Saspoort

The Flemish Environment Agency has started the renovation of the 'Grote Steunbeer' weir construction on the Saspoort in Diest. As a result, the water level in the Demer will rise and the river, which is now fed by the Begijnebeek, will really flow through the city again. The visible works will start in 2021 and last a few years.
(info source: VMM)

Through this 'water gate', which was completed in 1841, the Demer - which is currently almost completely vaulted in the town - used to flow into Diest. This is also the point where the water level of the moats can be regulated via a system of locks. For example, in the event of an attack, the trouser areas near the ramparts could be flooded.
Source: hiking guide: GR 5/E2 North Sea-Rivièra - route in the low countries

The Saspoort was also called "Demerpoort". This water gate was constructed during the construction of the 19th century defensive belt around Diest. The gate was fitted with an impeller sluice that could rotate against the current. In this way the rivers in the city and the fortresses could be supplied with water from the Demer (river) and at the same time ensured the drainage of the Begijnebeek. The water that flowed through the city via De Demer then left the city again along the Zichemsepoort
Source: diest-online.be (much more explanation there)

nice piece of history where you can walk on and in

The works are finished and you can easily pass by

The Great Buttress has been a protected construction for years. The old weir was built in 1835 and was intended to flood the surrounding land in the event of an imminent military conflict. This was to slow down the advance of the enemy. The historic weir structure is a valuable heritage and is archaeologically preserved and restored, as are the historic walls around and opposite the Great Buttress. Valuable plants such as polypody are also preserved. vmm.be/water/beheer-waterlopen/projecten/renovatie-grote-steunbeer-in-diest-1/wat-houden-de-werken-inWorking status in October 2022:Along the Omer Vanaudenhovelaan in Diest, works have been carried out near the Saspoort to allow the Demer to run through the center of Diest again. The construction enables better management of the flow of the Demer and of the Begijnenbeek. hbvl.be/cnt/dmf20221016_96788629