As I travel across the Bay watershed, I have become more convinced that from a stormwater perspective, terrain matters a lot more than state boundaries. How one approaches stormwater really depends on where you are. Local terrain matters — whether it is the coastal plain, piedmont, karst, the built environment of the big cities, or the more mountainous conditions that support high quality trout streams. The bulk of stormwater research, innovation and practice, however has taken place in only one terrain- the piedmont, just to the west of the I-95 corridor. By contrast, the other four terrains have received little or no attention. To avoid the piedmont bias, CSN is producing customized stormwater guidance for each kind of terrain, starting with the rain that drains the coastal plain.
It’s not easy to design effective stormwater practices in the coastal plain, much less make the big transition to environmental site design and runoff reduction. The list of site constraints in the coastal plain is remarkably long- flat terrain, high water tables, altered headwater drainage, poor drained soils, groundwater concerns for drinking water, shoreline development patterns, discharge to wetlands, mosquito control, shoreline buffers and the central role of highway drainage. Not to mention that stormwater managers often must focus on nitrogen and bacteria, which are extremely hard to remove from runoff. And, if the preceding constraints aren’t depressing enough, how about adding in hurricanes, tidal flooding and sea-level rise?
Over the past few months, I have talked with dozens of stormwater designers and researchers on how to come up with a better stormwater prescription for the coastal plain. The outcome of these conversations is summarized in a new Technical Bulletin. Please review the following slideshow for a visual of the key coastal plain concepts. As always, it is a first draft which will be improved and refined through Network critique and comments by the end of February. CSN also plans to work with several tidewater partners to convene a February workshop on the topic to get broader input.
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The new Technical Bulletin outlines a list of preferred practices for the coastal plain including constructed wetlands, shallow bioretention, wet swales, rain tanks, rooftop disconnection, permeable pavers, and filter strips. It also describes acceptable practices that may be feasible including sand filters, small-scale infiltration, urban bioretention, green rooftops and soil compost amendments. The restricted list includes wet ponds, dry ED ponds, grass channels, and large scale infiltration.
Yes, that’s right, it’s proposed that wet ponds, the most common practice in the coastal plain, be restricted…only the portion of the permanent pool above the seasonally high water table is considered eligible to meet water quality volume requirements. The main reason for the proposed restriction is that research has consistently shown a sharp decline in pond performance when they intersect groundwater.
Restricting wet ponds is certainly not a popular idea, as coastal plain designers like dugout ponds to acquire fill for use elsewhere at the site, among other reasons. In some tidewater communities, such as Newport News, Virginia, wet ponds treat more than 80% of the entire community. The new Bulletin also presents some proposed design modifications for every practice to make them work better in the coastal plain.
Folks may certainly disagree with the CSN coastal plain stormwater prescription, but everyone agrees that we need to design within the context of this difficult terrain. So, please share your ideas with the Network on how it can be improved and we can work toward a consensus document everyone can feel comfortable with.