Comparison of Acoustic Doppler Current Profiler and Price AA Mechanical Current Meter Measurements Made during the 2011 Mississippi River Flood

Abstract

The Mississippi River and Tributaries project performed as designed during the historic 2011 Mississippi River flood, with many of the operational decisions based on discharge targets as opposed to stage. Measurement of discharge at the Tarbert Landing, Mississippi range provides critical information used in operational decisions for the floodways located in Louisiana. Historically, discharge measurements have been made using a Price AA current meter and the mid-section method, and a long record exists based on these types of measurements, including historical peak discharges. Discharge measurements made using an acoustic Doppler current profiler from a moving boat have been incorporated into the record since the mid 1990’s, and are used along with the Price AA mid-section measurements. During the 2011 flood event, both methods were used and appeared to provide different results at times. The apparent differences between the measurement techniques are due to complex hydrodynamics at this location that created large spatial and temporal fluctuations in the flow. The data and analysis presented herein show the difference between the two methods to be within the expected accuracy of the measurements when the measurements are made concurrently. The observed fluctuations prevent valid comparisons of data collected sequentially or even with different observation durations. BACKGROUND The Tarbert Landing, Mississippi discharge range is located just downstream from the Old River Control Structures at Mississippi River Mile 306.3 (Figure 1). Tarbert Landing is the primary water and sediment discharge monitoring station for the Lower Mississippi River. This location is used to compute what is called “latitude” flow. Latitude flow is a term used by the US Army Corps of Engineers (COE) to describe the combined flows of the Mississippi and Red Rivers at latitude 30 degrees 58 minutes North. This latitude corresponds to the location of Red River Landing, LA at Mississippi River Mile 302.4. Discharge measured upstream at Tarbert Landing corresponds to the discharge at Red River Landing. Measured discharge at Tarbert Landing is used to compute the latitude flow and operate the flood control structures downstream: