Yjzhang at 17:31, 18 December 2014

2014-12-18T17:31:48Z

Yjzhang: Created page with "SELFE's wetting and drying capability has been carefully benchmarked and verified. There are 2 options: inunfl=0 as a 'light-weight' inundation scheme (you'd always start with..."

2014-12-18T17:31:02Z

Created page with "SELFE's wetting and drying capability has been carefully benchmarked and verified. There are 2 options: inunfl=0 as a 'light-weight' inundation scheme (you'd always start with..."

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SELFE's wetting and drying capability has been carefully benchmarked and verified. There are 2 options: inunfl=0 as a 'light-weight' inundation scheme (you'd always start with this option), and a 'fine-scale' inundation scheme (inunfl=1), which requires some care. In particular, since extrapolation is used with inunfl=1, you'd make sure that the grid is reasonably resolved in the wetting and drying regions. This scheme may also require a smaller time step to very accurately capture the inundation process.

The wetting and drying process is inherently nonlinear and challenging, and below are some tips.

==Bottom friction in shallow areas==

A critical parameter in shallow area is the bottom friction. Note that the bottom friction parameterizations are very different between 2D and 3D model, and so you cannot use same input. For details please read [http://ccrm.vims.edu/yinglong/wiki_files/Report-ChezyFlow-Sept2011.pdf this article].

From a physical point of view, the 2D and 3D models behave very differently. Consider a straight channel with variable depths, with flow coming from deeper part and going into shallower part. Fig. 1 shows the side views of 2D and 3D velocities.

[[File:WetDry-2Dvs3D.PNG|thumb|center|Fig. 1 Side view of channel flow for (a) 2D and (b) 3D cases. Volume conservation dictates larger velocities in shallow areas. Note that strong shear is possible in 3D model.]]

In 2D model, the velocity is depth-averaged and vertical shear is not represented. Strong friction merely translates into reduced velocity. In 3D model however, a large friction will lead to strong shear, although the depth integrated velocity value matches that from the 2D model. This problem is exacerbated by the use of terrain-following coordinates. This is because the log-drag formula, when bfric=1 is used, is:

Cd=[log(dz_b/z_0)/k]^-2

where z_0 is bottom roughness and dz_b is the bottom cell thickness. In shallow areas, dz_b~z_0 which leads to unrealistically large Cd. Note that Z-layer models do not have this problem as dz_b is not small there. A classical pathological velocity field obtained with SELFE is seen in Fig. 2. This problem is especially obvious when a large dt is used, as in the case of Fig. 2.

[[File:WetDry-issue-SELFE.PNG|thumb|center|Fig. 2 Noisy velocity field in shallow areas in SELFE 3D]]

From a grid generation point of view, you'd make sure that channels are not 'blocked', i.e., with at least 2 ''always-wet'' nodes.

Numerically, there is a quick fix to this problem by reducing Cd used in SELFE 3D. When using bfric=1, set a reasonable Cdmax=0.006, and in some tricky regions (e.g. with large acceleration or rapid bathymetric variations), you may need to further reduce Cd locally. If you used bfric=1, you can do so by simply setting the local roughness to a negative value (i.e. telling the code the value is not roughness but Cd!); we found -0.0025 to -0.001 usually works well.

==Depths at open boundary==
A very common crash is related to the wet/dry @ open boundary. SELFE does NOT allow the open boundary segments to become dry at ANY time. Therefore you need to make sure the depths there are deep enough compared to expected tidal range. An easy way is to impose a minimum depth near those segments.

Since the wet/dry rule inside SELFE is element-based, a node becomes dry if all surrounding elements become dry. As a result, you need to impose a minimum depth a few rows of elements into the domain, not just at open boundary nodes. This ensures that water can come into the domain without being blocked at the open boundary. On the other hand, wet/dry is allowed to occur at land/island boundaries.

@@ Line 1: / Line 1: @@
-SELFE's wetting and drying capability has been carefully benchmarked and verified. There are 2 options: inunfl=0 as a 'light-weight' inundation scheme (you'd always start with this option), and a 'fine-scale' inundation scheme (inunfl=1), which requires some care. In particular, since extrapolation is used with inunfl=1, you'd make sure that the grid is reasonably resolved in the wetting and drying regions. This scheme may also require a smaller time step to very accurately capture the inundation process.
+SCHISM's wetting and drying capability has been carefully benchmarked and verified. There are 2 options: inunfl=0 as a 'light-weight' inundation scheme (you'd always start with this option), and a 'fine-scale' inundation scheme (inunfl=1), which requires some care. In particular, since extrapolation is used with inunfl=1, you'd make sure that the grid is reasonably resolved in the wetting and drying regions. This scheme may also require a smaller time step to very accurately capture the inundation process.
 The wetting and drying process is inherently nonlinear and challenging, and below are some tips.
@@ Line 15: / Line 15: @@
 Cd=[log(dz_b/z_0)/k]^-2
-where z_0 is bottom roughness and dz_b is the bottom cell thickness. In shallow areas, dz_b~z_0 which leads to unrealistically large Cd. Note that Z-layer models do not have this problem as dz_b is not small there. A classical pathological velocity field obtained with SELFE is seen in Fig. 2. This problem is especially obvious when a large dt is used, as in the case of Fig. 2.
+where z_0 is bottom roughness and dz_b is the bottom cell thickness. In shallow areas, dz_b~z_0 which leads to unrealistically large Cd. Note that Z-layer models do not have this problem as dz_b is not small there. A classical pathological velocity field obtained with SCHISM is seen in Fig. 2. This problem is especially obvious when a large dt is used, as in the case of Fig. 2.
-[[File:WetDry-issue-SELFE.PNG|thumb|center|Fig. 2 Noisy velocity field in shallow areas in SELFE 3D]]
+[[File:WetDry-issue-SELFE.PNG|thumb|center|Fig. 2 Noisy velocity field in shallow areas in SCHISM 3D]]
 From a grid generation point of view, you'd make sure that channels are not 'blocked', i.e., with at least 2 ''always-wet'' nodes.
-Numerically, there is a quick fix to this problem by reducing Cd used in SELFE 3D. When using bfric=1, set a reasonable Cdmax=0.006, and in some tricky regions (e.g. with large acceleration or rapid bathymetric variations), you may need to further reduce Cd locally. If you used bfric=1, you can do so by simply setting the local roughness to a negative value (i.e. telling the code the value is not roughness but Cd!); we found -0.0025 to -0.001 usually works well.
+Numerically, there is a quick fix to this problem by reducing Cd used in SCHISM 3D. When using bfric=1, set a reasonable Cdmax=0.006, and in some tricky regions (e.g. with large acceleration or rapid bathymetric variations), you may need to further reduce Cd locally. If you used bfric=1, you can do so by simply setting the local roughness to a negative value (i.e. telling the code the value is not roughness but Cd!); we found -0.0025 to -0.001 usually works well.
 ==Depths at open boundary==
-A very common crash is related to the wet/dry @ open boundary. SELFE does NOT allow the open boundary segments to become dry at ANY time. Therefore you need to make sure  the depths there are deep enough compared to expected tidal range. An easy way is to impose a minimum depth near those segments.
+A very common crash is related to the wet/dry @ open boundary. SCHISM does NOT allow the open boundary segments to become dry at ANY time. Therefore you need to make sure  the depths there are deep enough compared to expected tidal range. An easy way is to impose a minimum depth near those segments.
-Since the wet/dry rule inside SELFE is element-based, a node becomes dry if all surrounding elements become dry. As a result, you need to impose a minimum depth a few rows of elements into the domain, not just at open boundary nodes. This ensures that water can come into the domain without being blocked at the open boundary. On the other hand, wet/dry is allowed to occur at land/island boundaries.
+Since the wet/dry rule inside SCHISM is element-based, a node becomes dry if all surrounding elements become dry. As a result, you need to impose a minimum depth a few rows of elements into the domain, not just at open boundary nodes. This ensures that water can come into the domain without being blocked at the open boundary. On the other hand, wet/dry is allowed to occur at land/island boundaries.

Simulating wetting and drying with SCHISM - Revision history

Yjzhang at 17:31, 18 December 2014

Yjzhang: Created page with "SELFE's wetting and drying capability has been carefully benchmarked and verified. There are 2 options: inunfl=0 as a 'light-weight' inundation scheme (you'd always start with..."