The floodplains of the city comprise a total area of 302 hectares. Situated in this zone are Sinalhan, Aplaya and Caingin, and portions of Tagapo, Ibaba, Labas, Pooc, Dila and Dita.
Slight seasonal run-off flooding is characterized by accumulated shallow run-off flood, which subsides within a short period ranging from a few hours to three (3) days. This occurs in low to moderately low flood plains situated in Kanluran, Malusak and Ibaba. Floods in these areas are due to run off accumulation coming from surrounding elevated areas, low physiographic positions, poor infiltration, permeability characteristics and drain ability outlet.
Moderate seasonal flooding is characterized by more frequent and deeper run-off water. It takes a week to a few months for water to subside. This condition is observed along the lakeshore in Caingin, Aplaya and Sinal han and in the adjacent northeastern portion of Ibaba, Market Area and Tagapo. These areas serve as the catchment of water coming from the upland areas and are characterised by poor drain ability outlets and very low infiltration and hydraulic conductivity.
When flash flood occurs, many areas in the city can be potentially affected (Figure 4- 6). These include barangays Sinalhan, Aplaya, Caingin, Market Area, Ibaba, Malusak, Kanluran, Labas, Tagapo, Pooe, Macabling, Balibago, Dila, Dita and Pulong Sta. Cruz. Among these areas, lakeshore barangays of Sinalhan, Aplaya and Caingin are most vulnerable to flash floods and could submerge in floodwaters from Laguna de Bay.
The province of Laguna can be affected by earthquakes given the presence of the West Valley Fault which traverses a north-south direction. The fault is disrupted by a number of minor steep faults along its course that is reflective of the volcanic activity relative to the area. Earthquake is a hazard in Santa Rosa as the West Valley Fault traverses Barangay Sto. Domingo.
Several areas in the city are susceptible to liquefaction (Figure 4-7). According to US Geological Survey (USGS), "liquefaction takes place when loosely packed, waterlogged sediments at or near the ground surface lose their strength in response to strong ground shaking." This can cause major damage in buildings and other structures during earthquakes. Barangays Sinalhan, Aplaya, Market Area, Kanluran, Ibaba, Malusak and Caingin are highly susceptible to liquefaction. 8arangays Tagapo and Labas have moderate susceptibility to liquefaction, while Pooc have moderate to high susceptibility. Areas of low susceptibility include barangays Macabling, Balibago, Dila and Dita.