Early September 2022, Bengaluru’s IT hub was in grip of one of the worst magnitude of disasters the city had seen in the recent past. Rains of 4th and 5th September were estimated to have been 131.6mm [ IMD Data]. It crippled the city and its infrastructure, causing great hardship and a massive financial loss to all its stakeholders.

The objective of this study is to identify the sources of flooding and its impact on the site. That includes calculating and quantifying the water ingress on site, analyzing existing infrastructure and mapping the same to arrive at a detailed problem statement. Strategies are then derived to tackle these problems either holistically or locally. Also, an attempt is made to cater for the worsening of extreme weather events due to climate change.

The site is affected by ingress of water from 3 sources:

• Fluvial flooding,

• Pluvial flooding and

• Incident rainfall.

Roof rain water incident is currently collected and stored for further usage. These storage tanks can hold up to 29mm rainfall worth of water. Beyond which the water is then runoff on the surface. The rest of site incident water and runoff make their way to the rajakaluve through drains and gaps in the retaining walls.

Pluvial water from the south and east flow through site to make its way into the rajakaluve. The threat arises when the rajakaluve is beyond its capacity and creates a back flow in the 5 meter drain.

Pluvial water from the north is currently meeting the Rajakaluve beyond the site, however, with upcoming development the runoff might flow towards the site. This will be a threat after 51mm of rainfall in a day.

The water in the rajakaluve breaks its barrier and swells into the site beyond 87mm of rainfall in a day. It is after this point, all other interventions for surface runoff on site fail and the site starts to flood.

Proposal for Flood Mitigation _ Strategies

Opportunities

1. To develop a waterfront campus.

2. Increase Bio-resilience of the campus to allow maximum percolation and help mitigate flooding.

These will reduce the water stagnation time on campus while also improving the natural environment quality.

Strategy 1 and Strategy 2 are combinable strategies that can mitigate surface runoff on site and pluvial flooding due to surface water runoff from the Southern and Eastern neighbors. The intent is to capture and percolate water to reduce dependency on the rajakaluve as the sole source of water egress from site.

The interventions include floodable spaces, that can act as water retention reservoirs. On normal (non-precipitation) days, these spaces can be used as landscaped congregation spaces. The Open air theatre acts as a balancing tank during a deluge. It also acts as a visual alert / indicator to initiate operational interventions.

It is proposed to convert all the existing road network into permeable paver blocks. Create bioswales along the road network while retaining the existing foliage. Introduce Rain-gardens at suitable locations.

The interventions proposed are to convert the road & open parking spaces along the Rajakaluve into floodable landscapes that can act as Social spaces during normal days and back up as floodable spaces when the Rajakaluve overflows. This will not only aid in beautifying the campus, but also as sound and odour absorbing spaces. This will also aid in visual alert/ indicator during deluge to initiate operational interventions.

Conclusion

• Rain Gardens:Shallow planted areas that collect clean runoff and allow it to infiltrate into the ground.

• Bioswales:Vegetated channels that slow, filter, and absorb stormwater runoff.

• Vegetation in Bioswales:Helps absorb water, reduce runoff, and enhance urban greenery.

• Bioswales + Recharge Pits:Combined systems that improve infiltration and boost groundwater recharge.

• Permeable Pavements:Pavement surfaces that let rainwater seep through to the ground, reducing runoff.

• Groundwater Recharge Pits:Deep pits that direct filtered stormwater to underground aquifers for groundwater recharge.

• Green Parking:Parking designs using green infrastructure to reduce heat and increase permeable surfaces.

• Storm-water Infiltration Trenches: Narrow ditches with permeable soil that quickly infiltrate rainwater from nearby areas.