Significance of tailwater level in culverts

Headwater level and tailwater level of culverts are significant parameters in hydraulic design. Headwater level can't be set too large, or else flooding upstream may take place leading to the loss of life and properties. Conversely tailwater level of culverts has to comply with the following requirements: 

(i)  For low tailwater levels at the outlet of culverts, small depths of flow can cause vital erosion of downstream channels.

(ii) For high tailwater levels, it can cause the culvert upstream to be flowing full or even under submerged condition. The headwater level is increased in order to flow through culvert and this in turn increases flooding risk related with high headwater level.

Fig: Tailwater level in culvert