Flood Hazard Assessment

Choekar Geowg, Bumthang

Choekhor is one of the four gewogs in Bumthang. The Gewog borders Chumey Gewog to the southwest , Ura Gewog to the southeast, Trongsa Dzongkhag to the west, Wangdue Phodrang Dzongkhag to northwest, Tibet to the north, Lhuentse Dzongkhag to the northeast, and Tang Gewog to the east.

Total area coverage is approximately 1,533 square kilometers. Its altitude ranges from 2600- 5800meters above sea level.

Many settlements in the gewog are located along the Chamkhar chhu which is one of the five river systems of Bhutan. With the river flowing through the gewog, areas are susceptible to the flood but nevertheless, there are other factors like precipitation, soil types, slope and elevation that too can influence the flood in an area even in the absence of a river.

Therefore, this study is aimed at producing flood hazard map for the whole Choekhor gewog irrespective of its location and nearness to the Chamkhar Chhu.

Methods

In order to achieve the final product for Flood Risk Mapping of Choekhor Gewog, the following steps and procedures has been followed. It is crucial that every step has been taken care so that final product is achieved without much difficulty and changes.

Step 1. Load DEM data, LULC data, Gewog boundaries and Precipitation data onto ArcGIS.

Step 2.Exporting of data to study area

Step 3. Extraction of Data

Step 4. Convert vector to raster data

Step 5. Reclassify raster data.

Step 6. Conduct a weighted sum analysis.

In order to carry out this flood hazard assessment, the data used are elevation, slope, LULC and precipitation. There are other potential factors (soil types and many more) that can be used to obtain the result but due to unavailable or in accessible data, we couldn’t acquire and use for this study purpose. Weighted sum overlay was used to create the final flood assessment for Choekhor Gewog. Different weightage is given for different data sets or factors that determine the flood risk in the area.

Relative precipitation from ArcGIS Online.

Flood Hazard Map

Settlements near the river are at very high risk of flood disaster. The places like Chamkhar town and Gyalaykhar are at higher risk of flood than other places. Having identified as high risk area, the people can plan mitigation works to reduce the destruction.

It is not always the rivers that causes floods. Though, the rivers influence the occurrence of the floods, there are other equally influential factors. For instance, there will be flood due to excessive rainfall or poor soil types. On the other hand, low areas are prone to floods than the places in the higher altitudes. Therefore, all the places can be susceptible for flood disaster.

In the above map, the northern part are at the higher elevation with high mountains. That is why it is indicated by red colour which means very low risk area. On the other hand, river valleys are said to be lowest point in the area which ultimately falls under very high risk area.

Impacts

This flood hazard map can be used for different purposes by different stake holders. Following are some of the stakeholders that can make use of Flood Hazard Map of Choekhor.

1. Gewog Administration

This map will be very useful for Gewog administration for they can plan and distribute budget based on which zone they fall under. They can also plan developmental activities accordingly.

2. Dzongkhag administration

With lot of developmental activities in the hands of dzongkhag administration, it is very important that they identify the sites wisely so that government budget does not go in waste. With this flood hazard map, it will help in proper planning of the developmental activities.

3. For the community

As a residents of the community, it is important to know flood risk mapping so that they can be alert in the first place and plan their cattle movements according to the season and weather conditions. They can also identify suitable site for community infrastructures developments.

Educational Applications

1. Based on this study, similar study can be done on any area with improved data sets and procedures for real application in the field. This study will act as an base for the for the future reference

2. We can use this Flood Risk map to discuss on flood management and mitigation in the class. By observing this map, students should be able to identify areas which are under high risk and which are at low risk

3. This map can be used to teach flood risk assessment reading to students. Through this map, students should be trained on how to observe and learn risk assessment so that they are prepared to read any kind of such maps in the future as well. This will enable students to build their skills in map reading

4. People who wants to carry out similar study on Choekhor Gewog can have data sets ready without having to extract and reclassify for them to use. When such more studies are done, there will be easily available data encouraging more people to study GIS and remote sensing

5. At least for beginners, the work flow and the steps described above can be useful and can be used even as a tutorial to achieve certain objectives in GIS.

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