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  • 21. EDI - Equal Discharge Increment

21. EDI - Equal Discharge Increment

Equal-discharge-increment method (EDI) samples are obtained from the centroids of equal-discharge increments across the cross section. This method requires some knowledge of the distribution of streamflow in the cross section, based on a long period of discharge record or on a discharge measurement made immediately prior to selecting sampling verticals. If this information can be obtained (such as with an ADCP), the EDI method can save time and labor; compared to the equal-width increment method. This is especially true on the larger streams, because fewer verticals are required (Hubbell and others, 1956) (Edwards and Glysson,1999).
 
A discharge measurement made with an ADCP immediately prior to the collection of EDI samples can be used to identify the location, depth, and mean velocity for each EDI sample. To improve the accuracy and efficiency of using ADCP discharge measurements for EDI sampling this function was written in Matlab to use the discharge measurements processed in QRev and perform the necessary computations.
 
Clicking on the EDI button opens the following window.
 
 
The user must select the transect that will be used to compute the equal discharge increments. It is important to select a transect that starts on the desired bank, as the computations will be relative to that bank. After selecting the desired transect, click OK. The following window will open.
 
 
The EDI program interface allows the user to change the default increments of 10, 30, 50, 70, and 90 percent of the discharge to any desired flow percentage between 0 and 100. The distance displayed in the user interface is from the starting bank of the selected transect and is computed as the Distance Made Good (a straight-line distance from the starting location) to the target ensemble location plus the starting edge distance and a user provided zero distance offset. Therefore, it is important that the transect traverse the channel in a straight line.
 
 If GPS data are available in the ADCP files, the longitude and latitude in degrees, minutes, and decimal minutes are displayed for each target sample-location. These data may be used with navigation software and devices to assist in navigating to and holding the boat on the desired locations. In addition, there is an option (checkbox Create TopoQuad File) to create a file compatible with Delorme TopoQuad software. The file is named "tqlatlon.txt", is stored in the same folder as the ADCP data, and contains the information to plot a yellow dot for each sampling location in the TopoQuad display.
 
When the user is satisfied with the Percent Q, Zero Distance Offset, and Create TopoQuad File settings clicking "Compute Stations" will complete the compuations  and display the results.
 
 
The location of the sample is determined by finding the first ensemble (referred to as the target ensemble) with a cumulative discharge that exceeds the target discharge. The target discharge is computed as the Target Percentage multiplied by the Total Discharge obtained from the selected transect. Both the target discharge and measured cumulative discharge to the target ensemble will be displayed in the user interface. If GPS data are available in the ADCP files, the longitude and latitude in degrees, minutes, and decimal minutes are displayed for each target sample-location.
 
The computations used in this QRev implementation of EDI uses averaging to determine the depth and velocity which is different form  standalone versions of EDI prior to EDI version 3.4. The depth and velocity displayed are an average of 1 percent of the total ensembles before and after the target ensemble. The depth used to compute the average is the final depth for each ensemble as computed by QRev. The velocity displayed is the magnitude of the mean velocity of the measured part of the velocity profile for the ensembles used in the average. The average is computed as the average of the u and v velocity components in each depth layer to form a mean profile with mean u and v components. Mean u and v components of the profile are then computed by vertically averaging the u and v profile components. Finally the magnitude of the mean u and v components is computed. No accounting for the unmeasured part of the velocity profile is included.