Module 1 :

Understanding the Wadis

John Ratsey [john.ratsey@ntlworld.com]

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

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

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 load

Reinforced 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