WSMU_MRSV_Email_20071024

//lrwd: Cherry Grove BMP Town of Mooresville]]
Subject:[Fwd:[Fwd:Cherry Grove BMP Town of Mooresville]]
From:Kelly Johnson<[e-mail]>
Date:Wed,24 Oct 2007 12:36:37-0400
To:[e-mail]
CC:Julie Ventaloro<[e-mail]>,Bradley Bennett<[e-mail]>
Bob,
If I understand correctly, you are reviewing this plan (and other plans?) on behalf of the town. The Division sent the attached letter out last week to local
governments which says that delegated programs have until March 1, 2008 to ensure that their BMP requirements meet those of the State. And, there will be
plan-review training available in the spring. Each local government can send one representative, and it will be first-come, first-serve. If you have been hired
to be the reviewer, then I would encourage you to speak with the town about attending the 2-day training.
In general, the State is happy to support specific technical questions as you have outlined below. However, due to resource constraints, we do not get into the
specifics of plan review. From your questions below, I have a few overall concerns about this project. First, retrofits are probably the hardest designs to do
and the town will have to use their judgment as to what is or is not acceptable. I have outlined some ideas below, but ultimately it will be the town's
decision (if they are a delegated program). The Division has established design requirements that we believe to be protective of water quality. For a new
project that is having trouble meeting the requirements in an application, the design will simply have to be modified. Of course if the development is already
built, and BMPs that are required will not fit on the available space, as appears to be the case for this project, then the town will have to decide on a
reasonable compromise rather than the state. Second, I would not recommend that the town decide to compromise in areas related to maintenance. A pond that
meets the design requirements but can not be maintained because equipment can not access it will fail. Having said all of that, here are some ideas on your
seven questions:
1. The subject of very deep ponds has come up several times, particularly the topic of possible thermal inversion (where small vertical "currents" are created
in deep ponds at certain times of the year). We have discussed this with the representatives of the BAE Department at NCSU and have reached the conclusion that
thermal inversion should not significantly affect the settling velocity of the particles. So, from a settling perspective the deep pond should work. However,
the SA/DA tables may not support a 15-ft deep normal pool, depending on the average depth. I see in question 3 below that retaining walls are being used below
the permanent pool. So, assuming that the average depth is deeper than the depth values on the SA/DA table, how is the surface area sized?
2. There will have to be a creative solution for the pond if there is concern that it will drain an adjacent stream. I am assuming that any stream buffer
requirements have been met. Perhaps require a slurry wall or other barrier that is designed under consultation of a hydrologist. Or, maybe use a pond liner. If
there is a pond liner I would be careful because it can be easily punctured during maintenance so I would not recommend plastic or similar material. Maybe a
traditional clay liner that is sufficiently thick so that it will not likely be punctured during maintenance. Or, maybe put rock on top of a thinner clay liner
so that the person performing maintenance will know that they have reached the bottom of the pond, and reference the rock in the maintenance agreement. This
will be a judgment call for the town.
3. See Figure 10-3 of the BMP Manual, http://h2o.enr.state.nc.us/su/bmp forms.htm. It sounds like you are describing using retaining walls in the pond below
the temporary pool, which is allowed. However, they have to meet other requirements as well. For instance they have to have a 10-ft shelf (wet pond requirement
816), and the permanent pool must fall at the mid-point of the shelf. This policy was enacted because we have found that the vegetation on the vegetated shelf
will die if the water is too deep. If the retaining wall option is used, the design must also have a 3:1 shelf above the 10-ft shelf to the edge of the
temporary pool. Again, if the pond is designed to not be maintained it will fail.
4. The right or the lack of rights to use the 1999 manual is the town's decision, not the developer's. In the attached letter mailed last week, DWQ gave
delegated programs until March 1, 2008 to ensure that their BMP requirements meet the State requirements. I would agree that using an alternate average depth
would be unacceptable. However, again, this is the town's decision to make and they may be willing to make allowances for this project given the situation.
Unfortunately, many ponds were designed without an effective "average depth" definition in the past.
5. The State doesn't have written guidance about allowing or not allowing pumping for maintenance. Given the other obstacles on this project, I see this as
being of the least concern.
6. Designs that ignore maintenance concerns will fail.
7. The state requires a 30-ft vegetated filter at,-the discharge of wet ponds. (The 50-ft reference in that manual is in regard to shellfish waters at the
coast). The Division offers only one exception (requirement N4 of the BMP manual). If a filter is not used, then the pond must be designed for 90% TSS removal
rather than 85% TSS removal (which means the surface area has to be bigger). Given the previous discussion about the questions regarding average depth and the
resulting difficulty of determining the surface area from the tables this may prove to be a difficult option to implement.
Kelly
Kelly Johnson
Environmental Engineer
N.C. Division of Water Quality
Stormwater Permitting Unit
1617 Mail Service Center
Raleigh, NC 27699-1617
Phone: (9f9) 733-5083 ext. 376
Fax: [phone]
Subject:[Fwd:Cherry Grove BMP Town of Mooresville]
From:Julie Ventaloro<[e-mail]>
Date:Wed,24 Oct 2007 09:26:30-0400
To:Kelly Johnson<[e-mail]>
Julie Ventaloro
Coordinator, Water Supply Watershed Protection Program
NC DENR-DWQ
NPS Assistance s Compliance Oversight Unit
1617 Mail Service Center, Raleigh, NC 27699-1617
Phone: [phone] ext. 530 Fax: [phone]
Subject:Cherry Grove BMP Town of Mooresville
From:"Robert Cooke"<Cooker@usi eng.corn>
Date:Tue,23 Oct 2007 12:35:47-0400
To:<[e-mail]>
Julie,
Thank you for your return call this morning. Please find attached a copy of the BMP that we have concerns about.
Again, a little history on the project. The site has been developed, and the Town is requiring the developer to install BMPs to
be in compliance with the Town watershed ordinance. Plan submittal for the BMPs should have occurred during the initial
subdivision review, but slipped through the cracks. The developer was aware that the BMPs were required and showed them
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on his initial site plans, but did not choose to comply until the Town forced the issue.
The plans for the project have already been submitted 4 times; this is the 5th submittal. Earlier designs were rejected because
the ponds encroached on the existing sanitary sewer or did not meet design standards. The design currently submitted is
questionable as to meeting the BMP requirements. The Town would like an opinion from your office on the design. As shown
on the attached plans the pond site is bounded to the west by a retaining wall to keep the pond off of the existing sanitary
sewer easement, to the north by a drainage swale, and to the east by existing home sites.
The pond has a drainage area of 57 acres, with 32% being impervious. Our concerns are as follows:
1. The pond is 25 feet deep from top of dam to bottom of pond. At normal pool the pond will be 15 feet deep. We are not
sure if the pond will function properly due to the excessive depth.
2. The pond is 5 feet below the adjacent creek bottom. Ground water would be a problem when dewatering the pond for
cleanout.
3. The insides of the pond are at 1:1 slopes and are concrete-lined. Access for cleanout would not be possible. The current
standards require 3:1 side slopes.
4. The developer has stated on the plans that he reserves the right to use the 1999 BMP standards. The plans are just
being submitted; we would not think this to be acceptable. For example, the developer does not want to use the method
for determining average pond depth (surface area at normal pool divided by volume at normal pool) as stated on the
bottom of page 10-6, 2007 BMP manual.
5. The pond bottom is 12 feet below the pipe outlet through the dam. The only means to drain the pond for dewatering
would be to pump out the pond. Typically the Town would allow only gravity drains due to maintenance issues.
6. The design as submitted has no access to the riser other than down 2:1 slopes. Typically a maintenance access route
must be provided (we think this could be worked out with some regrading).
7. The current standards require a vegetated buffer at the pond outlet. The design shows riprap all the way down to the
discharge point with no buffer.
We are unsure if modifications could be made to this design to come close to meeting the current standards. Any opinions or
guidance that you could provide us would be appreciated. If this design can not be modified to meet the current treatment
standards the Town is down to the point of requiring the developer to relocate the existing sanitary sewer to create more room
for the pond, or have the pond encroach into the building lots.
Thank you for your help. Please contact us if you have questions on the above.
Bob Cooke
USI
704-342-3007
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