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 HYDROEUROPE 2007 1 Prof. Ir. J.J.thisPeters are needed to see picture. VUB Consultant - River specialist
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 HYDROEUROPE 2007 2 Prof. Ir. J.J. Peters VUB Consultant - River specialist
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 HYDROEUROPE 2007 3 Prof. Ir. J.J. Peters VUB Consultant - River specialist
Bolivian Amazon Andes mountains HYDROEUROPE 2007 4 Prof. Ir. J.J. Peters VUB Consultant - River specialist
Pirai river basin HYDROEUROPE 2007 5 Prof. Ir. J.J. Peters VUB Consultant - River specialist
Slopes of the río Pirai, the alluvial fan starts in station 9 (inlet of canal Juan Latino) Middle reach Alluvial fan HYDROEUROPE 2007 6 Prof. Ir. J.J. Peters VUB Consultant - River specialist
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 HYDROEUROPE 2007 7 Prof. Ir. J.J. Peters VUB Consultant - River specialist
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 HYDROEUROPE 2007 8 Prof. Ir. J.J. Peters VUB Consultant - River specialist
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 HYDROEUROPE 2007 9 Prof. Ir. J.J. Peters VUB Consultant - River specialist
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! HYDROEUROPE 2007 10 Prof. Ir. J.J. Peters VUB Consultant - River specialist
Tests by SEARPI of retard structures (to protect the river banks by retarding the flow along these banks) HYDROEUROPE 2007 11 Prof. Ir. J.J. Peters VUB Consultant - River specialist
Destruction La Angostura bridge during 1983 flood HYDROEUROPE 2007 12 Prof. Ir. J.J. Peters VUB Consultant - River specialist
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 HYDROEUROPE 2007 13 Prof. Ir. J.J. Peters VUB Consultant - River specialist
Damages in the City Santa Cruz de la Sierra HYDROEUROPE 2007 14 Prof. Ir. J.J. Peters VUB Consultant - River specialist
Damages in the río Pirai (City Santa Cruz de la Sierra) HYDROEUROPE 2007 15 Prof. Ir. J.J. Peters VUB Consultant - River specialist
Defensive measures: gabion bank revetment Passage through the levee for crossing by wading HYDROEUROPE 2007 16 Prof. Ir. J.J. Peters VUB Consultant - River specialist
Bank revetment with gabion carpets Toe protection length was reduced from 40 to 10 m HYDROEUROPE 2007 17 Prof. Ir. J.J. Peters VUB Consultant - River specialist
HYDROEUROPE 2007 18 Prof. Ir. J.J. Peters VUB Consultant - River specialist
Correcting river bank alignment Have reduced the risk for attack of the bank HYDROEUROPE 2007 19 Prof. Ir. J.J. Peters VUB Consultant - River specialist
Retard structures - permeable groynes Have reduced the velocities along the banks HYDROEUROPE 2007 20 Prof. Ir. J.J. Peters VUB Consultant - River specialist
Retard structures - permeable groynes Have reduced the velocities along the banks HYDROEUROPE 2007 21 Prof. Ir. J.J. Peters VUB Consultant - River specialist
Construction of controlled inundation area In an abandoned branch, to store water and sediment River bank correction Permeable groynes Flow guiding structure Weir - dike HYDROEUROPE 2007 22 Prof. Ir. J.J. Peters VUB Consultant - River specialist
Construction of controlled inundation area Flow guiding structure HYDROEUROPE 2007 23 Prof. Ir. J.J. Peters VUB Consultant - River specialist
Construction of controlled inundation area Evolution of the structure (after one year) HYDROEUROPE 2007 24 Prof. Ir. J.J. Peters VUB Consultant - River specialist
HYDROEUROPE 2007 25 Prof. Ir. J.J. Peters VUB Consultant - River specialist
HYDROEUROPE 2007 26 Prof. Ir. J.J. Peters VUB Consultant - River specialist
Damage by oblique bank attack These aprons in gabions do not resist oblique attack HYDROEUROPE 2007 27 Prof. Ir. J.J. Peters VUB Consultant - River specialist
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 HYDROEUROPE 2007 28 Prof. Ir. J.J. Peters VUB Consultant - River specialist
HYDROEUROPE 2007 29 Prof. Ir. J.J. Peters VUB Consultant - River specialist
Avulsion phenomena in the alluvial fan History of avulsion phenomena in the vicinity of the city of Montero HYDROEUROPE 2007 30 Prof. Ir. J.J. Peters VUB Consultant - River specialist
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 HYDROEUROPE 2007 31 Prof. Ir. J.J. Peters VUB Consultant - River specialist
Avulsion phenomena in the alluvial fan Degradation of the La Madre riverbed (Lane balance) During normal floods, the railway dike worked as a dam HYDROEUROPE 2007 32 Prof. Ir. J.J. Peters VUB Consultant - River specialist
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 HYDROEUROPE 2007 33 Prof. Ir. J.J. Peters VUB Consultant - River specialist
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 HYDROEUROPE 2007 34 Prof. Ir. J.J. Peters VUB Consultant - River specialist
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. HYDROEUROPE 2007 35 Prof. Ir. J.J. Peters VUB Consultant - River specialist
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) HYDROEUROPE 2007 36 Prof. Ir. J.J. Peters VUB Consultant - River specialist
Santa Cruz de la Sierra now HYDROEUROPE 2007 37 Prof. Ir. J.J. Peters VUB Consultant - River specialist
Location where the river left its course (Juan Latino, 1990) HYDROEUROPE 2007 38 Prof. Ir. J.J. Peters VUB Consultant - River specialist
Location where the river left its course (Juan Latino, 1992) HYDROEUROPE 2007 39 Prof. Ir. J.J. Peters VUB Consultant - River specialist
HYDROEUROPE 2007 40 Prof. Ir. J.J. Peters VUB Consultant - River specialist
HYDROEUROPE 2007 41 Prof. Ir. J.J. Peters VUB Consultant - River specialist
Flood water drained to Montero treatment plant (1992) HYDROEUROPE 2007 42 Prof. Ir. J.J. Peters VUB Consultant - River specialist
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 HYDROEUROPE 2007 43 Prof. Ir. J.J. Peters VUB Consultant - River specialist
Erosion at the main road HYDROEUROPE 2007 44 Prof. Ir. J.J. Peters VUB Consultant - River specialist
Destruction of one of the road bridges HYDROEUROPE 2007 45 Prof. Ir. J.J. Peters VUB Consultant - River specialist
Reconstruction of the closure works in Juan Latino HYDROEUROPE 2007 46 Prof. Ir. J.J. Peters VUB Consultant - River specialist
HYDROEUROPE 2007 47 Prof. Ir. J.J. Peters VUB Consultant - River specialist
HYDROEUROPE 2007 48 Prof. Ir. J.J. Peters VUB Consultant - River specialist
Reconstruction of the closure works in Juan Latino HYDROEUROPE 2007 49 Prof. Ir. J.J. Peters VUB Consultant - River specialist
Avulsion of the Pirai in La Madre HYDROEUROPE 2007 50 Prof. Ir. J.J. Peters VUB Consultant - River specialist
HYDROEUROPE 2007 51 Prof. Ir. J.J. Peters VUB Consultant - River specialist
Avulsion of the Pirai in La Madre La Madre Pirai HYDROEUROPE 2007 52 Prof. Ir. J.J. Peters VUB Consultant - River specialist
Collapse of the railway bridge in La Madre river HYDROEUROPE 2007 53 Prof. Ir. J.J. Peters VUB Consultant - River specialist
Railway bridge useless above earlier Pirai riverbed HYDROEUROPE 2007 54 Prof. Ir. J.J. Peters VUB Consultant - River specialist
Splay phenomenon in the new canal Juan Latino HYDROEUROPE 2007 55 Prof. Ir. J.J. Peters VUB Consultant - River specialist
HYDROEUROPE 2007 56 Prof. Ir. J.J. Peters VUB Consultant - River specialist
Formation of meanders in the new canal Juan Latino HYDROEUROPE 2007 57 Prof. Ir. J.J. Peters VUB Consultant - River specialist
HYDROEUROPE 2007 58 Prof. Ir. J.J. Peters VUB Consultant - River specialist