‘Re-plumbing’ our watersheds
Rethinking roadside ditch management can reduce flooding and pollution, recharge groundwater and buffer the impacts of climate change
Until recently, concerns about climate change have focused mostly on warming temperatures. But there is growing recognition of the potentially more serious impacts from changes in precipitation patterns.
Drought in California, Texas and the Southeast has highlighted how these changing patterns affect drinking water supplies, crop and livestock production, ecosystem health and the very fabric of our society. Meanwhile, the northern Mississippi basin has been alternating between extreme floods and droughts for the past decade.
In the Northeast, we are already experiencing an increase in the magnitude and frequency of high-intensity rainfall events. These downpours bring more flooding, soil erosion and movement of sediment that pollutes our streams and rivers.
While progress on reducing greenhouse gas emissions moves at a snail’s pace, government officials at the federal level and in most states are recognizing the need to develop adaptation strategies to better manage water resources and increase the resiliency of communities to flood and drought.
Researchers at Cornell have identified roadside ditch networks as a critical driver of these adaptation strategies. Hundreds of miles of ditches crisscross every watershed. If we rethink how we manage these ditches, we can reduce flooding and pollution, recharge groundwater, and buffer our communities from the impacts of climate change.
The trouble with ditches
Over the past decade, I have led a research team that has documented the critical role of roadside ditches in sustainable water resource management. That team includes Todd Walter in the Department of Biological and Environmental Engineering, David Orr of the Cornell Local Roads Program, Dan Buckley in the Department of Crop and Soil Sciences, and graduate and undergraduate students,
Our studies show that ditches don’t just capture road runoff. They also intercept about 20 percent of the rainfall as it drains the hills upslope. The ditches then rapidly shunt that water downstream and discharge it like a high-velocity faucet directly into streams.
This discharge increases the magnitude of peak stream heights and contributes directly to flooding in the small- to mid-sized catchments that make up the majority of each larger watershed. Rapid shunting of surface runoff leads to less groundwater recharge and falling water tables. Headwater streams now routinely dry out as a result of the diversion of surface runoff and reduced baseflow between storms.
Ditches are also a significant source and conduit of suspended sediments. This is especially true when highway departments routinely scrape the ditches and leave miles of bare substrate vulnerable to erosion. Ditches also transport de-icer salts spread on roads.
Agricultural ditches and tile drains often discharge into roadside ditches. Fecal coliform bacteria from manure spread on headwater farm fields, as well nutrients from manure, fertilizers, leaking septic fields and other sources can reduce the quality of downstream drinking water supply systems and sometimes force closing of swimming areas.
Gravel and rocks tumbling along ditches during storms are deposited as deltas where ditches meet streams, redirecting stream flow and causing chronic scour and erosion along the stream banks.
Best Management Practices
Roadside ditch design and management has changed little in the last century. Ditches are a mundane element of our landscape and their role in water resource management is mostly overlooked. Highway departments are routinely left out of the water-management discussion.
That’s not good, because there are several practical ways we can “re-plumb” our ditches so that they become part of the solution:
- Disconnect ditches from streams and reroute ditch flow through under-road drains or into constructed wetlands or infiltration basins.
- Reconfigure and hydroseed ditches so that they can be mowed instead of scraped.
- Install check dams to slow down ditch flows and decrease sediment transport.
- Re-route agricultural ditches and tile drains to on-farm basins
Adopting these Best Management Practices (BMP) is essential for reducing erosion, improving water quality in streams and lakes, and ensuring good infiltration and groundwater recharge to maintain water tables and avoid summer dry-outs in homeowner wells. But Identifying the BMP is only half of the answer. It is equally important to identify stakeholders and overcome hurdles preventing the adoption of these practices.
Through the Cornell Local Roads Program, our team has given presentations, distributed factsheets and posters, and engaged nearly 2,800 town highway staff and local government officials across New York State in discussions on roadside ditch management. Their feedback has consistently identified several major barriers:
- Tighter highway department budgets limits funds for labor, equipment and other resources.
- Private property ownership complicates management, restricting options for ditch modification that are available to highway staff.
- Highway superintendents may be unwilling to use alternative management approaches that might anger the voters who elect them.
- Design and installation of agricultural tile drains are often overseen by Soil and Water Conservation Districts, which work independently of local governments.
- Highway departments consider routine ditch scraping their primary management tool. State or federal mandates or incentives may be necessary to change these long-used practices.
Carrots and sticks
Barriers like these are not unique to roadside ditch management. They also hinder adoption of other green infrastructure strategies, such as green roofs, bioswales, and rain gardens. These practices are similarly touted as solutions for stemming stormwater runoff.
This sluggishness is somewhat surprising given that EPA Phase II Stormwater Regulations have increased the top-down pressure on smaller communities, and federal funding is increasingly available for green infrastructure projects. A few studies have documented that key limitations are the paucity of data proving effectiveness of these strategies combined with a lack of guidance and training on how to construct them.
Since most grants require matching funds, towns with limited budgets and skepticism about practices are unlikely to take risks and default to traditional practices. Good demonstrations, more research documenting their effectiveness, and widespread training programs should help catalyze changes. Fortunately, studies now show that targeting resources to “high capacity” towns with strong leadership and experience at collaborative partnerships can have strong ripple effects to neighboring communities.
The traditional approaches to potential flooding under high intensity events has been to shunt rainwater out of the watershed through roadside ditches, drain wetlands which would store floods, build levees along rivers to prevent rising waters from reaching floodplains, and straighten and deepen the rivers, ultimately turning them into concrete sluiceways that race fresh rain water down to the sea.
A more sustainable approach is to capture this excess rainfall and store it in underground aquifers or aboveground, in ponds, wetlands, and reservoirs. These practices will reduce flood-potential right at the headwaters while also improving future drought resilience.
Our climate is changing rapidly and irrevocably. Even if we quickly reduce greenhouse gas emissions, we will still need sustainable approaches to capture more rainfall in aquifers, ponds, wetlands and underground reservoirs. Re-plumbing our watersheds can play a key role in buffering the impacts of climate change on humans and ecosystems alike.
Rebecca Schneider is an associate professor in the Department of Natural Resources, College of Agriculture and Life Sciences, Cornell University.
Category: Climate Change Forum