Calculation Of Bridge Scour Using The Erodibility Index Method Spreadsheet (www.theengineeringcommunity.org).xlsx

	A	B	C	D	E	F	G	H	I
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2	CALCULATION OF BRIDGE SCOUR USING THE ERODIBILITY INDEX METHOD
3
4	The worksheet presented below is based on the Erodibility Index Method developed by
5	Dr.George Annandale. See FHWA Manual HEC-18, 5th Edition, April 2012, page 7.43
6	for up-dated guidance on use of the method
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8	ENTER DATA ONLY IN THE SHADED CELLS. ALL OTHER INFORMATION WILL BE
9	COMPUTED BY THE PROGRAM.
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11
12	BRIDGE DESCRIPTON			FHWA HEC-18 SAMPLE PROBLEM 7.13.2
13				PAGE 7.45
14	ABUT/PIER DESCRIPTION
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16	BORING DESCRIPTION
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18
19	PART 1 ERODIBILITY INDEX
20	STEP 1 COMPUTE ERODIBILITY INDEX
21
22	ERODIBILITY INDEX (K) =			Ms Kb Kd Js				2,39
23
24	ELEVATION FOR COMPUTING ERODIBILITY INDEX							100
25
26	Ms		MASS STRENGTH NUMBER - PAGE 20					3,95
27
28	Kb =RQD/Jn		BLOCK SIZE FACTOR					7,33
29
30		RQD (FROM BORINGS)*						20
31		* Modify where appropriate with guidance from Geotechs
32		Jn Joint Set Number (TABLE 4 PAGE 22)						2,73
33
34	Kd = Jr/Ja		INTER-PARTICLE BOND SHEAR STRESS #					0,15
35
36		Jr Joint Roughness # ( TABLE 5 PAGE 27)						1,5
37
38		Ja Joint alteration #(TABLE 6 PAGE 28)						10
39
40	Js	RELATIVE GROUND STRUCTURE NUMBER						0,55
41		TABLE 7 PAGE 36
42
43	annandale
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48
49	FHWA HEC-18 SAMPLE PROBLEM 7.13.2
50	PAGE 7.45
51
52	PART 2 - COMPUTE SCOUR IN ROCK UNDER BRIDGE
53
54	STEP 1 CRITICAL STREAM POWER OF FLOW ,Pc,
55	Pc = K^0.75		1,92	KW/m^2 - K IS ERODIBILITY INDEX (cell H22)
56
57	STEP 2 STREAM POWER OF FLOW IN CHANNEL - Pa = tV;						Pa = gRS V
58			g	R	S	V	Pa = gRS V	Conversion	Pa
59			62,4	Hyd Rad	Energy	Velocity	English	factor	metric
60			# / ft^3	ft	Slope	fps	ft lbs/sec	See Note 3	KW/m^2
61	CHANNEL (1)		62,4	11,5	0,0035	9	22,6044	0,01455	0,329
62	BRIDGE (2)		62,4	11,5	0,0035	9	22,6044	0,01455	0,329
63	(1) JUST UPSTREAM OF BRIDGE (PIER ANALYSIS); (2) DOWNSTREAM FACE OF BRIDGE
64	(3) CONVERSION FACTOR				1 ft-lb/ sec = 0.001356 KW /ft^2
65					.001356 KW/ft^2 x 1/0.0929 = .01455 KW/m^2
66
67	NOTE: IS Pc > Pa ?; IF SO ROCK BED OF CHANNELSHOULD NOT SCOUR
68	Pc =	1,92	KW/m^2		Pa =	0,329	KW/m^2
69	SAFETY FACTOR =			(Pc/Pa)		5,84
70	( A safety factor of 2 or more is desirable when evaluating scour in rock)
71
72	PART 3 - COMPUTE SCOUR IN ROCK AT PIER
73	PIER SCOUR EQUATION (HEC-18)					P/Pa = 8.42 e^ -0.712(Ys/b)
74			Pa = local stream power at pier				= stream power just upstream of bridge
75		Pa	Ys	pier	Ys/b	P/Pa	P	Pc	P>Pc?
76			scour	width (b)
77			m	m
78		0,329	0	0,76	0,000	8,42	2,77	1,92	yes
79		0,329	0,2	0,76	0,263	6,98	2,30	1,92	yes
80		0,329	0,4	0,76	0,526	5,79	1,90	1,92	no
81		0,329	0,6	0,76	0,789	4,80	1,58	1,92	no
82		0,329	0,8	0,76	1,053	3,98	1,31	1,92	no
83		0,290	1	0,76	1,316	3,30	0,96	1,92	no
84					At a scour depth of one meter, : pc/p = safety factor = 2
85	NOTE 4:	Ys IS THESCOUR DEPTH BELOW STREAMBED. USE AN INITIAL VALUE OF ZERO; THEN
86		INCREASE Ys VALUES UNTIL CRITICAL ERODIBILITY INDEX (Pc) > STREAM POWER
87
88	NOTE 6:	COMPARE P (LOCAL STREAM POWER AT PIER) WITH Pc THECRITICAL
89		STREAM POWER NEEDED TO ERODE THE ROCK
90		IT IS DESIREABLE THAT A SAFETY FACTOR OF TWO (Pc/P) BE COMPUTED FOR THE
91		ESTIMATED SCOUR . A SAFETY FACTOR > 2 OCCURS AT A SCOUR DEPTH =1 METER
92
93	NOTE 5: There is currently no data available for computing Pc at abutments.	THERE IS CURRENTLY NO EQUATION AVAILABLE FOR EVALUATING ABUTMENT SCOUR
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