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Terrain Modeling in FEMA’s California Coastal Flood Studies
Jeremy Mull, BakerAECOM

FEMA coastal flood studies are large undertakings that often take advantage of a variety of accurate, high resolution data to analyze coastal vulnerability.  The 1% annual chance flood event for each community is determined with a combination of data including waves, tides, and bathymetry and topography.  A critical step in each study is the development of a terrain model, in which different survey datasets are combined into an accurate terrain surface that is used for coastal analyses.  Just as numerical models use source input data such as wave and wind data from offshore buoys to predict nearshore waves, terrain models incorporate source bathymetric and topographic data to approximate the geomorphology of the coastline.  Accurate terrain modeling is essential for a successful flood study and this article briefly describes some of the steps and challenges in this process. The figure below shows the general technical steps in a coastal flood study.

terrain photo 1.JPG

The California Coastal Analysis and Mapping Project/Open Pacific Coast (CCAMP/OPC) Study is utilizing a combination of source terrain datasets developed by various agencies at different resolutions.  The resolution of a particular dataset refers to the density of elevation points captured in the survey.  Higher resolution surveys have more data points per area, or smaller point spacing between each data point, and allow analysts to capture and analyze critical features.  Data used in the CCAMP/OPC Study include high resolution airborne topographic LiDAR data (1 meter point spacing), high resolution hydrographic survey data (2 meter point spacing), and moderate resolution bathymetric Digital Elevation Models (DEM; 10 to 100 meter point spacing).  Combining these datasets with different resolutions and accuracies into one surface presents unique technical challenges.  To build the foundation of the terrain for a particular area, engineers first develop a data hierarchy to prioritize the data from highest to lowest accuracy and resolution.  Each area is first filled with the highest quality data available and then gaps are subsequently filled with the next sequential dataset in the hierarchy.  In the CCAMP/OPC Study, all areas were first populated with the relatively accurate and recently collected LiDAR data and then the remaining bathymetric voids were filled with available hydrographic survey data.  An example of a merged bathymetric and topographic terrain surface for Santa Cruz County is shown in the figure below.

The timing of each survey can also impact how each terrain is modeled.  Because coastal flood studies typically cover large geographic regions, surveys are often conducted at different times depending on weather conditions, accessibility, funding, and other factors.  For example, LiDAR data for the CCAMP/OPC Study were collected in 2011, while nearshore hydrographic survey data were collected between 2004 and 2011 and offshore hydrographic survey data date back to 1920 in some areas.  Many beaches on the west coast are dynamic and can change seasonally so that combining data collected at different times can create data mismatches, or “seams”, at the edges of each source dataset.  These seams can be problematic since the equations used to model coastal flood processes such as wave runup, overtopping, and erosion are sensitive to the slopes and inflection points of beaches, bluffs, and structures.  If left in the terrain surface, seams can create erroneous slopes and artificial inflection points.  To address this issue and eliminate seams in the terrain surfaces created for the CCAMP/OPC Study, terrain analysts employed several techniques. For example, an analyst can shift the data merge location to an area where the source data match.  If that is not possible, the modeled surface can be interpolated across the seam.  Both techniques have been employed in the CCAMP/OPC Study to eliminate seams and avoid using older hydrographic survey data in the critical nearshore region.

Occasionally, the elevation data needed for a particular analysis are simply not in the data.  Airborne topographic LiDAR data is typically classified into two categories: (1) unclassified and (2) bare-earth.  Unclassified data include vegetation, buildings, and other structures while bare-earth data approximates the true ground surface.  Elevation points of coastal protection structures are critical to flooding analysis; however, since these points are often “unclassified”, they may not be represented in a terrain surface created from bare-earth data alone.  To obtain these elevation points for the CCAMP/OPC Study, FEMA reviewed the unclassified LiDAR data and identified coastal structures so they could be incorporated into the terrain models.  This procedure worked well for many larger structures that were adequately captured by the LiDAR survey; however, some narrower structures, particularly seawalls, were not captured fully in the survey.  Even well-designed seawalls are typically not wide enough to be fully resolved by LiDAR data with 1 meter spacing.  To obtain the correct dimensions (for example, crest elevation and width), engineers relied on field observations, aerial photographs, construction documents, and the limited coastal structure returns in the unclassified LiDAR data.  Using these sources in combination with the bare-earth data allowed engineers to accurately represent the dimensions of each structure for input to subsequent wave runup and overtopping analyses.

These are some of the challenges and complexities in terrain modeling for a FEMA coastal flood study. Like nearshore wave and water level modeling, terrain modeling is a key component to the study and accurate modeling is essential for quality flood elevation predictions.

 

 Coastal Beat Story Archive

 
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collapse Year : 2013 ‎(19)
<a href='http://www.r9map.org/Pages/EbulletinStory.aspx?storyID=27'>FEMA Partners with Oceanweather and Scripps Institution of Oceanography to Bring Modeling Expertise to CCAMP OPC Study</a>
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<a href='http://www.r9map.org/Pages/EbulletinStory.aspx?storyID=50'>Translating Coastal Flood Hazard Modeling Results into Floodplain Mapping</a>
<a href='http://www.r9map.org/Pages/EbulletinStory.aspx?storyID=60'>Terrain Modeling in FEMA’s California Coastal Flood Studies</a>
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<a href='http://www.r9map.org/Pages/EbulletinStory.aspx?storyID=56'>FEMA’s Annual Risk Awareness Survey: Findings from Previous Surveys and the Focus for the 2013 Survey</a>
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