Introduction to the Slope-Conveyance Method

Video Transcript

Detailed Description

This video provides an introduction to the slope-conveyance method of indirect measurement. Theory and background are described as well as high-water mark and cross-section placement.

Details

Date Taken:

Length: 00:05:28

Location Taken: NV, US

Video Credits

Office of Employee Development. Closed captions by Ruby Hurtado. Special thanks to Terry Kenney, Chris Morris, Todd Geiger, Tony Carson, and Chris Evans.

Transcript

Hi, this is Megan Poff and I’m the Field Office Chief at the USGS in Las Vegas, Nevada.  I’ll be talking briefly about the high-water marks and data you’ll need for a slope-conveyance measurement.  The information presented in this video can be found in various hydraulics textbooks such as Open Channel Hydraulics by Chow.  So, what is a slope-conveyance?  A slope-conveyance is one of the simpler methods of computing open channel flow, and it involves applying Manning’s equation to a single cross section.  The equation is defined as Q=(1.486/n)AR2/3S1/2, where Q is discharge, n is a roughness coefficient, A is the area of the channel cross section, R is the hydraulic radius, and S is the slope or energy gradient.  Hydraulic radius (R) is just the area of the channel cross section divided by the wetted perimeter.  Wetted perimeter is simply the perimeter of the cross section that is, well, wet.

I know you’re thinking, “Okay, Megan, how do these equations actually translate into me computing a discharge after a flood?”  Well, let’s think about it.  Based on the equations I just gave you, we need an area, a hydraulic radius, a slope, and a roughness coefficient.  We can do a field survey of a cross section and high-water marks to determine an area.  Then, we can determine a hydraulic radius based on the area and the wetted perimeter, which can easily be done with a spreadsheet.  With that same spreadsheet we’ll plot our high-water marks and determine a water surface slope, and finally we’ll estimate the roughness coefficient using pictures and tables.  See?  It’s really quite simple!

What I’m getting at here is that the slope-conveyance method needs a lot less data than the other indirect measurement methods that are described in this video series.  Why wouldn’t we do these for every indirect and save ourselves the trouble?  Danger, hydrographer!  Because a slope-conveyance relies on a single cross section, the flow estimates you compute can be 100% off of the actual flow.  Let me give you an example from one of our gages on the Virgin River for the flood of Dec 22, 2010.  The channel was a non-uniform channel, and we computed a different type of indirect measurement first.  However, this was an unusually large peak, and we wanted to check the original computation, so we ran several slope-conveyances.  For the same peak, separate slope-conveyances computed anything from 21,700 cfs to 41,400 cfs.  That’s a huge difference for the same flood on the same river!

Let’s move on to finding a good location for a slope-conveyance measurement.  Slope-conveyances, like any of the other indirect measurement methods outlined in this video series, assume that you have steady, uniform flow.  Steady flow means that your flow isn’t varying by time, and uniform flow means that your flow isn’t varying spatially, so the area and velocity should be the same.  Generally, that’s a safe assumption at the peak because we will have something that looks like steady and uniform flow after the rise and before the recession.  We can take this concept and apply it to our slope-conveyance indirect measurement.  All we need is a single cross section that is representative of the entire reach and will adequately represent the flow we are gaging, along with high-water marks.

Let me give you an example.  Let’s say we have a flow in a uniform concrete channel.  This kind of situation is perfect for a slope-conveyance computation because the area and velocities aren’t changing.  You will want to flag high-water marks on both sides of the channel.  For more information on flagging high-water marks, please see the high-water marks video.  The length of high-water marks can be nearly anything, but to be safe try to get a flood-channel-width of high-water marks on each bank.  For example, if your flood channel was 40 ft wide, try to get 40 ft of high-water marks on the banks.  Your cross section should be located approximately in the middle of your 40 ft of high-water marks.  If you can only find high-water marks on one bank, it isn’t a deal breaker.  Go ahead and flag what you find.

What if you are facing a situation where you have a channel that is only uniform for a very short distance and you need to determine a peak discharge?  This kind of channel might be a good place for a slope-conveyance as well, because the slope-conveyance method doesn’t require a long reach length.  Pick a cross section location, and then flag high-water marks about a flood-channel-width upstream and downstream from your cross section.

Here is one last example for when you might choose a slope-conveyance computation.  Let’s say you have a crest-stage gage only site, and you don’t have a stage/discharge rating defined for it yet.  The purpose of the crest-stage gage is to measure the peak of the year, and your peak of this particular year was just over the pin elevation. That’s a pretty minor flow, but we don’t have a rating and we need a discharge.  Doing a slope-conveyance will be better than nothing and will be more defensible than some sort of visual estimate of flow.  Plus, you’ll have a point to start defining your rating.  The slope-conveyance method can also be used for rating extension.  See Water Supply Paper 2175 for more information.

Let me stress that for the most part, you will want to use a more robust indirect measurement method, like a slope-area computation, or even any of the computations that involve a structure like a culvert or weir.  Slope-conveyances can be quicker than other indirect measurement methods, but they are less accurate and therefore should be used with caution.

For information on surveying a slope-conveyance indirect measurement, refer to the next slope-conveyance video in this series.  If you need help in the field, call your supervisor, surface-water specialist or indirect measurement specialist.