Case Study: Flooding near Bangsar South City

Post date: Jan 14, 2018 1:16:32 AM

What actually happened which causes the water to flood part of the road? Is it because of the earthwork in Bangsar South City?

The nuisance caused have nothing to do with the earthwork based on the aerial view. The silt fence was erected throughout the working perimeter and water and sediment are discharged into a large basin which would cater 10 years ARI storm without the need to clear for at least 6 months as the site is relatively small. Nevertheless the slope stability of the basin is questionable, so as the gradient provided for the basin. The Qpost is contained with lower Q exiting the site. They did well with urban storm water management and erosion and sedimentation control.

Nevertheless, how did water gushed and flood the road? What contributed to such inconvenience to road users and the incompetency of contractor to work with due care?

Drainage for the road is clogged by one of their external work. The sedimentation of clay due to secant retaining wall construction clogged the road drainage system. Most cannot see the secant wall construction but the reinforcement for bored piles and the irregular wall shape close to the road indicate such work. This secant wall is designed to be water tight and local roads are designed to be impermeable. Therefore, water will flow to the lowest point or to areas which have high infiltration rate until the soil starts to saturate.

Isn't the bored piles were meant for building's foundation construction? Yes, but the sequencing allows contractors to start with earthwork first. This early permit/approval can be granted by local councils before completing structural design review.

Based on Street, Drainage and Building Act 1974 Section 70A(4) and Section 86(a), such earthwork permit can be retracted if contractor caused danger and nuisance to the public. Again, the authority to do so is based on discretion of the local council.

Addendum: A reply to press conference by the Minister

1. EIA has little to do with this particular flooding as a whole. MSMA and ESCP submissions are under Drainage and Irrigation Department which forms only part of EIA. The whole bulk of this press conference is on EIA which is not the case of such incident.

2. Either you use a silt trap or a basin to manage silt or clayey soil. This assumption is made based on top soil horizon from soil taxonomy and soil group (HSG).

3. Photos indicate that the contractor have silt fence and sedimentation basin except for working area adjacent to the road. The muddy water is caused by small quantity of clay or mud which clogged the road side drain and not transgressed into the road from the working site. One of the photos i shared yesterday shows that the road level and the formation level of the earthwork.

4. The one characteristic of clayey soil particle when mixed with water is that, clay particles need distance and water height to settle down at appropriate flow speed.With energy produced (from rippling) of car passing by, clay particles will not settle but dispersed and that is causing water to look muddy for a period of time. Apart for that, clay particles will settle down with the use of chemical to assist flocculation processes. 

5. Hydraulic action from ripples acts in a way similar to scouring of the soil just like rainfall which leads to erosivity. Erosivity rate (R) depends on the level of energy and that is why erosivity map comes with kinetic energy - joule value.

6. Submission is shit if liable parties don't practice Best management practice (BMP).

7. Where are the local council or city hall and government agencies when they should send enforcement scouts going around monitoring projects? Enforcement should come and check if the basin was designed accordingly and to check the operation of the wet basin. If the discharge is muddy, it is clear that the contractor did not allow or introduce flocculation of clay particle in order to settle.

The right things mentioned in this press conference are clogged drain and mud (clay). You have missed the issue linking to watertight substructure built too close to the road, if not encroached the reserves. It is important to understand how over consolidated clay affects water movement and displace non-saturated clay which is dry from heavy pumping (yes, there is a dewatering pump in the photo)

** This is an educational article and should be use as it is.