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Jul 12 2011

Hydro Survey: The Process to a Map (Part 1)

We’ve talked a little about boats and the equipment set up for hydro survey and today we come to the question: How do we go from data collection to a final mapping product? This process does not start when you get back to the office but prior to even getting into the field.

Here is our case study: the San Joaquin River, just south of Stockton, California which is south of Sacramento. The project in this case was to survey nearly 300 miles of channel in San Joaquin system back in 1998. I’m going to show you a part of that and I’ll show you a typical set-up now not necessarily what was used back in 1997. The final product was a 2ft contour map for hydraulic mapping, not for design.

San Joaquin River near Stockton, CA

 

A few other pictures (oblique and from the water) of this channel to give you some perspective…

san joaquin

Where and How to Cut Sections

When single beam mapping for hydraulic modeling purposes, there are 2 key things to note:

1. Survey cross sections perpendicular to flow. This may seem like common sense but there are other approaches out there – like using a zig-zag pattern across the river. Perpendicular cross sections are key if you want to accurately represent the bend way and channel form.

2. Survey cross sections at channel width spacing. You can set cross sections ahead of time but it’s usually best to get in the field and eyeball it. If there are other significant changes in the channel or a scour hole, you’ll need more data there, but channel width is sufficient otherwise. For design purposes, tighter sections are needed (at least 50ft spacing usually).

Raw Collected Data

Extracting & Thinning

Now you might be thinking: What more is there? Don’t we have our data now and we can just come back to the office, triangulate it, and be finished? Sorry, you can’t just triangulate your raw data or you’ll wind up with a mess. Your contours will look awful and inaccurate, and you will not accurately represent the curvature in the river. You have to start by extracting just the perpendicular section data points.

Data thinned to just perpendicular section points

You’re still not finished getting the data processed at this point. Now, in order to build a digital terrain model for the bathymetry (and eventually merged with the overbank topography), you need a way to interpolate between the sections as well as account for the bend way. How do you do this? Let’s just say the key to our method is utilizing the thalweg centerline and temporally translating the curved alignment to a straight alignment. We’ll discuss this more in Part 2.

2 pings

  1. » Hydro Survey: The Process to a Map (Part 2) Hydraulically Inclined

    […] Survey: The Process to a Map (Part 2) In my last post (The Process to a Map (Part 1), I explained how the process to building a bathymetry map begins before you even get in the field. […]

  2. Hydro Survey: Multi-beam vs. Single Beam (Part 1) » Hydraulically Inclined

    […] see an example below that I’ve used in a previous post of what our single beam hydro survey results might look like before post-processing the […]

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