HY8:Limitations: Difference between revisions

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==Limitations==
{{TOCright}}
==Inlet and Profile Limitations==
 
===Entrance limitations===
 
Since HY-8 is not primarily a water surface profile computation program but is a culvert analysis tool, it assumes a pooled condition at the entrance to the culvert.
 
===Vena contracta assumptions===
 
In some cases, a drawdown of the water surface profile could occur in a culvert barrel since the culvert has the potential to act as a sluice gate at the entrance.  The location of maximum drawdown is called the [[HY8:Vena Contracta|''vena contracta']].  Computing the vena contracta provides numerical stability for the flow profile but does not affect the headwater computations.  The vena contracta is not yet computed for S2 curves but is computed if certain conditions exist on horizontal or adversely sloped culverts.  A coefficient that is generalized for circular and box culverts is used to compute the location and depth of the vena contracta for all culvert shapes.
 
===Brink depth===
 
For culverts with tailwater elevations below the outlet invert of the culvert, water flowing out of the culvert would theoretically pass through a brink depth after critical depth.  In this case, HY-8 uses critical depth to determine the final culvert depth and velocity rather than the brink depth.
 
===Culvert cross section===
 
HY-8 assumes the culvert cross section shape, size, and material does not change in the barrel except in the case of broken back runout sections, where the user can change the material and Manning's roughness in the runout (lower) culvert section.
 
==Hydraulic Jump Computations==
 
Hydraulic jump computations are supported in HY-8 7.3 and later versions.
 
===Computed outlet velocity and tailwater elevation===
===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, HY-8 assumes that the barrel flows full at the outlet and reports an outlet velocity corresponding to full flow.  A water surface profile for this case is shown below.
The user should be aware that for inlet control 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 [[HY8:Hydraulic Jump Calculations#Determining the Length of a Hydraulic Jump|in the section of this manual that describes hydraulic jump computations]].  A water surface profile for this case is shown below.




[[Image:HY8image59.gif]]
[[Image:HY8image59.png|500px]]




The water surface profile for these cases is not realistic.  In effect, HY-8 assumes a type of hydraulic jump occurs in the barrel.  This may or may not be the case; HY-8 does not include a momentum analysis to truly determine if and where a hydraulic jump might occur.  If, indeed, a hydraulic jump does occur, then the reported outlet velocity is correct.  If not, then the outlet velocity reported in HY-8 may be too low.  Caution must be exercised in these cases.
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.


When the elevation of the tailwater does not exceed that of the top of the culvert outlet, then a profile such as the one shown below is presented.  Note that the flow profile indicates that the outlet velocity is associated with the supercritical flow depth shown in the culvert barrel.  In effect, HY-8 is reporting that no hydraulic jump will occur.  If this is the case, then the outlet velocity shown is correct.  If not, the outlet velocity shown in the performance table will be too high and energy dissipation should be considered.
==Culvert Types==


===Newly supported culvert types===


[[Image:HY8image60.gif]]
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.


==Hydraulic Jumps==
HY-8 does not attempt to show the location of a hydraulic jump in the culvert profile.  Additionally, if more than one flow type exists for the culvert profile, HY-8 does not attempt to show multiple profiles.


NOTE: The user should be aware that when the tailwater elevation exceeds the elevation of the top of the culvert outlet, HY-8 assumes that the barrel flows full at the outlet and reports an outlet velocity corresponding to full flow. A more detailed description of this limitation is given in the Help document under the limitations topic.
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.


==Culvert Types==
The following types of culverts are still not available


There are several culvert types, such as CONSPAN or HDPE plastic, which were unsupported in MS-DOS HY-8 and will continue to be unsupported in HY-8 version 7.0.
Partially buried culverts or culverts with natural stream bottoms are supported in HY-8 version 7.1 and later versions.
 
===Inlet control computation limitations for selected shapes===


Partially buried culverts or culverts with natural stream bottoms remain unsupported in HY-8 version 7.0.
User Defined, Open Bottom Arch, Low-Profile Arch, High-Profile Arch, and Metal Box do not use, and may not have, original research that describes coefficients that can be used for their inlet control equations.  Instead, these shapes use [[HY-8:Polynomial Coefficients#Table_7._User_Defined.2C_Open_Bottom_Arch.2C_Low-Profile_Arch.2C_High-Profile_Arch.2C_and_Metal_Box_HW.2FD_Values.|an HW/D interpolation table]], defined by a chart in HDS-5, that can be used to determine headwater values at various values of Q/(AD^0.5).


==Broken Back Culverts==
==Broken Back Culverts==
Culverts with multiple slopes (broken back) and adverse slopes remain unsupported.
 
===Broken back culvert support===
 
Culverts with multiple slopes (broken back) and horizontal/adverse slopes are supported in HY-8 7.3 and later versions.
 
===Side and slope-tapered inlets===
 
Broken back culverts with side and slope-tapered inlets are not currently supported.
 
===High-slope sections===
 
The equations for broken back culverts used in HY-8 should not be applied to culvert sections with slopes greater than 55 degrees (143%).  These equations are not valid for very steep slopes and will give unrealistic results.
 
===MS-DOS HY-8 features===
The MS-DOS version of HY-8 performed reservoir routing and hydrograph generation.  These features were not converted to the windows version of HY-8 7.0.


{{HY8Main}}
{{HY8Main}}
[[Category:HY-8 General Information|L]]

Latest revision as of 16:15, 1 March 2018


Inlet and Profile Limitations

Entrance limitations

Since HY-8 is not primarily a water surface profile computation program but is a culvert analysis tool, it assumes a pooled condition at the entrance to the culvert.

Vena contracta assumptions

In some cases, a drawdown of the water surface profile could occur in a culvert barrel since the culvert has the potential to act as a sluice gate at the entrance. The location of maximum drawdown is called the vena contracta'. Computing the vena contracta provides numerical stability for the flow profile but does not affect the headwater computations. The vena contracta is not yet computed for S2 curves but is computed if certain conditions exist on horizontal or adversely sloped culverts. A coefficient that is generalized for circular and box culverts is used to compute the location and depth of the vena contracta for all culvert shapes.

Brink depth

For culverts with tailwater elevations below the outlet invert of the culvert, water flowing out of the culvert would theoretically pass through a brink depth after critical depth. In this case, HY-8 uses critical depth to determine the final culvert depth and velocity rather than the brink depth.

Culvert cross section

HY-8 assumes the culvert cross section shape, size, and material does not change in the barrel except in the case of broken back runout sections, where the user can change the material and Manning's roughness in the runout (lower) culvert section.

Hydraulic Jump Computations

Hydraulic jump computations are supported in HY-8 7.3 and later versions.

Computed outlet velocity and tailwater elevation

The user should be aware that for inlet control 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.


HY8image59.png


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.

Culvert Types

Newly supported 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 are supported in HY-8 version 7.1 and later versions.

Inlet control computation limitations for selected shapes

User Defined, Open Bottom Arch, Low-Profile Arch, High-Profile Arch, and Metal Box do not use, and may not have, original research that describes coefficients that can be used for their inlet control equations. Instead, these shapes use an HW/D interpolation table, defined by a chart in HDS-5, that can be used to determine headwater values at various values of Q/(AD^0.5).

Broken Back Culverts

Broken back culvert support

Culverts with multiple slopes (broken back) and horizontal/adverse slopes are supported in HY-8 7.3 and later versions.

Side and slope-tapered inlets

Broken back culverts with side and slope-tapered inlets are not currently supported.

High-slope sections

The equations for broken back culverts used in HY-8 should not be applied to culvert sections with slopes greater than 55 degrees (143%). These equations are not valid for very steep slopes and will give unrealistic results.

MS-DOS HY-8 features

The MS-DOS version of HY-8 performed reservoir routing and hydrograph generation. These features were not converted to the windows version of HY-8 7.0.