2.1.8 On-Site Implementation of Sponge toolkits

SPONGE COLLABORATIVE + WEAVING WITH WATER Team

MULLASSERY CANAL FRAMEWORK AND CANAL EDGE MASTERPLAN

Figure 36 - Section across AK Seshadri road illustrating how the components of Sponge Framework perform during 70% increase in Annual rainfall volume (Extreme Cloudburst event) (Image Credit: Sponge Collaborative)

Extreme Cloudburst Scenario with runoff overwhelming designed gray infrastructure capacities (~70% of Annual Rainfall Volume) During extreme rainfall events larger open spaces like playgrounds and plazas are needed to capture all the run-off from the larger catchment area and temporarily hold the water and delay the run-off. Thus preventing water clogging on adjacent streets and from burdening the existing stormwater pipes. These spaces are placed adjacent to other blue green infrastructure and are connected to main stormwater drains of the city. These large open spaces play a critical role in flood mitigation during storm events accompanied by high tide when the eciency of the canal and stormwater drains is highly reduced. Thus once the storm reduces these central spaces safely convey the water into the drains thus reducing huge property damages during flash floods.

2.1.8 On-Site Implementation of Sponge toolkits

Figure 37 - Diagram illustrating sponge streets suitability matrix and variables involved in implementing the components of sponge street typology (Image Credit: Sponge Collaborative)

SPONGE COLLABORATIVE + WEAVING WITH WATER Team

MULLASSERY CANAL FRAMEWORK AND CANAL EDGE MASTERPLAN

For all Sponge Street interventions, their respective capacities to handle stormwater runoff should be based on the contributing drainage area and the hydrologic group of the in-situ soil. Runoff reductions will be higher (upto 20%) for USDA soil categories of A and B and upto 10% for soil categories C and D. In most urban conditions, the soil within Sponge Street systems have to be engineered to meet the infiltration potential requirements. The layout of Sponge Street systems should ensure that the contributing drainage areas into the inlet points are evenly distributed. Sponge Street systems should be designed to handle 10 year flood events. Water flow path along Bioswale channels should be designed to maximize the time water spends in the swale. Sidewalk planters or rain gardens require a 0.75m - 1.2m deep planting soil bed, a surface mulch layer, and a 0.3m deep surface ponding area where the ponding area is calculated based on the size and porousness of the contributing drainage area. Tree Pits can meet local drainage if the landscape infrastructure planning considers the storage capacity of the soil voids in the cavity created for the root ball of the tree and the ponding area. The infiltration of the in-situ or engineered soil must be a minimum of 50mm per hour. The suitability matrix and key stakeholders diagram below suggest ideal sites and the network of institutional and financial collaborations needed to realize Sponge Streets.

Figure 38 - Diagram illustrating sponge open spaces suitability matrix and variables involved in implementing the components of sponge open space typology (Image Credit: Sponge Collaborative)

Sponge Open Spaces are the most compelling landscape infrastructure typologies in terms of their impact on improving the public realm and the habitat potential of the city. Existing open spaces can be converted into Sponge Open Spaces with strategic regrading efforts, planting designs, and co-ordination with the existing stormwater network. The resulting transformation can create enormous political and community goodwill while improving the resilience of urban neighbourhoods. As such, Sponge Open Spaces should be prioritized as pilot projects when possible. Siting Sponge Open Spaces and determining the typology