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ASEAN-German Mini Hydro Project (AGMHP)
Diversion Structure and Intake
1 Slide 1Consulting and Engineering
ASEAN-German Mini Hydro Project (AGMHP)
Intake
�Able to divert the design flow but discharge excess flows and reduce sediments and floating matter
�� submerged orifice type intake with sill
�Sediments should not choke the intake:
2 Slide 2Consulting and Engineering
not choke the intake: low weir height, sloped structure and flushing gate.
normal water level
ASEAN-German Mini Hydro Project (AGMHP)
The purpose of a dam or weiris to raise / control the water level in the stream so that sufficient quantities of watercan be diverted into the intake of the hydropower plant.
Purpose of the diversion structure
3 Slide 3Consulting and Engineering
of the hydropower plant. Upstream of the weir is no reservoir.
(NB: A dam has a more important size than a weir. The objective
of the dam is, therefore, to raise the water level significantly and
to allow for water storage. Upstream of the dam is a reservoir.)
ASEAN-German Mini Hydro Project (AGMHP)
The purpose of the water intakeis to abstract water from a riveror a pond and deliver it to a canal, penstock or storage basin.
The main challenge is that intakes must operate under a
Purpose of the water intake
4 Slide 4Consulting and Engineering
intakes must operate under a full range of flows from low to flood, sometimes handle large quantities of silt, sand and gravel or floating debris ranging from full grown trees to leaves and weed.
During normal operation, the water level at the weir should be kept at the highest possible level, without spilling water over the weir, to maintain the maximum head
ASEAN-German Mini Hydro Project (AGMHP)
Small weir with short canal:
The water level at the weir is controlled by the flow through the turbine. The flow through the turbine must be equal to
Overall design considerations
5 Slide 5Consulting and Engineering
the turbine must be equal to the flow in the river and the water level upstream of the weir should be at the weir crest. During high flows and floods, water spills over the weir or spillway.
ASEAN-German Mini Hydro Project (AGMHP)
Maintain maximum water level at the weir but do not spill water over weir unless discharge is higher than turbine capacity
Small weir with short canal
6 Slide 6Consulting and Engineering
power
house
ASEAN-German Mini Hydro Project (AGMHP)
Small weir with long canal followed by forebay: The water level in the forebayshould be kept at its maximum level. The flow into the canal is controlled by a
Overall design considerations (contd.)
7 Slide 7Consulting and Engineering
the canal is controlled by a headgate at the weir.
Dam with reservoir: Water levels in reservoir fluctuates because of storage operation. Maximize power generation following reservoir management plan.
ASEAN-German Mini Hydro Project (AGMHP)
Flow into canal can either be controlled by water level in forebay or by headgates at the intake into the canal
If the flow is controlled by water level in forebay, that water level needs to be lowered to allow larger flowsinto the canal and increased in the case of small flows to maintain maximum head
Small weir with long canal followed by forebay
8 Slide 8Consulting and Engineering
forebay
to maintain maximum head
ASEAN-German Mini Hydro Project (AGMHP)
Terminology of a typical intake with sand trap
Side spillway
Flushing gateSluice gate
Weir or diversion
structure
9 Slide 9Consulting and Engineering
Intake orifice
Gravel trap
Sand trap
Coarse rack
Headrace
Intake gate
Sluice gate
ASEAN-German Mini Hydro Project (AGMHP)
Terminology of a typical intake with sand trap (cont.)
Side Spillway
Flushing GateSluice Gate
Weir
10 Slide 10Consulting and Engineering
Intake Orifice
Sand trap Trash Rack
Headrace Pipe
Intake Gate
Sluice Gate
Intake Channel
ASEAN-German Mini Hydro Project (AGMHP)
The weir as diversion structure
11 Slide 11Consulting and Engineering
ASEAN-German Mini Hydro Project (AGMHP)
Weir Flow
sharp crested
free overfall
nappe
12 Slide 12Consulting and Engineering
submerged flow
rounded
broad crested
�adherent(if nappe is not sufficiently aeriated)
ASEAN-German Mini Hydro Project (AGMHP)
Flow over weirs
13 Slide 13Consulting and Engineering
ASEAN-German Mini Hydro Project (AGMHP)
Example: Flow over weirs
�Qflood = 50 m3/s
�b = 12 m
�free overflow -> c = 1.0
�µ = 0.7
14 Slide 14Consulting and Engineering
Q =2
3c µ b 2g H1.5
→ H = (3Q
2 c µ b 2g )
23
= (3 × 50
2 ×1.0 × 0.7 ×12 × 2 × 9.81 )
23 =1.60m
�H = ?
ASEAN-German Mini Hydro Project (AGMHP)
Seepage flow always happens.
Problem is progressive piping
Downstream erosion supports progressive piping
Leads eventually to failure
Failure mechanisms
15 Slide 15Consulting and Engineering
Seepage
flow
ASEAN-German Mini Hydro Project (AGMHP)
Watch seepage flows carefully, especially watch for increasing
seepage flows, this is a signal for possible failure
Watch for deflections and deformations of weir body
Watch for new fractures, cracks or changes of existing fractures
Watch for cavitation damage at spillways and stilling basin
Watch for erosion downstream of weir or spillway or stilling basin,
erosion behind wing walls, especially after floods
Monitoring and Maintenance
16 Slide 16Consulting and Engineering
erosion behind wing walls, especially after floods
ALL THESE OBSERVATIONS REQUIRE IMMEDIATE
EVALUATION BY AN ENGINEER
Maintain regular written protocols, such as monthly reports
ASEAN-German Mini Hydro Project (AGMHP)
Water Intakes
17 Slide 17Consulting and Engineering
Sound design of the water intake is decisive for proper functionality of the whole hydropower plant and important to reduce cost for operation and maintenance of the plant.
ASEAN-German Mini Hydro Project (AGMHP)
There are many different types of intakes, depending on specific situation:
• Free intake / bank intake without dam or weir
• Free intake / bank intake with temporary weir
Intake types
18 Slide 18Consulting and Engineering
• Side intake with solid dam or weir (Most common type of intake used for micro and mini hydropower development)
• Tyrolean weirs or Coanda type intake for steep mountain streams
ASEAN-German Mini Hydro Project (AGMHP)
As a basic principle, intakes should always be located on the outer side of a river bend to minimize sediment in headrace.
Location and design of the intake
19 Slide 19Consulting and Engineering
sediment in headrace.
Sluice gates in the intake are provided to allow flushing of deposited sediments from the intake
ASEAN-German Mini Hydro Project (AGMHP)
Bed load transport
20 Slide 20Consulting and Engineering
ASEAN-German Mini Hydro Project (AGMHP)
Typical layout of a side intake with cross weir
21 Slide 21Consulting and Engineering
ASEAN-German Mini Hydro Project (AGMHP)
The water level in the river is not dammed up for diversion (no cross weir or dam)
Costs for the headworks can significantly be reduced
Free intake / bank intake without dam or weir
22 Slide 22Consulting and Engineering
Can only be applied, if a small proportion of the dependable river flow is to be abstracted and if there is always enough water depth and head available in the riverfor diversion.
Ideal locations for free intakes are generally at natural control sections such as rock outcrops or big boulders.
ASEAN-German Mini Hydro Project (AGMHP)
A temporary weir is a low cost solution made of boulders, rocks and branches
Operators are required to rebuild the weir after every
Free intake / bank intake with temporary weir
23 Slide 23Consulting and Engineering
Operators are required to rebuild the weir after every major flood
The reliability of such an intake structure is seriously reduced.
ASEAN-German Mini Hydro Project (AGMHP)
Most common type of intake used for small hydropower development
Design difficulties of side intakes with weirs are related to the foundation of the weir or dam. Unless there is solid rock available in the river bed, the dam structure
Side intake with solid weir or dam
24 Slide 24Consulting and Engineering
solid rock available in the river bed, the dam structure requires large volumes of concrete or stone masonry in order to achieve sufficient resistance against sliding under flood conditions. Seepage through permeable ground underneath the dam or weir is a major cause of dam failure.
ASEAN-German Mini Hydro Project (AGMHP)
Typical layout of a side intake
25 Slide 25Consulting and Engineering
ASEAN-German Mini Hydro Project (AGMHP)
Section of a typical side intake
26 Slide 26Consulting and Engineering
ASEAN-German Mini Hydro Project (AGMHP)
Sections of side intake with cross weir
27 Slide 27Consulting and Engineering
ASEAN-German Mini Hydro Project (AGMHP)
Flow through submerged orifice
Q = µ a b 2g z
28 Slide 28Consulting and Engineering
ASEAN-German Mini Hydro Project (AGMHP)
Example: Flow through submerged orifice
�Qdesign = 0.5x1.2=0.6 m3/s
�z = 0.10 m
�a = 0.40 m
�b = ?
29 Slide 29Consulting and Engineering
Q = µ a b 2g z
→ b =Q
µ a 2g z
=0.6
0.8 × 0.4 × 2 × 9.81× 0.10=1.34m
�b = ?
ASEAN-German Mini Hydro Project (AGMHP)
Submerged intake orifice
30 Slide 30Consulting and Engineering
ASEAN-German Mini Hydro Project (AGMHP)
Most importantly, design has to be good to allow diversion of required amounts of water into canal or penstock with minimum possible headloss.
Trash and floating debris should be kept away and sediment should be kept from entering the headrace
Design
31 Slide 31Consulting and Engineering
• Submerged wall
• Floating bar
• Coarse trash rack
• Sluice gate
ASEAN-German Mini Hydro Project (AGMHP)
Has specifically been designed for mountain torrents (river gradients from 1 % to over 10 %) with coarse bed load during floods
Comprises a collecting channel across the river covered by a screen or a perforated plate.
The intake screen is
Typical layout of a Tyrolean Weir
32 Slide 32Consulting and Engineering
The intake screen is designed to work as a self-cleaning screen, however, this may not always be the case in reality
ASEAN-German Mini Hydro Project (AGMHP)
Typical layout of a Tyrolean Weir
33 Slide 33Consulting and Engineering
ASEAN-German Mini Hydro Project (AGMHP)
Typical arrangement of a lateral / Tyrolean Intake
34 Slide 34Consulting and Engineering
ASEAN-German Mini Hydro Project (AGMHP)
Typical sections and details of a Tyrolean Intake
35 Slide 35Consulting and Engineering
ASEAN-German Mini Hydro Project (AGMHP)
Intake with submerged perforated plate
Alternative Design
36 Slide 36Consulting and Engineering
ASEAN-German Mini Hydro Project (AGMHP)
Summary: parameters and requirements
• For stream gradients above 2.5% a Tyrolean intake should be used, for other cases or a lateral diversion is recommended.
• The water intake must divert the required stream flow at all times and all water levels in the river.
• The Intake location and design should reject bed load and
37 Slide 37Consulting and Engineering
• The Intake location and design should reject bed load and prevent excessive flood water from entering the system.
• An intake orifice (always submerged) should be provided to reduce the abstracted flow when the river / stream is in flood.
• Stop logs shall be provided in order to completely close off the system when required.
• Provision shall be made to exclude large floating debris, and gravel from the diverted water.
ASEAN-German Mini Hydro Project (AGMHP)
The designer has to provide a weir regulation scheme that defines how the weir must be operated at different flows, especially during floods
=> Operation follows the design
Operation
38 Slide 38Consulting and Engineering
Transport of silt and sediment occurs mostly during high flows, often only a few days per year, depends on size of sediment
No sediment should enter power canalWhen sediment transport occurs, open sluice gate at preset upstream water level to flush sediment downstream
ASEAN-German Mini Hydro Project (AGMHP)
Typical tasks for the operator include:
• Coarse trash rack at intake to be cleaned when necessary
• Remove floating debris from floating bar or submerged wall
• Close intake when sediment transport occurs during floods
• Watch for vortices that cause headloss, can depend on flow
Operation
39 Slide 39Consulting and Engineering
• Watch for vortices that cause headloss, can depend on flow
• Watch for debris and sediment deposition, remove manually or by opening sluice gate during high flows or while turbine is shut down
• Check movable gates regularly
• Check structure similarly like dam and weir, especially after floods
ASEAN-German Mini Hydro Project (AGMHP)
Watch for bank erosion behind wing walls, respond immediately if this happens
Open sluice gates to lower water level and allow
Operation during floods
40 Slide 40Consulting and Engineering
water level and allow flushing of sediment from behind weir
After flood, watch for downstream erosion and damage in stilling basin