CHAPTER 11 PRELIMINARY DESIGN AND COST ESTIMATE CHAPTER 11 PRELIMINARY DESIGN AND COST ESTIMATE 11.1 DESIGN FLOOD PEAKS, STANDARDS, AND WADI CHARACTERISTICS Problem of flash floods in wadis is the one of the major problems in planning road network in the Sultanate. This section discusses this problem from hydrological viewpoint. 11.1.1 General Floods in Oman are hazardous, and difficult to predict: Mean velocities, sometimes in excess of 5 m/s, and high sediment flowage rates are a feature of torrential wadis. Estimates of flood flow with probable various risks of exceedance are indispensable for a wide range of engineering applications including: • Highway bridge, culvert and dam spillway design. • Assessment of flood risk, including flood zoning/flood risk mapping. • Establishment of flood-resistant design and development of flood alleviation, and protection works. Ministry of Regional Municipalities, Environment & Water Resources (MRMEWR) is responsible for collecting flood data in the Sultanate of Oman, in addition to defining flood features, and providing instructions in specifying national engineering design standards. The Hydromteorological Monitoring Network embraces 138 wadi gauges 325 rainfall stations; including 216 Autographic recorders is presented in Figure 11.1-1. Channel cross section can be utilized and assessed with the historical information such as flood debris marks from major historic floods, and with the slope, to estimate flood peaks. The return period of the flood event which the structure is designed to withstand with full factors of safety is commonly taken as the design standard. In some cases (e.g. large dams) there may be two standards, one which the structure should last without any deterioration, and an advanced standard at which repairable damage is allowed, providing the structure remains basically intact. 11 - 1 Sultanate of Oman Wadis and Location of Wadi Flood Gauges MM2 MM3 MM1 MM4 Khasab Nataba Al Bih Khabb Sh SS3 SS4 Hatta MH4 Hatta MH3 SharmSharm Mahdah FizhFizh B UmarFizh Gh MH2 Al Ayn B Umar Gh Al Jizi Al Jizi SS5SS6 Ajran Salahi Hilti Ahin Ahin Ahin Sarami M 1 Al Fatah H Al Fatah SS7 Khubayb Sarami Shafan Hawasinah Dank Dank Bani Umar Bila Hawasinah Yanqul SS8 Bila Diyan SS9 B Ghafir B Ghafir Jifrah FaraFara Ibri BFaraB Kharus Kharus Lusayl B Ghafir Al Ayn Ma'awil MS13 SS10 Sahtan Far AlAl Abyadh Abyadh SS10 Fara Al Khawd Awabi Afi SS13 Sabt Aiden MisfahAdam Jaba Kawr 1 Lansab SS154 Aday SS12 Bahla Sayh Al 'Uqq Al Abyadh JannahMayh ManzariyahHaym Muaydin SK2 Halfayn Hayfadh Kalbu SarinSarin MS11 Miglas Umayri MS12 Tayyin Dayqah Samad Dayqah Arabiyin MS9 SK1 AndamBatha Ibra SK5 MS10 Hilm MSM6S8 SK3 B Khalid SK4 Rafsah MS5 MS7 B KhalidDidu Didu SK9 Batha MS4 SK8 SKSSKK111345 MS2 K6 MS1 SK67 BBK5 MS3 BK8 SK10SK12 BBKK34 SK11 BK7 BK1 SA2 BK2 MUM1U2 Dhahban AUHM125 Andur UM1 MU3 MU4 JarzizSahalnawt Rzat UM3 AH8 Darbat SA1 Darbat YT4 AH13 YYYTTT123 AH7 AH5 AH14 YYTT675 YT8 UM4 YT13 AH1 YT9 YT11 YT15 AHA3H2 BA2 YT16 BA10 SU1 BA12 BA14 NA3 NA4 Note: List of Flood Stations are given in Table 11.1-5 in Subsection 11.1.5 SHS6HS5H4 SSHH21 Figure 11.1-1 Wadis and Location of Flood Stations of Oman 11 - 2 (1) Wadi Gauging Stations One hundred and twenty (120) active gauging st former stations are now closed. Eight (8) wadi recharge dams which have tangible impact installation. It is noted that the limitation of flood storage in upper gorges means that flood peaks hold up in upper catc reaches can often be very small proportion of relative proportion of alluvial plain. It is reported that flood characteristics of Dhof in northern Oman. From the maximum reco that nearly all of the highest peaks relate to mountain gorge cases. (Figure 11.1-2) hment’s reaches and flood peaks in the lower catchment’s -gaugingations stations are currentlyare located in downstreamoperation, and of six on its flood peak char 25 20 the up stream floods, depending on the 15 acteristics since their Frequency ar appear to differ appreciably from those 10 rded flood peaks in Oman, it is observed 5 0 0 200 Figure 11.1-2 Catchment’s Oman Regional Size400 Distribution Flood 600 800 Figure 11.1-3 presents the maximum record 1000 selected on the basis of peak related to cat 1200 1400 greater than 10% of the world maximum related to area (Rodier,160 01984) are depicted, 1800 and they are plotted against the world maximum 2000 highest peaks relate to mountain gorge cases. Catchment220 0Area (km2) 2400 2600 The great floods recorded in Oman shown in the figure include the floods of Wadi 2800 3000 Dayqah at Mazara in 1927, and 1965, Wadi Ib 3200 3400 and Wadi Muaydin at Birkat al Mauz in 1951. Frequency Analysis, Flood3600 Data 3800 4000 4200 4400 4600 ed flood peaks in Oman. These have been 4800 chment’s area, such that only flood peaks curve. It is noted that nearly all of the ra in 1927, Wadi Halfayn at Izki in 1951 11 - 3 The implication of repeat of the event should be considered in any proposed and prospected development in the vicinity of the ad hoc areas, before any decision on final design parameters are taken. 16000 14000 World Maximum 12000 Mudayy 2 10000 Mazara Thumrait 2 8000 Mazara 6000 Flood Peak - m3/s Flood Peak Ibra Mazara Bayyad 4000 Izki Birkat al Mauz Mudayy 1 Habubiyah 2 2000 0 0 500 1000 1500 2000 2500 Area - km2 Figure 11.1-3 Greatest Recorded Flood Peaks in Oman Compared with World Maximum (Rodier 1984) (2) Rainfall Oman rainfall is caused by five principal weather mechanisms: • Cold Fronts. • Tropical Cyclones. • Monsoonal Currents. • Convective Storms. • Orographic Storms. Rainfall intensity, duration, frequency relationship, for use in determining design storm rainfall, was developed by Directorate General of Water Resources Assessment (DGWRA), MRMEWR in 1998. Further, 2000 Master Plan by MRMEWR filled the gap of rainfall data for the period 1975-96 utilizing the data from non-recording rain gauges, of some 2214 station years, from 126 stations including 11 from Dhofar, as well as the data of the study of Wheater, H. S and Bell, N. C, 1983. 11.1.2 Regional Flood Frequency Curves The methodologies and procedures of the antecedent ways and means for the regional flood frequency analysis have been developed and upgraded by the MRMEWR. The 11 - 4 regional flood frequency analysis for a number of combinations of regional groupings has been utilized in order to identify areas of homogeneity with respect to flood characteristics. The criteria for grouping the stations include administrative region, topography (hill and mountain/plain), catchment’s area, and catchment’s rainfall. The flood series at each of the selected sites are standardized by dividing each by its relevant mean annual flood. Probability weighted moments are computed for each of the flood series, averaged, and used to derive the parameters of the selected distribution. Dimensionless regional quantities or growth factors are computed for the derived distribution which is used to compute flood frequency curves for each of the sites included in the analysis. An example of regional growth curves of Northern Oman are summarized graphically in Figure 11.1-4. Compared with the Nov 2000 Flood Manual, the curve for Northern Oman derived in this guide gives lower values, -33% at 5 years and -18% at 20 year return period, then converges at around 47 years and thereafter gives significantly higher values, +22% at 100 years and +50% at 200 year return period. Oman Regional Flood Frequency Analysis - Regional Growth Curves (GEV Distribution) Hills/Mountains & Plains - Northern Oman - AM Return Period yrs 5 10 20 50 100 200 500 1000 40 35 30 25 All-p-am-nodams 356(21) All-p&np-am- 20 Q/MAF All-np-am 1206(63) 15 Flood-manual-Nov2000 10 5 0 012345678 Reduced variate y *p: Plain, np: Non-plain, am: Above Maximum, nodam: Cases where there is no dam in upstream Figure 11.1-4 Regional Flood Frequency Analysis, Growth Curves for Northern Oman 11.1.3 Mean Annual Flood Estimation Method The approach to deriving the mean annual flood (MAF) estimation method or MAF prediction equation is to carry out a multiple regression analysis of MAF on the 11 - 5 catchment’s characteristics. The mean annual flood is first calculated using the mean of the annual peak flows at the site. To date, it has been possible to correlate area (A), wadi length (L), wadi slope (S), and catchment’s percentage which is non-mountain/hill or alluvial plain (NM). The NM area has been taken as the flat area in the lower catchment, which is indicative of flood plain, recharge and flood routing effects. In the case of L and S, two versions have been tried, one taking the average for the major tributaries, and the second being the more traditional approach of simply using the longest tributary only. Undoubtedly the averaged lengths and slopes provide better estimates of catchment’s relative lengths, and slopes. The rational method is widely used around the world for flood estimation on small rural drainage basins, and particularly for urban drainage design. It is a very approximate deterministic model representing the flood peak that results from a given rainfall, with the runoff coefficient. It should be noted that the rational formula approach should not be used for areas greater than about 10 km2. The method theoretically assumes: • That there is no temporary storage of water on the surface of the drainage basin. • The frequency of occurrence for the peak discharge is the same as the frequency for the rainfall producing event.