Mapping and Assessment of Flood Risk in the Wadi Al-Lith Basin, Saudi Arabia

Flood risk mapping is vital in watershed management and planning, especially in reducing flood damages. In this study, a flood risk map was developed for the Wadi Al-Lith watershed (Saudi Arabia) by combining geographic information system techniques with a multi-criteria decision-making method known as the Analytical Hierarchy Process (AHP). Several factors were investigated in the study, including elevation, slope, topographic wetness index, drainage density, rainfall, soil and land use, and land cover. The watershed was divided into five regions: very high, high, moderate, low, and very low flooding danger areas. The results showed that 35.86% of the total watershed area is under high and very high flood risks, while 26.85% of the total area is under a moderate flood risk. Less than 38% of the total watershed area was under a low flooding risk. The results of the developed model were validated with the flooding event that occurred on 23 November 2018 in the study area. The model was also compared with the flood mapping of the 100-year return period generated by the HEC-RAS software. Both the developed model and the HEC-RAS software showed similar results. The findings demonstrated that the developed model could be used to develop flood risk maps, especially in watersheds that experience scarcity and shortages in the short-interval rainfall measurements and the stream flow gauges (e.g., Wadi Al-Lith watershed and other watersheds in Saudi Arabia). Additionally, the obtained results can provide helpful knowledge for the policy- and decision-makers to make the right decisions regarding the effectiveness of the protective structures of the study area against the risk of flash flooding in the future

HYDRAULIC JUMPS PROPERTIES ON CORRUGATED BEDS

Abstract Hydraulic jumps have been used for dissipation of kinetic energy downstream of hydraulic structures, such as spillways, chutes, and gates. It was found that if jumps were made to occur on corrugated beds, tail water and length of jumps reduced significantly. The effects of corrugations on the basic characteristics of jump were studied for a different range of Froude number from 3 to 7.5. Five shapes of corrugations (sinusoidal, triangular, trapezoidal with two side slopes and rectangular) of the same amplitude and wavelength were tested. Two values of relative roughness t/y1 of 0.36 and 0.72 were studied. It was found that, for all shapes of corrugated beds, the tailwater depth required to form a jump was appreciably smaller than that for the corresponding jumps on smooth beds. Further, the length of the jump on the different corrugated beds was less than half of that on smooth beds. For the same amplitude and wavelength, it was found that the effect of the shape of corrugations is relatively small. The results of this study confirm the effectiveness of corrugated beds for energy dissipation below hydraulic structures. Finally, attempts were made to draw some conclusions from this experimental study.Research Center, Faculty of Engineering, King Saud Universit

Rainfall Intensity-Duration-Frequency Relationship for some Regions in Saudi Arabia

Intensity-Duration-Frequency (IDF) relationship of rainfall amount is one of the most commonly used tools in water resources engineering for planning designing, and operation of water resources projects, or for various engineering projects against floods. The objective of this research is therefore, to derive IDF relationship of rainfall for two regions in the kingdom. These relationships are useful in the design of urban drainage works, culverts and other hydraulic structures. Two commonly frequency analysis techniques were used to develop the relationship between the rainfall intensity, storm duration, and return periods from rainfall data for two regions in Saudi Arabia. These techniques are: Gumbel and the Log Pearson Type III distribution. In order to do that, rainfall data was obtained for different durations. A set of curves was plotted using the intensity-duration-frequency data to be the basis of the analysis. An equation for calculating rainfall intensity for the every region was derived based on the results obtained from the IDF data. The results obtained using Gumbel distribution are slightly higher than the results obtained using the Log Pearson III distribution. In general, the results obtained using the two approaches are very close and have the same trend. Rainfall intensities are drawn from these two methods and the results obtained from that work showed good consistency with what has been done before in some parts of the study area. Further studies are recommended whenever there will be more data to verify the results obtained or updating those IDF curves.Research Center, Faculty of Engineering, King Saud Universit

Development of Rainfall Intensity, Duration and Frequency Relationship on a Daily and Sub-Daily Basis (Case Study: Yalamlam Area, Saudi Arabia)

Realistic runoff estimates are crucial for the accurate design of stormwater drainage systems, particularly in developing urban catchments which are prone to overland flow and street inundation following extreme rainstorms. This paper derives new intensity–duration–frequency (IDF) curves for the Yalamlam area in the Kingdom of Saudi Arabia. These curves were obtained based on daily rainfall measurements and, in some short durations, across the entire study area over 30 years. The study is based on applying two distributions—the Log-Pearson type III and Gumbel—to estimate the average rainfall for the different return periods. The results show that there are slight differences between the Log-Pearson type III distribution and the Gumbel distribution, so the average parameters were used to construct the IDF curve in the Yalamlam area. The maximum daily rainfall was converted into sub-daily intervals using two methods and compared with the observed value. The new ratios were calculated using the converting rainfall from daily to sub-daily. These ratios are recommended for application in the Yalamlam area if there are no short-time-interval data available. The following ratios for 1-day rainfall were proposed: 0.37, 0.40, 0.46, 0.53, 0.61, 0.66, 0.70, 0.76, 0.80, and 0.87 for 10 min, 15 min, 30 min, 1 h, 2 h, 3 h, 4 h, 6 h, 8 h, and 12 h rainfall, respectively. The developed IDF curve for the Yalamlam district was built based on the daily and sub-daily observed data

Experimental investigation of grain size effect on the temporal variation of local scour around bridge piers

The soil grain size may affect the volume of scour-hole and maximum scour depth substantially. In this paper, the effect of soil grain sizes (d50 = 3, 0.1, 0.002 mm) on the temporal variation of local scour around bridge piers, circular in shape, is experimentally investigated. The experiments were performed under clear-water scour conditions in a rectangular flume at different Froude numbers, pier diameters and varying soil grain sizes. Sieve analysis was conducted to determine grain sizes distribution of the studied soils and find their d50 values. The scour test results showed that the grain size affects the volume of scour-hole and maximum scour depth and plays a critical function in the scour process. The results also illustrated that, in general, the equilibrium scour depths in the cohesive (or clayey) soils is less than that in the non-cohesive (or sandy) soils

Spatiotemporal Variability of Intensity–Duration–Frequency (IDF) Curves in Arid Areas: Wadi AL-Lith, Saudi Arabia as a Case Study

In arid areas, flashflood water management is a major concern due to arid climate ambiguity. The examining and derivation of intensity–duration–frequency (IDF) curves in an urban arid area under a variety of terrain patterns and climatic changes is anticipated. Several flood events have been reported in the Al-Lith region of western Saudi Arabia that took away many lives and caused disruption in services and trade. To find and examine the extremities and IDF curves, daily rainfall data from 1966 to 2018 is used. The IDF curves are created for a variety of return periods and climate scenarios in three terrain variabilities. This research examines various distributions to estimate the maximum rainfall for several metrological stations with varying return periods and terrain conditions. Three main zones are identified based on ground elevation variability and IDF distributions from upstream in the eastern mountainous area to downstream in the western coastal area. These IDF curves can be used to identify vulnerable hotspot areas in arid areas such as the Wadi AL-Lith, and flood mitigation steps can be suggested to minimize flood risk