Manajemen Sungai dan Pengendalian Banjir
Dr. Dyah Indriana Indriana K, K, S.T., M.Sc. Ir. Mariyanto, MT Endro Prasetyo Prasetyo Wahono Wahono, S.T., M.Sc.
Part 1 Managemen Sungai Sungai dan Pengendalian Banjir Banjir
Hidrologi DAS DAS Hidrologi – Memahami Memahami perhitungan perhitungan hidrologi hidrologi yang yang diperlukan sebelum penelusuran penelusuran banjir banjir Flood Flood
frekuensi Conceptual model Conceptual Metode Rasional Rasional Metode
Konsep Managemen Managemen Sungai Sungai dan dan Pengendalian Pengendalian Konsep Banjir
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Banjir dan Manajemen Banjir
Kejadian banjir Resiko banjir Pembangkitan banjir Klasifikasi tipe-tipe banjir Analisis pembangkitan banjir Konsekuensi banjir
Hidrologi Daerah Aliran Sungai
Siklus hidrologi Analisis hidrologi (hujan dan banjir)
Flood frekuensi
Hidrograf banjir
Hidrograf satuan
Hidrograf satuan sintetik
Instrumentasi DAS
Instrumentasi DAS Rainfall gauge, AWLR
Pengenalan Investigasi sumber air
Pengenalan isotope untuk memahami sumber air
Kejadian Banjir
Banjir adalah kejadian hidrologi yang dicirikan dengan debit dan/atau muka air yang tinggi yang dapat menyebabkan penggenangan dari tanah di sekitar sungai, danau, atau sistem air (water body) yang lain. Biasanya yang dibicarakan adalah sungai dan saluran yang tidak mampu mengalirkan sejumlah air yang dihasilkan melalui runoff process, dan akibatnya terdapat limpasan air. Namun, banjir juga bisa disebabkan oleh ketidakmampuan air untuk melewati downstream disebabkan oleh muka air yang tinggi pada saluran di hilir. Kejadian banjir disebabkan oleh hujan yang lama dan berintensitas tinggi, kegagalan bendung atau tanggul, gempa bumi tanah longsor, air pasang tinggi aktivitas
Banjir di Indonesia
Resiko Banjir Banjir memiliki konsekuensi yang sangat luas terhadap bangunan, ekonomi, sosial dan lingkungan. Banjir, walaupun jarang terjadi, namun merupakan ancaman terhadap masyarakat, dan oleh karena itu berhubungan dengan sejumlah resiko. Salah satu cara untuk mengetahui resiko tersebut adalah dengan mengadopsi “source-pathway-receptor” model.
“Source-pathway-receptor” model Source
Di luar kendali kita
(hujan) Pathway (topografi dan kondisi tanah dan sungai) Receptor Manusia dan harta benda
Sedikit banyak masih bisa dikendalikan
Yang paling bisa dikendalikan di antara ketiganya
Risk assesment meliputi identifikasi potensial bahaya yang dapat menyebabkan kecelakaan maupun kerusakan, dan memperkirakan kemungkinan terjadinya dan konsekuensinya. Sehingga Resiko banjir untuk suatu komuniti tertentu akan meliputi : - Probabilitas bahaya banjir di daerah tersebut - Seberapa rentan daerah tersebut terhadap akibatakibat yang tidak diinginkan dan kerugian ekonomi
Biasanya seberapa parah suatu banjir, dinyatakan dengan kala ulang, misalnya banjir dengan kala ulang 100 tahun. Hal ini dibutuhkan untuk mengevaluasi sistem penanggulangan banjir dari sisi ekonomi (benefit-cost). Dapat dijadikan standard nasional untuk membuat skema perlindungan banjir Misalnya untuk daerah perkotaan maka bangunan pelindung banjir dirancang untuk banjir dengan kala ulang 50 tahun. Penentuan kala ulang banjir untuk suatu bangunan berkaitan erat dengan evaluasi ekonomi.
Banjir dunia Date 1421 1530 1642 1887
1900 1911 1931
1935
Location Holland Holland China Yellow River China Galveston Texas, USA Yangtze River, China Yangtze River, China Yangtze River, China
Death 100,000 400,000 300,000 900,000 5,000 100,000 145,000 142,000
Generation of floods Floods area a nature part of the hydrological cycle. Over thousands of year rivers have become adapted to the local geology and the frequency of flow events arising from regional climatic and geological processes. The river channel have adjusted their size and have attained a dynamic ‘equilibrium’. Although morphological changes may occur during a single flood (or extreme flow) event, most changes take place over may times the human life-span. The characteristic nature of rivers as having an identifiable channel with associated flood plains is due to the higher frequency of occurrence of formative flood events every one to two years (reference regime theory).
The ‘dominant discharge’ associated with these events is responsible for the dimensions and plan-form of the river channel together with the local geology. At higher less frequent flows the channel has insufficient capacity to contain the flow and water inundates the flood plains adjacent to the channel. Given the lower frequency of the higher flood events, the flood plains tend to form a less well-defined river valley, though sometimes glaciers will have carved these out during ice ages millennia ago
.
Floods are generated in most circumstances by prolonged and intense rainfall, though occasionally natural embankments, such as created by receding glaciers, can break due to increased water stored behind them. In the latter case huge amounts of water can be suddenly released, creating large damages as in the case of the breaking of artificial structures such as dams and dykes. A proportion of the rainfall soaks into the ground infiltrating down to the local water table or is eventually lost through evapotranspiration.
The remainder finds its way into streams and river channel as overland flow or through groundwater. This is termed runoff. The overland flow generally contributes to what is called the ‘fast’ or ‘direct’ runoff. As a proportion of the total runoff, the fast component depends on the nature of the geology of the and the degree of saturation of the ground surface. Normally the proportion of the total runoff during a severe storm will be between 0.2 and 0.45 if, however, the catchment is already very wet before the start of the storm, infiltration may be limited and the proportion can rise to be as high as 0.7.
The flow in the river resulting from a storm event will vary according to the spatial and temporal pattern of the rainfall and the preceding rainfall. Therefore, there not necessarily be a direct correspondence between frequency of the rainfall and the frequency of the runoff. This makes the analysis of design rainfall events corresponding to a certain return frequency of flood events complicated. Once a flood has been generated in a long river, it can be said to propagate along the river towards its mouth, though the nature of the propagation may be distorted by additional runoff entering the river along its length.
Generally, a flood builds up rapidly in the headwaters of the river, but may take several days or even weeks to reach the sea or lake to which it discharges. It is important to distinguish between the travel time of the water and the travel time of the flood. Generally, the latter is faster then the former. The speed of propagation of the flood peak is dependent on the gradient of the riverbed and the extent of flooding. The flatter the river and the wider the extent of flooding on adjacent flood plains, the slower the speed of the flood peak. These factors affecting the speed of travel are a function of two important concepts: storage and conveyance. Water in the channel and on the flood plains can be said to be ‘stored’ dynamically. Storage is significant in affecting the rate at which a flood peak decreases as it propagates downstream.
Conveyance refers to the ease with which water (rather than the flood disturbance) moves downstream. Some flood plains convey floodwater in a downstream direction and therefore add to the conveyance of the river channel. The degree of flood plain conveyance depends on the topography of the flood plain and obstructions such as hedges and boundary walls, embankments etc. The propagation of the flood is intimately connected with the conveyance. Artificial intervention in rivers through, say, structural aspect of flood plains. There is growing concern that the cumulative consequences of river management actions down the centuries have adversely affected the natural performance of flood plains leading to increased flood risk.
SIKLUS HIDROLOGI
Kondensasi Presipitasi
Evaporasi air hujan
Aliran permukaan Infiltrasi
Evaporasi air danau, kolam
Muka air tanah
Transpirasi
Evaporasi air laut
Evaporasi air sungai
Aliran air tanah Mata air Danau
Aliran air tanah
Laut Sungai
• Perubahan guna lahan : lahan terbuka / hutan, sawah pemukiman, kawasan industri, dll. tanpa kompensasi pengganti resapan akan mengakibatkan kenaikan debit puncak sampai 25 kali.
Resapan
Misal: Debit Puncak = 10 m3/dt Resapan = 5 m3/dt
Akibat perubahan guna lahan bisa menjadi Debit Puncak = 75 m3/dt Resapan = 0,5 m3/dt
Intersepsi (menangkap & menyimpan sementara) f(A) Evapotranspirasi f(t, A) Memperlambat aliran f(n) Meningkatkan infiltrasi f (t, I) Meningkatkan limpasan permukaan
Perubahan hidrograf banjir Limpasan 55%
Limpasan 74%
20
20
) t d / 3 m ( t i b e d
) t d / 3 m ( t i b e d
10
10
0
Limpasan 89%
20 ) t d m ( t i b e d
/ 3
10
0 0
30
60 waktu (menit)
Daerah pedesaan masih mempunyai cukup simpanan dan retensi
0 0
30
60 waktu (menit)
Daerah pengembangan, kapasitas simpanan menurun, limpasan meningkat. Penduduk da n fasilitas meningkat
0
30
60 waktu (menit)
Penduduk da n fasilitas meningkat ba hkan sampai di daerah rawan banjir. Kapasitas simpanan menurun terus, limpasan meningkat pesat. Terjadi tanah longsor da n banjir
Classification of types of floods Floods may be the result of a number of causes and result in particular damages :
Flash floods that build up rapidly, usually in step terrain
Lowland or plains floods that build up slowly and with more predictable onset
Floods from highly localized rainfall events (thunderstorms)
Floods from natural events such as the collapse of a natural embankment
Floods generated by the failure of a flood defence infrastructure
Flooding arising from raised groundwater levels
Floods exacerbated by recent previous rainfall events that have contributed to the saturation of the ground before the storm event, thereby increasing runoff
Flooding from inadequate urban drainage, or the inability of drainage water to escape to swollen receiving waters
Coastal or estuarial flooding due to tidal surges or a dyke collapse due to wave overtopping
Flood generation analysis Estimation of flood discharge is done in one of several ways:
Empirical formulae Frequency analysis Regional flood analysis Probable maximum flood (PMF) methods Conceptual modeling The best known of the empirical formulae is the Rational method, which has been particularly successful in the area of urban drainage.
Qp = debit puncak banjir, m3/detik C = koefisien limpasan, merupakan fungsi guna lahan, bervariasi antara 0 – 1. I = intensitas hujan, mm/jam A = luas daerah tangkapan air, ha
Koefisien limpasan, C Diskripsi lahan/karakter permukaan
Business perkotaan pinggiran Perumahan rumah tunggal multiunit, terpisah multiunit, tergabung perkampungan apartemen Industri ringan berat Perkerasan aspal dan beton batu bata, paving Atap Halaman kereta api Taman tempat bermain Taman, pekuburan Hutan datar, 0 - 5% bergelombang, 5 - 10% berbukit, 10 – 30%
Koefisien limpasan, C
0,70 - 0,95 0,50 - 0,70 0,30 - 0,50 0,40 - 0,60 0,60 - 0,75 0,25 - 0,40 0,50 - 0,70 0,50 - 0,80 0,60 - 0,90 0,70 - 0,95 0,50 - 0,70 0,75 - 0,95 0,10 - 0,35 0,20 - 0,35 0,10 - 0,25 0,10 - 0,40 0,25 - 0,50 0,30 - 0,60
Such formulae are easy to use but contain many oversimplifications and unrealistic assumptions, which make their use limited primarily to design problems. Frequency analysis is essentially a matter of fitting a suitable probability distribution to the flood data, in particular, to extrapolate to the low frequency range of occurrence. The reliability of these techniques is dependent on sufficient length and completeness for thehistorical flood data records. Such techniques are well established in operational in hydrology.
Where the record is short, uncertainties in the accuracy of the flood is the charges, the selection of the appropriate distribution and the estimation of its parameters can limit the reliability and accuracy of the predictions. These can be supported using unconventional data such as historical flood reconstruction (modeling), botanical methods (e.g. tree rings), sediment characteristics, etc. Another way of dealing with the shortage of data on one catchment is to pool the data within the region using a regional flood analysis.
Normally, this in done by first estimating the flood distribution and its shape parameters (e.g. a design unit hydrograph), estimating the parameters based on observable physical characteristics of the catchment (area, stream length, etc), and finally determining the mean annual flood using appropriate design rainfall for the region. Given sufficient catchments with data in the region then the variability from basin to basin dominates the sampling errors. The regional analysis then becomes extremely valuable in estimating floods for ungauged catchment and sites.
An estimate of the probable maximum flood (PMF) is important for major structures at risk, including dam spillways, nuclear power stations and major bridges. The approach is to maximize the meteorological processes and catchment response and to deduce the maximum, physically feasible flood discharge in conjunction with suitable statistical or conceptual modeling techniques. An estimate of the probable maximum flood (PMF) is important for major structures at risk, including dam spillways, nuclear power stations and major bridges. The approach is to maximize the meteorological processes and catchment response and to deduce the maximum, physically feasible flood discharge in conjunction with suitable statistical or conceptual modeling techniques.
The use of conceptual modeling based on knowledge of explicit soil moisture accounting is probably one of the most widely used approaches for flood analysis. There are many conceptual models produced for flood estimation, prediction and forecasting. Generally these models account for
Rainfall over the catchment
Computation of rainfall based on hydrological abstractions
Routing the rainfall and snowmelt excess over the cacthment
The model can be conceived in either a lumped or distributed form over the whole of the catchment, and address single storm events or a continuous record of precipitation (including dry periods). Increasingly, because of the complex physical processes, both in terms of poor knowledge of the physics and the associated spatial and temporal distributions of key parameters, more attention is being given to various forms of data modeling for flood estimation (see other lecture notes).
Many forecasting models are restricted to data driven on lumped conceptual models (perhaps for separate sub catchments) in that the emphasis is more on the updating procedure rather than modeling the physical processes in the catchment in detail (see other lecture notes). Nevertheless, where inventions are being planned in the catchment or prediction is being done with design rainfall, for example, then there can be good reasons to apply more detailed physically based models of the rainfall runoff process.
Obviously this requires sufficient data be available, especially of the topography and land use of the catchment. Here, models such as MIKE-SHE become very valuable tools. These models have now become very sophisticated in their use of remote sensed geophysical data including geomorphology, soil type, land use, vegetation, soil moisture and appropriate calibration data for rainfall, groundwater levels and runoff.
Flood hydraulics
Calculations of flood hydraulics are nowadays done using sophisticated simulation models based on solutions of the full or approximate forms of the de Saint Venant equations for gradually varying flow in open channels. Such models are usually one or two-dimensional. In the first instance a 1 D model can address flood flows in open channels, provided the water level across the channel normal to the flow is approximately uniform (horizontal). The channel here can include the flood plain. Difficulties come when the channel is embanked and water levels on the flood plain are markedly different from those in the channel.
In this case,1 D models treating flow on flood plain from a storage of view only can sill be used, or the net work-type of 1 D model can treat the flow on the floodplains as being in separate channels. More generally, 2D models are needed for this case. Because of their physical basis, these models only require data for calibration and appropriate input data for event or time series modeling.
Physical (laboratory) models can also be used as an alternative to mathematical models in certain cases where there is a strong need to convince decision makers who an unfamiliar with mathematical models, or there are hydraulic structures for which it is difficult to deduce appropriate head-loss coefficients, such as through dense urban areas.
Consequences of flooding The impact of flooding on human beings includes
Short and long term mental and emotional distress
Acute and chronic disease from water borne pathogens and residual damp in buildings
Death whether from the inability to escape from the flood or from accidents associated with the unexpected opportunity for unusual recreation that a flood may afford
The economic impact of flooding is measured by the damage to national assets and disruption to normal economic activities within the flood-affected area and its surrounding economic links. Individual impacts of flooding are measured by financial losses through damage to personal or business assets, loss of employment, loss of crops and livestock, inability to sell property in a flood affected area, etc. Health is recognised as an important issue affected by the occurrence or even the threat of floods.
Besides the extreme event of loss of life, many experience stress from the social and personal disruption due to loss of personal items, the damage done by silt, and occasionally large bounders, to building and contents, or the contamination of the silt with sewer discharges. In hotter climates the lack of clean water during floods and the exposure to water borne diseases can be very serious for the health of the survivors. This may be exacerbated by water supplies being disrupted or contaminated, and the failure to receive emergency supplies.
But floods are not wholly bad. They can lead to river rejuvenation, beneficial siltation on flood plains, enchanced wetlands, improved bio-diversity, and vegetation and stripping of channel and flood plains. The ecological consequences of floods are of increasing importance, calling for an integrated approach to river basin management. In addition, flood alleviation schemes can introduce indirect benefits to the local region such as higher land values, better private and commercial development.
This course will touch on some of the subjects above, though others by necessity will have to be put to one side for others to present or debate. For example, the ecological effects of flooding are being recognised as an important area for the future. This is linked with the generation of pollution through flooding. The interest is in factors such as the spatial and temporal changes or responses of the local ecology to individual floods events or the changing frequency of flooding. In addition, there are significant morphological consequences of flooding in countries like Bangladesh, but again this topic is outside the scope of the course.
