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FOREBAY
Page 1 of 14
DESIGN OF FOREBAY
PROJECT ABC PROJECT
RIVER ABC RIVER
REGION PAKISTAN
DESIGN ENGINEER M. ASIF
INPUT
- BASIC DATA
Minimum diverted water - Headrace = 1.20
Design discharge - Penstock = 3.00 ( = Peak discharge)
- INFLOWS
Length of headrace = 5000.0 m
Diameter/Bed width of headrace 2.50 m
Depth of flow in headrace 1.18 m
X-sectional area of flow = 2.94
Velocity in headrace = 0.41 m/s
Volume of headrace = 14710
Time of travel (upto forebay) = 12258 sec
- OUTFLOWS
Length of penstock = 500.0 m
Diameter of penstock = 1.00 m
X-sectional area of flow = 0.79
Velocity in penstock = 3.82 m/s
Volume of penstock = 393
Time of travel (upto turbines) = 131 sec
- PEAKING TIME
Peaking time = 0.1 hrs
Non-peaking time = 23.9 hrs
Volume of water required for peaking = 1080
- FOREBAY DIMENSIONS
Width of forebay (at start) = 20.0 m
Width of forebay (at end) = 12.6 m
Length of forebay = 23.1 m
QH m3/s
QP m3/s
LH
DH
dH
AH m2
vH
VH m3
tH
LP
DP
AP m2
vP
VP m3
tP
TPK
TN-P
VPK m3
WFs
WFE
LF
FOREBAY
Page 2 of 14
Depth of forebay (including freeboard) = 8.0 m
Freeboard in forebay F.B = 1.5 m
Slope for vertical transition z = 1.0 1v : zh
Length of vertical transition = 7.5 m
Δx of vertical transition = 5.3 m
SIDE WALLS GEOMETRY
Width of side walls at top = 0.5 m
Width of side walls at bottom = 0.8 m
Extension of bottom slab at each end = 0.8 m
Thickness of bottom slab at each end = 1.0 m
Thickness of bottom slab in center = 0.5 m
Length of thicker bottom slab = 10.0 m should be < 10
TRANSITION LENGTH (PLAN VIEW)
Diameter/Bed width of headrace = 2.5 m
Transition angle β = 15 degree
Transition length Ref: L.W. Mays
Transition length - horizontal (required) = 32.7 m
Transition length (provided) = 33.0 m
- CONCRETE VOLUME (FOREBAY STRUCTURE)
Thickness of top slab = 0.3 m
PCC blinding layer (0.1m thick) YES
Top slab NO
X-sectional area of top slab = 0.00
X-sectional area of bottom slab = 22.60
X-sectional area of side walls = 12.35
Total x-sectional area = 34.95
Concrete volume of main structure = 808.8
Concrete volume of u/s transition = 576.68 (approx)
Concrete volume of spillway = 128.99 (approx)
dF
LVT
ΔxVT
wt
wb
ws
tsE
tsC
lb
DH
LHT
LHT
tt
Ats m2
Abs m2
Asw m2
AXS m2
Vms m3
Vut m3
Vsp m3
tan2HF
T
DWL
FOREBAY
Page 3 of 14
Total concrete volume of forebay = 1514.51
PCC quantity in blinding layer = 53.69
- HEADLOSSES
FRICTION LOSS
Friction headloss (Manning)
Flow area A = 130.00
Wetted perimeter P = 33.00 m
Hydraulic radius R = 3.94 m
Flow velocity V = 0.023 m/s
Friction headloss in forebay = 4.5E-07 m
ENTRANCE LOSS Ref: Mosonyi
K = 0.10
Entrance headloss = 0.002 m
GATE SLOT LOSS
Where Ref: Mosonyi
"Vol-I: page # 530"
and Weisbach coefficient
Velocity (just before the slots) = 0.8 m/s
Width of entrance flume B = 2.50 m
Depth of entrance flume h = 2.50 m
Width of gate slot = 0.30 m
Depth of gate slot = 0.10 m
For gate slots = 0.06 m
Coefficient (gate slots) = 0.933
Weisbach coefficient (gate slots) = 0.930
Head loss at gate slots = 0.0004 m
TOTAL HEADLOSS IN FOREBAY (Entrance + Friction + Gate Slot)
Total headloss in Forebay = 0.002 m
VTF m3
CPcc m3
m2
hf
hLE
vin
eg
dg
if d > 0.2e then y* = 0.2e
& if d <= 0.2e then y* = d
yg*
βg
αg
hLG
hLT
3/4
22
R
LVnh f
g
vKhL 2
2
2
22
,
11
22.1
g
vh sl
yBhyBh
Bh
F
F
y
*2
337.063.0
FOREBAY
Page 4 of 14
- LONGITUDINAL SECTION
Headrace invert / BL at end = 1457.62 m asl
Water level at end of headrace = 1458.80 m asl
(Wall top level will be constant throughout the entire length of forebay including u/s transition)
Wall top level = 1460.29 m asl
Bed level of forebay at start = 1452.29 m asl
Longitudinal slope of forebay S = 1.0 %
Bed level of forebay at end = 1451.79 m asl
Maximum water level in forebay (end) = 1458.79 m asl
Allowable fluctuation in water level = 1.4 m
Minimum water level in forebay = 1457.39 m asl (provided)
Minimum water level in forebay = 1456.24 m asl (required)
- PEAKING CAPACITY CHECK
Useful capacity of forebay (designed) = 648 (live storage provided)
Flow available from headrace = 432
Useful capacity (required)
Useful capacity (required) = 648 (live storage required)
satisfactory for mechanical governors. For digital governors the control volume can
be further reduced
= O.K. ۩
Water level in forebay masl = 1452 1456 1457 1458 1459
Useful volume in forebay = 0 671 1065 1458 1851
B.Lhr
WHR
ELWT
B.Ls
B.Le
Wmax
σallow
Wmin
Wmin
VF m3
VHA m3
VF-R m3
Normally a volume of QP x 120 m3 (or two minutes at maximum plant flow) will be
Check VF ≥ VF-R
m3
0.0 5.0 10.0 15.0 20.0 25.0 30.01446.01448.01450.01452.01454.01456.01458.01460.01462.0
Longitudinal Section
Distance (m)
Elev
ation
(m a
sl)
HPPKRF QQTV 3600
FOREBAY
Page 5 of 14
Total volume in forebay = 0 1828 2222 2615 3009
Time Spill
(hrs)
0 0 648 0 648 0
1 6:PM 1.2 648 3 0 0.00
2 7:PM 1.2 648 3 0 0.00
3 8:PM 1.2 648 3 0 0.00
4 9:PM 1.2 648 3 0 0.00
5 10:PM 1.2 648 1.2 648 0.00
6 11:PM 1.2 648 1.2 648 0.00
7 12:PM 1.2 648 1.2 648 0.00
8 1:AM 1.2 648 1.2 648 0.00
#NAME? 9 2:AM 1.2 648 1.2 648 0.00
Lower W.L. 1456.24 masl 10 3:AM 1.2 648 1.2 648 0.00
11 4:AM 1.2 648 1.2 648 0.00
12 5:AM 1.2 648 1.2 648 0.00
13 6:AM 1.2 648 1.2 648 0.00
14 7:AM 1.2 648 1.2 648 0.00
15 8:AM 1.2 648 1.2 648 0.00
16 9:AM 1.2 648 1.2 648 0.00
17 10:AM 1.2 648 1.2 648 0.00
18 11:AM 1.2 648 1.2 648 0.00
19 12:AM 1.2 648 1.2 648 0.00
20 1:PM 1.2 648 1.2 648 0.00
21 2:PM 1.2 648 1.2 648 0.00
22 3:PM 1.2 648 1.2 648 0.00
23 4:PM 1.2 648 1.2 648 0.00
24 5:PM 1.2 648 1.2 648 0.00
- TRASHRACK
Rack Cleaning Machine RCM = no (yes or no - in small letters)
Width of trashrach Ref: GTZ Publication
"High head Hydropower"
Number of rack bars
Width of rack bars t = 12.0 mm
Clearance between rack bars b = 150.0 mm
Angle of bars with horizontal α = 76.0 degrees
Width of Trashrack = 2000.0 mm
Channel width / diameter = 1863.0 mm
m3
Forebayinflow
Initial forebayvolume
Forebayoutflow
Final forebayvolume
(m3/s) (m3) (m3/s) (m3) (m3/s)
bra
bch
freeboard
Peak storage
Min. operating WL:
penstock
chra bntb
1b
bn ch
FOREBAY
Page 6 of 14
Number of rack bars n
" " " n = 11.4 no.
Effective head from center of penstock = 6.8 m
Diameter of bellmouth for penstock = 1.5 m
Entrance velocity in bellmouth = 1.70 m/s (calculated)
Ref: Mosonyi
Average flow velocity between bars "Vol-I: page # 537"
Average flow velocity between bars = 0.81 m/s
Ref: ESHA Guide
Minimum trashrack area (required) = 0.80
Minimum trashrack area (required) = 0.38
Trashrack area provided = 4.00
= O.K. ۩
- SPILL SECTION
Provide Spilling Arrangement YES
Crest level of spill section = 1458.79 m asl
Width of spill section (crest length) = 10.0 m
Allowable surcharge over crest H = 0.75 m
Discharge over ogee crest Ref: C.V. Davis
" " " " Q = 14.17
= O.K. ۩
he
Dbm
vbm
vbars
Arack m2
Arack m2
Check Arack-provided ≥ Arack-min
ELC
LS
m3/s
Check Q ≥ 2.0QP
2/3HCLQ S
b
btvv ebars
sin
11
erack v
Q
bt
tA
DDbm 5.1
FOREBAY
Page 7 of 14
- SPILL CHANNEL OPTION
= 156.0 m
Bed width of spill channel = 1.62 m
Depth of water in spill channel = 1.26 m Ref: C.V. Davis
Free board in spill channel = 0.50 m
Transition angle β = 15.0 degree
Transition length (horizontal) = 16.0 m
Bed elevation of channel at forebay = 1458.3 m asl
Bed elevation of channel at end = 1457.5 m asl (required)
Slope of spill channel = 0.0050 m/m
Friction Coefficient (concrete) n = 0.015
0 1460.1
0 1458.3
1.6 1458.3
1.6 1460.1
0 1459.6
1.6 1459.6
Manning's Equation
Flow velocity in spill channel = 2.94
Discharge of spill channel = 6.03
= O.K. ۩
Unit discharge of spill channel = 3.72
Critical depth in spill channel
" " " " " = 1.121 m
Rating curve at given distance x = 0.0 m (from start)
Length of spill channel (Δx) LS
wS
dS
f.b.S
LT
BLS
BLE
SS
vS m/s
QS m3/s
Check QS ≥ 1.5QP
qs m3/s/m
yc
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.81457
1457.5
1458
1458.5
1459
1459.5
1460
1460.5
0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.001457.0
1457.5
1458.0
1458.5
1459.0
1459.5
1460.0
1460.5Rating Curve (Spill Channel Option)
Flow (cumecs)
Wat
er E
leva
tion
(m a
sl)
3/2
2/1
2
1
SS
SSSSS dw
dwSdw
nQ
cs yfb 4.0
s
ss w
3/12
g
qy s
c
FOREBAY
Page 8 of 14
Elevation Flow
1458.29 0.00
1458.47 0.37
1458.64 1.06
1458.82 1.89
1459.00 2.81
1459.17 3.79
1459.35 4.81
1459.52 5.85
1459.70 6.92
1459.88 8.00
1460.05 9.10
CONCERETE VOLUME FOR SPILL CHANNEL OPTION
Thickness of top slab = 0.20 m
Width of side walls at top = 0.30 m
Width of side walls at bottom = 0.40 m
Thickness of bottom slab = 0.30 m
Extension of bottom slab at each end = 0.30 m
Total depth of headrace channel = 1.77 m
PCC blinding layer (0.1m thick) YES
Top slab NO
X-sectional area of channel (RCC area) = 2.15
Concrete quantity in bottom slab = 141.34
Concrete quantity in side walls = 193.28
Concrete quantity in top slab = 0.00
Total concrete quantity (RCC) = 334.62
PCC quantity in blinding layer = 47.11
0.0 1458.0 2.3 1458.3
0.0 1458.3 2.3 1460.1
tt
wt
wb
ts
ws
hc
Axc m2
CB m3
CS m3
Cts m3
CRcc m3
CPcc m3
0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.001457.0
1457.5
1458.0
1458.5
1459.0
1459.5
1460.0
1460.5Rating Curve (Spill Channel Option)
Flow (cumecs)
Wat
er E
leva
tion
(m a
sl)
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.51456.5
1457
1457.5
1458
1458.5
1459
1459.5
1460
1460.5
LtwwbC ssbB 22
LwwhhwC tbcctS 22
1
LtwbC ttts 2
FOREBAY
Page 9 of 14
0.3 1458.3 2.6 1460.1
0.4 1460.1 2.7 1458.3
0.7 1460.1 3.0 1458.3
0.7 1458.3 3.0 1458.0
2.3 1458.3 0.0 1458.0
- SPILL PIPE OPTION
Diameter of spill pipe = 2.00 m
Depth of flow = 1.50 m
= 156.0 m
Mannings friction factor (steel) n = 0.0130
Flow area A = 2.527 Ref: C.V. Davis
Wetted perimeter P = 4.189 m
Hydraulic radius R = 0.603 m
Top width T = 1.732 m
Hydraulic depth D = 1.459 m
Ref: C. Nalluri
f = 0.87
a = 4.189
h = -0.500
2θ = 240.00
Mannings equation
Flow velocity V
" " " " " = 3.88 m/s
Discharge capacity = 9.82
Maximum discharge capacity = 11.58
= O.K. ۩
Rating curve at given distance x = 0.0 m (from start)
DS
ys
Length of spill channel (Δx) LS
m2
QS m3/s
QSmax m3/s (at y = 93.8% of di)
Check QS ≥ 1.5QP
2.00
yθ
z
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.51456.5
1457
1457.5
1458
1458.5
1459
1459.5
1460
1460.5
3/22/11RS
nV
FOREBAY
Page 10 of 14
Elevation Flow
1458.3 0.00
1458.5 0.22
1458.7 0.94
1458.9 2.11
1459.1 3.63
1459.3 5.38
1459.5 7.23
1459.7 9.01
1459.9 10.52
1460.1 11.47
1460.2 11.58
STEEL QUANTITY FOR SPILL PIPE OPTION
Thickness of steel sheet (provided) = 8.0 mm
Density of steel = 7850.0 Ref: Flowadvisor
Minimum thickness of steel sheet (K = 500 & D in mm)
Ref: Warnick
Minimum thickness of steel sheet = 6.25 mm
Corrosion allowance = 1.00 mm
Effective thickness of penstock = 7.00 mm
= O.K. ۩
X-sectional area of steel = 0.050
Volume of steel = 7.87
Weight of steel = 61.8 tons Ref: Mosonyi
Weight of steel (5% increse for joints) = 64.9 tons
Width of steel sheet w = 6.33 m
Length of steel sheet l = 6.0 m
No. of steel sheets n = 26 no.
- ENERGY DISSIPATION FOR SPILL CHANNEL/PIPE
Froude Number Ref: C.V. Davis
ts
ρs kg/m3
tmin
ca
te
Check te ≥ tmin
As m2
Vs m3
Ws
Ws
0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.001457.0
1457.5
1458.0
1458.5
1459.0
1459.5
1460.0
1460.5Rating Curve (Spill Pipe Option)
Flow (cumecs)
Wat
er E
leva
tion
(m a
sl)
gD
VF R
400min
KDt
4410002 2
2
i
si
s
dt
dA
FOREBAY
Page 11 of 14
(Where V & D are velocity and depth of flow entering the jump)
Froude Number - before the jump = 0.84 dimensionless
Ratio of conjugate depths Ref: L.W. Mays
Depth before the jump = 1.26 m
Depth after the jump = 0.99 m
Length of stilling basin =
" " " " = -1.4 m
Alternatively
Length of stilling basin
" " " " = 4.77 m
Ref: L.W. Mays
Height of walls of basin =
" " " " = 1.4 m
Height of baffle blocks =
" " " " = 3.2 m
Number of baffle blocks n = 2
Width of baffle blocks = 0.41
SCOUR DEPTH BELOW WATER SURFACE
Scour depth of rivers subject to fluctuating flood throughout the year
Scour depth below water surface (in British unit system)
Scour depth below water surface R = 0.88 m 2.00
- FLUSHING PIPE
Flushing discharge = 0.60
Diameter of flushing pipe = 0.80 m
Length of flushing pipe = 8.5 m
Mannings friction factor (steel) n = 0.0130
Effective head from center of pipe = 6.1 m (maximum)
Minimum effective head = 3.6 m
Ref: L.W. Mays
Flow velocity (orifice flow) 0.60
FR1
(Recommended Froude Number 4.5 ≤ FR1 ≤ 9, for stable & steady hydraulic jump)
D1
D2
LB α (D2 - D1)
LB
LB
HB D2 + 0.1 * (D1 + V1)
HB
hB 2.5 * D1
HB
wB
Cr =
Qf m3/s (20% of QP)
Df
Lf
he
hemin
Cd =
gD
VF R
38.025.4
RB F
DL
ed ghCV 2
10/3
78.0
rC
QR
1815.0 21
1
2 RFD
D
FOREBAY
Page 12 of 14
Flow velocity (orifice flow) V = 5.0
Discharge (orifice flow) = 2.52
= O.K. ۩
STEEL QUANTITY FOR FLUSHING PIPE
Thickness of steel sheet (provided) = 6.0 mm
Density of steel = 7850.0 Ref: Flowadvisor
Minimum thickness of steel sheet (K = 500 & D in mm)
Ref: Warnick
Minimum thickness of steel sheet = 3.25 mm
Corrosion allowance = 1.00 mm
Effective thickness of penstock = 5.00 mm
= O.K. ۩
X-sectional area of steel = 0.015
Volume of steel = 0.13
Weight of steel = 1.01 tons Ref: Mosonyi
Weight of steel (5% increse for joints) = 1.06 tons
Width of steel sheet w = 2.53 m
Length of steel sheet l = 6.0 m
No. of steel sheets n = 2 no.
- IMPACT TYPE STILLING BASIN FOR FLUSHING PIPE
This is an impact-type energy dissipator equipped with a hanging-type, L-shaped baffle,
contained in a relatively small boxlike structure, which requires no tail water for successful
performance.
The use of the impact-type stilling basin is limited to installation where the velocity at the
entrance to the stilling basin does not greatly exceed 30 ft/s (9.1 m/s). For discharges
m/s
Qo m3/s
Check Qo at hemin ≥ Qf
ts
ρs kg/m3
tmin
ca
te
Check te ≥ tmin
As m2
Vs m3
Ws
Ws
exceeding 10 m3/s, it may be more economical to consider multiple units side by side.
ed ghCV 2
400min
KDt
4410002 2
2
i
si
s
dt
dA
FOREBAY
Page 13 of 14
Diameter of flushing pipe = 2.62 ft 0.80 m
Flushing discharge = 88.89 2.52
Df
Qf ft3/s m3/s
FOREBAY
Page 14 of 14
Flow velocity (orifice flow) V = 16.43 ft/s 5.0
Froude Number = 1.79 dimensionless
W / D = 4.70 dimensionless
Where:
Depth of flow entering the basin D = 2.33 ft = 0.71 m
(D = square root of flow area)
Inside width of stilling basin W = 10.93 ft = 3.33 m
Alternatively:
W = 6.00 ft = 1.83 m
Loss in energy (from graph) = 53 %
Select the larger value of width i.e.
Inside width of stilling basin W = 10.93 ft = 3.33 m
The dimensions of various components as shown in the above figure, are given below;
H = 8.20 ft = 2.50 m
L = 14.58 ft = 4.44 m
a = 5.47 ft = 1.67 m
b = 4.10 ft = 1.25 m
c = 5.47 ft = 1.67 m
d = 1.37 ft = 0.42 m
e = 1/12 W e = 0.91 ft = 0.28 m
t = 0.91 ft = 0.28 m
= 0.55 ft = 0.17 m
= 10.93 ft = 3.33 m
m/s
FR1
W / D ratio (from DOSD graph)
Inside width of stilling basin (from graph)
EL/E1
Total depth of basin at start (H = 3/4 W)
Total length of basin (L = 4/3 W)
Distance of baffle from inlet (a = 1/2 W)
Height of baffle (b = 3/8 W)
Depth of basin at end (c = 1/2 W)
Length of baffle top (d = 1/8 W)
Thickness of baffle (t = 1/12 W)
Riprap stone size (drock = 1/20 W) drock
Length of riprap (Lrr = W) Lrr