Minute 319 Pulse Flow

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The minute 319 pulse flow experiment flood in the Lower Colorado River Delta allows scientists the opportunity to study many different aspects of a flood. With the help of CSAs we had the opportunity to actually back calculate the roughness value at the times that our technicians in Yuma measured discharge.


Date Taken:

Length: 07:06:00

Location Taken: Colorado River, MX


Brandon Forbes: 
The minute 319 pulse flow experiment flood in the Lower Colorado River Delta allows scientists to have the interesting opportunity to study many different aspects of a flood: from water quality, to recharge, rates and volumes, to the actual flood itself. And Jeff Cordova and I were able to install CSA or Continuous Slop Area sensors in a reach just downstream of the dam that was opened up where the flood was released. These sensors allow us to estimate what the discharges is and normally we put these gages or Continuous Slop Area sensors in areas that are impacted by wildfire or are just hard for our technicians to reach. So it's hard to actually hard to get to a spot and measure and a difficult thing to estimate when we do this kind of method is the roughness that the vegetation actually pushes against the water during the flow event. And you have to estimate this while you're in the field and this goes into the Manning's equation so we can estimate discharge. In this unique event in the pulse flow, in the Colorado River, we were able to put these sensors in and we knew when the flood was coming and how big it was going to be so we could prepare before the flood and collect as much data as possible during the event. We also had the ability for our scientists in the Yuma field office to make high quality and very highly accurate discharge measurements using ADCPs or Acoustic Doppler Current Profilers, and they did this at multiple times during the event which allowed us to have very accurate discharge measurements where we could compare this with the CSA or Continuous Slop Area sensors that we had deployed in the reach. This gives us the opportunity to actually back calculate the roughness value at the times that our technicians in Yuma measured discharge. 
During the installation of the CSA sensors, all the sensors in the reach were programmed to collect stage, or the depth of water, every five minutes throughout the duration of the flood event.  
Once the sensors were properly programmed to collect data at the correct intervals, which can be done in the field, the sensors were anchored in the channel to prevent them from moving during the flood event. 
Once the CSAs are fully installed, we mark each cross section associated with each sensor in the CSA reach so that a survey can be conducted to record the shape of the channel and the height of the sensor at each cross section.
Here we are surveying the channel geometry at each cross section using a Total Station.  This is an accurate tool used to measure the height of each sensor as well as the shape of the channel at each cross section in the CSA reach. The Prism at the top of the survey rod is used as a target for the Total Station; when using this equipment, there needs to be line of sight between to the Total Station and the Prism, this is not always possible give the winding nature of rivers and the vegetation you can see here. In such cases, a reference mark is used. Here it is a stake hammered into the riverbed and is used as the reference mark.  The mark can be seen multiple cross sections when the Total Station is moved. This ensures that the survey between the two points where the Total Station has been set up remains on the same plane.  
Yeah this is going to be CSGX4; the pin will be four. Yeah I'll do the top of the T post then I'll try and get the sensor level. Ready.
This is Brandon Forbes, April 25th.  But I'm going up to our upper most sensor in the slope area reach; the water's about six feet deep here. The channel changed quite a bit, but it's way deeper than it was when we first installed the sensors. Yes! And there she blows. She looks good, stable; it's in this clay layer right here. Very very nice. 
Now that the flood is over and the dam is now shut off, we have conducted three surveys of the channel bottom and the heights of the four sensors that we deployed in the reach, and we've also measured the water with ADCPs multiple times during the event. Now we're looking at all the data and we're going to compare the two and see what we get. It's very important to survey after the flood, because a lot of times the channel can change, and what we're seeing already in that the channel is filled with fine sediment that was carried from the dam and down into the reach where we measured. And this can affect the cross-sectional area of our estimate, so it's very important to capture this now right after the event occurred so we can have the closest estimate of what the channel looked like while the event was happening. 
This is an efficient tool to measure the channel profile and the channel characteristics that we use in a lot of streams in Arizona. It's called an RTKGPS and this is a high accurate survey grade GPS unit that we can take out into the field and measure channel properties without line of sight. With traditional survey equipment like the Total Station, you need line of sight and it's far more difficult to get through the canopy and through the vegetation that we have on the banks of the streams. The RKTGPS allows us to go to any point where we can get a GPS signal, and that we want to know the exact XYZ coordinates of that point. 
In Arizona it's vet difficult to get to a flood while it's occurring because floods happen so fast. In this unique opportunity we knew the flood was going to happen which gave us an incredible heads up on we could collect during the event and now process after the event has occurred to try to learn as much as we can from the pulse flow event. Now we are compiling, collecting, and analyzing all the data we collected during the flood event and we hope to have interesting results we can share in the very near future.