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Flood Mitigation

Typical Flood Problems and How to Solve It.

Flood Case Study- Flood at SMK Tudoh Apoh, Telan Usan, Sarawak

It was reported in the local Borneo Post that SMK Tutoh Apoh school in Telan Usan which caters for many students from remote areas of Sarawak has been under flooding monthly. The Government has allocated $50 million to relocate and build a new school.

Flood Problems in SMK Tudoh Apong, Telan Usan, Sarawak

Looking at the picture of the school compound from aerial view, it appears that the existing School is extensively built-up and would have many students here. To move or relocate such a huge school would take considerable time & resources but would be an ideal situation in the long run. However, the many existing facilities and buildings would be a waste if just abandoned later.

This article would look at ways to minimize flooding and maintain usability of the area as a School, thereby saving $50 million which could have been used to instead bring more teachers, better food, better or more facilities to the school, or provide scholarships to students to pursue further education. Similar approach to solving Flood Prone areas, can be taken.

From an aerial view and news report, the following are noted:-

  1. Flood occurs Once a Month. This is by no mean serious if property damage do not occur. Interruption of school activities once-a-month is not serious as these can be rescheduled. It is uncertain if the flood is once a month during the wet season or throughout the year. From photo observation flood appears to coincide with high river water level, and this should coincide with high rainfall during wet season. Thus it can be deduced that during normal weather, flooding would not occur and if so, it is caused more by lack of local drainage system and outlets. A detail local drainage pattern within the school would solve some of the normal weather flooding problems, and will minimize impact of wet weather flooding.
  2. Some areas within the school compound are dry lands, ie above the flood water level. This is good as these areas can be identified by survey and demarcation and utilized for more flood critical buildings or facilities.
  3. Those areas which are flood prone should be utilized for activities or facilities requiring lots of water. Such can include a fish pond, a water storage pond, a flood detention pond, an irrigation pond or a wet padi and similar water borne plants. Water storage for irrigation, washing, drinking can be a huge savings on metered or potable water, especially in drier weather.
  4. From the photo it also appears that most of the buildings and walkways are above flood levels. This is a normal and desirable practice in rural areas where timber are used extensively. Raised walkway, raised ground floor, stilted buildings are all common building practices and are cheap and practical as the ground conditions in these areas are mostly wet, water logged, peaty and would be more expensive to condition. Any attempts to fill up the existing ground could cause more settlement ad adverse effects, so the stilted approach to buildings are perfect in these environment. As stated above, some high grounds can still be used for specific ground  based facilities.

Flood Mitigation Measures

In such an environmental, flood water control and management is important. It has to be a holistic approach, with nature and costs as primary consideration, ie, minimum interrumption to the existing environment and keeping cost of mitigation works low.

Retention Ponds – a serious of ponds can be carved out from the flood plain and these be made into retention ponds for runoff storage. Some earth bunds may need to be constructed to contain the flood water. Outlets and collecting channels need to be integrated. Offsite retention pond can also be constructed remote from here but related to inflow water. This will stop water from entering the school compound.

Perimeter Earth Drains – these are important to cut-off incoming runoff and bring the surface water towards discharge into nearby river or into the above retention ponds.

Flood Gates – if the discharge outlets can be identified, simple timber or concrete flood gates can be installed with or without retention pond. These will slow incoming flood water from  the river which will in turn ease water discharged from the school compound. Flood gates does not have to be totally water tight. For this to work, all outlets and inlets should be identified and managed collectively.

Widen Drainage Outlets – very effective way to minimize upstream flooding is to widen existing earth drains or streams, buy creating flood plains within the shoulders of the drains or streams. These will put additional storage for incoming water, and ease outfall congestion.

Procudures for a Detail Study

The procedures outlined below can be used for the above school or any flood assessment. This is prelimary and can be used for a more detail study to be carried out.

Obtain Data - obtain all relevant information available for the affected area and the surroundings such as Topo Maps, Present Township/Development Maps, Future Development Plans, Natural Rivers & Streams, Man-Made Drains, Hydrology Maps & data, Geological/Soil Maps, Vegetation/Land Use Maps. A detail layout of the school is necessary. If proper survey plans are not available, then aerial photos can be used.

Visit Ground Zero - make a visit to the site and obtain local information about floods, water levels in rivers, drains, in and outside the school compound. Look at upstream and downstream developments and make assessment whether these impact the flood conditions in the school.

Initiate Survey - detail topographical, spot levels, contour survey is required so a Survey Plan can be produced which will show different levels of the site. Ensure also Upstream and downstream be surveyed. Vegetation survey, soil survey should also be carried out.

Catchment Studies - using 1:10,000 or 1:50,000 maps, conduct a catchment study of the areas concerned.

Hydrology Studies - using data of rainfall and catchment characteristics initiate runoff calculations and assess short term and long term flood conditions.

Mitigation Measures - using data obtained above, assess flood damages and possible solution to the problems, starting with the cheapest and easiest to be implemented measures.

Preliminary Drawings & Reports - prepare a preliminary drawings and flood data showing flood waters in several state and measures taken. A Report can be made to highlight all aspects of the study so the appropriate authorities be made aware of situations. These Reports can include Inplementation Schedule, Costs Implications, & Damage Assessment. It may also recommend for any Environmental Impact Studies to be carried out if size of affected are is significant as to affect ecology and environment.

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Authur- Philip Goh (B.Sc, MIEM, P.Eng. MICE, C.Eng)
MEC Engineers, Civil & Structural Engineer
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Authur- Philip Goh (B.Sc, MIEM, P.Eng. MICE, C.Eng) MEC Engineers, Civil & Structural Engineer HP: 016-8672189, Email: Widget: email cloaker