Removing tidal aliasing in estuarine discharge data; the big reveal

Justin Stockley - Xylem Analytics , 28 June, 2018

A major waterworks in South Malaysia draws its source water from the lower reaches of the Johor River. During high tides and in dry seasons, a salt water wedge pushes far enough upstream, causing saline raw water to affect water supply operations. A dam was constructed such that the upstream water could be regulated and selectively released to manage this issue, however with the development of the surrounding area it was recognised that there was a need to have a better understanding of the existing baseline flows along the major rivers. An expected outcome of this study was to determine travel times of released dam water to the water works in order to optimise dam operations. 

A series of gauging stations was established along the river which measured water level and velocity. The peak discharge from the dam release was clearly tracked through the first two stations. The third station was affected by tidal flux and as such the movement of the peak discharge was effectively masked. This paper looks at the methods employed to collect the information as well as the subsequent analysis of the data to assist the operation of the dam and waterworks. In particular, the use of a low pass digital filter to remove tidal aliasing is demonstrated and a broader context comment on the handling of discharge data subjected to tidal influences is discussed referencing current USGS policy.