bln, 2158 mm/th
Wh: lama tjdnya 1x hujan, mis: 42 menit, 2 jam
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Ih: banyak hujan yg jatuh dlm periode ttt, misal: 48mm/jam dlm 15’, 72mm/jam dlm 30’
Fh: kemungkinan tjdnya besaran hujan yg melampaui suatu tinggi hujan ttt, mis: ch 115 mm/hr akan tjd atw dialampaui 1x dlm 30 th, 2500 mm/th akan tjd atw dilampaui dlm 10 th
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Tabel 1: Juml penakar hujan pd suatu daerah yg diwakili
Sumber: Wilson (1974:17)
Luas (km2) Juml sta penakar hujan262601300260052007800
2612152040
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Tabel 2: Network stasiun hujan di Indonesia
Sumber: Murni D., Sri (1976:6)
Daerah Juml stasiun Km2/staIndonesia
Jawa Sumatra
Kalimantan Sulawesi
+/- 4339+/- 3000+/- 600+/- 120+/- 250
+/- 440+/- 44+/- 790+/- 4500+/- 760
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Pencatatan manual, tdr dr corong 8”, tabung pengukur, dan penyangga, mis: standard 8” precipitation gauge (US National Weather Service), didapat data hujan harian
Penakaran otomatis, didapat data hujan mingguan pd kertas grafik, mis:1. weighing bucket rain gauge2. tipping bucket rain gauge3. syphon automatic rainfall recorder
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Bentuk tabel Bentuk diagram Bentuk grafik
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Tahun R (mm) Tahun R (mm)19701971197219731974
13311775
150154
19751976197719781979
161220129160120
Sumber: Data hujan pd stasiun Bantaran G. Kelud Jatim
Tabel 3: Hujan harian maksimum
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R (mm)
15
10
5
10 11 12 13 14 15 16 t (jam)
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R (mm)
150
100 50
0 t (bulan) J P M A M J J A S O N D
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Prosedurnya adl sbb (7 langkah): Hitung tot hujan utk n penakar hujan Hitung rata2 aritmatik hujan di daerah
aliran Hitung juml dr kuadrat utk n data hujan Hitung variansnya Hitung koefisien variasinya Juml penakar hujan yg optimum N yg
diperlukan utk memperkirakan hujan rata2 dgn % kesalahan (p)
Jumlah penakar hujan yg hrs ditambahkan Contoh soal lihat Sholeh (h.34)
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Cara rata2 aritmatik Cara rasio normal Cara korelasi
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Penyebab: Stasiun hujan dipindah Tipe penakar hujan diganti
Utk mengecek digunakan: Analisa Double Mass Curve (perub kemiringan korelasi menunjukkan adanya perubahan)
Contoh Double Mass Curve lihat Sholeh (h.37)
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Variasi tahunan Variasi bulanan Variasi harian
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R (mm)10000
Mass Curve7500
Massa hujan rata25000
2500
90 91 92 93 94 95 96 97 98 Disebut tahun basah apabila kemiringan mass
curve > kemiringan massa hujan rata2, begitu pula sebaliknya
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Bulan basah (100 mm <…) Bulan kering (…< 60 mm) Bulan normal (60 – 100 mm)
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Konsentrasi hujan yg berbeda tiap2 jamnya
Berlangsung setiap hari dalam satu bulan
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CARA ARITHMATIC MEAN CARA THIESSEN POLYGON CARA ISOHYET
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Dipakai pd daerah yg datar Banyak stasiun penakar hujan Curah hujan bersifat uniform R = 1/n . (R1 + R2 + R3 + … + Rn)
dimana:R = tinggi hujan rata2 daerah aliran (area rainfall)R1,R2,R3,…,Rn = tinggi hujan masing2 stasiun (point rainfall)n = banyaknya stasiun hujan
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Tdp faktor pembobot (weighing factor) / koefisien Thiessen
Besar faktor pembobot tgt luas daerah yg diwakili sta yg dibatasi oleh polygon2 yg memotong tegak lurus pd tengah2 grs penghubung
R = A1/A .R1 +…+ An/A . Rndimana:A = luas daerah aliranAi = luas daerah pengaruh stasiun iRi = tinggi hujan pd stasiun i
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Isohyet: grs yg menunjukkan tinggi hujan yg sama
Isohyet diperoleh dgn cara interpolasi harga2 tinggi hujan local (point rain fall)
Besar hujan antara 2 isohyet: R1,2 = ½(I1 + I2)
Hujan rata2 daerah aliran: R = A1,2/A . R1,2+…+ An,n+1/A . Rn,n+1
dimana:Ai,i+1 = luas antara isohyet I1 dan I1+1Ri,i+1 = tinggi hujan rata2 antara isohyet I1 dan I1+1
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Intensitas: kemiringan dr grafik pencatatan hujan (harga tangen)
I = R/tdimana: I = intensitas hujan dlm mm/jamR = hujan selama interval (mm)t = interval waktu (jam)
Pola intensitas = hyetograph (gambar 4.16, Sholeh, h.43)
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Uniform pattern Advanced pattern Intermediate pattern Deleyed Pattern Gambar 4.17, Sholeh, h.44
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Talbot (1881) Sherman (1905) Ishigoro (1953) Mononobe Utk perumusan intensitas memerlukan
data hujan jam-jaman hingga 24 jam Data hujan harian tdk bs digunakan
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Utk hujan dgn waktu < 2 jam I = a /(t + b)
I = intensitas hujan (mm/jam)t = waktu hujan (jam)a,b = konstanta tgt kead setempat
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Utk hujan dgn waktu > 2 jam I = c / tn
dimana: c,n = konstanta yg tgt kead setempat
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Utk data hujan harian I = R24/24 . (24/t)m
dimana: I = intensitas hujan (mm/jam)R24 = tinggi hujan max dlm 24 jam (mm)t = waktu hujan (jam)m = konstanta (2/3)
Baca juga Sosrodarsono, Suyono (2006) Hidrologi untuk pengairan, hal 32-36
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Tinggi hujan : Waktu 1-10 hari Waktu 1-24 jam Waktu 0-1 jam
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Menggunakan perumusan Haspers 100.R/R24 = 362 log (t+6) – 206
dimana: t = banyaknya hari hujan R = tinggi hujan (mm) R24 = tinggi hujan dlm 24 jam100.R/R24 = dlm prosentase
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(100.R/R24)2 = 11300.t/(t + 3,12)dimana: R, R24 dlm mmt dlm jam 100.R/R24 = dlm prosentase
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R = a.R24 / (R24 + b)dimana: R, R24 dalam mma,b = konstanta utk hujan dg waktu ttt spt pd tabel 4.7, Sholeh hal 48
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Adalah: kemungkinan tjdnya / dilampauinya suatu tinggi hujan ttt dlm massa ttt pula yg jg disebut sbg massa ulang (return period)
Frekuensi hujan dpt berupa harga2 tinggi hujan max dan tinggi hujan min
Tinggi hujsn ekstrim max dan min didapatkan melalui pendekatan statistik
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Diperlukan utk perenc bangunan air, proyek2 pengemb SDA, gorong2, saluran irigasi, sal drainase, dll
Tinggi hujan renc diambil yg mendekati tinggi hujan ekstrim max, shg resiko kecil
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Murni D., Sri (1976), Hidrologi I, Jakarta: Fakultas Teknik UI
Sholeh, M (1984), Diktat Hidrologi, Surabaya: Teknik Sipil ITS
Sosrodarsono, Suyono & Takeda, Kensaku (2006), Hidrologi untuk Pengairan, Jakarta: Pradnya Paramita
Wilson (1974), Engineering Hidrology, Macmilan