The Gonsbach

In spring 2013, work began on renaturalizing the Gonsbach Valley in the Mainz-Gonsenheim district. Over a length of 1.2 kilometers and an area of six hectares, the Gonsbach Valley—from Mainzer Straße to Lungenberg—was restored to its original state.

The stream was freed from its concrete channel and now flows freely through the valley once again. Native shrubs and trees such as willows, alders, ash trees, and oaks will grow here again in the future, along with reeds and rushes on the banks. Walkers can enjoy peace and relaxation in the midst of nature on public paths.

However, what looks like a pretty little stream in many places is still part of a wastewater treatment plant, even after renaturation. Therefore, please do not swim, play, or drink the water. We will be happy to explain why.

When it starts to rain in Finthen and Gonsenheim, rainwater from the roofs and streets collects in the sewer system together with wastewater from the kitchens, bathrooms, and toilets in Finthen and Gonsenheim. During a thunderstorm, a large amount of water can accumulate very quickly. This initial surge of water, also known as a dirty surge, is directed entirely through the sewer system to the wastewater treatment plant for purification.

If it continues to rain, meaning that more and more water is coming in, the wastewater becomes very diluted. This is now referred to as mixed water.

If it continues to rain, the Gonsbach stream comes into play. It serves as a buffer. If more water comes than the canal can hold, heavily diluted mixed water is directed into the stream to make room.

This procedure is used throughout Germany. It is approved and monitored by the respective structural and approval authority, in our case the SGD Süd.

Of course, water protection is our top priority. By installing several fine screens in the inlet, we ensure that only the highly diluted liquid components pass through, while the solids are screened out beforehand.

However, more and more people are mistakenly using their toilets as waste bins. Food scraps, wet wipes, cotton swabs, or cigarette butts cause the screens to clog. When a thunderstorm comes, fecal matter and toilet paper can also overcome the barriers and end up where they shouldn't be, e.g., in the Gonsbach stream.

Wouldn't larger sewers be better? The size of sewers is determined by the number of connected households and the local precipitation situation, i.e., how often and how heavily it rains.

In order to transport solids such as human excrement, but also sand, dust, and tire abrasion from roads that end up in the sewer via the drain to the sewage treatment plant, the wastewater needs a certain flow. To ensure this even in times of low rainfall, the diameter of sewers cannot be increased indefinitely. This is because the larger the diameter, the smaller the flow.

During the renaturation work, Roman finds were unexpectedly unearthed.

Several building remains and a circular arena were found in the former sewage basin on Angelrechweg.

The size and structural quality of the remains indicate military use. Comparable facilities suggest either use as a training area (ludus) for gladiators or as a dressage arena (gyrus) for horses.

Some wall remains and larger finds remained on the site. Today, a viewpoint with information boards explains the history of this place.

Good to know: The Romans developed the first flood protection measures as early as 2000 years ago.

There is another attraction located on the Gonsbach. At the very end, where the Gonsbach flows into the Rhine: in Mainz's Neustadt district. More precisely, on Gaßnerallee.

Here, a rectangular, anthracite-colored cube rises almost nine meters into the air. It was awarded the Rhineland-Palatinate Architecture Prize in 2011 and houses one of the most modern flood pumping stations in Europe.

Up to 22,000 liters per second (!) – in other words, all the rainwater from the entire city center during heavy rainfall – can be channeled into the Rhine via a canal under the building.

When the Rhine floods, the 7.20-meter-high lift weir closes and protects Mainz city center from the backwater of the Rhine flood into the sewer system. Now the pumps start up and pump the rainwater from the city center above the flood level into the Rhine. And the Gonsbach? It also flows into the Rhine right here.

...if you are wondering why part of the path on Lungenberg—below Hartenberg—is a little bumpy due to large rocks, it is because the path is not actually a path at all. You are standing in the middle of a flood emergency overflow.

To prevent flooding on the Gonsbach during storms, the water is diverted into various collection facilities, such as rainwater retention basins or this flood overflow. The stones in the basin ensure that the ground does not slip away, and the distance is important so that the water can seep away there.

Nice story, isn't it? Feel free to pass it on during your next walk.