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High Tunnels and Stormwater Management

Farmers in Lancaster and York counties have requested assistance from CWQE to help them better understand local stormwater management requirements for the addition of high tunnels — temporary hooped structures that extend the growing season from March to December.

The problem? Some municipalities are looking at proposed high tunnels as permanent building structures that require the submission of detailed stormwater management plans including hydrological routing to design the stormwater facilities. And stormwater structural controls designed for the farm often lack a watershed approach in design and sensitivity to protecting soil health.

“Regulations look at high tunnels like a building,” says Jeff Stoltzfus, Penn State Extension Educator. “They are impervious to the extent that the plastic covering is impervious, but there are other distinctions. High tunnels are temporary structures rotated every 1-5 years, and there’s no soil excavation.

On March 10, 2022, CWQE provided a forum for farmers and municipalities to learn more about the benefits of high tunnels, the state stormwater plan exemption, appropriate stormwater practices for agricultural lands, and what may need to change in both state and local regulations to best accommodate high tunnels as an agricultural practice. For a summary of the workshop presentations and guidance on high tunnel stormwater management, visit High Tunnels & Stormwater Management - PA-CWQE.

Holistic Look at Stormwater BMPs

In places like northeastern Lancaster County, some farms are moving from animal agriculture to specialty vegetable crops that bring a premium price at local markets. Agricultural high tunnels boost small farm profits by extending the growing season. The productivity they bring also aids food security, an issue recently elevated as a result of the COVID pandemic.

The growing popularity of high tunnels calls for a fresh look at what can be done to control stormwater runoff most fitting to this type of agricultural practice.

“It’s important to look comprehensively at a site’s watershed characteristics to identify opportunities to integrate stormwater management,” says Sally Holbert, CWQE Manager.  “You don’t want to install practices that get in the way of farm production – requiring stone infiltration trenches, for example, will interfere with the rotation of high tunnels. Farmers don’t want stones in their fields.

“Look around at possible ways to manage runoff and improve water quality.  Is there a stream where a riparian forest buffer could be installed to intercept runoff flow? Is there a building that was built prior to current stormwater regulations that could help offset runoff volumes expected from new construction?  In other words, if there is a sizable structure without stormwater controls, why not allow BMPs on that structure to mitigate controls placed on a planned high tunnel? Municipalities could allow something as simple as rainwater harvesting using cisterns around older buildings; collected runoff could then be used to water the high tunnel vegetables.”

Stormwater Management Benefits of Biochar

Another promising approach includes the use of biochar, a carbon-rich product created by roasting organic materials in an oxygen- limited environment, much like the process used to make charcoal. Byproducts made of organic matter like wood chips, green waste, grain hulls, and manure are upcycled into biochar in tightly controlled commercial systems which can be sold and integrated into a variety of Industries. Different types of biochar are made for different applications.

For stormwater management, biochar can help retain and infiltrate stormwater, purify water, restore degraded soils, remediate contaminated landscapes, and increase productivity of agriculture and nursery operations. 

Biochar can be used in a dozen different BMPs approved by PA DEP for reducing nutrients and sediment loads to the Chesapeake Bay. BMPs include barnyard runoff control, biofilters, stream restoration, forest and grass buffers, tree plantings, lagoon covers, manure composting, and poultry litter amendment. Biochar is also an approved soil carbon amendment under NRCS standards in many states.

CWQE, in collaboration with Ecotone, is evaluating an innovative stormwater management strategy for one farmer’s high tunnels that would use biochar between the high tunnels to improve the soil’s porosity and its long-term capacity for infiltration, runoff attenuation and pollution removal.  If the farmer decides to go back to large row crop production in the location of the high tunnels at a later date, it’s an easy transition and the biochar will offer soil health benefits as opposed to having to remove stone-filled infiltration trenches.

Another example project was completed by Brandywine Conservancy and Ecotone in Chester County, PA. Biochar was used on a farm to manage stormwater runoff from a new building.

The project’s goal was to capture wash water from two barns and a small, paved area between the barns. They installed the swale, a trench two feet deep, two feet wide, and approximately 150 feet long, lined it with geotextile and included a perforated pipe wrapped in filter sock to complete the infiltration system. The trench was then filled with a mixture of 85 % green woodchips and 15% biochar, and the surrounding area was stabilized and re-seeded with native meadow species. At the inlet an open top concrete box was formed up to allow for sediment settlement and to make for easy cleaning while at the lower end, a stone outlet seep was constructed. Ecotone had the project fully installed in just under three days. 

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