Countermeasures for large-scale landslide dams caused by Typhoon No. 12 in September 2011 1,* 1 Wataru SAKURAI and Ryo SAKAI 1 Kii Mountain District Sabo Office, Kinki Regional Development Bureau, Ministry of Land Infrastructure Transport and Tourism (1681 Sanzaicho, Gojoshi, Nara, 637-0002) *Corresponding author. E-mail: [email protected] The record-breaking heavy rains caused by Typhoon No. 12 in September 2011 caused deep seated landslides that blocked drainages at 17 points in Nara and Wakayama Prefectures. At five of these locations, the damage was so severe that the landslide dam reached 100 meters in height, and erosion-control countermeasures are currently being undertaken by the Kii Mountain District Sabo Office under the direct control of the Japanese Government. At present, all emergency work, including the installation of temporary overflow channels, has been completed so that construction of the sabo dams and other works can be implemented. However, these countermeasures are an attempt to remedy unprecedented large-scale landslide dams. Moreover, ongoing rainfall triggered by typhoons has caused numerous difficulties with construction. This paper reports on progress and policies regarding the abovementioned countermeasures. Key words: landslide dam, countermeasures plan, Sabo facilities 1. Introduction The following reports the progress in working out the measures. A great number of large-scale debris disasters such as deep-seated landslides were caused by the heavy rainfall triggered by Typhoon No. 12 mainly in Nara and Wakayama Prefectures of Kii Peninsula in September, with the result that these places were seriously damaged. Landslide dam occurred at 17 positions. In five of these points where the damage was particularly serious, countermeasures are Akadani in Nagatono in currently being taken by the Kii Mountain District Nara Prefecture Nara Prefecture Sabo Office under the direct control of the Government (Fig. 1). At present, the emergency work of installing temporary overflow channel for ensuring provisional safety has been terminated. Full-scale measures such as construction and improvement of the sabo dam and overflow channel are currently being taken. For the study of the full-scale measures against the unprecedented large-scale Kuridaira in landslide dam, "Study Committee for Measures Nara Prefecture against Landslide Dam " (headed by Prof. Takahisa MIZUYAMA of the Postgraduate course, Graduate Kitamata in Iya in Wakayama School of Kyoto University) was established. The Nara Prefecture Prefecture policy of the countermeasures was worked out under the guidance and instruction of this Committee. Fig.1 Scope of study area 394 2. Implementation of emergency work fabric-based formwork for filling with ready-mixed concrete if the latter could be delivered to the site Emergency work began on countermeasures at (Photo 4). Almost all measures were completed by five locations: Akadani Area, Gojo City, Nara mid-June 2012. At present, the emergency work has Prefecture (Photo 1) and Iya Area, Tanabe City, been supplanted by the implementation of full-scale Wakayama Prefecture on September 16; Kitamata countermeasures for building sabo dams and Area, Nosegawa Village, Nara Prefecture on overflow channels. September 30; and the Nagatono (Photo 2) and Kuridaira Areas (Photo 3), Totsukawa Village, Nara 3. Status of damages at the relevant positions Prefecture on October 8, 2011. during the period of deluge The aim of the emergency work was to build temporary overflow channels to accommodate any On June 19 and September 30, 2012, the flood on the level of 2-year probability of emergency overflow channels in each area were exceedance before the rainy season in the following subjected to the disaster brought about by Typhoon year so as to prevent damage by overtopping and No. 4 and 17, respectively. The following addresses resulting erosion of the landslide dams. With the in particular Akadani and Kuridaira Areas, which intent to complete the work by the time of the next suffered heavy damage. In Akadani area, the upper year’s deluge, the structure of the temporary portion of a deep-seated landslide that had occurred overflow channels in each area was designed with in September 2011 adjacent to the temporary consideration given to restrictions in the material overflow channel was remobilized by the rainfall of locally available and delivery of equipment and Typhoon No. 4 on June 19, 2012. This storm material to the site. The latter included a mat for generated continuous precipitation of 127.5 mm and loading local boulders in a cage, soil cement to be a maximum of 17.5 mm/h, according to the Akadani formed by a mixture of debris and cement, and area rain gauge for of the Ministry of Land, Photo 1 Akadani Landslide Dam Photo 3 Kuridaira Landslide Dam Cage mat Fabric-made formwork (Nagaｔono) (Nagaｔono) Complete view of overflow canal Soil cement sprayed with mortar (Kuridaira) (Kuridaira) Photo 2 Nagatono Landslide Dam Photo 4 Type and shape of temporary overflow 395 Photo 4 Type and shape of temporary overflow Infrastructure, Transport and Tourism. 2013, a large-scale slide again occurred. On this Approximately 100,000 m3 of debris flowed into the occasion, the culvert again proved effective, and the temporary overflow channel, resulting in the water level of the flood reservoir did not increase. deposition of debris over the entire area of the Kuridaira Area suffered the greatest damage of all channel, whose construction had almost been the sites where measures were taken, with a completed (Photo 5). landslide dam about 100 m high. In this area, the This was followed by continuing debris rainfall from Typhoon No. 17 on September 30, flows issuing from the collapsed slope. At present, a 2012, caused large-scale erosion of the huge huge deposit of debris blankets the temporary landslide dam. overflow channel. This exemplifies the likelihood of This storm generated continuous precipitation of disaster if facilities with an open water channel are 231.5 mm and a maximum of 54 mm/h, according built as countermeasures immediately below an to the Kuridaira Area rain gauge of the Ministry of unstable slope; the maintenance of such facilities Land, Infrastructure, Transport and Tourism. will be made difficult by the inflow and deposition Two-thirds of the temporary overflow channel with of debris. Meanwhile, water is discharged from the an overall length of 576 m was discharged (Photo 7 flood reservoir through an underground culvert and Fig. 2). The analysis of images from the below the temporary overflow channel bottom. closed-circuit television installed at the site suggests There has been no increase in the water level of the that scouring overwhelmed the temporary overflow flood reservoir. This suggests that the installation of channel due to running water with a head of 94 m, a culvert may be effective for a landslide dam and this spread over the entire landslide dam adjacent to an unstable slope when countermeasures [SAKURAI et. al.,2014 (2)]. The terminus of the are taken (Photo 6) [SAKURAI et. al.,2014(1)]. temporary overflow channel had been subjected to Furthermore, after Typhoon No. 18 in September scouring by the running water during Typhoon Before damage by Typhoon No. 4 (May 31, 2012) Scope of repair after Typhoon No. 4 Remaining temporary overflow channel work After damage by Typhoon No. 4 (June 22, 2012) Temporary L=185m drainage Head of channel temporary work overflow L=576m channel work Erosion of H=94m landslide dam Photo taken on July 30, 2012 Photo taken on October 2, 2012 Photo 7 Comparison of temporary overflow channel before ← Debris deposit and after Typhoon No. 17 of September 2012 Volume of deposited Volume of deposited Volume of erosion debris：17,000ｍ3 debris：296,000ｍ3 debris：305,000ｍ3 Scope of deposition Scope of deposition Photo 5 Debris deposit in the temporary overflow channel Crown of Flood in downstream river immediately below Scope of erosion landslide dam reservoir of Akadani area channel eroded area 270m 185m 870m 320m Transverse line Deposit width:100m Erosion width:120m Culvert drain pipe m Scope of deposition on Scope of deposition Crown of 650 downstream river immediately below Scope of erosion landslide dam 600 channel eroded area 270m 185m 870m 320m 550 Deposit gradient:：2.3° Deposit gradient ：11.8° 500 Maximum erosion depth ：37m 450 Maximum deposit depth ：24m 400 <Legend> 350 100m Before damage (measured on June 23, 2012) After damage (measured on October 5, 2012) 300 Photo 6 Discharge from the culvert drain pipe Fig. 2 Erosion of Kuridaira landslide dam 396 No. 4 on June 20, which may have weakened the and suitable facilities installed. structure. The riverbed 65 m downstream of the terminus was reinforced by a net filled with 2 t of 4.2.2 Scale of the project concrete blocks and cobblestones (Photo 7). The scale of the countermeasures project However, shortly after the start of flood overflow assumes a flow rate resulting from rainfall on the from the reservoir, the greater part of the temporary level of 100-year probability of exceedance. The overflow channel was discharged, even though the facilities to be built as countermeasures against this channel bottom had been reinforced with soil flow rate include overflow channels and sabo dams. cement to a maximum thickness of 4.6 m. This resulted in large-scale erosion of the landslide dam. 4.2.3 Policy for installation of facilities This example in Kuridaira Area shows that when an The basic policy for the countermeasures overflow channel is installed in a landslide dam with is to minimize the overflow elevation of the flood a great difference in hydrostatic head, it is important reservoir and to backfill the flood reservoir with to build a sabo dam at the terminus of the overflow excavated debris without sacrificing the stability of channel simultaneously with the overflow channel adjacent collapsed sites or soil deposition areas.