Yesterday and today I couldn’t make it out to Al Baydha because we had 22 mm of rain in Jeddah. That’s less than 1 inch. However, 1 inch of rain in this city wreaks a certain kind of havoc, as you can see in the photos below:
Part of the issue with flooding is that mountain valleys due east of Jeddah direct water to the city, so it’s not like an inch of rain on the city is the only source of the water.
But a major part of this flooding is that Jeddah has no buffer zones where water can drain to; the whole city is concrete and pavement, with 100% runoff rates. This is a massive failure of design; there are numerous spaces that if they had been done differently, could have greatly ameliorated the floods in Jeddah, and those spaces are duplicated in patterns throughout the city.
Here’s example #1:
There are 3 borders to the water on this street–to the left of the barrier is another 2 lane street, with another elevated sidewalk next to it. Total width of the streetscape is probably 17 meters, with 12 dedicated to the four lanes, and 5 dedicated to elevated walkways and trees.
The solution is obvious: lower the sidewalks and the planted medians enough to absorb the water on the street. 22 mm of rain over 12 square meters of road is 264 liters of water for every meter of road. For the medians and the sidewalks to absorb that much water, they would only have to be sunken 53 mm, a little more than two inches. With trees that perform hydraulic redistribution they could absorb more than that. But you could also sink them down a full foot–5 times deeper than you need for that particular road, so that it can absorb runoff from other roadways. This median dynamic is repeated throughout the city, on most major roadways. The other aspect of this poor design is that within 2 days, a water tanker with desalinated water in it will drive by to water these trees.
This may seem like a tremendously minor issue, but roads are currently the lowest spaces in the cityscape, and without a sink for the water, it multiplies over kilometers. Let’s take one of the main thoroughfares of Jeddah, Malik Road. Malik road runs north-south along the coastal side of Jeddah, and after driving most of it today, i couldn’t really see anywhere for water to leave the road until it reaches an underpass.
A look at the roads shows the total width to be about 80 meters, the main dividing median to be 15, and two other dividers splitting up the main road from service roads that are about 3 meters each. This doesn’t include the sidewalks that exist, as they’re not everywhere. Here is what that median looks like up close:
Notice that the median is raised about 50 cm, with grass, palm trees, and shrubbery planted thereon. The other medians are also raised, which means this road is essentially a sloped container when it rains. There are a few places for water to flow off onto side roads, but those are just other roads that are also flooding. The total width of this street is 80 meters, 59 of which is hardtop, and let’s take a 5 KM length. With 22 mm of rain, that is 6500 cubic meters of water finding the low point of that road. Multiply that by all the streets in Jeddah, and it’s no wonder that less than an inch of rain causes such problems: the whole streetscape is upside down.
Here’s where that road’s lowest point occurs: an underpass with water at least 3 meters deep at the bottom, after 22 mm of rain.
Yet the solution is simple: sink the medians and the dividers enough to absorb all the runoff.
The other major source of runoff is from rooftops, but this also has a simple solution. Almost all buildings already have a water storage tank, that are accessible to trucks that bring in desalinated water. Drain the roofs into these tanks, and a huge amount of runoff will be eliminated. Simple filters can handle the first flush eliminating dust and debris, after which the water is clean.
In a city where a gallon of water is more expensive than a gallon of oil, and where desalination takes up 20% of national energy consumption, it makes no sense to create a cityscape that makes no use whatsoever of free water falling from the sky. Yet that is exactly how Jeddah functions now, and the solution put forward by multinationals has been to drain that water into the sea as quickly as possible.
The principle for handling water in cityscapes in dryland situations is to take advantage of run-off from all surfaces, by elevating roads and buildings, and lowering green spaces to act as sinks and catchment for that runoff. By that standard, Jeddah’s streets are upside down. By putting them right side up, it could dramatically reduce road-scape flooding, seasonally decrease street-scape watering from tankers carrying desalinated water, and reduce damage, delays, and inconveniences caused by rain.