Using FEMA’s recently adopted Pacific Coast Floodplain Mapping Guidelines, NHC produced new flood hazard mapping for over 50 miles of coastline in Juneau, Alaska. This was the first mapping project on the Pacific Coast to utilize these guidelines, thus it served as a pilot project where the procedures and subsequent results could be tested. NHC was also a major contributor to the development of these guidelines for FEMA.

A key component of the new Pacific Coast Floodplain Mapping Guidelines is the use of response statistics rather than event statistics. Previously, wind, water level, and waves corresponding to a 1-percent annual chance event were used to determine total water level. In the new procedure, total water level is determined by using data obtained from several large storms each year for 30 or more years. A statistical analysis is then conducted on this series of data, including measured winds, waves, and water levels that occurred during each storm event, to determine the 1-percent total water level. Based on both physical processes and measured data, this is a more robust approach, especially on the Pacific Coast, where a variety of correlated and uncorrelated processes can occur simultaneously to influence total water levels.

Two priority shoreline reaches were identified by the City and Borough of Juneau. Wave estimation in these areas required an examination of hydrodynamic conditions in Gastineau Channel, Auke Bay, and parts of Stephens Passage. This was accomplished using a 2D SWAN model. Once the modeled waves reach the shoreline, the total water level for each event is computed along a series of transects normal to the shoreline. A statistical analysis was then completed to determine the 1-percent annual chance total water level at each transect. Digital work maps were then prepared showing the appropriate coastal inundation limits and flood hazard risk zones.