Rain gardens, detention ponds, and permeable paving are all sized around a catchment’s runoff behavior — this tool works through that sizing logic.
| ARI | Intensity (mm/hr) | Peak Q (L/s) |
|---|
| Segment | Length (m) | Slope (%) | Velocity (m/s) | Time (min) |
|---|
| Pollutant | Typical Removal % |
|---|---|
| Total Suspended Solids (TSS) | 85% |
| Total Phosphorus (TP) | 65% |
| Total Nitrogen (TN) | 45% |
| Heavy Metals (Zn, Cu, Pb) | 80% |
| Bacteria / Pathogens | 70% |
How Sustainable Drainage Systems Are Actually Sized
Sustainable Drainage Systems have moved from an optional green feature to a routine planning requirement in most jurisdictions, and understanding how the underlying hydrology calculations actually work is genuinely useful for meeting that requirement with confidence rather than guesswork. This guide walks through the core reasoning.
Why time of concentration is the foundation of every other calculation
Time of concentration, the time it takes runoff to travel from the most hydraulically distant point of a catchment to its outlet, is the foundational input almost every other stormwater calculation depends on, because it determines the design storm duration relevant to that specific catchment, and a smaller, steeper, more efficiently drained catchment will generally have a considerably shorter time of concentration than a larger, flatter catchment with a longer, more meandering flow path, meaning the same rainfall intensity can produce a genuinely different peak runoff response depending on how quickly that particular catchment actually concentrates its runoff at the outlet.
The logic behind the widely used Rational Method
One of the most widely used and internationally recognised approaches for estimating peak runoff from small to medium catchments works on a genuinely intuitive principle, that peak runoff rate is the product of a runoff coefficient, representing the proportion of rainfall that actually becomes surface runoff rather than infiltrating or evaporating, the rainfall intensity relevant to the catchment’s time of concentration, and the catchment area itself. The runoff coefficient is where land use and surface type genuinely matter, since a heavily paved, impermeable surface produces a runoff coefficient close to the maximum possible value, meaning nearly all rainfall becomes runoff, while a well-vegetated, permeable surface produces a considerably lower coefficient, meaning a meaningful proportion of the same rainfall infiltrates into the ground rather than running off, which is precisely the underlying mechanism that makes replacing hard paving with permeable or vegetated surfaces such an effective stormwater management strategy in its own right, before any dedicated drainage feature is even added.
Sizing a rain garden or bioretention area
A rain garden or the larger-scale bioretention basin it scales up into is sized around managing a target volume of runoff from a defined contributing catchment, generally an adjacent area of hard, impermeable surface, by providing enough ponding depth and planted filtration media area to temporarily store and gradually infiltrate or filter that runoff volume rather than allowing it to discharge immediately and at full force into the wider drainage system. The relationship between contributing catchment area and required rain garden footprint is not fixed, but depends on the specific ponding depth and infiltration rate of the planted media specified, meaning a rain garden built with a more free-draining growing medium and a deeper ponding allowance can manage a proportionally larger contributing catchment area within the same footprint than a shallower, less permeable design.
How permeable paving contributes to stormwater attenuation
Beyond the structural considerations covered elsewhere, permeable paving’s sub-base layer can be specifically designed with additional void space to provide genuine stormwater storage capacity, temporarily holding a design storm event’s runoff within that void space before it either infiltrates into the ground below or discharges at a controlled, restricted rate into the wider drainage system, functioning essentially as a distributed storage and attenuation feature integrated directly into the paved surface itself rather than requiring a separate standalone structure.
Sizing a detention pond to meet a target discharge rate
A detention pond, generally the largest-scale stormwater management feature in this toolkit, is sized around a specific target, reducing peak post-development runoff discharge back down to a defined maximum rate, most commonly the pre-development or greenfield runoff rate the site would have produced before any development took place, which is a very common planning requirement precisely because it prevents development from increasing flood risk downstream compared to the site’s undeveloped condition. The required pond storage volume is essentially the difference between the peak inflow volume during a design storm event and the reduced peak outflow volume the pond is designed to release, held temporarily within the pond until it can be released at the controlled target rate without exceeding it at any point during the storm event.
Disclaimer
The information and any results referenced on this page are provided for general educational and concept design purposes only, based on the Rational Method and general Sustainable Drainage System design principles. This is not a certified drainage design, flood risk assessment, or planning compliance document. All stormwater management systems must be designed and verified by a qualified civil or drainage engineer in accordance with local planning conditions, drainage regulations, and design storm data before construction.