WEBVTT Kind: captions Language: en 00:00:07.240 --> 00:00:12.070 Hi, my name is Nick Stasulis and I am a hydrologic technician with the Maine office of the New 00:00:12.070 --> 00:00:14.120 England Water Science Center. 00:00:14.120 --> 00:00:18.520 In this screencast I will discuss the basics of reviewing extrapolation methods for midsection 00:00:18.520 --> 00:00:21.500 measurement data collected in RiverSurveyor Stationary Live. 00:00:21.500 --> 00:00:26.990 It’s important to note that the USGS extrap program cannot be used to evaluate extrapolation 00:00:26.990 --> 00:00:34.100 methods in midsection software, it is a manual process, as we’ll discuss. 00:00:34.100 --> 00:00:39.080 -Before reviewing extrapolation in RSSL, be sure the screening distance is set appropriately 00:00:39.080 --> 00:00:43.390 in the software, as this will cause data near the surface to be removed, and might change 00:00:43.390 --> 00:00:46.160 your evaluation of the methods used. 00:00:46.160 --> 00:00:51.420 As a reminder, the screening distance is set to 0.52 plus the transducer depth for open 00:00:51.420 --> 00:00:54.780 water measurements. 00:00:54.780 --> 00:00:58.750 For ice measurements, the screening distance is still based on the water surface, but must 00:00:58.750 --> 00:01:00.320 include ice information. 00:01:00.320 --> 00:01:06.470 So, you would add the 0.52 to the transducer depth below water surface, which would vary 00:01:06.470 --> 00:01:10.659 for each station if ice information was entered into the software. 00:01:10.659 --> 00:01:17.579 -When evaluating extrapolation methods for any ADCP data, it is useful to have some knowledge 00:01:17.579 --> 00:01:22.530 of the site conditions to aid in your understanding of an appropriate extrapolation method. 00:01:22.530 --> 00:01:27.009 For example, it’s useful to know about the bottom substrate type and size in relation 00:01:27.009 --> 00:01:31.249 to the channel depth, the channel shape and hydraulic features upstream and downstream 00:01:31.249 --> 00:01:33.180 of the cross section. 00:01:33.180 --> 00:01:37.060 Certain channel shapes can tell us something about what the expected velocity profile might 00:01:37.060 --> 00:01:40.149 be, as we see in this diagram from Chow. 00:01:40.149 --> 00:01:44.749 It shows that some site conditions might cause us to expect a bend back at the surface, indicated 00:01:44.749 --> 00:01:49.409 by the blue arrows, while some might cause to expect an increase in velocity towards 00:01:49.409 --> 00:01:51.899 the surface, indicated by the red arrows. 00:01:51.899 --> 00:01:57.399 Also, the roughness of the bottom could tell us something about the expected power exponent. 00:01:57.399 --> 00:02:01.710 If the average depth is 4 ft and there were 1 ft boulders on the channel bottom, you would 00:02:01.710 --> 00:02:06.079 expect the profile to trend towards zero higher in the water column than if the channel were 00:02:06.079 --> 00:02:07.710 a sand bottom. 00:02:07.710 --> 00:02:12.280 The rougher the bottom, the higher you would expect the extrapolation power fit exponent 00:02:12.280 --> 00:02:16.250 to be. 00:02:16.250 --> 00:02:20.030 -Start your evaluation of the extrapolation method by scrolling through each vertical 00:02:20.030 --> 00:02:23.320 and looking at the velocity profile plot. 00:02:23.320 --> 00:02:27.880 This is easily done using the left and right arrow keys. 00:02:27.880 --> 00:02:33.080 Remember that the blue measured data is always used, and we want to evaluate how the green 00:02:33.080 --> 00:02:38.530 top fit and orange bottom fit line up with the measured data and extend beyond that data 00:02:38.530 --> 00:02:40.860 into the unmeasured areas. 00:02:40.860 --> 00:02:46.780 -If you start noticing a consistent trend of measured data and the extrapolated areas 00:02:46.780 --> 00:02:50.400 not agreeing, it’s likely a change is needed from the power/power default. 00:02:50.400 --> 00:02:54.760 If a change in slope to the measured data will help align the extrapolation, it may 00:02:54.760 --> 00:03:00.770 be as simple as increasing or decreasing the exponent, RSSL software labels the exponent 00:03:00.770 --> 00:03:03.650 as coefficient. 00:03:03.650 --> 00:03:08.590 A lower coefficient will cause a more vertical profile (due to a smoother bed), while a higher 00:03:08.590 --> 00:03:12.700 coefficient will flatten the profile (due to a rough bed). 00:03:12.700 --> 00:03:18.600 If the coefficient alone doesn’t allow a proper fit, you might need to change the method, 00:03:18.600 --> 00:03:21.690 as well. 00:03:21.690 --> 00:03:25.900 Note that when changing the exponent for a power fit, the coefficient value needs to 00:03:25.900 --> 00:03:29.910 be changed for both the top and bottom method. 00:03:29.910 --> 00:03:35.220 -Constant/no slip will take the upper most bin and carry that velocity to the surface 00:03:35.220 --> 00:03:38.930 and fit a power curve through the lower 20% of the data. 00:03:38.930 --> 00:03:44.250 The cells to use for the constant method is typically not changed, left at 1, and the 00:03:44.250 --> 00:03:48.620 % of the cells used for the no slip method should typically not be changed, either. 00:03:48.620 --> 00:03:52.390 It is important to note that you can, and should, change the coefficient for the no 00:03:52.390 --> 00:03:58.120 slip method if the standard exponent doesn’t follow the trend of the measured data. 00:03:58.120 --> 00:04:02.930 -Let’s make a few other points on the available methods. 00:04:02.930 --> 00:04:08.660 No slip should only be used on the top for ice measurements, never for open water data. 00:04:08.660 --> 00:04:13.760 Also, constant should never be used on the bottom, for any measurement, as we know the 00:04:13.760 --> 00:04:19.310 velocity eventually goes to zero. 00:04:19.310 --> 00:04:23.210 3-point will fit a line through the top three bins, and is useful in conditions where strong 00:04:23.210 --> 00:04:27.199 winds cause a severe bend towards zero on the surface. 00:04:27.199 --> 00:04:33.180 Use of this method should be confirmed with detailed field notes. 00:04:33.180 --> 00:04:37.479 -While many measurements will have the same extrapolation method used for the entire measurement, 00:04:37.479 --> 00:04:41.310 some cross sections may require different methods for each station. 00:04:41.310 --> 00:04:46.180 If the portions of your cross section vary significantly in bed roughness, bed type, 00:04:46.180 --> 00:04:51.680 and/or flow direction the velocity profile for some sections may also vary. 00:04:51.680 --> 00:04:56.960 In RiverSurveyor Stationary Live you can specify extrapolation methods, including the power-fit 00:04:56.960 --> 00:05:02.129 exponent, for individual stations when the shape of the velocity profile varies from 00:05:02.129 --> 00:05:06.960 station to station. 00:05:06.960 --> 00:05:11.360 -After your evaluation is complete, and you’ve selected the method that you feel is appropriate, 00:05:11.360 --> 00:05:13.229 there are a couple steps left. 00:05:13.229 --> 00:05:18.979 First, apply the newly determined extrapolation settings to each station, or the entire measurement, 00:05:18.979 --> 00:05:20.789 as appropriate. 00:05:20.789 --> 00:05:24.629 Take a look at the change that resulted to the final discharge, as this will help determine 00:05:24.629 --> 00:05:28.319 the uncertainty associated with your new extrapolation method. 00:05:28.319 --> 00:05:31.909 If you are confident in the selected method and the change in final discharge is quite 00:05:31.909 --> 00:05:36.659 small, it’s likely the change in extrapolation method will have little influence in how you 00:05:36.659 --> 00:05:38.419 rate the measurement. 00:05:38.419 --> 00:05:42.199 If the change is quite large or uncertain, you should likely down-rate your measurement 00:05:42.199 --> 00:05:45.580 due to the increased uncertainty with the extrapolation method. 00:05:45.580 --> 00:05:51.600 Also, any time you apply a change to the extrapolation methods, make sure that the change is documented 00:05:51.600 --> 00:05:52.490 in field notes.