1. IMPROVED TRADITIONAL SYSTEMS
6.3
IMPROVED TRADITIONAL SYSTEMS

2. Promoting improved traditional systems – two strategies
Promote effective, innovative traditional systems from one area to another
Improve existing traditional systems with new techniques

3. Examples Diversions Flood channels Traditional diversion structures
Fixing of river intakes
‘Algama and damien’
Soil and gabion bunds
Bed stabilizers
Bank protection
Flood channels
Channel intakes
Channels
Escapes
Drop structures
Flow dividers

4. DIVERSIONS

5. Diversions/ free intake

6. Diversion structures / deflector type

7. Diversion/ bunds or weir

8. ALL THESE TRADITIONAL STRUCTURES BREAK IN VERY HIGH FLOODS – THE COMMAND AREA IS THEREFORE AUTOMATICALLY PROTECTED FROM THE HIGH FLOOD DAMAGE AND MOST COARSE SEDIMENT IS FLUSHED DOWN THE RIVER BED

9. MOREOVER THESE TRADITIONAL STRUCTURES HAVE FLEXIBILITY – AND THEIR POSITION CAN BE ADJUSTED TO CHANGES IN THE LOCATION OF THE LOW FLOW CHANNEL OR CHANGES IN RIVER BED LEVEL

10. Diversion structures/ algamas
Conical stone structures with circular base of 3-4 meters,
foundation 2 meter deep, large stones filled with smaller ones
Used for:
- bank protection
- hard points for canal entrance or rejection spillway

11. Diversion structures/ damien (stone bund)
Channel
Flow
Large stones
to control scour
Stepped downstream face
dissipates energy

12. Diversion structures/ soil and gabion bunds
Soil bund:
in river with low gradients and soft materia
at angle with flow / soften the impact
guide water over large distance
easy to construct and low cost
manipulate river bed levels (force bed level up)
gabion sections

13. Bed stabilizers
Where river beds degrade, one looses command and peak floods do no longer naturally spill over
Use gabion bed sills with surface skin of concrete
Can control retrogression of river and stabilize river bed level
Less useful in aggrading rivers (rising river beds)
Yandofero River, Ethiopia
Retrogression of soft bed levels
could be controlled by
combination of bed stabilizers
and soil bunds

14. Vegetative Bank Protection
Traditional bank protection
Natural vegetative bank protection
prevents the river from breaching and
taking new course
In many system there are bans on
cutting the bank vegetation

15. Man-made vegetative bank protection

16. Man-made bank protection using tamarix (Sheeb, Eritrea)
Existing bank with
Tamarix aphylla
New bund
to prevent
water from
spilling back
to main river
Newly planted tamarix

17. CHANNELS

18. Improved flood channel intake
Wadi Siham, Yemen
Enforced river embankment
Channel intake and bed level fixed
Traditional diversion spur
reinforced with gabions

19. GATED OR UNGATED?

20. Construction: Ungated Intake
No gate provided
Design: gated intake
Farmers do not want
Agency (TDA) to control the intake

21. Avoid stoplogs on gated intakes:
Difficult to operate

22. In other cases gated intakes are useful
Can regulate flow into flood channel
Take largest possible size for manual handling – so trash can pass
Radial gates easier to operate (manually up to 5 meter) but expensive
Vertical gate up to 2 meter can be operated manually
Avoid stop logs – very difficult to handle in floods
Use of breast walls:
to set maximum flow limit
to minimize size and weight of the gate
Make provisions for rising command levels
Accept that gates will never be fully closed:
Farmer do not want to loose water
Difficult to quickly open and close

23. Improved side spillway
Wadi Rima, Yemen
Head section of the canal
To limit flows to flood channels
Most effective if flow control structure is provided in channel downstream

24. Traditional channels - slopes
Usually constructed on prevailing landslope – usually do not need drop structures to reduce slope and velocity
Bed slopes often around 3.5 to 4.5 meter/km – much steeper usually than canals in modernized systems
Do not suffer from siltation
Do not suffer from scour either, even though they run at high velocity because discharges usually far less than the maximum

25. Escapes in flood channel (Hadramawt)
Stones are removed
to prevent that large
floods enter and
damage command
area

26. Drop structure
Farmer-built drop structures