The document describes the design of a forebay for a hydropower system. It begins by outlining the key components and functions of a forebay. It then provides design guidelines and parameters to consider, such as volume, depth, width, and spillway size. Two design examples are presented. The first designs a forebay with a discharge of 2 cubic meters per second and the second designs one with a discharge of 12 cubic meters per second conveyed by two penstocks. Both examples calculate the necessary dimensions and design characteristics of the forebay based on the given parameters.

1. 1

2. Design of Forebay
PRESENTED BY HADIQA QADIR
2K22-MS-HIE-02
CIVIL ENGINEERING DEPARTMENT (CED), UCE&T, BZU, MULTAN
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3. OUTLINE Of The Topic
 Components Of Hydropower
 Introduction Of Forebay
 Function Of Forebay
 Components Of Forebay
 Hydraulic and Hydrological design aspects
 Design Flowchart
 Design Guidelines
 Design Steps
 Design Parameters
 Design Case I
 Design Case II
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4. Components of Hydro Power System
The scheme in which water is supplied to hydro power system has the following components.
Main parts:
1. Head work
2. Intake structure
3. Head race (canal)
4. Forebay
5. Head pond
6. Penstock
7. Power house
8. Tail race
4

5. Introduction of Forebay
Forebay is a structure like a small reservoir located at the end
of the water passage from the reservoir and before the water is
fed to penstock.
We can define it as,
An impoundment immediately upstream of a diversion dam
or hydroelectric plant intake, where water is temporarily
stored before going into penstock.
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6. Introduction of Forebay
 A forebay is required in the case of run-of river plants at the upstream of
diversion work.
 When canal leads water to the turbines the section of the canal in front of
turbines is enlarged to create forebay.
 The reservoir acts as forebay when penstock takes water directly from it.
 Some projects, such as those associated with a large dam having a deep
power intake, may have no specifically designed forebay. Again, other
projects may need more than one forebay; for example, a forebay at the
entrance to the headrace canal and a second forebay at the power intake at
the downstream end of the headrace canal
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7. Introduction of Forebay
7

8. Introduction of Forebay
 The forebay temporarily stores water for supplying the same to the
turbines.
 The storage of water in forebay is decided based on required water
demand in that area. This is also used when the load requirement
in intake is less.
 It forms the transition between the reservoir or conveyance canal
and the power intake and as such is designed to facilitate the
necessary entry flow conditions at the power intake.
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9. Components of Forebay
 Entrance Bay or Basin/Tank Body
 Escape weir/spillway
 Fine Trash rack
 Flushing gate
 Water level control system
 Penstock Inlet
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spillway

10. Flushing Gate
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11. Components of Forebay
Escape weir/spillway
 Spillway is constructed to act as a safety valve. It discharge the
overflow water to the d/s side when forebay is full. The preferred
location for the escape weir is in the rim of the forebay tank. Where this
is not practical for topographic reasons the escape weir should be
located at the nearest suitable site upstream of the forebay tank.
 A simple overflow weir is recommended with a design head that can be
contained within the normal canal freeboard. Weir discharge should be
routed towards a natural water course of adequate capacity or a ditch
provided that is suitably protected against erosion.
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12. Components of Forebay
Fine Trash rack
The fine trash racks are used to protect the turbine from small debris. They
can also, in some cases, prevent fish or eel to pass through it.
Flushing Gate
The accumulated debris has to be flushed out from time to time. It is used
to facilitate flush out of any sediment or debris that might settle in the
bottom of the forebay tank and can be drawn in to the penstock. This can
be done by opening the flushing gate or valve. In this manner the material
is allowed to return to the river bed.
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13. Components of Forebay
Water level control system:
A water level control system requires that real time water level
measurements in the forebay tank and tailrace canal be transmitted
to the turbine governor. In the water level control mode the governor
will estimate the inflow to the forebay tank and adjust the wicket
gates to correct for difference between turbine and canal flows so as
to maintain forebay tank levels within a prescribed range. A float
type water level gauge with electronic data transmitter is used for
this purpose.
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Wicket gate is are the series of adjustable
vanes used to guide water in turbines.

14. Functions of Forebay
 Provide a volume of stored water to permit water level control of turbine operation.
 Reduces the entry of air into penstock pipe, which in turn could cause cavitation (explosion of the trapped
air bubbles under high pressure)
 Flow adjustment: the forebay tank and escape weir facilitate the adjustment of turbine discharge due to
system load changes by diverting surplus flow over the escape weir back into the river.
 Water level control: For small hydro plants connected to the grid it is convenient to match turbine output to
available flow, thereby maximizing use of available water.
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15. Hydraulic and Hydrological design
aspects
 Fish exclusion
 Flow characteristics including flow patterns and velocity distribution in the forebay, and particularly at the
approach to the power plant intake.
 The submergence required at the intake and any depth requirements at sluiceways
 Minimization of hydraulic losses
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16. Flow Chart to design forebay
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 Submergence head is the minimum level of
water required above the penstock pipe to
prevent the entry of air into penstock pipes
 Retention time is the time during which
turbine will be shutdown and water will be
stored in forebay. It is taken as 3-4 minutes.

17. PLAN & SECTION OF FOREBAY
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18. Design Guide Lines of Forebay
 The design discharge for power generation is linked to the turbine and the diameter of the pipe and the
velocity of the flow in the penstock linked to the penstock design.
 Set the width of the forebay. As a thumb rule, to start the design, it can be assumed that the length of the
forebay will be 2 to 2.5 times this size. However according to the required volume of water above the
penstock and to meet the site conditions the designer may have to change this ratio.
 Set the clearance of the penstock from the bottom of the forebay. This is to avoid that particles and
sediments settled in the forebay get in the penstock. The minimum clearance is 0.30 m. While, 0.50 to 1 m
is a common and reasonable value to use.
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19. Design Guide Lines of Forebay
 For the design of the forebay it is considered that it should be available (to cope with the flow variations in
the turbine during normal operation conditions) a buffer volume equivalent to 15 seconds of supply at the
design flow. This is also automatically calculated as well as its corresponding depth of water.
 Set the discharger coefficient of the spillway. This depends on the shape of the crest and spillway.
 Set the depth of water over the crest of the spillway. Have in mind that the higher the water depth the
smaller will be the crest length of the spillway. The spillway is designed to spill all the design flow in
case the powerhouse is shut down and no flow is going through the penstock.
 Set the freeboard for the spillway. This is a safety margin in case higher flows than the design flow arrive
to the forebay. Consider a value that is half the water depth over the weir crest. This will increase, if
necessary, in 50% of the discharge capacity of the spillway
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20. Design Guide Lines of Forebay
 The length of the spillway is automatically calculated using the previous input data and the weir equation.
It has to be noted that this value must be smaller than the length of the forebay.
 Note that the crest of the spillway will be placed 0.05 m above the Normal Water Level(NWL) so that
small changes in the flow or fluctuations or turbulence in the water surface (like wind) do not cause an
immediate spilling.
 The spillway should be sized such that it can release the entire design flow when required. This is because
if the turbine valve is closed during emergencies, the entire design flow will have to be spilled from the
forebay until the operator reaches the intake or other control structures upstream of the forebay.
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21. Design Guide Lines of Forebay
 The trash rack at the forebay should be placed at 1:3 slope for both efficient hydraulic
performance and ease of cleaning.
 To minimize headless and blockage, the recommended velocity through the trash rack is 0.6 m/s.
but a maximum of 1 m/s could be used.
 Set the angle of the transition walls at the entrance of the penstock. This transition must be
smooth so mild angles are recommended (around 20%). However to fit site conditions the values
can be adjusted.
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22. Design Steps of Forebay
Calculate
discharge
through
forebay
Assume
detention
time if not
given
Calculate
volume of
forebay
Assume
free
board
and
settling
Height
Calculate
submerge
nce head
above
penstock
Calculate
total
depth of
forebay
Calculate
width of
forebay
for normal
and worst
case
Calculate
length of
forebay
for total
head
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23. FOREBAY DESIGN PARAMETERS
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 Design Discharge = Qd
 Forebay Discharge= Qf = 2(Qd)
 Volume Of Forebay = V = Qf x t x 60
 Detention time = t (3-4 minutes)
 Limiting Velocity In Forebay = Vf (0.2-0.6 m/s)
 Submergence Head = Hs, Hs >1.5
𝑉𝑝2
2𝑔
Or 0.5 Vp 𝐷𝑝0.5
 Diameter of Penstock = Dp or 𝜑
 Total Head/ Depth Of Forebay = H = H-drawdown + Hs + dia of penstock
+ Freeboard + minimum bottom height H
 Total Head in worst condition = Hw = Hs + dia of penstock
 H-drawdown/H-downsurge = Vp
𝐿 𝐴𝑝
𝑔 𝐴𝑓
, but we will take it 0 in our design examples

24. Design Case-1
Design a forebay with Design discharge of 2mᶟ/s, flow is carried into a penstock of diameter 1.5m. Limiting velocity is
0.3m/s.
Solution
 Step1: Discharge through the forebay = Qf =2(Qd) = 2x2=4 mᶟ /s
Assume detention time, t = 3 min
 Step 2: Volume of forebay = V = Qf * t*60 = 4*3*60 =720 mᶟ
 Step 3: Determination of Total Head/depth of forebay = H-downsurge + Hs + Diameter of Penstock + Freeboard +
minimum bottom height
take Free board = 1m Settling height/Bottom height = 1m,
Diameter of Penstock = 1.5m (given)
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25. Solution
 For Submergence Head, Hs
Hs >1.5
𝑽𝒑𝟐
𝟐𝒈
Or 0.5 Vp 𝑫𝒑𝟎.𝟓
(Select Greater Value)
Vp = Velocity In Penstock,
We Know that Q=A*V ; Area Of Penstock = (𝜋𝐷2
)/4 = 𝜋*1.52
/4 = 1.766m²
Vp = Qd/Ap = 2/1.766 =1.1325 m/s
Then, Hs is
=(1.5 *1.13^2)/(2*9.81)=0.0977m
Or
= 0.5*(1.13)*(1.5)^0.5=0.69m
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26. Solution
Taking Greater Value,
Hs = 0.69m ≈ 0.7m
Also, assume H-downsurge = 0
 Total Head/depth of forebay = H-downsurge + Hs + Diameter of Penstock + Freeboard + minimum bottom
H = 0+0.7+1.5+1+1 = 4.2m
 Step 4: Determination of Forebay Width
For normal case B= Qf/(H*Vf) = 4/(4.2*0.3) =3.175m
Also assumed Vf = 0.3 m/s
For worst case
Total Head against worst cond = Hw = Hs + diameter of Penstoke = 0.7+1.5 = 2.2m
B՛ = Qf/(Hw*Vf) = 4/(2.2*0.3) = 6.06 m (Select greater value of B)
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27. Solution
 Step 5 Determination of Length of forebay against worst case
L= volume/(Bw*H) = 720/(6.06*2.2) = 55m
Dimensions of forebay = (55m*6.06m*4.2m)
 Step 6 Determination of Spillway Length (Ls)
As we know that discharge eq for spillway is
Qf = Cd*L* (𝑯𝒄)𝟏.𝟓
Take head over crest of spillway, H-spillway = 0.5m, Cd = 1.7
So, Ls = Qf/(Cd*(𝑯𝒄)𝟏.𝟓
) = 4/(1.7*0.5^1.5) = 6.65m ≈ 7m
L-spillway < L-forebay
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28. Solution
 Step 7: Design Check
Check 1 Since, L-spillway < L-forebay (OK)
Check 2: Let’s check for limiting velocity, for which forebay and
settling depth are not considered,
Vf = Qf/(Bw*Hw) = 4/(6.06*2.2) = 0.3 m/s (OK)
Note: Here “V” which we calculated is the average horizontal flow
velocity of the water inside the forebay after the water enters in the
forebay. It is the velocity of water along the length of forebay.
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29. Design Case-2
Design a forebay with turbine discharge 12 m3/s. Water conveyed from forebay to powerhouse by 2
penstock of 2m diameter each. The retention time is 3 minutes and limiting velocity is 0.2m/s.
Solution
 Qd = 12 mᶟ/s
Diameter of penstock = 2m
Discharge in each penstock = qd = 12/2= 6mᶟ/s
Discharge for forebay = 2* Qd = 2*12 = 24mᶟ/s
Given retention time, t = 3 min
Area of penstock = Ap = (pi*2^2)/4 = 3.1415 m²
Velocity in penstock, Vp = qd/ap = 6/3.1415= 1.91m/s
Given Limiting velocity (Vf) = 0.2m/s
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30. Solution
 Volume of forebay (V) = Qf *t*60
= 24*3*60 = 4320 mᶟ
 For height (H)
take Free board = 1m Settling height = 1m,
And submergence head, Hs
Hs >1.5
𝑉𝑝2
2𝑔
Or 0.5 Vp 𝐷𝑝0.5 (Select Greater Value)
Then, Hs is =(1.5 *1.91^2)/(2*9.81)=0.278m Or = 0.5*(1.91)*(2)^0.5=1.47m
Take greater value, Hs = 1.47m
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31. Solution
 Total head/depth of forebay = H- downsurge + Hs + diameter of penstock + freeboard +minimum bottom
height
H = 0+1.47+2+1+1 = 5.47m
 For Width,
For normal case, B = Qf/(H*V) = 24/(5.47*0.2) = 21.937m
For worst case,
Total head against worst case= Hw = Hs + diameter of penstoke = 1.47+2 = 3.47m
B՛= Qf/(H*V) =24/(3.47*0.2) = 34.5 m
 For length
L= volume/(Bw*H) = 4320/(34.5*3.47) = 36m
Dimensions of forebay = (36m*34.5m*5.47m)
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32. Solution
 For Length of spillway (Ls),
Qf = Cd*L* 𝑯𝒔𝟏.𝟓
Take head over crest for spillway= Hc = 0.5m
Coefficient of discharge = Cd = 1.7
Then, Ls = Qf/(Cd*𝑯𝒄𝟏.𝟓) = 24/(1.7*0.5^1.5) = 9.98m ≈ 10m
 Since, L-spillway < L-forebay (OK)
 Let’s check for limiting velocity, for which forebay and settling depth are not considered,
V = Qf/(Bw*Hw) = 24/(34.5*3.47) = 0.21 m/s > 0.2m/s (OK)
Note: Here “V” which we calculated is the average horizontal flow velocity of the water inside the forebay after the
water enters in the forebay. It is the velocity of water along the length of forebay.
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33. Air Vent Pipe at inlet of penstock

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34. References
 BPC Hydroconsult. (2012). Civil Works Guidelines for Micro-Hydropower in Nepal.
 Chow, V. T. (1959). Open-Channel Hydraulics. McGraw-Hill.
 Micro/Mini Hydropower Design Aspects, Micro/Mini Hydropower Design Aspects - Pakistan
Poverty ...www.ppaf.org.pk › hre › publications › vol 11 mhp design-final-05-12-13, Carlos Martins,
Ajoy Karki, Ulrich Frings, 2013.
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