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Archive for January, 2019

Back in the 1970’s and 1980’s, the U.S. Army Corps of Engineers had the foresight to know the value inherent in floodplains. Consequently, over the course of several years, this agency went about purchasing floodplain land in the Charles River watershed. The Army Corps coined the term “Natural Valley Storage Areas” (NVSA’s) which is indeed what intact and functional floodplains do. The homepage for this project can be viewed here.

Fast forward to 2017 to when the Army Corps presented their Draft Master Plan for the project to the public. The Corps conducted an economic analysis of the benefits of these storage areas. The estimated prevented losses from flood damages was almost $12 million in unadjusted dollars. The Corps also estimated the economic benefits of visitors and recreation between $3.2 an $4.6 million.  Based on this analysis, one could say the natural capital in these areas protected areas is at least $15 million.

At the public meeting,  the Army Corps presented the hydrograph for the 2010 floods in late March and early April. As soon as I saw the data, I was reminded of the Otter Creek analysis I had written about.  My only problem with the analysis that the Army Corps presented, was that the discharge was not normalized by drainage area.  After obtaining the data from the gauges in Dover, MA and  Medway, MA, the story appeared to be pretty similar to Otter Creek’s.

The data show that for the upstream gauge in Medway, the peak unit discharge was 26.8 cubic feet per second (cfs) per square mile (csm) on March 31, 2010. Note how much more rapid the rising limb of the Medway unit hydrograph is compared to the Dover gauge.  On April 1st and 2nd the Medway runoff is receding, whereas the Dover runoff is still increasing, albeit at a slow rate.  The peak discharge in Dover occurs on April 2nd, but only at a unit runoff of only 15 csm.  Just as with the Otter Creek analysis, we can do a ‘what if’ analysis and simply assume that Dover had very similar unit runoff as Medway. Under this hypothetical scenario, Dover could have experienced a discharge of just over 4,900 cfs (26.8 csm X 183 square miles).  Fortunately, the peak mean daily discharge at the Dover gauge was 2,760 cfs, which is 56% of the theoretical peak, had the same unit runoff occurred throughout the watershed.  The two USGS gauges and the Natural Valley Storage Areas can be seen in this ArcGIS Online presentation slide.

A cursory review of the NVSA’s indicate that while floodwaters can and indeed are stored, it would be perhaps a bit too generous to attribute all of the flood attenuation between Medway and Dover to these areas. The 8,095 acres of storage area converts to 12.6 square miles, or roughly 6.9% of the Dover gauge’s watershed area.  In addition, at only 65 square miles, one would expect a certain amount of flashiness (i.e. rapidly increasing and decreasing flows) at the Medford gauge.  Nevertheless, as a thought experiment,  had the Natural Valley  Storage Areas been developed and the percent impervious area dramatically increased in these areas, one could safely assume that  the Dover gauge would very likely have seen a unit discharge higher than 15 csm.  The NVSA’s do provide important floodwater storage and the Army Corps analysis does indeed indicate a diminution in flood damages.

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