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

Qq

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