HY8:Limitations
Limitations
Computed outlet velocity and tailwater elevation
The user should be aware that when the tailwater elevation exceeds the elevation of the top of the culvert outlet, the barrel may or may not flow full at the outlet. HY-8 determines a water profile using the direct step method in each direction and the sequent depth associated with each of the steps. If the sequent depth associated with the forward profile matches the depth along the backward profile through the culvert, a hydraulic jump occurs and the length of the jump is calculated from that location. Since the lengths of jumps have not been tested for all culvert sizes and slopes, only a limited set of equations are available for computing the lengths of jumps in HY-8. More information on the jump length computations is available in the section of this manual that describes hydraulic jump computations. A water surface profile for this case is shown below.
In this case, the hydraulic jump length computed by HY-8 may or may not be correct since the equation used to compute hydraulic jump length is for box culverts only, but is applied to all the other possible HY-8 culvert shapes. If a hydraulic jump occurs inside the culvert and the end of the hydraulic jump is located outside the culvert, HY-8 assumes the hydraulic jump occurs outside the culvert and a hydraulic jump is not shown in the profile. If both the beginning and end of the hydraulic jump occur inside the culvert barrel, the hydraulic jump is shown in the profile and is reflected in the profile computations, as shown in the image above.
Hydraulic Jumps
Hydraulic jump computations are supported in HY-8 7.3 and later versions.
Culvert Types
Previous versions of HY-8 did not fully support CON/SPAN culverts, HDPE culverts, or culverts installed with a natural stream bed as the bottom.
CON/SPAN (Concrete Open-bottom Arch) culvert types are supported in HY-8 7.3 and later; HDPE plastic culvert types are supported in HY-8 version 7.1 and later.
Partially buried culverts or culverts with natural stream bottoms were supported in HY-8 version 7.1 and later versions.
Broken Back Culverts
Culverts with multiple slopes (broken back) and horizontal/adverse slopes are supported in HY-8 7.3 and later versions.
