my notes from a 3/5/09 meeting at DHEC-OCRM:
Shannon Hicks stated: “All ponds must be built according to the approved construction plans.” However, DHEC enforces only certain aspects of the approved plans. DHEC is concerned with storing runoff (quantity) and treating the “first flush” (quality). They are not concerned with construction materials. DHEC will not enforce the “structures must be precast concrete” requirement in the approved construction plans. DHEC does not regulate algae or odors in ponds.
Amendments to the approved construction plans
DHEC’s regulations require them to review proposed changes to the construction plans, and to approve them, if the changes meet their regulations. A representative of the Property & Grounds Committee asked if a plan change for a 3 ft. minimum depth might be approved. Shannon Hicks responded that since DHEC does not require a minimum pond depth in their regulations, that this is possible.
DHEC will require as-built plans following reconstruction. The required certification is as follows (taken from DHEC’s “PROJECT CERTIFICATION SITE INSPECTION REQUEST” form):
“I, as a registered professional, certify construction of the stormwater management system at the above referenced project has been completed in accordance with the approved plans and specifications. This certification is based upon periodic observations of construction and an inspection for design compliance by me or a representative of my office who is under my supervision. Any changes from the approved plans are shown on the attached as-built drawings (if applicable).”
Shannon Hicks said that it is common practice here in the Lowcountry to build stormwater management ponds by digging a hole and not placing embankment along the wetland’s side, allowing water to spill out over long stretches into the wetlands. Phase 5 SCHH ponds that empty into the wetlands are built this way. I pointed out that rip rap is placed sporadically along the edge that spill out into wetlands, not along the entire edge. This design (allowing water to spill out over a hundred or more feet) results in a continuous maintenance problem.
Leaking control structures
We reported to DHEC that control structures leak. Photographs taken after the recent heavy rains show the water level in a lagoon dropping close to a foot over a few days after the rain.
Limits of excavation
DHEC considers the bottom elevation shown in the approved plans to extend over the entire bottom, not just part of it. Pond banks should slope at approximately 4:1 to the bottom elevation, which should extend across the entire bottom of the pond.
We told DHEC that we are concerned with damage to private property during reconstruction. We said that we are concerned that private property will not be restored completely or in a timely manner.
We expressed our concern that DHEC closely monitor the reconstruction work. DHEC performs a final inspection after reconstruction is complete. DHEC is working with Pulte to obtain periodic reports of the reconstruction. Shannon Hicks said that SCHH residents can contact DHEC with their reconstruction concerns.
Final approval process
When the reconstruction is complete, DHEC will perform a final inspection. After DHEC confirms that the construction conforms to the approved plans, including any amendments, DHEC will approve the reconstruction. After DHEC’s approval, operation, maintenance, and repair of the lagoons are the community’s responsibility.
A schedule for the reconstruction work has not been submitted. We discussed our concern with the schedule for the work, including how it relates to the January 12, 2010 expiration of the DHEC permit for Phase 5.
Dead trees in wetlands
DHEC said that the Corps of Engineers will allow removal of dead trees in wetlands that threaten private property. Removal of dead trees that threaten private property should be coordinated with Common Area Maintenance and the Corps of Engineers. A representative from the Property & Grounds Committee said that guidance for residents on removal of dead trees that threaten private property will be forthcoming.
Pulte told DHEC that as they pump water out of the lagoons to wetlands, irrigation, etc., that they pump it back into them from wells to maintain water levels.
Subject: SWM ponds
Date: 3/7/2009 6:40:54 A.M. Eastern Standard Time
Thank you for meeting with us last Thursday, and thank you for meeting with SCHH residents on January 22.
I was surprised to hear you say at the meeting this past Thursday that it's common practice here in the Lowcountry, with respect to stormwater management ponds, to dig a hole, not build an embankment on the discharge side, and allow the water to run out along the edge of the hole over long stretches.
What’s to prevent “backwash”? With water flowing over the edge of the hole, potentially in both directions, what's to prevent the erosion and sloughing of this edge into the pond, even with rip rap protection? What’s to prevent the edge from “moving” into the wetlands? from trees falling into the hole? Given enough time, isn’t it reasonable to expect that the edge of the hole will collapse and the hole (pond) will simply fill up with sediment? Eventually, won’t the surface return to what it was before the hole was dug?
All of the ponds that I designed had embankment all the way around with freeboard. Overflows were at an elevation above the natural ground line at discharge, preventing “backwash”. A pond with embankment and freeboard all around, and an overflow at an elevation above the natural ground line, is controlled. A pond with a side without embankment and freeboard, where water is allowed to spill over the edge (potentially in both directions), is uncontrolled. Such a pond doesn’t even meet the definition of what I believe that a stormwater management pond is supposed to be.
I would be interested to see experience data for these ponds without embankments, with one side simply being a dug edge, and with water flowing out onto natural ground over long stretches. What has been their operating and maintenance experience over a period of 25 years or longer? I will appreciate you furnishing such data, or referring me to a source where I can obtain it.
Again, thank you for meeting with us on Thursday.