Flow Measurement at an Industrial Outfall Using the Index-Velocity Rating Method

Channels with a large depth to width aspect ratio present a challenge when it comes to accurately measuring flow rates in real time.  The index velocity method provides a solution for real time flow rate reporting in situations where traditional stage-discharge ratings are not possible.  The index velocity method uses a mathematical model to calculate the wetted cross sectional area at a given stage as well as the index velocity so that the flow rate can be calculated by multiplying the cross sectional area by the velocity.

Over the past 1.5 years C.C. Lynch has been working with an industrial customer in south Louisiana to fine tune their index velocity rating utilized for regulatory reporting of their total daily discharge at their final outfall.  In the winter of 2010 the customer contacted us and indicated that the flow meter at their final outfall was significantly underreporting flow rates.  As a baseline comparison the customer was totalizing flow rates from flow meters located at smaller outfalls within the plant that discharge into this 90 foot wide X approximately 6 foot deep channel.  Upon further investigation we determined that the facility had recently reduced their output of wastewater and strong north winds from winter cold fronts pushed water out of the surface water body they discharge to resulting in their existing flow meter coming out of the water at times and therefore significantly underreporting the flow rate. In addition, the channel had been lined with concrete changing it’s geometry since the last channel cross section survey was performed therefore the stage-area rating being used by the flow meter to calculate the wetted cross sectional area was incorrect.  An updated channel cross section survey was performed and the data was used to create an updated stage-area rating.

In the spring of 2011, the Teledyne RDI ChannelMaster ADCP used as the index velocity meter at the final outfall was relocated and installed deeper in the water column to prevent it from coming out of the water during low water level events.  The ChannelMaster ADCP is a “side looking” ADCP which mounts underwater on the side of a channel and sends two directional acoustic signals across the channel measuring velocities in discrete “velocity bins” all the way across the channel or until the signal encounters an object such as the channel bed or water surface.

 Each month, discharge measurements were collected using a Teledyne RDI StreamPro or RiverRay ADCP while simultaneously logging discharge measurements with the ChannelMaster.  The StreamPro and RiverRay ADCPs are both float mounted acoustic Doppler current profilers that are used to calculate discharge by running transects across the channel.  These ADCP’s measure velocities in discrete velocity bins from top to bottom and bank to bank.  Since the outfall of this facility is tidally influenced, discharge measurements used to develop the index velocity rating were spread out to capture seasonal tidal fluctuations.  A setback occurred in the late summer of 2011 when algae growing up from the channel bottom entered into the zone where the ChannelMaster was measuring velocities.  This skewed the velocities low since the velocity of relatively stationary algae was being measured as opposed to the velocity of suspended particles traveling in the water column.  As a result, the ChannelMaster was raised so that the acoustic beams would not encounter the algae growth occurring in the late summer months.

Subsequent discharge measurements have been utilized to develop the index velocity rating for this site.  A total of 77 discharge measurements have been used in the index velocity rating to date.  These include verification measurements done every two months to verify that conditions have not changed and that the index velocity meter reports flow rates within 10% of flow rates reported by a StreamPro or RiverRay ADCP.  The average difference between discharge measurements collected with a StreamPro or River Ray ADCP vs. the ChannelMaster index velocity meter is less than 0.012%.

Additional information and guidance regarding index velocity rating development can be found at our Resource Center under the heading “Stream Flow Measurement”.