Yearly Information
2003-2004 Wet Season 2002-2003 Wet Season 2001-2002 Wet Season 1996-1997 Wet Season
October 20, 2004
The graphs above show precipitation conditions at 3:00 p.m. PDT (2:00 p.m. PST) on October 20, 2003, relative to three precipitation thresholds for the occurrence of landslides that have been developed by the USGS for the Seattle area. The top graph shows conditions in relation to the three-day and fifteen day antecedent precipitation threshold. The middle graph shows conditions in relation to a precipitation-intensity and pre-storm antecedent precipitation threshold. The lower graph shows conditions in relation to a precipitation-intensity and duration threshold. The two precipitation intensity thresholds were developed by Jonathan Godt of the USGS on the basis of statistical analysis of landslide-triggering precipitation events and point out the significance of rainfall intensity, antecedent conditions, and total precipitation in causing landslides in the Seattle area. A six-hour running average is the basis for rainfall intensity used in Godt's thresholds; although somewhat arbitrary, the 6-hour average intensity gave the statistically best fit to the data on landslide occurrence. A report describing these thresholds is currently in review.
Heavy precipitation on October 20 resulted in conditions that exceeded all three of the precipitation thresholds. Hourly precipitation amounts between one-quarter and one-half inch during the morning of October 20 resulted in a maximum 6-hour average intensity of about one-third of an inch per hour at the time these graphs were plotted. The number of landslides that occurred in Seattle and its immediate vicinity is still undetermined. The storm of October 20 came at the end of a summer drought and the soil was still dry. Precipitation earlier in the month had not wetted the soil significantly below the ground surface.
October 15-24, 2004
Explanation
This graph shows the trajectory history of cumulative rainfall and rainfall intensity in the northeast Seattle area during the period October 15 to October 24, 2003 (color coded line) in relation to the 3-day/prior 15-day cumulative precipitation threshold for landslides (dashed red line). Rainfall intensity categories are shown in the table of color coded values on the right side of the graph. Arrows indicate beginnings of days in the period. The cumulative rainfall totals and rainfall intensities indicated by the trajectory history are based on hourly rainfall data from City of Seattle rain gauge no. 2 located in northeast Seattle.
Record short-term rainfall fell in parts of the Seattle area on October 20. In the vicinity of rain gauge no. 2 the 3-day/prior 15-day threshold was exceeded at approximately 11:00 a.m. on the 20th. That afternoon, high intensity rainfall occurred raising the 3-day cumulative to over 3.0 inches. The 3-day cumulative peaked the morning of October 21 at 4.64 inches. The 6-hour average rainfall intensity peaked at 0.33 in/hr at 2:00 p.m. on the afternoon of the 20th.
At least five small landslides reportedly occurred in the Seattle area. Two very small landslides (triangles beside color-coded line) occurred along the Burke-Gilman trail in northeast Seattle at approximately 2:00 PM on October 20. As shown conditions were above the threshold and the 6-hour average intensity was at or near its peak of 0.32 in/hr. A slope failure that occurred in south Seattle (Rainier Ave. S) reportedly occurred at approximately 1:00 PM on October 20. Data (not shown) from the nearest operating rain gauge (no. 17) indicate conditions were above the threshold. Also, the data indicates that the 6-hour average intensity at the time of the failure was between 0.25 and 0.30 in/hr.
December 10-21, 2002
This graph shows the trajectory history of cumulative rainfall and rainfall intensity in east-central Seattle during the period December 10 - 20, 2002 (color coded line) in relation to the 3-day/prior 15-day cumulative precipitation threshold for landslides (dashed red line). Rainfall intensity categories are shown in the table of color coded values on the right side of the graph. Arrows indicate beginnings of days in the period. The cumulative rainfall totals and rainfall intensities indicated by the trajectory history are based on hourly rainfall data from City of Seattle rain gauge no. 20 located in east-central Seattle.
As indicated by the trajectory, a series of frontal systems brought rainfall to the area during the period December 10 - 17. During the period, 6-hour mean rainfall intensity reached a high of 0.11 in/hr the evening of December 14 (orange line segment). By the afternoon of December 16, the 3-day cumulative rainfall total had risen to 2.51 inches, very nearly reaching the 3-day/15 day threshold (red dashed line). During the remainder of the period (December 17 to December 20), there was litte or no accumulation and conditions remained below the threshold as the 3-day cumulative precipitation dropped to zero and the 15-day total prior to the 3-day rose to nearly 4 inches. To date, no reports of landslide occurrence have been received for this storm period.
November 13-26, 2001
This graph shows the trajectory history of cumulative rainfall and rainfall intensity in northeast Seattle during the period November 13 - 26, 2001 (color coded line) in relation to landslide occurrences (triangles) and the 3-day/prior 15-day cumulative precipitation threshold for landslides (dashed red line). Rainfall intensity categories are shown in the table of color coded values on the right side of the graph. Arrows indicate beginnings of days in the period. The cumulative rainfall totals and rainfall intensities indicated by the trajectory history are based on hourly rainfall data from City of Seattle rain gauge no. 2 located in northeast Seattle.
As indicated by the trajectory, an initial storm front brought rainfall to the area during the period November 13 - 15. During that interval the 3-day cumulative rainfall total rose above the 3-day/15 day threshold (red dashed line) to 3.3 inches. The 6-hour mean rainfall intensity reached a high of 0.13 in/hr (light red line segment in color coded line) on the afternoon of November 14. Landslides (triangles) occurred in northeast Seattle on November 14 and 15. All but one of the slides occurred when the 3-day/prior 15 day cumulative rainfall totals were at or above the threshold. The initial storm ended November 15 and little or no rain fell between November 16 and 19. On November 19 the 3-day cumulative had dropped to zero and the prior 15-day cumulative total had risen to nearly 4 inches.
A series of storm fronts brought additional rain to the area during the period November 19 - 23. As shown on the graph, the 3-day cumulative total rose to 2.1 inches and the prior 15-day rose to over 4.5 inches. Six-hour mean rainfall intensity reached a maximum of 0.11 in/hr on November 22. Landslide activity occurred on November 21 and 23 and during the weekend of November 24 -25 when conditions were above the 3-day/prior 15-day precipitation threshold.
March 15-April 3, 1997
This graph shows the trajectory history of cumulative rainfall and rainfall intensity in the West Seattle area during the period March 15 to April 3, 1997 (color coded line) in relation to the 3-day/prior 15-day cumulative precipitation threshold for landslides (dashed red line). Rainfall intensity categories are shown in the table of color coded values on the right side of the graph. Arrows indicate beginnings of days in the period. The cumulative rainfall totals and rainfall intensities indicated by the trajectory history are based on hourly rainfall data from City of Seattle rain gauge no. 14 that, in 1997, was located in West Seattle.
As shown on the graph the 3-day/prior 15-day threshold was exceeded at the end of March 17. Subsequently, high intensity rainfall on March 18-19 resulted in a 3-day cumulative total of 4.51 inches the evening of March 19. The 6-hour average rainfall intensity peaked at 0.18 in/hr the evening of March 18.
It was reported that as many as 36 landslides occurred in the City of Seattle during the interval March 18-19. Additional slide activity reportedly occurred on March 20, 23 and on the 2nd of April. Triangles on the graph represent landslides with known times of occurrence.
