A culvert that lowers a channel conveyance typically results in which outcome?

• A. Increased roughness values
• B. Decreased downstream velocity
• C. Decreased head loss
• D. Increased headwater depth

Answer: (D)

A culvert that lowers a channel conveyance generally causes an increase in headwater depth. When a culvert is installed, it creates a restriction in the flow of water; this constriction can lead to a situation where water backs up behind the culvert, effectively raising the water level upstream. This rise in water level is characterized as increased headwater depth.

In cases where the flow through the culvert is impeded, the upstream area experiences higher water levels as it is unable to maintain the same flow conditions that existed prior to the installation of the culvert. As a result, the increased headwater depth reflects the need for the water to rise in order to achieve a balance with the reduced capacity of the culvert.

To put this in the context of the other options: increased roughness values typically refer to the frictional resistance experienced in a channel, and while a culvert might affect these values, they are not a direct outcome of lowering channel conveyance. Decreased downstream velocity is not a guaranteed outcome since velocity changes depend on various factors, including the downstream profile and flow characteristics. Lastly, decreased head loss would imply a more efficient flow, which contradicts what usually occurs with various hydraulic structures like culverts that generally introduce additional resistance