Cross-sections at Culvert:

Cross-section:1 The most downstream cross-section (cross-section 1) should be located where the effect of culvert on flow width is diminished. It should be located where the flow is fully expanded from its constriction caused by the culvert.

Cross-section:2 The cross-section immediately downstream of the culvert opening (cross-section 2) should be little wider than the width of the culvert at the exit. Why? Well, the flow will expand after it exits the culvert. Ok, how much wider? Well, each situation is unique but as a rule of thumb: Width of cross-section 2 = (1.33 x distance from the culvert exit to the cross-section 2) + (Width of the culvert opening at the exit). (Where does this 1.33 comes from? The answer is: Rule of thumb tells that it takes 1.5 units of travel length for the flow to expand 1 unit in width. Therefore, the total increase in width for 1 unit of travel is (1/1.5). But this increase occurs on both sides of the river bank. Therefore, 2*(1/1.5) = 1.33 units is the total increase in width of flow per unit travel length from the culvert exit). These cross-sections 1 and 2 information is used by the HEC-RAS program to compute the losses (friction and expansion) in the channel reach downstream of the culvert.

Cross-section:3 The cross-section immediately upstream of the culvert opening (cross-section 3) should be located where it allows some flow contraction. Therefore, it should be little wider than the width of the culvert opening. How much wider? Again, each situation is unique but as a rule of thumb: Width of cross-section 3=(2.0 x distance from the culvert opening to the cross-section ) + (Width of the culvert opening at the entrance). (Where does this 2.0 comes from? The answer is: Rule of thumb tells that it takes 1.0 unit of travel length for the flow to contract 1.0 unit in width. But this contraction occurs on both sides of the river bank. Therefore, 2*(1.0/1.0) = 2.0 units is the total decrease in width of flow per unit travel length to the culvert entrance).

Cross-section:4 The most upstream cross-section (cross-section 4) should be located where the flow width is unaffected by the culvert. It should be located where the flow is fully expanded.

Based on the description above, you can tell that the flow contraction is rapid compared to flow expansion. Generally, expansion causes more energy loss than contraction.