Proceedings of the 1st International Congress on Hydroclimatology pages:12 p.
International Congress on Hydroclimatology edition:1 location:Cochamamba, Bolivia date:24-28 August 2009
Santa Cruz is located on the east of Bolivia. It has more than 1.5 million inhabitants. The city is built along the
Piraí River; which has tremendous discharge variability and is part of the Amazonas basin. Santa Cruz is protected
by structures and in some areas by secondary structures as well.
The flood of March 1983 partially destroyed Santa Cruz; as a result 100 persons died and close to 900 were declared as disappeared. The cost of the damages in infrastructure, housing and agricultural lands reached 37 million US dollars (Peters, 2000).
The Piraí is a braided river and it is wider than deeper, sub divided into several branches. The width goes from 50
to 500 metres, the river basin elevation ranges from 2600 m.a.s.l. to 250 m.a.s.l. The study area covers the subbasin from La Angostura till La Belgica (90 km). Between La Angostura and Espejos the river is still geologically controlled by material from the Tertiary or by colluviums. From La Espejos to Santa Cruz, the river loses its geological control and in some places it is prone to avulsion.
This paper analyses the flow dynamic process and the quantification of the peak discharges by using a hydrodynamic model (HEC-RAS). Flooding in Piraí River obeys entirely to flash floods and therefore unsteady flow has to be simulated. It was done by using derived hydrographs from measured hourly water levels as boundary conditions.
A hydraulic model of the river is built to calculate inundated areas, evaluate critical sites and calculate water
depths. Three historical events were selected for the present study: January 1986, January 1988 and March 1988. It
was found that the roughness coefficient decreases with increasing water level and flow. The calibrated Manning
coefficients range from 0.035 to 0.0175 for the main channel. The computational time step of the model is 5 seconds and the distances between cross-sections is in average 50 m.
Due to the lack of a complete inflow input data or hourly rainfall-runoff models calibrated; only 60% of the basin
area was considered, and therefore the results show underestimations in the determination of the peak flow in La
Belgica station. However, the shape and the time of occurrence of the peak flow were well represented.