Technical Note No. 98/4 - Task 3 (Tide Gauge) Explained

Technical Note No. 98/4 - Task 3 (Tide Gauge) Explained

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 3, DOBIE samples pressure in bursts and at the end of each burst calculates and stores burst-averaged total pressure.

Mean water depth is computed by the PEDP from the mean total pressure as:

where

is mean (i.e. burst-averaged) water depth in metres, is mean (i.e. burst-averaged) total pressure 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.

Technical Note 98/3 explains how time series of water depth computed by the PEDP from time series of total pressure (Task 2) are not necessarily true water depths. This happens when waves are present and is because the conversion from pressure to depth used by the PEDP in that case does not properly account for the way the fluctuating component of pressure that is due to the waves is attenuated by depth. This is not an issue with time-averaged data because the average over time of the wave-induced pressure fluctuations is zero. Thus, mean water depth calculated by the PEDP from Task-3 data is in fact true water depth.

MOG
June, 1998