Selecting the design return period for urban drainage
Updated: Jul 31
Selecting a design return period is, in essence, accepting the flood risk level for the urban area. The chosen criterion may reflect standards established by a municipality or by national regulations. In Brazil, for example, federal funding programs generally require a minimum 25-year design return period, although no explicit regulation formally mandates this level of protection. Drainage literature also provides recommended return periods based on watershed size and the consequences of flooding, but these recommendations remain largely qualitative.
Selecting a lower level of acceptable risk—that is, designing for longer return periods—inevitably increases the capital investment required for stormwater infrastructure. Such investments must be supported by a sustainable funding mechanism, which remains one of the sector's greatest challenges. Although Brazilian legislation provides for cost recovery, municipalities rarely have effective mechanisms to finance either capital expenditures (CAPEX) or operating expeditures (OPEX). In Porto Alegre, the Urban Drainage Master Plan adopted a conceptual design based on a 10-year return period for each of the city's 27 urban watersheds. Estimated investments ranged from approximately US$1.5 million/km² for solutions relying primarily on detention storage facilities to US$4.5 million/km² where conventional conduits and channels were required. Higher design return periods (low risk) increase these costs substantially.
Design storm characteristics vary considerably across a country the size of Brazil. Using the 10-year, one-hour design rainfall as a reference, rainfall intensity generally increases toward the Equator. In Porto Alegre (approximately 30°S), the corresponding rainfall intensity is about 52 mm/h, whereas in Teresina it reaches 78 mm/h. Comparable values are observed in other tropical cities, including Panama City (80 mm/h), Dhaka, Bangladesh (78 mm/h), and Accra, Ghana (80 mm/h). Under these conditions, peak discharges may nearly double, making projects designed for the same return period significantly more expensive and, in some cases, economically unsustainable.
A practical way to define the design return period is through a benefit–cost analysis. By combining the reduction in flood damages associated with increasing the return period with the corresponding cost of flood-control infrastructure, it is possible to identify an economically optimum return period (T*). This point represents the minimum value of the total annual cost curve, which combines construction costs and residual flood damages. Beyond T*, the marginal reduction in damages becomes smaller than the additional construction cost, meaning that further investment is no longer economically justified. For urban drainage systems, this optimum is typically around a 10-year return period, when residual damages have generally been reduced to approximately 10–20% of their original value.
A project developed in Teresina provides a practical example of this approach. Because rainfall intensities in the city are considerably higher than in southern Brazil, the estimated construction cost was exceptionally high. Since the project relied on federal funding, a 25-year return period was required. Our solution was to adopt a 10-year design return period while verifying system performance under the 25-year event, ensuring that floodwaters would not reach private properties. This approach allowed part of the inherent design safety margin to be used during verification, resulting in a meaningful reduction in project costs.
Ultimately, selecting a design return period means defining the level of flood risk a community is willing to accept, and that decision has a direct influence on the total investment required for urban drainage infrastructure. Once municipalities establish dedicated funding mechanisms for both CAPEX and O&M—or secure stable public funding sources—the definition of the economically optimum return period (T*) will become an even more important decision-making criterion for allocating investments efficiently.
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