2020-12 - Urban Flooding due to Slope Drain Failure

Over-topping of slope drain for super-critical flow with: (i) 45-deg bend; and (ii) presence of two trash racks is simulated by OpenFOAM. This animation demonstrates how the presence of bend and obstruction in steep slope drain can result in significant over-topping.
This simulation is for a real flooding case at Smithfield, Kennedy Town in Hong Kong back in 2005. The failure of slope drain caused significant amount of stormwater overflowing down the slope and caused severe flooding and disruption to traffic at Smithfield which is a very steep road. The webpage from the University of Hong Kong (HKU) provides some background information below.
www.civil.hku.h...
Based on HKU physical model results, two deficiencies are identified resulting in failure of this slope drain:
(i) Two trash racks with frame made of 75mm rectangular section
(ii) Insufficient length of covered section downstream of 45-deg bend
For (i), it is the main reason of stormwater over-topping. Size of this stepped channel is 1.5m(w)x1.6m(h), and the frame of trash rack was made of 75mm rectangular section. Size of the frame is small compared to the channel cross-section. It surprises me that the top horizontal bar of the trash rack frame can split and cause serious overshooting of flow when approach flow is super-critical flow during that rainstorm event.
For (ii), it is a relatively minor reason of over-topping. Super-critical flow does not follow bend in the channel resulting in over-shooting. Although channel downstream of the bend is covered by concrete cover, its length is not long enough to confine spiral flow and still resulting in some over-topping.
It is encouraging that CFD results concur with physical model results, and this gives us more confidence to use CFD to investigate this type of complex flow problems.
Two improvement works were recommended by HKU to rectify the stepped channel, and no flooding is reported afterwards.
(i) Replace frame of trash rack by 16mm dia. steel bar
(ii) Increase length of the covered section after 45-deg bend
Hong Kong has a hilly terrain, and there are still flooding at other locations, such as Repulse Bay Road, Tai Tam Road and Shek O Road, which are located adjacent to slope. Flooding at these location is probably also caused by failure of slope drain, and CFD is the powerful tool to tackle these problems.

[@martinfeliperamirezmunoz]

Hi eddy, great work

[@CFD-EddyLau]

Thank you! Cheers!