The underpasses are typically built as corrugated steel culverts. Drainage pipes and culverts have been used as dens. Bullfrogs can also occasionally be found inhabiting urban areas near ditches and culverts. Airport authorities blamed the incident on corroded steel structures supporting the drainage culverts. The railway also required the construction of 1,524 culverts for drainage. Infrastructure features included limestone culverts, head walls, and limestone or concrete sidewalks. Roads, bridges, and culverts were also lost. They were also lost when they were buried beneath sewers or culverts. A series of ditches and culverts carries the water from Purley to Croydon. The process of removing culverts to restore an open-air watercourse is known as daylighting. Both rivers have now been diverted into underground culverts. There are also 120 bridges, 9 galleries and 440 culverts. Contributing structures to the listing include one bridge, one overpass and four concrete box culverts. To carry small streams under the canal, 182 culverts, usually of masonry, were built. Water passing through undersized culverts will scour away the surrounding soil over time.