The shape of the culvert is defined by picking one of the nine available shapes. These shapes include: circular; box (rectangular); arch; pipe arch; elliptical; high profile arch; low profile arch; semi-circular; and ConSpan. The size of the culvert is defined by entering a rise and span. The rise refers to the maximum inside height of the culvert, while the span represents the maximum inside width. Both the circular and semi-circular culverts are defined by entering a diameter.
The inside height (rise) of a culvert opening is important not only in determining the total flow area of the culvert, but also in determining whether the headwater and tailwater elevations are adequate to submerge the inlet or outlet of the culvert. Most box culverts have chamfered corners on the inside, as indicated in Figure 6-6. The chamfers are ignored by the culvert routines in computing the crosssectional area of the culvert opening. Some manufacturers' literature contains the true crosssectional area for each size of box culvert, considering the reduction in area caused by the chamfered corners. If you wish to consider the loss in area due to the chamfers, then you should reduce the span of the culvert. You should not reduce the rise of the culvert, because the program uses the culvert rise to determine the submergence of the culvert entrance and outlet.
All of the arch culverts (arch, pipe arch, low profile arch, high profile arch, and ConSpan arch) within HEC-RAS have pre-defined sizes. However, the user can specify any size they want. When a size is entered that is not one of the pre-defined sizes, the program interpolates the hydraulic properties of the culvert from tables (except for ConSpan culverts).
HEC-RAS has 9 predefined Conspan arches. Conspan arches are composed of two vertical walls and an arch. Each predefined span has a predefined arch height, for example the 12 ft arch has an arch height of 3.07 ft. For the 12 span, any rise greater than 3.07 ft can be made by adding vertical wall below the arch, when a rise is entered less than the arch height, the arch must be modified as discussed below. RAS has the ability to produce a culvert shape for rise and span combinations not in the predefined list. The following is a list of the pre-defined ConSpan sizes.
Predefined Spans | Arch Heights |
|---|
12 | 3.00 |
14 | 3.00 |
16 | 3.53 |
20 | 4.13 |
24 | 4.93 |
28 | 5.76 |
32 | 6.51 |
36 | 7.39 |
42 | 9.19 |
If a span is requested that is not in the list of predefined shapes, then one is interpolated geometrically from the bounding predefined shapes. The plot below shows an interpolated 21 ft arch from 20 and 24 predefined arches.
If the span is less that the smallest predefined arch, then the smallest arch is scaled to the requested span, similarly, if a span is entered larger than the largest predefined arch, then the largest arch is scaled to the requested span.
If a rise is entered that is less that the predefined arch rise, then the vertical ordinates of the arch are scaled down to the requested arch rise and no vertical segments are added. In the plot below, a 20 ft span was requested with a 3 ft rise. The arch height of the 20 ft span is 4.13 feet so all the vertical distances were multiplied by 3 / 4.13.
