Debris Flow — Haines Borough, Alaska

2020-12-01 to 2020-12-03 · near Haines, Haines Borough, Alaska

Event narrative

As the atmospheric river moved over the far north central inner channels of Southeast Alaska it produced significant amount of precipitation from December 1st-2nd. Heavy to moderate rainfall persisted over 48 hours with the highest amounts coming during the afternoon hours of December 1 through late morning of December 2nd. By mid morning on December 1st, near 2 of precipitation already fell. The snow levels were low at the start of the event with a few inches at sea level in Haines but as much as 12 was reported at 800 feet by the Haines40NW COOP and 20 at the 2500 foot Flower Mt SNOTEL site by the morning of December 1st. Closer to Haines, the Ripinski ridge weather station at 2560 feet reported an estimated snow accumulation of near 59 from before the warm air moved into the area. The snow level rose rapidly through the evening of December 1st to be above the mountain tops by the early morning hours of December 2nd. This warming at elevation melted the fresh snow that fell with Ripinkis Ridge losing close to 30 and Flower Mt losing near 20 by the afternoon of December 2nd. By the end of December 1st the Haines airport reported a 24 hour precipitation amount of 5.49 which was a 25 to 50 year event and broke the all time daily precipitation record from 1946. The Haines COOP reported at 7am AKST on December 2nd a 24 hour amount of 6.62 which also broke the all time daily precipitation record from 2005. Haines40NW COOP along the Haines Hwy reported at 7am AKST on December 2nd a 24 hour precipitation amount of 5.23 a new all time daily precipitation record beating out 1999 and a 25 year event. From December 1st to the 2nd precipitation amounts from the downtown Haines COOP were 8.54', Haines Airport 10.26' a 200-500 year event, and the Haines40NW COOP 8.12' a 50-100 year event, were all time 48 hours precipitation records. From December 1st to 3rd 10 to 10.50 of precipitation fell from downtown Haines to the Haines border with a 100 to 200 year event. All of this heavy rainfall and snow melt produced significant impacts in the Haines area. In the early hours of December 2 after all-time record rainfall, saturated soils, and a deep snowpack, a second impulse of heavy precipitation moved over the area, and the snow level rose above 2500ft. The rising snow level began to melt the fresh snowfall at high elevation to produce significant amount of runoff that overwhelmed drainage ditches and culverts. The intense runoff began to erode the roads at the base of Ripinski Ridge and by day break there was significant infrastructure damage to all roads near Ripinski ridge which left Piedad Rd, Cathedral View Dr, Allen Rd, Comstock Rd, 4th ave,Young Rd to name a few that were impassable as the roads were washed away from erosion. There was also significant flooding away from the steep terrain from poor drainage not keeping up with the amount of runoff. Up to 4 feet of water flooded the Haines Hitch-Up RV park along with flooding across places at the Haines airport essentially closing it down to flights. To go along with the major road infrastructure damage there were a number significant major debris flows from the intense precipitation, super saturated soils and strong winds to help trigger the slides that blocked roads and damaged homes near steep terrain. Haines was cut off from the Canadian Border and the airport due to debris flows blocking the Haines Hwy. There were also multiple debris flows along Lutak Rd that damaged the Lutak fuel Dock, blocked the road in multiple locations and isolated residence in a subdivision at the end of the road. A few homes at the end of Lutak Rd we significantly damaged from debris flows coming down behind their homes with one getting pushed off its foundation, another with debris in the house and another with minor damage. Mud Bay Rd was blocked by a debris flow which isolated another part of town. A large avalanche came down into Chilkat Lake out the Haines Hwy that damaged a few homes. Residence in the affected areas and other locations near steep terrain were evacuated to emergency shelters throughout the morning of December 2. Heavy rain continued into the afternoon hours and around 130pm AKST a massive landslide occurred along Beach Rd that took out a 200 yard section of the road, destroyed 2 homes while causing major damage to another. One of the destroyed homes was not occupied but the other had to 2 people inside. The 2 people perished from the landslide or what the geologist are calling a geologic mass wasting event. There were about a half dozen homes on the south side of the slide that were evacuated due to the threat of another very large landside. It continued to be very wet with freezing levels going up and down to keep increase runoff not just from the precipitation but also from snow melt for the next week with a 7 day total of 14.85 which is a near 150 year event. The continue precipitation on the super saturated soils kept the threat of more debris flows high through December 7th. There was some minor movements of a home near an old landslide from 2012 called the slump area. This was one of the most impactful flooding and debris flow event since 2005 or ever and the data backups extreme impacts. The city of Haines declared a disaster for this event from the widespread and extensive infrastructure damage, 2 homes destroyed, 7 homes with major damage, 16 homes with minor damage with another 12 home adversely affected from the area damage and the 2 deaths. The governor of Alaska also declared a disaster emergency for Southeast Alaska from the impacts of this event.

Wider weather episode

A large plume of concentrated moisture or Atmospheric River (AR) with an associated low-pressure system moved across the Pacific Ocean and transported tropical moisture from the southwest which impacted Southeast Alaska (SEAK) from December 1st through December 2nd. This event brought widespread significant, and sometimes historic record setting rainfall to locations across Southeast Alaska. There were multiple waves along the main front to produce a quasi-stationary front and allowed heavy precipitation to persist over the panhandle for a long duration. Rainfall amounts were as high as over 15 at Little Port Walter with most other locations reporting 5 to 12 over the 2 days. Juneau, Skagway and Haines airports broke their all time daily precipitation record on December 1st along with the all time 2 day total when both days are combined. Rainfall intensity recurrence intervals ranged from a 50 to 200 year event when looking at 6,12,24,48 hour time periods. In some locations the precipitation started out as snow, minor accumulations but then changed quickly over to moderate/heavy rain. Significant snowfall did occur mainly at high elevations and along the Haines and Klondike highways before transition to rain as temperatures rose above freezing all the way up to 6000 feet with the impacting warm front. Snow accumulations were very impressive from December 1st into the 2nd with 18 at 800 feet near the Haines US customs border station and a staggering 59 at 2560 feet on Mount Ripinski above Haines. There was persistent precipitation that occured the previous weeks to keep the soils on the wet side. The heavy precipitation and warm temperatures melted snow that had already accumulated on the ground at sea level then as the snow level climbed above the mountain top this increased the amount of runoff significantly from the mountains. All the excess runoff resulted in significant and in places record flooding of roads and homes, erosion that damaged roads and infrastructure, and widespread mass wasting events such as landslides and debris flows. Most of these impacts were reported to have started in the early morning hours of December 2nd and persist into December 3rd or longer. The northern half of Southeast Alaska saw most if not all of these impacts with a few specific rivers, Salmon River in Gustavus and Jordan Creek in Juneau, seeing record river stage values. Widespread damaging debris flows impacted Juneau, Skagway and Haines that took out roads, destroyed homes and caused 2 fatalities in Haines. To go along with the heavy precipitation there was also very strong damaging winds as the waves moved along the front over the panhandle. These strong winds damaged power lines from falling trees.In total there were about 7 communities (Haines, Juneau, Skagway, Gustavus, Hoonah, Tenakee Springs, and Hyder) that declared a disaster from this event.

View location on OpenStreetMap → (59.1249, -135.4334)

Source: NOAA Storm Events Database, event_id 931869. Narrative written by the NWS forecast office that issued the report.