What three purposes do under-drains serve?

Under-drains primarily serve to support the filter media in a drainage system. This function is critical because the filter media is designed to separate sediments and pollutants from the water while facilitating drainage. If the filter media is properly supported, it maintains its structure and efficiency, ensuring that water can pass through effectively without clogging.

Additionally, under-drains provide a pathway for excess water to flow away, which helps maintain the proper moisture balance within the surrounding soil. When soil becomes overly saturated, it can lead to a reduction in the effectiveness of the filter media, which could compromise water quality. Thus, the role of under-drains in supporting filter media is fundamental for both the operational efficiency of the drainage system and the protection of water quality in the surrounding environment.