What's with the stone-lined ditches?

Residents on Tucker Hill Road in Thetford were mystified by the significant amount of time, labor and materials poured into re-shaping a roadside ditch that had never seemed problematic, turning it into a V- profile cut lined with broken rocks. A contrast was drawn with an abutting town where a ditch had recently been reworked to a gentle U-profile without the stone lining. Instead there were periodic small dams of stones that would serve to slow the water flow, presumably to allow it to better soak into the ground. Why the difference?

Blame a state program known as the Municipal Roads General Permit for Stormwater Discharges from Municipal Roads (MRGP), that was adopted in 2017 and updated in January 2023. This permit is required under the federal National Pollutant Discharge Elimination System (NPDES) and is issued by the State of Vermont in its role as administrator of the NPDES under VT statute 10 V.S.A. § 1264. The permit essentially gives a town the authorization to discharge road stormwater runoff, under conditions laid out in the MRGP document. Each town pays a prorated fee for this, based on population and road miles.

As stated by the VT Agency of Natural Resources (ANR) the MRGP is intended to protect streams, rivers and lakes from the pollution and other effects of road runoff and "is intended to achieve significant reductions in stormwater-related erosion from municipal roads, both paved and unpaved."

Road or stormwater runoff is defined as "rain or snowmelt that runs across the land surface and discharges to rivers or lakes." Natural areas have spongy soil and most stormwater is absorbed into the ground, evaporates, or is taken up and transpired by plants. However, when the land cannot absorb rainfall because it is paved, covered by buildings with impervious roofs, or compacted by mowing or agricultural machinery, stormwater runoff unnaturally swells streams and increases their velocity, exacerbating streambank erosion and endangering  homes, roads, and other infrastructure.

The increase in outbreaks of toxic blue-green algae in lakes and ponds and the decrease in clarity of lakewaters are other effects of stormwater runoff, because it flushes excessive amounts of phosphorus that feeds algae into waterbodies.  Likewise, pollutants including sediment, road salt, oil, grease, coolants and toxic tire dust from automobiles contribute to poor water quality and harm aquatic life.The problem is magnified by the hilly terrain of VT that has historically resulted in most roads clinging to the banks of streams and rivers.

The MRGP, which was updated in January 2023, states " Municipalities will implement a customized, multi-year plan to stabilize their road drainage system. The plan will include bringing road drainage systems up to basic maintenance standards...."

What this means is the town must identify all its "hydrologically connected" road segments defined as " ...a road segment, equal to 100 meters in length, where the Secretary has determined that road and drainage characteristics indicate a likelihood of discharges to surface waters or wetlands."  The town is obligated to perform a Road Erosion Inventory (REI) of those sections.  Furthermore, any town road section "...uphill from, and draining to, a municipal road that bisects a water of the State or wetland, should be included in the REI to accurately capture the extent of the stormwater watershed."

The 2023 MRGP sets a deadline for a town to comply. The completed REI reassessment must be submitted by October 31, 2027. The VT Department of Environmental Conservation (DEC) uses this data to identify "Very High Priority" road segments and sets a schedule for the town to bring them into compliance with MRGP standards. Altogether "the municipality shall upgrade at least 7.5% of non-compliant road segments (including Very High Priority sections)  ....each year."  Fully compliant roads must be achieved by December 31st 2036.

As for those all-important road drainage ditches - the standard for ditch construction depends on the slope of the road.  Grass-lined ditches with a U-shaped cross section are allowed on low slope roads (0-5% slope, where a 5% slope means 5ft of vertical rise over 100 horizontal feet.) As the road slope increases the recommendation shifts towards stone-lined ditches. Thus at road slopes between 5 and 8% a town can choose grass-lined ditches with stone check-dams or completely stone-lined ditches. Once the slope is above 8%  the only option is the stone-lined ditch. And "broken rock" is specified because it does not pack tightly, leaving many spaces where water can infiltrate.

According to the VT Department of Lakes and Ponds, the "efficient removal of runoff from the roadway will help preserve the road bed and banks. Well designed ditches provide an opportunity for sediments and other pollutants to be removed from runoff before it enters surface waters. This can be achieved by controlling, slowing and filtering the water through vegetation, stone, or other structures. In addition, a stable ditch will not become an erosion problem itself."

And there's more. A ditch may not discharge road runoff where it could flow directly into surface waters, after all the whole point of the MRGP is to reduce water pollution. Thus "roadway runoff shall flow in a distributed manner to grass or forested filter area..." and drainage ditches "shall be turned out to avoid direct outlet to surface waters." Attending to these details makes ditch construction more complicated for the road crew.

Ditches, as we may observe at this time of year, collect leaves. Eventually the filtering spaces between the broken rocks will become clogged. The only solution is to dig out all the rocks and replace them with new. The old rocks cost the town money, so they would be saved and stockpiled somewhere, to be turned occasionally by the ever-patient road crew. The leaves and other debris would dissipate through decay and rainfall washing. By that time the replacement rocks would probably themselves need to be replaced.

Anybody want to host a rock pile?