’De Watersnood van 1953’, the largest Dutch flood in recent history, caused the death of 1795 people in the Netherlands directly from the flood conditions, while in the UK, 315 were recorded. Most of them were among those whose residence collapsed due to high water depth, quick rise rate of the water or strong flow velocity. Based on historical data of floods with similar flood characteristics and comparable buildings, mortality functions were developed to estimate the number of fatalities. These functions are still used, but the correlation between the flood characteristics and the damage observation is not clear according to multiple studies. The current study contributes to improving these functions by investigating which flood conditions may lead to collapse of the residences in the current Dutch building stock. From the BAG-registration (in Dutch: Basisregistratie Adressen en Gebouwen) it is found that 50% of the Dutch live in terraced houses (in Dutch: rijtjeshuizen), which is similar in the areas which are most likely to be affected by flooding. Most of these residences are built in the period of the housing shortage between 1965 and 1975 and the energy crisis between 1975 and 1994, which are considered as ’the typical Dutch residence’. This residence type consists of cavity walls with a load-bearing leaf of concrete or unreinforced masonry (URM), which can be clay or calcium-silicate. This inner leaf is tied to the outer leaf of URM consisting of perforated clay units, wood-based materials, or concrete. Stability is provided by piers in the façades in case of the URM walls or rigid connections between the concrete floor and walls. To define the properties of the building materials, existing experimental research on the masonry is used.
Experiments with a physical model were conducted herein to measure the quasi-steady load in the form of pressures acting on different elements of the residence. This enables the comparison of the quasi-steady flood load and the lateral load due to wind on different elements of a building. Similar to FEMA (2011), it was found that the pressure coefficient decreases when the width-to-water depth ratio decreases. However, higher coefficients are found from the experiments than those provided by FEMA, resulting in higher hydrodynamic loads. Furthermore, the orientation of the residence compared to the flow direction changes the angle of attack. When the flow is perpendicular to the wall, the pressure coefficient is the largest. Decreasing the angle of attack causes a decrease of the pressure due to equal flood conditions. The pressure coefficients obtained from the experiments are used to define the hydrodynamic load due to flooding. The resistance of the load-bearing cavity walls, windows and piers were compared to the acting moment due to different depth-flow velocity combinations. The resistance of out-of-plane bending of the load-bearing wall is the critical failure mechanism for typical Dutch residences. Residences with calcium-silicate masonry walls and system floors have a higher resistance than residences with clay masonry walls and timber floors. Cracks start to develop at a small lateral load resulting in zero tension strength after cracking and an eccentricity of the normal force. This makes the influence of the dead weight carried by the wall, in combination with the compression strength and the thickness, more important than the flexural bending strength.
All types of residences, using design values, already collapse before the hv-product (water depth times flow velocity) of 7 m2/s is reached according to Clausen (1989). A water depth of ±1.2 meters for the older residences (1965-1975) and ±1.8 meters for the newer residences (1975-1994), already cause the design moment resistance of the wall without taking the velocity or wave action into account. If the flood water has a flow velocity of 2 m/s or waves are generated by a wind speed of 29.5 m/s over a fetch of 100 m, the critical water depth reduces to respectively ±0.9 and 1.5 meters.

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.