The Landslide dams, resulting from the deep-seated landslides that occurred during Typhoon No.12 in September 2011, differed in terms of their internal structure. These differences affected hydrological conditions such as the water level of flooding ponds, their leakage flow rate, frequency of overflow, and severity of erosion. We investigated the relationship between the internal structure of landslide dams and their hydrological and erosion conditions to establish effective countermeasure plans. This was achieved by outlining debris deposition, grain size distribution, and permeability through geological surveys performed in the area of Akadani, Nagatono, and Kuridaira. These landslide dam characteristics were then related to the hydrological and erosion conditions. This study demonstrates that the permeability of a landslide dam has a significant influence on the groundwater level and degree of water level fluctuation within the flooding pond. A thick layer of slide rock masses results in lowered water permeability, which increases the frequency of dam overflow and the corresponding danger of erosion. The risk of erosion is also increased by the accumulation of fine-particle debris at the foot of landslide dams. It is important to understand how erosion-prone sediments are deposited, and what the internal structure of landslide dams is likely to be, such that effective countermeasures can be planned.