BAE Stormwater Researchers Update and Q & A – June 2020

About the Update

This update is 1.5 hours and covers the following topics: Bioswales, Regenerative Stormwater Conveyance, Bioretention, Wet Pond Retrofits, Nutrient Crediting, and Subsurface Gravel Wetlands.

Webinar Materials

Webinar Recording

Webinar Presentations

Bioretention Cells and Wet Ponds Presentation Subsurface Gravel Wetlands Presentation Regenerative Stormwater Conveyance Presentation

Webinar Q&A Responses

Bioretention Cells Questions

Q. Is that true for sodded cells as well as those planted with native shrubs/trees?

A. The root zone for sod will not be as deep as for bioretention with shrubs and trees; however, it may have a greater surface area within the available root zone. That could be a potential future research opportunity.

Q. What size forebay to overall bioretention area is a good recommendation and is the forebay planted as well with the same plants?

A. It is recommended to have a forebay equal to 10% of the surface area of your bioretention cell.

Q. What difference in performance do you see for BRC with a liner and underdrain vs BRC without a liner?

A. Using a liner will restrict exfiltration to native soils and potential volume reduction. As volume reduction is key for load reduction, it is best to maximize potential volume reduction.

Q. For bioretention cells, how important is weed removal; is use of cover crops such as clover vs. mulch advisable?

A. Weeds, and other vegetation management, mostly comes down to aesthetics and line-of-sight preservation. When not sodded, mulch is advised to give immediate erosion protection of bioretention cells while providing additional pre-treatment of runoff through filtration and even helps with removal of metals.

Q. If a bioretention site does not have room for a forebay what are some other recommended pre-treatment options

A. Pre-treatment can also include a grass and gravel verge. For example, 8 inches of gravel leading to 3-5 ft of sod. This slows water down and allows particulates (and pollutants bound to them) to fall out.

Q. Referring to the C:N graph for bioretention, what explains the variability of data within a specific year? Any seasonal effects?

A. The C:N data represents a C:N value at an individual bioretention cell of a particular age. Variation is going to occur based on multiple factors including the rate of carbon cycling into the system from vegetation and the watershed and the carbon demand from microbial respiration (which includes N removal mechanisms).

Wet Pond Questions

Q. How much of the pond should be covered with the floating wetlands. Is it true that roughly 25% of the pond needs to be covered with floating wetland?

A. Current guidance recommends 20-50% surface area coverage with FTWs to achieve a noticeable water quality benefit.

Q. Do the floating wetlands need to be permitted by the State?

A. As they do not (yet) receive credit from the State, the implementation of floating wetlands should be a local issue.

Q. Referring to the non-vegetation parts of the floating wetlands, in what ways do they decompose? 

A. It depends on the material. Typically, floating wetlands are made of PVC or material similar to Styrofoam have a very long life-span.

Q. When the plants die back seasonally on floating wetlands, do they release P and N back into the wet pond system?

A. To some degree, yes. Seasonal senescence will release some N and P back to the water column. However, from early research, the amount appears minimal.

Q. How those floating wetlands perform in the winter?

A. Vegetative uptake will be reduced in the non-growing season. However, treatment opportunities still exist via sedimentation as water flows through the root zone.

Q. How are the floating wetlands maintained?

A. Quarterly monitoring is recommended, particularly during plant establishment. Typical maintenance activities include:

  • Replacing plants to ensure adequate coverage,
  • removal of invasive species and weeds without proper root density,
  • removal of volunteer tree species as (1) their roots can reach pond sediment and root-in and (2) their height and weight can cause FTWs to tip over, and
  • cleaning gross solids (plastic bottles, trash, branches) that get trapped by the FTWs

Q. Any information on floating wetland performance when planted with plants with persistent foliage (sedges and rushes) versus plants that die back in winter (traditional shallow water species such as Pickerelweed)

A. The short answer would be that it’s a good idea to include both. Some research has explored plant selection for optimal nutrient uptake. The key factor is to ensure a nice dense root zone to maximize surface area for contact within the water column below FTWs. Also, consideration needs to be given to avoid plants with roots that grow deeper than the depth of the pond to avoid rooting-in to the pond sediment. FTWs need to be able to rise with water levels within the pond. Planting a diverse mix of plant species is recommended.

Q. Can a littoral shelf filter be planted like a bioretention?

A. Littoral shelf filters need to treat as much water as can be routed through them to maximize polishing potential. To ensure maximum infiltration rates, it is best to avoid vegetation and remove volunteer species as they present.

Q. Thoughts on positioning littoral filters at inlet versus outlet, best flow path over littoral filter?

A. If within retrofit constraints (e.g., available space and pond geometry), positioning littoral shelf filters adjacent to outlet structures would ensure maximum treatment potential by maximizing hydraulic residence time.

Q. Our development has 4, 30-year-old wet ponds that flow into Lake Benson.  They appear murky and have been collecting lawn runoff for 30 years.  Since Lake Benson is the endpoint for the Raleigh water, should we be concerned?

A. It is always a good idea to have individual sites inspected by a certified professional if there is a concern. At the very least, sediment levels should be checked to ensure sufficient volume is available from runoff capture.

Subsurface Gravel Wetlands Questions

Q. In reference to Slide 7 – Was there any barrier between the sand and #78 stone? did you find mixing with the sand and #78 stone caused clogging? 

A. (Answered during the Q&A portion of the webinar) – There wasn’t a barrier between the sand and #78 stone; this type of layering is used in bioretention cells and the design guidance for gravel wetlands doesn’t include a barrier between the media and gravel. We are attributing the clogging to the watershed (the watershed contributes a lot of sediment) and lack of vegetation (vegetation helps to prevent clogging). 

Q. In reference to Slide 7 – Would a bigger particle sand work better for clogging? 

A. (Answered during the Q&A portion of the webinar) – Yes, a bigger particle of sand would help with the clogging. We used ASTM C33 sand because it works well for sand filters and is readily available. 

Q. In reference to Slide 9 – can you monitor treatment of metals, PAH, etc.? 

A. (Answered during the Q&A portion of the webinar) – Yes, and that is something we will definitely consider. Thanks for the suggestion! 

Q. How large is the forebay for the gravel wetland? Is that where a lot of the TSS removal is occurring? 

A. The forebay is about 25% of the gravel wetland’s total surface area, and we believe that’s where a lot of TSS removal is occurring. 

Regenerative Stormwater Conveyance Question

Q. In relation to content around to Slide 45 – how did you measure and quantify seep?

A. (Answered during the Q&A portion of the webinar) – We measured inflow and outflow. We calculated seepage (by measuring drawdown rates inside the cells), we used Penman-Monteith to calculate/estimate ET loss. The only unknown after that was seepage, which we found using this equation: Runoff In = Flow Out + ET Loss + Infiltration + Seepage. 

Q. Does RSC treatment train to SSGW where it gets wet?

A. (Answered during the Q&A portion of the webinar) – Yes, that is a possibility. Another would be just to design a stable channel to prevent erosion. 

Q. Are there any photos of old RSCs?

A. (Answered during the Q&A portion of the webinar) – Yes, Please contact Biohabitats in Baltimore, MD, who is associated with some RSC’s that are ~15 years old in Anne Arundel County, Maryland. 

Q. How does the wood in the cell dividers hold up as far as rotting? 

A. Great question. That wood was only for monitoring purposes. That wood will likely not hold up well, but it is not a detriment to the structure/stability of the RSC. 

Our Instructors

William F. Hunt, III, Ph.D., PE

Dr. Hunt is a Professor and Extension Specialist in North Carolina State University’s Department of Biological and Agricultural Engineering. Dr. Hunt holds degrees in Civil Engineering (NCSU, B.S.), Economics (NCSU, B.S.), Biological and Agricultural Engineering (NCSU, M.S.), and Agricultural and Biological Engineering, (Penn State, Ph.D.). He is a registered PE in North Carolina.

Since 2000, Hunt has assisted with the design, installation, and/or monitoring of over 90 stormwater best management practices (BMPs), including bioretention, stormwater wetlands, innovative wet ponds, green roofs, permeable pavement, water harvesting/cistern systems and level spreaders. He teaches 20-25 short courses and workshops each year on stormwater BMP design, function, and maintenance throughout North Carolina and the US.

Sarah Waickowski, PE

Sarah Waickowski grew up in the great town of Fellsmere, FL, and attended the University of Florida. She earned her B.S. in Civil Engineering with a specialization in Water Resources in December 2012. Her interest in stormwater grew after competing in the Florida Water Environment Association (FWEA) Student Design Competition. She helped to treat runoff from a bridge by designing the conveyance network and identifying the site’s hydrologic characteristics.

Jeffrey Johnson, Ph.D., PE

Dr. Jeffrey Johnson is an Extension Associate working alongside Dr. Bill Hunt as a member of the Stormwater Engineering Group. He provides extension, research, project management, and engineering design for stormwater projects in North Carolina and works alongside collaborators on projects both nationally and internationally.

Dr. Johnson’s research is multifaceted and includes topics such as the impacts of aging on existing stormwater infrastructure, optimizing green infrastructure and stream restorations for nutrient and pathogen removal, and enhancing/improving flood mitigation. Dr. Johnson also leads the Stormwater Engineering Group’s efforts in testing proprietary New Stormwater Technologies (NEST) for approval via the NC Dept. of Environmental Quality’s NEST Program. He has also provided engineering design, bidding, and construction oversight on multiple stormwater control measures in North Carolina and Washington, D.C.