Storage-based + source-control measures for urban drainage — detention basins (temporary storage with controlled release), retention ponds (permanent water body), Sustainable Urban Drainage Systems (SUDS): permeable pavement, bioswales, rain gardens, green roofs, blue-green infrastructure. Volume sizing, design rules, AMRUT 2.0 + Smart Cities incentives, integration with urban landscape.
Size a detention basin to attenuate peak inflow Q_in (from chapter 3 Rational Method) down to an allowable outflow Q_out (downstream pipe capacity OR pre-development discharge). Uses simplified triangular hydrograph approximation; refine with SWMM/MIKE for large catchments.
Detention, retention, + SUDS represent the modern paradigm shift in urban drainage — from 'collect + convey + discharge' to 'slow + spread + infiltrate'. The CPHEEO 2019 manual + AMRUT 2.0 + Smart Cities Mission all push this approach.
Why the shift: Sizing pipes for full peak flow is expensive, and the runoff still ends up in the receiving water body (often as polluted first-flush). Detention + SUDS reduce peak by 30-50 % (saving downstream capacity), improve water quality (treating first 25 mm), recharge groundwater, + provide urban amenity (parks, ponds, gardens).
Detention basin = temporary storage that drains down between storms. Sized to limit peak release to pre-development discharge OR downstream system capacity, whichever lower. Outlet = orifice (slow drain) + overflow weir (emergency). Drain-down time 12-48 hours so basin is available for next storm. Cost: ₹500-1500/m³ storage.
Retention pond = permanent water body (urban lake / pond / wetland). Captures runoff into the wet body. More expensive than detention, but doubles as amenity + bird habitat + groundwater recharge zone. Smart Cities Mission likes these for visibility.
SUDS (Sustainable Urban Drainage Systems) is the umbrella for distributed source-control:
- Permeable pavement: porous concrete or interlocking blocks let water infiltrate. Works in sandy subgrade; struggles in clay. Requires periodic renewal (5-10 yr).
- Bioswale: linear vegetated channel captures + treats first 25 mm. Best for parking lots, road medians, parks.
- Rain garden: small landscaped depression captures rooftop + driveway runoff. ₹15-50K per Indian villa.
- Green roof: vegetated roof retains 25-75 mm rainfall. Structural loading is the constraint.
Rainwater harvesting is mandatory in most Indian cities per municipal building bye-laws — typically for plots > 300 m². 5-15 m³ underground sump + recharge pit, ₹50K-2L per house. Penalties for non-compliance vary; enforcement is often weak but tightening.
The financial case: AMRUT 2.0 offers up to 25 % capital subsidy for SUDS components in approved master plans. Smart Cities Mission funds blue-green infrastructure separately. Combined, SUDS often becomes cheaper than equivalent grey infrastructure — and the maintenance burden shifts from municipal pipe network to landscape O&M (which has its own challenges, but distributable).
The reality check: SUDS requires inter-departmental coordination (drainage + parks + horticulture + planning) that many ULBs lack. Permeable pavement requires sweeping + jetting that many municipalities don't budget for. Green roofs require structural design + waterproofing expertise. SUDS is proven globally + scaling rapidly in India, but execution is uneven.
Where this chapter sits: SUDS is the modern alternative to grey-only conveyance. Evaluate SUDS FIRST (per CPHEEO 2019 clause), then size grey infrastructure for the residual peak. Yields cheaper, more resilient, more attractive cities — when implemented with adequate O&M commitment.