Using early 80s as reference, people of Olongapo City,(Zambales,Central Luzon, Philippines) can not pin point a recorded instance of worst flood. Torrential rains have never been threats to this silent and united community.
The mid 80's through the 90s and onward however witnessed the tremendous topographical and environmental changes in the city. The mountains and hills around became barer and balder.No more original and first growth forests. The once verdant forests teeming the mountains became grassy rolling highlands. River beds, creeks and natural water ways and tributaries became silted due to erosion from the uplands.
Torrential rains had caused landslides in barangays or communities carved out of mountain foots, sides and tops; along ridges; river and creek banks and other natural water outlets to the sea. Of the 16 barangays that comprised the city, 11 are located in relatively flatlands which were formerly marshes and swamps.
These are Banicain, Pag-Asa, New Kalalake, New Kababae, Ilalim, East Tapinac, West Tapinac, West and East Bajac-Bajac and East Bajac-Bajac and Sta. Rita are located on relatively flat lands which were formerly marshes and swamps.
The rest of the communities are on the highlands like Bangal, Boton, New Cabalan, Old Cabalan, Tabacuhan, Gordon Heights, Kalaklan and Barretto.
In August, 1985 the worst landslide of the decade (as residents described the tragedy in a random interview of 10 residents who had witnessed it) happened in Upper Kalaklan just opposite of the present Barangay Hall Complex across the highway. At the height of typhoon Rosing that year it caused a side of the mountain to roll down the hill and buried a house along with a pregnant woman and two of her children.
Kalaklan River was minimally silted. Then, City Mayor Richard J. Gordon embarked on periodic dredging of the river from Filtration, Sta. Rita bridge down to its mouth on Subic Bay. Nevertheless, siltation went on and on.
Then, Mount Pinatubo Big Bang after being dormant for 600 years happened on June 12, 1991. That was the single largest factor that altered the negative susceptibility of the city against floods. Millions and millions of volcanic debris were dumped over and beyond city limits. River beds, creeks, esteros canals and drainage channels were all filled with volcanic sands. There was a ten year period dredging/desilting program of city natural water waterways that went on until today.
On May 17, 1997 the unusual rain during that time summer time brought flood waters and covered the city with mud as its aftermath. That was the first time the city had witnessed the worst flood. The Subic-Tipo Expressway (http://en.wikipedia.org/wiki/Subic-Tipo_Expressway)
was then, newly built and became operational. That unusual flood of mud many residents attributed it to the construction of the highway in which cut log debris clogged the natural drainage channels.
The original depths of natural drainage channels in and around the city were never restored. They all became shallower and shallower each year.
Concurrent City Mayor James “Bong” Gordon said that dredgings deeper than the present banks of Kalaklan and other rivers in the city are not advisable because banks have the tendencies to collapse toward the river beds.
The construction of Subic-Clark-Tarlac-Expressway (http://en.wikipedia.org/wiki/Subic-Clark-Tarlac_Expressway) was another culprit that is being blamed for the annual floods that hit Olongapo City; Dinalupihan and other towns in Bataan the highway because has impaired if not altered the natural water pathways.
Obviously, the difficulty of instilling education among the citizenry is another factor. Despite of the very efficient and effective garbage disposal system the city government have implemented through the years, dumping of garbage in drainage channels, canals, esteros, rivers and waterways could not be avoided by irresponsible and unconcerned citizens.