Directional Spreading Index, n and DMWD

Torstein,

How to derive:
1. Directional spreading index, n
2. Maximum deviation from mean wave direction, DMWD (deg)

Thank you.

Lee

Hello Lee,

Could you give me a little more information about what you are interested in deriving? I am not familiar with the directional spreading index or DMWD.  Maybe these quantities have another name?

kind regards,
Torstein

Torstein,

I am using Mike 21 (from DHI) to model wave condition along coastal waters. I need the directional spreading index, n and maximum deviation for mean wave direction, MWDM value. Please have a look at the attached picture in compressed format.

Thanks.

Lee

Hello Lee,

Thank you for the screen shots of the MIKE 21 software, it was helpful in understanding your question.

From what I can see, the Spreading Index is a function of the Directional Standard Deviation (DSD).  This is presented in the table of image p1.  The DSD is simply what we call the Spreading.  If you are using the QuickWave software this value is reported at the peak frequency and found in the file *.WAP.

This means that you will have to use the values in the table of spreading index to convert from the spreading estimates to the spreading index.

Unfortunately, Maximum deviation from mean wave direction, DMWD (deg) is a parameter that we do not report.  Do you have a description of this value in the MIKE 21 software?  Maybe we can find a way so that you may get this value " />'>

I hope this answers your questions,
Torstein

Hi Torstein,

Nevermind the description in Mike 21. I need to verify how true it is that any value of Hmax divide by Hs is 1.85 ?

Two of my consultants from Bangladesh (contractual project consultant) told me that the data collected using AWAC AST is not true as the value Hmax/Hs is only 1.5+.

I have the WaveExtract software. I have downloaded the QuickWave from your support page. If it is not too troublesome can you please mail to me the serial number to hllee@nahrim.gov.my. Thank you.

Hello Lee,

I must apologize that I have not seen your question and answering until now (2 years later!!!).  Very sorry.

 

I suspect that you have probably moved on to other projects by now, but for the sake of our other forum browsers, I will add my comments.

 

The relationship between Hs and Hmax is one that is quite variable.  I think that if you jump to our bibliography page (http://www.nortek-as.com/support/bibliography), and download the paper by Justin Vandever which is titled "Evaluation of wave parameter estimates in coastal environments", you will see a pretty clear description of the problem.

 

Justin documents quite clearly that there is quite a bit of variability with this relationship.  In fact, he shows that it ranges from rougly 1.6-2.0.  This is quite broad and shows that the traditional approach of just scaling Hs to Hmax may not be adequate. 

 

This scaling has commonly been used for bottom mounted wave instruments that do not have a direct measure of the sea surface (such as the AST).  Instead a scaling constant such as 1.67 has been used to take these inferred estimates and attempt at getting an estimate of Hmax.  This once again shows how useful a the Acoustic Surface Tracking really is

-Torstein

Dear Lee,

I have been using MIKE 21 NSW model for the same purpose and I'm having your problem too. Have you find out any solution? How did you find the relationship between Spreding index and DSD?