In dit onderzoek zal antwoord worden gegeven op de vraag: Moet het falen van gemalen mee worden genomen in de watersysteemtoets? In de 6-jaarlijkse toetsing van de waterschappen wordt bepaald of het watersysteem voldoet aan de door de provincie opgestelde eisen voor wateroverlast. Op dit moment wordt er in deze toetsing geen rekening gehouden met het feit dat gemalen kunnen falen en dit falen invloed kan hebben op de wateroverlast. In dit onderzoek zijn de onbeschikbaarheid van gemalen en de gevolgen van deze onbeschikbaarheid in kaart gebracht. Met deze gegevens is het risico dat gemalen vormen voor het watersysteem bepaald.

The Addicks and Barker Reservoirs, built in the forties, are located in Houston and collect precipitation and run-off from upstream areas to reduce flood risks along Buffalo Bayou to protect downtown Houston. During Hurricane Harvey (August 25 - August 30, 2017), the precipitation reached a new record of 910 mm [36.2 inches] in a 4 day period in Houston. The gates of Addicks and Barker Reservoirs were opened during the night of 27-28 August which led to major damages due to downstream flooding. Besides, non-government owned land upstream was flooded due to high water levels in the reservoirs. In this report, new design water levels for Addicks and Barker Reservoir are calculated based on inflowing discharge into the reservoirs and precipitation directly onto the reservoirs, including data of Hurricane Harvey. These calculated design water levels are compared with the critical water levels calculated based on the failure mechanisms of the dams. This study shows that the original design water level of the dams, based on the Probable Maximum Flood, are 2.83 m and 1.01 m higher than the critical water level for which failure of the dams can occur due to piping for Addicks and Barker Reservoir. However, the maximum allowed water level which is currently maintained by the United State Army Corps of Engineers, is 2.19 m and 2.46 m below the calculated critical water level. During Hurricane Harvey, these maximum allowed water levels were exceeded with 3.46 m and 1.93 m. The damage of residential properties upstream and downstream of the reservoirs are minimized based on the distribution of excess volume from the inflow of creeks and precipitation onto the reservoirs. The ratio of the amount of volume which should remain upstream of the dams and the volume discharged into the Buffalo Bayou is calculated for every considered event with its duration and return period. The ratio of Addicks Reservoir is the dominant ratio, which should be used for both reservoirs. Run-off alone already produces damage, especially for the 12h and 24h precipitation, so the Addicks and Barker Reservoirs should not release discharge into the Buffalo Bayou for small durations. For events with a longer duration, it would cause less damage to open the outlets of the reservoirs than to keep them closed. However, if the water level in the reservoir exceeds the critical water level for piping, it is advised to discharge more to the downstream area to prevent breaching of the dams. Since the critical water level is reached for approximately 25% of the events at Addicks Reservoir, mitigations against piping should be taken to improve the minimization of damage. For Barker Reservoir, the critical water level is not reached in the optimization. During big events, people living upstream will be more affected by the flooding than people living downstream since this optimization is based on the damage minimization of residential properties.