Contents. Introduction Flash floods events Eastern desert Risk map. Mitigation of study. Literature Review. Methodology. Results.

Transcription

1 Flash Floods Risk Assessment in The Eastern Desert By Eng. Mona M. Mohamed Faculty of Engineering, Ain Shams University Cairo, Egypt Contents. The main goal Flash floods events Eastern desert Risk map Mitigation of study INTRODUCTION The events in Egypt and its effect. (1969,1980,1984,1985,1994) Flash floods events Eastern desert Risk map Mitigation of study INTRODUCTION Eastern Desert as a semi arid region is the study area. Heavy rainfall has caused floods across Egypt including Al Arish

2 Flash floods events Eastern desert Risk map INTRODUCTION Flash floods events Eastern desert Risk map INTRODUCTION Mitigation of study Mitigation of study yet no risk map for the whole Eastern Desert region was established. 1-The Red Sea Coastal Plain 2-The Red Sea Mountain 5 6 By get the response of All watersheds A flood risk map with prioritization for the locations subjected to flooding in the Eastern Desert to the Same pseudo storm. towards the red sea coast using morphological parameters

3 MITIGATION of STUDY 1-GIS Get morpholog ical parameters 2- MCA Calculate SRF 3- HEC Get Q & Tp 4- Excell -Correlation bet. Q & Tp Vs SRF -get WSRF - Definition : Sharp and unexpected "are the two best words to use to characterize a flash flood and its hydrograph. - Properties of : The hydrograph of a flash flood has very specific characteristics: It generally has -a single very high peak discharge, -the flood volume is not necessarily important, -the duration of the entire flood event is short, and -the time to peak is within six hours Definition : Although there is no agreement among hydrologic experts on the distinct classification of arid and semi-arid based on their annual rainfall, the following categories may be generally identified (Soliman 2010): 1. Areas where the annual total rainfall is less than 70 millimeters per year and evaporation exceeds the yearly rainfall may be classified as extreme desert areas. Two-thirds of the Middle East region can be classified as desert. 2. Areas where annual total rainfall is between 70 and 200 millimeters per year with sparse vegetation are called arid. 3. Areas where total annual rainfall is between 200 and 450 millimeters are classified as semi-arid. The Mediterranean Sea coast is classified as a Mediterranean zone with rainy and moderately warm winters and dry summers and can be considered to be between arid and semi-arid Definition : the terms arid and semi-arid zone are applied to those areas where rainfall will not be sufficient for regular rain-fed farming (FAO, 1981; Walton, 1969) UNESCO has taken the ratio of precipitation to potential evapotranspiration (ET) as an aridity index: P/PET<0.03 hyper arid zone 0.03<P/PET<0.2 arid zone 0.2<P/PET<0.5 semi-arid zone 12 3

4 - Climate and natural characteristics : The natural geography of the arid and semi-arid zones is complex and differs from site to site. infrequent rainfall serious soil loss and erosion Drought low cover ratio Poor vegetation cover - Example of arid zones : It is located at the western side of the Gulf of Suez, Eastern Desert, Egypt. Wadi Ghoweibba represents one of the largest basins in the Eastern Desert of Egypt. It covers an area of 3043 km2 with 70 km length and basin perimeter of 298 km. Parts of the arid and semi-arid of Egypt are located in Sinai and Red Sea Governorate. This is distinguished by the scarcity of rain and relative high moisture content. The average rainfall received by the Eastern Desert annually ranges between 2.75 and >50mm annually. Precipitation is most concentrated between September and October, with storms of short duration and limited area Example of arid zones : - Definition : Receives small amounts of rainfall (25.0 mm/year) mainly in autumn and winter (El-Rakaiby, 1989). A watershed can be defined as the area of land that catches water from precipitation and snowmelt. The water then drains to a common waterway, such as, a stream. Each stream has its own watershed. refers to the physical characteristics of the watershed

5 - characteristics and its effect : - characteristics and its effect : Q AREA Large Area Length Q Slope Steep Slope Shape Small Area Q Shorter Length Mild Slope Time Longer Length Time Time characteristics and its effect : - characteristics and its effect : Drainage Density Q Less Rough Hydraulic roughness Q High Density More Rough Time Low Density Time

6 Risk Gha dir El-G em al 53 most dangerous dangerous medium dangerous less dangerous not dangerous Study Area 7 1 Sam ada y2 N akry U rayar Um Abbas Ghso un R ing a Lah m y Qul aan Al Adaya W N S E - El-Shamy ( 1992) - Elmoustafa (2012) used a Weighted Normalized Risk Factor (WNRF) for floods risk assessment. The four parameters used are: AREA, SLOPE, time of concentration and runoff volume A weight coefficient (W) was assumed constant for all factors and equal to 1/ (No. of parameters) Class A (high groundwater potentiality and low flooding) Class B (low groundwater potentiality and high flooding,) Class C (intermediate groundwater potentiality and flooding) It was noticed during the analysis for a case study in the Eastern desert that the drainage basin area has a great effect on the floods generated at its outlet while other factors have less effect than the drainage area such as the slope and roughness RESOURCES TECHNOLOGY - RESOURCES TECHNOLOGY This Study used 9 morphological parameters risk map for watersheds that affect on Area from marsa alam to Ras Banas. Morphological parameters was used : 1-Area of watershed (A),2-Slope,3-Drainage Density,4-Drainage Frequency,5-RB=Nw-1/Nw 6-Rt = Nw/p 7-Roughness factor Rn = R*D where ; R= heights difference in km and D = Drainage Density 8-Shape factor =1.27A/P2 where ; A=watershed Area and P =watershed perimeter 9-Heights Factor equal where ; R=maximum height difference and LB distance between them 23 Study Area & the Problem Definition RISK VALUE=4*((X X Min. )/(X Max. X Min.))+1 Then summation value of risk was calculated and gets risk for every watershed, watersheds were classified in to five categories: most dangerous (5) dangerous (4) medium dangerous (3) less dangerous (2) not dangerous(1) Kilometers 6

7 Area ( km2) Length. Longest flow path. Slope( ). Streams number(1-1216) Streams Lengths -The Digital Elevation Model (DEM) from NASA Shuttle Radar Topographic Mission (SRTM) has provided digital elevation data with a 90 m resolution Drainage frequency (F)= Streams No./A Drainage Density (D)= Streams Lengths/A Surface flow length (L o ) = 1/2D Area Standardized Risk Factor (ASRF) =( Area Area Min. )/(Area Max. Area Min.) The box plot technique was applied to avoid extreme high values Area (km2) Shape factor (Ish) = 1.27(A/P^2) Time of concentration (Tc)= ( (L^0.77)/S^0.385) number

8 w4*dsrf w3*fsrf w5*losrf w2*ssrf w1*asrf WSRF w6*isrf w7*tcsrf Precipitation. Loss method(scs curve number) Transform method (The SCS unit hydrograph). WSRF= Σ (W i x SRF i ) Time to peak 1 0,93 0,9 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0 0,21 0,85 0,14 0,21 0,02 0,02 Peak discharge

9 0,45 0,40 0,35 0,30 0,25 0,20 0,15 0,10 0,05 0,00 0,43 0,00 0,39 0,06 0,10 0,01 0,01 Low Risk Low to moderate Risk Moderate Risk High Risk Very High Risk W= correlation value / Σ (correlations) ,2 0,1 0-0,1-0,2-0,3-0,4-0,5-0,6-0,7 1/Area Slope 1/Tc 1/F -0,57-0,67-0,6-0,2 Shape factor 1/Lo 1/D -0,25 0,11-0,17 0,30 0,25 0,20 0,15 0,10 0,05 0,00 0,27 0,23 0,24 1/Area Slope 1/Tc 0,10 0,08 0,00 1/F Shape factor 1/Lo 0,07 1/D W= correlation value / Σ (correlations)

10 Low Risk Low to moderate Risk Moderate Risk High Risk Very High Risk Low to moderate Risk Moderate Risk High Risk Very High Risk The Egyptian eastern desert is having a long coast along the Red sea very high risk high risk moderate moderate to low 9% 13% 43% of more than 1000 km length, Sixty eight main watersheds discharging eastward into the red sea were studied, area of watersheds were found to range between 38 km 2 and 41,474 km 2 and the slope were found to range between and and the relation 35% A new criterion was developed to evaluate the risk factor for the floods resulting from watersheds generated within the Red Sea Mountains and flowing to the Red Sea. These criteria could be used in other places with similar characteristics. The box plot test represented a very useful, easy to use and quick tool when trying to exclude extremely high parameter that may lead to unrealistic risk factor

11 Eastern Desert watersheds discharging into the Red Sea are classified into four categories where 9% of watersheds have very high risk, 13% has high risk, 35% has moderate risk and 43% has moderate to low risk. The drainage basin area is the morphological parameter that has the highest effects on the peak floods generated followed by time of concentration and slope; other factors have less effect such as the shape factor, drainage frequency, drainage density and surface runoff length. For future studies, The Weighted Standardized Risk Factor (WSRF) obtained can be used during the design of flood protection measurements and/or the calculation of design peak flows for crossing structures. This may lead to more economic design procedure that can be adopted in drainage design guidelines and manuals. Studies should be carried out to investigate how to implemented this in the design procedure It is recommended to study curve number as an important morphological parameter in future assessment that affect on the generated hydrograph realistic approach. The produced Risk map is helpful to know the locations that have high flood risk in order to prevent loss of life and minimize damages to property. The drainage basin slope is the morphological parameter that has the highest effect on the time to peak followed by the shape factor. This technique is to be investigated in other places that importance to our future development plans such Sinai coast Ashraf M. Elmoustafa 2012, Weighted Normalized Risk Factor for Floods Risk Assessment Ain Shams Engineering Journal (ASEJ), April El-Shamy, I. Z. 1992, New approach for hydrological assessment of hydrographic basins of recent recharge and Thank you for your attention! flooding possibilities. 10th Symp. Quaternary and Development, Egypt, Mansoura Univ., 18 April, p Baptista et al. "Multi criteria evaluation for urban storm drainage", First SWITCH scientific meeting University of Birmingham, UK, 9-10 Jan Horton, R. E., Drainage basin characteristics, Trans. Am. Geophysics. Union, vol. 13, pp ,