After storm Sendong and the landslide in Compostela Valley, several maps propped out in the open. Experts flaunt their respective vulnerability and hazard maps. LGUs supposedly knew this and were blamed for not having implemented precautionary measures to avert these disasters. I guess much have been said about poor governance and the biophysical vulnerabilities we are in. In this case, I will try to digest the relevance of these maps and try to analyze why despite all of these information, we are caught unaware when disaster strikes.
Flood maps supposedly showed inundation areas once 1-meter, or 2 meter flood levels come rushing in. Maps are good for planning. They give us idea of the spatial dynamics answering the important question of "where disaster may strike". But what these maps fail to say is "when disaster may potentially strike". Addressing temporal dynamics is also an important consideration in addressing disasters.
I think there is a disconnect between our appreciation of maps and digesting forecasts. What I mean is that we know where areas can be inundated by a 1-m flood stage but we do not know when this flood level may occur. At what rainfall can we say that 1-m flood stage may occur. For example, 180 mm would be destructive for CDO and Iligan. This would mean that rainfall forecast beyond 180 mm has to be watched out for CDO and Iligan (please see my previous article for Iligan: how rainfall can save from flooding). Every watershed respond differently, so preparedness would be different. For example, 180 mm would be minimal if situated in the Cagayan River in Luzon but would be destructive for Cagayan river in Mindanao. So communities have to be careful in listening to forecasts. Previously, I called for revising our alert system by focusing mainly on rain instead of wind (typhoon warnings: switching from wind to rain). This time, I believe it is high time to categorize our watershed and other water bodies that correspond to its response to rainfall volume, intensity, and duration.
For example, a watershed can be categorized as high risk if floods may occur at very small rainfall amounts. Conversely, areas which can absorb high volume of rainwater can be categorized as low risk. It is not enough to say where areas can be flooded by a certain flood height but also when this height could possibly occur given rainfall amounts. I longed for weather forecast that states, "we expect rainfall up to 50 mm therefore areas under category A should be vigilant". I guess stating just "low lying areas" is not enough to warn because other low-lying areas may not have the topography vulnerable to floods.
In categorizing watersheds, topography and land use has to be considered. Areas with the impounding potential and "choked" topography would be different from really flat areas. Cultivated areas would respond differently from closed forest. Our current hazard maps are static in nature and does not into account changes in land cover. Hazard maps have to be revised as often as possible to reflect actual land use. Actually, maps generated ten years ago may not be applicable today because substantial land use change might have occurred during this duration. On the other hand, if vulnerability maps were based on historical hydrological data, it may not be able to capture future scenarios brought about climate change.
There is a substantial stride in climate forecasting in recent times. For example, it has been predicted that above than normal rainfall is expected in Mindanao for December, January and February (La Nina). "Above than normal rainfall" may not sound alarming. But we need to understand that DJF period is when floodings "normally" occurred in Maguindanao, Caraga, and Davao. In this case, this would mean "above normal flooding".
My point here is that forecast is nothing if we do not reflect that with the vulnerability on the ground. We need to repackage our alert systems enough to warn. Vulnerability maps are nothing if we cannot connect that with forecasts. PAGASA, LGUs, and the so-called climate experts should try ways on how to connect their information that is understandable by the common tao. Please do not flood us with bytes, grids, pixels and weder-weder lang yan.
Monday, January 9, 2012
Tuesday, December 20, 2011
The heart of Mindanao needs hydrological surgery
Hydrologically speaking, the most ideal land use is that of closed forest stands represented by dark green colors in Figure 1. Unfortunately, only the northeastern part of Bukidnon and the eastern part of Lanao del Sur (as of 2003) have closed forest covers. What I mean of hydrologically ideal situation is the capacity of a certain land cover type to retain or delay runoff caused by rains. Once these closed forest stands are disturbed its retention capacity is diminished significantly (in the order of 30 to 50%). These are the areas represented by light green or the open forest stands in Figure 1. Even reforestation cannot revert the hydrological capacity back to its original capacity. Virgin forest once lost has no hope for recovery. Closed forests have an ecosystem structure perfected through centuries of evolution. The hydrological capacity is not only about trees but also include the species assembly underneath the canopy. Thus, reforestation is better if done in such a way that shrubs, grasses, bushes are included in the program not just trees. In biosystems engineering, we call it biomimicry.
Similar in hydrological capacity to open forest is the wooded grasslands represented by the peach color. Though not as effective as the closed forest, this land cover type is better than the cultivated annual crops cover represented by yellow colors. It is worthy to note that wooded grasslands, closed and open forests in the eastern portion of Bukidnon have made the hydrology of Davao river stable and tamer. But once the agricultural activities in the Cabanglasan and San Fernando in Bukidnon intensifies, it will be a nightmare for Davao City. I heard that a road is being constructed in the areas that would connect Bukidnon with Davao del Norte. This would mean intensified economic activities in this Bukidnon territory. And if Davao City will open their northern frontier to economic activities (conventional agriculture, etc.), it would be a disaster waiting to happen.
The prevalence of annual crops (yellow areas) has disabled ecosystem services that Bukidnon provides in terms of hydrological regulation (flood delay, and water during drought). This makes the receiving provinces of North Cotabato, Maguindanao and CDO at the mercy of Bukidnon waters (from different watersheds!). This is demonstrated by the perennial problem of flooding in Maguindanao and Cotabato City. Unfortunately, we tried to solve Cotabato floods by controlling the water lilies. Personally, I believe it is better to start seriously looking at the land use practices in the heart of Mindanao. The erratic nature of weather patterns needs adaptive interventions that transcends boundaries and academic disciplines.
Here are some weak points, I believe we need to address:
1) We need to review the different land use plans of the municipalities (CLUP). Does it conform to the whole river basin hydrodynamics? Does the projected land use take into account the possible consequences it may impose on our hydrology? Several accounts would point to CLUPs being a "copy and paste" plans. This is where the LGUs need to craft their respective land use plans that conforms with the ecological dynamics beyond their territorial limits.
2) There is a need to delienate the final limits of forest lands and those alienable and disposable. It can be gleaned in Figure 1 that a lot of forestlands were already cultivated. For example, in Bukidnon, 491,579 hectares were classified as forestlands but only about 170,000 hectares are with forest covers with over 100,000 hectares already open.
3) DENR, DAR, and DA should work hand in hand in developing upland agriculture that focusses on land and water conservation. DA and DAR refuse to work on forestlands eventhough agricultural activities are evident simply because it is within the domain of DENR. DENR are "tree - oriented" and may be not technically conversant with appropriate agricultural technologies that farmers need. On the other hand, agricultural professional are "plantation-oriented" and may not be technically equipped in dealing with smallholder sloping agriculture. If only DENR, DAR, and DA creates a liason office that deals with rigors of upland and sloping agriculture or agroforestry. I think it is no longer possible to reforest the whole 491,579 forestlands but agricultural and environmental agencies can work together in developing economic activities in the uplands that can maintain vital ecosystem water resources services.
4) Built up areas (red colors) should be redesigned that considers storm water regulation. Our preference to pavements in real property development makes a concrete jungle that has the worst hydrological capacity. This is demonstrated by Typhoon Ondoy. Rainwater harvesting and stormwater retention facilities can be embedded in urban structures.
5) Enforce forestry laws to maintain the integrity of the last remaining closed forest. I guess a total log ban is in proper. A log ban for both legal and illegal logging of natural forests. In order to supply our timber needs, tree planting should be encouraged in non-critical parts of the watersheds.
6) Downstream provinces and cities need to reconsider their "dikes and dams" mentality as flood control. Engineering designs are usually based on hydrological analysis. Unfortunately, some of our historical data (if there is such data existing for specific locality) do not capture uncertainties brought about by climate change and the faster land use transitions brought by human activities upstreams. And so our usual dam and dikes solution may not work in the long run. There should be plan B - and that is adaptation and mitigation. What I learned from Dutch water engineering is that water is difficult to control and so they make do of floating structures (bridges, houses, etc.). Instead of relying much on dikes, they make more "room for rivers" and "space for waters". They keep on studying their situation and propose solutions beyond engineering realms.
There is a need to rehabilitate our river basins. Let the heart of Mindanao pump life again. A transboundary natural resources management is imperative that addresses the biophysical and social dynamics of the river basins. A participative and adaptive mechanism that connects stakeholders from different spatial scales and technical backgrounds. Let us remember that water knows no boundary.
Monday, December 19, 2011
flashfloods and weather forecasting
After Typhoon Sendong, rains may now be viewed as agents of disasters. But knowing rainfall forecast can actually save us from harm. We need only to know better our surroundings. This blog tries to explain how we should look at weather forecast with the end in view of better assessing risk vulnerability of our respective communities. I am emphasizing that we should not rely heavily on storm signals because they are all based on winds (please read switching from wind to rain). We should start listening to rainfall forecast and make sense of it. Later, we will use the Iligan case for illustration purposes.
So how rainfall forecast can save you? If you live near the river or at the footslopes of a mountain, you need to listen to the following information:
a) RAINFALL INTENSITY
b) SPEED AND DIRECTION OF MOVEMENT OF RAIN CLOUDS
c) SWATH OR DIAMETER OF THE CLOUDS
Our common mistake is we only focus on the eye of a typhoon and don't bother about how wide is the rain cloud system. More often we associate typhoon signal as the strength of rain. Which is why a lot of us would wonder why certain areas have signals in place but there are no rains. We sometimes ridicule PAGASA about this. But actually PAGASA based their warnings on wind strength. That is why we need to know the information about the rain and the cloud system behavior.
So here we are. Rainfall intensity gives us idea on how heavy is the rain. Usually it is expressed in mm/hr. So that if rain pours at 25 mm/hr for 8 hrs, then we expect to have received 200 mm. By the looks of it, you may say 200 mm or 20 cm is not that deep, right? About 2/3 of your child's ruler. But let us reserve that info later.
For Sendong, it was projected to have 10 to 25 mm/hr of intense rain. How do we predict the duration of the rain? This is where the swath of clouds come in. Usually, you can estimate the diameter by looking at the satellite imagery (usually red in color) or Kuya Kim will sometimes tell you this. For example, Sendong has been predicted to have a cloud diameter (swath) of 400 km. We should know also the speed of its movement. PAGASA pegged it at 24 km/hr. From here, we will compute how long will it take for a cloud system to pass a particular place from end to end. For Sendong, it would be 400 km / (24 km/hr) = 16.67 say 16 hrs of continous rain. If we expect 10 to 25 mm/hr for 16 hrs, the expected rain would be in the range of 160 mm to 400 mm. That means any particular place passed by Sendong potentially can receive 160 to 400 mm of rainfall depth. If I am not mistaken, CDO received 180 mm.
So what do we do with this 160 to 400 mm? This is were the concept of hydrological unit would come into play. Always remember that a certain place will respond differently given a certain estimated volume. This is where the role of PAGASA ends and where the community's vigilance begun. Let us use Iligan City as our case. I use Google Earth to render a 3D visualization.
The satellite image above was captured in July 2009. At first glance, Iligan is so pristine with all the lush green and mountains around it. These mountains in fact can shield the city from strong winds cause by typhoons. But beyond this imposing mountains lies a big container of water or watershed. Iligan was not hit by the winds, it was hit by waters that rushed in from the mountains. Now let me give you a tour what lies behind this Iligan beauty.
As you can see above, there are two watershed that drains to Iligan. One that has an area of approximately 65,000 hectares and one with around 7,800 hectares (estimated only through GE Path). The bigger watershed drains water from as far as Talakag in Bukidnon and Kapai and Tagoloan II in Lanao del Sur. The smaller watershed drains partly the towns of Tagoloan, Baloi, and Pantaran in Lanao del Norte. All of these watersheds drains to an area approximately 1,500 hectares within Iligan proper. It is like having two large buckets pouring their contents to a very small one.
Suppose 160 mm poured over these watersheds, we can use this to compute the flooding volume. We will use 160 mm because we do not know the exact observation there. Anyway, if others have the exact figure from any of the rain gauges in the city or on its watershed they can just follow the procedure herein and compute for themselves a precise figure.
Expected flooding volume = 160 mm (65,000 ha + 7,800 ha) = 0.16 m (728,000,000 m2)
that would be 116,480,000 m3 potential water volume. But of course, not all of these water will rush in. Since the watershed looks intact with lush forest, let us say only 50% (higher if without forest cover) of these waters flows into Iligan. So that would be around 58,240,000 m3. What does this figure mean for the Iliganos? Let us relate that to their low lying area (potential floodplains) which is around 1,500 hectares. Let us divide 58,240,000 m3 with 1,500 hectares: 58,240,000 m3/ 15,000,000 m2 = 3.88 m. This can be the estimated flood height. Enough to pile up three cars over each other. In short, if 160 mm of rain pours over their watersheds, Iliganos expects 3.88 m of floods (If the rainfall was 80 mm, then expected floods would be 1.94 m). Then the next to do is to stay away from the areas that can be inundated (can be reached by possible flood level).
The foregoing discussion illustrates how rainfall forecast can be used to project potential flood level without sophisticated flood warning system. We deviate from the usual hydrological computation and use only the basic principle. This is the predicament of limited hydrological data in an area which is by the way expensive to conduct as well. While there are flood hazard maps in the area, these are without meaning in disaster preparedness unless linked with rainfall forecast. This heuristic approach is good for areas without the resources to buy flood warning systems. This is a crude way of localizing weather forecasts, something that makes sense in our localities.
The question is how about your area? Do you have any clue what would happen if you expect 160 mm rainfall? Communities should also be aware of their sorroundings. How large is the watershed area? What is the shape of the river channel? Is its mouth constricted (similar to downspout) like that of Iligan? Sometimes, it pays to listen to the rhythm of the falling rain.
(The figures above are estimates and is meant for discussion puposes only. Please also read: topography of flashfloods and typhoon warnings: switching from wind to rain)
So how rainfall forecast can save you? If you live near the river or at the footslopes of a mountain, you need to listen to the following information:
a) RAINFALL INTENSITY
b) SPEED AND DIRECTION OF MOVEMENT OF RAIN CLOUDS
c) SWATH OR DIAMETER OF THE CLOUDS
Our common mistake is we only focus on the eye of a typhoon and don't bother about how wide is the rain cloud system. More often we associate typhoon signal as the strength of rain. Which is why a lot of us would wonder why certain areas have signals in place but there are no rains. We sometimes ridicule PAGASA about this. But actually PAGASA based their warnings on wind strength. That is why we need to know the information about the rain and the cloud system behavior.
So here we are. Rainfall intensity gives us idea on how heavy is the rain. Usually it is expressed in mm/hr. So that if rain pours at 25 mm/hr for 8 hrs, then we expect to have received 200 mm. By the looks of it, you may say 200 mm or 20 cm is not that deep, right? About 2/3 of your child's ruler. But let us reserve that info later.
For Sendong, it was projected to have 10 to 25 mm/hr of intense rain. How do we predict the duration of the rain? This is where the swath of clouds come in. Usually, you can estimate the diameter by looking at the satellite imagery (usually red in color) or Kuya Kim will sometimes tell you this. For example, Sendong has been predicted to have a cloud diameter (swath) of 400 km. We should know also the speed of its movement. PAGASA pegged it at 24 km/hr. From here, we will compute how long will it take for a cloud system to pass a particular place from end to end. For Sendong, it would be 400 km / (24 km/hr) = 16.67 say 16 hrs of continous rain. If we expect 10 to 25 mm/hr for 16 hrs, the expected rain would be in the range of 160 mm to 400 mm. That means any particular place passed by Sendong potentially can receive 160 to 400 mm of rainfall depth. If I am not mistaken, CDO received 180 mm.
So what do we do with this 160 to 400 mm? This is were the concept of hydrological unit would come into play. Always remember that a certain place will respond differently given a certain estimated volume. This is where the role of PAGASA ends and where the community's vigilance begun. Let us use Iligan City as our case. I use Google Earth to render a 3D visualization.
The satellite image above was captured in July 2009. At first glance, Iligan is so pristine with all the lush green and mountains around it. These mountains in fact can shield the city from strong winds cause by typhoons. But beyond this imposing mountains lies a big container of water or watershed. Iligan was not hit by the winds, it was hit by waters that rushed in from the mountains. Now let me give you a tour what lies behind this Iligan beauty.
As you can see above, there are two watershed that drains to Iligan. One that has an area of approximately 65,000 hectares and one with around 7,800 hectares (estimated only through GE Path). The bigger watershed drains water from as far as Talakag in Bukidnon and Kapai and Tagoloan II in Lanao del Sur. The smaller watershed drains partly the towns of Tagoloan, Baloi, and Pantaran in Lanao del Norte. All of these watersheds drains to an area approximately 1,500 hectares within Iligan proper. It is like having two large buckets pouring their contents to a very small one.
Suppose 160 mm poured over these watersheds, we can use this to compute the flooding volume. We will use 160 mm because we do not know the exact observation there. Anyway, if others have the exact figure from any of the rain gauges in the city or on its watershed they can just follow the procedure herein and compute for themselves a precise figure.
Expected flooding volume = 160 mm (65,000 ha + 7,800 ha) = 0.16 m (728,000,000 m2)
that would be 116,480,000 m3 potential water volume. But of course, not all of these water will rush in. Since the watershed looks intact with lush forest, let us say only 50% (higher if without forest cover) of these waters flows into Iligan. So that would be around 58,240,000 m3. What does this figure mean for the Iliganos? Let us relate that to their low lying area (potential floodplains) which is around 1,500 hectares. Let us divide 58,240,000 m3 with 1,500 hectares: 58,240,000 m3/ 15,000,000 m2 = 3.88 m. This can be the estimated flood height. Enough to pile up three cars over each other. In short, if 160 mm of rain pours over their watersheds, Iliganos expects 3.88 m of floods (If the rainfall was 80 mm, then expected floods would be 1.94 m). Then the next to do is to stay away from the areas that can be inundated (can be reached by possible flood level).
The foregoing discussion illustrates how rainfall forecast can be used to project potential flood level without sophisticated flood warning system. We deviate from the usual hydrological computation and use only the basic principle. This is the predicament of limited hydrological data in an area which is by the way expensive to conduct as well. While there are flood hazard maps in the area, these are without meaning in disaster preparedness unless linked with rainfall forecast. This heuristic approach is good for areas without the resources to buy flood warning systems. This is a crude way of localizing weather forecasts, something that makes sense in our localities.
The question is how about your area? Do you have any clue what would happen if you expect 160 mm rainfall? Communities should also be aware of their sorroundings. How large is the watershed area? What is the shape of the river channel? Is its mouth constricted (similar to downspout) like that of Iligan? Sometimes, it pays to listen to the rhythm of the falling rain.
(The figures above are estimates and is meant for discussion puposes only. Please also read: topography of flashfloods and typhoon warnings: switching from wind to rain)
Subscribe to:
Posts (Atom)