intro story Coast / Estuary

Coast / Estuary

Coastal systems are among the most dynamic physical systems on earth and are subject to a large variety of forces. The morphodynamic changes occurring to coastlines worldwide are of great interest and importance. These changes occur as a result of the erosion of sediments, its subsequent transport as bed load or suspended load, and eventual deposition. 
 
Estuaries are partly enclosed water bodies that have an open connection to the coast. Estuaries generally have one or more branching channels, intertidal mudflats and/or salt marshes. Intertidal areas are of high ecological importance and trap sediments (sands, silts, clays and organic matter).
Within the Delft3D modelling package a large variation of coastal and estuarine physical and chemical processes can be simulated. These include waves, tidal propagation, wind- or wave-induced water level setup, flow induced by salinity or temperature gradients, sand and mud transport, water quality and changing bathymetry (morphology). Delft3D can also be used operationally e.g. storm, surge and algal bloom forecasting. 
 
On this discussion page you can post questions, research discussions or just share your experience about modelling coastal and/or estuarine systems with Delft3D FM. 
 

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Sub groups
D-Flow Flexible Mesh
DELWAQ
Cohesive sediments & muddy systems

 

 

Message Boards

Onshore sed. trans. in Delft3D, and use of MORFAC in very shallow water

Ben Williams, modified 8 Years ago.

Onshore sed. trans. in Delft3D, and use of MORFAC in very shallow water

Jedi Knight Posts: 114 Join Date: 3/23/11 Recent Posts
Hi,

I have a question about the best way to get reasonably accurate morphology change in the near-shore zone. I have a 2D (depth-averaged) morphology simulation for a beach area that is micro-tidal (~1m), and a d50 of 0.7mm. The beach profile is pretty steep, as you might expect, down to a depth of about -2m MSL and then roughly 1 in 50 after that. The wave condition I am considering is roughly 1.5m Hm0 and 10 second Tp. The currents in the area are almost exclusively wave-driven, although there is a very small tidal component (~0.1m/s) from a nearby jettied river outlet.

I am having two issues that I'd like to ask:

1) It seems that, no matter what I do, I cannot reproduce onshore transport in Delft3D. I have found this in several other areas where you'd expect the gradual onshore migration of sediment in gentle wave conditions. Although admittedly the beach profile in this case is quite steep, I have an area of artificial shoreline 'drawback' where the contours are quite gentle and I still do not get onshore movement of sand in this location, even though in nature you would expect this. When I look at the sediment transport vectors, they are all along-shore or directed slightly offshore. I am using the Soulsby-Van Rijn transport formula as I have found this to give me quite reasonable answers in the past for areas of strong tidal currents and reasonable bathymetry gradients. Is there a mechanism for realistic onshore movement of sand in Van Rijn (2000) or Van Rijn (2004,2007)? Or is there something that I might need to change in the FLOW module?

2) A separate issue: I am trying to simulate 'annual morphology change' by use of a 'representative' wave condition and applying a morphological factor. I appreciate that there are quite some limitations to using MORFAC in the nearshore zone, but in my case the simulation runs at almost real-time so I don't have the luxury of running, say, 1 week/2weeks/28 days and then applying a MORFAC to that to get the annual change. I am limited to a 26 hour simulation (2 full tidal cycles) and then applying a rather large MORFAC (>70) to the results. I therefore get rather rapid morphology change that is quite unrealistic. Essentially any small currents seem to scoop out the sediment in the nearshore, causing large scour holes in the shallow water zone. This immediately feeds in to the hydrodynamics, sustaining its own current that directs the sediment to areas that you would not expect.

I am pretty confident that this issue is largely attributable to the MOFAC used, as when I output results every 60 seconds (4560 seconds prototype time), you can literally see the sediment crawling along the beach. This can't be good for the hydrodynamics as the bed level change will be quite large per hydrodynamic time step. Has anyone formed any guidelines for using MORFAC when trying to hindcast wave-driven morphology within very shallow areas? Is Delft3D even the best choice of model for this environment?

Kind regards,

Ben
Ben Williams, modified 8 Years ago.

RE: Onshore sed. trans. in Delft3D, and use of MORFAC in very shallow water

Jedi Knight Posts: 114 Join Date: 3/23/11 Recent Posts
Just to mention that I am aware of Roshanka Ranasinghie's paper in Coastal Engineering (Ranasinghie et al, 2011, Morphodynamic upscaling with the MORFAC approac: Dependancies and sensitivities. Coastal Engineering 58, pp806-811).

That paper I find a bit simplistic (whilst a good discussion), as in the majority of cases you are going to have a hump (such as a swash bar) in the very shallow zone. And typically you also look at sediment transport on beaches, which as the paper acknowledges, is rather more complex.

Is anyone else looking at this type of problem (critical MORFAC; how to get onshore transport in Delft3D)?

Cheers,

Ben
JH
Jeff Hansen, modified 8 Years ago.

RE: Onshore sed. trans. in Delft3D, and use of MORFAC in very shallow water (Answer)

Padawan Posts: 29 Join Date: 7/12/11 Recent Posts
Hi Ben,

In regards to your first question, the amount of cross shore sediment transport you get in a 2DH model is largely dependent on what value you choose for the "wave-related suspended transport factor" and "wave-related bed-load transport factor" in the morphology tab (BedW and SusW in the mor file). The default value of 1 is entirely unrealistic for a 2D model. In my experience a good starting value is ~0.25 for both. These are tuning parameters that allow you to make the transport be what ever you want since you can't resolve cross-shore wave transport in 2D model (i.e. just because you can get the cross-shore transport to agree with you observations it doesn't mean the model has physics right, just that you turned the knobs correctly).

For your second question a morfac of ~70 seems very high, but sounds like you don't have much of a choice. Something you could try is to just use a morfac of 1 and then try and upscale in post-processing which will remove the unrealistic feedback.

Good luck,,

Jeff
Ben Williams, modified 8 Years ago.

RE: Onshore sed. trans. in Delft3D, and use of MORFAC in very shallow water

Jedi Knight Posts: 114 Join Date: 3/23/11 Recent Posts
Hi Jef,

Thanks for the reply - it was useful. I have come to the conclusion that it is silly to apply a MORFAC in this particular occasion, and therefore will try alternative approaches.

With regards to trying to get the onshore sediment transport correct - I am sure that I am not the only one that has puzzled over this, and I am grateful for your suggestion of tweaking Sus, Bed, SusW and BedW. I had wondered what these were and they are not exactly described clearly (or easily found) in the manual. Once you know what they are, however, it is very clear and suddenly the comment on page 342 of the FLOW manual makes sense - although quite how you are supposed to notice this as a 2 line comment buried away in such a large manual, escapes me. And you don't always know what papers to go hunting the information in.

To promote discussion amongst other Delft3D users, I did a quick Google search (other search engines are available) and came across the following papers that describe calibrating the morphology file.

A great paper is here: http://journals.tdl.org/ICCE/article/viewDownloadInterstitial/1237/pdf_258

Another good one is this: Grunnet et al (2004) Process-based modelling of a shoreface nourishment. Coastal Engineering 51, pp 581-607
http://www.sciencedirect.com/science/article/pii/S0378383904000729#

Other Deltares reports are here.
Deltares (2008) report Z4479, Monitoring and modelling of a shoreface nourishment
Deltares (2004) report Z3748.21, Shoreface nourishment scenarios.

Another brief discussion of calibrating the MOR file free parameters is given in Coastal Planning and Engineering Inc (2006): South Palm beach-Lantana Shore Protection Study, Delft3D model calibration.
ftp://ftp.dep.state.fl.us/pub/ENV-PRMT/bma/2_Palm%20Beach%20County/Coastal%20Engineering%20and%20Planning%20Documents/200706_Final%20town%20of%20Palm%20Beach%20Erosion%20Control%20study/Appendix%20A%20-%20Delft3D%20Model%20Calibration%20Report.pdf

To quote Grunnet (2004) (pg 600), "A smaller value of fBED corresponds to less onshore bedload transport, and a smaller value of fSUS corresponds to less onshore wave-related suspended sediment transport. A physical interpretation of the combination of these parameters relates to the underpredicted undertow, hence to simulate the observed bed level changes, parameters were adapted in such a way to increase net offshore sediment transport."

What kind of "realistic" values have other Delft3D users had, for various environments (e.g. beach slope, D50 etc), after a thorough calibration of the morphological free parameters? Often you don't have data to calibrate your model with if it is a quick engineering study, so there must be a better approach than "Well this looks vaguely OK"....

Cheers,

Ben