In general, this document has many omissions or word changes form regulatory text, incorrect references, or incorrect data that generates much more confusion than it does clarity. 

2.302 Temporary Sediment Trap

D. Runoff coefficients or curve numbers used in runoff computations should correspond to a
bare earth condition. See Section 5.303 herein. This language does not apply to Sediment Traps per 9VAC25-840-40 #6. Recommend removal of this language as it pertains to Sediment Traps.

2.303 Temporary Sediment Basin

F. Runoff coefficients or curve numbers used in runoff computations should correspond to a bare earth condition. See Minimum Standard #6, 9VAC25-840-40 6, and Section 5.303 herein.  This does not meet current regulation as written as it omits language from MS#6.  MS#6 also allows for those conditions expected to exist while the sediment basin is utilized.  Please include this language as allowed by regulation as there are circumstances where the entire drainage area to a basin may not be disturbed.

3.301 DESIGN STORMS AND HYDROLOGIC METHODS

C. The Natural Resources Conservation Service (NRCS) synthetic 24-hour rainfall distribution models, including but not limited to, NRCS Technical Release 55 and NRCS Technical Release 20, should be used to perform the water quantity design computations herein. Hydrologic and hydraulic methods developed by the U.S. Army Corps of Engineers may also be used. See 9VAC25-870-72 C.  This omits language from the VSMP regulation and therefore does not meet current regulation as written.  See 9VAC25-870-72 C.  Regulation also allows for other standard hydrologic and hydraulic methods.  Please include this language as allowed by regulation.

3.302.1 DISCHARGES TO MANMADE STORMWATER CONVEYANCE SYSTEMS

C. Channel Protection Computations.

3. Permissible (non-erosive) velocities Please provide a source for this standard for RCP.  This is not practical and may preclude the use of RCP.  What about other pipe types?  What about other open channel linings?

3.302.3 DISCHARGES TO NATURAL STORMWATER CONVEYANCE SYSTEMS
“Natural stormwater conveyance system” means the main channel of a natural stream and the
flood-prone area adjacent to the main channel. This includes any natural or perennial or
intermittent streams, unimproved ephemeral channels, wetlands, or swales. A manmade lake or
reservoir not created for the purpose of managing post-development stormwater should be
considered a natural stormwater conveyance system. 

This does not meet current VSMP regulation.  Please see 9VAC25-870-10. Definitions. Every other definition provided in this guidance document appears to be from the VSMP regulation but this one has been added to as highlighted above.  DEQ cannot arbitrarily change or add to the definition of items defined in the VSMP regulation in a guidance document. In addition, VSMP regulation clearly defines “manmade” and the technical criteria that apply. 

3.302.4 DETERMINATION OF FLOOD-PRONE AREA

A.The limits of a natural stormwater conveyance system’s flood-prone area should be
determined by mapping the water surface elevation associated with the pre-development
1-year 24-hour storm event. Computations supporting the natural system’s 1-year 24-
hour water surface elevation should be provided with the plan.

B. DEQ may consider discharges to other areas adjacent to main channel (e.g., mapped
Federal Emergency Management Agency (FEMA) floodplain, FEMA floodway, FEMA
floodway fringe, wetlands, riparian buffers, Resource Protection Areas) on a case-by-case
basis. 

This section appears to be attempting to redefine a “flood prone area” from VSMP Regulation.  As the definition of a “flood prone area” includes the items in B.  DEQ cannot consider these on a case by case basis as it goes against VSMP regulation.

3.305.1 DISCHARGES OF SHEET FLOW FROM PERVIOUS OR DISCONNECTED
IMPERVIOUS AREAS

A&B1 - 10-yr 24 storm velocity requirement – This should be 2-yr velocity. 10-yr is against VSMP regulation as even the VSMP regulation evaluates erosion for the 2-yr storm.  In addition, See 9VAC25-870-72 C which lists the required methods to conduct the analysis in VSMP regulation.  Standard methodology would be to evaluate velocities for the 2-yr storm.

B2 - 10-yr 24 storm depth of sheet flow of 0.1’ requirement. - See 9VAC25-870-72 C which lists the required methods to conduct the analysis in VSMP regulation.  The 0.1’ is from reference manuals with regards to Time of concentration section which is for the 2-yr storm from the NEH or TR-55.  Applying the 0.1’ depth requirement to the 10-yr storm would not be a standard hydrologic or hydraulic method and would be against VSMP regulation.  Suggest changing this to the 2-yr storm.

3.305.2 DISCHARGES OF SHEET FLOW FROM LEVEL SPREADERS
A. Piped or channelized stormwater runoff converted to sheet flow prior to discharge should
comply with 9VAC25-870-66 B (see Section 3.302 herein) and 9VAC25-870-66 C (see
Section 3.303 herein) or “Safe Harbor” (see Section 3.304 herein).  This doesn’t not meet VSMP regulation.  9VAC25-870-66D clearly allows sheet flow from the “…physical spreading of concentrated flow through level spreaders…” that do not have to meet the requirements stated in A. This should be removed from this section.

B - 10-yr 24 storm velocity requirement – Same as above in 3.305.1 In addition, the values in this table are incorrect and do not match the same table and reference elsewhere in the document.

C. 10-yr 24 storm depth of sheet flow of 0.1’ requirement – Same as above.

Additionally, there are several conflicts with DEQ BMP Specifications on the clearinghouse that need to be resolved.

4.404 STEP 4: OFFSITE COMPLIANCE OPTIONS
A. Design professionals should investigate the use of offsite compliance options if the
required total phosphorus load cannot be achieved onsite through the implementation of
ESD, RR, and PR practices.  This appears to ignore the fact that VSMP regulation lists specific instances where offsite compliance options shall be allowed.  A strict interpretation of this would be against regulation.

5.200 PRE- VS. POST-DEVELOPMENT DRAINAGE AREAS / DIVIDES
A. 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. 
B. Post-development drainage areas should deviate from the pre-development condition by
no more than plus or minus 10%. 

Where is the source for 10%?  This appears to be arbitrary and will be an unobtainable and impracticable limit in a lot of cases. 

This entire section should be removed as it does not meet regulation.  The language in A is not the language from the referenced code.  The code states “… runoff rate of flow and characteristics that replicate, as nearly as practicable, the existing predevelopment runoff characteristics and site hydrology…”.  Runoff characteristics and site hydrology encompass so much more than just drainage areas/divides.  Attempting to limit one specific parameter is a complete misinterpretation of the referenced code.