August 26, 2022

Ms. Melanie Davenport

Commonwealth of Virginia

Department of Environmental Quality

Director - Division of Water Permitting

1111 East Main Street

Richmond, VA 23219

Re:    Guidance Memo No. 22-2012

         Stormwater Management and Erosion & Sediment Control Design Guide

Dear Ms. Davenport:

As an active participant of many erosion control & stormwater permit applications within the Department’s jurisdiction, we appreciate the opportunity to review the aforementioned guidance memo and offer the following comments: 

Chapter 2.000 Erosion and Sediment Control

2.302 Temporary Sediment Trap

B. Consider eliminating wet storage, providing full availability of designed sediment storage capacity. 

2. 303. Temporary Sediment Basin

B. Consider eliminating wet storage, providing full availability of designed sediment storage capacity. 

D.1.c. Should this also state that the analysis should consider the 2-year 24-hour storm event?

D.3. Should this also state that the analysis should consider the 2-year 24-hour storm event?

F. Elaborate on definition of “structural integrity” to meet this requirement. Perhaps a check list could be provided.

Chapter 3.000 Stormwater Management – Water Quantity

3.302.2 Discharges to Restored Stormwater Conveyance Systems

In the second paragraph definition of “Flood-prone area,” include “Resource Protection Area,” similar to its inclusion of the definition section of 3.302.4.B

3.302.3 Discharges to Natural Stormwater Conveyance Systems

In the second paragraph definition of “Flood-prone area,” include “Resource Protection Area,” similar to its inclusion of the definition section of 3.302.4.B

C. Clarify whether Qforest and RVforest are for the pre- or post-development drainage area boundaries.

3.302.4 Determination of Flood-Prone Area

A. The selection of a 1-year storm event to determine flooding seems inappropriate, as most “bank-full” stream analysis contemplate a 2-year storm event.  Therefore, it would seem more appropriate to select the 10-year storm event to consider the determination of flooding.  This would be consistent with the 10-year storm selection in 3.303.1.A.

3.303 Flood Protection

This section defines the term “localized flooding.”  Could you also provide a definition “flood-prone area” to differentiate these two terms? 

Provide additional clarification/examples of what constitutes localized flooding for the purposes of evaluating an existing conveyance system. Some conveyances are more clearly defined than others (i.e. ditch vs swale).

D.1. Three cross sections within 150ft of the outfall seems excessive especially when large portions of a conveyance are on property. Such a close interval is also unlikely to yield additional useful information. Suggest that a minimum of 1 representative cross section be provided within 150ft or wherever there is a substantial change in channel geometry, roughness or slope.

3.305 Sheet Flow

3.305.1 Discharges of Sheet Flow from Pervious or Disconnected Impervious Areas

B.1/B.2 Sheet flow velocity should be evaluated using the 2-year event (consistent with Manning’s assumptions as listed in the NEH).

B.2. Can you provide clarification on selection of 0.1’ flow depth for sheet flow analysis?

3.305.2 Dishcarges of Sheet Flow from Level Spreaders

A. This seems inconsistent with the code. Sections B & C directly apply to Concentrated Flows. Section D says that increased volumes of sheet flow may be discharged through physical level spreading if they will not cause downstream flooding or erosion. If those conditions are met, then the analyses set forth by Section B and C are not required.

B. The table provided in this section references Virginia Erosion & Sediment Control Handbook, Table 5-14.  This same table is referenced in 3.305.1.B.1, however they are different tables.

E.2. Can you provide criteria for analysis of “adversely impact” to downgradient environmental features?

G. Soil Compost Amendment should not be required if no Water Quality credit is intended by use of level spreader/sheet flow area.

H. Design storm selected for this analysis is the 10-year event.  Typical level spreader design storms for many municipalities are for “first flush” runoff volumes (i.e. runoff from 1” storm), with bypass provided for larger storm event.  Consider reducing the design storm requirement to calculate the length of the level spreader.

M. Consider including treated timber as an acceptable material (Henrico and Loudon County details)

3.307 Drainage Easements

D. Please include code reference that requires easements that cover the discharge of sheet flows.

Provide a definition for “down-gradient stormwater conveyance system.”

Chapter 4.000 Stormwater Management – Water Quality

Pursuant with the guidance provided in Section 5.500 Solar Panel Arrays, additional consideration should be given to lowering post-development nutrient loading when calculating the entire solar array as disconnected impervious.  It is reasonable to deduce that the nutrient loading from the solar array, underlaid by pervious managed turf, is not the same condition as other impervious surfaces (i.e. rooftops, roads, parking lots).

4.301 New Development

D. Will this review modify the calculation in the VRRM spreadsheet?  If so, consideration should be given to the implementation period for these modifications, as they relate to projects that are currently in design and/or permitting.

Chapter 5.000 Good Engineering Design Practices

5.200 Pre- vs. Post-Development Drainage Areas/Divides

A./B. Va. Code §62.1-44.15:28 A 10 “require that VSMPs maintain after-development runoff rate of flow and characteristics that replicate, as nearly as practicable, the existing predevelopment runoff characteristics and site hydrology.”  Item B exceeds the regulatory requirement, by introducing a finite amount of change to a specific design parameter, which the current regulation does not contemplate.  It appears as though the regulatory language already provides sufficient clarity in regard to equalizing post-development runoff to pre-development conditions.

5.301 Pre-Development Curve Number Selection

D. Provide clarification that existing Agricultural land would meet the definition of “previously disturbed soils,” as they are “native soil profiles that have been mixed or removed,” thereby allowing for the prescribed adjustment of HSG by at least one factor.

5.302 Post-Development Curve Number Selection

D. This section defines that “disturbed soils,” which is indicative of all areas within the Limits of Disturbance (LoD), will need to meet the criteria of this section.  In previous discussions with VDEQ reviewers, these criteria only applied to areas of “fill” in the proposed design. Can you provide clarification of why this would apply to areas of “cut” and/or areas where the existing grade is not modified (i.e., coastal plain sites)? Following construction, stripped and stockpiled topsoil is applied to cut areas and ungraded areas are decompacted and surface roughened prior to application of temporary/permanent stabilization.

D. Consideration should be given to the implementation period for these modifications, as they relate to projects that are currently in design and/or permitting. It will have profound consequences to developable acreage.

E. Correct the last section to be item “E.”

E. The inclusion of “Forest Cover (adjusted)” is likely not necessary, due to the definition of “disturbed soils” would constitute the removal of trees.

E. Chapter 9, Part 630, NRCS National Engineering Handbook Table 9-5 defines Open space to include lawns.  It is unclear what differentiates Open Space from Managed Turf.  Please include definitions of the categories, so it is clear to designers for their selection of CN.

5.500 Solar Panel Arrays

D. We have designed and built hundreds of utility scale solar projects in North Carolina and Virginia over the past decade and have not encountered rain-sensing technology.  Please provide any industry contacts who manufacture and integrate this product offering.

Additionally, we offer the following feedback/considerations for this guideline:

• For single-axis tracker systems, typical rotations are 52° from horizontal (up to a maximum of 60°).  Provide clarification that this meets guidance to “rotate to a vertical position”
• In the case where “vertical position” is achieved during applicable rain events, please clarify that all structural support elements (posts, torque tube, etc) will still be considered impervious surfaces even though they are located beneath the modules which would effectively be pervious.
• Recommend providing clarification on the minimum interval to deploy rain-sensing technology due to large variability of weather patterns over the footprint of utility scale projects.  It is likely that only portions of the site would have rainfall to meet the deployment criteria.
• Single-axis tracker systems have variable stow characteristics (depending on the manufacturer) during wind and/or snow events.  Most elevated wind events (typically triggered by ≥35 mph, depending on manufacturer) are accompanied by rainfall.  To prevent torsional galloping of the structure, many manufactures stow at 0° (horizontal) to 30° to lower the profile of the structure.  Therefore, it may be difficult for many systems to comply with this regulation during high wind events.
• Most manufacturers now offer various software algorithms to improve energy production on cloudy days (diffused light).  These strategies can tilt all of the trackers in various directions, depending on sensors deployed to monitor light levels.  The introduction of rain-sensing technology would most likely negate the effectiveness of these technologies.
• Single-axis trackers move very slowly, mostly dictated by the speed at which they need to rotate to track the sun throughout the day.  Frequent movement of the tracker due to potentially intermittent showers on cloudy days would significantly disrupt overall energy production, lowering the overall availability of the energy from the plant

Thanks for your time and consideration.

Sincerely,

Brent T. Niemann, PE

Director of Civil Engineering

Strata Clean Energy