Abutment Riprap Design

Abutment Riprap Design (for bottomless culvert see special note at the end of the page):

This utility is based on The FHWA Publication HEC-23, March 2001 edition, Design Guideline 8 Section 8.7, Sizing Rock Riprap at Abutment. These equations, along with additional guidance are also contained in the SHA Hydrologic and Hydraulic Manual Chapter 11, Appendix D.

To size the riprap at an abutment, the user needs to know the characteristic average velocity in the contracted section. This velocity depends on the setback distance of the abutment, which will be used to determine whether the flow at the abutment is a mixture of channel flow and overbank flow, or whether it is comprised only of overbank flow.

As explained in the F-1 help information, the method of computing the depth and velocity of the flow depends on the setback of the abutment from the channel.

Case a: Short setback on both abutments

If the setback is less than five times the hydraulic depth of flow in the channel, the flow is defined as mixed flow. This means that the channel flow is mixed with the overbank flow. For this case use the hydraulic flow depth in the channel at the downstream side of the bridge. The velocity of flow (V) is defined as V= Q/A where Q = the total flow through the bridge and A = the total waterway area of the flow.

Case b: Medium or long setback

If the setback is greater than five times the hydraulic depth of flow in the main channel, the characteristic flow depth is defined as the average depth of flow (hydraulic depth) in the overbank area between the abutment and the edge of the channel. Similarly, the flow velocity is defined as the average flow velocity in the overbank area and is computed as follows:

 V = Q/A = Q overbank/(overbank hydraulic depth x overbank top width between the abutment and the channel).

Case c: Short setback on this abutment but medium or long setback on other abutment

The average flow velocity is defined as the average mixed flow of this overbank area and the channel flow.

V=(Q overbank + Q channel)/(Overbank flow area + Channel flow area).

Normally, the data can be imported from the abutment scour analysis simply by pressing the [Import] button. If an abutment scour data file is not available, then the characteristic flow depth and flow velocity for the abutment can be input into the cells provided to calculate the D50 riprap size. Select these values using the guidance in the F-1 tabs.

Selection of size of riprap.

The Utility program provides the minimum diameter riprap size to resist the calculated velocities. If the calculated value of D50 is 16 inches or less, use Class 2 riprap. If the calculated value of D50 is significantly over 16 inches, use Class 3 riprap.

Special Note For Riprap at Bottomless Culvert:

This method applies to subcritical flow within the limits of a bottomless culvert. It applies to protection for the upstream headwalls. It does not apply to supercritical flow, particularly as may occur at the culvert outlet.