3 rd Dec’24
By: Pradeep CS, Aun Abdullah
Stormwater management is essential when developing cities or designing buildings, especially in tropical regions like Mumbai. Each building or a neighborhood typically has a stormwater drainage system that connects to a nearby stream or municipal drain. This setup is crucial for carrying away rainwater and preventing local waterlogging during heavy rains. Let’s break down how this works, especially in the context of Mumbai.
In Mumbai, all buildings are connected to the municipal drainage system. The internal drainage of a plot is designed to handle rainfall intensity, which is traditionally set at 70 mm/hour[1]. One key design consideration is ensuring that the discharge point from the property is positioned higher than the High Flood Level (HFL) of the municipal drainage system or any nearby water body, such as a stream. This elevation is critical in preventing water from flowing back into the property during heavy rains. The HFL is typically estimated based on flood observations or existing flood markers, often placed on bridge piers by government authorities.
To meet this requirement, site levels may need to be raised. However, raising the ground level can have unintended consequences, especially in areas where it could exacerbate regional flooding. This requires careful planning and appropriate mitigation measures to avoid contributing to larger-scale flooding, while also considering the social and environmental impact of such changes.
While there may be a natural inclination to quickly discharge stormwater from the property, this approach is not ideal for storm water drainage. Rapid discharge places considerable strain on the municipal drainage system, which is already struggling to handle the increasing intensity of rainfall caused by climate change. A more sustainable solution would involve slowing the flow, enhancing water percolation, or retaining rainwater for future use. However, for smaller projects, implementing these measures can be challenging, especially when a larger proportion of the site consists of impermeable surfaces with high runoff. This makes it difficult to ease the burden on existing drainage infrastructure.
Return Period / Frequency noted above is a crucial factor in Storm Water Drainage Design. While local authorities often do not specify a return period, CPHEEO guidelines typically recommend using a 2-year or 5-year return period for stormwater drainage (SWD) design, it is the probabilistic measure of high intensity rainfalls that can happen in the future. However, when determining the formation level based on the Highest Flood Level (HFL), it’s essential to carefully select the return period. Opting for a higher return period can sometimes significantly affect the financial viability of the development due to the grading and mitigation costs for the plot.
In even larger projects, or city scale developments such as Palava, we must also consider that water tends to remain on-site longer as the project size increases. This is due to the longer distance the water needs to travel before it exits the property. This brings us to the concept of time of concentration—the time it takes for water from farthest catchment to flow through the drainage system to the outfall. The longer the water stays on the property, the larger the drain needs to be. Also by making multiple discharge points, the sizes of the outfall drains can be minimised avoiding the flood risk (backflows into the property)
Recently, we’ve begun using forward-looking data based on Global Climate Models aligned with the IPCC’s SSP2[3] scenarios. While one can debate on the level of accuracy of these models, they offer better directional insights into future climate conditions, helping us build resilience against changes that historical data may not fully capture. A sample snapshot below:
Frequency precipitation using Gumbel distribution: The Gumbel theory of distribution is the most widely used distribution for IDF analysis owing to its suitability for modelling maxima. It is relatively simple and uses only extreme events (maximum values or peak rainfalls). The Gumbel method calculates the 2,5, 10-, 25-, 50- and 100-year return intervals for each duration period and requires several calculations.
Frequency precipitation ꭕt (in mm) for each duration with a specified return period T (in year) is given by the following equation:
ꭕt = Mean + SD * Kt
where, Mean is the average of the maximum precipitation corresponding to a specific duration. SD is the standard deviation of the maximum precipitation corresponding to a specific duration.
Kt is Gumbel frequency factor given by:
Kt = (Yt – Ῡn)/ Sn
Estimation of Rainfall Intensity (mm/hr.): The frequency factor (Kt), which is a function of the return period and sample size, when multiplied by the standard deviation gives the departure of a desired return period rainfall from the average. Then the rainfall intensity, It (mm/hr) for return period T is obtained from:
It = ꭕt / Td
With the ever-increasing impacts of climate risks, the management of urban flooding has become a critical issue in urban resilience. It must be addressed holistically and through a multi-disciplinary approach. Recently, we have witnessed the effectiveness of early warning systems in managing flood risks. Early warnings are particularly valuable in cases of flash floods, which have become more frequent in the Himalayan rivers and their tributaries. Similarly, such systems are crucial in mitigating flooding caused by cyclones and storm surges in coastal areas.
The strategies outlined in this note can assist designers in ensuring a resilient stormwater network for managing urban storm water. This is particularly important as municipal drains have limited capacity to handle increasing storm intensities, which are also being exacerbated by climate change.
[1] In response to the 2005 floods, a committee led by Dr. R. R. Chitale recommended the 70 mm/hour standard for stormwater design in Mumbai. One must note that most existing storm water drains in Mumbai were designed during the British era and cater to much lower rainfall intensities, many being under 25 mm/hour.
[2] Decision of type of rainwater harvesting system shall be made considering ground water level fluctuations, shallow ground water levels, rising /declining trend of groundwater. Shallow ground water levels might affect the drainage system.
Hydrogeological investigations play a vital role in the decision making process of rainwater harvesting scheme
[3] IPCC: Intergovernmental Panel for Climate Change, SSP2: Shared Socioeconomic Pathway - Middle of the Road