Wednesday, September 2, 2015

Flash Floods - Manage the Risk

At a river in the jungles of West Papua. Tell-tale signs of a possible flash flood: floating debris, murky water, swelling river

The recent incident in Tarlac gave us another pause to think about mountain safety.  With 6 fatalities, and all too familiar cause – flash flood, some ask what could have been done differently.

Flash flood, like its cousin - avalanche, is an ‘instant killer’.  It’s probably the number one cause of death in the Philippine mountains, not counting war or other man-made ‘disasters’.  It’s not always “timely predictable”, but there are known risk areas to avoid.

It is likely that experienced hikers or mountaineers in the tropical setting had some experiences with this roaring force of water – in some ways or another.  In fact, heavy rains – especially during monsoon, is nothing but common and hence the consequential rush of water from the mountain slopes,  surging down the tributaries and river system.  

What is it?

Flash flood is nothing more but a ‘sudden flood’,  sudden could be counted in mere seconds to few minutes depending on the terrain, amount of rainwater,  drain/canal/river structure, vegetation, etc.   The simple way to visualize is to first imagine a funnel.  If one is to look at a topographic map (or just Google geo image), the watershed or basin (the funnel) is normally defined by the largest and tallest ridge sections of the mountains, typically a river is somewhere at the center - the lowest in-between section.   Rain water, in millions of gallons - pour down the slopes of the mountains, trickling down from the highest ridges, to the lower slopes, to tributaries (like little canals and/or lowest point-line in the slopes) to main river systems.  As the water flows down, the water coming from different slopes converges and increases the volume and so is the flow intensity (the rush).  Rivers with narrow gorges will obviously restrict the flow and enhances the surge-force of water.  Simple logic dictates that the higher you are in the funnel, the less volume and energy, and the lower you are – well you can imagine the magnified effect.  Unfortunately, most rivers where hikers or locals frequent to bathe, wash or simply to cross are in the lower section of the mountains.  And if that spot is ‘tight’ and narrow or sometimes with rock walls (gorges) – the more the risk as escape is almost impossible.

Flash flood can occur within minutes, but it’s also possible to occur after a day or even couple of days.  One can be tricked to believe that crossing or swimming a river on a sunny day is safe, where in fact a surging, roaring volume of water is streaming down from the far and higher slope of the mountain.  If I recall it right this is one possible/ concluded reason why 4 of my own club’s experienced members perished in a flash flood – 30years ago (in Mt Guiting Guiting).

Experience makes you more aware.

My first flash flood experience was in Mt Maculot.  UP and UPLB Mountaineers joined hands in an effort to ‘stabilize’ the continually eroding and deepening trails of the mountain.  This was (I think) 1992 and lacking whether forecasting tool/ news (i.e. no internet then!) we didn’t know there was an incoming typhoon.  It was sunny during our walk up but started raining during the late evening. We were hammered that Saturday night, lost our tent fly from strong winds, and endured a cold wet night inside the flooded and broken tent.  I recall Banny – one of my tent mates squeezing water out of his super soaked sleeping bag.  But still smiling!

At first light, we packed up and hurried down the mountain.  There at the lower section of the mountain, a gushing brown water had ‘erased’ part of the slope (which later revealed large stones – and now part of the regular trail).   Our senior members were still somewhere up the mountain, and we (juniors) tried to figure out what to do.  It’s a no-no to cross ANY flash flood, but the weakening force of water allowed us to cross a 2-meter wide section of the ‘river’.  (NEVER DO THIS).  Looking back, one just needs to wait out and let the surge of water to pass. It’s normally a matter of minutes, or sometimes an hour or a couple.  

How can we mitigate the risk, how to manage?

1.     Plan ahead.   Identify risk areas (primarily based on past incidents) and double check conditions (up and down the mountains) before venturing in risk areas. PREVENTION is the only sure way to avoid death/injury due to flash floods.

2.     Use local knowledge.  Use local guides if available.  People who had lived in the area since childhood possess very specific knowledge about a river’s behaviour.  They know water color changes (and what it means), they know rate of swelling (how much reaction time you have), they know if a particular kind of heavy rain is dangerous, etc.

3.     Ridges are windy, but valleys are wet.  Establishing trail is an art but also follows some safety rules.  But if you’re just re-using an old trail, at least mind the danger areas.  Ridges (the highest part of a slope) are generally safe against surges, offer better view or pictures, but sometimes cold-windy (i.e. exposed). Valleys are typically more covered (but could be windy-cold in river sections),  usually near or around water sources, but could be dangerous for sudden flooding.   Avoid narrow river trails especially with gorges (walls) during rainy days.

4.     Check for signs. When crossing or walking by the river, check water color, presence of debris, and flow rate changes.  A sudden change from clear to brown-murky water could mean an incoming surge.  Depending on the terrain, changes in flow rate or color may give you from few seconds to few hours of reaction time; generally, the steeper and shorter the slope, the faster the flow and shorter the time.

5.     Preserve the forest, plant forest trees.  Denuded mountains will readily allow water to trickle down and fast without trees and undergrowth to slow them down.  Deforestation also causes landslides which also kills.

6.     Use of safety device when crossing rivers.  While wearing helmet and PFD (personal floatation device) can only do much when caught in a flash flood – it can still help in increasing one’s chance of survival.     

7.     When caught in a flash flood...  It will be unfortunate if this happens, but do what you can to survive!  Swift water rescue and white-water rafting courses teach students how to swim in raging rivers correctly – feet first,  float on your back, tilt head forward to navigate, steer with your hands behind/under you.  Try to maintain feet-first position lest you risk hitting your head on a rock which will knock you out and drown you in the process.  PRAY!  

No comments: