Does a developed property likely require stormwater control and containment devices if the pre-development flow is 1 m3/s and post-development flow is 1.5 m3/s?

Does a developed property likely require stormwater control and containment devices if the pre-development flow is 1 m3/s and post-development flow is 1.5 m3/s?

• A. yes, because control and containment are always provided when post-development exceeds pre-development flow
• B. yes, because post-development flow is considerably greater than pre-development flow
• C. no, because post-development flow is less than twice pre-development flow
• D. no, because post-development and pre-development flows are very small

Answer: (B)

The main idea is that development changes how much water runs off a site, and engineers look at how much the peak discharge increases. Here, the peak flow rises from 1 m3/s to 1.5 m3/s—a 50% increase. That is a substantial rise, and stormwater controls are typically used to attenuate such increases so downstream channels, floodplains, and drainage systems aren’t overwhelmed. So this situation is the kind that would likely require stormwater control and containment devices to manage the higher post-development runoff. The other options aren’t as fitting. It isn’t guaranteed that any exceedance triggers controls in every jurisdiction, and the rule isn’t simply “only if it doubles.” Saying the flow is very small overlooks the fact that a 50% increase can still have significant downstream impacts, and the chosen scenario reflects a meaningful rise that warrants management.

The main idea is that development changes how much water runs off a site, and engineers look at how much the peak discharge increases. Here, the peak flow rises from 1 m3/s to 1.5 m3/s—a 50% increase. That is a substantial rise, and stormwater controls are typically used to attenuate such increases so downstream channels, floodplains, and drainage systems aren’t overwhelmed. So this situation is the kind that would likely require stormwater control and containment devices to manage the higher post-development runoff.

The other options aren’t as fitting. It isn’t guaranteed that any exceedance triggers controls in every jurisdiction, and the rule isn’t simply “only if it doubles.” Saying the flow is very small overlooks the fact that a 50% increase can still have significant downstream impacts, and the chosen scenario reflects a meaningful rise that warrants management.