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HomeMy WebLinkAboutFDP201900012 Correspondence 2020-05-20SHIMP ENGINEERING, P.C. Design Focused Engineering May 21, 2020 Benjamin Kaiser, PE, CFM Revisions Manager CopmassPTSJV FEMA LOMR Review Regarding: Case No. 20-03-0443P — Cliff Fox LOMR Dear Benjamin, We have revised Case No 20-03-0443P — Cliff Fox LOMR per your comments provided on March 17, 2020 in Summary of Additional Data Required to Support a Letter of Map Revision (LOAM). In this letter you will find a narrative of the application detailing the mapping strategy and recent changes, as well as a detailed response to each of your review comments. Second Submittal Narrative This LOMR application has been adjusted since the previous submittal to provide more accurate topographical modelling. This was achieved by creating a TIN surface model of the original survey points provided by the licensed surveyor using the Carlson software system. We then used Carlson to generate cross sectional data from the TIN model for each of the following cross sections: Sta. 2860, Sta. 2965, Sta. 3026, Sta. 3136, Sta. 3166, and Sta 3186. We then used LIDAR data to complete the portion of the cross sections that were not surveyed. The LIDAR data only accounts for the outer portion of the sections, which well above the 0.2%-annual-chance flood elevations. Thus, LIDAR data is not used to determine flood elevations, but were only used to complete the outer portion of the sections. The only other cross section that has been revised is Sta. 3284. The only revision to this section is the elevations within the stream banks, as this was the only portion that was surveyed. We reviewed the portions of the cross sections against the LIDAR data, and concluded that Sta 3284 was an acceptable estimate of the topography at that cross section, excluding the revised channel. One important point to note about the mapping at the culvert section: the boundaries look erratic here because the width was mapped based on the elevation as resulted from the model. Since the road is crowned, and has curb and gutter, the elevations mapped laterally across the culvert cause the resulting boundaries. For reference, at the upper end of the culvert, the WSE is 675.08 for the 1% annual, and 675.90 for the 0.2% annual; at lower end of the culvert, the WSE is 674.91 for the 1% annual, and 675.47 for the 0.2% annual). To finish the mapping between sections 3284 & 3334 (limit of upstream revision) and between 2860 & 2365 (limit of downstream revision), we connected the 1%-annual-chance flood, 0.2%-annual-chance flood, and the floodway boundaries to smoothly merge the revised boundaries with the current effective boundaries. The conclusion for this flood study remains that based on the surveyed topography, the 1%-annual-chance and 0.2%-annual-chance floodplains should be revised in the following ways within the limits of analysis: -the floodplain elevations are generally lower -the floodplain boundaries should generally be shifted west, to the right-hand overbank of Powells creek (relative to HEC-RAS Cross Sections). 912 E. High Sr. Charlottesville, VA 22902 1434.227.5140 1 shimp-engineering.com -the floodplain area on the development parcel is reduced -the floodplain area on the adjacent parcels served by Haden Ln is increased -the floodway is narrower and is now better centered on the surveyed stream centerline. One final administrative item, we wish to acknowledge the email sent by Daniel Koehler, EIT on Thursday April 30, 2020 at 7:41 AM that the fee exemption request was granted for this LOMR, and that there is no fee required for this application. Comment Response Based on the comments listed in "Summary of Additional Data Required to Support a Letter of Map Revision (LOMR)", which are listed in grey below, please refer to our responses listed in bold below. The issues listed below must be addressed before we can continue the review of your request. 1. Our review revealed that the submitted models and plans do not have descriptive names. Please label the effective plans as "Effective Multiple Natural Run" and "Effective Floodway Analysis" and label the revised plans as "Revised Multiple Natural Run" and "Revised Floodway Analysis." Please remove any extra plans and unlabeled plans from the HEC-RAS project file. We are assuming that the effective hydraulic plans are also pre -project conditions hydraulic plans for this case. If not, then please provide clearly labeled plans as "PreProject Conditions Natural Run" and "PreProject Conditions Floodway Analysis." The pre -project conditions model should reflect any modifications that have occurred within the floodplain since the date of the effective model but prior to construction of the project for which the revision is being requested. If no modification has occurred since the date of the effective model, then this model would be identical to the corrected effective model or duplicate effective model. Please submit a pre -project conditions hydraulic model for Powells Creek or verify that the submitted effective model represents existing conditions. We agree that the names were not adequately descriptive. These files & plans have been revised in the following manner: Changed file name to "Powells—Effective" for the files that represent the effective (and pre -project) conditions. The Effective Plan names are "POWELLS CREEK Effective M1 Floodway" for the current floodway analysis, "POWELLS CREEK Effective M4 Floodway" for a current additional Floodway analysis, (Note: this was provided by FEMA, however we are unsure if this plan is relevant. We left this plan in the effective project to match original data from FEMA) "POWELLS CREEK Effective Multiple Natural" for the current floodplain analysis. Changed file name to "Powells_Revised" for the files that represent the revised conditions shown in the LOMR. The Revised Plan names are "POWELLS CREEK REVISED FLOODWAY" for the revised floodway analysis, "POWELLS CREEK REVISED MULTIPLE NATURAL" for the revised floodplain analysis. There is no pre -project condition required, as this is represented by the effective model. The revised analysis reflects the as -built topography around the Jarmans Gap Culvert Crossing. Virginia Department of Transportation replaced the culvert after the effective model was adopted by FEMA in 2005. However, in this LOMR there are no future alterations planned in the revised model — this revision only represents as - built conditions. 2. Our review revealed that there are increases in the 1-percent-annual-chance (base) flood elevations (BFEs) when we compared the revised (post -project) conditions model with the effective (preproject) conditions model. Paragraph 60.3 (d)(3) of the National Flood Insurance Program (NFIP) regulations prohibits encroachments within the adopted regulatory floodway, including fill, new construction, substantial improvements, and other development, unless it has been demonstrated that the proposed encroachment would not result in any increases in flood levels within the community during the base flood. Please review these cross sections carefully. If your analysis reveals that increases do not occur, please submit a revised pre -project and/or post -project conditions hydraulic model demonstrating that no increases occur. The sections have been carefully reviewed, and are mapped precisely. The results are still generally the same as the first submittal — the BITE increases slightly (max. 0.36') in the area near the Jarmans Gap Rd culvert crossing. the BFE mapped extents also shift, and are wider on the western side of Powells Creek in the area downstream of the culvert. This represents the current topographic data, and to the best of our knowledge, is correct. 3. The submitted certified culvert plan, entitled "Cliff Fox LOMR Culvert Plan", prepared by Shimp Engineering, P.C., certified December 20, 2019, did not reference the vertical datum. Please provide certified plans of the as -built hydraulic structures that reference the vertical datum such as the North American Vertical Datum of 1988 (NAVD 88) or the National Geodetic Vertical Datum of 1929 (NGVD 29). The vertical datum of NAVD 88, along with the benchmark, is now shown on the Culvert Plan. 4. Our review revealed that there are interpolated cross sections at Cross Sections 2965 and 3136 in the revised conditions HEC-RAS hydraulic model. Please revise the model to include the field run and aerial geometries of the above -mentioned cross sections, or remove them from the model. Interpolated cross sections are not appropriate at locations where the bounding cross sections have greatly varied geometry. These cross sections have been remapped based on the field survey data, and are no longer interpolated. 5. According to the HEC-RAS Hydraulic Reference Manual, the typical contraction and expansion loss coefficients are equal to 0.3 and 0.5, respectively, at bridge and culvert Sections 2, 3, and 4 and are equal to 0.1 and 0.3, respectively, at all other sections. Please revise the submitted revised hydraulic model so that the contraction and expansion loss coefficients are equal to 0.3 and 0.5, respectively, at Cross Section 3334, or provide an explanation why the contraction and expansion loss coefficients used in the model were chosen. Contraction and expansion loss coefficients at Cross Section 3334 have been revised to he 0.3 and 0.5, respectively. 6. The submitted topographic work map, entitled "Cliff Fox LOMR Work Map -Cross Secuons do Surveyed Topo," prepared by Shimp Engineering, P.C., certified December 20, 2019, does not provide some of the essential information required to complete our review of this request. Please submit a revised topographic work map, certified by a registered Professional Engineer (P.E.), which shows all applicable items listed in Section C of Application/Certification Form 2, entitled "Riverine Hydrology and Hydraulics Form," including the following information. All items should be labeled for easy cross-referencing to the submitted revised conditions hydraulic model. Noted, see individual responses below: a. The effective and revised 0.2-percent-annual-chance floodplaln delineation is not shown on the work map. Please show the boundaries of the currently effective conditions 0.2-percent-annual- chance floodplain as they are shown on the Flood Insurance Rate Map (FIRM) panel 51003CO229D and show the revised 0.2- percent -annual -chance floodplain. For clarity, please show the effective and revised delineations in different line types and color. Effective and revised 0.2-percent-annual-chance floodplain delineation is now shown on the work map, with different linetypes and colors. b. Please show smooth graphical tie-ins between the revised and effective flood hazard boundary delineations at the upstream and downstream ends of the revised reach. Please ensure that the revised delineations tie-in directly to the effective delineations and that the tie-ins occur a short distance upstream of the upstream most cross section in the revised conditions hydraulic model and a short distance downstream of the downstream most cross section. For clarity, please clearly mark the locations of upstream and downstream tie-in location on the work map. Please ensure that there is no revision in the revised model and floodplain delineations outside the tie-in locations marked on the work map compare to the effective hydraulic model and effective floodway and floodplain delineations. Smooth graphical tie-ins between revised and effective flood hazard boundaries are provided a short distance above the upstream revised cross section and a short distance below downstream revised cross section. These are now marked on the work map. There are no revisions beyond the tie-in points. There are no resulting BITE differences beyond the limits of study. c. Please show and label the topographic contour information used for the boundary delineations of the revised base floodplain and 0.2—percent-annual-chance floodplain on both overbanks. Contours are not shown on the right overbank. Please ensure that enough contours are labeled so that the floodplain delineations can be verified. Contours on the right overbank are now shown and labelled. d. Please relabel the revised delineations as "Revised" instead of "Proposed" on the work map as culvert installation is already done. Noted, this is updated throughout. e. Please show and label the locations and alignments of all cross sections used in the hydraulic model that are within the revised area. All revised cross sections are now shown. f. Please show the stream centerline as one line and for the entire revision area. The stream line is not shown for the cross sections upstream of the Jarman's Gap Road. Please clearly show the stream line and if the revised stream centerline is different from the effective stream centerline, please ensure that the revised stream centerline ties -in to the effective centerline at the upstream and downstream ends of the revised reach. Noted, the surveyed stream banks were shown. This has been replaced with the centerline for the entire revision area. g. Please reference the vertical datum such as NAVD 88. NAVD 88 vertical datum now referenced. h. The topwidths of the base floodplain and floodway computed in the revised conditions hydraulic model do not match the floodplain and floodway topwidths shown on the topographic work map at the cross sections listed below. Please revise the work map or hydraulic model as appropriate to resolve these discrepancies. The geometry of the cross sections in the revised conditions hydraulic model should reflect the topography 4 shown on the work map. We have utilized better software to profile and map the cross sections. The new sections are accurate and the updated topwidths have been reviewed and are now consistent with the HEC-RAS revised model. 8. Please submit an updated annotated FIRM that shows the revised boundary delineations of the base floodplain, 0.2-percent-annual-chance floodplain, and regulatory floodway as shown on the updated work map and how they tie-in to the boundary delineations shown on the effective FIRM at the downstream and upstream ends of the revised reach. Please use different colors to differentiate the proposed and effective boundary delineations. Also, please show the title block of the effective FIRM on the annotated FIRM. An updated annotated FHtM has been provided which shoes revised boundary, 0.2% change floodplain, floodway, tie-ins, and a legend. Linetypes, colors, and lineweights have been updated for clarity. 9. Effective February 20, 2015, the Federal Emergency Management Agency revised the fee schedule for reviewing and processing requests for conditional and final modifications to published flood information and maps. The current fee schedule is available for your information on the FEMA website at https://www.fema.gov/flood-map- related-fees. In accordance with this schedule, the fee for your request is $8,000 and must be submitted before we can continue processing your request regarding installing the box culverts at Jarman's Gap Road. Payment of this fee must be made in the form of a check or money order, payable in U.S. funds to the National Flood Insurance Program, or a credit card payment (Visa or MasterCard only). For identification purposes, the case number referenced above must be included on the check or money order. Per our email correspondence, finalized on Thursday April 30, 2020, we are now exempt from the $8,000 fee. 10. Our review of the submitted draft property owner notification revealed that the draft needs revising prior to distribution. Once we are confident that there will be no further changes to the modeling and/or mapping, we will provide our comments on the draft so that it can be finalized and distributed. Please do not distribute the property owner notifications until it is approved by us. Noted, we will await your comments on this. If you have any questions about the updated LOMR application or any of the revisions herein, please contact me at: keane@shimp-en ing eering com or by phone at 434-299-9843. Regards, 00. ar %"IF,IM Keane Rucker, EIT Shimp Engineering, PC