Evaluating Seismic Effects on a Water Supply Network and Quantifying Post-Earthquake Recovery

R. R. Biswas / IJE TRANSACTIONS B: Applications Vol. 32, No. 5, (May 2019) 654-660 655 that supply high-quality water. As the water quality is good, there is no need for water treatment, resulting cheap water supply to customers. As shown in Figure 1, there are seven major water supply pressure zones in Christchurch. Zone 1 (also called Central Zone) operates with highest operating pressure range of 70 to 80 m. All the other major zones are typically operates between 55 to 65 m pressure range. 4. EARTHQUAKES IN CHRISTCHURCHa Christchurch was hit by an earhquake with a magnitude of 7.1 Richter on 4 September 2010 [4]. It was centred approximately 50 km west of the city centre [4]. A second earthquake with a magnitude of 6.3 hit Christchurch on 22 February 2011 [4]. The second earthquake caused major damage to the water network as its epicentre was near to ground level and closer to the city centre. Another earthquake with a magnitude of 6.3 hit Christchurch on 13 June 2011 [4]. There have been numerous earthquake aftershocks since then. 5. QUANTIFYING EARTHQUAKE EFFECTS The earthquakes caused widespread liquefaction, lateral spreading and ground deformation [3]. Christchurch’s water supply network was damaged mainly due to the February 2011 earthquake. The eastern suburbs of Christchurch – such as Wainoni, Aranui, New Brighton, and Bexley – suffered the most due to the earthquake. 5. 1. Impact on the Operation of Pressure Zone In Christchurch, there are large water supply zones; for example, the central zone (zone 1) supplies water to around 60% of Christchurch customers. There are a number of small pressure zones in the hills that supply water to around 7% of Christchurch customers. The earthquakes of 2010–2011 highlighted a number of issues related to the operation of water supply zones and the vulnerability of the network due to the open nature of the supply zones. It was difficult to manage large water Figure 1. The pressure zones currently comprising Christchurch’s water supply (Seven major pressure zones) supply zones during the emergency response immediately after the earthquake given the multiple failures of assets (pumps, pipes and reservoirs). It took a long time to return relatively undamaged areas to full service by isolating the area from the large pressure zone (damaged significantly). Smaller zones were found to be more manageable and it was easier to restore full service during the emergency response time. In reality, large water pressure zones were converted to smaller zones for efficient management of water supply network during post-earthquake emergency. [7] 5. 2. Water Supply Pipelines It was estimated that approximately 240 km of water supply mains and 320 km of sub-mains needed to be replaced or repaired due to the earthquakes [6]. The water supply pipes were damaged due to cracks, breaks and joint failures. During post-earthquake recovery works, SCIRT (Stronger Christchurch Infrastructure Recovery Team) developed a number of damage criteria to define which pipes require repair and/or replacement. In some areas, the network was operating with reduced delivery pressure and capacity due to pipe damage. In Christchurch, pipe materials also varied considerably. Some of the predominant pipe materials are polyvinyl chloride (PVC), polyethylene (PE), steel (St), galvanised iron (GI), and asbestos cement (AC). As shown in Figure 2, asbestos cement and PE were the dominant pipe materials in Christchurch’s water supply network. The impact of the three major earthquakes on different pipe materials was assessed. As shown in Figure 3, the February earthquake had significant effects on pipe damage and AC pipe was found to be the most vulnerable pipe material during ground movements. Flexible pipe materials such as PVC and PE performed much better than other pipe materials such as steel, GI, and AC. Figure 2. Water pipe materials (in km) in Christchurch Figure 3. Water supply pipe affected by earthquakes 656 R. R. Biswas / IJE TRANSACTIONS B: Applications Vol. 32, No. 5, (May 2019) 654-660 The choice of appropriate pipe materials can make a significant difference on the performance of pipes during seismic activities. 5. 3. Pump Stations, Wells and Reservoirs There were 57 pump station sites in pre-earthquake Christchurch. Of these, two water pump station sites were damaged significantly and needed to be rebuilt. Damage to pump stations was related to the structural damage to the building and floor slabs due to liquefaction [6, 7]. There were a total of 175 wells in the city and, of these, 110 wells were damaged due to the earthquakes [6, 7]. Of the damaged wells, 25 had to be either rehabilitated or redrilled. The remaining 85 damaged wells required minor repairs to make them fully functional [6]. In preearthquake Christchurch, there were eight main storage reservoirs and 37 service reservoirs. Of these, 12 reservoirs needed repairs whereas two reservoirs needed to be rebuilt [6]. 5. 4. Customer Complaints Pre-earthquake 2009 (June 2009 to May 2010) customer complaints related to water supply network is compared with the 2011 (June 2011 to May 2012) post-earthquake customer complaints. All the customer complaints have been broadly categorized into four major categories: a) Low pressure complaints, b) Pipe-burst/water leakage complaints, c) Water quality complaints, d) Other water service related complaints. Figure 4 outlines the customer complaints comparison in graphical format. Low water pressure and pipe burst related complaints had increased by around 200% in 2011. Overall, there was a significant increase in customer complaints during the postearthquake 2011/2012 time period. It also needs to be noted that the post-quake customer complaint data may not be a true reflection of reality as a lot of customer complaints were not documented properly immediately after the earthquake. 6. WATER NETWORK MODEL AND POSTEARTHQUAKE RECOVERY IN CHRISTCHURCHb Water network model (also called as water network hydraulic model) is a very powerful tool to assess the Figure 4. Customer complaints related to water supply network 2009 pre-earthquake vs. 2011 post-earthquake performance of a water supply network [8-10]. Water network modeling is a modern technology that has been adopted by many water authorities in New Zealand to assess different what/if operational and future planning scenarios of a water network. There are a number of sophisticated software available in the market where the real water supply network can be replicated and visualized using different GIS and hydraulic modeling calculation engine. Asset data (pipe network, pump stations, nodes, water meter etc) are imported in the modeling files and then extensive calibration and validation works are carried out so that the water network model replicates the reality (water pressure, operation of the network etc). 6. 1. Water Network Model Development Method Hydraulic models were built for pre and post-earthquake water supply network of Christchurch. Water network asset data (pipes, pumps, reservoirs, wells, nodes etc.) were imported into hydraulic modelling simulation software Infoworks WS. Infoworks modelling software is one of the most popular hydraulic modelling softwares available in the market [9]. Once water network asset data were imported, asset data verfication and connectivity checks were carried out. The operating features of the water network is replicated in the model by changing control information of different water assets (valves, pumps, reservoirs etc.). Water supply network model development methodology has been outlined in Figure 5. The pre-earthquake and post-earthquake models were calibrated/validated with pre and post-earthquake flow data respectively. Data for pre and post-earthquake water network models are further explained in section 6.1.1 and 6.1.2. As part of this research, the performance of the preearthquake hydraulic model was compared with the December 2011, post-earthquake hydraulic model to understand earthquake damage 6. 1. 1. 2010 Pre-quake (Pre-Q) Model This model was developed to assess and understand the performance of Christchurch’s water supply network before the Figure 5. Hydraulic model building and calibration method R. R. Biswas / IJE TRANSACTIONS B: Applications Vol. 32, No. 5, (May 2019) 654-660 657 September 2010 earthquake. The model was developed by taking pre-earthquake information such as network survey files, GIS files, billing and demand data, pressure and flow-monitoring data and as-built files (as also shown in Table 1). 6. 1. 2. 2011 Post-quake (Post-Q) Model This model was developed to assess post-earthquake performance of the network and also to test emergency response activities in Christchurch. Please refer to Table 1. 6. 2. Performance notes 2010 Pre-Q Network Model vs. 2011 Post-Q Network Model Hydraulic modelling simulations were run to assess the performance of Christchurch’s water network immediately after the June 2011 earthquake. Both pre and post-earthquake water supply models were run for a peak summer day and the results are compared. 6. 2. 1. Minimum and Maximum Pressure In case of 2010 pre-Q model, minimum pressure at the property boundary is noted as around 27m (meter) for the combined water supply network (7 major pressure zones). In the hill suburbs minimum pressure for water supply drops to around 23m. Maximum pressure at the property boundary is found as around 80 meter. In case of 2011 post-Q model, minimum pressure at the property boundary is around 16m for the combined water supply network (7 key pressure zones). In the hill suburbs minimum pressure for water supply drops to around 14m in some areas. Maximum pressure at the property boundary is found as around 67m. 6. 2. 2. Fire-flow Availability Building footprints, sprinkler systems and hydrant locations were assessed as part of this investigation. New Zealand’s firefighting water supplies code of practice has been used to TA