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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
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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 Flow Turbulence caused by flows in two channels converging
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 Total flow volume Baseflow volume Flood 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
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
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
