Vernal Pool Design/Build
We have a client that requested the design-build of a vernal pool for herpitological (“herp” short for amphibian and reptile) breeding and habitat diversification. The selected site was formerly a utility hub building, which was razed, then with the site being stabilized.
A herp pond should be capable of sustained hydrology for the early part of the growing season, during the months of February, March, April and May, so that most amphibian species can complete their life-cycle stages. Most vernal, ephemeral, seasonal ponds/pools may dry-up from June through to October, during the low groundwater months. The drying process assures that no predatory fish reside in the pool. Some pools may have persistent hydrology and only dry out during extreme drought periods, which will likely be the case for our built system.
The plan shown is the site existing conditions, with the building removed, which was used for our baseline studies. The back toe-of-slope of the study tract had a wetland bench seep, which flowed along the base of the incline, ultimately flowing to a road culvert, and then directly to a receiving fresh tidal marsh. The site proper, was an upland with poor quality, disjunct soils, due to the building extraction and remediation process.
Our first job was to perform a formal wetland delineation, followed by a regulatory pre-application meeting for wetlands/waters permitting. We then performed a detailed water budget analysis to document proposed hydrology. Typically we would install a groundwater monitoring well or two with automated Hobo readers, but we did not have the luxury of time, nor the budget. Our base flow determinations throughout the growing season suggested that we would have more than enough groundwater discharge to support our wetland.
The plan above is our final grading plan for the constructed wetland.
This photo represents pre-construction existing conditions, an over-compacted upland, devoid of topsoil and natural soil profiles, then with a wetland bench seep along the wooded perimeter of the site.
Our first design considered an island, but we reconsidered due to the possible clothes-lining effect that could dry the wetland. Instead, we decided on a pool, with basking log and woody debris for initial habitat. This photo shows the finished grade, which included both the interception of seasonal groundwater and overland flow from the bench seep.
When constructing a pond or pool, the Maryland Department of the Environment (MDE) will not allow for an in-line structure, such as a pond built within a stream (intermittent R4 or perennial R2). We must pass the base flow of a stream, but are allowed to have the difference, bypassed into an adjacent pond built in uplands, using a bypass structure. In the case of this originating headwater, MDE allowed us to construct two bleeders along the edge of the bench seep channel, to allow some flow into the constructed pool.
I’m sure that you have heard the old adage that “its better to plant a one-dollar tree in a two-dollar hole, than a two dollar tree in a one-dollar hole.” We made sure that the contractor used 6-inches of quality top soil at the surface of the finished grade.
The legal definition of top soil is that it have 2.5% organic content. Our site was amended to have a minimum of 4% organic content, and hence the slightly tannic color of the otherwise clear, cold pool.
All three of the above photos are from a north to south orientation, the first, a pre-construction photo from September of 2014, the second, the completed project in September of 2015, and the third a photo from October of 2015, representing the finished and “charged” wetland pool.
This graphic is the planting plan, where the site was seeded with an Ernst seed mix consisting of obligate (OBL) and facultative wetland (FACW) forb species, such as swamp milkweed, Virginia wild rye, lurid sedge, fox sedge, soft rush, boneset, green bulrush, softstem bulrush, bur-reed and others, all of which are endemic to this coastal plain site. I fully expect that next spring the plants will segregate themselves based of hydrophytic tolerance/preference, and that we will also have a degree of volunteerism, as marshland occurs nearby. I have every expectation that a forb community will become established along the littoral fringe, saturated and slightly inundated zones of the pond.
A very important design element of the pool is a grade control structure at the outlet of the pond, which is a rock weir that will hold the grade, prevent head-cutting from downstream to upstream, and act as the spillway, maintaining our desired normal pool elevation. The combination of a roadway culvert and this rock weir will also reduce the chances of predatory fish being introduced to the pool from downstream.
This set of pictures is looking from south to north, with the first photo from September of 2014 (pre-construction), the second photo taken at project completion in September of 2015, and the third photo of initial grow-out stabilization in October of 2015.
The rock weir is evident in the foreground right, in which all of the interstitial spacing was filled with soil to fill the voided areas, and better support vegetation. The weir represents the “bath tub plug,” which maintains the water elevation of the pond. On this particular day, the erosion fencing and seep bypass pipe was removed.
As I walked to the pool edge in October of 2015, three frogs hopped into the water (future recruitment). I observed a turtle for just a moment, as it swam under the woody debris to hide.
Standing back from the pool, a solitary sandpiper (Tringa solitaria) flew in to walk the margin in search of an opportunistic meal.
I also observed a red swamp crayfish (Procambarus clarkii) along the edge of the pond.
The site looks predisposed for success. Why? Amphibians will imprint to a pool if the species is able to successfully run the course of its breeding cycle. The pool has sound hydrology of high quality spring seepage, and I expect that the pool littoral fringe will grow out with a combination of native grass, sedge and forb species.
Now for the possible downside. Nearby I have observed Murdannia (marsh dayflower) and Arthraxon (hairy joint grass or carpet grass), two non-native plant species that may compete along the shoreline. The red crayfish is a native to the lower southeast, and is the crayfish of crayfish boils and the preferred eating crayfish of southern dishes. The species is not endemic to Maryland, and is directly competing with other native crayfish species. They are omnivores and will feed on any/all herps, through any portion of their life-cycle, if and as available. And I thought that the only likely predator would be the occasional heron or raccoon stalking the quiet waters of the pool.
Thank you to Stephanie for preparing the design and obtaining the permits, and to Steve Warner and John Keister of Coastal Construction.
Through the years, we have designed several vernal pool systems. A few books that I have in my library that I would recommend when considering the design and build of vernal pools include:
Wetland Restoration and Construction: A Technical Guide, Thomas Biebighauser, 2011, Upper Susquehanna Coalition
A Guide to Creating Vernal Pools, Thomas Biebighauser, USDA Forest Service
A Field Guide to the Animals of Vernal Pools, Leo Kenny and Matthew Burne, 2001, Massachusetts Division of Fisheries & Wildlife, Natural Heritage & Endangered Species Program