Surveying High-Water Marks and Cross-Sections for Indirects

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Detailed Description

This video describes how to survey high-water marks and cross-sections for indirect measurements. Included are tips and tricks for getting across non-wadeable sections of the stream.
 

Details

Image Dimensions: 1920 x 1080

Date Taken:

Length: 00:07:15

Location Taken: Las Vegas, NV, US

Video Credits

Todd Geiger, Hartley Delvalle, Carly Venghaus, Michael Steiner, Office of Employee Development
 

Transcript

Hi, this is Megan Poff and I’m the Field Office Chief at the USGS in Las Vegas, Nevada.  Other videos in this series have covered indirect measurement theory, where to flag and survey high-water marks and cross sections for the various methods, and surveying techniques.  In this video, I’ll provide recommendations for specifically how to survey high-water marks and cross sections. 

I’d like to start off with the high-water marks.  We want to survey high-water marks to a vertical precision of 0.01 ft.  Let’s say we go out to survey an indirect measurement.  We already had someone come through and flag the high-water marks.  That person could have flagged the high-water marks with stakes, nails, wire flags, regular flagging, green USGS marking tabs, or marks made by permanent markers, spray paint, or even a chisel.  For information on flagging high-water marks, please see the high-water mark video in this series, or consult the USGS Techniques and Methods Report 3-A24: Identifying and Preserving High-Water Mark Data. 

So, here we are in the field.  Before I start surveying anything, I want to walk the flagged reach so I know what I’m dealing with.  Let’s take a look at some of these flagged high-water marks.  Here’s a nice spray paint line – great!  It’s bright and easy to see.  However, we have a problem.  It has been a few weeks since the flood and all of the actual high-water marks have degraded, so all I’m left with is this spray paint.  It’s a couple of inches wide.  Did the flagger mean for me to survey the top of the line?  The bottom of the line?  The middle?  These options will give us pretty different numbers if we’re surveying to a vertical precision of 0.01 ft.  The moral of the story is, talk to the flagging team before you go out to do the survey, or at the very least, consult the notes they took during the flagging. 

I’ve now walked my reach and have come up with a plan and set up my instrument.  For specific information on the notes you will need to take during the survey, see the video called Indirect Measurement Field Notes and Metadata.  Let’s start surveying!  For our spray paint line, I looked at the flagger’s notes and determined that the bottom of the line represented the actual high-water mark.  Great!  Let’s get the rod person set up to get a shot on that high-water mark.  Notice that the bottom of our rod is pointed, and it’s sinking into the ground.  Do you think we have a problem here?  If we can switch rods or put a flat foot onto the current rod, we’ll be able to survey our mark with good precision.  Continue to survey all the high-water marks. 

I’m going to move on to talk about surveying cross sections.  This assumes you have already plotted your high-water marks in the field and determined the cross-section locations.  The specific mechanics of plotting high-water marks in the field is discussed in another video.  Cross sections should be surveyed to the nearest 0.1 ft.  Data loggers that we use with our instruments often log to 0.01 or 0.001 ft. While this exceeds our convention, we can accept the excess precision and round the values later if necessary.

First, I’d like to talk generally about how we’ll go about surveying the cross sections.  Cross sections should be surveyed perpendicular to the flood channel.  Be careful!  Your low flow channel may be at a different angle than the flood channel.  Visualize the flood, and make sure your cross sections are perpendicular to those flow lines.  Next, we want to make sure we survey each cross section as straight as possible.  By surveying in a wavy line, you will actually add length and area to your cross section.  Because this entire computation will be based on the area we survey, we want to get it right and not introduce more error.  The software does adjust for drift, but we’d rather survey cross sections correctly in the field.  There are various methods you can use to keep your rod-person going in a straight line.  For example, you could set up the instrument over one of the cross-section end hubs, thereby allowing you to help your rod-person.  You might be able to string a tag line, or you could even use a spotter to keep the rod person on-line based on visual markers across the channel. 

Ideally, hub stakes or rebar should be set at the end of each cross section, above the elevation of the high-water mark. Survey the top of stake, then ground at stake at each end of the section.  Why would we do this? Well, as much as we want to get it right the first time, sometimes more data needs to be acquired.  If someone has to go back for any reason before the indirect is completed, they’ll be able to find the cross section again.

We’ve now determined our cross section locations and set our hub stakes or rebar at each one, so now let’s talk about getting across the cross section.  Here in southern Nevada, we typically don’t have a problem getting across our dry or shallow stream channels, but we recognize that most of the rest of the country deals with actual water.  Now, what if the cross section is too deep to wade?  There are a few options you have.  You could get an ADCP and take it across to help you determine depths and distances.  What if you don’t have an ADCP?  You could attempt taking a boat across and do manual soundings while holding yourself still on a tagline.  If the stream is too wide for a tagline, you could locate the boat at sounding points using a range finder and shooting to a stable point on the bank that you’ll survey later.  What if you don’t have a boat at all?  Are we going to just give up and not survey the part of the channel that’s in the water?  No!  USGS TWRI 3-A1, General Field and Office Procedures for Indirect Discharge Measurements, presents a cool option known as “diddling.”  You get a board that’s 6-12 ft long, and mark the foot marks on it.  Attach a rope to the top using a hole, hook, or staple.  You will need two people for this operation.  Have one person on each bank hold an end of the line, walk upstream a short distance with the line taut and the board floating on the water, then quickly lift the ends of the line while turning downstream.  This will force the loose end of the board down.  At the moment the board is vertical, read the depth of water on the board, estimating the tenths. 

Okay – we have solved the problem of getting across the stream.  Time to survey!  My instrument is already set up.  How many points should I take?  A good rule of thumb is to try to get around the same number of points as you would for a midsection measurement – around 25 points if you can, although some channels are complex and will need more points than that, and some simple channels may need less; the main objective is to define significant changes in the cross-section geometry.  Let’s look at what happens if you don’t survey enough points.  Here’s a relatively simply-shaped channel on the Amargosa River.  I should be able to get away with surveying a minimum number of points, because it’s hot outside and I want to go back to the office.  I’m going to survey five points, and here it is in a graphing program.  Looks good, right?  Given the depth of flow at this location, I computed an area of 3200 square feet.  Now, let’s say I had decided to survey the middle point here instead of at the location I just showed you.  Here is what that might look like.  I now compute an area of 3400 square feet.  That’s a 200 square-foot difference just from surveying one point differently!

That’s not really what happened though for this cross section.  We actually surveyed 20 points, and it looks like this.  The 20-point survey gives us a cross-sectional area of 3300 square feet.  Yes, it’s only a difference of plus or minus 100 square feet from our computations with the 5-point cross sections, but this is a relatively simple cross section and there are already so many assumptions and unknowns in an indirect measurement.  Do you really want your cross-sectional area to contribute excessive error to your computation?  Do your best and survey it right the first time. When you’re finished surveying, make sure you’ve closed the circuit properly before turning off the instrument.

If you need help in the field, call your supervisor, surface-water specialist, or indirect measurement specialist.