Tropical cyclones generate risk of compound flooding in coastal watersheds due to both precipitation and storm surge. For regions prone to this phenomenon, it is clear that in order to properly quantify flood hazard and flood risk to later use these estimates in the implementation of mitigation, adaptation and prevention measures that are effective on decreasing the flood risk, it is necessary that compound flooding is taken into account since not contemplating the simultaneous occurrence of different flood drivers and their joint dynamic interaction, will immediately leave out the worst case scenario.

Nonetheless, this is not the regular practice carried out by risk agencies as is the case of the Federal Emergency Management Agency (FEMA) of the United States, which delineates floodplains considering only one flood driver at the time and has ignored the co-occurrence of both mechanisms. Recent studies have shown that in the United States, the currently estimated flood hazard generally underestimates the actual flood losses in coastal areas (Blessing et. al. 2017). This is highly problematic since the delineation of the flood hazardous zones drives policy decisions such as urban planning, flood mitigation measures, and most relevant for the U.S case, the decision whether or not to take flood insurance policies which are all important strategies to effectively reduce and distribute flood risk.

This thesis aims to delineate flood hazard and flood risk in a coastal watershed including the effect of compound flooding using the SFINCS model; a semi-advanced 2D model which was developed to solve all relevant processes in coastal catchments with computational efficiency (Leijnse, 2018). The research is divided into four phases: (1) an overview of the Clear Creek coastal watershed and validation of the SFINCS model for two major hurricane events triggering compound flooding in the area, (2) delineation of boundary conditions for a compound flood analysis based on synthetic data, (3) new estimation of flood hazard and flood risk for the Clear Creek watershed, and (4) discussion on how relevant it is to include compound flooding in a flood risk assessment and how the results can be used to improve actual floodplain delineation and insurance rate estimates in the area.

As population levels protected by large dams have risen in the last decades, accurately assessing flood hazard and risk will become increasingly important for developing and sustaining flood mitigation policies. Recent research has shown that the degree of control and protection offered by retaining structures largely depend on their operation strategies. As such, now, more than ever, it is crucial to be prepared to operate a reservoir under extreme hydrological circumstances, when the consequences of an operational error could be very serious. However, the operation schedules currently use in the engineering practice lack on mechanisms for evaluating and balancing the potential risks associated with storage and release decisions. This results in reservoir flood control strategies that, from the perspective of flood risk mitigation, are distant from the optimal. This graduation project aims to improve existing methods for developing emergency operation schedules by including the concept of risk into the optimization of flood control operations. To address this topic, the research is divided into: (1) development of a methodology for including reservoir operation effects in the risk analysis of dam-reservoir systems; (2) combination of the risk analysis procedure with an optimization algorithm to devise optimal risk-based emergency operation schedules.

The thesis culminates with a case study in which the emergency operations are optimized for the Barker Reservoir system in Houston, Texas. Susceptible to Hurricanes like Hurricane Harvey (2017) and intense precipitation events such as Tax Day (2016), the Barker system presents an operational dilemma requiring trade-offs between released flows and stored volumes. Using the methods developed in this thesis, the flood risk analysis shows that a change in the operational strategy would contribute greatly to reducing the total risk of the system. Under extreme hydrological events, an operation strategy with releases starting at the first stages of the flood event display a reduction of almost a 32% on the total risk of the system as compared with the current operation strategy, including a 40% decrease on the risk associated with the structural failure of the dam. The new operating policy, however, increases the frequency of downstream damages during non-structural low frequency failure scenarios. Therefore, an increase of the downstream channel capacity along Buffalo Bayou and adequate measures to strengthen the dam are further recommended to reduce downstream damaging flooding and diminish the failure probabilities of the structure.

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.

Flooding is one of the most frequent and terrible natural disasters. For the coastal areas connecting inland with the open sea, the occurrence of flooding could be caused by high flows from inland rivers or high-water levels at open sea (e.g., from tsunamis or storm surge). There is also one extreme case that high sea level and high inland flow happen simultaneously, which is also known as the compound flood event. If a compound flood event happens, it has the potential to cause huge losses.

The Houston Ship Channel (HSC) is an important passage connecting “Greater Houston” area to the Gulf of Mexico which plays a critical role in the economic development of Texas and even the whole country. However, because of its special location, the HSC area is prone to flooding caused by both storm surge and heavy rainfall, which has been demonstrated by many historical flood events e.g. Tropical Storm Allison (2001), Hurricane Ike (2008) and Hurricane Harvey (2017). In the wake of Hurricane Ike, the Severe Storm Prediction, Education, and Evacuation from Disasters (SSPEED) Center at Rice University proposed to build a storm surge barrier near the downstream of Fred Hartman Bridge for the protection of the industrial facilities along the HSC from storm surges. One of the major questions related to the design of the storm surge barrier is ‘what are the probable boundary conditions associated with compound flood events (i.e., the combination of storm surge and upstream rainfall-runoff) in the Houston Ship Channel?’. Because once the barrier is closed, the upstream flow cannot flow out to the Galveston Bay, there is the potential to cause the flooding behind the barrier when the closure time is long and upstream discharge is large enough. Therefore, in this thesis, it is focused on the exploration of potential combinations of compound floods in the downstream reach of the HSC.