A comparison of rated discharge at Jewfish Creek using a 0.5 MHz and a 1.5 MHz Argonaut-SL using different sampling volumes

Mark Zucker, Jeff Woods, Christian Lopez, Carrie Boudreau, and Stephen Huddleston

Abstract

Flow in Jewfish Creek near Key Largo has been monitored since 1998 to quantify the volumetric transport of surface water between Barnes Sound and Blackwater Sound. Jewfish Creek is approximately 250 ft wide, 1.0 mi in length, and 8-12 ft in depth. Flow through Jewfish Creek acts as an important flushing mechanism for northeastern Florida Bay. Recent algal blooms reported for this area coincide with an active 2005 hurricane season and road construction along Highway US-1. These recent events reinforce the need for real-time hydrologic and water-quality data.

Index velocity methods for tidal rivers are well established, but comparisons between various acoustic Doppler velocity meters (ADVM) that sample different volumes of the creek channel are lacking. The current index velocity ratings at Jewfish Creek utilize a 1.5 MHz Argonaut-SL with a 1.0-meter sampling cell. Concerns over the limited sampling range were evaluated using a 0.5-MHz Argonaut-SL "boomer" with a 35-meter sampling cell. Eighty discharge measurements were collected using an acoustic Doppler current profiler (ADCP) and used to correlate two ADVM velocities and measured mean channel velocity. The relations were used to compute two discharge time series to determine how changing the sampling range affects discharge determination. A statistical analysis was performed to determine if in the differences in sampling ranges resulted in significant differences in discharge determination.

	To the left is a picture of the 1.5 MHz Sontek SL. To the right is a picture of the 0.5 MHz Sontek SL "Boomer." Images obtained from www.sontek.com. Below is a map of Jewfish Creek and the surrounding area. [click on images for larger versions]

Index Ratings and Acoustic Meter Configuration Settings

1.5 MHz Sontek SL Velocity vs. Measured Velocity
[larger image]	Sontek SL 1.5 MHz - Argonaut User Setup DefaultTemp (deg C) -- 20.00 DefaultSal (ppt) ---- 30.00 TempMode ----------MEASURED DefaultSoundSpeed (m/s) ---- 1515.80 CellBegin (m) ------ 0.99 CellEnd (m) ------ 1.99 BlankDistance (m) ------ 1.00 CellSize (m) ------ 5.00 Number of Cells -- 3 AvgInterval (s) ------ 60 SampleInterval (s) ------ 60

	0.5 MHz Sontek SL "Boomer" Velocity vs. Measured Velocity
Sontek SL 0.5 MHz - Argonaut User Setup DefaultTemp (deg C) -- 20.00 DefaultSal (ppt) ---- 30.00 TempMode --------- MEASURED DefaultSoundSpeed (m/s) ---- 1481.60 CellBegin (m) ------ 1.50 CellEnd (m) ------ 35.00 AvgInterval (s) ------ 60 SampleInterval (s) ------ 60	[larger image]

Above are the index velocity ratings created from the ADCP measurements collected on Nov. 8, 2006 as compared with the 1.5 MHz Sontek SL (A) and the 0.5 MHz Sontek SL "Boomer" (B)

Statistical Analysis

t-Test: Paired Two Sample for Means
	Variable 1	Variable 2
Mean	729.544	749.0508
Variance	4488372	4531711
Observations	8734	8734
Pearson Correlation	0.983229
Hypothesized Mean Difference	0
df	8733
t Stat	-4.68562
P(T<=t) one-tail	1.42E-06
t Critical one-tail	1.645028
P(T<=t) two-tail	2.83E-06
t Critical two-tail	1.960236

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Above left is a photograph of a typical ADCP discharge measurement. Above right is a photograph of the gage site at Jewfish Creek. The 1.5 MHz Sontek SL is attached to a vertical 2" PVC pipe on the station while the 0.5 MHz Sontek SL is installed on a separate tripod to the south of the gage box (not shown). [click on images for larger versions]

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Conclusions

From December 23, 2006 through March 26, 2007, there were 8,734 data points examined over a range of tidal velocity conditions (-1.9 ft/s to 2.84 ft/s). Velocity data collected from the 1.5 MHz and the 0.5 MHz acoustic Doppler velocity meter were highly correlated during the calibration performed on November 11, 2006 and over the three month period of data collection listed above. Uncertainty in the index ratings developed from the November 11, 2006 data were due to wind conditions that can minimize a typical tidal cycle routinely observed at Jewfish Creek. For example, the maximum positive velocity observed during the ADCP calibration equaled 0.55 ft/s versus a maximum negative velocity of 1.87 ft/s. A paired sample T Test was performed on the rated discharges from each instrument with no significant difference in the mean discharge. The study has shown that the accurate computation of discharge at Jewfish Creek using a 1.5 MHz acoustic meter with a 1.0 meter cell when compared to a 0.5 MHz unit with a 35.0 meter cell are equivalent. Follow up calibration measurements incorporating a more representative tidal cycle should support the above findings.

For more information:

Poster presented April 2007, at the National Surface Water Conference and Hydroacoustics Workshop

Related information:

SOFIA Project: Freshwater Flows to Northeastern Florida Bay

SOFIA Project: Coastal Gradients of Flow, Salinity and Nutrients