This comment was made during comment period for GM 22-2012 and it was not addressed properly then and is now carried over into this SWM HB with little back up.

A.3.2.2, A.5.6.1.1 & A.5.6.2 The 10-year 24-hr post development sheet flow velocity shouldnt need to be below permissible velocities in Table 5-1 & A-42 as that table was originated for the 2-yr storm velocities from Table 5-14 of the VESCH and was meant for grass lines channels. This table is also excluding alot of other resources out there that establish permissible velocities methods like VDOTs Drainage Manual (VDM) Table 7-1, VDM Appendix 7D-6, 7D-2, and VESCH Table 5-22. There should be language here that accepts other standards that maybe not be listed in the table. If it is suggested to use this table as a guide, another alternative is to rephrase this as a suggestion and offer designing a level spreader that mimics pre-development velocities that may be greater than the permissible velocities listed in the table as existing condition doesnt demonstrate erosion. And velocity design should be limited to the 2-yr storm as state code 9VAC25-870-66, ESC Handbook, as well as engineering standards and even earlier references in the handbook that specifically state that erosion is checked based on 2-yr events, NOT 10-yr events. Also note that their is a typo in the table notes stating the erodible soils are considered high when greater than 0.35, when previous VESCH Appendix 6a page VI-44 stated it was 0.36, not 0.35

A.3.2.2, A.5.6.2, & Table P-SUP-08-1. stating to limit sheet flow depth, requiring it to equate to 0.1 feet or less for the 10-year storm is incorrect and not how NRCS originally established it. In determining sheet flow, instructions in TR-55 Chapter 3, page 3-3 specifically states it’s for ABOUT 0.1 feet or so, NOT capped at 0.1 or less. And, TR-55 specifically bases sheet flow off the 2-year 24-hr rainfall event, NOT the 10-year event, as seen in equation 3-3 of TR 55. Velocities is continued to be calculated using the 2-yr event as further seen in Figure 3-2 example 3-1 of the TR-55 handbook. THIS IS A CHANGE TO STANDARD/HISTORICAL ENGINEERING CALCULATIONS.

A.5.6.3 & Table P-SUP-08-1. Clarity on the length of sheet flow. This calculation should be established for the maximum length of sheet flow before it starts to turn into shallow concentrated flow, but shallow concentrated flow naturally can maintain shallow for extended lengths before it concentrates into a stormwater conveyance system, especially in rural areas where you have 100's of acres of land that contain swales everywhere, but no defined channel. Shallow concentrated flow like swales are not credited as defined channels/stormwater conveyance system even though they are well established and better for the environment then manmade conveyance system so it really should be included especially if they maintain non-erosive conditions by their existing vegetation. Local regulating agencies get very confused that this means that you must reach a defined channel/conveyance system within the calculated maximum length of sheet flow ignoring/negating shallow concentrated flow in long lengths of swales. They arent counting swales in this limit to reach a conveyance system which they should. DEQ and local agency's should not ignore shallow concentrated flow and be forced to have a level spreader enter a more concentrated conveyance system by the max length calculated Lsf for simple driveway culverts that are just trying to convey large amounts of runoff from one side of a swale to another in a rural area. Otherwise this forces applicants to destroy existing vegetated large swales in rural areas to create manmade channels miles downstream of level spreaders which is worse for natural infiltration, increases time of concentration/flows due to manmade channelization of previous shallow wide flows that had high existing vegetation, and its worse for Environmental Site design. Like said previously, the handbook is viewed as law by many agencies and needs to be rephrased as suggestive otherwise this handbook cannot predict every single application that doesnt fit into the black and white criteria listed in the handbook. Local agencies wont provide variances or waivers from language written in the handbook like this section because it written so black and white without considering all applications out there.

Table P-SUP-0801 Level Spreader Design length is designing it for the 10-year storm, when mentioned previously that TR-55 NRCS Handbook defines sheet flow based off a depth of about 0.1 for the 2-yr storm, so the Q should be the 2-yr storm when trying to replicate sheet flow in a level spreader design. Not 10-year. Also stating a maximum 30 cfs in a level spreader contradicts a maximum level spreader length of 200 feet with an assumed 0.1 ft max permissible head, which means more than 0.1 ft of head should be allowed of 0.127 feet to allow 30 cfs and not provide a ~290 linear foot spreader. Again this all should be designed based on the 2-yr and everything really should be rephrased to not set a strict cap.

A.5.1.1.1 Permissible velocities have actually been accepted by the industry to be 18 ft/s for RCP and should actually rely more on manufacturer specifications. this should be rewritten in a way as a suggestion and not as a strict requirement, as Ive made my point previously only the way strict language is interpreted as law.