Which of the following risks require aggregate systems to remain dry when not flooded?

The correct answer is related to the fact that algae thrive in wet environments. When aggregate systems are allowed to remain flooded or wet for extended periods, they can become a breeding ground for algae growth. This can not only cause aesthetic issues but also lead to further complications such as reduced effectiveness of the aggregate system due to clogging or deterioration over time.

Keeping these systems dry when not flooded is essential to prevent algae from establishing and spreading. In contrast, while other risks like damage, wood rot, and leaks can be associated with wet conditions, they do not specifically necessitate the requirement for keeping aggregates dry as a primary concern, as algae growth is directly linked to moisture and can have compounding effects on system performance.