DPWES Letters to Industry
Land Development Services
12055 Government Center Parkway
Fairfax, VA 22035-5503
703-324-1780, TTY 711
March 13, 2006
Water Quality Impact Assessment for Water Dependent Development in a Resource Protection Area
Issue: Application of Section 118-2-1(a) of the Chesapeake Bay Preservation Ordinance (CBPO) for storm sewer or conveyance channel outfall improvements within a RPA with respect to the submittal and Director approval process of a WQIA.
Background: Outfall structures of storm sewer systems and stream bank stabilization measures, among other facilities, are included in the CBPO as “water-dependent development”, which is ‘the development of land or a facility that cannot exist outside of a RPA and must be located within a RPA, either in whole or in part, by reason of the intrinsic nature of its operation’ [CBPO 118-1-6(ff)]. When these structures are proposed to be located within a RPA, approval of a WQIA by the Director is required [CBPO 118-2-1(a) and 118-3-3(a)]. The purpose of the WQIA is to demonstrate that the proposed RPA encroachment is necessary and that impacts are identified, minimized, and mitigated to the maximum extent practicable. The design details of storm sewer system outfalls and conveyance channels must be included in construction plans as defined in Chapter 104 of the County Code. These same details are also to be included in the WQIA, which upon approval, must subsequently be incorporated into the final approved construction plan.
Most all WQIAs for outfall improvement or construction contain similar information and mitigation methods. The clearing limits are restricted around standard storm drainage easements, super silt fence is used during construction, revegetation of disturbed areas with ground covers, and channel stabilization with biostabilization materials or other energy dissipators have been consistently addressed in each WQIA for a storm system outfall within a RPA. In an effort to reduce review and processing time, and eliminate redundant review steps, incorporation of the WQIA directly into the construction plan for review and Director approval is desirable. This will also reduce inadvertent omissions of design details on the construction plans.
Procedures: Effective immediately, development plans proposing construction of a storm sewer or conveyance channel outfall in a RPA may include the required WQIA within the development construction plan for Director approval. The WQIA is to be incorporated in the overall stormwater management plan, erosion and sediment control plan and landscape plan addressing the required items of CBPO 118-4-3. The assessment shall be labeled on the development plan as ‘Water Quality Impact Assessment Narrative’ and shall:
- Display the boundaries of the RPA as well as include a description of the source of the RPA boundary, e.g., adopted County map or approved RPA delineation study.
- Display (on the grading plan) and describe the location and nature of the proposed RPA encroachment and impacts to the RPA (in narrative form), including the area of the clearing and grading (i.e., in square feet).
- Provide a justification statement for the proposed encroachment and impacts to the RPA.
- Display and describe the extent and nature of any proposed disturbance or disruption of wetlands.
- Display and discuss the type and location of proposed best management practices to mitigate the proposed RPA encroachment or adverse impacts.
- Describe the extent to which the proposed construction will comply with the performance criteria of the CBPO.
- Describe the runoff in the outfall in terms of flow rate (in cubic feet per second) and velocity (in feet per second) for the 2-year, 2-hour storm event and the 10-year, 2-hour storm event.
- Describe, quantify and display any erosion protection measures used in the channel, such as rip rap energy dissipaters, or to protect the RPA buffer from degradation due to point discharge or sheet flow discharge.
- Provide an outfall channel profile, cross-sections (with equal horizontal and vertical scales) and information consistent with Chapter 5 of the latest edition of the Virginia Erosion and Sediment Control Handbook under the ‘Determination of Adequate Channel’ section.
The “Plan Approval Information” chart, Item #8 Water Quality Impact Assessment ‘Required’ box on the plan Cover Sheet shall be marked and the ‘Comment/Sheet No.’ shall contain a note as to which plan sheet contains the narrative statement.
With review and Director approval of the construction plan, approval of the WQIA will be deemed to have been met.
If you have any questions or need additional information, please call Valerie Tucker or Qayyum Khan at 703-324-1720, TTY 711.