Landslides and rivers

The prevailing logic is that landslides occurred due to logging. Logged-over areas and areas of illegal logging did slide on an extensive scale during the disaster. Often, these landslides were not very deep as the soils were already eroded and had limited vegetation to contribute as dangerous debris. These areas slid down and added to the devastation, along with hidden logs from illegal activities just waiting for the ride down river. During the two weeks of near continuous and heavy rain, hundreds of landslides occurred, and surprisingly perhaps for some, in areas of full forest cover (primary forest). All the soil between the forest roots and the bedrock liquefied, and with the final downpour, not even the forest roots could withstand the "meltdown" or liquefaction. Liquefaction occurs in saturated soils, that is, soils in which the space between individual particles is filled with water and physical stability is lost to the power of gravity. The liquefied mass, drawn by gravity to a downward position, puts pressure on the surface cover that is holding it in place, and then breaks out taking with it the materials no longer held in place above the break, as well as what is movable in its path. All the soil and forest vegetation that become debris are flushed down the slope until they reach a river.

Where there is enough material, this can create a temporary dam that then breaks, causing a surge in the river and making for an even more dangerous flood event. Landslides add to the flood volume with soil and debris that ram into bridges or housing, and create surges in the flood flows.

The earth's landscape is not as it was formed in ‘Genesis' times.' The geology of an area is not only active when there are earthquakes or volcanic eruptions. Geological activity is ongoing and the landscape continuously evolves. Rivers take the path formed by the geology: the faults and fractures, the slopes and depressions that extenuate the form. Where rocks are exposed to the sun, the air, and water, they weaken, crack, loosen, and lay there for centuries. In the unseen part of the river below the surface of the riverbed, active change occurs through the presence and movement of water. The material washed out in a storm not only comes from landslides and existing sediments carried by the river, but also from the riverbed itself that gives up rock material as it cuts itself deeper in the underlying geology.

There are many scenes along the Sierra Madre where primary and degraded forest areas were dislodged and washed out by the heavy rains. The deep soils below having liquefied and the weathered and well-fractured rock materials now loosened were ready to move. Entire valley beds cascaded down to spread out and smother all activity - agricultural lands and coastal bays. The debris is disgorged from the mountains as metres-deep alluvial fans and contributes to silting up riverbeds in the floodplains and deltas. When standing in the post-flood riverbed, one stands in awe of the height from where vegetation was ripped off both sides showing how high the torrent was, with the water level being even higher. Looking from the silted-up riverbed in a coastal town up towards the mountain sources, one can see literally square kilometres of boulders and gravel excavated and spewed out by the river from top to bottom.

Clearings and logged-out forests show a high percentage of soil loss and generally small landslides, apart from those near sheer slopes. These landslides though have not yet been comprehensively analyzed and are the aspects of logging that need further analysis in relation to flooding. Though forest cover is no longer considered scientifically as significant in reducing the flood waters by the effect of infiltration, analysis may well show in the future that there are significantly more landslides in logged areas. These landslides may be, on average, shallower than those in undisturbed forest as erosion has occurred but may be shown to be more widespread. If that is the case, the argument for the value of forests (and I emphasise natural forests over any plantation form) during large-scale events is very strong. Landslides contribute significant material during a flood event, through temporary damming create surges and leave behind a level of sediment with extensive costs for clean-up and reconstruction.