6.7Improved Traditional Systems

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

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

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/ algamasConical 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 vegetative bank protection

Man-made bank protection using tamarix (Sheeb, Eritrea)

Existing bank withTamarix aphylla

Newly planted tamarix

New bundto preventwater fromspilling backto main river

CHANNELS

Improved flood channel intake

Wadi Siham, YemenEnforced river embankment

Traditional diversion spurreinforced with gabions

Channel intake and bed level fixed

GATED OR UNGATED?

No gate provided

Farmers do not want Agency (TDA) to control the intake

Construction: Ungated Intake

Design: gated 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

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

Drop structure

Farmer-built drop structures