Wadi Muzairah · flood study
FS-01 / Shuaib Muzairah catchmentwater paths · 131 km² · Tuwaiq escarpment
24.7808°N 46.3174°E · 766 m
particles follow the downhill flow · colour = water gathered

Where the water goes

A flood and climate study of one plot on Shuaib Muzairah, west of Riyadh: twenty-three years of Google Earth, eighty-six years of rain, heat and wind, the dam downstream, and a flash flood simulated hour by hour.

10Flood simulation2D · 15 M · SCS UH

A flash flood past the plot, in 3D

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 its north, west and south-west.

11 · 3D viewpick a storm · play
12 · Flood map1.4 KM · N↑
13 · Flow down the wadiM³/S
Solid: past the plot. Dashed: coming in 3 km upstream.
14 · Deepest water against the wallsM
15 · Four stormsrain in a day
20Image historyGOOGLE EARTH · MAXAR · AIRBUS

Twenty-three years over the plot

Google Earth has pictured the site 21 times since 2003. Under each, the heaviest rain since the picture before.

21 · The plot1 KM · N↑
22 · Before / after 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: the Riyadh floods of 16-19 November 2013, and 31 mm on 6 January 2014, six days before the second.
30Al-Muzairah dam · سد المزيرعة1.9 KM DOWNSTREAM · 570 M

The dam downstream is a rain gauge nobody installed

An earth embankment across the same wadi. Each picture with water behind it shows the wadi ran.

31 · The dam1.2 KM · N↑
32 · Pond size by water levelHA

40RainERA5 1940– · NASA POWER 1981–

Eighty-six years of the wettest days

No gauge stands at the site. Two free records cover it; both average rain over cells tens of kilometres wide, so a storm over the escarpment falls harder than they show.

41 · Each year's wettest dayMM
■ ERA5■ NASA POWER┄ design day
42 · How much in a day, how oftenat a point

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 26-27 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, so the study keeps 75 mm only as a stress test.

50HeatERA5 · 1940–2025

Each stripe is a year, coloured by how much warmer or cooler it was than 1961-1990. ERA5's cells are about 30 km across, so it flattens the hottest afternoons: Riyadh's stations have seen 50 °C where ERA5 never passes 46.

51 · Warming stripes
1940
2025
52 · The day and the year
53 · Highs, lows and records°C
54 · Hot days and hot nightsa year
■ days ≥ 42 °C■ nights ≥ 30 °C
60Wind

Where the wind comes from

Each rose shows how often the wind blows from each direction, coloured by its speed. Floods come with storms; the wind brings the dust and decides which walls take the sand.

m/s
66 · Wind through the dayM/S · %
━ wind speed━ humidity
67 · Strongest gustsM/S
70Dust, sun and evaporationCAMS 2022– · ERA5

71 · Water in, water outmm a year
72 · Dust in the air by monthPM10 µG/M³
73 · Sun and evaporation by monthMJ/M² · MM
74 · Dustiest daysCAMS model, PM10, an hour's peak
80Seen from spaceSENTINEL-2 · SWIR B12

Wet ground after rain

Wet ground darkens in short-wave infrared. The picture just before a rain against 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
90Limits and method

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.