Kimley-Horn has reviewed the provided Guidance Memo No. 22-2012 – Stormwater Management and Erosion & Sediment Control Design Guide dated July 1, 2022 (Appendix B).  As an active participant in the design community, Kimley-Horn appreciates the opportunity to provide comments and request clarifications on the guidance document.

2.303 D.

• • INTERPRETATION:
    • It is our understanding that the new requirement is an analysis showing that the downstream conveyance system will contain and not be eroded by the total flow (off-site and on-site) from the 2-yr 24-hr storm event, considering the land cover to be the “worst-case” condition – meaning bare earth with impervious areas (panels and road, in the case of solar) installed (see section 5.500 A below). This analysis would document that the conveyance system can contain the flow within the system without overtopping, and that the velocity generated from the 2-yr 24-hr storm event is non-erosive for the lining of the downstream conveyance system.
  • CLARIFICATION:
    • If a sediment basin or trap is not modeled, how will DEQ determine the peak outflow from the facility?
    • Based on the current VESCH design guidelines, if a sediment basin is modeled, it should be modeled assuming the dewatering method is clogged. Is this the intent when calculating the peak flow from the sediment basins for the downstream analysis?
    • Is DEQ expecting to see any final improvements (roads, inverters, foundations, panels, etc.) included in the hydrology calculations for the contributing drainage areas?
    • We further request that DEQ provide clarification for the extent of downstream analysis of adequacy for the 2-YR 24-HR Storm.
    • Please clarify what the process to extend the period of use beyond 18 months would be.
  • CONSEQUENCE:
    • We anticipate this new requirement to cause sediment basins and traps to be larger, for new downstream improvements required for receiving channels, and for design costs and time to increase.
  • SUGGESTION:
    • Due to the temporary nature of sediment basins and traps, a minimum threshold for contributing drainage area could be established under which an analysis would not be required.
    • DEQ provides formulas for calculating allowable peak discharge (1-year 24-hour) into natural conveyance systems in Section 3.302.3, but does not provide similar formulas for the E&S condition.  DEQ could provide a similar formula/approach for the 2-year 24 hours E&S condition.
    • DEQ could offer a tabular methodology to estimate peak discharge for sediment basins and traps considering the routed effect but avoiding level pool modeling. This could be summarized in a spreadsheet using rational method.

2.303 G.

• • INTERPRETATION:
    • This section specifies the land cover for hydrologic calculations during the E&S phase but is inconsistent within the guidance document on the land cover conditions which should be used for temporary E&S calculations.
  • CLARIFICATION:
    • Section 5.500 A. specifically outlines that solar projects should consider the panels as impervious (and therefore included) when computing runoff computations for E&S controls. 
    • Please clarify the rationale for and basis of requiring a new hydrologic analysis methodology based on land use.  There are other large land uses (industrial and single-family developments, for example) that take significant time for pervious areas to stabilize but are not mentioned here.
    • Additionally, please clarify that it is the intent of this document to continue accepting rational method as a suitable method of computing runoff for the temporary E&S condition. There have been recent (project specific) comments from the department that this may become unacceptable in the future.
  • CONSEQUENCE:
    • We anticipate this new requirement to cause sediment basins and traps to be larger and for design costs and time to increase
  • SUGGESTION:
    • Any consideration of proposed impervious area should be limited to the final hydrology.

3.302.1 A.1.

• • INTERPRETATION:
    • It is our understanding that this section requires analysis of channel protection up to and including the point of analysis.
  • CLARIFICATION:
    • It has been general practice to use 9VAC25-870-66B.1.a. of the state code to satisfy channel protection if your limits of analysis (LOA) is reached while simultaneously reaching a “natural stormwater conveyance system”.   If this is no longer the case, more outfalls will need to meet energy balance which will result in larger basins or eliminating potential developable areas.
    • Please clarify if the intent of this document is to include the receiving channel at the point of analysis (a stream with localized flooding, for example) in the channel protection analysis, or if this section is consistent with previous policy.
  • CONSEQUENCE:
    • This requirement could create the need for increased design costs and time.
    • This requirement could also create increased construction costs for improvement of downstream receiving channels.
  • SUGGESTION:
    • Provide clarification that if the limit of analysis for channel protection calculations is reached while simultaneously entering a natural stormwater conveyance system, 9VAC25-870-66B.1.a. has been met.

3.302.1 C.2.a.-b. , 3.303.1.D.1. & 3.303.2.C.1.  

• • INTERPRETATION:
    • It is our understanding that these sections require “field surveyed cross-sections” at a specific frequency to analyze the adequacy of the receiving channel.
  • CLARIFICATION:
    • Provide clarification if analysis done at the specified frequency with cross sections of flown/LiDAR topographic surveys would satisfy the department, or if manually surveyed cross-sections would be required.
  • CONSEQUENCE:
    • This will create a significant expense and potentially schedule constraints for large projects which typically utilize flown/LiDAR topographic surveys for design.
    • There are risks that adjacent property owners may not provide access required for ground field survey of the stormwater conveyance.
  • SUGGESTION:
    • Allow for flown/LiDAR topographic surveys to be utilized for the analysis of receiving channels.

3.302.4 A-B

• • INTERPRETATION:
    • It is our understanding that this section identifies how to determine a conveyance system’s flood-prone area by calculating the water surface elevation during the 1-yr 24-hr storm event.
  • CLARIFICATION:
    • Please confirm that the definition of “flood-prone areas” considers what is flooding in the 10-yr 24-hr storm event.
    • Note that the purpose of identifying a “flood-prone area” is to determine potential limits of analysis (LOA) for flood protection calculations, which consider the 10-yr 24-hr storm event.
  • CONSEQUENCE:
    • Considering flood prone areas based on the 1-yr storm would result in significantly more modeling and documentation to comply with flood protection requirements.
  • SUGGESTION:
    • Confirm that the definition of “flood-prone areas” considers what is flooding in the 10-yr 24-hr storm event.

3.303.1.C.3. & 3.303.2.B.3.

• • INTERPRETATION:
    • It is our understanding that this section identifies how to determine the limit of analysis for flood protection calculations.
  • CLARIFICATION:
    • Please clarify that the intent is to also include “flood-prone areas” as an adequate limit of analysis, as mentioned in 9VAC25-870-66.C.3.c.
  • CONSEQUENCE:
    • As written, this section of the guidance document is not consistent with the language of the Virginia Code.
  • SUGGESTION:
    • Revise language to match 9VAC25-870-66.C.3.c.

3.305.1.A.

• • INTERPRETATION:
    • It is our understanding that this section requires analysis to confirm there is no increase in both volume and velocity.

• • CLARIFICATION:
    • Based on section 9VAC25-870-66.D. of the Code, only increased volumes of sheet flow require identification and evaluation for down-gradient effects.
    • Please confirm if DEQ intends to require analysis of sheet flow that is not an increase in volume.
  • CONSEQUENCE:
    • This will require additional analysis and documentation beyond what is required by the Code.
  • SUGGESTION:
    • Limit analysis only to increases in sheet flow which increases runoff.

3.305.2.A.

• • INTERPRETATION:
    • It is our understanding that this section is implying that sheet flow leaving a level spreader cannot be analyzed for quantity as sheet flow but must meet channel and flood protection requirements.
  • CLARIFICATION:
    • Please confirm if sheet flow leaving a level spreader may be analyzed for quantity as sheet flow.
  • CONSEQUENCE:
    • This will create increased design costs and time, and potentially increase construction costs for improvements to outfalls that otherwise would have been considered sheet flow that honors natural drainage patterns.
  • SUGGESTION:
    • This guidance indicates that the department does not consider discharge from a level spreader to be “sheet-flow”, without consideration for the calculations which could show it induces flow that meets sheet flow characteristics.

3.307

• • INTERPRETATION:
    • It is our understanding that this section will require an off-site easement to be recorded if an area that is currently leaving the site as sheet flow is designed to maintain the existing drainage patterns.
  • CLARIFICATION:
    • Please provide further explanation why an off-site easement would be required for an existing outfall. 
    • Please clarify how this guidance would reconcile with existing code in local facility manuals. 
    • Please clarify if drainage easements will be enforced with standard provisions and deed instruments, and if DEQ is requiring local MS4 Permit holders to become maintainers of last resort.
  • CONSEQUENCE:
    • Without an enforcement mechanism, adjacent landowners could refuse to grant an easement on existing outfalls. 
    • This section of the document will therefore increase the likelihood of deviating from existing drainage patterns when easements are impossible to obtain, or otherwise render projects otherwise fully entitled impossible to build without full consent from adjacent properties for outfalls already functioning.
  • SUGGESTION:
    • Require easements only for newly created, man-made convenance systems.  Defer to local jurisdictions when facility manuals already specify requirements for easements.

4.301.C

• • INTERPRETATION:
    • It is our understanding that this section would require separate analysis of phosphorus load and therefore removal requirements for projects spanning multiple HUCs.
  • CLARIFICATION:
    • Please clarify the intent of separating pollutant load calculations.
    • Please clarify that phosphorous loads can continue to be met with credit purchases in adjacent HUCs.
  • CONSEQUENCE:
    • Separating calculations will require additional design cost and time.
  • SUGGESTION:
    • Require separate calculations for phosphorous load requirements only if the project spans multiple HUCs AND is non-contiguous. 
    • Allow for a percentage of the phosphorous reduction required be achieved on-site in the adjacent HUC.

Chapter 5: Good Engineering Design Practices

GENERAL CLARIFICATION: Please provide an explanation for the section.  Is DEQ attempting to suggest best practices for Engineers, establish a framework for policy guidance, or setting the stage for upcoming code?

5.200 B.

• • INTERPRETATION:
    • It is our understanding that this section gives general guidance for preservation of existing drainage patterns only.
  • CLARIFICATION:
    • Please confirm that this is a recommendation not a requirement.
  • CONSEQUENCE:
    • If this section becomes a strictly enforced requirement, it will lead to increased design cost and time. Given the strict requirements surrounding sheet flow and adequate outfall locations, this is going to be a difficult parameter to maintain.
  • SUGGESTION:
    • Make this a recommendation not a requirement. 

5.301 D. 1. & 5.302 D.1.

• • INTERPRETATION:
    • It is our understanding that these sections require adjustment of all pre-development soil groups by one factor of improved drainage, then taking the “adjusted” pre-development HSG from section 5.301 D.1. and further adjusting for the post-developed condition.
  • CLARIFICATION:
    • Please clarify the intent and justification of this requirement.  Does DEQ not want engineers to utilized NRCS soil survey data?
  • CONSEQUENCE:
    • This provision would require more detention of onsite runoff, which would increase construction cost by increasing the size of basins.
  • SUGGESTION:
    • Adjust onsite hydrologic soil groups only for projects where testing from recent adjacent projects have demonstrated inaccurate soils in the NRCS soil surveys.   

5.302 D.3.-4.

• • INTERPRETATION:
    • It is our understanding that this section requires field-testing the on-site HSG after construction has been completed but prior to project close out.
    • It is further our understanding that should the field investigation return results that support a higher HSG than what was proposed in the design plans, the design must be updated to account for the change in HSG.
  • CLARIFICATION:
    • Please explain the precedence and intent of this requirement. 
    • Does DEQ desire to update post-development hydrology after construction, then modify constructed facilities?
  • CONSEQUENCE:
    • This is a significant risk to construction costs.
    • This testing would be performed once perimeter stormwater measures are designed, installed, and functioning, which is itself a significant cost increase on large sites.
    • If the field testing returned data that showed the HSG was divergent, it presents a significant risk that the design and ultimately what has already been built and stabilized would need to be expanded to “handle” the field-tested HSG values. This could have unnecessary impacts to the environment if disturbance of previously stabilized areas is required to meet this guidance.
    • There is also no protection here for an iterative reconstruction and testing of drainage areas.
  • SUGGESTION:
    • Limit onsite testing of hydrologic soil groups to one option for analysis of sites that are not performing after construction, rather than an expected procedure for every drainage area for every land development project in the state.

5.500 A.

• • INTERPRETATION:
    • It is our understanding that this section requires all installed solar panels to be considered unconnected imperious area per 210-VI-TR-55, Chapter 2.

• • CLARIFICATION:
    • This section makes the assumption that the land beneath the horizontal projection area of the solar panel is impervious, but does not account for any initial abstraction nor infiltration of the soil, porosity, and roughness of the stabilized meadow condition that will and does establish under those panels.
    • Please clarify how the sheet flow under the panel over grass is being accounted for.
    • Please clarify if any case studies have been performed by DEQ or research reviewed to conclude that the sites demonstrate hydrologic performance comparable with traditional unconnected impervious area.

• • CONSEQUENCE:
    • This will create increased design cost and time to model the increased number of retention and treatment facilities.
    • This will create increased construction costs due to the increased number and size of retention basins to handle additional runoff.
    • This will create increased construction costs due to the increased number and size of BMPs to handle additional treatment.
  • SUGGESTION:
    • Enact a study period to monitor the performance of several solar facilities in Virginia to measure an empirical characterization of runoff compared to traditional unconnected impervious. 

5.500 B.

• CommentID: 127479