Why closed storm water drain is not a good idea?
Storm Water drains on either sides of the road in residential layout that usually run parallel to a road and feature long, uncovered channels to carry free-flowing storm water, that are made using stone slabs, built at level lower to the road & were left open allowing the rain water to flow into drains easily.
In recent years, BBMP (Bruhath Bengaluru Mahanagara Palike) have been redoing the open storm water drains with RCC based drains covered with RCC Slabs with fewer or no holes for water to enter the drains from the road.
In this story we will review how the new storm water designs could be causing the flash floods, water logging in residential layouts of Bangalore.
Rainfall & Water Runoff
Rapid urbanisation of cities has left very little ground space to soak the failling rainwater as shown in drawing below more than 95% of the water falls on building, asphalted roads, footpaths etc., that should flow into road side storm water drains that needs to flow into large storm water drains and flow into watershed areas / lakes.
If either the stream network from road / rooftop water > storm water drains > watershed areas are not designed well or the rainwater doesn’t discharge properly into drains, this can lead to flooding or temporary water logging causing inconvenience for people to commute.
Cause for flooding
In Chapter 5 of Tender S.U.R.E (Specification for Urban Road Execution) 2012 report talks about Urban Drainage & Flooding, that explains the consideration taken for measuring the peak rainfall, number of openings to storm water drains & width / height of the same. BBMP seem to have selectively taken the design of closed storm water drains from Tender SURE roads (done in Central Bangalore) & left out the important guidelines for opening into storm water drains.
BBMP should revise the guidelines to contractors, to provision a hole of at least 3-4 feet wide for every 15 feet of the road, it seems doing such wide openings should not cause any increase in construction (it could rather reduce, as we will not need a few curb stones at these openings). Wide openings along with rain water harvesting pits (as mentioned in section below) every 30ft should help reduce the amount of water that stays on road should reduce.
Closed vs Open Storm Water Drains
Table below shows the comparison of various factors between a closed & open storm water drains
Let’s analyse what happens in peak rainfall condition — there is a lot of water on buildings, roads that is trying to get into storm water (needless to say, small openings will cause lot of water to remain on road for a while), if the storm water drains remained non-concrete based built using stone slabs, a good percentage of water would have been sucked in the gaps by soil, acts as rain water harvesting. Ok, we agree the new closed drains are good (since its done in western countries) — we should have at least planned a rain water harvest pits every 20–30feet of the drain? a good depth of open area below the drains that would absorb some rain water.
We can barely see any benefits of doing the closed drains, other than walkable area for pedestrians in roads that don’t have footpath & an attempt to make the neighbourhood look beautiful by building closed drains.
BBMP seems to be just wasting tax payers money in the name of city development (beautification) that neither remains in the same form after a few years as lot of the closed drains get damaged at the time of house construction.
BBMP is not thinking from a long term, good solution perspective, just trying to put a bandage every monsoon season around the flooded area, Is it possible to review the guidelines for gutter / storm water drains design & take corrective measures.
