The determinants of land use as regard urban-rural usage can be attributed to flooding hazards. Barangay Sto. Niño 1st has a perennial flooding problem. This barrio is the traditional catch basin of the poblacion area being situated on the periphery of the poblacion at the northwest portion. Spring water is abundant in these areas such that a one (1) 6.00 meter pipe strategically placed on the ensuing soil strata will yield flowing water.
Flood in this area are contributed by run-off accumulated from the uplands or surrounding elevated areas of Sitio Pinagcuartelan and usually occurs during the rainy season (July-September). Also, a local water impounding reservoir in the same site tends to overflow during flash floods, and this overflowing water find its way into the barangay.
There are other barangays which are considered as flood hazards due to their slope. Heavy and prolonged downpour causes floods in some parts of lowly located barangays. Rainwater which could not be contained by the soil tends to find its course to lowlands, additionally, creeks passing through these areas tends to overflow causing moderate floods to the run down areas; Kita – Kita, San Agustin, Palestina and going down finally to San Mauricio. Part of Barangays Caanawan and San Mauricio are also moderately considered flood hazards. The 2009 typhoons Ondoy and Pepeng proved the vulnerability to floods of these 5 named barangays.
However, moderate seasonal flooding, which takes as long as a week to a few months to subside is non-existent in the city.
Erosion and Siltation
Based on the Environmental Impact Assessment (EIA) conducted by Ecosys Corporation, the degree of erosion in the city has been categorized based on slope and the different factors that affect erosion such as the following:
• Farming method
• Intensity of rainfall
• Soil characteristics
• Vegetation/ Land use
The urban and sub-urban areas of the city experience no apparent erosion. These areas are situated on a flat to nearly level terrain and the slope under this class ranges from 0-3% with no gullying.
Slight to moderate erosion occurs in the northeastern barangays of the city. In slightly eroded areas, the slope ranges between 3-8% and the soil profile is slight. These are commonly utilized for upland crops. Moderately sloping to rolling relief on the other hand, has moderate dissection and moderate soil profile depth. Slope under this class varies from 8-30%.
The mountainous areas in the northern part of the city experience severe erosion. The slopes are steep to very steep and strongly slanting. This erosion is characterized by slopes having 30% or greater with severe dissection and shallow soil profile.
Seismic and Volcanic Activities
There are three (3) major faults converging at the northeastern section of the city. The convergence zone covers the barangays of Tayabo, Villa Marina, San Juan, Pinili, Porais and Villa Joson.
Another is Dig-dig Fault System which is the cause of the 1990 earthquake. The fault line traverses San Jose City from the northeastern to the southeastern section passing along the mountainous area of the city. The category of this fault line is of the strike slip active earthquake fault system.
Lastly, a fault system runs from the southeastern corridor of the city to the northwestern portion passing through the town of Lupao and to the province of Pangasinan. This fault line is described as a normal fault by PHILVOCS. In the design and construction of infrastructure such as buildings, roads, bridges etc., it is therefore imperative that the soil structure be considered as a critical factor to avoid casualties and severe damage during earthquakes.