Further, small culvert-sized streams in Maryland generally have well vegetated overbank areas. For worst case scour conditions, much of the flood flow will be contained in these overbank areas in the upstream approach section. As a result, the bed load delivered to the culvert from overbank flow is likely to be small.
The headwall and side walls of the bottomless culvert are similar to the geometry of the abutment arrangements for a bridge, and it is considered appropriate to utilize abutment scour equations in estimating scour in bottomless culverts. It is suggested that the Engineer's estimate of scour depths be conservative but reasonable.
If the designer is confident that live bed scour conditions will prevail for worst case scour conditions, the live bed scour conditions can be selected for the scour analysis. However, in view of the potential for clear water scour, it is recommended that preliminary analyses consider both live bed and clear water scour conditions.
The cross-section of a typical bottomless culvert installation can be accessed by clicking on the link at the end of this discussion. The actual channel cross-section is converted to the ABSCOUR cross-section in the same manner as is done by the ABSCOUR Program for a bridge opening. Scour at the culvert walls corresponds to scour at the bridge abutments, and is computed in a similar manner. Therefore, the terms 'abutment' and 'culvert wall' are used interchangeably in the help screens.
Click here for a definition sketch of a bottomless culvert or bridge cross-section
The following guidance pertains to selection of the datum point for the culvert section.
The contraction scour for a bottomless culvert is computed in the same manner as the vertical wall abutment scour case for a bridge opening. (See also Appendices A, B and C of Chapter 11).
Wall (Abutment) Scour
The wall area at the culvert inlet is a region of higher velocity flow due to the rapidly contracting flow and the resulting vortex action. This is similar to the flow at a vertical wall abutment, resulting in localized scour that is deeper than the contraction scour in the channel. The SHA abutment scour equations can be used to estimate the scour depth at the culvert wall near the culvert entrance. Please note that the computed abutment scour represents the total scour at the abutment. It should not be added to the contraction scour. This is accomplished as follows: the contraction scour depth y2 computed above is multiplied by the correction factors, Kv and Kf to account for the higher velocity and vortex flow near the culvert wall. These correction factors are computed by the following equations (See also Appendices A, B and C of Chapter 11):
Kv = 0.8(q1/q2)1.5 + 1 Kf = 0.13 + 5.85F1 (clear water) Kf = 0.46 + 4.16F1 (live bed)Where:
The term Kv is related to the effect of the higher flow velocity which occurs near the culvert wall.
The term Kf is related to the effect of vortex flow on scour at the corner of the culvert. The limits of the Kf value range from 1.0 to 4.0. If the value computed by the above equation is less than 1.0, use a value of 1.0. If the value computed is greater than 4.0, use a value of 4.0.
The scour depth at the culvert walls, y2a can be written as:
Scour depth, y2a = Kp Kf (y2) Kv(k2)Where:
Pressure flow:
ABSCOUR 10 Introduces a new procedure for estimating the effect of pressure flow in increasing the extent of contraction scour at a bridge:
Limitations to the bottomless culvert scour analysis.
In order to use this method of analysis for a culvert, there must be a downstream control that will maintain flow depths inside the culvert so that velocity decreases as scour increases. If there is no control downstream, then it is likely that channel degradation will occur downstream, through and upstream of the culvert. If the user encounters this flow condition in the culvert, the ABSCOUR bottomless culvert module should not be used to design the culvert since it does not account for the effect of anticipated increases in the velocity of flow in the culvert.
Hydraulic flow conditions should be checked for several discharges to verify that the downstream controls are effective for low and intermediate flows as well as the design flow.
For the flow conditions described above, including supercritical flow, the user is encouraged to consider one of the following options:
The contraction and abutment (or wall) scour is based on the use of the ABSCOUR cross-section as described in the Users Manual (See the Appendices to Chapter 11 of the Office of Structures H&H Manual). The user will need to compare the ABSCOUR cross-section with the actual cross-section and make a judgment as to the need to modify the scour values computed by the program to account for the actual field conditions.