Promoting improved traditional systems – two strategies
1. Promote effective, innovative traditional systems from one area to another
2. Improve existing traditional systems with new techniques
Examples
Diversions 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
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
MOREOVERTHESE 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
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
Diversion structures/ damien (stone bund)
Large stones to control scour
Stepped downstream facedissipates energy
Channel
Flow
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
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, EthiopiaRetrogression of soft bed levelscould be controlled by combination of bed stabilizersand soil bunds
Vegetative Bank Protection
Natural vegetative bank protectionprevents the river from breaching andtaking new course
In many system there are bans on cutting the bank vegetation
Traditional bank protection
Man-made bank protection using tamarix (Sheeb, Eritrea)
Existing bank withTamarix aphylla
Newly planted tamarix
New bundto preventwater fromspilling backto main river
Improved flood channel intake
Wadi Siham, Yemen
Enforced river embankment
Traditional diversion spurreinforced with gabions
Channel intake and bed level fixed
No gate provided
Farmers do not want Agency (TDA) to control the intake
Construction: Ungated Intake
Design: gated intake
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
Improved side spillway
Head section of the canalTo limit flows to flood channelsMost effective if flow control structure is provided in channel downstream
Wadi Rima, Yemen
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
Escapes in flood channel (Hadramawt)
Stones are removedto prevent that large floods enter and damage commandarea