HomeMy WebLinkAboutSP201000038 Legacy Document 2012-06-28 (9)pF A
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COUNTY OF ALBEMARLE
Department of Community Development
401 McIntire Road
Charlottesville, Virginia 22902 -4596
Phone (434) 296 -5832 Fax (434) 972 -4126
Project: Badger Industrial [SP201000038]
Plan preparer: Mr. Justin Shimp, PE; Shimp Engineering, P.C.
Owner or rep.: HMC Holdings, LLC
Plan received date: 5 July 2011
Date of comments: 15 July 2011
Reviewer: Phil Custer
Engineering has reviewed the FEMA models associated with the second submittal of the Special Use
Permit Application for fill within a floodplain (SP- 2010 - 00038). The models were received 5 July 2011.
The following comments require further clarification from the applicant or modifications to the model:
1. For each model ( "Corrected," "New," and "with Fill "), please list each modification you have
made to the existing model. For instance, in the "Corrected" model, please note how many feet
each side of the floodway limits were moved and for which cross - sections. Another example, for
the "New" model, please clarify which models were updated with county topography (it looks as
though none of the existing cross - sections in any of the new models were) and what "among other
items" meant in the Readme file. It is difficult to determine what modifications were made to each
file by exploration.
2. Why does adding the three new crosssections (without fill) raise the floodplain elevation
throughout the model? The difference between the output tables of "Existing" and "New" is
between 0.6ft and nearly 2ft. This seems to indicate that something may be wrong with the entire
model if a crossection, several thousand feet upstream, is affected by simply entering new cross -
sections at the Badger site.
3. Similarly, the "with Fill" floodplain model shows higher flood elevations throughout the whole
model than the "New" model. I believe FEMA requires the study be carried to the point where the
impact of the fill has no affect on the existing flood elevations
4. Though the fill reduces the flood carrying capacity of the river, the model "with Fill" shows that
the floodplain is actually lowered in the area of fill when compared to the "New" model. This is
counterintuitive. Was there another modification or assumption in this model that might affect the
floodplain elevation in this area of fill?
5. When determining the floodway location, it seems Method 4 or 5 should primarily be used, as
stated in the Hec -RAS user's manual. These methods require that the floodplain carrying capacity
be reduced equally on both sides of the cross - section. In one of the cross - sections, the floodway
has been moved farther south and east away from the proposed development. It seems that Hec-
RAS can generate the location of these limits of the floodway internally and that the user override
(Method 1) should only occur at the very last step for "further refinement ": "The input data for
method 5 consists of a target water surface increase and a target energy increase. The program
objective is to match the target water surface without exceeding the target energy." I would
suspect FEMA would not accept any modification to the floodway that was not primarily relying
on Method 4 or 5. The modication to the floodway should be reviewed /approved by FEMA before
the models are submitted to the county for review.
7 -15 -2011 2nd submittal FEMA Model Review Memo.doc