hello everyone!!!
we are doing rainfall runoff mdelling Using SWAT.
we have two gauging sites where levels are measured.
one locations is weir and another one is controll crossestion.
both the locations are very close to sea. levels measured at both the sites are affected by the sea level variations.
i would like to correct these levels before using it for callibration and validation (we have stage disgharge relationship to convert levels into discharge) .
Note: we dont have the sea levels data.
I have not tried to do this. Most places have approximate tide elevations by date and hour calculated by moon influence. Wind speed and direction can affect tides. You should probably determine the exact elevations when tidal influence begins, and how much time after especially high tides like new or full moon does it take to drain the brackish water and return to freshwater. Since freshwater is less dense, obtaining salinity values is unlikely to help due to poor mixing. During non-storm periods, you can probably recover flow data by smoothing the recession curve before tidal influence with recession curve when tidal influence is gone.
If tidal influence is regular, such as twice a day, you might be able to use the past daily tide tables and using periods of low freshwater flow, determine the influence of the tides in reversing flow direction, flooding weir or upstream sections or floodplain with tidal water, and then the time it takes when the tide drops to return to baseflow. You might need to calibrate flows over a weir that has backwater, rather than a free drop in elevation. The flow velocities through culverts are different between inlet control, and outlet control, partially to fully submerged.
If your long term goal were only measuring freshwater response at these sites, an efficiently placed dike downstream at high tide elevation may be used to control salt water entry. A water control structure sometimes called a trunk, can limit salt water entry, but allow for freshwater passage at lower tide. Another option could be to raise the weir outlet so salt water influence is avoided or less frequent. Some of the advantages of a weir are lost if the v notch control is affected by backwater. As the backwater elevation increases, the effective cross section of the notch is reduced from free fall by gravity to push back by water, velocity slows.
Periods of storm hydrographs affected by high tides would be most difficult to recover. Storm hydrographs not affected by tides will be the best data. Flow data affected by tides will likely be of lesser quality. Placement of a transducer or other water level indicator upstream of tidal influence and downstream of the weir and/or gauged cross section would help differentiate better the timing of salt water influence and help improve quality of recovered data.
I do not know of any model that can help, but would not be surprised if there was one that could be calibrated or modified to assist. If you have stream gauging equipment or an agency such as US Geology Survey, I think a respectable level of calibration and recovery of flow data would be possible.
I appreciate and recommend the explanation of Dr. William. Usually to overcome the effect of wave action due to wind, the water level indicator will be housed within a closed chamber or by inserting a pipe. However if the water level fluctuation is due to tidal influence, then you have to follow the directions of Dr. William.
Dear Rathod:
First, I totally agree with William suggestions. I think that with your available data you can try to model the reach between the sea and the gauge stations, for instance, using HEC RAS if the problem is a 1D problem or HECRAS 2D instead. It is not the same if the tidal varies 1 m or 5 meters. Depending on the amount of the variation of the variables you could not consider the wind effect and the model precision will vary. Regards.
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