Debris Flow and Large-Scale Landslide Disaster Prevention Information Network

The Council of Agriculture commissioned National Cheng Kung University to complete the first debris flow potential stream survey, totaling 485 potential streams across Taiwan. The delineation method mainly referred to and modified Yasuo Takahashi's (1991) scoring method, primarily based on three indicators: valley slope, streambed slope, and effective catchment area. The main delineation criteria were a valley slope greater than 15°, a streambed slope greater than 15°, and a catchment area greater than 5 hectares. After the survey was announced, the fan area was added to the 485 potential debris flow streams.

After the 921 Earthquake, the Soil and Water Conservation Bureau (SWCB) re-conducted the survey of potential debris flow streams in the 921 reconstruction areas (Miaoli County, Taichung County, Taichung City, Nantou County, Changhua County, Yunlin County, Chiayi County). The number of potential streams in the reconstruction area increased from 133 to 370 by adding 237 streams, resulting in a total of 722 potential debris flow streams across Taiwan. The stream numbers included "A" for distinction, starting the numbering from 001, e.g., Nantou A001. For non-reconstruction areas re-surveyed after 2001 (Year 90 R.O.C.), "A" was also added for distinction, but the numbering continued from the original sequence. For instance, Tainan County's original numbering in 1996 (Year 85 R.O.C.) was up to Tainan 002, so re-surveyed streams started from Tainan A003. The Taipei area, originally numbered up to Taipei 060, started from Taipei A061.

Due to changes in geomorphological conditions caused by Typhoon Toraji and Typhoon Nari, the Bureau re-conducted surveys nationwide by dividing the country into North, South, and East regions, completing the second national survey in March 2002 (Year 91 R.O.C.). Considering the severe topographic changes in Taiwan caused by earthquakes and heavy rainfall from typhoons, which resulted in the accumulation of large amounts of sediment material in mountainous areas and lowered the conditions for debris flow occurrence, the original criterion for a catchment area of 5 hectares or more was revised to 3 hectares or more to be considered as having debris flow potential. Other criteria remained unchanged. In 2002, surveys for the North, South, and East potential areas were completed and disclosed, totaling 1,420 potential debris flow streams nationwide, with Taipei County having the most (214 streams), followed by Nantou County (199 streams).

Re-evaluated the priority treatment index for the 1,420 potential debris flow streams and delineated the affected areas.

The Soil and Water Conservation Bureau (SWCB) commissioned a NTU team to re-survey the affected areas, completing the following tasks:

• (1) Overlaying color orthophoto maps and requesting local governments to confirm whether there are indeed households within the potential area.
• (2) Reorganizing the attribute table and adding the locations of safeguarded households.
• (3) Creating the PR20L table and adding overflow point data.
• (4) Producing 1/5,000 to 1/3,000 affected area maps using orthophoto maps as the base map.
• (5) Adding temporary numbers for newly added potential debris flow streams (e.g., Nantou N010).

After evaluation, 158 potential debris flow streams were deemed to have a low risk of debris flow disaster and no safeguarded households, and their risk level was adjusted to "Continuous Observation."

Following the 72 Flood and Typhoon Aere disaster, the SWCB sent maps and data to local governments and requested them to establish registers of safeguarded households.

Approximately 80% of safeguarded households were registered, totaling over 28,000 people. To date, local governments regularly update the register of safeguarded households before the flood season each year, and the data is established in the system, with the current number of safeguarded individuals recorded in the system being 32,865.

Considering the environmental changes in recent years, the positioning of potential streams was shifted from the middle section of the stream to focusing primarily on safeguarded households. Since some potential debris flow streams' safeguarded households span across two villages, fields for a second county/city, township/district, and village were added. Furthermore, due to administrative boundary adjustments for some villages, relevant field data in the debris flow potential stream layer were updated to timely reflect current disaster response needs. After adjustment, the total number of potential streams remained at 1,420, with Taipei County having the most at 215 streams.

During this period, local governments proposed new potential debris flow streams, which were field-investigated by experts and scholars commissioned by the Soil and Water Conservation Bureau of the Council of Agriculture, Executive Yuan, and assigned temporary numbers (e.g., Chiayi N001). Local governments were requested to establish safeguarded household registers beforehand. At the end of 2008 (Year 97 R.O.C.), Tainan County Government and others subsequently proposed new potential debris flow streams after Typhoons Kalmaegi and Sinlaku. After field investigation by experts and scholars convened by the SWCB of the Council of Agriculture, Executive Yuan, an adjustment to the potential debris flow stream update work was recommended.

To conduct the potential debris flow stream update work in 2009 (Year 98 R.O.C.), the Bureau's 18th Soil and Water Conservation Planning and Management Task Force meeting approved on April 8th to adopt a new coding method for potential debris flow streams starting from 2009 (Year 98 R.O.C.): "Simplified County/City Name (two characters) + DF + three-digit serial number" (e.g., "北市DF001"). In conjunction with the Regulations on the Disclosure of Debris Flow Disaster Potential Data, potential data disclosure operations were carried out. Two briefing sessions regarding the addition, coding, and map data update of potential debris flow streams were held on April 14th and May 5th, gathering various levels of government, relevant units, and experts/scholars. The review was completed on May 25th in accordance with Article 5 of the aforementioned regulations. A total of 83 new potential debris flow streams were added, increasing the updated number of potential streams in 2009 (Year 98 R.O.C.) to 1,503, distributed across 19 counties (cities), 159 townships (districts), and 643 villages (lis).

In response to the severe damage caused by Typhoon Morakot in August 2009 (Year 98 R.O.C.) to Southern Taiwan, which altered the topography and landscape of mountainous areas, work on adding and updating map data for potential debris flow streams was carried out. A briefing session was held on March 26, 2010 (Year 99 R.O.C.), with relevant government units and experts/scholars. Based on the "Regulations on the Disclosure of Debris Flow Disaster Potential Data," the first review meeting of debris flow disaster potential data for 2010 (Year 99 R.O.C.) was convened on April 7, 2010, where it was resolved to add 49 new streams, increasing the total number of potential debris flow streams to 1,552, distributed across 19 counties (cities), 159 townships (districts), and 657 villages (lis).

For the potential debris flow streams reported by local governments in 2010 (Year 99 R.O.C.) as suspected debris flow disasters within their jurisdiction, and those actively investigated by the Bureau, the Bureau carried out work on adding new potential debris flow streams and updating map data. A briefing session was held on November 9, 2010, with various levels of government and relevant units. After the local governments were requested to re-verify and confirm no objections on March 24, 2011 (Year 100 R.O.C.), the 2011 (Year 100 R.O.C.) Debris Flow Disaster Potential Data Review Meeting was convened on April 26, 2011, inviting relevant agencies and experts/scholars. It was resolved to add 26 new streams, increasing the total number of potential debris flow streams in Taiwan to 1,578, distributed across 17 counties (cities), 159 townships (districts), and 662 villages (lis).

To strengthen the debris flow potential stream survey work, the Bureau used satellite imagery to screen and identify potential debris flow disaster locations. Based on the debris flow potential data completed by the Central Geological Survey and overlaid with household data, locations requiring addition to the list of potential debris flow streams were selected for investigation. Briefing sessions on the survey results were held on October 4 and October 19, 2011 (Year 100 R.O.C.), inviting relevant units. Following this, two Debris Flow Disaster Potential Data Review Meetings were convened on November 22, 2011, and February 8, 2012 (Year 101 R.O.C.), inviting relevant agencies and experts/scholars, where it was resolved to add 82 new streams. The total number of potential debris flow streams in Taiwan increased to 1,660, distributed across 17 counties (cities), 159 townships (districts), and 680 villages (lis).

The Bureau conducted work on adding new potential debris flow streams and updating map data for suspected debris flow disasters within jurisdictions reported by local governments in 2011 (Year 100 R.O.C.) and 2012 (Year 101 R.O.C.), and for potential streams actively investigated by the Bureau in 2012 (Year 101 R.O.C.). The survey results were presented at a briefing session on November 7, 2012 (Year 101 R.O.C.), inviting relevant units. In accordance with the "Regulations on the Disclosure of Debris Flow Disaster Potential Data," a Debris Flow Disaster Potential Data Review Meeting was convened on December 10, 2012, inviting relevant agencies and experts/scholars, where it was resolved to add 4 new streams. The total number of potential debris flow streams in Taiwan reached 1,664, distributed across 17 counties (cities), 159 townships (districts), and 681 villages (lis).

The Bureau investigated and reviewed the newly reported suspected debris flow disasters by all levels of government in 2013 (Year 102 R.O.C.) and the already disclosed potential debris flow streams. The relevant evaluation results were presented at a briefing session on November 14, 2013, inviting experts, scholars, and local governments. In accordance with the "Regulations on the Disclosure of Debris Flow Disaster Potential Data," a Debris Flow Disaster Potential Data Review Meeting was convened on December 12, 2013, inviting experts, scholars, relevant agencies, and local governments, where it was resolved to add 7 new streams. The total number of potential debris flow streams in Taiwan reached 1,671, distributed across 17 counties (cities), 159 townships (districts), and 684 villages (lis).

The Bureau investigated and reviewed the newly reported suspected debris flow disasters by all levels of government in 2014 (Year 103 R.O.C.) and the already disclosed potential debris flow streams. The relevant evaluation results were presented at a briefing session on December 11, 2014, inviting experts, scholars, and local governments. In accordance with the "Regulations on the Disclosure of Debris Flow Disaster Potential Data," a Debris Flow Disaster Potential Data Review Meeting was convened on January 8, 2015, inviting experts, scholars, relevant agencies, and local governments, where it was resolved to add 2 new streams. The total number of potential debris flow streams in Taiwan reached 1,673, distributed across 17 counties (cities), 159 townships (districts), and 684 villages (lis).

The Bureau investigated and reviewed the newly reported suspected debris flow disasters by all levels of government in 2015 (Year 104 R.O.C.) and the already disclosed potential debris flow streams. The relevant evaluation results were presented at a briefing session on November 26, 2015, inviting experts, scholars, and local governments. In accordance with the "Regulations on the Disclosure of Debris Flow Disaster Potential Data," a Debris Flow Disaster Potential Data Review Meeting was convened on January 4, 2016, where it was resolved to add 14 new streams. The total number of potential debris flow streams in Taiwan reached 1,687, distributed across 17 counties (cities), 159 townships (districts), and 685 villages (lis).

The Bureau investigated and reviewed the newly reported suspected debris flow disasters by all levels of government in 2016 (Year 105 R.O.C.) and the already disclosed potential debris flow streams. The relevant evaluation results were presented at a briefing session on November 2, 2016, inviting experts, scholars, and local governments. In accordance with the "Regulations on the Disclosure of Debris Flow Disaster Potential Data," a Debris Flow Disaster Potential Data Review Meeting was convened on December 8, 2016, where it was resolved to add 18 new streams. The total number of potential debris flow streams in Taiwan reached 1,705, distributed across 17 counties (cities), 159 townships (districts), and 686 villages (lis). The debris flow warning criteria value for 2017 (Year 106 R.O.C.) was also announced at the same time.