Barrier islands can protect the mainland from flooding during storms through reduction of storm surge and dissipation of storm generated wave energy. However, the protective capability is reduced when barrier islands breach and a direct hydrodynamic connection between the water bodies on both sides of the barrier island is established. Breaching of barrier islands during large storm events is complicated, involving nonlinear processes that connect water, sediment transport, dune height, and island width among other factors. In order to assess the impacts barrier island breaching has on flooding on the mainland, we modified a storm surge model, GeoClaw, to impose a Gaussian bell-curve on the barrier island that opens during a hurricane simulation and deepened over time. We added a new method of generating storm surge with storm forcing inputs in the form of wind and pressure fields to expand GeoClaw's current utilization of best track information so that storm forcing from planetary boundary layer models can also be utilized in simulations. We created a statistical method to assess the sensitivity of mainland storm-surge to barrier island breaching by randomizing the location, time, and extent of a breach event across the barrier island at Moriches, NY. My results show that total mainland inundation is affected by the changes in location, size, timing and numbers of breaches. Total inundation has a logarithmic relationship with total breach area and breach location is an important predictor of inundation and bay surge. The insights from this study can help prepare shoreline communities for the differing ways that breaching affects the mainland coastline. The model updates created can also allow others to use this framework to study differing regions. Understanding which mainland locations are vulnerable to breaching, planners and coastal engineers can design interventions to reduce the likelihood of a breach occurring in areas adjacent to high flood risk. / Doctor of Philosophy / Storm surge is one of the most deadly and expensive parts of a hurricane. Storm surge can be reduced if a barrier island exists near the mainland coastline. The sand dunes, beach vegetation, and size of the island aid in reducing the waves and storm-surge generated by large storms. When a barrier island breaches it causes a channel of water that connects the ocean and the bay that separates the island from the mainland. This channel of water allows waves and storm surge to be directly pushed into the bay, increasing the storm surge along the mainland coast. Breaching is complex with many factors such as dune size and total island width determining when or where a breach will form. In order to study how breaching affects the mainland during a hurricane, we chose a program that simulates storm surge from a hurricane, made a simple breaching calculation that opens a hole in a sand dune on a barrier island so that the ocean and bay are directly connected and the storm surge can pass through this hole, and enabled the program to use storm data that is pre-computed rather than generated during the simulation. In order to understand how breaching impacts the mainland coast at Moriches, NY, we simulated a hurricane and varied the number, size, and time of different breaches. We learned that total of all breach dimensions has a linear relationship to the amount of coastal flooding up to a threshold, where the breaches cover most of the island. Coastal communities can use the results of this study to determine where to put interventions in place that will reduce the impacts of barrier island breaching, update flood risk maps, and warn community members of changes in their location's risk assessment. Additionally, other regions with barrier islands nearby can use the software updates we created to study their own regions and assess different risk patterns than this study.