Identification of hydraulic instabilities in a storm sewer system – an experimental investigation of Alvaro Chaves Penstock.

Juliana Kaiber da Silva, Rogerio Dornelles Maestri, Ana Luiza de Oliveira Borges

Thursday 2 july 2015

8:45 - 9:00h at South America (level 0)

Themes: (T) Water engineering, (ST) Experimental facilities and instrumentation

Parallel session: 10C. Engineering - Industrial

Between 2005 and 2008, a storm sewer system named Alvaro Chaves Penstock (ACP) was built in Porto Alegre (Brazil) aiming to overcome recurrent floods in main avenues. The system drains a highly populated region (2,8 km2) and expected discharges ranging from 20 to 30 m3/s. The system has an unusual design, combining an on-line detention reservoir with a penstock in the same structure. An unique rectangular cross-sectional area decreases to downstream direction (7,50m with heights ranging from 5,74 m to 2,50m along 112m; 6,25m x 2,50m x 220m; 3,00m x 2,50m x 540 m; 2 conducts of 2,00m x 1,50m x 1060m). Besides, abrupt sharp edges conduit contraction (vertical and horizontal) and 90° bend with a reduction of the rectangular cross-sectional area. Both the reservoir and penstock are unventilated. In February 2013, during a strong rainstorm (nearly 70 years return period considering local IDF curve), ACP has presented a structural collapse. The precast concrete cover was displaced and soil above was removed causing local damages. To study the rupture of penstock, a Perspex-glass partial physical model of the detention reservoir and the short upstream portion of the penstock was built in the Instituto de Pesquisas Hidráulicas of the Universidade Federal do Rio Grande do Sul. The geometric scale factor is 1:15, resulting in a 26 m length, variable height (16cm - 42cm) and width (20cm - 50cm). Considering unusual design options, some research hypotheses concerning flow features were derived, such as: transition instability between open channel flow and pressurized flow, moving hydraulic jumps, translational waves, air entrapment and water hammer. Up to now, with only preliminary tests the initials hypotheses were already confirmed. For a few boundary conditions in preliminary tests with discharges lower than the project discharges, oscillatory and translational waves with moving hydraulic jumps, air entrapment and another problems were observed. A chaotic behavior of the system is also suggested, determined by the initial discharge and water levels (upstream and downstream).