New Fairfax County Requirement — Surveying Downstream Ponds

Fairfax County announced in a March 31, 2003 Letter to Industry new policies and procedures for land development projects upstream of existing impoundments. All first submission site plans, subdivision plans, rough grading plans, bonded grading plans (where the limits of disturbance have not been included in the subdivision plans), public improvement plans, infill plans, and minor site plans, will require a pre-construction bathymetric survey of certain ponds downstream of the site prior to plan approval. This policy is in response to several fights in the last few years over how much sediment left certain construction sites and was deposited in downstream ponds.

The new requirements are effective May 12, 2003. They include impact area determination standards, developer/engineer responsibilities and Fairfax County DPWES responsibilities. The intent of the new policy is to quantify the pre-construction condition of downstream impoundments so that future conflicts regarding “impacts” to those impoundments can be minimized.

Most significantly, the new policy will require a bathymetric survey (conducted using specific U.S. Army Corps of Engineers standards) of impoundments located downstream of development projects to be conducted prior to plan approval to establish existing conditions. The amount of additional work and time can be significant and requires some specialized surveying techniques and equipment. In addition, developers or engineers will be responsible for: (1) identifying all downstream water impoundments within the Potential Influence Area (PIA)1; and (2) providing certification on the plan that indicates if a water impoundment is or is not located within the potential influence area of the proposed land-disturbing activity. In addition, under certain circumstances, post-bathymetric surveys may be required.

Developers may be released from performing pre- and/or post-development bathymetric surveys if: (1) the owner of the water impoundment does not respond within 30 calendar days to the developer's written request to perform a pre-development bathymetric survey; (2) the owner of the water impoundment does not allow the developer reasonable access to perform the bathymetric survey after reasonable efforts to secure that access by developer have been made; or (3) the requirement is waived or modified in writing by EFRD on behalf of the Fairfax County DPWES Director.

While this new policy will require additional up-front effort to establish pre-construction bathymetric data on downstream impoundments, benefits to the development community should be realized. For example, WSSI has been involved in many regional projects in which both real and perceived sedimentation issues could have been easily resolved at much less expense had pre-development data been established. With procedures such as these, countless man-hours can be saved on modeling pre-development conditions of ponds, sampling the in-situ sediments, and in arguing with owners about pond conditions prior to the commencement of upstream construction.

For further policy details refer to Fairfax County's “Policy and Procedures for the Evaluation of Downstream Impoundments".

If you have specific questions on this topic, contact: Frank Graziano at fgraziano@wetlandstudies.com or Bill Nell at bnell@wetlandstudies.com.



  1. Defined as: (1) the point at which the total drainage area is at least 100 times greater than the maximum proposed disturbed area, or (2) an offsite water impoundment downstream of the proposed land-disturbing activity where the cumulative total water surface area of the downstream water impoundment(s) exceeds 10 percent of the proposed disturbed area.