Calibration/Safety Factor

For the NCHRP 24-20 method, a calibration/safety factor of 1.0 should be selected for all conditions.

For the ABSCOUR 10 method, the following is presented as a guide to assist the user in selecting an appropriate calibration/safety factor.

In developing the ABSCOUR equations for estimating abutment scour, available information from laboratory studies collected by the consultant firm of GKY and Associates was used as a means of evaluating the model. These laboratory tests were conducted in simple rectangular straight channels (laboratory flumes) with uniform flow. A total of 126 data points were used to develop the envelop equation describing the value of the coefficient for the spiral flow adjustment factor, kf. These initial studies were augmented by a second set of flume studies conducted by the FHWA in 2004.

Natural rivers are not accurately represented by the simple flow conditions modeled in a laboratory flume.  For practical design, use of a safety factor is suggested to take into account the effect of complex flow patterns which can be expected to occur at bridges abutments. The ABSCOUR calibration/safety factors have been evaluated on the basis of the USGS comparison study of ABSCOUR computed scour values vs. measured abutment scour at South Carolina bridges.  Current guidance in selecting a calibration/safety factor, based on both the flume and field studies, is presented below.

SELECTION OF BASE CALIBRATION/SAFETY FACTORS

100-YEAR FLOOD PLAIN WIDTH CHANNELS AND FLOOD PLAINS WITH FINER BEDLOADS CHANNELS AND FLOOD PLAINS WITH COARSER BEDLOADS
D50 < 2 MM
D50>2MM
LESS THAN 800 FEET
0.8
1.0
GREATER THAN 800 FEET
1.1
1.0

SELECTION OF INCREMENTAL CALIBRATION/SAFETY FACTORS BASED ON SITE CONDITONS

Channel Description at Bridge Site Incremental Safety Factor
Straight channel with uniform flow.
Add 0.0
Moderately meandering upstream channel
Add 0.0
Severely meandering upstream channel, or
Add 0.1
Channel with complex approach flow conditions (Sharp upstream bend in channel, confluence, unstable reach, lateral movement, etc.)
Add 0.2
Non-tidal river with wide flood plains and complex two dimensional river and flood plain flow patterns that may change with river stage where a 2-D analysis is appropriate but not available
Add 0.1
Tidal river with wide tidal flats or wetlands and complex two dimensional river and flood plain flow patterns that may change with river stage where a 2-D analysis is appropriate but not available
Add 0.1

This table is used in the following manner.  The user reviews the site conditions or descriptors which are present at the bridge site under consideration, and selects the factor in the table that best describes the crossing site under consideration.  The engineer may select a higher safety factor if it is considered necessary to reflect a high risk crossing site.

Please note that the current scour evaluation described in Chapter 11 of the Manual directly calculates the potential effect of channel movement and degradation.  This calculation serves to decrease the need for reliance on a safety factor to account for lateral channel movement and degradation.