DESIGN OF RIPRAP FOR PROTECTION AGAINST. SCOUR ...

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) :: ;..•..... «-." - .... ~ . '.'.. (80) TWO APPROACHES DESIGN OF RIPRAP FOR PROTI:CTION AGAINST SCOUR AROUND BRIDGE PIER VOL. 16, (No.1) The size of non-movable material in riprap can be determined by either critical velocity approach or critical shear stress approach. In critical velocity approach, the size of riprap (D) can be obtained relating it to average velocity of flow (U), depthof flow (Y) and difference in specific weights of sediment and ~ater (f1y s)' if one assumes that viscosity is not important. Investigators such as Ishbash (1935), Garde (1970), Bonasoundas (1973), Quazi and Peterson (1973), Maynord et al. (1989), Richardson et al. (1991), Garde and Kothyari (1995) Parola(1995), Chiew (1995) have suggested equations for computation of size of riprap. Most of these equations are for the computation of size of riprap in unobstructed flow.Further, it may be noted that when water flows, shear or velocity distribution around the bridge pier is affected and instantaneous values of these two parameters vary and much greater than time averaged values. These two aspects are not considered in these equations. Alternately one can specify that /). D in case of riprap layer should be less Ys 50 than a specific value for it to be stable and control scour. Garde and Kothyari (1995) recommended this value as 0.03 for riprap in a channel. Here 'to is average shear stress in the channels equal to RS. However, in case of bridge pier, local shear around the pier is greater than the average shear stress in the channel. Assuming 'to is proportional to l.P, experimental evidence indicates that the time averaged local shear stress around the pier can be 3 to 5 times the average shear stress in the channel. Measurements of shear stress around pier by Hjorth (1975) and Darghi (1987) indicate that instantaneous shear around pier can be as high as 11 to 12 times 'to. These two facts need to be taken in to account while developing the method for design of riprap. As mentioned by Chiew and Melville (1989) the effect of sediment gradation is negligible when standard deviation ( (Jg) of riprap is less than 2. Hence, it is desirable to have standard deviation ( (Jg) of riprap between 2 and 3. Filter underneath the riprap layer is usually required to prevent leaching of base material, which takes place due to penetration of turbulence in the riprap layer. This takes place due to penetration of turbulence in the riprap layer. However, considering the difficulties in laying the filter layer, Worman (1989) has indicated that two or more layers of graded riprap can be designee in such a way that provision of filter is not needed. This approach is adopted here. BRIEF REVIEW Generally, the size of riprap is determined by using one of the equations available for critical velocity. Worman (1989) has used Ishbash (1935) equation in which diameter of riprap D is expressed as function of critical velocity as given below. ISH JOURNAL OF HYDRAULIC ENGINEERING, VOL. 16.2010, NO.1
I VOL. 16, (No.1) DESIGN OF RIPRAP FOR PROTECTION AGAINST SCOUR AROUND BRIDGE PIER u, =O.8sl2g p,;p D Hereu =2U. c On the basis of small-scale experiments, Suzuki (1992) suggested that the thickness (T) of the riprap be obtained from Eq. (1). where r., and 'to are dimensionless critical shear stress given by (~ 1"~ J and r, 50 ~Ys D5~ respectively. Kulkarni (1993) has recommended that, thickness of riprap (T) can be obtained from the equation. U 2 T=--:--~ 2 g (:; -IJ (3) (81 ) (1) (2) , l. • j. , t l. r I . . ':'-:----0.'-' v-, :.: -. :.: where U is average velocity in unobstructed flow. This equation is to be used for protection of bed and banks in the river. On the basis of analysis of the laboratory data collected, Worman (1989) has proposed the following equation for thickness of riprap as, ... "--- -. where T is thickness of riprap, p is coefficient of friction for turbulent flow, CD is the drag coefficient of particle of bed material and n is porosity the value of which is taken as 0.38. Inserting values of p, PS' Pf' Co and n, this equation reduce to ( U ~6(~) 2 ) gT c DI5 If (d85 ) > 0.15 , DI5 It may be mentioned that in Eqs. (4) and (5), Worman uses U= 2U o where U, average velocity in unobstructed channel, in order to account the fact that scouring ISH JOURNAL O~HYDRAULIC ENGINEERING, VOL. 16.2010. NO.1 (4) (5) ~ .-' ., :.. -'~. .~:- -." :-: ." .... - .. -.: :.: :. :-,:,:::,:~:",:.- ':::;':~::.::::::':
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