IMPROVED TRADITIONAL SYSTEMS 6.3 IMPROVED TRADITIONAL SYSTEMS
Promoting improved traditional systems – two strategies Promote effective, innovative traditional systems from one area to another Improve existing traditional systems with new techniques
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
DIVERSIONS
Diversions/ free intake
Diversion structures / deflector type
Diversion/ bunds or weir
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
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
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) Channel Flow Large stones to control scour Stepped downstream face dissipates energy
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, Ethiopia Retrogression of soft bed levels could be controlled by combination of bed stabilizers and soil bunds
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
Man-made vegetative bank protection
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
CHANNELS
Improved flood channel intake Wadi Siham, Yemen Enforced river embankment Channel intake and bed level fixed Traditional diversion spur reinforced with gabions
GATED OR UNGATED?
Construction: Ungated Intake No gate provided Design: gated intake Farmers do not want Agency (TDA) to control the intake
Avoid stoplogs on gated intakes: Difficult to operate
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 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
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 removed to prevent that large floods enter and damage command area
Drop structure Farmer-built drop structures