Why do arid areas flood more frequently?

Arid regions like the middle east are really confusing to any observer.

Why?

As a region with under 150 – 200 mm / annual rainfall, we always see the media fall with views of flooding cities, such as Jeddah, Fujairah, Amman, Beirut …

There are numerous reasons,

Let’s start with hydrology, I was lucky to complete my studies in Europe and know well the humid climate, arid storms are quite different from humid regions storms. It is aggressive, intense, brief, and localized whereas in humid climates is gentle, longer in duration, and less intensity.

More intense storm means quick runoff peaks which our traditional stormwater drainage systems cannot handle.

So, why cannot handle it?

Also, it is another story, let’s start with the literature.

All methodologies used in designing stormwater drainage systems in Arid regions are based on humid climate-developed methods.

I will explain one example, The most frequently used methods in flood estimations on small catchments are the Rational Method (RM) and the Soil Conservation Service (SCS) method (SCS,

1986).

The main parameter for the RM is the runoff coefficient, C, and for the SCS method the curve number, CN.

In many applications, the C value is considered constant, but in nature, it changes with time, and especially in the calculation of the design discharge average recurrence interval, it plays the single most important role.

Estimation of the C value is difficult and is the major source of uncertainty in many water resources projects. The coefficient must account for all the significant factors affecting the peak flow to average rainfall intensity, not restricted to area and response time.

In any stormwater design in the Middle East region, the engineers take Cs from tables based on a set of drainage features. They are chosen in a rather vague manner and largely include personal judgment rather than actual field data.

Generally, the C increases as the average recurrence interval of rainfall increases, thus allowing for nonlinearity in the runoff response of the drainage basin.

Because considerable judgment and experience are required in selecting satisfactory Cs for a design, there is a need to check values against observed runoff data.

I know it is a long example (sorry), but another principle that is important to understand is that even if the traditional drainage system is designed well in arid regions using the humid – based methodologies,

It is not the case,

Why,

?

Because these systems (inlets, pipes, and outfalls) are logically not practical. Imagine you would convey huge runoff volumes from its fall for a long distance just to flush it in a pond or other outfall after these volumes of runoff increase significantly and loaded crazily with sediments and pollutants.

Add to this that the majority of stormwater drainage systems in arid regions are vulnerable to clogging by sediment accumulation and other kinds of trash, anyway, I am sure also that the maintenance plans are quite poor if it is existed in some places.

What are the solutions?

Actually, I am still in the research phase. I know you would think me saying green infrastructure.

Green infrastructure is great in vegetation sustain environments such as Humid regions.? And could be a good solution for the Arid region, we need more research and more trying with different solutions.

The future of stormwater drainage systems would be based on green spots and green corridors that handle the storm's runoff efficiently. The Middle East needs movement towards changing the design methodologies or technologies used.

Thanks for reaching this line, I hope my ideas are delivered in a clear flow.