Site Selection for Indirect Discharge Measurements

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Detailed Description

This video demonstrates how to select an appropriate location for an indirect measurement. Theory and initial reconnaissance are discussed. 

Details

Image Dimensions: 4608 x 3456

Date Taken:

Length: 00:05:46

Location Taken: Las Vegas, NV, US

Video Credits

 

Todd Geiger, Michael Steiner, Hartley Delvalle, Terry Kenney, Office of Employee Development
 

Transcript

Hi, this is Megan Poff and I’m the Field Office Chief at the USGS in Las Vegas, Nevada.  In this video, I’ll be discussing site selection for indirect measurements.  The major types of indirect measurements and their survey requirements are discussed more in detail in the Overview of Indirect Measurements - Survey Requirements video.  You can find more information on this topic in TWRI 3-A1: General field and office procedures for indirect discharge measurements.

Let’s say that a flood just occurred at one your streamgages.  The rating doesn’t extend that high, and your supervisor just assigned you to “go flag for an indirect.”  What does that even mean?  An indirect measurement is a method for determining the discharge at a peak that uses hydraulic equations which relate discharge to the water-surface profile, the channel geometry, and roughness.  All of the USGS indirect measurement methods have been proven to work in a variety of different floods in different places.  We’ve been using these techniques with good accuracy and repeatability since the late 1940s!   

I want to briefly touch on the theory behind an indirect measurement.  Don’t tune out!  This is important so you understand what you’re doing and why you’re doing it.  Indirect measurements make use of the energy equation for computing discharge.  What’s the energy equation?  Basically, it says that the total energy at point one is equal to the total energy at point two if all aspects are accounted for such as energy losses and conversions.  We can apply this concept, the conservation of energy, along with some basic assumptions related to steady and uniform flow conditions to predict discharge in a stream reach. The specific equations for each method of indirect measurements differ depending whether you have open-channel flow or some sort of structure such as a culvert.  Generally, you need three things to compute an indirect measurement: 1) physical characteristics of the channel, 2) water-surface elevations at the time of the peak, and 3) hydraulic factors such as roughness coefficients and discharge coefficients.

Back to our big flood at our field site.  Before you do anything, including that flagging assignment your boss just gave you, you need to complete a thorough reconnaissance of the flood area.  The first thing you should do is check the Station Description.  It’s possible that someone in the past wrote up where a good indirect measurement reach is located near the gage.  If you have that, you’ve already saved a lot of time.  The next thing to do is to look through the office files for the site and see if someone else did an indirect measurement for that gage, and if so, where it was.  If you don’t have information in the Station Description or an old indirect measurement to look at, the next stop is an aerial photograph, GIS, or even a program like Google Earth. 

For this example, I’ll show you how a recon can be done in Google Earth.  Let’s zoom into Amargosa River at Tecopa.  Here’s the gage.  The first thing I’m going to look for are inflows.  Obviously, if we are gaging the flow that goes past the gage here, we don’t want to add more water from inflows, because it won’t be the same water as what went past the gage.  Let’s avoid those.  So, the first thing I’m seeing is this channel entering just downstream from the gage on the left bank.  Uh oh.  We have an inflow at the gage!  That’s not good.  Let me drag the screen over and zoom out some.  The inflow actually just appears to be a minor road drainage ditch, so I can probably disregard it.  Let’s look at what else we might have here.  There’s another inflow just upstream of the gage and what looks like a more major inflow just up from there.  All these inflows tell me that upstream from the gage isn’t a good spot for an indirect measurement.  Let’s look downstream.  Oh wow – check it out right here at the road.  We have a set of five culverts.  You can use culverts for an indirect measurement.  That could be a great location.  Let’s continue downstream.  Look – there’s an area that’s pretty straight with similar widths which suggests minimal expansion and contraction and no additional inflows.  That could be good too!  So now, just from looking at Google Earth, I’ve identified a couple of potentially good areas for an indirect measurement.  Now I can go to the field and pick the best place.

Once I get to the field, I realize very quickly that the culverts at the gage provide the best indirect measurement location.  When I look downstream, I see that not only is the vegetation very thick, but there’s a floodplain on the left bank that I didn’t see on Google Earth.  It appears that the floodplain had a lot of standing water during the peak, which means we might have had an undesirable amount of energy loss at that location.  Therefore, for this particular gage, I decide to flag high-water marks for a culvert indirect measurement as described in the culvert indirect measurement videos.

What if we look at Google Earth and realize there are no good indirect measurement locations near the gage?  You may have to go a significant distance away from the gage to find a place for an indirect measurement.  Even if you have no additional inflows, you may have to account for storage at a far-away location, and those adjustments become arbitrary unless you have detailed information about the flood wave.  Whatever you do, make sure you document your choices and take plenty of photographs. 

Another good strategy once you’re in the field is to hike up a hill and see if you can get the lay of the land.  Sometimes you’ll be able to see good indirect measurement locations just by getting above the flood and looking down.  For the truly extreme events, it may be advisable to do the recon by air.  If you have to do that, make sure you note access routes and locations on a map.

Before attempting any indirect measurements, make sure you see the individual videos on each technique.  Each type of indirect measurement requires different data, and you want to make sure you have an idea of what you need before you go to the site.  If you need help in the field, call your supervisor, surface-water specialist, or indirect measurement specialist.