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Module 1 :. Understanding the Wadis. John Ratsey []. Wadi Hydrology. The classic feature of wadi hydrology is the floods (also called spates), which rise very quickly and then recede over a period of hours or days

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Module 1

Module 1 :

Understanding the Wadis

John Ratsey []

Wadi hydrology
Wadi Hydrology

  • The classic feature of wadi hydrology is the floods (also called spates), which rise very quickly and then recede over a period of hours or days

  • The peak of the spate will progressively reduce once it is in the flood plains and the water is diverted or can naturally spread

Wadi mawr hydrographs
Wadi Mawr Hydrographs

The recorded rise time is usually the time step of the equipment

A wadi rima spate
A Wadi Rima Spate

Often another spate arrives before the first has receded

Hydrological statistics based on upstream gauging stations (peak flow, base flow, flow volumes) may not be completely applicable to the diversion sites

Floods have energy
Floods have energy


Turbulence caused by flows in two channels converging

Floods do not like obstacles
Floods do not like obstacles

Flood water hitting bridge piers has nowhere to go except upwards!


Wadi zabid frequency of floods
Wadi Zabid – Frequency of Floods

An average of 5.7 floods per year between 50 and 100m³/s. 5.7 + 2.4 +1.1 + 0.4 = 9.6 floods per year exceeding 50m³/s

Annual flow wadi zabid
Annual flow – Wadi Zabid

Wettest years have over four times the volume of the driest years

Wadi zabid annual number of floods and flood volumes
Wadi Zabid – Annual Number of Floods and Flood Volumes

Less than 20 to more than 80 spates per year


  • There may also be a base flow in the wadi which may be continuous through all (annual) or part (seasonal) of the year

  • There can be difficulty distinguishing between base flow and the last part of a flood recession

  • In many wadis the base flow has reduced or disappeared during the past 30 years due to increased abstraction of surface and groundwater upstream

  • The proportion of water coming as floods therefore increases

Baseflow volume
Baseflow volume

  • The total volume of a small baseflow for a long time can be substantial

  • 1m³/s for 365 days per year = 31.5 million m³

  • But it does not require much small-scale upstream development to use 1m³/s

Flood flow and base flow
Flood flow and base flow

Total flow volume

Baseflow volume

Flood flow volume

Wadi zabid flood volume distribution
Wadi Zabid – Flood Volume Distribution

80% of water is in flood flows of less than 100m³/s

Damage mechanisms
Damage mechanisms

  • Abrasion

  • Scour

  • Force and energy of water

  • Trash blockage

  • Trash caught in and breaking gabions

  • Seepage and washouts


  • Sediment is a major feature of the floods

  • The sediment includes fertile soil which the farmers want

  • And also includes sand / gravel / boulders which can damage structures and block canals

Abrasion from sediment load
Abrasion from sediment load during a major flood

Reinforced concrete slab being eroded

Sediment can erode concrete
Sediment can erode concrete during a major flood

Concrete being eroded once the stone protection layer has been removed

Baffle blocks are very vulnerable
baffle blocks are very vulnerable during a major flood

Baffle blocks in sluiceways are very vulnerable to damage

Sediment can block intakes
Sediment can block intakes during a major flood

And choke canals
and choke canals during a major flood

Sediment size distribution
Sediment size distribution during a major flood

Trash during a major flood

  • Intakes are usually designed to catch the cleaner surface water

  • However, the cleaner surface water is where the floating trash is concentrated

  • Trash screen collects big trash

  • Bigger trash collects medium trash

  • Medium trash collects small trash

  • Intake blocked

  • Try to provide intakes which can pass big trash without any screens but also make provision for trash removal

Trash can obstruct intakes
Trash can obstruct intakes during a major flood

Scour during a major flood

  • Scour is one of the major mechanisms of flood damage

  • Scour is the movement of bed or bank material during floods

Scour locations
Scour locations during a major flood

  • Scour may happen anywhere in the wadi channel but is most severe on the outside of bends or where obstacles in the wadi cause flow turbulance

  • The depth of scour may not be evident after the flood since scour holes tend to be infilled during the recession – only the consequential damage remains

Scour damage to spur
Scour damage to spur during a major flood

Nose of spur has been undermined and has collapsed

Finally if the engineering is not good
Finally, if the engineering is not good ..... during a major flood

This weir was blown up because it interfered with base flows