BASIC SEDIMENT TRANSPORT AND RIVER MORPHOLOGY

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BASIC SEDIMENT TRANSPORT AND RIVER
MORPHOLOGY??????? ??? ???????? ?????????? ?????
2
STATEMENT IN SPATE IRRIGATIONMANAGING
SEDIMENTATION IS AS IMPORTANT AS MANAGING WATER
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RIVER MORPHOLOGY

• Steep slopes and scarce vegetation in the
  catchments result in high concentration of
  sedimentation in the floods
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• The upper reaches of the wadi typically have
  steep slopes and they carry a wide range of
  sediments load from silts and clays to
  boulders
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• When the wadi reaches the plain wadi beds
  often increase by at least a factor 3
  (sedimentation) and sediment transport capacity
  is reduced
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THE WIDE FLOOD CHANNEL IN THE UPPER SECTION
TYPICALLY CONTAINS A NUMBER OF LOW FLOOD CHANNELS
THAT ARE FORMED BY THE PRECEEDING HIGH FLOWS
AND THE RECESSION FLOWS ?? ???? ????? ??????
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IN THIS SECTION WADIS CAN BE QUITE UNSTABLE AND
EVEN CHANGE DIRECTION IN A HIGH FLOODS UNLESS
THEY ARE CONTAINED IN THESE AREAS EMBANKMENTS
AND GUIDE BUNDS ARE IMPORTANT ?? ??? ????? ??
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In middle section?? ????? ??????

• RIVERS ARE TYPICALLY INCISED AND CONTAINED
  VEGETATIVE
• PROTECTION GENERALLY SUFFICES
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  ??????? ?????

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BECAUSE OF SEEPAGE, CHANNEL STORAGE AND
IRRIGATION ABSTRACTIONS, FLOODS FLOW REDUCE AS
THEY PASS DOWN AND THE WADI OFTEN REDUCES IN
DEPTH IN THE LOWER SECTIONS ???? ?????? ????
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IN MANY WADIS THERE IS HEAVY SCOUR AND
SEDIMENTATION???? ?? ??????? ??? ????? ??????? ??

• In Wadi Rima (Yemen ) measurements showed wadi
  bed levels dropping 1.5 meter and the filling
  again during high floods
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  ?????? ????? (1.5m ) ?? ???? ??? ???????? ????
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• During the flood season changes in loca bed
  level were on average 0.3 meter, with peaks
  meeasured value is 1 meter
• ???? ???? ??????? ?????? ?? ????? ????? ??
  ??????? ( (0.3m ?? ???? ???? ???? (1m)

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In some Wadis farmers try to influence the
scouring and sedimentation of the Wadi???????
????????? ??????? ??????? ??? ????? ???????? ??
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IN KHORAKAN RIVER (KHARAN, PAKISTAN) FARMERS
TRIED TO BLOCK THE RIVER WITH STRONG SOIL BUNDS
TO FORCE THE RIVER BED TO COME UP AGAIN. IT HAD
BECOME TOO DEEP AND PEAK FLOODS DID NO LONGER,
NATURALLY SPILL OVER. THIS HAD MADE IT IMPOSSIBLE
TO CONTROL THE HIGH FLOODS ?? ????? ???????
(????? ???????) ????????? ??????? ?? ????? ?????
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YANDOFERO RIVER, KONSO, ETHIOPIA
NEW BED LEVEL ????? ??????
OLD BED LEVEL ????? ??????
RIVER MORPHOLOGY GONE WRONGAFTER THE CUTTING OF
A RIVERAINE FOREST THE YANDOFERO RIVER CHANGE
ITS DOWNSTREAM COURSE AND BED LEVELS DROPPED 1-3
METERS ????????? ????? (???? ?????) ??? ??? ?????
????? ??? ????. ??? ??????? ??? ????? ?? ???????
??????? ????? ????? ??? ????? (1-3 m)
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SEDIMENT TRANSPORT?????? ????????
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SEDIMENT LOADS OF 5 OF WEIGHT ARE COMMON IN
FLOODS HIGH FLOODS MAY CARRY AS MUCH AS 10 ???
???????? ??????? ?? ????????? (5) ??? ????? ??
????????? ??????? ??? (10)
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SEDIMENT SETTLES DOWN AS FLOODWATER LOSES
SPEED???????? ????? ????? ???? ??????? ????
??????
FINE MATERIAL SETTLES ON THE LAND, MIDDLE FINE
MATERIAL IN FLOOD CHANNEL, COARSE MATERIAL IN
RIVER BED ?????? ??????? ????? ?? ????? ???????
?????? ??????? ?? ????? ???? ??????? ???????
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13
WITHIN THE WADI SYSTEM A MIX INCLUDING
COARSE MATERIAL IS USUALLY FOUND IN THE UPPER
SECTIONS ?? ???? ?????? ???? ????? ??? ??????
?????? ???? ????? ?? ??????? ??????? ??????
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CLAYS AND FINE MATERIAL SETTLE MORE IN THE TAIL
????? ??????? ??????? ????? ???? ?? ???????
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SEDIMENT RISE CAN BE QUICK AND EVEN
PROBLEMATIC?????? ???????? ???? ???? ????
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The fine sediment in one irrigation may reach 15
cm height! ??????? ??????? ?? ????? ??????? ??
??? ???????? ??? 15 ??
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SEDIMENT EXCLUDERS HAVE BEEN INTRODUCED IN A
NUMBER OF MODERN SYSTEMS???? ???????? ??? ?? ???
?? ??????? ???????
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SEDIMENT EXCLUDERS SOME EXPERIENCES??? ?????
?????? ????????
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Traditional mechanisms of managing high sediment
loads??????? ????????? ?????? ???????? ???????
?????

• Traditional systems often fail in time and
  floods with high sediment loads are not
  routed into the command area
• ??????? ????????? ????? ?? ????? ?????????? ??
  ??????? ??????? ?? ???????? ?? ???? ??? ???????
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• In some system floods with high (and
  infertile ) sediments are kept out of the
  command area by closing / breaking diversion
  structures
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  ???? ???? ???? ??????? ??????? ???? ?? ???
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• When there is field to field irrigation and
  enough slope a lot of sediment moves out of the
  command area with the breaking and erosion
  of the field bunds
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• Elsewhere exces s sediment is used to raise
  bunds
• ???? ?? ??????? ???? ???? ?????? ????? ??????
  ???? ??????
• When fields rise the intake is moved
  further upstream
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  ???????
• In the worst case an area is abandoned and a
  new area is developed
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