ManajemenManajemen SungaiSungai dandanPengendalianPengendalian BanjirBanjir

Dr. Dr. DyahDyah IndrianaIndriana K, S.T., K, S.T., M.ScM.Sc..Ir. Ir. MariyantoMariyanto, MT, MT

EndroEndro PrasetyoPrasetyo WahonoWahono, S.T., , S.T., M.ScM.Sc..

Part 1Part 1ManagemenManagemen SungaiSungai dandan

PengendalianPengendalian BanjirBanjir

HidrologiHidrologi DASDAS MemahamiMemahami perhitunganperhitungan hidrologihidrologi yang yang diperlukandiperlukan

sebelumsebelum penelusuranpenelusuran banjirbanjir Flood Flood frekuensifrekuensi Conceptual modelConceptual model MetodeMetode RasionalRasional

KonsepKonsep ManagemenManagemen SungaiSungai dandan PengendalianPengendalianBanjirBanjir

Silabus

Hidrologi Daerah Aliran Sungai (DAS), Instrumentasi DAS, Pengenalan Investigasisumber air (water source) dengan menggunakanIsotope, Karakteristik sungai, morfologi sungai, pemodelan sungai (model matematik dan model fisik), drainasi perkotaan, pengendalian banjir, bangunan-bangunan sungai.

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 sumberair

Kejadian Banjir Banjir adalah kejadian hidrologi yang dicirikan dengan

debit dan/atau muka air yang tinggi yang dapatmenyebabkan penggenangan dari tanah di sekitarsungai, danau, atau sistem air (water body) yang lain.

Biasanya yang dibicarakan adalah sungai dan saluranyang tidak mampu mengalirkan sejumlah air yang dihasilkan melalui runoff process, dan akibatnya terdapatlimpasan air.

Namun, banjir juga bisa disebabkan olehketidakmampuan air untuk melewati downstream disebabkan oleh muka air yang tinggi pada saluran dihilir.

Kejadian banjir disebabkan oleh hujan yang lama danberintensitas tinggi, kegagalan bendung atau tanggul, gempa bumi, tanah longsor, air pasang tinggi, aktivitasmanusia, termasuk pengoperasian sistem pengendalian

Banjir di Indonesia

Resiko Banjir

Banjir memiliki konsekuensi yang sangat luas terhadapbangunan, ekonomi, sosial dan lingkungan.

Banjir, walaupun jarang terjadi, namun merupakanancaman terhadap masyarakat, dan oleh karena ituberhubungan dengan sejumlah resiko.

Salah satu cara untuk mengetahui resiko tersebutadalah dengan mengadopsi source-pathway-receptormodel.

Source-pathway-receptor model

Source

(hujan)

Pathway

(topografi dan kondisitanah dan sungai)

Receptor

Manusia dan harta benda

Di luar kendali kita

Sedikit banyak masihbisa dikendalikan

Yang paling bisadikendalikan di antaraketiganya

Risk assesment meliputi identifikasi potensial bahayayang dapat menyebabkan kecelakaan maupunkerusakan, dan memperkirakan kemungkinanterjadinya dan konsekuensinya.

Sehingga Resiko banjir untuk suatu komuniti tertentuakan meliputi :

- Probabilitas bahaya banjir di daerah tersebut- Seberapa rentan daerah tersebut terhadap akibat-

akibat 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 banjirdari sisi ekonomi (benefit-cost).

Dapat dijadikan standard nasional untuk membuat skemaperlindungan banjir

Misalnya untuk daerah perkotaan maka bangunan pelindung banjirdirancang untuk banjir dengan kala ulang 50 tahun.

Penentuan kala ulang banjir untuk suatu bangunan berkaitan eratdengan evaluasi ekonomi.

Banjir dunia

Date Location Death

1421 Holland 100,0001530 Holland 400,0001642 China 300,0001887 Yellow River China 900,0001900 Galveston Texas, USA 5,0001911 Yangtze River, China 100,0001931 Yangtze River, China 145,0001935 Yangtze River, China 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 arisingfrom regional climatic and geological processes.

The river channel have adjusted their size and have attained adynamic 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 isresponsible for the dimensions and plan-form of the riverchannel together with the local geology.

At higher less frequent flows the channel has insufficient capacity to contain the flow and water inundates the flood plainsadjacent 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 prolongedand intense rainfall, though occasionally natural embankments, such as created by receding glaciers, canbreak 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 groundinfiltrating down to the local water table or is eventually lostthrough 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 contributesto what is called the fast or direct runoff. As a proportion of the total runoff, the fast componentdepends 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, thecatchment 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 toreach 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, thelatter 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 flatterthe river and the wider the extent of flooding on adjacent floodplains, 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 storeddynamically.

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

LautSungai

Danau

Evaporasi air laut

Evaporasi air sungai

Evaporasi air danau, kolam

Transpirasi

Evaporasi air hujan

Muka air tanah

Aliran air tanah

Mata air

Presipitasi

Infiltrasi

Kondensasi

Aliran air tanah

Aliran permukaan

Eksploitasi sumberdaya lahan yang berlebihan: Perubahan guna lahan : lahan terbuka / hutan, sawah

pemukiman, kawasan industri, dll. tanpa kompensasipengganti resapan akan mengakibatkan kenaikan debit puncaksampai 25 kali.

Misal: Debit Puncak = 10 m3/dtResapan = 5 m3/dt

Resapan

Debit Puncak = 75 m3/dtResapan = 0,5 m3/dt

Akibat perubahan guna

lahanbisa menjadi

Fungsi tanaman penutup lahan

Intersepsi (menangkap & menyimpansementara) f(A)

Evapotranspirasi f(t, A) Memperlambat aliran f(n) Meningkatkan infiltrasi f (t, I)

Meningkatkan limpasanpermukaan

Perubahan hidrograf banjir

Limpasan55%

Daerah pedesaan masih mempunyai cukup simpanan dan retensi

0

10

20

0 30 60waktu (menit)

d

e

b

i

t

(

m

3

/

d

t

)

Limpasan74%

Daerah pengembangan, kapasitas simpanan menurun, limpasan meningkat.Penduduk dan fasilitas meningkat

0

10

20

0 30 60waktu (menit)

d

e

b

i

t

(

m

3

/

d

t

)

Penduduk dan fasilitas meningkat bahkan sampai di daerah rawan banjir.Kapasitas simpanan menurun terus, limpasan meningkat pesat.Terjadi tanah longsor dan banjir

Limpasan89%

0

10

20

0 30 60waktu (menit)

d

e

b

i

t

(

m

3

/

d

t

)

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 defenceinfrastructure

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.

Persamaan Rasional

Qp = 0,00278.C.I.A

Qp = debit puncak banjir, m3/detikC = koefisien limpasan, merupakan fungsi guna

lahan, bervariasi antara 0 1.I = intensitas hujan, mm/jamA = luas daerah tangkapan air, ha

Koefisien limpasan, CDiskripsi lahan/karakter permukaan Koefisien limpasan, C

Business perkotaan 0,70 - 0,95 pinggiran 0,50 - 0,70

Perumahan rumah tunggal 0,30 - 0,50 multiunit, terpisah 0,40 - 0,60 multiunit, tergabung 0,60 - 0,75 perkampungan 0,25 - 0,40 apartemen 0,50 - 0,70

Industri ringan 0,50 - 0,80 berat 0,60 - 0,90

Perkerasan aspal dan beton 0,70 - 0,95 batu bata, paving 0,50 - 0,70

Atap 0,75 - 0,95 Halaman kereta api 0,10 - 0,35 Taman tempat bermain 0,20 - 0,35 Taman, pekuburan 0,10 - 0,25 Hutan

datar, 0 - 5% 0,10 - 0,40 bergelombang, 5 - 10% 0,25 - 0,50 berbukit, 10 30% 0,30 - 0,60

Such formulae are easy to use but contain manyoversimplifications and unrealistic assumptions, whichmake 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 ofoccurrence.

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 ofthe flood is the charges, the selection of the appropriatedistribution and the estimation of its parameters can limitthe reliability and accuracy of the predictions.

These can be supported using unconventional data such as historicalflood reconstruction (modeling), botanical methods (e.g.tree rings), sediment characteristics, etc.

Another way of dealing with the shortage of data on onecatchment is to pool the data within the region using aregional flood analysis.

Normally, this in done by first estimating the flooddistribution and its shape parameters (e.g. a design unithydrograph), estimating the parameters based onobservable physical characteristics of the catchment(area, stream length, etc), and finally determining the mean annual flood using appropriate design rainfall forthe region.

Given sufficient catchments with data in theregion then the variability from basin to basin dominates the sampling errors.

The regional analysis then becomes extremely valuable in estimating floods for ungaugedcatchment and sites.

An estimate of the probable maximum flood (PMF) isimportant for major structures at risk, including damspillways, nuclear power stations and major bridges.

The approach is to maximize the meteorological processesand catchment response and to deduce the maximum,physically feasible flood discharge in conjunction withsuitable 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 withsuitable statistical or conceptual modeling techniques.

The use of conceptual modeling based on knowledge ofexplicit soil moisture accounting is probably one of the most widely used approaches for flood analysis. There aremany conceptual models produced for flood estimation,prediction and forecasting. Generally these modelsaccount for

Rainfall over the catchment

Computation of rainfall based on hydrological abstractions

Routing the rainfall and snowmelt excess over thecacthment

The model can be conceived in either a lumped ordistributed form over the whole of the catchment, andaddress single storm events or a continuous record ofprecipitation (including dry periods). Increasingly, becauseof the complex physical processes, both in terms of poorknowledge of the physics and the associated spatial and temporal distributions of key parameters, more attentionis being given to various forms of data modeling for floodestimation (see other lecture notes).

Many forecasting models are restricted to data driven onlumped conceptual models (perhaps for separate subcatchments) in that the emphasis is more on the updatingprocedure rather than modeling the physical processes inthe catchment in detail (see other lecture notes).Nevertheless, where inventions are being planned in thecatchment 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 thecatchment. Here, models such as MIKE-SHE become veryvaluable tools. These models have now become very sophisticated in their use of remote sensed geophysicaldata 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 thefull or approximate forms of the de Saint Venantequations 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 openchannels, provided the water level across the channel normal to the flow is approximately uniform (horizontal).The channel here can include the flood plain. Difficultiescome when the channel is embanked and water levels onthe flood plain are markedly different from those in the

channel.

In this case,1 D models treating flow on flood plain from astorage of view only can sill be used, or the net work-typeof 1 D model can treat the flow on the floodplains asbeing in separate channels. More generally, 2D models areneeded for this case. Because of their physical basis, thesemodels only require data for calibration and appropriate input data for event or time series modeling.

Physical (laboratory) models can also be used as analternative to mathematical models in certain cases wherethere 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 mayafford

The economic impact of flooding is measured by thedamage to national assets and disruption to normaleconomic activities within the flood-affected area and its surrounding economic links. Individual impacts offlooding 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 experiencestress from the social and personal disruption due to lossof personal items, the damage done by silt, and occasionally large bounders, to building and contents, or the contamination of the silt with sewer discharges. Inhotter 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, andthe failure to receive emergency supplies.

But floods are not wholly bad. They can lead to riverrejuvenation, 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 riverbasin management. In addition, flood alleviation schemescan introduce indirect benefits to the local region such ashigher 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 sidefor 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, thereare significant morphological consequences of flooding incountries like Bangladesh, but again this topic is outside the scope of the course.

HOMEWORK

PELAJARI

ANALISIS HIDROLOGI

ANALISIS FREKUENSI

Manajemen Sungai dan Pengendalian BanjirPart 1Managemen Sungai dan Pengendalian BanjirSilabusBanjir dan Manajemen BanjirHidrologi Daerah Aliran SungaiInstrumentasi DASPengenalan Investigasi sumber airKejadian BanjirBanjir di IndonesiaResiko BanjirSource-pathway-receptor modelBanjir duniaGeneration of floodsSIKLUS HIDROLOGIFungsi tanaman penutup lahanPerubahan hidrograf banjirClassification of types of floods Flood generation analysis Persamaan RasionalKoefisien limpasan, CFlood hydraulicsConsequences of floodingHOMEWORK