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Archive for November, 2012

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.

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I was fortunate enough to be able to attend the FGM workshop.  There were a number of great talks and the organizers of the event have done a nice job of making all the PowerPoint presentations available online here:

http://wrrc.umass.edu/fgm-presentations

If there were one file to view, I would suggest looking at Mike Kline’s presentation.  A good chunk of this blog has been about various tools that are available to better understand the hydrologic and geomorphic settings of a project site.  What I like about Mike’s presentation is that it highlights the benefits that can be realized from a fluvial geomorphic assessment.  FEMA’s flood insurance maps are (as the name suggests) designed for the insurance market.  A fluvial geomorphic assessment and resulting fluvial erosion hazard map, indicates the areas within a valley context that are subject to erosion hazards.  In some settings, this will be narrower than a mapped FEMA floodplain, and in other places, it will be wider.  Vermont’s fluvial geomorphic assessments and subsequent river corridor plans provides water quality, habitat and public safety benefits.  They provide important context for restoration projects and they help prioritize restoration projects as well.  That’s the real benefit of the program.  Getting to where Vermont is took several years, dozens of partners were hired to walk the streams and load the data into the database, and several VT DEC staff have worked hard at leveraging that data by writing river corridor plans and making the data available via an online map viewer.

The main thrust of the FGM workshop was to start to generate some interest in developing a program similar to what Vermont has developed.  I’m keeping my fingers crossed that Massachusetts will be able to follow suit.

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