Which of the following provides a safer stormwater design to avoid flooding while protecting the environment?

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The selection of a 100-year storm design frequency combined with a retention period of 120 minutes offers a more robust stormwater management strategy to mitigate flooding risks while enhancing environmental protection.

Designing for a 100-year storm frequency means that the system is built to handle severe rainfall events that statistically have a 1% chance of occurring in any given year. This is a critical level of safety for infrastructure because it prepares for less common but more intense storms, minimizing the risk of flooding during severe weather conditions.

In addition, a 120-minute retention period allows for more extended detention and treatment of stormwater, which is crucial for enhancing water quality. This retention time enables more solids and pollutants to settle, reducing their likelihood of entering local water bodies. It also enhances groundwater recharge, which is important for sustaining local ecosystems.

Together, these two components—frequency and retention time—work synergistically to create a design that is not only effective in managing stormwater but also prioritizes environmental health. The combination is more resilient against flooding and more responsible in terms of ecological impact compared to the other options listed.