Road cross-drainage structures also control upstream water levels in floodplains. An important consideration is the level of the bed sills in bridges and culverts. These will effectively determine the water level in the upstream section of the road. Because these bed sills define the level of the main drainage outlet, they effectively determine the water level in a very large area. This creates the conditions for wetland development, ensuring adequate water levels for submerged or aquatic crops such as rice or sugarcane. Bed sills of bridges and culverts are an important factor in the drainage and waterlogging of large floodplain areas.
One can move a step further and regulate the water levels in the upstream area where land is used productively. This will enable the land to be used, for instance, for submerged crops such as rice and sugarcane, for aquatic crops, or for aquaculture under varying degrees of regulation.
To manage water levels on the land for these productive uses, gates may be provided on the culverts. This will make it possible to either raise the water level or drain the land. The placement of such gates should be determined in close cooperation with the land users. It is also important to assess the effect of impounding water upstream on the integrity of the road body. Reinforcement may be provided, if required. Generally, the preferred type of gate is the stop log with wooden planks sliding in a railing. These wooden logs can be maintained by the land users and will be less prone to theft and vandalism than permanent gates.