Technical Note No. 98/3 - Task 2 (Time Series) Explained; A Warning About "Water Depth"

Technical Note No. 98/3 - Task 2 (Time Series) Explained; A Warning About "Water Depth"

When deployed in still water (i.e. no waves), DOBIE experiences a total pressure that is composed of two parts: one part due to the weight of the overlying water (hydrostatic pressure) and another part due to the weight of the overlying atmosphere (atmospheric pressure). When waves are present there is yet another, fluctuating, pressure component that is due to the waves. When executing task 2, DOBIE stores burst time series of total pressure.

"Water depth" (we will explain the quotes shortly) is computed by the PEDP from total pressure as:

where h(t) is the water depth time series in metres, p[t] is total pressure time series in pounds per square inch sensed by DOBIE, p_a is atmospheric pressure also in pounds per square inch and z_p is elevation of DOBIE above the bed. g is acceleration due to gravity (9.81 m/s²), r is water density (1025 kg/m³ typical seawater density) and F is a factor (6895 kg/[s²m]/psi) used to convert pressure in pounds per square inch to pressure in kg/(s²m). The PEDP requests from the user values for DOBIE elevation, water density and atmospheric pressure.

The time series of water depth, as defined, will only be true water depth when there are no waves present. This is because the conversion from pressure to depth as stated does not account for the way the fluctuating component of pressure that is due to the waves is attenuated by depth.

Note that the same conversion is used by the PEDP on Task-3 data (Tide Gauge), but in this case the water depth that results is the true water depth because the conversion is applied to a time-averaged pressure. This is so because the time average of wave pressure fluctuations is zero. Also note that Task 4/0 (Wave Gauge) properly accounts for the attenuation of wave-induced pressure fluctuations in calculating wave statistics.

MOG
June, 1998