Trench Weir Design

Design of Trench Weir
Design Discharge	Q40	1.47
Design Flood Discharge	Q100	120.00
Length of river	L	20.00	m
River bed level	Z	1020.00	amsl
Ratio of area of opening to total area of trashrack surface over trench	E1	0.50
Ratio of opening in the trashrack likely to be clogged	E2	0.40
Coefficient of discharge through broad crested weir	C	1.53
Coefficient of discharge through opening	Cd	0.46
Assumed length for trench	L'	10.00
Discharge to pass through weir	Q	2.21	m3/s	2.2
Specific Energy at any section of the stream	E	0.17
Width Required to pass design discharge	W	0.65		0.65
Width by Considering velocity
Velocity	v	0.75	m/s
Width	W	0.15	m	0.15
Width required for 50% clogging	W	0.30	m
Minimum width Required	Wmin	1.25	m
Final width of weir at top	W	1.25	m
Final Length of trench	L	10.39	m	10
constant depending on bottom profile of channel	η	0.50
Depth required	d	1.00	m
Area	A	1.25	m2
Velocity head at the end of trench	H	0.17	m3/s
Discharge	Q	2.26	m3/s
Final Depth	d	1.00	m
Slope of trench		0.06
Depth at end		1.625		1.63
Depth at Start (from Intake Criteria)		1.00
Trench bottom slope level at start		1019.00	amsl
Trench bottom slope level at end		1018.37	amsl
Check for adequency of waterway
Average depth		1.31
Area		1.64
Wetted Peimeter		3.25
Slope Provided	S	0.06
Mannings Coefficient		0.0018
Discharge	Q	25.22	m3/s
Check for adequency of capacity with 50% area clogged
Average depth		0.66
Area		0.82
Wetted Peimeter		2.56
Slope Provided	S	0.06
Mannings Coefficient		0.00
Discharge	Q	13.16
Slope of trashrack		0.10
Width of trashrack		1.30
Design flood discharge (Qflood)		120	m3/s
Discharge Passing through the Spilling portion		0	m3/s
Provide height of the weir above river bed level           =		0	m
Qweir(flood)	=	Cw * L weir * H overtop 3/2
coefficient of discharge		2.2	assumed
Weir Crest width L weir =		17
Head over crest =		2.18	m
River Bed level at Weir axis          =		1020.00	m
Crest level of weir                        =		1020.00	m
High Flood Level                          =		1022.18	m
Scour Depth Calculation
Design Flood	Q25		84.0	m3/s
Width of Stilling Basin @ Q25	B		17.0	m
Specific Discharge	q		4.94	m3/s
Average Particle size	D		7.28	mm
Silt Factor	f=1.76*D50^0.5		4.7
Lacy's Scour Depth	R=1.35 *(q^2/f)^(1/3)		2.3	m
Adopted U/S Scour Depth= Cutoff wall	1.25R		2.91	m
Adopted D/S Scour Depth= Cutoff wall	1.5R		3.5	m
Design flood discharge (Qflood)			85.0	m3/s
Discharge Passing through the Spilling portion			0	m3/s
Provide height of the weir above river bed level           =			0	m
Qweir(flood)	=	Cw * L weir * H overtop 3/2
coefficient of discharge			2.2	assumed
Weir Crest width L weir =			17
Head over crest =			1.73	m
High Flood Level			1021.73
Bottom of cut off U/S			1018.82
Height of Cutoff U/S			1.18	1.16
Bottom of cut off D/S			1018.23
Height of Cutoff D/S			1.79143	1.75