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Using GIS to Assess Water Supply Damage from the Northridge Earthquake

by Thomas O'Rourke and Selcuk Toprak

The research reported in this article was supported by NCEER's Lifeline Project. It is also included in the proceedings from the NCEER-INCEDE Workshop on Earthquake Engineering Frontiers in Transportation Facilities, which will be available later this year as NCEER technical report 97-0005. The authors wish to express their deep gratitude to Mr. H. Dekermenjian of LADWP for his interest and assistance in obtaining pipeline and pipeline repair data. Thanks are also extended to Mr. N. Blaze of EQE, Inc., Mr. C. Davis of LADWP, and Mr. D. Wright of MWD for their assistance in obtaining valuable information. Comments and questions should be directed to Professor Tom O'Rourke at Cornell University, at (607) 255-6470; email: tdo1@cornell.edu.

Introduction

The 1994 Northridge earthquake led to significant disruption of the water supply system of Los Angeles. It caused damage at 15 locations in the three transmission systems providing water from Northern California, 74 locations in water trunk lines (nominal pipe diameter > 600 mm), and 1,013 locations in the Los Angeles Department of Water and Power (LADWP) distribution pipeline network. The damage was distributed over approximately 1,200 km². Damage of such a widespread nature invites questions about its spatial variability and its relationship with parameters such as earthquake intensity, peak acceleration, peak velocity, groundwater levels, and areas of permanent ground deformation (PGD). Large water supply systems are composed of pipelines constructed with different materials, diameters, and joint characteristics. Hence, there also are questions about how the damage patterns are influenced by the different material and mechanical characteristics.

Questions related to spatial variability are well suited for evaluation with geographical information systems (GIS). This article provides a description of how a GIS database was assembled for water supply damage caused by the Northridge earthquake. The earthquake-induced damage to water pipelines and the database developed to characterize this damage have been described by O'Rourke, et al. (1996). Graphical information is provided regarding the overall statistics and spatial patterns of pipeline damage. Various spatial relationships between earthquake damage and seismic intensity are explored. Local patterns of damage and repair are examined relative to groundwater levels and zones of liquefaction-induced PGD.

Northridge Earthquake Damage

Figure 1 shows the portion of the Los Angeles water supply system most seriously affected by the Northridge earthquake superimposed on the topography of Los Angeles. The water supply system includes transmission lines, trunk lines and distribution lines. All large diameter pipelines upstream of the treatment plants are considered to be transmission facilities.

Figure 1: Map of Los Angeles water supply system affected by the Northridge earthquake

The aqueduct systems that supply water from northern California are the Foothill Feeder, operated by MWD (Metropolitan Water District), and Los Angeles Aqueducts No. 1 and 2, operated by LADWP. A description of damage sustained by LADWP and MWD transmission lines has been provided by Lund (1995). In total, there were 11 repair locations in LADWP aqueducts No. 1 and 2, of which four involved either circumferential cracks or compressive buckling at welded slip joints. Excessive axial pullout occurred at two Dresser couplings.

Damage to the MWD transmission system occurred near the Jensen Filtration Plant at a welded slip joint of a 2,160 mm steel pipeline and as cracks and leakage in a reinforced concrete conduit. Damage was sustained by another steel pipeline at a sleeve-type coupling and in an area of differential settlement and horizontal movement adjacent to the Jensen Plant. Sixty-seven of the 74 trunk line repairs were located in the San Fernando Valley, with the highest concentrations of damage in the Van Norman Complex, near the intersection of Balboa Blvd. and Rinaldi St., and along Roscoe Blvd.

Figure 2 presents a map of distribution pipeline repair locations and repair rate contours for cast iron (CI) pipeline damage. The repair rate contours were developed by dividing the map into 2 km x 2 km areas, determining the number of CI pipeline repairs in each area, and dividing the repairs by the distance of CI main in that area. Contours were then drawn from the spatial distribution of repair rates, each of which was centered on its tributary area. The 2 km x 2 km grid was found to provide a good representation of damage patterns for the map scale of the figure. These contours are especially well suited for comparing damage with the spatial distribution of strong motion parameters.

The overall statistics of pipeline damage are summarized in figure 3 in the form of pie and bar charts. Note that the bar chart scale is logarithmic. Most repairs to trunk lines occurred in steel pipelines, with 80% of all repairs in riveted and continuous wall steel piping. Sixty-six percent of repairs were in continuous wall steel pipe, whereas only 56% of all trunk lines were composed of this type of pipe. The steel trunk lines were heavily damaged by compressive wrinkling of welded slip joints, with this type of damage recorded at 20 locations. There also were 10 reported locations of pullout at compression couplings.

Seventy-one percent of the distribution line repairs were in CI pipelines, which constitute 76% of the system. Twenty-two percent of the repairs were in steel pipelines, which constitute only 11% of the system. The relatively high concentration of steel pipeline repairs is associated with various types of steel, such as Mannesman and Matheson steel, which are prone to corrosion, as well as damage at certain types of elastomeric joints that are vulnerable to creep and leakage.

Earthquake Damage vs. Seismic Intensity

Figure 4 shows the repair rate contours for CI pipelines superimposed on zones of Modified Mercalli Intensity (MMI) mapped by USGS (Dewey, et al., 1995). The locations of highest repair rate coincide with areas of MMI IX and MMI VIII.

Figure 5 and figure 6 show the repair rate contours for CI pipelines superimposed on zones of peak acceleration and velocity measured by free-field strong motion instruments.

The free-field records used to plot peak acceleration and velocity zones are identical to those described by Chang, et al. (1996) in their evaluation of the engineering implications of the earthquake motion. The records from approximately 240 rock and soil stations were used. The maximum horizontal acceleration of 1.78 g measured at the Tarzana-Cedar Hill Nursery was removed from the database prior to GIS evaluation to avoid distortions from possible topographic influences. In addition, records from stations at dam abutments were screened when a station downstream of the dam was available, again to minimize distortion from topographic effects.

The zones of highest peak acceleration coincide reasonably well with the locations of highest repair rate, especially near the northern edge of the San Fernando Valley, the Santa Monica Mountains, and the Los Angeles Basin. The zones of highest peak velocity show similar spatial correlation with repair rate concentrations. The velocities do not coincide as well with repair concentrations in the Los Angeles Basin, but correlate better with CI pipeline repairs in the western and central portions of San Fernando Valley.

It is notable that high concentrations of repair rates occurred in the northern portion of the Santa Monica Mountains in the Sherman Oaks area. This location coincides with a zone of extensive slope movements, ground fissures, and cracking of artificial fill described by Barrows, et al. (1995). Ground failure of this type is likely affected by acceleration levels, which were very high in the Sherman Oaks area, as illustrated in figure 5. Zones of high acceleration, therefore, would be expected to correlate with locations of ground failure and thus PGD effects on pipelines. In contrast, zones of high velocity would be expected to correlate well with locations of high transient ground strain and be correlated less directly with PGD.

Local Patterns of Damage

Figure 7 shows CI pipeline repair rate contours in the western and central portions of San Fernando Valley superimposed on the outline of high groundwater level zones that were taken from Tinsley, et al. (1985) and Los Angeles County (1990). The highest repair rate concentrations occur in pre- and post-1944 zones with groundwater approximately 3 m deep. Holzer, et al. (1996) reports that locations within this area experienced PGD from both liquefaction and failure of soft clay sediments. It appears that the high groundwater table helps to delineate locations of liquefiable sands and soft clay, both of which are susceptible to large transient strains associated with site amplification. They are also susceptible to ground failure.

Conclusions

Geographical information systems (GIS) are well suited for evaluating the spatial relationships between water supply system damage and factors such as seismic intensity, peak acceleration, peak velocity, groundwater levels, and locations of ground failure. Spatial correlations between water pipeline damage and each of these parameters were examined for the 1994 Northridge earthquake. Good correlations between pipeline repair rates and both peak acceleration and peak velocity were found, although neither parameter was observed to provide consistently strong correlations at all locations of concentrated repair. It appears that zones of high peak acceleration coincide with locations of ground failure and thus PGD effects on pipelines. In contrast, zones of high velocity would be expected to correlate well with locations of high transient ground strains. Zones of high groundwater levels in the western and central portions of San Fernando Valley help to delineate locations of liquefiable sands and soft clay, both of which are susceptible to large transient strains associated with site amplification. They also are susceptible to ground failure. There is a strong spatial correlation between pipeline repair rates resulting from the Northridge earthquake and high groundwater levels in the San Fernando Valley.

References

Barrows, A.G., Irvine, P.J., and Tan, S.S. (1995), "Geologic Surface Effects Triggered by the Northridge Earthquake," The Northridge, California, Earthquake of 17 January 1994, Special Publication 116, M.C. Woods and W.R. Seiple, Eds., California Division of Mines and Geology, Sacramento, CA, pp. 65-88.

Chang, S.W., Bray, J.D., and Seed, R.B., (1996), "Engineering Implications of Ground Motions from the Northridge Earthquake," Bulletin of the Seismological Society of America, Vol. 86, No. 1B, Feb., pp. 5270-5288.

Dewey, J.W., Reagor, B.G., Dengler, L., and Moley, K. (1995), "Intensity Distribution and Isoseismal Maps for the Northridge, California, Earthquake of January 17, 1994," U.S. Geological Survey Open-File Report 95-92, U.S. Department of the Interior, Washington, DC.

Holzer, T.L., Bennett, M.J., Tinsley, J.C. III, Ponti, D.J., and Sharp, R.V. (1996), "Causes of Ground Failure in Alluvium During the Northridge, California Earthquake of January 17, 1994," Proceedings, Sixth Japan-U.S. Workshop on Earthquake Resistant Design of Lifeline Facilities and Countermeasures Against Soil Liquefaction, Technical Report NCEER-96-0012, National Center for Earthquake Engineering Research, Buffalo, NY, pp. 345-360.

Los Angeles County (1990), Technical Appendix to the Safety Element of the Los Angeles County General Plan, Hazard Reduction in Los Angeles County, Department of Regional Planning, Dec.

Lund, L. (1995), "Water Systems," in Northridge Earthquake Lifeline Performance and Post-Earthquake Response, A.J. Schiff, Ed., TCLEE Monograph No. 8, ASCE, August, pp. 96-131.

O'Rourke, T.D., Toprak, S., and Sano, Y. (1996), "Los Angeles Water Pipeline System Response to the 1994 Northridge Earthquake," Proceedings, Sixth Japan-U.S. Workshop on Earthquake Resistant Design of Lifeline Facilities and Countermeasures Against Soil Liquefaction, Technical Report NCEER-96-0012, National Center for Earthquake Engineering Research, Buffalo, NY, pp. 1-16.

Tinsley, J.C., Youd, T.L., Perkins, D.M., and Chen, T.F. (1985), "Evaluation of Liquefaction Potential," Evaluating Earthquake Hazards in the Los Angeles Region - An Earth-Science Perspective, U.S. Geological Survey Professional Paper 1360, J.I. Ziony, Ed., U.S. Department of the Interior, Washington, DC, pp. 263-316.

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