Monitoring of the reconstruction process in a high mountainous area affected by a large earthquake and subsequent debris flows

Recovering from major earthquakes is a challenge, especially in mountainous environments where 15 post-earthquake mass movements and floods may cause substantial impacts. We monitored the reconstruction of Longchi town in Sichuan, China, over a period of 11 years, following the 2008 Wenchuan earthquake. Seven inventories of buildings, land use, roads and mitigation measures were made by using remote sensing image interpretation and field surveys. Most of the buildings were rebuild by 2010 and reconstruction was completed by 2012. The total economic value of the new 20 buildings in 2010 was much more than the pre-earthquake situation in 2007. Unfortunately, post-seismic hazards were not sufficiently taken into consideration in the recovery planning before the catastrophic debris flow disaster in 2010. As a result, the direct economic loss from post-seismic disasters was more than the loss caused by the earthquake itself. The society showed an impact adapt pattern, taking losses from disasters and then gaining resistance. 25 https://doi.org/10.5194/nhess-2019-202 Preprint. Discussion started: 10 September 2019 c © Author(s) 2019. CC BY 4.0 License.

In this study we generated seven inventories of elements-at-risk from satellite images covering a period 90 of 13 years (2005 -2018) and conducted several field surveys to study the recovery of Longchi valley, located close to the epicenter of the Wenchuan earthquake. Image interpretation was carried out based on a series of satellite images collected between 2005 and 2018. The study aims to demonstrate and analyze the process of post-disaster recovery in an unstable geo-environment disrupted by a major earthquake. 95

Study area description
The study was conducted in the Longxi watershed, located within 20 km from the epicenter of the 2008   Before interpreting built-up areas, we also consulted OpenStreetMap, in order to evaluate if data from this platform could be used. Unfortunately, the information in OpenStreetMap was very general for the Wenchuan earthquake-affected area, and was limited to the main roads, and general polygons of settlements. Given the current difficulty to digitize and store data in OpenStreetMap from different time periods we decided to generate our database outside of the platform. 175 We used the above mentioned data to interpret and digitized manmade features, including buildings, farmlands, plantations, roads and mitigation works. Inventories were made for the following years: 2007, 2008, 2010, 2011, 2013, 2015 and 2018. The inventory of 2007 was made first, then the 2008 inventory was created based on modifying the earlier inventory using the aerial photograph of 2008.
The inventory of 2010 was derived by modifying the inventory of 2008 using the Wolrdview-2 image 180 from 2010, and the inventory of 2011 was derived from the 2010 inventory, and so on. Digitizing in such a manner allowed us to keep consistency among the multi-temporal inventories. A series of attributes listed in Table 2 were acquired for the digitized features through image interpretation, field mapping, and interviews.
Institutional buildings refer to public service buildings like schools, hospitals and water pumping 185 stations. Commercial buildings accommodate shops and local companies. Agricultural buildings are used for storage of livestock, agricultural products and farming equipment. Shelters are temporary residences, including pre-fabricated houses, tents and shacks.
Farmlands were classified into crops for food or commercial crops. Commercial crops are several local plant species, including kiwifruit, tea, and magnolia officinalis, that were widely cultivated and 190 exported to benefit the local economy. Crops for food are the vegetables grown for local consumption.
Roads were categorized into: major road, which were wide and built by the national government; secondary road, which is narrower than the major road and could be either local-build or constructed with help from the government; dirt roads are roads without asphalt or concrete layer. Several bridges and tunnels were mapped as well.  The status of a building is determined by the attributes of damage level, damage type and usage status.
The damage level indicates the magnitude of damage a building receives and was assigned based on both image observation and interviewing local people and authorities. If a building is not damaged, level 0 is assigned. 210 Moderately damaged (level 1) means a disaster-affected building was damaged and restored its function after repair.
If a building was damaged beyond repair and not collapsed, it was considered as severely damaged flooding. Under certain circumstances a building could be affected by more than one hazard type, for instance by ground shaking and landslide impact at the same time. The usage status indicates if a feature is functioning normally, is temporary not been used, or completely abandoned. It is assigned based on field mapping and interviews. The geometrical attributes (area or length) were calculated automatically in ArcMap, based on the polygon (buildings or land parcels) or line (road) features. 220 Floor space was calculated by multiplying the number of building floors with the footprint area. The unit price is the cost to construct buildings per square meter and was obtained through interviews, and literature study. The replacement value of a building was estimated by multiplying the unit price with the floor space. All the economic values in this study were converted to US dollar (USD) with a 10-years-average exchange rate of 1 dollar = 6.51 Chinese Yuan. 225 We investigated economic recovery by interviewing the local inhabitants and village authorities.
Unfortunately, most of them were not willing to share information regarding their income, thus we could only make a descriptive analysis. Each of the interviewees represents one family in the analysis.
A total of 113 persons were interviewed in 2018.

Monitoring reconstruction 230
In this section we analyze the changes of the built-up environment caused by human activities and disasters from 2007 to 2018. The statistics of each year are shown in Table 3.

Pre-earthquake (2007)
We created the 2007 inventory based on a Quickbird image from 2005 and an IKONOS image from 2007. The attributes of the inventory were based on the memories of our interviewees. 417 buildings 235 were identified from the images (Figure 3). Many buildings were constructed along the river due to easy access to the main road. Most of the buildings were self-built residences (304), and more than half of them used WB structure (186). Buildings with a tourism function were the second class in terms of number (87), and most of them consisted of RCM types (75). Only 16 buildings were constructed by the government, including 12 RCM apartment buildings and 4 RCF institutional buildings (Table 3). 240 Most of the buildings were not properly designed to withstand a major earthquake, because most construction was informal and no earthquake resistant building practices were applied by local people.
The last major earthquake in this area dates back from 1933 (Deixi earthquake), and there were no eye witnesses alive of that event anymore in 2007. Even though there were many RCM buildings, it appears that only the ones built by the government applied a certain standard against ground shaking, 245 https://doi.org/10.5194/nhess-2019-202 Preprint. Discussion started: 10 September 2019 c Author(s) 2019. CC BY 4.0 License. triggered by road construction could be observed on the hill slope to the south.

The impact of the earthquake (2008)
From 2007 to 2008, prior to the earthquake, 6 buildings were removed and 33 buildings were constructed by the local residents. A total of 444 buildings were affected by the earthquake, of which 250 142 buildings were completed destroyed. Based on the 2009 SPOT image and the 2010 Worldview-2 images a total of 221 buildings were severely damaged and subsequently removed.         The local residents mostly constructed the shelters next to their destroyed houses, even when this was very close to co-seismic landslides. The largest planned settlement with pre-fabricated buildings along with some native shacks was sited on the lower part of the alluvial fan of one of the largest sub-watersheds, the Bayi catchment, which later posed a high debris flow threat, as 29% of its 305 watershed area was covered by co-seismic landslides (Figure 4 B).
It was difficult to estimate the accommodation status of the survivors since many of them went to relatives outside the area and many workers and soldiers stayed in the area to carry out the relief.    Some self-built buildings were removed during the construction process. The total numbers of functioning buildings were reduced to 678 (Table 3). Many mitigation measures, such as check dams, sediment retention basins, and debris flow early warning systems, were implemented in the three most dangerous sub-catchment and concrete embankments were installed along parts of the river (Figure 11). Due to the construction of the mitigation works the total road length increased to 38.1 km. 390

Value estimation
The unit prices for different building types and roads were acquired through interviews with local builders and local government officers (Table 5). The unit prices of buildings increased after the 425 earthquake due to several reasons: higher building standards, large consumption of building materials in the earthquake-hit areas, and currency devaluation. The price of mitigation structures were estimated based on the mitigation design of a catchment in the neighboring watershed     Farmlands continued to expand from 2010 to 2018, reaching 98 hectares, which was 15 hectares more than in 2010. Since the temperature in the Longchi valley is always lower in summer than the nearby cities (Dujiangyan and Chengdu) and most of the landslides were stabilized, the tourism started to recover since 2015. The closure of the tunnel connecting Longchi town with the highway increased the fuel cost to transport goods and reduce potential tourism. Till the end of 2018, the government did not 520 announce any plan to repair the tunnel. The economy of the Longchi watershed is not likely to be fully recovered before the reopening of the nation park.

Discussion & conclusions
We monitored the changes in the Longxi valley during a ten year period after the Wenchuan earthquake and the subsequent recovery process, with seven inventories from different years containing buildings, 525 roads, land use and mitigation measures. Most of the stronger building construction types were only implemented after the earthquake, and mitigation structures were only installed after being impacted by debris flows and floods. A greater awareness to avoid living in hazard prone areas was observed after https://doi.org/10.5194/nhess-2019-202 Preprint. Discussion started: 10 September 2019 c Author(s) 2019. CC BY 4.0 License. the 2010 debris flows. Despite the extensive and repeated damage, the earthquake, and subsequent landslides, debris flows and floods gave Longchi town a chance to increase its resistance to these 530 hazards in future, and to improve economically.
Due to the direct involvement of the Government of the city of Shanghai, who supported Longxi town financially and with expertise, the recovery was fast, considering the large loss and the mountainous terrain in the area impacted by the Wenchuan earthquake. The lack of experience of dealing with post-earthquake landslides was the largest flaw in the recovery planning. The damage caused by 535 post-seismic landslides was not only restricted to Longxi, but was reported across the entire earthquake affected region. The post-earthquake disasters did not significantly slow down the reconstruction process because of the strong economy of China, and the large amount of funding that was invested in reconstruction and protection using mitigation structures. However, recovering the economy through tourism was a failure in Longchi town, because post-seismic debris flow activity was underestimated. 540 Many resources were wasted, for example the destroyed and abandoned hostels, the destroyed main road, and the revoked highway entrance. Similarly many unused and often destroyed tourism facilities can be seen all over the earthquake affected area. Among all the towns that had planned tourism, Longchi had one of the worst failures, because its biggest attraction was the national park which could not be reopened. The recovery would have been much more efficient if it included the awareness of 545 dealing with post-seismic hazards. However, the question remains if these reactivations could have been predicted and mitigated properly.
The Longxi valley shows a typical example of the concept of dynamic risk, as both hazard and element at risk were constantly changing. Post-seismic landslides were controlled by the available amount of co-seismic loose materials and vegetation growth (Fan et al., 2018;Tang et al., 2019;Tang et al., 550 2016;Yang et al., 2018), which were different from time to time. Floods were closely related to landslide activities as they could dam rivers in the short term and caused raising of the riverbeds in the long term. The reconstruction made the total value of the built-up features nearly 4 times higher than it was in 2007, and many of them were reconstructed in the hazard prone zones. The risk was very high in 2010 as the reconstruction created many potentially exposes buildings and there had not been a 555 major event to deplete the co-seismic landslide materials. The direct economic loss caused by the https://doi.org/10.5194/nhess-2019-202 Preprint. Discussion started: 10 September 2019 c Author(s) 2019. CC BY 4.0 License. debris flows alone was more than that caused by the earthquake. It is recommended to update hazard and risk map in an earthquake-hit region frequently, depending on the occurrence of hazards, human activities and environment recovery.
It is important to take multi-hazard effects into consideration in planning recovery. The Longxi 560 watershed experienced an earthquake, slope failures, debris flows, and floods. The four disasters showed directly or indirectly interactions, which caused a high difficulty for hazard assessment. In such a mountainous region it is recommended not to re-build near the outlet of catchments containing many co-seismic landslides. Avoiding reconstruction too close to rivers is also recommended to avoid the floods caused by riverbed raising and landslide dams. However, in a mountainous region the flat lands 565 are most likely created by historical landslides and river terraces, leaving very limited options to planners. One possible solution could be to delay reconstruction and wait for environment to recover to an acceptable point. Careful monitoring and analysis of post-earthquake hazards and risk is essential.