Sunday, December 18, 2011

topography of flashfloods

Amidst these news of flash floods, I explored the topography of the flooding sites namely that of Cagayan de Oro (CDO) after Typhoon Sendong (December 2011) and that of Davao City (last July 2011). I used 3D rendition via Google earth to see what is the current land use and topography. I do not have an exact knowledge about the actual land use but satellite imagery more or less can suggest actual land use. This however need to be verified on the ground. I do not know the names of the places so I just tag them as points A, B, C, D, etc.

Here are some screenshots for Cagayan de Oro City:

The wide swath of the typhoon can render runoff from points A, B, and D going to the vicinity of C which is approximately the position of Cagayan de Oro City.



I am not so sure if this is the flooded area, but based on videos on the internet, the features resembled that of the actual flooded area. Well, this rendition is just for discussion purposes on how land use can affect hydrology and how urban planning can render populace vulnerable to risk. Take for example this image above, it shows the topography like a trough. Unfortunately, most of the residences are within this trough.


Going back to our larger picture, we can actually see two watershed draining to the vicinity of C. We added point E because it seems D have its own pathway but still in the vicinity of C. Here, we illustrated that floodwaters coming from A and B are essentially choked at C (like a trough) and eventually flush out water (illustrated as a three pronged arrow). Floodwaters from D would eventually settle on the E floodplains. By the looks of it, it may not be as destructive as in point C.


So we have here (see image above) a situation that a large watershed drains to a small floodplains (marked CDO).


It is wide that point A to C is approximately 50 km and B to C is around 20 km. By the looks of it point A is already in Bukidnon.





Examining satellite images, we can see that land uses can be agricultural fields in point A and B. Looking closer at point A, it can be gleaned that it is of intensive agricultural land use as evidenced by its brown patches. That is also true with slopes at point D. Although at point D, we cannot discount possible logging activities there due the presence of forest patches. What is crucial here is to know (ground truthing) the actual land use activities on point A and B. Agricultural activities that need to be reviewed are vegetable and corn farming - considered to be erosive agricultural land practices. Are agricultural activities in those areas employ some sort of a soil and water conservation to arrest runoff?  If farmers do not consider this, dwellers in C will be forever at the mercy of floods. This large watershed may encompass several municipalities in Misamis Oriental and Bukidnon. Negotiating proper land use to protect CDO might be an uphill struggle but worth the try.

Zooming in on C, you will find several property development. Locally, if these subdivisions are constructed in such a way that drainage connects to the main river, it will add to the problem. This is particularly true if streets are paved and residences lawns are paved as well. I suggest that they employ some sort of water retention facilities (in agricultural engineering we called it bioretention) to contain storm water to at least delay the runoff towards to the river. Or design houses with rainwater harvesting or green roofs. For point C, they have no choice but to start building houses with second floor to adapt to the situation. Our forefathers are smarter than us. They built houses on stilts!

More or less, the Davao City incident has some pecularities with CDO case:



But unlike CDO, the Davao flashflood was triggered by localized rainfall on a smaller watershed. But the same mechanism is still in place and that is the presence of a "choked point". Looking at satellite imagery, the watershed were possibly plantation crops (banana, fruit trees). Unlike that of CDO, this is less erosive provided planted properly. What I mean by planted properly is that it should consider the contour and slope. But looking closely (please try Google Earth) some of this plantation crops was planted along straight lines. In this case, runoff is still high.



I could not discount the possibility of the contribution of property development or quarrying on top of the Matina floodplains. But I guess the biggest contribution comes from the agricultural plains in the north. A larger watershed is needed to impound such large volume of water to flush the floodplains. I mean flush not just flash.

I just hope that this will help explain some phenomena we experience nowadays. Hope that policy-makers will consider hydrology in crafting their respective land use plans. Again, these illustrations are meant for discussion purposes. We still need to verify on the ground the actual land use before we could definitely give conclusions.

Please read also: using rainfall to save yourself from floods and typhoon warnings: switching from wind to rain . If you think this is helpful, you are free to share.

6 comments:

  1. felipe velez abrogarDecember 20, 2011 at 4:32 PM

    yes,i believe all of these coz even w/out your images i've already imagined it since 1970s.now w/ these images you've posted, people who can't understand me before will now think i'm not crazy,hehe. thank you very much.

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  2. Thanks. This study helps.

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  3. This is a good source of information to start discussions leading to legislation. Local government to invite our experts from DPWH, Private planners / consultants, DOST, local government engineers, Department of Agriculture, DENR, Pag-asa, Private Organizations etc. must collectively consider this and start discussing the issue seriously to come up with the best solution to this recent tragedy in Cagayan de Oro City, Iigan City, etc…

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  4. marilyn3113@gmail.comJanuary 5, 2012 at 12:10 PM

    i agree w/u garry.pags. i hope this will not fall into deaf ears lest CdeO will be erased from Google map. God forbids!

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  5. Suggested direct solutions to mitigate these flood problems are as follows: widen the river channel at its narrowest point and dredge the existing river from the mouth up to the Taguanao Bridge Upstream. Construct Earth dikes along the river bank on both sides of the river from the mouth of the river up to the upstream side of the Taguanao Bridge.This should delineate the flood area from the non-flood area. Construct relocation area and mass housing for the flood victims and/or residents of the flood area. Cost estimates is between 3Billion to 5 Billion pesos-concerned cdo resident

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  6. This information is very helpful. I think this should be featured on TV for public information.I think the most difficult problem we face with Cagayan de Oro is not the flood itself but the seemingly-appearing ignorant city officials. Nagma-ang-maangan lang sila. anong bang klaseng opisyal kayo!!!Binoto kayo ng mga tao para sila ay serbisyohan ninyo pero tingnan niyo mas ignorante pa kayo sa kanila. Why do they keep on denying that mining and logging do not caused the flood?You better step down from office if even basic environmental awareness you do not know. Buti pa ang mga elementary graders alam nila. Di ba yon ang tinuturo natin sa eskwela? logging and mining are destructive to our environment? Buti kung di kayo mga Attorney o kayay college graduate di niyo alam. Ni illegal man o legal ang mga activities na mga ito ay talagang masama sa environment. Kayo lang naman ang nag-legalize ng mga iyan so that para sabihin di binabawal ng batas. Mga CDO officials ano kaya kung kayo mismo magpasimuno nga mga tree planting activities. Make it a requirement to all government, non-government and other private offices/companies, in schools, churches or to any simple residents. If Cagayan de Oro has a total population of 500,000 and all residents will be required to plant then we will have 500,000 trees in a month. If Bukidnon has 1.1 M population and the same will plant then they will have 1.1M trees in a month. If these be maintained up to 10 years then trees will flourish and green environment will likely to come back.

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