Pirai river (Bolivia) - · PDF filePirai river (Bolivia) ... new types of hydraulic structures to control the stability of the river course, ... permeable “groynes”

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are needed to see this picture.HYDROEUROPE 2007 1 Prof. Ir. J.J. Peters | VUB | Consultant - River specialist

Pirai river (Bolivia)

Confluent of the Amazon river which average discharge is only 6 m3/s, but with peak discharge over 5000 m3/s, a challenge for river basin management

and for flood control



How the project began • The Pirai river experiences regularly flash floods because of its

location along the edge of the Andes (river basin ranging from 2600 m to 250 m)

• Erosion in the upper catchment produces a lot of sediment that

deposits in the lower catchment as an large alluvial fan where the river course changes very often

• The area of the alluvial fan is an important agricultural area with a

relatively (and unimportant …) city of Montero • The German Cooperation had a long-term project for designing

new types of hydraulic structures to control the stability of the river course, especially in the alluvial fan



How the project began • In 1983, a flash flood occurred in the Pirai river basin which

inundated the (rich part) of the (rich) city of Santa Cruz de la Sierra, the most important economic centre of the country

• The Bolivian Government requested the assistance of the

European Commission for building flood defences along the city (the emergency phase) and to set up a master plan (the long-term management phase), project accepted for financing in 1986

• In 1987, a consultancy consortium presented the plans for the city

defences at a cost = 220% the available budget … based on modelling and theoretical formulas for riverbed scouring



Andes mountains

Bolivian Amazon



Pirai river basin



Slopes of the río Pirai, the alluvial fan starts in station 9 (inlet of canal Juan Latino)

Middle reach Alluvial fan



A “turbion” in 1992, with velocities higher than 6m/s A meandering flow in a braided river, with very high velocities in only a part of the cross section



A very special river … • The flash floods start from an almost empty river till discharges of

the order of thousands cubic meters per second in less than one day

• The riverbed is very wide (300 - 500 meters), with cross-sections

almost horizontal (bed level variation less than 1 m) • The riverbed material observed during dry periods is very fine,

about 0.3 mm, what is exceptional for a river with such strong slopes (> 3 m/km), but under the top layer are very coarse materials



Modelling this very special river … • The first 1-D modelling for designing the flood defences required

the use of extreme flow resistance coefficients • A second modelling for the Master Plan with a different numerical

model also needed “roughness” smoother than glass … • Nobody could understand, but everybody (or almost) was happy

because it had been possible to ‘calibrate’ the models • Observing the river’s behaviour, the damages caused by the floods

and the peculiar composition of the bed sediments, we suspected a strong reworking of the river bed during the floods



Modelling this very special river … • We designed a system to measure the scouring (erosion) of the

riverbed during a flood: coring (drilling tubes) deep into the bed, removing the sediment and replacing it by an artificial sediment (moulded bricks) coloured in red and wait for the flood

• The idea was that the coloured material would be washed away

down to the deepest scouring depth • Results from significant (but not extraordinary) floods in 1992 gave

fantastic results: the riverbed eroded locally to depths of more than 8 m

• The change in cross-section geometry makes that water level do not rise very much … Modelling the floods with an adapted geometry did not require any adjustment of the “roughness” coefficients!



Tests by SEARPI of “retard structures” (to protect the river banks by retarding the flow along these banks)



Destruction La Angostura bridge during 1983 flood



The role of the Project Adviser … • The first task was to establish a potamological diagnostic • The results of this was that the inundation had not been provoked

by overtopping of the banks, but by outflanking of a channel • This outflanking had been due to the deflection of the flow by the

shape of the opposite bank, itself caused by bank protection structures more upstream

• The consultant had proposed to build 3 large groynes (big

structures), but we instead suggested to correct the shape of the opposite bank, which was done successfully at almost no cost …



Damages in the City Santa Cruz de la Sierra



Damages in the río Pirai (City Santa Cruz de la Sierra)



Defensive measures: gabion bank revetment Passage through the levee for crossing by wading



Bank revetment with gabion carpets Toe protection length was reduced from 40 to 10 m





Correcting river bank alignment Have reduced the risk for attack of the bank



Retard structures - permeable “groynes” Have reduced the velocities along the banks



Retard structures - permeable “groynes” Have reduced the velocities along the banks



Construction of controlled inundation area In an abandoned branch, to store water and sediment

River bank correction

Permeable groynes

Weir - dike Flow guiding structure



Construction of controlled inundation area Flow guiding structure



Construction of controlled inundation area Evolution of the structure (after one year)







Damage by oblique bank attack These aprons in gabions do not resist oblique attack



Avulsion phenomena in the alluvial fan • The Pirai river has always developed new courses close to the city

of Montero that grew quickly due to increased agricultural activities in the area with fine, rich sediments

• Construction of a road and a railway perpendicular to river courses

and drainage system induced upstream sediment retention of sediments (according to Lane balance)

• Already in 1973, the river had abandoned its bed and was diverted

close to the city of Montero • A project of the German cooperation brought the river back almost

in its previous course





Avulsion phenomena in the alluvial fan

History of avulsion phenomena in the vicinity of the city of Montero



Avulsion phenomena in the alluvial fan Impact of the construction of the road Santa Cruz de la Sierra to Cochabamba and of the Beni railway



Avulsion phenomena in the alluvial fan • Degradation of the La Madre riverbed (Lane balance) • During normal floods, the railway dike worked as a dam

…



Avulsion phenomena in the alluvial fan • … and produced a breach in the dike • The Pirai flow followed then the La Madre river course, producing

even more regressive riverbed degradation up to the road



Avulsion phenomena in the alluvial fan • An extraordinary flood produced an avulsion in an upstream bend,

with formation of a kind of a “delta”, upstream of the road dike …



Avulsion phenomena in the alluvial fan • The excavation of an artificial course in the Juan Latino canal

brought the Pirai river back almost in its previous course, although it had insufficient hydraulic capacity for major flood discharges.



What did we learn from this project? • Inundations are not only due to water levels, also attack and

erosion of river banks can be very dangerous (happened in the Var river in 1983 in the Industrial Zone of Carros)

• Modelling is usually made with the geometry measured after the

flood, not during it; the geometry may change a lot but how do we take this into account in the modelling (I have proposed for the Var experiment similar to the one in the Pirai, but this has been considered as not feasible …)

• Preventive and recurrent measures are often more effective and

economic than heavy engineering (but decision makers don’t like them because they prefer inaugurate big works)



Santa Cruz de la Sierra now



Location where the river left its course (Juan Latino, 1990)



Location where the river left its course (Juan Latino, 1992)







Flood water drained to Montero treatment plant (1992)



Drainage to La Madre (1992) • The small canal from the Montero

treatment plant became the Pirai river and destroyed de road bridges (old and new ones)

• The flow, free of sediment (retained by the sedimentation upstream from the road dikes) eroded even more the La Madre riverbed

• River works were build to bring the Pirai river in its previous course



Erosion at the main road



Destruction of one of the road bridges



Reconstruction of the closure works in Juan Latino …







Reconstruction of the closure works in Juan Latino …



Avulsion of the Pirai in La Madre





Avulsion of the Pirai in La Madre

Pirai La Madre



Collapse of the railway bridge in La Madre river



Railway bridge useless above earlier Pirai riverbed …



“Splay” phenomenon in the new canal Juan Latino





Formation of meanders in the new canal Juan Latino



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Pirai river (Bolivia) - · PDF filePirai river (Bolivia) ... new types of hydraulic structures to control the stability of the river course, ... permeable “groynes”