Post Storm Assessment In the Colby Fire Impact Area
February 26th, 2014 - March 2nd, 2014
Initial forecasts by the National Weather Service early in the week of February 23, 2014, indicated the front passing through could produce rain intensity levels of an event likely to occur only once every 10 years (a so-called “10-year storm”). Based on forecasts and analysis by the Los Angeles County Flood Control District, US Forest Service, and the United States Geological Survey (USGS), the City of Glendora raised the alert status to Red on Thursday, February 27, 2014.
With the forecast, the Flood Control District issued a Phase 2 debris/mud flow warning. A Phase 2 flow would travel approximately ¼ to ½ mile on streets such as Yucca Ridge, Rainbow, Easley Canyon, Englewild and Loraine Avenue. Analysis also showed that a number of properties backing up into the steeper hillsides could experience significant flows during a 10-year storm event.
Based on the analysis, the City of Glendora implemented a wide variety of mitigation measures for this and future storm-related events of the next three to five years. In less than 84 hours, the City installed over 1.6 miles of concrete k-rails and 72 feet of pipe-and-timber structures, placed to assist in deflecting flows from affected properties in the Colby Fire Impact Area. Additionally, over 50,000 sandbags were distributed to residents and placed upon various public right-of-way locations within the Impact Area.
During the two-storm period, it appears that many of the storm cells missed the Colby Fire burn area by turning north when they reached the 605 Freeway area. Downtown LA and Monrovia received rain total amounts between 4.52 and 5.19 inches, with a recorded intensity of nearly 1 inch per hour. Totals in and around the Colby Fire area reached approximately 4.4 inches. The highest one-hour rain rate was approximately .4 inches. The threshold for severe debris/mud flows by the Weather Service and USGS was set at .5 inches an hour, so the initially forecasted intensity did not materialize in Glendora’s Impact Area. With its more isolated debris/mud flows, Glendora’s storm was assessed as a Phase 1 incident, rather than a more severe Phase 2 incident.
Because atmospheric conditions shifted many of the more intense storm cells, Glendora received rainfall more typical of an annual rainy season than of the 5- or 10-year storm levels experienced in much of Los Angeles County.
During the storms and afterward, City crews removed nearly 20,000 cubic yards of mud from Glendora streets. Five nearby debris basins operated by Los Angeles County Flood Control captured approximately 97,000 more cubic yards. A standard dump truck can carry approximately 10 - 20 cubic yards.
As a result of the storms, five properties reported structure damage (three garages and two accessory structures), and approximately 22 properties experienced moderate mud flows on their yards. One resident was extracted from her car, caught in a flow when she attempted to leave during one of the Saturday lulls between rain cells. There was no serious injury or loss of life.