Evaluation of Urbanization Impacts on Flood Generation in the Adigrat Watershed, Northern Ethiopia

The City That Learned to Run Too Fast

Imagine a watershed as a giant, natural sponge. When it rains, this sponge soaks up the water, letting it seep slowly into the ground, feeding rivers gently over time. This is how nature usually handles a storm. But cities are different. When we build roads, houses, and parking lots, we cover that soft, spongy earth with hard, unyielding concrete and asphalt. In the world of hydrology—the science of how water moves across the land—this is a big deal. These hard surfaces are called "impervious," meaning water can't pass through them. Instead of soaking in, the rain has nowhere to go but straight over the top, rushing downhill like a race car on a track.

Scientists use special computer programs to predict what happens when these natural sponges get covered in concrete. One of the most popular tools is called HEC-HMS. Think of it as a digital time machine that lets researchers rewind and fast-forward through years of weather, changing the map of a city to see how the water would react. They look at things like "Curve Number," which is basically a score telling us how much water a specific patch of land will let soak in versus how much will run off. The higher the score, the more water runs off. When cities grow, these scores go up, and the rivers get angry faster. This matters because when a city grows too fast without planning for the water, the result isn't just a puddle; it's a flood that can wash away homes and roads.

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The Adigrat Experiment: When the Sponge Turned to Stone

In the northern part of Ethiopia, there is a town called Adigrat. Like many places around the world, it has been growing quickly, with more buildings and roads popping up every year. A team of researchers from Aksum University wanted to know: exactly how much has this growth changed the way floods behave in the local watershed? They didn't just guess; they built a digital twin of the Adigrat watershed using a 30-meter grid of the land's shape and maps of what the land looked like in two very different years: 2000 and 2018.

Since there wasn't a water gauge right inside the Adigrat watershed to measure the flow, the scientists had to be clever. They borrowed data from a nearby station called Sluh and used a math trick called the "area-ratio method" to estimate what the water flow would look like in Adigrat. They fed this data, along with soil maps and rainfall records, into their HEC-HMS computer model. They calibrated the model (tuned it like a radio) using data from 1998 to 2012 and then tested it (validated it) on data from 2013 to 2017 to make sure it was telling the truth. The model worked well, showing a strong match between its predictions and the real-world data.

Then came the main event: the comparison. The researchers ran the model twice. First, they simulated what would happen if the watershed still looked like it did in 2000, before the big urban boom. Then, they ran it again with the 2018 map, where more land had been turned into buildings and roads. The results were a clear signal that the city had changed the water's behavior.

The Speed and the Surge
The most striking finding was how much faster and bigger the floods became. In 2000, the peak flow of water leaving the watershed was 6.1 m³ s⁻¹. By 2018, after years of urbanization, that peak had jumped to 8.8 m³ s⁻¹. That is a 30.68% increase in the speed and force of the water hitting the outlet. It wasn't just a little faster; the water was rushing out with significantly more power.

It wasn't just the speed, either; the total amount of water flooding the area grew, too. The volume of floodwater increased from 166.49 × 10⁶ m³ in 2000 to 215.09 × 10⁶ m³ in 2018. That's a 29.19% increase in the total amount of water. To put it simply, the watershed was holding less water and dumping more of it all at once.

Why Did This Happen?
The paper explains that this happened because the "Curve Number" (the runoff score) went up. As the town expanded, the amount of "impervious surface" (concrete and rock) increased, while the "initial abstraction" (the amount of rain the ground can soak up before it starts running) went down. The water had nowhere to hide.

The model also showed that the "lag time" and "time of concentration" got shorter. Imagine a relay race where the runners used to stop to tie their shoes at every station; now, they are sprinting straight through. The water reached the bottom of the watershed much faster. For example, in one sub-basin called W600, the time it took for water to travel dropped from 14.87 minutes to 13.48 minutes. In another, W710, it dropped from 7.46 minutes to 5.53 minutes. The water was moving faster than ever before.

Not Every Corner Was the Same
Interestingly, the flood didn't get worse everywhere in exactly the same way. The researchers found that the biggest jumps in flood peaks happened in the center of the watershed, where different streams meet at junctions like J99 and R330. In these central spots, the peak discharge jumped by about 2.0 to 2.6 m³ s⁻¹. However, in a tiny sub-basin called W710, the peak discharge didn't change at all (it stayed at 0.2 m³ s⁻¹), likely because it was too small to feel the full effect of the city's growth. But even there, the water arrived faster.

The Hidden Cost: Less Water Later
There was a downside to this speed. Because the water rushed off the surface so quickly, it didn't soak into the ground to recharge the groundwater. The model showed that the "low flow" (the water left in the river after the storm passes) dropped dramatically. At the watershed outlet, the residual flow fell from about 6.0 m³ s⁻¹ to 3.4 m³ s⁻¹, a 43.3% reduction. The city was getting a bigger flood, but the river was drying up faster afterward.

What This Means for the Future
The authors conclude that the rapid urbanization in Adigrat has fundamentally changed the hydrology of the area. The city has become less like a sponge and more like a slide. They suggest that if the town continues to grow without planning for these changes, the floods will get even worse. The paper recommends that future urban planning must include "hydrological considerations," such as keeping natural drainage paths open, building detention basins (like giant sponges to hold water temporarily), and using permeable pavement.

The study admits it has some limits. Because they had to borrow data from a nearby station, there is some uncertainty. Also, they only looked at two years (2000 and 2018) and assumed the rain stayed the same, not accounting for climate change. But the simulation is clear: the concrete city is making the water run faster, harder, and in greater volumes. The message is that if you want to keep a city safe, you have to plan for the water before you pour the concrete.