Middle Earth – slip sliding away

Today’s song is Landslide by Fleetwood Mac featuring the amazing Stevie Nicks.

Lord of the Rings movie afficionados get to experience parts of the Middle Earth landscape as they undertake the pilgrimage through New Zealand visiting various filming locations. From Queenstown, you can easily drive to one of the more popular destinations at Glenorchy, which lies at the head of Lake Wakatipu. By car, it is a short, spectacular drive yet remains surprisingly far removed from the Queenstown tourist squeeze. But we had travelled the hard way – overland, on foot, through some stunning, but rugged, New Zealand back country. And not just from Queenstown. Our journey had begun 42 days and 170km ago, at the base of the South Island, on the beach at Te Wae Wae Bay. We had planned to follow the north-east trending backbone of the main divide, as closely as practicable, for the full length of the island. It was December 2012 and we were to make the Southern Alps our home for the next five months.

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The snow covered Southern Alps stretch almost the full length of the South Island and are clearly visible in this NASA satellite photo.

During the first 12 days in Fiordland we saw no-one. We became content in the soothing quiet of the bush as our feet traced the undulations over the surface of the land. The hills are steep sided and the terrain is mobile, difficult and unrelenting. Heavy rain had dogged us, for much of the trip, thwarting our plans at every turn. Slips were commonplace and swathes of trees frequently lay broken and piled up before us, blocking the way forward.  Cool moist earth and splinters regularly took refuge under our fingernails as we clambered and cursed our way over the fallen. It is disconcerting walking across steep, loose ground. And it is certainly made all the harder when it is wet. Most Kiwi holidaymakers will tell you that December weather is wet and changeable. They frequently have to pack up their soggy tents and head homewards, prematurely. Unfortunately, we didn’t have that luxury. By the time we reached the north branch of the Routeburn River, which runs north along the western edge of the Humboldt mountains, we were wishing for some relief from drizzle and downpours. We sure got it! At Routeburn Flats Hut, we replenished our food supplies at food drop number 11 of 27. When we finally got going, the heat of the sun scorched our faces as we toiled across the golden grass flats and then heaved our groaning backpacks up and over the tortuous hillsides. Our muscles straining, we struggled for breath. Partly, it was because of the sheer effort in moving our loads against gravity. Yet, when we paused to gaze around us, what little breath remained was whipped away. The beauty of this country is utterly mindblowing. Even when viewed from afar the landscape is fully deserving of its superlatives. But when you are embedded in it, moving to its rhythm and navigating through it on its terms, it becomes deeply personal and intensely emotional. The lofty heights of the alpine summits, the smell of the damp earth, the micro detail of the vegetation texture and hue, and the twitter of rifleman, bombarded us until we were punch drunk with its richness. But in the grandeur there is deception. It tricks us into feeling as though it has always been this way. Yet, despite the scale and apparent permanence, the landscape around us is constantly changing. We see evidence of it everywhere. And there’s a key element at work here.

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High rainfall coursing down sheer slopes produces spectacular waterfalls in New Zealand’s mountains.

The western side of the main divide has a phenomenal rainfall record. Strong westerly winds carry warm air towards the verdant mountain slopes. Along the way, the air has picked up moisture evaporated from the sea. The Southern Alps form a formidable barrier and the air has nowhere to go but up and over the top. As the air rises, it cools and the air can’t hold as much moisture. So, you guessed it, it drops its load on the way up. More than we could do! But all of that rain has consequences.

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The South Island lies fully within a belt of strong westerly winds, situated between 40 and 50 degrees south, known as the Roaring Forties. The pink belt is superimposed on a NASA photo.

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High camp at Lake Nerine.

After the hard work of reaching a high camp at Lake Nerine in the Humboldts, we had a few days enjoying the scenery before following the Rock Burn down the eastern side of the range to its confluence with the Dart River. We were to pick up our next food drop and stay at Rock Burn Hut but we arrived to find that an enormous tree had landed on it, rendering it uninhabitable. A storm the previous week had taken its toll. The Dart River, which has a typical flow of about 80 cumecs (cubic metres per second), had been running at 1600!  The water, normally clear and sparkling, was grey and opaque.

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Our crumbling mountains are far from solid and stable. Steep sided slopes and high rainfall shapes our landscape.

As we rounded the next bend we could see the culprits – signs of active slips. Further up the valley, the scene that greeted us was different from our previous trips into the area. Grey sediment covered the valley floor creating an eerie, surreal landscape.

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A constant supply of sediment clouds the river.

 

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Walking up the Dart Valley. Photo credit: Markus Milne.

The combination of steep slopes and high rainfall make landslides a common occurrence in New Zealand. Just take a look at the one at Green Lake in Fiordland – truly world class! Any downslope movement of material which occurs when the force of gravity is greater than the friction of the material holding it in place, is called mass wasting. It includes landslides but also subsidence, creep, heave and flow. It can happen slowly or suddenly over large areas, or small, with consolidated rock or loose material, on steep slopes or gentle. And the strength of the material changes with the structure of the rock and the addition of water delivered by those downpours. These affect the type of movement and the result. We use the term landslide, in general sense, to mean any downslope movement of rock, soil, or mud . But for scientists the term is more precise. It refers to rock masses that slide along planes of weakness without flowing. These planes can be straight or curved: landslide   On the other hand, many of the slips in the Dart Valley are debris flows. They often result after heavy rain saturates rock fragments and silt causing it to behave as a fluid. The mix flows downhill rapidly, usually confined to an existing channel, with the consistency of wet concrete. GNS have a great video of one in the Dart Valley that happened a year after we were there. Depending on the amount of material they can flow a long way and cover a large area. The recent Washington landslide started off with a rotational slump and, once in motion, the saturated material progressed into a debris flow which engulfed the town.

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