AVALANCHE/LANDSLIDE IMPACTS

STANDARD OF REVIEW

According to the CBJ Mining Ordinance (49.65.135(a)(4)), the CBJ shall require:

• "the operator conduct all mining operations...so as to minimize to the extent reasonably practicable safety hazards and to control and mitigate adverse impacts on the public and neighboring properties, such as from...avalanches, landslides..."

The CBJ Land Use Code, at CBJ 49.70.300(a), requires that:

• "(1) Development in all landslide and avalanche areas shall minimize the risk of loss of life or property due to landslides and avalanches."

Policy 3.11 of the CBJ Comprehensive Plan states:

• "It is the policy of the CBJ to minimize the threat to human safety and development posed by landslides and avalanches."

Also, the Juneau Coastal Management Program states:

• CBJ 49.70.910(b) "Development in areas having known hazards may not be approved until siting, design, and construction measures for minimizing property damage and protecting against loss of life have been provided."

  CBJ 49.70.910(e) "Mitigating measures are required for development in areas of moderate hazard. These may include dissipating structures or dams, appropriate structural engineering, or other techniques that respond to the specific site hazards."

The objective throughout should be to eliminate risks to life or property and/or the degradation of environmental quality caused by avalanches or landslides.

ANALYSIS

Avalanches are a commonplace phenomenon in the Sherman Creek drainage area, to judge by the extensive avalanche tracks in the area. Evidence of landsliding, or mass wasting, is less obvious. However, according to the Final EIS (1992; pages 3-7 and 3-8), most of this area is assigned a moderate to high mass wasting hazard owing to the very steep slope geometry that predominates. Other factors contributing to the risk of mass movement include poorly drained soil conditions, unfavorable bedrock dip orientations, vegetative cover, and high snow loads.

Avalanche

Comprehensive information about avalanches in the project area is contained in a report entitled Snow Avalanche Hazard Analysis and Mitigation Recommendations for the Kensington Mine (1994) by Fesler and Fredston, Alaska Mountain Safety Center, Inc. The 1994 report was written before plans for the applicant's proposed project were developed, and so it does not consider certain avalanche impacts peculiar to this new project (Alternative B in Final SEIS).

Most of the avalanche tracks in the Sherman Creek basin are well away from the proposed mine facilities or activities. There are some small tracks on the face of Horrible Hill directly above the "850" mine portal. Fesler and Fredston (1994) recommend employing a protective shed at this portal, which is included in the current mine plan.

The main risk of avalanche comes with the Ophir Path, which descends almost 5,000 vertical feet on the west flank of Lions Head Mountain, terminating at a point 500 feet beyond the proposed Ophir Creek diversion and well within the footprint of one of the proposed sand and gravel borrow pits (see attached figure). Active extraction of sand and gravel from this will coincide with construction of mine-related surface facilities, at the conclusion of which the pit will be reclaimed.

As described by Fesler in a 1987 report entitled Preliminary Snow Avalanche Hazard Evaluation of the Ophir Creek and Sherman Creek Mill and Portal Sites, the Ophir Path is a "major path covering an area of approximately 0.75 square miles...and there is the potential for avalanches of great size, particularly if one considers the entrainment potential within a 0.75 square mile area (this is the potential for a moving avalanche to entrain additional unstable snow as it descends its path)". Out of concern for the safety of mine workers, Fesler recommended leaving a 450-foot wide stand of timber between the toe of the path and a "safe zone" for the workers. Among Fesler's general recommendations was that a "Snow Safety Plan" be developed by a qualified avalanche specialist to help assure the safety of mine personnel working outside what he terms the "safe zone".

As presently designed, portions of the Ophir diversion and a large sand and gravel borrow source both appear to lie outside such a "safe zone". Given that the toe of the Ophir Path already extends well within the boundary of the borrow pit, sand and gravel extraction will have to be scheduled to avoid exposing workers to avalanche hazards.

The Ophir Path crosses the planned route of the Ophir Creek diversion channel at a point roughly 500 feet up slope from the toe of the path. The channel is thus subject to being plugged by snow and perhaps physically damaged by avalanche impact, impacts Fesler predicted in the 1994 report. A portion of the channel several hundred feet in length could be filled by a single event, and clearing the channel of so much snow would be a time consuming and dangerous undertaking. Until the channel is cleared, any water entering upstream of the blockage would fill and then overtop the channel embankment and flow into and collect in the borrow pit.

Avalanche risks might be mitigated to some extent by providing a large-capacity catchment basin and deflecting berms of the sort illustrated in attached Figure 2-1 of the Draft SEIS (page 2-7) at the confluence of Ophir Creek and the diversion channel. However, Ophir Path avalanches are potentially of such great volume that it would be impractical to construct a basin big enough to hold them. A more realistic approach would be to accept that avalanches will block the diversion channel and that avalanche conditions would remain too dangerous to attempt clearing the blockage. Anticipating this requirement, the project plans incorporate an armored overflow spillway in the crest of the embankment of the borrow pit access roadway. This spillway is positioned so it would direct any overflows that might come down to the original Ophir Creek channel safely past the leach field and other vulnerable facilities flanking that channel.

Landslides

Landslides can either prove destructive in their own right or as the trigger to major flood-borne debris torrents. The only portion of the project site steep enough to be at risk of direct impact from a major landslide is the mine portal area. However, the risk there is considered to be manageable provided basic precautions are taken, such as the preservation of the vegetative cover of the hillside. There is some concern that rock cuts along the mine access road be properly designed so as not to undermine otherwise stable slopes in that area. If they occur, slope failures along the road are more likely to be small, localized events.

Debris torrents are a more serious threat at the Kensington site. They are caused when a relatively minor landslide, occurring in the course of a heavy rainfall event, falls into a sediment filled stream channel. The landslide material mixes with the flood water creating a dense, viscous fluid that destabilizes and mobilizes the streambed sediments it encounters as the growing mass of debris moves downstream. The accumulated sediment of decades is thus transported en masse in a very brief interval of time. Debris torrents of as much as 100,000 tons per event have been estimated for both upper Sherman and Ophir Creeks once a century on the average. A single event of that magnitude would completely overwhelm and bury the Ophir Creek diversion structure, putting it out of operation for an extended period of time before sediment and debris excavation and diversion reconstruction could be effective.

Debris flows at least equal in size to the flows that hit the AWARE Shelter and Calhoun Avenue in September 1996 could be expected to come down the Horrible Hill drainages behind the mine process area surface facilities. Stormwater from the mine process area is supposed to be treated in the settling ponds before discharge to Sherman Creek. Facilities to be sited within the process area include bulk fuel storage and distribution, power generation, ore processing and processing reagent storage, warehousing, equipment maintenance and administrative offices. Although some project drawings suggest otherwise, a profile drawing of the process area shows that the ground will be sloped to drain back towards Horrible Hill. Stormwater from the process area will be carried to the settling ponds via a drain channel that will be constructed alongside the diversion ditch at the base of the mountain. In the event a debris flow from Horrible Hill blocks the diversion ditch, the slope of the process area will prevent the encroachment of mud or floodwaters into sensitive facilities within the process area. Instead, the sloping ground will tend to force flows back towards the drain channel and/or diversion ditch and away from the process area.

Given its large drainage area, Ophir Creek has the potential of delivering debris flows orders of magnitude greater in volume and destructive capacity than the September 1996 Juneau area flows. Without a large sediment trapping basin to intercept the sediment above the diversion channel, a debris flow will plug the channel and the debris and subsequent stormwater flow will continue un-checked on down the valley. Such flows will encounter the till borrow haul road and be deflected by the armored overflow spillway away from sensitive facilities such as the sewage treatment plant leach field, the mine water settling ponds and the topsoil stockpile.

Any attempt to control debris flows as they occur would be an exercise in futility, if not downright dangerous. They come without warning, move swiftly (often in surges), and their sediment load arrives at a much faster rate than it can be disposed of. The keys to controlling the adverse impacts of these flows are intercepting the material in catch basins before it can block normal drainage channels or providing auxiliary drainage channels to handle water flows once primary drainage facilities become blocked. Generally speaking, it is the latter approach that the mine operator is relying upon in the basic project design.

STAFF FINDINGS

• 1. Is the large mine permit application complete with respect to avalanche/landslides? (CBJ 49.15.330(e)(1)(B))

  Yes.

  2. Will the avalanche/landslide aspect of the project endanger public health or safety?

  (CBJ 49.15.330(f)(1))

  No. Scheduling of construction-related activity in the large sand and gravel borrow pit can eliminate the exposure of mine employees to avalanche hazards.

  3. Will the proposed mine project be in general conformity with the comprehensive plan, thoroughfare plan, or other officially adopted plans as they may apply to avalanche or landslide hazards? (CBJ 49.15.330(f)(3))

  The proposed mine project will comply with CBJ Comprehensive Plan Policy 3.11, which calls for minimizing threats to human safety and development posed by landslides and avalanches. Compliance is met by restricting worker access to areas during avalanche conditions and by the mine operator providing auxiliary drainage facilities to control flows in the event of blockage of diversions by landslide or avalanche.

  4. Will the proposed development minimize risk of loss of life or property due to landslides and avalanches? (CBJ 49.70.300(a))

  Yes, provided that the proposed auxiliary drainage facilities are constructed and maintained so that they control stormwater flows in the event that the proposed diversions become blocked.

  5. Will the proposed mine project be sited, designed, and constructed to, as well as include mitigating measures that, minimize property damage and protect against loss of life?

  (CBJ 49.70.910(b) and (e))

  Yes, provided that the operator constructs and maintains auxiliary drainage facilities to control stormwater flows in the event of blockage of the proposed diversions.

  6. Will the mining operation be conducted so as to control and mitigate adverse impacts on the public health and safety from avalanches and landslides? (CBJ 49.65.135(a)(4))

  Yes. The major risks to health and safety from landslide and avalanche are confined to the construction phase of the project.

STAFF RECOMMENDATION

Staff recommends that the following condition be included in the large mine permit:

• 1. The operator shall prohibit job site access of workers to the Sand and Gravel Borrow Pit when avalanche conditions are present.

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