Re: Where to deploy an AWAC near a harbor entrance

The following post is taken from a recent email about locating an online AWAC for wave measurements.  After writing it I realized it would be of interest to many others out there.  I welcome others to comment with differing opinions or add further considerations:

I have been asked to contact and perhaps assist on finding the optimum location for your AWAC. There are several things to consider in the problem and in the end you will have to weigh the options.

The following considerations should be made:

1) Any wave measurement guage placed in front of a breakwater will measure both the reflected and incident waves. This will obviously be worse for the AWAC if deployed in front of the breakwater and least effected along the axis of the breakwater. Any location between will result in included reflected waves energy dependent on the breakwaters reflection characteristics to wave height, period, and direction.

I should note that reflected wave energy will bias the wave direction. I will not go into the details now, but you can see that severity of the problem if you download the paper (N4000-716 "Resolving Transformed Wave Directions Near Coastal Structures") from our bibliography:

http://www.nortek-as.com/en/knowledge-center/bibliography


2) The AWAC may be located along the axis of the breakwater. This, however may mean that it is in the channel of shipping traffic. I would not say that this is a huge issue but two things to consider is (a) ships passing above the AWAC will block the acoustic surface tracking measurement and lead to suspect data, (b) Any planned dreding will mean that the AWAC will have to be moved, (c) if sedimentation is heavy in this location then a periodic inspection should be included in the maintanance plan - however this may be checked by evaluating chaning tilt and acoustic amplitude return (reduced when burried).



I like to think that there is a balance between the two options that can be found, where the AWAC could be positioned near a channel and a little off of the breakwater axis. This would lead to only mind reflection during times of uncharacteristic wave directions and perhaps a safe distance from the channel.

I am attaching an image showing where an AWAC has been located at Hwa Lien harbor in Taiwan. I believe they had the luxury of several hundred meters of cable but not necessarily much more then you. They did not have sedimentation nor dredging but did have to locate it where fishermen where not operating nor anchoring. The data has been good for over 8 years, and is similar to 5 other location around the country.


As for the waves from passing ships, this is something that you are not going to avoid. The waves will likely propagate over the AWAC no matter where it is located. Our experience is that you will see some of the long waves (but low amplitude) and some of the bow wake which is again low amplitude. The fact that it is low amplitude means that it will only be noticeable when the sea state is calm - this is usually the time when waves are less of a concern. Issues associated with passing ships can usually be identified easily by comparing with other wave measurements over time - i.e., they will appear as a spike.

Kind regards,

Torstein

PS the AWAC in the image is located in the faint red box in the lower left