TABLE OF CONTENTS Steady state salt intrusion models can be divided into three types, depending on their derivation: low water slack (LWS) models, e.g. Ippen and Harleman (1961); tidal average (TA) models, e.g. Van den Burgh (1972); and high water slack (HWS) models, e.g. Savenije (1989). A LWS model is calibrated on measurements carried out at LWS and a HWS model with measurements carried out at HWS. For the calibration of a TA model, the average salinity has to be derived from measurements carried out during a full tidal cycle at several points along the estuary axis, which, if done well, is very elaborate and time consuming.
Although HWS models are not commonly used, they are the most practical models because the best time instant to carry out a salt intrusion measurement is at HWS; this is for the following reasons:
The moment that HWS occurs is easily determined. The observer measures the salinity, when the in-going current slacks. Although the same advantage applies to LWS, the accessibility at LWS is generally poor. The estuary is shallow and sometimes, it is very difficult to reach the waterside. Inaccessible mud flats often separate the river from the banks.
If the salinity at the downstream boundary is not known, it can easiest be estimated at HWS. At HWS, the salinity at the estuary mouth is generally equal or almost equal to the sea salinity, which is not, or not much, affected by...
