Ditching Best Management
DITCH MAINTENANCE DECISIONS
David Orr, PE, Director
Folks who work at local Soil and Water Conservation Districts (SWCD) like to remind us that we all live in a watershed. For highway departments, ditches are an integral component of the watershed that are also critical for the highway. Proper maintenance is important for the road and the environment.
The Value of Ditches
Ditches carry collected surface water to natural streams. They also collect some subsurface water from the base of the roadway or surrounding land. If maintained properly, they keep water away from the roadbed which helps the road last longer. Ditches can also divert water to and from a highway.
A properly managed ditch is relatively shallow and only drains a small area. If the ditch has to carry additional flow, it is best to get help from the local SWCD or other professionals to make sure the ditch will do the
Roadside ditches are also used by surrounding landowners to carry the flow of water away from fields. If done properly, this can work fine, but it needs to be done carefully in coordination with both the landowner or farmer and the local highway department. If done improperly, pollutants and sediment from the field can get into, and clog up the ditch. Again, the local SWCD is a great resource to make sure this is done well.
If a ditch is too deep, it can become a safety hazard. Ditches only need to be deep enough to carry the surface flows, and possibly drain the base of the roadway. Most ditches are already as deep as they need to be and making them deeper does not help. If there is a driveway along the highway, the size of the culvert under the driveway controls the capacity of the ditch to carry flow in most cases.
Intercepting Natural Flow
Just as ditches help keep a roadbed dry, they can also intercept natural drainage flow. Ditches not only drain the road surfaces, but they also efficiently capture runoff from adjacent hill slopes. Research has shown that ditches can increase the runoff in natural streams by as much as 300 percent which can lead to flooding. In fact, over 20 percent of the natural flow in the watershed may be diverted by the ditch system! This increased flow can lead to increased erosion and flooding downstream which is especially problematic when the ditch is over-scraped and not properly seeded or protected from erosion.
Ditches can also help carry pollutants and sediments downstream. There is some evidence that increases in pollution from ditches may be increasing the number of hazardous algae blooms (HABs) seen in the last few years across the state. If ditches intercept natural flows, the effect can be to lower groundwater tables, induce drier streams, empty wells, increase stream bank erosion, and add more pollution to our drinking water supplies.
Ditch Maintenance Practices across NYS
A significant portion of the work undertaken by most highway departments is ditch management. A few years back, Cornell University Professor Rebecca Schneider, and her graduate student Anthony Johnson, surveyed all of the town and county highway officials across New York State about their ditch management practices. They received a very high response rate (41%) from the 999 highway staff surveyed.
A third of the agencies reported using full scraping or reshaping without any reseeding as their primary method of ditch management and half the agencies scraped their ditches on average once every one to four years. Based on these numbers, one-third to one-half of the roadside ditches across upstate New York are more prone to erosion and increased flow beyond what would be expected if seeding and best management practices were followed in every case.
This translates to thousands of miles of exposed substrate that is more vulnerable to storms and acts as a source of sediment and pollution. Limited resources including time, labor, equipment, and money were the primary reasons given for the less than ideal management practices used. Additional challenges identified included interactions with landowners over ROWs, farm-field drainage, and increasing frequency of downpours.
The study also identified barriers to the implementation of recommended best practices to improve ditch management. A majority of those who were surveyed reported conducting ditch maintenance:
A) When convenient,
B) With scheduling, most commonly based upon existing maintenance plans,
C) Prior to roadwork and resurfacing projects, and,
D) In response to constituent complaints.
Ditch management is not just about what happens within the ditch, but also about where the ditch discharges. While 35 percent reported that most of their ditches discharged straight into streams, the majority (65%) of respondents stated that less than one quarter of their ditches did so. Over two-thirds (67.8%) of the respondents stated that they were concerned about reducing sedimentation and water pollution and would consider redirecting existing ditch water outflow away from surface water sources and into infiltration basins or other sources. Of those who would not consider redirecting ditch water outflow away from surface water sources, the perceived costs, time, equipment and space needed to make such changes were the primary reasons.
Respondents also noted an increase in the use of roadside ditches for farm field drainage, including increasing drain tile flow and sedimentation from exposed farm fields. Several agencies mentioned that weather, and specifically the increase in severe rains, is a growing problem.
BMPs for Ditches
There are many Best Management Practices (BMPs) for ditch maintenance. These practices can generally be categorized as addressing separate, but overlapping goals, i.e. draining the road, decreasing flooding and reducing water pollution. There are plenty of times when a ditch needs to be reshaped or rebuilt to slow down the flow of water. In some cases it may be important to redirect the ditch so it does not discharge directly into streams or lakes. See the resources at the bottom of the article for some ideas. In most cases, however, the most critical activity to improve the performance of a ditch is proper maintenance. Here are some of the more critical items to keep in mind as you perform basic ditch maintenance:
Keep Ditches Shallow…
Ditches, for the most part, do not need to be that deep. Even a few inches into the subgrade is usually enough to drain the road base. If there is a driveway in the ditch, the capacity is controlled by the driveway pipe and making the ditch deeper or wider is likely to lead to more erosion, not more capacity to carry flow.
Clean, Don’t Scrape…
Remove as little material as needed to reestablish the flow line of the ditch. Do NOT scrape the fore and back slopes. If possible, survey the work ahead of time or use a laser level to set the grade. Work uphill so the ditch is not over scrapped. Going downhill also leads to flat spots that hold water and can actually encourage mosquitoes.
The last step is not the last truck-load of removed material, it is reseeding the ditch. Many SWCD have hydro-seeders, but even seeding by hand can be very effective if done as part of the initial cleaning operation. Waiting a couple of weeks will be too late so seed immediately!
If the ditch is too steep, rock or check dams may be warranted, but too much rock can lead to overheating of the water and can be difficult to maintain long-term. If using rock, erosion blankets, or any other erosion protection method, be sure these methods are ready to go as part of the initial ditching work.
Mow if Possible Instead...
In many case, just mowing the ditch is enough to get the flow reestablished. Look for alternatives when you cannot mow often enough to keep the ditch flowing. Try to make mowing your primary ditch maintenance tool.
When in doubt, GET HELP...
There are lots of sources to help. As previously mentioned, the local Soil and Water Conservation District is a primary resource. You just have to give them the most valuable commodity you have: time.
Training on the Horizon
The survey found almost half of the respondents (48%) did not consider revegetation of ditches to be a priority. This lack of understanding about the contribution of sediment to water pollution indicates a critical gap. Highway staff need to understand how the roadside ditch networks, and the highway department's management of the networks, may be directly contributing to flooding and pollution problems.
Recently, there have been new grants received by Cornell to develop tip sheets, help with ditch inspection and inventory, and teach more of the BMPs used with ditch management.
Also, Cornell is looking at ways to replumb ditches to allow them to serve both the needs of the road system, and the overall need to slow the water down and put water back into the natural drainage pattern. This replumbing work also has the added benefit of shallowing up ditches to make them less of a safety issue. More details will be coming in the future, but if your department is willing to help by installing a prototype, please let us know.
Stormwater Management, Cornell Local Roads Program,
CLRP No. 14-03
Roadway and Roadside Drainage, Cornell Local Roads Program, CLRP No. 98-5
Roadside Ditches: Best Management Practices to Reduce Floods, Droughts, and Water Pollution
Understanding Ditch Maintenance Decisions of Local Highway Agencies for Improved Water Resources across New York State
Rebecca Schneider, PhD, David Orr, P.E., Ph.D., Anthony Johnson
Additional CLRP Ditching Resources:
Understanding Ditch Maintenance Decisions of Local Highway Agencies for Improved Water Resources across NYS (pdf)
Rebecca Schneider, PhD, David Orr, P.E., Ph.D.,