What feature allows access to subsurface areas in drainage systems?

The feature that allows access to subsurface areas in drainage systems is the manhole. Manholes are vertical access points that provide maintenance personnel with entry into underground drainage systems, such as sewer lines, stormwater conduits, or various utility tunnels. They are designed to facilitate inspection, cleaning, and repair of the systems below ground.

Manholes are typically equipped with covers that can be opened to allow for entrance, making them an essential aspect of effective stormwater management and drainage system maintenance. Their design not only ensures that workers can safely access the subsurface areas but also allows for efficient monitoring of system performance.

Although other options like access ports and inspection closets can offer access or observation points, manholes are the most common and robust features used specifically for subsurface drainage systems. An underdrain, while functional in directing water flow, does not provide access for maintenance or inspection in the same way that manholes do.