{"id":1574,"date":"2017-03-01T01:21:34","date_gmt":"2017-03-01T01:21:34","guid":{"rendered":"http:\/\/blogs.lincoln.ac.nz\/gis\/?p=1574"},"modified":"2023-05-07T00:49:52","modified_gmt":"2023-05-07T00:49:52","slug":"waiho-scanning","status":"publish","type":"post","link":"https:\/\/blogs.lincoln.ac.nz\/gis\/waiho-scanning\/","title":{"rendered":"What Do Cave Orcs and Riverbeds Have in Common?"},"content":{"rendered":"

In this post we look at how a hand-held laser scanner is being used in the water lab to capture high-resolution elevation data of a scale model of the Waiho River.<\/em><\/p>\n

Lincoln has a long history of using physical models of to study gravel bed river systems, and the West Coast’s Waiho River in particular.\u00a0 At the moment, we’ve got a postgrad, Rose Beagley, doing just that (and working with Tim Davies<\/a> from Canterbury University).<\/p>\n

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For many years now, the Waiho River has been rapidly aggrading, and large floods threaten to overtop the existing stopbanks, putting a holiday park, amongst other things (like Franz Josef township, as if they didn’t have enough to worry about..<\/a>.), at high risk.\u00a0 (And Tim would happily tell you that the stopbanks have created the problem.)\u00a0 Rose is interested in looking at the long term effects of potential new stopbank and highway alignments and a 1:3,000 scale physical model is just the ticket.\u00a0 With a physical model, we can experiment with a range of different configurations without spending much money or time and no one ends up in danger.\u00a0 The image below shows the area of interest – Rose’s model covers the area from the true right side of the Waiho (the Franz side) and out to the furthest-out set of yellow\/white broken lines:<\/p>\n

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Photo courtesy of Rose Beagley<\/em><\/figcaption><\/figure>\n

The model itself is pretty simple.\u00a0 To get the braiding effects we see on gravel bed rivers, all one really needs to do is let sediment laden water run out over a sloped surface and, with enough time, braiding just naturally occurs.\u00a0 And the basic idea is that what happens on the scale of the Rakaia River<\/a> (or the Brahmaputra<\/a> for that matter) is the same as what happens on the scale of a small flow of water running over Sumner Beach, or a model in the water lab (sounds very fractal<\/a>, you might say.\u00a0 I wouldn’t disagree).\u00a0 Here’s what Rose’s model looks like:<\/p>\n

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Water and sediment are mixed together at the top of the model (the far end of the image) and then flow downslope within the confines of the polystyrene walls (which simulate the natural landform boundaries of the floodplain).<\/p>\n

Using riverbed elevation data is an effective way of comparing the results of different scenarios.\u00a0 In the old days, data were collected by placing a graduated beam across the model and measuring the distance from the beam down to the riverbed surface at regular intervals, say every 10 cm or so: time consuming, prone to error and very low resolution.<\/p>\n

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Several years back, we purchased a hand-held laser scanner from an outfit in town, Applied Reseach Associates NZ (ARANZ<\/a>).\u00a0 The early Weta Workshop had used their gear to scan in scale models of things like cave orcs and then animate those models to great effect in the Lord of the Rings movies.<\/p>\n

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http:\/\/www.hollywoodjesus.com\/lotr_fellowship_making.htm<\/em><\/figcaption><\/figure>\n

We thought we could do the same thing with our scale models.\u00a0 The system is similar to LiDAR <\/a>in that it uses a laser to capture the elevations.\u00a0 The laser is mounted in the middle of a “wand” with two video cameras mounted on the wings.<\/p>\n

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The cameras capture the laser beam striking the surface and can take those images and determine how far away from a fixed point that location is.\u00a0 With the aid of radio transmitters linking all the components together, the collected points fit into a 3-D coordinate system\u00a0 that’s accurate to +\/- 1 mm: perfect for our purposes.<\/p>\n

To make this all work, Rose first runs the model for a period of time so that it gets to some equilibrium.\u00a0 Here’s a short movie of the model in action (21 seconds):<\/p>\n