The Mink Creek Earthflow March, 1995
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Forest Sciences Prince Rupert Forest Region Extension Note # 04 The Mink Creek Earthflow March, 1995 Research Issue Groups: Forest Biology Forest Growth Soils Wildlife Habitat Silviculture Timber Harvesting Ecosystem Inventory and Classification Biodiversity Figure 1. The Mink Creek earthflow retrogressed about 600 meters. Ecosystem Management Background Hydrology (sediments of glacial origin deposited We generally associate slope failures in the sea) flowed and slid rapidly Geomorphology with steep, mountainous areas. Flat into Mink Creek, near Terrace, BC. to rolling, subdued terrain is thought Spoil from the earthflow crater filled the incised channel of Mink Creek, Extension of as having a relatively low risk of landslide occurrence. This is not extending downstream approximately always the case. Sometime, between one kilometer. The debris dammed Forest Engineering mid-December, 1993 and early the creek, raised the water level by 10 January, 1994, approximately 23 meters and backed water upstream hectares of glaciomarine sediments 1200 meters. Forest Sciences, Prince Rupert Forest Region Bag 5000, Smithers, BC V0J 2N0 847-7500 (FAX 847-7217) ocean water filled the valley, creating a fjord. Glacial retreat up the Kitsumkalum-Kitimat Trough was relatively rapid, although there were several periods when the ice front was stable. During one such period, the snout of the stable, calving glacier was situated near the present location of Thornhill. Sediment- laden meltwaters poured from the ice front. Gravels and sands were deposited near the snout of the glacier forming a large sandur, or outwash plain (Terrace Airport). Smaller clay and silt particles remained suspended in the sea much longer and were deposited further from the ice front, forming silty-clay deposits such as those found at Mink Creek. Figure 2. Exposed failure plane in centre of flowbowl. Spoil lineation in the centre of photo parallels the direction of flow. How Marine Sediments Become Sensitive Cause The headscarp (upper edge of the failure) rapidly retrogressed away Isostatic rebound (uplift of the A helicopter pilot, who followed from the initiation point, filling and earth's surface in response to a dirty water in the Lakelse River to its damming Mink Creek with landslide decreased load) following source in Mink Creek, first observed debris. deglaciation elevated the fjord floor the earthflow site January 9, 1994. approximately 200 meters near What specifically triggered the While the conditions that led to the present day Mink Creek. Elevated earthflow at Mink Creek is not failure took thousands of years to sediments that had settled in a saline, known. No earthquakes occurred or develop, the entire event may have marine environment, underwent were felt in the vicinity of Mink occurred in a few minutes. For leaching by freshwater which Creek during the months preceding example, the time elapsed from the gradually lowered the material's the failure. Factors contributing to initial failure to the final expulsion salinity. the failure were probably: 1) of debris of the 1962 Lakelse Lake sensitive marine parent material; 2) earthflows was estimated between Clay particles are plate-like, and in high stream flows (observed one and ten minutes. most freshwater situations, settle November, 1993); and 3) high pore face down as individual particles. In water pressures due to a wet, mild Deglaciation a saline solution, however, clays fall and early winter. form aggregates where individual The Mink Creek earthflow occurred particles are randomly aligned. This The failure probably started at the in rolling to flat, gullied aggregated structure gives the site of a small bank failure in the glaciomarine terrain in sediments material a higher than normal ratio down-cutting Mink Creek when a that were deposited approximately of pore space to solids. This high zone of sensitive clay suddenly 10,000 years ago during deglaciation void ratio enables the soil to have an liquefied, causing a complex (glacial retreat). The weight of the abnormally high water content. sequence of flowing and sliding glacier depressed the ground below along gently dipping failure planes. sea level, such that when it retreated 2 There are two distinctive areas in the main flowbowl. The eastern section of the failure contains a relatively small amount of landslide debris. Extensive areas in its center reveal an exposed failure plane (Figure 2). Where the failure plane is exposed, bedding dips down-flow at about 3°. Debris sliding along a dipping slip- surface travels more rapidly than debris on a horizontal slip-surface. Lineations of spoil on a clean surface, trending parallel to the flow direction, are possibly indicators of rapid, high energy debris transport. These features may be the result of flakesliding (where a large mass of material slides simultaneously rather than retrogressively). Ridges and Figure 3. Ridge with horizontal bedding slid towards trees along a slightly intervening wedges, indicating dipping failure plane. retrogressive failure, are more common in the western part of the Interparticle bonds in marine clays much strength that it changes into a failure (Figure 3). Generally ridges are relatively strong at high slurry, enabling the earthflow at trend perpendicular to flow direction. salinities. These sediments become Mink Creek to occur. "sensitive" when the salinity falls The flowbowl contains numerous below a certain threshold. At low Material Description ridges, and remarkable pyramidal salinities, interparticle bonds structures (Figure 4) - portions of weaken, leaving clay aggregates A stiff, fissured, weathered crust, 3 - ridges - that have slid along a failure prone to collapse. 4 m thick, overlies sensitive gray plane. Bedding in these structures is silty-clay. Bands of silty-clay are horizontal, or nearly so, indicating Sensitivity is simply a ratio of the separated by thin (generally less than there was no rotational failure. strength before and after structural 1 mm thick) silt seams and an Between ridges, intervening wedges collapse of the clay aggregates. The occasional sand seam. The of debris collapsed under their own greater the difference between the sensitivity of the material beneath the weight. In some cases wedge undisturbed and the remoulded crust decreases with depth. The material rotated off ridge flanks. strength of a material, the greater its decrease in sensitivity probably sensitivity will be. coincides with increasing salinity. There are multiple slip-surfaces Marine shells are common in the within the earthflow. This is The sensitive sediments of Mink material. significant because it demonstrates Creek are relatively strong when there are many zones of weakness undisturbed, but become nearly Description of Flowbowl and potential slip surfaces in the liquid when disturbed, as in the material. Failures possibly occurred failure process. The crater left by the earthflow is along silty, or coarser textured shaped like a cloverleaf and is seams. At one location an exposed Marine sediments become sensitive bounded by gullies to the east, and to slip-surface was littered with small under leached conditions due to the west. Some of these gullies are pebbles. decreasing salinity. It is the low scars of older earthflows. salinity that allows the soil to lose so 3 Research To Date Morphological features in the earthflow have been extensively examined to determine mechanisms of failure. In addition, detailed pre- and post-failure topographical maps have been prepared for the site. Shear strengths were measured at two locations in the crater, and also above the headscarp. Drill hole samples were collected above the headscarp. Samples were analyzed for strength, particle size, morphology, and other geotechnical properties. Mineralogical and chemical analysis is currently underway. Measurements from two piezometers, installed in the drill hole, are being recorded on a regular Figure 4. Remarkable, horizontally banded prism slid along a nearly level, basis. to slightly dipping, failure plane. Prism is approx. 4.5 m high. Management Implications Post-Failure Erosion Lakelse Lake. Smaller flows have The Mink Creek and Lakelse been triggered in recent years through earthflows were significant events. Post-failure erosion of the crater and construction activities, generally with Just because an area has already spoil is significant. Nine months the loading of materials and vibration failed, is no reason to conclude it after the failure, 5-cm high pedestals by equipment (Kitsault, Kitkiata, won't fail again. Further downcutting were common in exposed deep Crow Lagoon). Drill records indicate of Mink Creek, for example, could material such as on the failure plane that sensitive sediments also occur set up the conditions for another and gray spoil surfaces. Spoil from near Alwyn Creek. Older, undated earthflow. the stiff upper crust appeared to be earthflow scars have been identified less erodible than the deeper on airphotos at a variety of locations Activities that accelerate stream material. Near the snout of the in the Kitsumkalum-Kitimat trough. erosion can also accelerate earthflow debris, Mink Creek had re-routed a occurrence. Similarly, loading of course through the adjacent forest. Flowbowls in clearcut areas are slopes near erosional waterbodies The creek had cut a channel more relatively easy to pick out on air may trigger earthflows. than 2 m deep, and in one location, photos, but they are very difficult to Channelization of water may also had eroded