In 2008, the National Academies Press released a book titled ‘Mapping the Zone: Improving Flood Map Accuracy. The book is available for free if you are willing to read it online here: http://www.nap.edu/openbook.php?record_id=12573
In September 2012, Brandt and Lim wrote a paper titled ‘Importance of river bank and floodplain slopes on the accuracy of flood inundation mapping.’ The paper can be downloaded from this site: http://hig.diva-portal.org/smash/record.jsf?pid=diva2:553169
The conclusions from these two documents seem to tell a different story.
‘Mapping the Zone’ has this statement in its conclusion: “Improving the accuracy of flood maps by using higher-quality topographic data as well as updated hydrology and hydraulics enables communities to more accurately portray flood hazard and mitigate the risk to existing structures.”
Brandt & Lim have this statement in their conclusion: “Intrinsic uncertainties possessed by flood risk maps were investigated by relating the disparity distances and the average side slopes between a modeled map and reference data. The results show that although the quality of the DEM impacted the flood extents, the characteristics of the slope of the floodplain, perpendicular to the river flow, affected the ambiguities of the boundaries produced. In flatter regions, uncertainties in flood predictions were greater, regardless of the resolution of the DEM used. In flat plains, this uncertainty becomes infinite and restricts the capabilities of the hydraulic models in delineating the desired inundation extent thus limiting the reliability of flood risk maps for providing accurate information for flat areas” (emphasis added)
Granted, it should be noted that Brandt & Lim used HEC-RAS which is a 1-dimensional model. However, this model is widely used. ‘Mapping the Zone’ talks extensively about mapping in coastal areas which tend to be fairly flat areas. The results from Brandt & Lim certainly raise some questions about the benefits of increased resolution data which are becoming more and more available through the USGS and state-level GIS data repositories.
I’ve given short thrift to both documents which are worth reading. The results and conclusions from Brandt & Lin do raise some important questions. Comments on this are encouraged.