Module 1 :
description
Transcript of Module 1 :
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
The recorded rise time is usually the time step of
the equipment
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
Turbulence caused by flows in two channels
converging
Flow
Floods do not like obstacles
Flood water hitting bridge piers has nowhere to go
except upwards!
Flow
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
Wettest years have over four times the volume of the driest years
Wadi Zabid – Annual Number of Floods and Flood Volumes
Less than 20 to more than 80 spates per year
Baseflow• 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
• 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 volume
Baseflow volume
Total flow volume
Wadi Zabid – Flood Volume Distribution
80% of water is in flood flows of
less than 100m³/s
Damage mechanisms
• Abrasion• Scour• Force and energy of water• Trash blockage• Trash caught in and breaking gabions• Seepage and washouts
Sediment• 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
Floods can transport sediment – 10% or more by volume during a major flood
Abrasion from sediment loadReinforced
concrete slab being eroded
Sediment can erode concrete
Concrete being eroded once the
stone protection layer has been removed
baffle blocks are very vulnerable
Baffle blocks in sluiceways are very
vulnerable to damage
Sediment can block intakes
and choke canals
Sediment size distribution
Trash• 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
unless provision is made to keep it out
Scour• Scour is one of the major mechanisms of
flood damage• Scour is the movement of bed or bank
material during floods
Scour locations• 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
Nose of spur has been
undermined and has collapsed
Size distribution of large bed sample (Wadi Laba, Eritrea)
D50 = 40mm
Finally, if the engineering is not good .....
This weir was blown up because it
interfered with base flows