I am creating my own impervious layer, as the NLCD layer is not as current or detailed as my ortho-photo. Do I need to include roads in the impervious layer? Thanks.
That is your choice. Most impervious layers I have worked with do include roads.
The impervious layer simply breaks out dry land categories into developed and undeveloped. If the "protect developed" flag is set then developed lands are assumed protected from inundation effects and remain dry lands. This can look a bit strange if low-lying developments are combined with high SLR scenarios (developed land persisting but surrounded by water). If protect-developed is true then the developed lands also form a hard barrier against marsh transgression. However, marshes can generally migrate inland past developed lands unless the elevations of those dry lands prevent water from connecting to the lands behind them.
Best regards -- Jonathan
Thanks for the quick response Jonathan. So most likely major roads would be protected and minor roads not so?
That seems a reasonable assumption! -- J