Question

Discuss using the continuity equation why culverts change flow capacity

Explain why having pressurized culvert flow can be problematic for natural channels

Explain what channel conveyance is and the effects culverts have on it

Describe the basic concepts of open channel flow around using culverts

Answer 1

Hope it helps.

A culvert is a conduit of small length which is generally design to transport water underneath roadways. The opening of culvert kept large to avoid water head at the entrance. According to continuity equation i.e. { Q= AV} , it says greater the area larger will be the flow . So If a culvert is designed with a small opening then the discharge through it would be lesser which will create a water head in the upstream. To avoid this, engineers design entrance of culvert to be larger than rest of the conduit segment and hence the discharge ( flow capacity) changes in culvert.

Pressurised flow in culverts caused by many reasons such as deposition of sediments, plugging of sediments and large wood, high velocity flow. These may cause channel scoring. Now we are discussing about the natural channel, which have channel bed consist of soil and vegetation which can easily wear off due to high velocity and pressure present inside a culvert which may cause flood in worst cases.

Channel conveyance is permanent waterway just to guide water flow directon and to ensure that the flow in culvert should be smooth. To achieve this, culvert design with a small entrance. The effect of channel conveyance on culvert is that the water capacity entering in the channel is lesser which will reduce the chances of high velocity flow, channel scouring and flood.

Open channel flow in channel having culvert has same concept of specific energy and super critical or subcritical flow. Generally culverts are laid above the slope and the slope is downward.

Subcritical flow in inlets and supercritical flow in outlet of the culverts.