Compliance to GRIHA Criteria 4 – Site Imperviousness Factor
GRIHA (Green Rating for Integrated Habitat Assessment) is a national rating system for green buildings in India. Compliance with the GRIHA criteria is mandatory for all new government buildings and is recommended for all other types of buildings. In this blog article, we will discuss the step-by-step procedure to comply with the GRIHA Criteria 4 – Site Imperviousness Factor.
The intent of this criterion is to ensure implementation of site design measures which assist in the reduction of the overall site imperviousness factor. High imperviousness on site leads to rapid runoff of rainwater, reduces urban rainwater recharge and contributes to conditions of urban flooding. Therefore, it is necessary to take appropriate measures to reduce the site imperviousness factor.
Appraisals:
There is only one appraisal for this criterion, and it states that if the net imperviousness factor of the site meets the NBC 2005 norms, and the site is designed such that post-construction stormwater discharge from the site is zero, then the project is eligible for 1 point.
Net Imperviousness Factor:
To calculate the net imperviousness factor, we need to know the runoff coefficient for various surfaces and the effective impervious area of each surface. The table given below provides the runoff coefficients for various surfaces along with the surface area and effective impervious area of each surface.
| Surface Type | Runoff Coefficient | Surface Area (sq.m) | Effective Impervious Area (sq.m.) |
|---|---|---|---|
| Roof | |||
| Conventional | 0.95 | 6,050 | 5,747.5 |
| Roof garden < 100 mm thick | 0.5 | 0 | 0 |
| Roof garden 100 – 200 mm thick | 0.3 | 0 | 0 |
| Roof garden 200 – 500 mm thick | 0.2 | 0 | 0 |
| Roof garden > 500 mm thick | 0.1 | 0 | 0 |
| Impervious Paving on site | |||
| Concrete/ Asphalt/ Kota Paving | 0.95 | 22,000 | 20,900 |
| Gravel | 0.75 | 5,234 | 3,925.5 |
| Brick Paving | 0.85 | 0 | 0 |
| Vegetation | |||
| Slope 0-1% | 0.1 | 21,622.58 | 2,162.258 |
| Slope 1% to 3% | 0.2 | 0 | 0 |
| Slope 3% to 10% | 0.25 | 0 | 0 |
| Slope > 10% | 0.3 | 0 | 0 |
| Turf slopes | |||
| 0% to 1% | 0.25 | 1,850 | 462.5 |
| 1% to 3% | 0.35 | 0 | 0 |
| 3% to 10% | 0.4 | 0 | 0 |
| >10% | 0.45 | 0 | 0 |
| Total Impervious Effective Area (sq.m.) | 56,757 | 33,197.758 | |
| Net Impervious Factor (%) | 58.49082081 |