Wadi Muzairah floods

A flash flood past the plot, in 3D

Pick a storm and play it hour by hour. The ground is Google Earth's picture of the site; the red frame is the plot.

The wadi

131 km² of the Tuwaiq escarpment drains past the plot

Shuaib Muzairah gathers its water from the high ground to the west and south-west and runs past the plot's north-east corner, about 50 m away, then on to Al-Muzairah dam 1.9 km downstream.

━water courses┅the longest one●the plot▮the dam
    Simulated floods

    Where the water goes in a storm

    The catchment turns each storm into a flood coming down the wadi; a 2D model then spreads it over the ground round the plot. The wadi's west bank overflows at the bend north-west of the plot, and a shallow sheet of water crosses the open ground to the plot's north, west and south-west.

    Flow down the wadi
    Solid: past the plot. Dashed: coming in 3 km upstream.
    Deepest water against the plot's walls
    History

    Twenty-three years of Google Earth

    Google Earth has pictured the site 21 times since 2003. Slide through them: under each, the heaviest rain since the picture before.

    Before and after the wet season of 2013-14
    2013-02-122014-01-12

    The channel north-east of the plot is freshly scoured, and the ground to its west is stained where water spread. Between the two pictures: the Riyadh floods of 16 to 19 November 2013, and 31 mm on 6 January 2014, six days before the second.

    Al-Muzairah dam

    The dam downstream is a rain gauge nobody installed

    A 570 m earth embankment across the same wadi, 1.9 km downstream. Each picture with water behind it shows the wadi ran.

    How big its pond gets
    Area behind the dam at each water level, from the 30 m heights. Dots: ponds measured in Google's pictures.
    Rain

    86 years of the wettest days

    No rain gauge stands at the site. Two free records cover it: ERA5 from 1940 and NASA POWER from 1981. Both average rain over cells tens of kilometres wide, so a storm over the escarpment falls harder than they show.

    ■ ERA5 ■ NASA POWER
    Was 2013 really that wet? ERA5 puts 74 mm on 29 April 2013, its wettest day since 1940, but NASA POWER has 15 mm over 26 and 27 April and nothing on the 29th. Late April 2013 was a real storm across the Kingdom, and November 2013 brought Riyadh's worst floods in years. The 74 mm itself is probably ERA5 overdoing one cell: this study keeps 75 mm only as a stress test.
    Seen from space

    Wet ground after rain, from Sentinel-2

    Wet ground darkens in short-wave infrared. Comparing the picture just before a rain with the one just after shows where water ran: blue is ground that got wetter than the scene as a whole. 6 km round the plot.

    Where it got wetter
    Limits

    How far to trust it

    For anything built on these numbers, such as gate thresholds, drains or a wall's footing, a civil engineer should check them with a level survey and the Ministry's design rainfall for Riyadh.

    Method and sources