Section

Comment

2.303 (D)

Repeat Comment: This section should include meeting the channel and flood protection requirements of 9VAC25-870-66 or meeting the Safe Harbor provisions as options for compliance for sediment basin discharges in lieu of analysis of the downstream channel to the limit of analysis.  This is stated in Minimum Standard 19(n) (9VAC25-840-40).

Follow-up:  This was added under item d. (natural conveyance systems), but the applicable compliance methods above would apply to manmade or restored conveyance systems as well.  This should be a separate item, not contained within item d.

3.302.3

Repeat Comment: The Guidance Document should clarify (as is done in Section 3.302.1(A)(1)) that the energy balance requirement applies only if a natural conveyance system is reached before the limits of analysis.  As an example, this is clearly explained in Section 11.5.2.1.7 of the VDOT Drainage Manual:

“The point where the limit of analysis occurs does not need to be included in the evaluation of adequacy, but the system from the outfall or point of discharge up to (but not including) the limits of analysis must be analyzed. If there is a natural channel anywhere in the system from the outfall up to the limit of analysis (but not including the point defining the limit of analysis), then the energy balance will apply.”

The suggested text keeps in line with the long-standing Virginia Statewide Stormwater Runoff Standard (Virginia Erosion and Sediment Control Handbook, Chapter 4) in which a receiving channel may be assumed adequate if the total drainage area to the point of analysis is 100 times greater than the contributing drainage area of the project site.  If the channel is adequate, there remains no reason or benefit to apply the energy balance reduction.  This section may also be improved by creating a list of specifically excluded water bodies, such as tidal waters, and rivers such as the James, the New, the Dan, the, Roanoke, and the Rappahannock.  However, at some point, the exclusion must still come back to a limit based on a comparison of the site area to the watershed area (considering the headwaters of these rivers) – in lieu of regulatory changes, this must be 100 times the site area, unless a locality has adopted more stringent technical criteria.

Follow-up: DEQ’s response did not address this change in technical criteria and long-standing state stormwater standard that conveyances receiving greater than 100 times the site area may be assumed adequate and excluded from contributing to the technical requirements that the site is subject to (i.e. energy balance).  The application of 9VAC25-870-66(B)(3) in the above described scenario violates and represents more stringent technical criteria than the limits of analysis per 9VAC25-870-66(B)(4).

3.302.3

Repeat comment: “A manmade lake or reservoir not created for the purpose of managing post-development stormwater should be considered a natural stormwater conveyance system.”

This modifies the definition of a natural conveyance system (as defined in 9VAC25-870-10) and adds an unnecessary and wasteful burden on development, particularly when the receiving waterbody has a watershed greater than 100 times the site area and is controlled by a release structure.  A suggested improvement would be to either clarify the applicability based on the 1% limits of analysis as described above in VDOT’s Drainage Manual and/or create a list of specifically excluded controlled water bodies such as Kerr Lake, Smith Mountain Lake, Lake Gaston, Lake Anna, Claytor Lake, etc.

Follow-up: DEQ’s response states that the definition of a natural conveyance channel is not modified with this guidance, however, the guidance definitively subjects additional systems (manmade lakes and reservoirs) to the more restrictive detention requirements of 9VAC25-870-66(B)(3).  Manmade lakes and reservoirs cannot reasonably be discerned as an intended subset of the definition of a “natural stormwater conveyance” in the current regulations and thus this represents new technical criteria and burden on development.

3.302.4

Repeat comment:  The 1-year storm is not a channel forming or flooding event in natural channel design parameters.  A more appropriate flood event to analyze and determine the flood-prone area would be the 2- or 10-year storm event.

Follow-up:  No change was made to this section of the document and no justification for the use of the 1-year storm event for channel forming or flooding analysis was provided.

3.305.1

3.305.2

Repeat comment: All references in these sections to 10-yr 24-hour storm velocity are inconsistent with the regulations.  The VSMP Regulations and all other VA state standards evaluate erosive velocities based on the 2-yr 24-hour storm.

Follow-up: No change was made to this section of the document and no justification for the use of the 10-year storm event for velocity analysis was provided.  Use of the 10-year storm to evaluate velocity is a departure from current technical criteria.

3.305.2

Repeat comment: “The length of sheet flow path to the down-gradient stormwater conveyance system should be less than or equal to the following…”

This requirement is not stated in the regulations and ignores the condition where the pre-development downstream flow path may be shallow concentrated flow.

Instead of tying the issue of post-development stormwater discharges to NEH Part 60 Chapter 15 which speaks in terms of Time of Concentration, this section should focus on whether the discharge mimics the pre-development rate and volume in line with Va. Code § 62.1-44.15:28 A 10.  For example, the pre-development runoff may be shallow concentrated flow across a rolling farmland or karst swale; or, a sheet flow discharge to a wetland or RPA may be more beneficial (provide greater infiltration) when located more than 100 feet from the main channel.

3.307

Repeat comment: All discharges that are made in accordance with the Virginia Stormwater Management Act and/or VSMP Regulations and do not require/propose construction, improvement, or modification of downstream flowpaths should not be subject to obtaining easements or agreements from downstream property owners.  This section should specifically exclude discharges to an existing stormwater conveyance system that is not being modified.

Chapter 5

Repeat comment: This section of good engineering practices should be more specifically defined in section 5.100 as suggestions and not requirements.  Good engineering practices relies on the judgement of engineers through the design practice and do not always include the steps or processes described in this section.  This chapter should be emphasized as suggestions for the engineering community and not requirements for DEQ or local authorities to adhere to in their reviews

5.200 (A) and (B)

Repeat comment: “Post-development drainage areas and drainage divides should replicate, as nearly as practicable, the pre-development drainage areas and drainage divides. See Va. Code § 62.1-44.15:28 A 10.”

This statement misrepresents Va. Code § 62.1-44.15:28 A 10.  It replaces the parameters of rate of flow, runoff characteristics, and site hydrology with “drainage areas and drainage divides” and ignores the provision that the law provides for improvement of existing erosion and flooding conditions.  It should be noted that the existing channel and flood protection requirements already address Va. Code § 62.1-44.15:28 A 10, including the change in runoff volume incurred by changes in drainage divides. 

“Post-development drainage areas should deviate from the pre-development condition by no more than plus or minus 10%.

Because after-development runoff can be exponentially changed whether drainage areas/divides are modified or exactly the same, a 10% change in area is substantively irrelevant to the intent of the Virginia Stormwater Management Act and VSMP Regulations or the effects of stormwater runoff on the Commonwealth.  Varying pre-development conditions including topography, site size, and geographic location, urban versus rural settings, and linear development projects make application of this new criteria highly impractical.

A better application of this concept would be to use a ±10%-20% change in area to trigger an evaluation of the impacts by way of the adopted technical criteria:

• Areas that increase must meet the criteria of channel and flood protection, Safe Harbor, or 9VAC25-870-66(D) for sheet flow.  Any requirements in addition to the adopted technical criteria are not warranted as there is otherwise no restriction on the allowable increase in runoff volume caused by development in Virginia (whether that increase be from change in land cover or change in watershed area is irrelevant)
• Areas that decrease must either demonstrate a runoff volume that is similar to the pre-development condition or describe how the reduction in runoff volume on the adjacent/downstream properties does not have a negative impact (for example, demonstration that there are no ponds or wetlands immediately downstream that rely on the runoff water from the site to fill/hydrate).

Follow-up: As discussed above, a stipulation on maximum change in drainage areas is not a valid association to the statutory authority referenced in Va. Code § 62.1-44.15:28 A 10.  DEQ’s response did not address the disassociation between drainage areas and other factors that influence site hydrology.  Additionally, DEQ’s response did not validate or justify the selection of 10% as this new technical criteria.  Stakeholders object to the implementation of this requirement without further study.  While DEQ Staff has made attempts to assure that such “good engineering practices” would be implemented as “guidance and guidelines to fast-track review”, the reality is DEQ Regional Offices have already begun enforcing this as a requirement for plan approval, not an option or suggestion.

Further discussions with industry professionals, including VSMP authorities, and DEQ’s response and modification to this section continue to shed light on the short-sighted nature of this requirement.  No evidence has been provided as to the prevalence or practicality of this concept outside of urban environments.  In rural areas, shifts in drainage divides are often done for the purpose of routing stormwater discharges to existing or proposed stormwater conveyance systems where other drainage basins do not have a defined downstream channel.  The application of the sheet flow analysis in other parts of this guidance memo further restricts the ability to discharge to basins that do not have defined downstream channels.  The updated guidance in 5.200 (B) states that >10% deviations may be permitted where post-development volumes do not exceed pre-development volumes but that caveat in itself is often highly impractical in new development sites where impervious covers are increased. 

5.301(D) 5.302(D)

Repeat comment: “The hydrologic soil group (HSG) used for post-development CN selection should be based on the ultimate development condition of the subject land-disturbing activity…For disturbed soils (e.g., pre-development soil profiles that will be mixed or removed or fill material from other areas will be introduced), the design professional should:

1. Adjust the pre-development HSG by at least one factor (i.e., HSG A to HSG B; HSG B to HSG C; HSG C to HSG D) when selecting the post-development CN. See Section 5.302 D herein; or …”

A requirement to adjust the post-developed CN for disturbed soils is not stated in the VSMP Regulations, current and previous Virginia Stormwater Handbooks, or previous Agency guidance (Virginia Runoff Reduction Method, GM 16-2001).  Universal application of an adjustment for any disturbed soil would translate to a substantial additional burden on land development, particularly in rural areas where pre-development lands are undisturbed.

Modification of the pre- to post- HSG is not possible with the Agency provided VRRM spreadsheets.  If you do not provide matching soil area totals, the VRRM worksheet returns an error and does not compute.

Full implementation of DEQ Spec. No. 4 (Soil Compost Amendment) per the specifications is not practical in western parts of the state due to the slope limitations in the specification.

A suggested modification to these sections would be to provide further clarification limiting the applicability of HSG adjustment.  For example, limiting pre- and post- development adjustment to situations where pre-construction infiltration testing was performed.

An alternate suggestion would be clarification that implementation of an adjustment is optional at the discretion of the design professional (as long as if the adjustment is applied it is applied based on consistent criteria from pre- to post- development).  Hydrologic soil grouping, like all stormwater computations, is based on a set of assumptions including but not limited to large scale soil mapping from USDA; there must be understanding regarding the level of accuracy relative to the engineering factor of safety present in stormwater design.

Follow-up: DEQ’s response states that “it is common that site developments adjust the post-development HSG classification as a result of construction activities”.  The prevalence  of negative feedback provided as part of the public comment period during the initial publication of GM22-2012 regarding this section in particular would suggest that this is not standard practice across the Commonwealth.  Additionally, no explanation was provided in DEQ’s response as to the clear incompatibility of this guidance with the VRRM worksheets.