Hydraulics for Causeway 

yogendra borse

Here is an example for quick calculations of Hydraulics of Causeway, which can be done immediately for estimate purpose.

HYDRAULIC DATA

1	Catchment Area	11.25	sq.mile
2	Bed width of define cross section	25	metres
3	Bank width of define cross section	42	metres
4	H.F.L.	102.42	metres
5	Lowest bed level	98.905	metres
6	Discharge by Inglis formula	508	Cumecs
7	Protected bed level	100.9	metres
8	R.T.L.	99.96	metres
9	Annual rainfall	1250	mm

DESIGN OF VENT

Calculation of area on define C/S at RTL = 100.765

Chainage	G.L.	R.T.L.	Diff. between R.T.L. and G.L.	Mean Diff.	Length	Weighted area
-13.8	100.765	100.765	0	-	-	-
-10	100.125	100.765	0.64	0.32	3.8	1.21
-5	100.045	100.765	0.71	0.67	5	3.35
0	99.955	100.765	0.81	0.76	5	3.8
5	99.695	100.765	1.07	0.94	5	4.7
10	99.365	100.765	1.4	1.23	5	6.15
15	99.995	100.765	0.76	1.08	5	5.4
19.8	100.76	100.765	0	0.38	4.8	1.82
Total =						26.43

1. Provide vent area 30% of area on defined C/S of R.T.L. i.e. 26.43 sq.m

= 26.43 x 30% = 7.977 Sq.m.

2. No.of 1000 mm. dia. pipe.

    =  7.977/ (ㄫ/4 x 1.00 ²) = 10.15 Nos 

Say 11 Nos.

Calculation of area on define C/S at RTL = 101.705

 

Chainage	G.L.	R.T.L.	Diff. between R.T.L. and G.L.	Mean Diff.	Length	Weighted area
-22.2	101.705	101.705	0	-	-	-
-20	101.445	101.705	0.26	0.13	2.2	0.28
-15	100.695	101.705	1.01	0.63	5	3.15
-10	100.125	101.705	1.58	1.29	5	6.45
-5	100.045	101.705	1.66	1.62	5	8.1
0	99.955	101.705	1.75	1.7	5	8.5
5	99.695	101.705	2.01	1.88	5	9.4
10	99.365	101.705	2.34	2.17	5	10.85
15	99.995	101.705	1.7	2.02	5	10.1
20	100.795	101.705	0.9	1.3	5	6.5
25	100.605	101.705	1.09	0.99	5	4.95
30	101.165	101.705	0.53	0.81	5	4.05
35	101.615	101.705	0.09	0.31	5	1.55
45	101.705	101.705	0	0.04	10	0.4
Total =						74.28

Calculation of area between R.T.L. & P.B.L. on existing C/S

 

Chainage	G.L.	R.T.L.	Diff. between R.T.L. and G.L.	Mean Diff.	Length	Weighted area
-34	100.9	100.9	0	-	-	-
-16	99.96	100.9	0.94	0.47	18	8.46
0	99.96	100.9	0.94	0.94	16	15.04
16	99.96	100.9	0.94	0.94	16	15.04
34	100.9	100.9	0	0.47	18	8.46
						47

3. Provide 16 No. of 1000 mm. dia. pipe as pipes can be accommodated in bed width i.e. 25 metres at define cross section.

PERCENTAGE OBSTRUCTION

A₁ = Area provided through vent

16 x 0.7854 x 1.00 ² = 12.56 Sq.m.

A2 = Area of protected bed

i.e. ares between Road top and top of Protected bed.

B = X + Y = Area of define cross section corresponding to protected bed top.

B = 74.24 Sq.m.

%bstruction = (1 - (A1 + A2)/B) x 100

= (1-(12.56 + 47.00)/74.24) x 100

= 19.77% < 30% hence Safe