Antecedent Conditions and Damage Caused by 2015 Spring Flooding on the Sagavanirktok River, Alaska
Publication: Journal of Cold Regions Engineering
Volume 31, Issue 2
Abstract
Alaska’s economy is tied to oil production, with most of the petroleum coming from the Prudhoe Bay oil fields through the Trans-Alaska Pipeline System. Deadhorse, an industrial town located on the North Slope, provides support to the oil industry. The Dalton Highway is the only terrestrial connection between Deadhorse and other cities in Alaska. During winter 2015, the road was impassable twice for 12 days total due to winter overflow from the Sagavanirktok River. During spring breakup, unprecedented flooding occurred near Deadhorse, caused by ice accumulation near the road and fast snowmelt due to warm air temperatures, which resulted in the road’s closure for 18 days, the first time since the highway was built in 1976. Presented in this article are antecedent hydrometeorological conditions in the Sagavanirktok and neighboring watersheds, a summary of infrastructure protection measures implemented before breakup, hydrologic data collected during breakup, and damage caused by the spring flood to the infrastructure.
Introduction
Early in 2015, winter overflows and subsequent aufeis development from the Sagavanirktok River (Figs. 1 and 2) in Alaska were detected in several places along the Dalton Highway in the 35-km stretch of road south of the town of Deadhorse, Alaska. The overflow eventually overtopped the road, causing road closure for a cumulative period of 12 days (March 30–April 2, and April 5–12). In response to these winter events, the Alaska Department of Transportation and Public Facilities (AKDOT&PF), which is in charge of maintaining the road, built a series of snow berms and channels in the ice to protect the road. However, all temporary berms collapsed during the early breakup stages. Because of this failure, the road was breached in many sections along the Dalton Highway. Road damage produced by the flowing waters was extensive; the highway was closed to traffic for nearly 3 weeks (May 18–June 4). A gas line, which supplies energy to the pump stations, and a crude oil pipeline, both part of the Trans-Alaska Pipeline System (TAPS), are located near the road in this area and experienced damage. However, this damage did not threaten the physical integrity of either pipeline.
This article presents the meteorological and hydrological conditions prior to spring breakup in 2015, and a summary of work done by AKDOT&PF and the Alyeska Pipeline Service Company (APSC) to protect the road and the TAPS, respectively. Additionally, hydrological measurements carried out during breakup are presented and discussed.
Geographic Setting
The Sagavanirktok River originates in the Brooks Range of Alaska and flows north, parallel to the Dalton Highway and the oil pipeline for approximately 160 km, reaching the Beaufort Sea near Deadhorse (Fig. 1). The highway is mostly an unpaved, two-lane, gravel road, built in 1976 to support the construction of the TAPS. North of the Brooks Range, the Dalton Highway is generally elevated 1.5–2 m above the original ground level. The pipeline, which is owned by the major oil producers in Alaska, is 1.22 m in diameter and 1,300 km in total length (from Deadhorse to Valdez). In this article, the customary United State milepost system is used to identify specific locations along the highway and/or pipeline. Specifically, the pipeline starts at Prudhoe Bay on the Arctic Coast at pipeline milepost (PLMP) 0 and ends at the port of Valdez at PLMP 800. The pipelines were built between 1975 and 1977 (APSC 2016) and have been operated by APSC since 1977. The Dalton Highway milepost (MP) count begins at the Elliott Highway near Fairbanks and ends at Deadhorse, MP 414.
The Dalton Highway and the pipeline from PLMP 0 to PLMP 63 are on the west side of the Sagavanirktok River floodplain. At PLMP 18.8, the oil pipeline crosses the Dalton Highway from west to east and enters the active floodplain of the Sagavanirktok River (Fig. 2). The oil pipeline is located on the east side of the Dalton Highway next to the Sagavanirktok River an additional 71 km south of PLMP 18.8. As the buried pipeline enters the Sagavanirktok River floodplain, it descends a low terrace into an active zone that is frequently overtopped by the Sagavanirktok River between PLMP 18.8 and PLMP 23. At this location, a series of eight spur dikes from 182 to 457 m long protect the oil pipeline from the west branch of the Sagavanirktok River by keeping the main channel away from the pipeline.
The Sagavanirktok basin encompasses three major areas: coastal plain, foothills, and mountain regions (Kane et al. 2014). The watershed is characterized by low gradients in the coastal plain and high gradients in the headwaters area. The basin area is about , with the majority of it in the Brooks Range (), and only 20% of it on the coastal plain (Toniolo et al. 2015). The mountain region has two subbasins: Upper Sagavanirktok and Ivishak located in the west and east sides of the watershed (Fig. 1). The subwatershed areas are approximately (Upper Sagavanirktok) and (Ivishak).
The entire North Slope of Alaska is characterized by a scarcity of meteorological and discharge data. In fact, only a few meteorological and/or gauging stations are available in the area. For instance, discharge is measured by the USGS on the Sagavanirktok River, near Pump Station 3 (Station 15908000, period of record 1982–present), where the stream presents a single channel. This gauging station captures runoff from approximately (USGS 2016).
Approximately 8 km downstream of the gauging station, the stream begins a braided pattern that continues to its mouth. The USGS gauging station is located upstream from the confluence with the ungauged Ivishak River. No recent continuous discharge record is available downstream of Pump Station 3. The USGS also gauges the Kuparuk River near Deadhorse (Station 15896000, period of record 1971–present).
Air temperature at the west side of the watershed (near the Dalton Highway) is collected at stations maintained by the National Resource Conservation Service (NRCS), University of Alaska Fairbanks (UAF), and USGS. The period of record from these stations is variable.
Meteorological and Hydrological Conditions Prior to Breakup
The analysis was conducted considering several timescales and variables: (1) annual (cumulative runoff); (2) monthly (winter and spring breakup discharge); and (3) hourly (air temperature). Details on the conditions of these variables are given in the following subsections.
Cumulative Runoff
The total volumetric runoff during the open water season for the Kuparuk River basin at Deadhorse was calculated from the period of record, which ranges from 1971 to present (USGS Station 15896000). The Kuparuk basin is an adjacent watershed west of the Sagavanirktok River (Fig. 1).
The Kuparuk River originates in the foothills of the Brooks Range and flows north through the coastal plain to the Arctic Ocean. It is a medium-gradient basin with an area of approximately . Nearly 62% of the basin area is within the foothills region, and 38% is within the coastal plain (Kane et al. 2014). The basin elevation ranges from 1,464 m at the headwaters (Kane et al. 2004) to sea level at the outlet, with an average basin elevation of 245 m (McNamara et al. 1998). The stream length is 330 km (D. L. Kane, personal communication, 2016) and the basin length is approximately 250 km. Selected years representing the lowest and highest cumulative runoff are shown in Fig. 3. Data for 2014, when approximately 276 mm of water left the watershed, represents the highest volumetric runoff for the entire period of record, which has an average value of 150 mm. This high cumulative runoff for 2014 was attributed to a deep end-of-winter snowpack and a wet summer (Toniolo et al. 2015). Cumulative runoff during 2013 was the second highest year on record with approximately 250 mm of water.
Due to high uncertainties in the data reported by USGS Station 15908000 during breakup, a similar analysis to the one carried out for the Kuparuk River was not performed. These uncertainties arise from the reduced number and specific time of measurements. A different approach was taken herein when reviewing the runoff for USGS Station 15908000. The summer monthly mean flows were examined, and it was found that 2014 was the third highest flow on record () for mean monthly discharge in the months of July and August. A similar analysis was completed for other nearby rivers such as the Colville () and the Kuparuk () (Table 1). Thus, one could conclude that there was a water surplus (i.e., water storage system was full) in the region’s watersheds at the beginning of winter 2014–2015.
USGS | Record (years) | Rank | |||||
---|---|---|---|---|---|---|---|
River | Station number | 1 | 2 | 3 | 4 | 5 | |
Kuparuk at Deadhorse | 15896000 | 45 | 2002 | 2014 | 2003 | 1989 | 1984 |
Sagavanirktok | 15908000 | 33 | 2003 | 2010 | 2014 | 1994 | 1999 |
Colville | 15875000 | 13 | 2003 | 2006 | 2014 | 2004 | 2012 |
Winter and Spring Breakup Discharge
Table 2 shows the monthly mean discharge for the USGS gauging station. Reported 2014–2015 winter monthly values (i.e., October–May) were above the mean of monthly discharge for the entire record, with increasing percentages as the winter progressed (last rows in Table 2). In addition, Table 2 indicates that flows during winter 2013–2014 were also high. Discharge corresponding to June 2015 was below the mean discharge for the entire record, indicating that breakup consisted of one major, single pulse. In other words, snowmelt happened rapidly in the entire watershed. This argument can be supported by the air-temperature plots shown in Fig. 4. The figure indicates that air temperatures throughout the watershed, from low to high elevation, were above freezing most of the time beginning on May 8, 2015, and continuously above 0°C from May 15 to May 30, 2015, in the foothills and mountain areas. The available energy melted the majority of the snowpack in a short time frame, producing a swift breakup.
Year | January | February | March | April | May | June | July | August | September | October | November | December |
---|---|---|---|---|---|---|---|---|---|---|---|---|
Discharge () | ||||||||||||
2013 | — | — | — | — | — | — | — | — | — | 20.1 | 10.1 | 8.5 |
2014 | 6.7 | 5.2 | 4.5 | 4.2 | 53.9 | 254.5 | 203.9 | 131.5 | 73.4 | 28.7 | 14.0 | 8.7 |
2015 | 7.3 | 6.2 | 6.1 | 6.2 | 174.5 | 129.9 | 85.7 | 106.9 | 60.6 | — | — | — |
Historical mean monthly discharge (MMD)a | 2.0 | 1.4 | 1.2 | 1.2 | 42.8 | 166.5 | 138.2 | 111.9 | 51.5 | 17.7 | 7.3 | 3.4 |
Percentage of flow with respect to MMD (%) | ||||||||||||
2014 | — | — | — | — | — | — | — | — | — | 162 | 192 | 256 |
2015 | 364 | 444 | 505 | 519 | 408 | 78 | — | — | — | — | — | — |
a
Period of record: September 1, 198–September 30, 2015.
Air Temperature
The plot in Fig. 5 represents the historical monthly mean (period of record: 1987 to 2015) and the 2014–2015 winter hourly air temperatures at the UAF Franklin Bluffs meteorological station. The region experienced a warm spell during mid-February to early March 2015. While the prediction of aufeis (in terms of location and temporal/spatial development) is still obscure to the scientific community (Kane et al. 2013; Yoshikawa et al. 2007), Kane (1981) associated warm temperatures with aufeis formation and growth. The unseasonal temperatures recorded in 2015 coincided with the initial overflow of the Sagavanirktok along the Dalton Highway. Hence, it could be argued that the warm temperatures exacerbated aufeis formation, which eventually reached the Dalton Highway, causing the initial winter flooding of the highway (Fig. 6).
In summary, the available hydrological data clearly indicate that watershed storage was full heading into 2014–2015 winter. When winter overflow began on the Sagavanirktok, plenty of water was available in the system to develop a massive aufeis field from the river to the Dalton Highway. The areal extension of aufeis in both the vertical and horizontal directions was significant. Near the Dalton Highway at MP 395, the river channel and floodplain are constrained to the east by Franklin Bluffs, which caused the overflow to spread more toward the highway and the oil pipeline to the west. In many cases, the ice elevations were above the road’s centerline elevation (Fig. 7).
Damage to Infrastructure
A summary of major countermeasures to protect key infrastructure, such as the TAPS and the Dalton Highway, and a summary of damage to the pipelines and road that occurred during the 2015 spring breakup are presented in this section.
Widespread flooding induced by unprecedented aufeis and winter overflow from the Sagavanirktok River occurred along the Dalton Highway during spring 2015. The magnitude of the event was the largest recorded in the area and resulted in state declarations of disaster, road closures, and a 2.5-month emergency response by AKDOT&PF, with costs to the state of $15.5 million.
Trans-Alaska Pipeline System
Historically, with almost 40 years of monitoring, the reach of the Sagavanirktok River that flooded has not been an area prone to significant aufeis accumulation. On February 19, 2015, during a pipeline surveillance flight, aufeis was detected at PLMP 23. From the initial discovery, the aufeis accumulated rapidly. Follow-up ground surveillance on February 27 revealed some spur dikes with only 15 cm of freeboard. The top of dikes are typically 1.8–3.7 m above ground level. By March 14, the aufeis had completely overtopped and encased the first two upstream dikes with 1 m of ice.
The principal methods used to contain and deflect additional aufeis accumulation involve building trenches through ice and using the excavated ice to construct berms. This approach has been effectively implemented for the TAPS in the past at other floodplains. The ice trenches, which are directed down-valley, intercept overflows, providing a drainage path for water.
As the aufeis infills the floodplain, there is a risk that spring melt will be forced outside the active floodplain, potentially altering channel flow patterns. This scenario may ultimately lead to channel avulsions, which can pose an integrity risk to infrastructure. The ice berms, therefore, serve an additional function of containing and guiding the spring flow to the existing active floodplain, reducing the potential for channel migration.
High flows began on May 17, 2015, following a period of record-high ambient temperatures, as previously described. Historically, at the Sagavanirktok River downstream of the confluence with the Ivishak River, breakup occurs in several cycles of high flows followed by a lull where flows subside. The duration of breakup can last 4–6 weeks, with three or more cresting and falling flood waves in between. Floodwaters on May 18 rose rapidly, and the resulting backwater extended about 8 km beyond the farthest upstream limit of the aufeis field. The ice berms and trenches built to contain the flow were outflanked near PLMP 23.6, and the Dalton Highway overtopped (Fig. 8). Floodwaters reached tundra on the west side of the highway and flowed 40 km downstream towards the oil fields of Prudhoe Bay (Fig. 9).
Along the TAPS, which is located west of the Dalton Highway from MP 395 to MP 413, the floodwaters that overtopped stretches of the roadway exposed a 25.4-cm-diameter gas line at five locations and the crude oil pipeline at three locations. However, the pipes incurred no mechanical damage, so the integrity of the TAPS was not an issue. Supporting infrastructure was damaged: maintenance roads, components of the pipe-corrosion protection system, drainage structures, and ground insulation used to protect the permafrost. Costs for repairing damage associated with TAPS were significant.
On the active floodplain of the Sagavanirktok River, where the oil pipeline crosses the highway (PLMP 18.8) and is buried adjacent to the river for several miles, damage to the pipeline was relatively minor. None of the pipe was exposed by erosion. The ice berms and trenches shielded the area from main channel flows.
Dalton Highway MP 395 to MP 413
Prior to spring breakup—on March 13, 2015—water began flowing over the Dalton Highway from aufeis accumulations in approximately eight locations. Water depths reached 0.3 m on top of the highway, and covered stretches 60 m long. Crews of the AKDOT&PF began working to construct snow berms and dig trenches in the ice to divert water from the road. During late March and early April, water continued to flow over the top of the river ice and breached snow berms, forcing traffic to use only one lane and causing intermittent road closures. Crews worked around the clock to keep the Dalton Highway open to traffic. Road conditions deteriorated with the onset of warmer temperatures in early May. Impacted areas along the Dalton Highway continued to expand, and water depth and ice thickness continued to increase. Breakup flows began in mid-May, and by May 18, AKDOT&PF was forced to close the Dalton Highway indefinitely because water was crossing the road (water was up to 0.6 m deep on top of the road in several sections), causing severe road damage and washouts (Figs. 8–10).
Additional workforce resources were engaged by AKDOT&PF, joining the department’s personnel to protect the highway. Up to 30 pieces of equipment including graders, bulldozers, excavators, and side dumps were onsite constructing snow berms, digging trenches, and installing temporary culverts to help facilitate floodwater movement back to the river’s main channel. Water levels in the flood zone continued to rise through spring breakup. On May 22, 2015, Alaska’s Governor declared the Dalton Highway a state disaster. Extreme flooding occurred from approximately MP 395 to MP 413. Floodwater crossed the Dalton Highway to the west and encroached on an airport about 1 km outside of Deadhorse. Flooding had minimal impact on airport operations. Work crews with AKDOT&PF cut breaches across sections of the highway as an outlet to carry water back to the east towards the main channel of the Sagavanirktok (Fig. 10). The Dalton Highway reopened on June 5, 2015, after an 18-day closure. Motorists drove in typical construction conditions, with the road rough and narrow. The extent of the road damage was not entirely known until water levels dropped. Table 3 shows the location and main characteristics of each breach along the Dalton Highway. The total volume of borrow material needed to bring the road back to preflood conditions surpassed .
MP | Length of road damage (m) | Volume of borrow material () |
---|---|---|
412.7 | 259 | 3,823 |
412.0 | 91 | 1,185 |
402.5 | 15 | 344 |
401.5 | 24 | 344 |
400.9 | 18 | 497 |
400.1 | 49 | 459 |
398.7 | 91 | 994 |
398.2 | 12 | 153 |
397.5 | 180 | 2,294 |
396.8 | 174 | 2,294 |
394.5 | 610 | 5,734 |
Fieldwork during Breakup
During a 3-week period (May 15 to June 5, 2015), a hydrology team from UAF monitored the river conditions. Tasks performed by UAF included discharge measurements near MP 395 (Fig. 2), sediment sampling, and the installation of an observation station on the east bank of the river to track water-level changes in the main channel.
Based on the river conditions, discharge measurements were carried out using two types of acoustic Doppler current profiler (ADCP): a Teledyne (Poway, California) RD Instruments (RDI) Rio Grande 1,200 kHz and a Teledyne RDI StreamPro 2,000 kHz. The ADCPs were mounted on the side of an aluminum boat that was equipped with a jet motor, ideal for low water depths such as the Sagavanirktok River in the measurements reach. A base and rover differential global positioning system (GPS) set was also used during the measurements. While discharge measurement standards are available (Mueller et al. 2013), the river conditions during breakup are generally rough, characterized by anchor ice, floating ice, river braiding, and unsteady flow. Fig. 11 shows the temporal variation of water levels at the east bank station. Additionally, measured and estimated discharge values are depicted in the graph. The graph indicates that the water level changed 3 m during breakup. The maximum measured discharge was , roughly coinciding with peak water level, which occurred on May 20, 2015.
Considering a flood frequency study carried out by PND Engineers in 2003 (PND 2005) and assuming an equal discharge distribution between the east and west channels, which fairly coincides with discharge measurements done by UAF during breakup and is in the order of magnitude of previously published percentages—45% west channel and 55% east channel (Veldman and Ferrell 2002)—the corresponding return period for the 2015 breakup flood is approximately 5 years. However, water levels reached unprecedented elevations in the area, as a result of significant ice accumulation in the floodplain. Thus, based on the current record (that the road was built nearly 40 years ago) and assuming an empirical probability distribution, a single occurrence in the record of extensive road closure due to overtopping indicates a return period in terms of water levels of approximately 40 years.
Grab water samples were collected when the UAF field crew was on site. In addition, a series of water samples were collected by a Teledyne ISCO (Lincoln, Nebraska) model 4,700 automated sampler from May 19 to June 1. The time interval varied from 6 h (May 19–25) to 12 h (May 25–June 1). Suspended sediment concentration (SSC) was calculated at UAF’s lab following ASTM standards D3977-B (ASTM 2013) and D2974-C (ASTM 2014). Grain-size distributions were determined for selected samples. Figs. 12 and 13 illustrate the temporal variation of SSC and the relationship of SSC and discharge, respectively. Fig. 13 shows a counterclockwise progression in the relationship between SSC and discharge, a rare condition for rivers during breakup according to Tananaev (2015). The average grain size from the available sediment distributions corresponds to silt-sized particles. Specifically, these values ranged from 10 to 14 μm, indicating a small variation in terms of particles in suspension.
Conclusions
Alaska’s economy is highly dependent on oil production, with most of the petroleum originating at the Prudhoe Bay oil fields and moving through the TAPS. Deadhorse, the single oil town located on the North Slope, can only be accessed by ground via the Dalton Highway. The highway is heavily used to move supplies to and from the oil fields.
In late March to early April 2015, winter overflow from the Sagavanirktok River crossed the Dalton Highway near Deadhorse at several points. Consequently, the road was closed for 12 days. Work crews with AKDOT&PF excavated channels along the highway and towards the Sagavanirktok River west channel to stop the flow of water/ice to the road. During spring breakup in mid-May, the Dalton Highway was flooded by the Sagavanirktok River in several locations from approximately MP 395 to MP 413 (Deadhorse). The magnitude of this event, the first of its type in recorded history (i.e., since the road was built), was such that the road was closed for 18 days. Total costs to the state to repair the highway were about $15.5 million.
The extent of aufeis and resulting spring flooding was a rare and potentially catastrophic event. A segment of the TAPS that crosses the Sagavanirktok River floodplain and is considered most vulnerable because of its proximity to the river’s main channel had only minor damage, and none of the pipe was exposed by erosion. The absence of any major damage to the oil pipeline in the active floodplain is due to proactive measures taken by the APSC and AKDOT&PF, which had installed an extensive network of ice berms and trenches. These protective structures and the thick ice layers over the floodplain shielded the pipe from erosive forces. No significant channel changes were detected after the flood. While the flood did not threaten the oil pipeline’s integrity, costs for repairing damage associated with the TAPS were similar to the state costs.
An analysis of summer cumulative runoff for other watersheds in the region indicates that in 2014, extremely high flows were recorded. Additionally, an unseasonal period of warm air temperatures was recorded during mid-February to early March. In May, an extended and continuous period of above-freezing temperatures was recorded in stations located throughout the watershed. While the specific conditions responsible for this unprecedented flood would be difficult to point out, it is the authors’ opinion that the previously mentioned high runoff, the thick accumulation of ice within the floodplain, and widespread warm air temperatures, which melted all the snow at once, certainly contributed to the flood event and its damage to the Dalton Highway.
Data collected during breakup show that water levels in the river changed approximately 3 m at an observation station located on the east bank (MP 395). Discharge measurements ranged from nearly 400 to , with the maximum measurement roughly coinciding with the peak water level. Representative sediment sizes () ranged from 10 to 14 μm. Suspended sediment concentrations ranged from a few (clear water in early flooding stages) to approximately .
Acknowledgments
The access and use of data collected by different institutions such as the National Resource Conservation Service, United States Geological Survey, and University of Alaska Fairbanks (under several research projects led by Douglas Kane, Sveta Stuefer, and Christopher Arp) is acknowledged by the authors. Jeff Stutzke and Alex Lai recognize the support provided by AKDOT&PF and APSC. The UAF authors were partially supported by AKDOT&PF. The authors appreciate the reviewers’ suggestions, which improved the manuscript.
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Received: Feb 12, 2016
Accepted: Nov 14, 2016
Published online: Jan 30, 2017
Published in print: Jun 1, 2017
Discussion open until: Jun 30, 2017
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