d.01 civil structures en vol 1

ASEAN-German Mini Hydro Project (AGMHP)

Diversion Structure and Intake

1 Slide 1Consulting and Engineering


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:


not choke the intake: low weir height, sloped structure and flushing gate.

normal water level


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


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.)


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


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


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


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.


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


power

house


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.)


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.


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


forebay

to maintain maximum head


Terminology of a typical intake with sand trap

Side spillway

Flushing gateSluice gate

Weir or diversion

structure


Intake orifice

Gravel trap

Sand trap

Coarse rack

Headrace

Intake gate

Sluice gate


Terminology of a typical intake with sand trap (cont.)

Side Spillway

Flushing GateSluice Gate

Weir


Intake Orifice

Sand trap Trash Rack

Headrace Pipe

Intake Gate

Sluice Gate

Intake Channel


The weir as diversion structure



Weir Flow

sharp crested

free overfall

nappe


submerged flow

rounded

broad crested

�adherent(if nappe is not sufficiently aeriated)


Flow over weirs



Example: Flow over weirs

�Qflood = 50 m3/s

�b = 12 m

�free overflow -> c = 1.0

�µ = 0.7


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 = ?


Seepage flow always happens.

Problem is progressive piping

Downstream erosion supports progressive piping

Leads eventually to failure

Failure mechanisms


Seepage

flow


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


erosion behind wing walls, especially after floods

ALL THESE OBSERVATIONS REQUIRE IMMEDIATE

EVALUATION BY AN ENGINEER

Maintain regular written protocols, such as monthly reports


Water Intakes


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.


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


• 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


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


sediment in headrace.

Sluice gates in the intake are provided to allow flushing of deposited sediments from the intake


Bed load transport



Typical layout of a side intake with cross weir



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


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.


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


Operators are required to rebuild the weir after every major flood

The reliability of such an intake structure is seriously reduced.


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


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.


Typical layout of a side intake



Section of a typical side intake



Sections of side intake with cross weir



Flow through submerged orifice

Q = µ a b 2g z



Example: Flow through submerged orifice

�Qdesign = 0.5x1.2=0.6 m3/s

�z = 0.10 m

�a = 0.40 m

�b = ?


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 = ?


Submerged intake orifice



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


• Submerged wall

• Floating bar

• Coarse trash rack

• Sluice gate


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


The intake screen is designed to work as a self-cleaning screen, however, this may not always be the case in reality


Typical layout of a Tyrolean Weir



Typical arrangement of a lateral / Tyrolean Intake



Typical sections and details of a Tyrolean Intake



Intake with submerged perforated plate

Alternative Design



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


• 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.


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


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


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


• 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


Watch for bank erosion behind wing walls, respond immediately if this happens

Open sluice gates to lower water level and allow

Operation during floods


water level and allow flushing of sediment from behind weir

After flood, watch for downstream erosion and damage in stilling basin

d.01 civil structures en vol 1

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