Stream/Storm Surge Gauge
Build and deployment guidance for using an ultrasonic distance sensor to monitor stream level and storm surge.
Overview
The 3D-PAWS Stream/Storm Surge Gauge uses a MaxBotix MB7363 or MB7364 HRXL-MaxSonar-WRLS ultrasonic distance sensor mounted above the water surface.
The sensor measures the distance between itself and the water surface. Changes in that distance can be used to monitor changes in water level.
Typical applications include:
stream stage monitoring
flood monitoring
storm surge monitoring
The MB7363 provides a measurement range of up to approximately 10 m, while the MB7364 provides a range of up to approximately 5 m.
How the Sensor Works
The ultrasonic sensor emits an acoustic pulse toward the water surface and measures the returning echo.
As the water level rises, the distance between the sensor and the water surface decreases. As the water level falls, the measured distance increases.
Because the measurement depends on an unobstructed acoustic path, the location and mounting geometry of the sensor are important.
Instruction Slides for Assembling the Stream Gauge
Use these slides for step-by-step assembly of the ultrasonic sensor and associated components.
Stream Gauge Frame
The mounting frame can be adapted to different installation environments. The sensor should be positioned so that its acoustic beam has a clear path to the water surface.
Ultrasonic Sensor Boom Geometry
The ultrasonic sensor emits a narrow acoustic beam. Nearby structures such as the mast, bridge, seawall, or other mounting surfaces can produce unwanted reflections if they enter the beam path.
The boom should extend far enough from nearby structures to keep the sensor's acoustic beam clear between the sensor and the water surface.
For planning purposes, this guidance assumes a conservative 20° full beam angle.
Simple Rule for Boom Length
If the mast is installed at the edge of the measurement area:
Every 1 m of sensor height requires approximately 0.18 m of horizontal boom extension.
where:
L = minimum boom length (m)
h = sensor height above the water surface (m)
Recommended Boom Lengths
1 m
0.18 m
0.25 m
2 m
0.35 m
0.45 m
3 m
0.53 m
0.60 m
4 m
0.71 m
0.80 m
5 m
0.88 m
0.95 m
6 m
1.06 m
1.15 m
7 m
1.23 m
1.35 m
8 m
1.41 m
1.55 m
9 m
1.59 m
1.75 m
10 m
1.76 m
1.95 m
The recommended values include approximately 50–100 mm of additional clearance to account for mast movement, uneven surfaces, and variations in beam spread.
If the Mast Is Set Back from the Edge
If the mast is installed some distance from the edge, add that distance to the calculated boom length:
where:
L = minimum boom length (m)
h = sensor height above the water surface (m)
dₑdge = horizontal distance from the mast to the edge (m)
The sensor must have a clear acoustic path to the water surface. Before installation, check that the mast, boom, bridge, seawall, vegetation, or other nearby objects do not enter the sensor's measurement beam.
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