COUNTY OF ALBEMARLE

PLANNING STAFF REPORT SUMMARY

 

Project Name:  SP 2006-00011 Mosby Mountain Stream Crossing

Staff:  Tamara Jo Ambler

Planning Commission Public Hearing: 

July 11, 2006

Board of Supervisors Public Hearing: September 6, 2006

Owners:  Evergreen Land Company

Applicant: Beights Development Corporation

Acreage: 40.898 (open space within the existing Mosby Mountain subdivision)

Special Use Permit for:  Amendment to previously approved special use permit in accordance with Section 30.3.05.2.1(2) and 30.3.05.2.2(3) of the Zoning ordinance to allow fill in the floodplain to allow a culvert stream crossing in a Flood Hazard Overlay District.

TMP:   90E-A0

Location:   Approximately 200 feet from the intersection of Ambrose Commons Drive and Old Lynchburg Road (Route 631)

Existing Zoning and By-right use:  R-1 Residential – low density residential uses; FH Flood Hazard Overlay District – agricultural, recreational, and utility location uses which will not pose a danger to life or property in the event of a flood; EC Entrance Corridor Overlay District – all uses permitted by right in the underlying districts 

Magisterial District:  Samuel Miller

Conditions:  Yes

Proposal: The applicant proposes to install a quadruple (4-barrel) 10X10 box culvert instead of the previously approved ConSpan prefabricated bridge over a tributary to Biscuit Run.

Requested # of Dwelling Units:  N/A

 

DA (Development Area): X

RA (Rural Area)

Comprehensive Plan Designation:

The stream divides the Development Areas in Neighborhood 5 from the Rural Areas in Rural Area 3.  The stream crossing is located within the Development Areas portion of the site.

Character of Property:  The site is within the Mosby Mountain subdivision.  The stream crossing would complete the northern entrance (Ambrose Commons Drive) into the subdivision from Old Lynchburg Road, and allow residents to access the interior of the subdivision from this northern entrance.  This northern entrance and the interior portion of Ambrose Commons Drive are constructed except for the stream crossing.  Currently the only access to the interior of the subdivision is by means of Singleton Lane, which is accessed further south on Old Lynchburg Road.

Use of Surrounding Properties:  Adjacent property to the east, across Old Lynchburg Road, is the location of the Southwood Estates Mobile Home park, zoned R2 Residential.  Adjacent property to the west is vacant property zoned RA approved for a subdivision of 37 lots (Mountain Valley Farm).  Adjacent property to the north contains a horse farm zoned R1 Residential.  Adjacent property to the south is vacant property zoned PRD Planned Residential Development.  The immediate area can be generally characterized as low-density residential.

Factors Favorable:  The Virginia Department of Transportation considers a box culvert as an acceptable alternative structure to the ConSpan proposed under the original special use permit.

The applicant has coordinated unsuccessfully with the Virginia Department of Transportation to obtain approval of a specific crossing design utilizing the ConSpan structure.

Factors Unfavorable:  Staff has determined that there would be potentially greater long term impacts to the stream from the box culvert versus the ConSpan.

The applicant would need to provide a new floodplain analysis for the box culvert to assure that there would be no rise in the elevation of the 100-year floodplain.

RECOMMENDATION: Staff recommends approval of the amendment to SP 2001-065 with revised conditions


 

PETITION:  Amendment to a previously approved special use permit (SP 2001-065) in accordance with Section 30.3.05.2.1(2) and 30.3.05.2.2(3) of the Zoning ordinance to allow fill in the floodplain to allow a culvert stream crossing in a Flood Hazard Overlay District.

 

PLANNING AND ZONING HISTORY:  On March 26, 2002 the Planning Commission recommended approval of the original special use permit request (SP 2001-065) to install a ConSpan bridge crossing at this location to provide a northern entrance to the Mosby Mountain residential development (Staff Report – Attachment G; Minutes – Attachment F).  Relevant prior history can be found in that attached staff report.  On May 1, 2002 the Board of Supervisors approved the request (Action Letter – Attachment C).  On March 30, 2004 the Planning Commission recommended approval of an extension of the special use permit (SP 2003-087), since the stream crossing had not yet been constructed due to delays in obtaining approval from the Virginia Department of Transportation on the specific design of the ConSpan crossing.  However, it was later determined that the extension was not necessary because of the extent of construction that had occurred on the Mosby Mountain development, and this extension request was subsequently withdrawn prior to hearing by the Board of Supervisors. 

 

STAFF COMMENT: 

Staff’s full analysis of a stream crossing at this location can be found in the original staff report (Attachment G).  The applicant is now requesting to amend the special use permit (2001-065) to allow the construction of a quadruple (4-barrel) 10X10 box culvert in place of the ConSpan bridge that was originally approved.  The applicant has indicated that significant unexpected issues with the use of the ConSpan at the site have arisen and have made the approval of the bridge by the Virginia Department of Transportation, and the actual construction of the bridge, difficult.  Specifically, the applicant has advised that the soils at the site are fine and unstable, which would require greater stream disturbance than expected because VDOT would require the installation of pilings for the foundation of the ConSpan and the installation of riprap scour protection the length and width of the streambed under the bridge.  The applicant has submitted various designs for the ConSpan crossing over the course of several years, but has not been granted approval for a specific crossing design that satisfies VDOT’s concern for foundation stability and scour protection.  Staff requested clarification from VDOT regarding whether or not a ConSpan structure would be approved at this location.  In an email dated May 15, 2006 (Attachment I) Mr. Michael Viar, Hydraulic/Plan Review Engineer with the VDOT Culpeper District office indicated that “as long as the proper foundation is designed for the footings and scour protection is provided, a ConSpan structure will be acceptable.  However, a box culvert will also satisfy the hydraulic requirements for this roadway.”   In a letter dated May 19, 2006 (Attachment J) Mr. James Utterback, Residency Administrator with the VDOT Charlottesville Residency advised that “a standard box culvert…would provide an acceptable alternative.  Therefore, the Department strongly recommends the County allow the use of a standard box culvert at this site.”  Mr. Utterback’s statement and recommendation were also in the applicant’s special use permit application as being the recommendation of Mr. David Pearce, Structure and Bridges Engineer with the VDOT Culpeper District office.

 

Because it appeared that conflicting recommendations were being received from VDOT, the County Engineer spoke with Mr. David Pearce to obtain clarification of VDOT’s recommendation for the use of a box culvert at this site.  Mr. Pearce clarified that he had no issue with the use of either the ConSpan or the box culvert system – his primary concern is for the unstable soils which were identified by the geotechnical report for the area adjacent to the ConSpan foundation, and the need to provide adequate scour protection.  The scour protective measures that have been submitted by the applicant attempt to protect the foundation while minimizing impacts to the stream.  Mr. Pearce indicated that adequate scour protection can only be obtained by armoring the stream channel under the ConSpan with a thick layer of riprap, and that a design and analysis including that method of scour protection had not yet been submitted to VDOT by the applicant.  It should be noted that the conditions of approval of the original special use permit would not prevent the installation of this scour protection.   

 

The County Engineer has determined that there would be a notable difference in long term stream impacts between the uses of the box culvert and the ConSpan structures, even with the extensive scour protection under the ConSpan.  Although the installation of this riprap would result in immediate impacts to the natural stream channel, there are still advantages to using the ConSpan.  The bridge will still provide for a large open channel through the structure.  The riprap scour protection can also be shaped to provide a primary or normal flow channel through the center of the span.  Over time some resemblance of a natural streambed may be restored.  In addition, the stream channel upstream and downstream of the bridge can remain undisturbed.  By contrast, once four box culverts are installed the open channel will be permanently replaced with four concrete channels.  These culverts are intended to flush sediment through them, so there is less likelihood of the redevelopment of a streambed within the culvert.   Significantly more grading and stream disturbance upstream and downstream of the box culverts will be required, because the upstream area will need to be graded to transition and distribute flow to the four culverts.  Similarly, the downstream outflow will need to be transitioned from the four culverts back to the single natural channel.  Riprap will need to be installed at the inlet and outlet ends of the box culvert for erosion control and velocity dissipation.  In addition, the applicant will need to submit a new flood plain analysis for the box culvert to assure that there will be no rise in the floodplain elevation.  The applicant has indicated that the proposed box culvert provides the same hydraulic opening as the previously approved ConSpan and will not result in a rise in the floodplain elevation.  However, these computations have not yet been submitted to staff for review and confirmation.

 

The general debate between the uses of box culverts versus bridge spans is ongoing and well documented.  While it is generally believed that culverts are more detrimental to streams than are bridges, and that culverts are more economical than bridges, the controversy remains to be resolved through additional studies.  (Source:   North Carolina State University, Raleigh - Department of Civil Engineering – Study Abstract A Comparison of the Impacts of Culverts Versus Bridges on Stream Habitat and Aquatic Fauna).  No quantified comparison of stream impacts or costs was provided for this project by the applicant for the amendment request.  The applicant did submit a qualitative report developed by an environmental consultant to compare the environmental impacts of a span versus a box culvert at this location.  The report concluded that the permanent impacts to the stream from a span structure and a box culvert would be the same.  However, the report discussed the use of a double barrel box culvert, and not the much larger quadruple barrel box culvert as proposed, and therefore the impacts from the larger box culvert structure were not considered.

 

To provide a basic illustration of the difference between the ConSpan and a 4-barrel box culvert, standard drawings are shown below for each:

Figure 1 – ConSpan detail drawing (Source:  ConSpan website)

 

Figure 2:  Four-Barrel Box Culvert detail drawing (Source:  VDOT Structure and Bridge Standards)

 

 

The following photographs show representative examples of the two types of structures as installed in the field.  The ConSpan was recently installed at Old Trail Drive within the existing Old Trail development in Crozet.  The three-barrel box culvert was installed several years ago at Mill Creek Drive, off of Route 20.

 

 

 

Photograph 1:  Transition of stream into a ConSpan bridge within the Old Trail development.

 

 

 

Photograph 2:  View of stream channel through the ConSpan

 

Photograph 3:  Downstream of ConSpan structure

 

 

 

 

Photograph 4:  Transition of stream into a three-barrel box culvert at stream serving Mill Creek Drive. 

 

Photograph 5:  Close up of transition into center barrel from photo above

 

 

Photograph 6:  Downstream of three-barrel box culvert

 

 

 

 

SUMMARY:

The County Engineer has determined that there would be greater long term stream impacts if a quadruple box culvert were installed instead of the ConSpan structure, even with the extensive scour protection under the ConSpan.  Therefore, staff continues to prefer a span structure over a box culvert structure.  However, staff acknowledges the efforts expended by the applicant thus far to obtain VDOT approval of a site design utilizing the ConSpan.   Staff coordination with VDOT indicates that applicant has not yet submitted a design and analysis to VDOT showing full armoring of the streambed under the ConSpan, and if such a design were submitted, it would be acceptable to VDOT.  Therefore staff recommends that the applicant must first submit to the Virginia Department of Transportation one final proposal for the ConSpan structure with the riprap scour protection provided under the extent of the structure.  If VDOT indicates in their review of this final submittal that the riprap cannot satisfactorily address scour concerns and does not approve the final proposal in a timely manner, then the quadruple 10X10 box culvert may be installed in lieu of the ConSpan.  The box culvert must be cast in place or otherwise constructed to provide for countersinking of one barrel of the culvert to provide for normal and low flows in a manner satisfactory to the approved U.S. Army Corps of Engineers water quality permit.

 

RECOMMENDED ACTION:

Staff recommends approval of the request with the following revised conditions:

 

1.      The applicant must first submit to the Virginia Department of Transportation one final proposal for the ConSpan structure with the riprap scour protection provided under the extent of the structure.  If VDOT indicates in their review of this final submittal that the riprap cannot satisfactorily address scour concerns and does not approve the final proposal in a timely manner, then the quadruple 10X10 box culvert may be installed in lieu of the ConSpan

2.      Albemarle County Engineering Community Development Department approval and VDOT approval of final grading plans and bridge or box culvert and road plans and computations;

3.      Albemarle County Engineering Community Development Department approval of an Erosion and Sediment Control Plan to include stabilization of fill;

4.      Albemarle County Engineering Community Development Department approval of mitigation plans for disturbance of the stream buffer;

5.      If utilized, the ConSpan bridge system must be installed per manufacturers’ specifications.  If a box culvert is utilized it must be cast in place or otherwise constructed to provide for countersinking of one barrel of the culvert to provide for normal and low flows in a manner satisfactory to the approved U.S. Army Corps of Engineers water quality permit.

6.      The final subdivision plat should reflect any changes to the floodplain and floodway limits, and tThe applicant must provide computations supporting any such changes to the floodplain and floodway limits, as well as copies of the correspondence demonstrating FEMA approval of the revised floodplain or no changes in flood plain levels can occur.

7.      In an effort to minimize environmental degradation, no soil shall be removed from the stream to compensate for any fill.

 

ATTACHMENTS:

Attachment A: Location Map

Attachment B: Application

Attachment C: Action letters dated April 6, 2004, May 9, 2002, and March 28, 2002, respectively

Attachment D: Minutes from March 30, 2004 Planning Commission meeting

Attachment E: Minutes from May 1, 2002 Board of Supervisors meeting

Attachment F: Minutes from March 26, 2002 Planning Commission meeting

Attachment G: Original staff report for SP 2001-065

Attachment H:  Staff report for extension (SP 2003-087)

Attachment I:   May 15, 2006 email from VDOT Culpeper District office

Attachment J:  May 19, 2006 letter from VDOT Charlottesville Residency office

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