The Diversion: A BMP to Control Erosion, Sedimentation, and Runoff

By Paul Solomon

A diversion consists primarily of an earthen embankment, which is constructed with back and front slopes and contains a channel at its base to catch and slow runoff.

Recently I became the owner of an abandoned 21-acre York County farm, now called Timberidge Farms. This neglected property contained a number of both natural and man-made surfaces which were conduits for stormwater. This runoff had the potential to cause damage to my own property as well as to pollute local and regional waterways, including an adjacent stream. I considered the precipitation that fell on and traveled across my land to be both my responsibility and my opportunity. Introducing stormwater best management practices (BMPs) became one of my first priorities as the new property owner.

In order to determine which BMPs to consider, my first steps were to study prior stormwater damage and the quantity and direction of runoff following multiple intense storms. Damage was especially evident in the farm’s driveway, which was badly washed out. Stormwater rushed along this driveway during each heavy rainstorm, depositing pollutants into the adjacent stream. In places along the driveway, sediment from prior storms had been deposited. Capturing and retaining this water on-site would prevent property and ecological damage, while providing a plentiful water supply to support the farm’s well and any future rain gardens, ponds, and various landscaping enhancements.

Preventing stormwater damage to local waterways is particularly important. In my situation, the stream located adjacent to the farm is a first order, perennial, spring-fed stream. This watercourse exists as a tributary to Little Falls, which extends south through York County and into Baltimore County, MD. Little Falls, in turn, is a major tributary to the Gunpowder Falls, a major public water supply source for metropolitan Baltimore. These waterways are classified as cold water fisheries with exceptional water quality. In addition to their vital role in area water supplies, these waterways provide recreational opportunities to Pennsylvania residents and visitors and support native brown and brook trout populations.

To develop a stormwater mitigation strategy, I sought out the advice of the professionals at The Natural Resource Conservation Service(NRCS), located in York County. (Similar offices are located in most other Pennsylvania counties as well.) NRCS prepared a Soil Conservation Plan for my farm, which recommended a number of practices designed to reduce runoff. This expert technical advice, which was available free-of-charge, enabled me to prioritize and plan the various improvements so that the most important and impactful work was scheduled first. (While I did not make use of it, cost sharing for design and construction is also often available.) The NRCS technical experts designed a diversion as the first intervention.

A diversion consists primarily of an earthen embankment, which is constructed with back and front slopes and contains a channel at its base. This channel serves to catch, slow, infiltrate, and direct runoff into a spreader, or adjacent area of land, where the runoff can infiltrate slowly into the ground. In general, a diversion prevents or reduces soil erosion and concentrated runoff flows by “walking” the collected runoff along the channel, which is built at a 1 to 2 percent grade, to the spreader located at the end of the diversion. This outfall must be carefully designed to ensure that it will not cause erosion.

Timberidge Farms Diversion

Steps in Constructing a Diversion

1. Strip all topsoil from footprint of diversion.
2. Make a cut using heavy construction equipment to form a channel and obtain subsoil to establish embankment.
3. Redistribute stockpiled topsoil uniformly over entire diversion.
4. Fertilize, seed, and mulch front and back slopes.
5. Establish spreader at end of channel into which excess runoff traveling along channel can exit.

Diversion Schematic

A diversion typically can be constructed on any farmland which contains a slope over some distance. With my diversion, which was 55 feet in width, I seeded the filter strip located above the channel, with a grass seed mixture, and the embankment itself with a mixture of pollinator-friendly wildflowers and native tall grasses.

In effect, the diversion divided the 21-acre farm into two watersheds, an upper 15-acre and a lower 6-acre watershed. The lower watershed contained all the farm buildings, driveway, and parking area as well as other areas vulnerable to damage from uncontrolled runoff. The diversion was put into place prior to any improvements that were likely to cause extensive earth disturbance, including restoration/ construction of farm buildings, relocation of the driveway, landscaping, as well as any other earth moving activities.

In addition to the diversion, other BMPs were later put into place to manage runoff on the property. These included: rain gardens; small retaining basins; grassed waterways; cover crops; and, contour strips across fields. On any property, the first intervention should be selected and designed with future projects in mind, considering how, collectively, they might work together to manage runoff.

As a conservation-minded property owner, I have been gratified to observe that very little runoff from my farm now enters into the adjacent Little Falls tributary. Enabling the stormwater that collects to percolate into the soil on my land raises the local groundwater table, which, in turn, feeds the seeps and springs, which create and maintain the baseflow of the tributary. Increasing or maintaining the baseflow is vital for sustaining stream flow and ecosystems through dry periods. Strong baseflow contributes to the stability of stream channels and banks, attractive wildlife habitat, water quality, and stream aesthetics as well as reducing erosion. Doing my part to maintain this ecologically important cycle has enabled me to contribute to clean water in the local watershed, while at the same time allowing me to enjoy clean water, beautiful spaces, and productive agriculture on my own land.

With weather forecasters predicting an increase in both the number and intensity of storm events, stormwater volume is likely to increase. For owners of properties containing sloped areas, diversions are one of the many simple yet highly effective, tools available for controlling stormwater flow. Any landowners who are concerned about stormwater flow and how it may affect their properties, their communities, and the environment in general would be well advised to seek expert guidance on stormwater BMPs that they can put into place on their land.