Hi. I'm using delft3d Flow to simulate a storm surge event on Manila
Bay, Philippines. The tidal simulation has good results but when I
added the wind component of the storm, the result goes bad. Attached
is a picture of the results using QUICKPLOT. The blue line is the
tidal simulation (astronomic forcing only, no wind) and the red line
line is the cyclonic model with a spiderweb file used for wind. There
is a dip in water level that should have occurred. Please help.
Attached here is my model.
Hi. What do you think is wrong with your results?
There shouldn't be a deep decrease in water level before the storm
surge, as simulated at July 16.
If needed, here are my boundary conditions and wind model as well.
A few things to check like: your horizontal viscosity is very low
for a model of this scale, think about maybe 50 rather than 1. In the
wind file, the wind is in knots, should it be in m/s??? The area
covered by the wind file is quite large compared to your quite small
model area so perhaps there is not enough resolution / detail of the
wind? Is the wind direction convention correct (i.e. blowing to or
Perhaps someone with more experience of this kind of study might come
along and have some thoughts as to whether your model area is too
small to reproduce what you are trying to model?
Not sure if there is anything wrong with the model. If you look at
the figure that I attached, you'll see that Manila experienced strong
offshore winds at the moment (July 16th 00h00) you see the dip in
water levels. These offshore winds are causing wind set-down in Manila
Bay. The model does probably overestimate the set down somewhat, as
the effects of land roughness (which typically reduce offshore blowing
winds) in the Holland wind model of Delft Dashboard are not taken into
account. Your model inputs look fine to me (don't worry about units of
wind speed (indeed knots, as Phil pointed out) in the cyc file, as
Delft Dashboard converts these to m/s when creating the spiderweb
file). The one thing I would change are the settings for the wind drag
coefficient Cd. It is now commonly excepted that the wind drag
coefficient increases up to 25-30 m/s and then starts to decrease
again. I have used the following setting in the mdf file in many
Wstres= 1.0000000e-03 0.0000000e+00 2.800000e-03 2.5000000e+01
Thank you so much for the insightful response, sir Marteen. However,
the model would then diverge significantly from the actual data I'm
comparing it with, as to assess its validity. Attached is the picture
(and data file) of the actual measured data as compared with the
forecasted tidal level generated by Delft dashboard at IHO tide
stations. While there is a dip in water level, it isn't as significant
as the simulation's. I'm not sure if the effect of surrounding
topography is enough to explain such large discrepancy.
Your positive surge is pretty well reproduced, so the model isn't far
off. In this context, for the negative surge, Maarten's suggestion of
the lee effect of topography is sensible. Once you've dealt with
that, along with changing wind drag in line with his suggestions and
increasing hev in line with mine, then getting the positive surge
profile spot on becomes simply a part of the normal calibration process.
Your model resolution is very low by the way; seemingly not important
in this case for modelling levels, but if you want flow speeds it will
need to increase significantly. Good luck. :-)
Thank you, sir. Will implement these changes. On the resolution, this
is actually just my macro-grid. I will still nest a finer micro-grid
(upto 500x500m resolution at the coastline) focused on my study area.
Hopefully, my research gets approved soon. Really appreciated the
feedback, quick and on point <3 <3
When you do come to refine your grid, make sure that orthogonality
and smoothness are good, and also check your timestep in Quickin by
using the Courant Number function. For your present coarse grid, for
example, you can probably have a much larger timestep than the 0.5
minutes you're using. Although the model probably runs very fast
anyway, as you have very few cells! :-) Good luck. Phil.
Just noticed that you're using spatially-varying bed roughness with
Manning's n values of about 0.1 inside the bay. That is extremely
high! It should be more in the order of 0.024. When I run with a
spatially constant value of 0.024, the peak surge level goes up to
~1.2 which is more in line with the observations. The set-down just
before the peak surge also increase a bit, which of course is not
good. Unfortunately, it is not straightforward to fix this issue with
current versions of Delft3D and Dashboard. They simply do not take
into account the land effect on the wind field...
Sorry, I'm back again. My delft3d seems to be encountering an error
during nesting. It can create the observation file for the macro-grid
properly. Attached is the picture of the micro-grid within the
macro-grid. When I run the nesting tool (1), it only creates one
observation point very far from the micro-grid.
Here are the grid and enclosure files of the macro-and micro-grid, if needed.
Why are you nesting? Why not just increase the resolution of the
macro grid? You have plenty of scope to do so.
The example given at the manual (attached) were also overlapping. At
this point, editing the macro-grid just seems too much work as
everything else would have to follow. Also, I managed to capture the
nesting command line during processing and here's what it reads (see
2nd attached screenshot).
I believe the forcing in the bnd file needs to be set to time series
(T) instead of astronomic (A) for the first step of the nesting to work.
West Z T 1 17 1 2
5.0000000e+002 South Z T 2 1 37 1
5.0000000e+002 East Z T 38 2 38 26
The overall and detailed grid are indeed usually overlapping.