How is scour depth calculated?

How is scour depth calculated?

Normal scour depth is measured from the maximum flood level in the river.

1. As per Lacey’s Theory, Normal scour depth is given by:
2. C = 1.35 and q is the discharge per unit width.
3. Various formulas deduced by lacey is given below:

What is called scour depth?

The total scour depth is determined by adding three scour components which includes the long-term aggradation and degradation of the river bed, general scour at the bridge and local scour at the piers or abutment.

What is Lacey’s silt factor?

Lacey`s silt factor (f ) : A factor introduced by Lacey in 1930 based on the theory that in all regime channels of the same silt grade the ratio V2/R in which V denotes the mean velocity and R the hydraulic mean depth, characterizes turbulence and is constant.

What is afflux in bridge?

Afflux is an increase in water level that can occur upstream of a structure, such as a bridge or culvert, that creates an obstruction in the flow. Afflux is shown as the maximum increase of water level above normal depth in the undisturbed stream.

How is scour calculated?

Scour depth should be less than or equal to 2.4 times the pier width if Froude number is less than 0.8. Scour depth should be less than or equal to 3.0 times the pier width if Froude number is greater than 0.8. Note: If K4 is not 1.0, K4 = 0.4 (VR)0.15. From HEC-18 Equation 6.5.

Why is scour depth over predicted?

When applying the existing empirical equations for predicting bridge pier scour to field cases, the scour depths are overpredicted (Babaeyan-Koopaei and Valentine 1999). This means increased construction and maintenance costs as the foundation levels are required to be deeper than it should be.

What is Lacey scour depth?

When equation (15) is applied to rivers in flood the value of R gives the normal depth of scour. Hence this formula is very useful determining the levels of foundations, vertical cutoffs etc. This formula is popularly called Lacey’s scour depth formula.

What is Lacey’s formula?

Lacey’s equation reads:(1) P=4.8Q b 0.5 In alluvial rivers, the wetted perimeter P is only slightly larger than the surface width B. About a century after the debate on regime theory started, there appears to be a convergence of opinion, which is presented by Cao and Knight (1996).

What’s the best way to calculate scour depth?

If the bed is sand bed streams we can use the empirical Zeller Equation for estimation of scour depth; Other empirical methods are also there like Neil, Blench and Lacey for estimation of the scour depth.

How is scour depth related to the risk of failure?

Although scour this is an inspection-related input, the parameter to cause failure is the resultant stress. This factor uses measured scour depth (ft or m) divided by the design scour depth (ft or m). Table 8.20 presents a score between 0 and 10, where a higher score signifies greater risk.

What is the maxm Deoth of scour from hhwl?

Ksf = Silt factor for a representative sample of bed material obtained uptothe level of anticipated deepest scour. Maxm Deoth of Scour from HHWL (2dsm) = 5.372 m ( As per clause No. 703.3.1.1 of IRC:78-2014)

What causes a pier to have a high scour depth?

A subroutine in the HEC-RAS program does scour analysis. However, in some cases high values of scour depth are obtained for contraction or local scour. Local scour at piers is caused by horseshoe vortices forming at the base of the pier. Obstruction of flow by a pier results in a stagnation line on the front of the pier.