Quantity of stone in launching aprons of guide bank/Groyne/Revetment

Design of launching apron for revetment/guide bank shank/U/S shank of spur
Design data
Q.         =20000 Cumecs
f.          = o.96
V.        = 3 m/s
HFL.   = 100 m
LWL.  =  92 m
Bed RL=91m
Ss.       = 2.65
Protection slope 2:1
Design as per guidelines of BIS Codes
Size of stone for pitching.                     Ds = 0.25m
Thickness of pitching                           "t " = 0.50 m
Lacey's scouring depth.                        D.  = 13.015 m
Max. Designed scouring depth 1.5D.         = 19.523 m  & R L = 80.477 m
Apron laying level.                                     = 91 m
Des.scouring depth for apron.      Dmax    = 91 - 80.477 = 10.523 m

Thickness of launhing apron 1.25 to1.5 t.  = 0.625 to 0.75 m.  (for loose stones)
Launching slope of loose stone.                 = 2:1
Qty.of stone per mtr.of apron.                    =5^1/2 * t * Dmax
                                                                   =14.706 to 17.648 Cum/ m
Width of apron to be laid.                         = 1.5 * Dmax = 15.785 m
Av. thickness of apron                        "T"= 0.932 to 1.118 m
Thicness of apron at toe of pitching.        = 0.8 T.  
-------do-------------- river side end.          = 1.2 T
Aprons of loose stone boulders are to be laid in Wedge shape as per guidelines of IS-14262 & 10751      

RIVER TRAINING & CONTROL WORKS GROYNES/SPUR/STUD-- GUIDE BANKS-- REVETMENT/PITCHING

BIS has published 3 separate Codes for titled works
IS 8408-1994 for Groynes , IS 10751-1994 for Guide Banks & IS 14262-1995 for Revetment.
Though guidelines for important designing features have been elaborated in above codes ,but in a very sketchy and confusing manner as discussed below:-
WEIGHT & SIZE OF STONE FOR PITCHING & LAUNCHING APRONS 
weight of stone                       "W" =0.02323 x Ss x V^6 / K (Ss - 1 )^3
size of stone                           "Ds" =0.124 x (W/Ss) ^ 1/3
min.thickness of pitching         " T " =V^2 / 2g (Ss - 1 )
IS 14262-1995 guides as para 3.4 " Minimum dimension of the stones should not be less than Ds as obtained above.From the worked out wt. & known specific gravity,the volume of stone should be calculated."
IS 10751-1994 guides as para 5.7.3 (b) " Quarried stones of size 350 mm and / or weighing 40-70 kg should be used up to an average velocity of 3.5 m/s.
But IS 8408-1994 does not guide in above manner regarding minimum wt & size.
The formula to calculate Ds has been derived for a " exactly round " stone , where as such type of natural / quarried stones are not available for use.The theoretical wt. & size of exactly round stone with Ss =2.65 & for avg velocity of 3 m/s for slope 2:1 comes out 22.34 kg & 0.2525 m respectively.But actual wt of available round stone (with acceptable flat surface) with 0.2525 m Ds at its mid section has been observed between 35 to 40 kg.But in absence of guidelines of minimum wt of stone in IS 8408 [as in IS 10751 para 5.7.3 (b) ] as well as guidelines in Illustration on Nomograph page of each code "Provide two layers of stone weighing 20 kg ; Ds 0.25 m " has encouraged the some Designers as well reputed Designing Institutions for suggesting stone weighing 23 kg , sufficient , for pitching to with stand the avg velocity of 3 m/s as well as launching apron of Submerged Groyne & Revetment for Bank Protection ,though this wt does not full fill the criteria of Ds.for any type of available stone.
CONCLUSION :--
As the functions of Pitching & Launching Apron for all type of works discussed above , are same  .hence guidelines for Wt & Size of Stone for Pitching and Launching Apron , Thickness of Pitching , Thickness of Launching Apron , Shape & Size of Launching Apron , Designed Slope of Launching Apron in launched position , should be same for these works.