Documentation of the morning rain event on Saturday, August 26, 2023 in Brno - Řečkovice. A montage of the double relief in the relief chamber Novoměstská II on the combined sewer trunk collector C15 near Novoměstská Street. On August 26, 2023, a low pressure area moved over the Czech Republic, behind which a narrow spur of higher air pressure then expanded. Storm activity activated at the pressure interface in the morning hours. Until then, the front had manifested itself overnight as slight precipitation activity mainly in Bohemia. With the rising sun, the storms initiated a short distance before Brno and then quickly arrived over the city from the west. I arrived in Brno on Saturday night because the forecasts for morning storms varied and I didn't want to come to the event. It was a bit uncomfortable to wait out the rest of the night until morning. The storm surprised me just as I was resting on a bench in the dog park on Novoměstská Street and, looking at the radar, I didn't give it much of a chance... The intensifying thunder forced me to quickly run to the shaft into the relief sewer at the Hradecká expressway entrance. When I was closing the hatch above me, the first drops were falling. It took me a while to climb the landing pad and get ready... By then, the storm was already in full swing outside. The only way to film the relief in this slit relief chamber is from a ladder into the shaft to the surface. However, liters of rainwater instantly started flowing through the concrete hatch above onto the hanging backpack and down my neck, which made the already uncomfortable filming even more complicated. The beginning of the relief is therefore filmed quite badly and miraculously after the torrential rain subsided, the back part of the storm came with another downward flow, which was no longer as strong, but the already wet OK Novoměstská II catchment area sent a similar amount of water into the sewers again. As a result, the footage is probably quite usable and so in the end the video is only a few minutes long. The first relief has begun. As I mentioned, the C15 sewer passes through the chamber. However, it is already impassable in the next shaft and falls into the main large sewer from the left as a pipe with a diameter of DN 1000 with a fairly decent flow rate. The sewer has a small diameter mainly because there are countless branches along its path and together with the other Rečkovice sewers it forms a network of sewers. The large sewer with a diameter of approximately DN 2000 turns left against the flow and heads towards Banskobystrická Street. It collects water from this street and the surrounding area, but also water from the throttling lines of the relief chambers Jabloňová I and II, whose relief sewers lead to the Medlánecký stream - parts of the CH sewer and also the sewers passing through OK Novoměstská I and continuing to sewer C. OK Novoměstská I and II have a common relief sewer to Ponávka. But I'm digressing a bit and confusing the minds of the uninitiated. Exploring this area is otherwise in separate videos on my channel and on the channel of my colleague zemi02, who also has beautifully documented documentation of these places. Back to the relief. A significant storm surge came a moment after the overflow began. The first relief lasted several minutes and I managed to film its end even from the overflow area, when the current was not so strong as to sweep me into the fall. It didn't take long before I noticed that drops of water started falling through the manhole again through the hatch. Immediately after that, the increase in the flow rate through the C15 sewer was also noticeable, which again reacted to the rain first. I climbed back up the ladder and the relief began to flow just as quickly as the first time. I can't estimate what the flow rates were in the chamber, but the inflow could have been as much as 6-8 cubic meters per second, and about half of that flowed into the relief sewer. The throttle line only has a diameter of DN 1000, but the gap takes in a lot of water and the drain eats up more than it would seem at first glance. There is a theory that during some extreme flows (such as the microburst on June 21, 2021 in Řečkovice), the DN 1800 relief sewer in OK Novoměstská I does not take all the water and the entire space fills up, to the point that part of the relief sewer from OK Novoměstská II to the spillway goes into pressure mode (traces in the shaft to the OS) - however, this is only an assumption. Otherwise, it was fun to watch how the turbulent flow of water behaves in the space designed in this way. In the spillway, the speed of the water was such that it hit the opposite wall and sometimes splashed almost to the ceiling. Towards the end of the relief, I watched how the water behaved in the gap and on the edge of the overflow. All you need to do is slightly change the amount of flowing water and the flow will go from aggressive splashing to ca