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Dike

Started by epihua, April 09, 2013, 12:55:36 AM

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epihua

Hi,

According to the Technical Documentation, (1) enclosed wetlands are assumed to be maintained in their initial conditions until there is greater than 2 meter inundation, at which time small dikes are assumed to fail (page 18), and (2) when land is protected by a dike, the accretion or sedimentation is assumed to be zero (page 14).

Should I assume that any diked wetland will have no change in its elevation and extent until a 2-m (or higher) sea-level rise is modeled? If I am only interested in scenarios below 2 meters, could I simply exclude diked wetlands from the modeling domain?

Also, why is the dike height set to be 2 meters. Could I change it? Could I even make dike height specific to each sub-site?

Thanks,

Stephen



Jonathan S. Clough

This model limitation has been improved in the latest version of the model:  I will email to you a version of this model that is currently available by request only, awaiting final approval from our client before becoming publicly available.  More information follows --

In the traditional model, if a cell is defined as protected by a dike or levee it is not permitted to change. The existence of these dikes can severely affect the ability of wetlands to migrate onto adjacent shorelines. Diked wetlands are assumed to be subject to inundation when relative sea-level change is greater than 2 m, although that assumption can be changed. In SLAMM 6.2 it is also possible to enter the elevation of the levees or dikes on a cell by cell basis or to use a connectivity algorithm along with cell elevations to determine when a dike is overtopped.

Levee and Dike Data Input

The presence of dikes protecting wetlands and dry lands may be partially determined from NWI data. NWI dike data are often incomplete, though, especially for dikes that protect non-wetland areas. Additional data sources should be utilized to augment the NWI dike coverage such as USGS topographical maps, Army Corps of Engineers layers, and local sources of information.

Levees and dikes are entered as an input raster. Using the "classic" SLAMM dike model this input grid identifies protected cells (non-zero entries represent protected regions; zero or no-data entries otherwise). Not only dike locations, but also lands protected by these dikes must be specified as part of the dikes layer. This model assumes that these areas will not be inundated given RSLR below 2 m. For backward compatibility, this option is maintained.

In SLAMM 6.2 it is also possible to enter the elevation of the levees or dikes on a cell by cell basis. When levee elevations are provided with respect to NAVD88, SLAMM combines the DEM and levee/dike elevation data to obtain the overall elevation for each cell. In this case, only the levee locations must be specified, rather than identifying areas that are protected or unprotected by levees or dikes. During the simulation, SLAMM searches for water inundation paths using the connectivity algorithm that is checked by default.

Finally, it is possible to combine this new levee model with the classic SLAMM dike model in a single simulation. This can be useful when using mixed data sources. For example, NWI data are more compatible with the "classic" dike model as they indicate whether a wetland is protected by a
levee or a dike but do not include elevation data. To use both models simultaneously, both data types must be combined into a single raster with negative numbers representing regions that should be protected using the "classic" dike model, and a positive number representing dike elevations. When using both models combined, the user should characterize this hybrid raster as "dike location raster" within the file-setup interface.

If future plans for dike removal or dike addition are known, or can be estimated, a time-series of dike rasters may be specified. See the User's Manual for more details on this procedure.

epihua

Very grateful for your detailed explanation. It is exciting to know the improvement on dike features in the new SLAMM. My only concern is how to get the information of dike height. Except for deriving it from LIDAR data, do you know any methods or databases that may lead to a reliable regional map of dike height, particularly for the SF bay? 

Jonathan S. Clough

Derivation from LiDAR or from dike construction specifications or related data layers.  Unfortunately we have no data for SF bay.  Best regards -- Jonathan

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