William Lord
Area Environmental Agent
NCCES
Ditches are everywhere: from roadsides, to farm fields, to back yards in residential Neighborhoods. Most people ignore ditches, or at best, try to stay out of them, but ditches are usually where our waterways begin, and we should take care of them if we want clean water.
Ditches are dug for drainage. Highways are built with a crown in the center of the road so water will drain to the road's edge and then go into a ditch to be carried away. Farmers dig ditches to drain water away from fields, and the eastern part of North Carolina is full of small and large networks of drainage ditches moving water away from fields. But, if you consider the fact that ditches are connected to our streams, rivers, lakes, and oceans, are we taking the best care of the headwaters of our water resources?
The water entering roadside and urban ditches is called stormwater. Stormwater, even urban stormwater, entering storm drains is seldom treated. It runs directly to the nearest stream or ditch. Thus, any roadside trash, cigarette butts, oil, or grease on the road travels into and down our ditches straight to the nearest stream. Some people have been known to park their cars over ditches to change the oil, dumping the oil into the ditch, creating an instant oil spill in the nearest stream. Used oil should be recycled, and never dumped into a ditch or onto the ground.
Water is a powerful substance. Think of the Grand Canyon - it started as a ditch but was created by running water. Water always flows downhill, but as it flows it can gain in velocity, power, and volume. Since ditches are the first place water collects during rainfall events, bad things can happen to poorly constructed or poorly maintained ditches. If a ditch is V shaped and has bare soil in the bottom, water will be concentrated in the bottom of the ditch and the odds are the water will cut away the bottom of the ditch. This is called entrenchment. As the ditch becomes entrenched the water may begin to back-cut the ditch. Back-cutting occurs when a small waterfall begins to cut away the bottom of the ditch and the waterfall cuts its way back up the ditch, cutting away soil and rock and making the ditch bigger and deeper.
The best way to build and maintain ditches is the make and keep them with a well vegetated, flat bottom. Flat bottoms make water spread out and slow down, giving the water a chance to infiltrate into the soil and allowing sediment and other pollutants to settle out. Vegetation also slows down the water and helps clean pollutants from stormwater. The roots of grass and trees and shrubs offer the best protection from erosion. Roots are constantly renewed by plants and over time become stronger and stronger. If your ditch has a grassy bottom then let the grass grow to 4inches or higher. The taller the grass the better it can hold the soil and treat stormwater runoff. One common mistake people make in ditch maintenance is to spray the ditch to kill all vegetation. This is done to make mowing and trimming easier, but if all vegetation is killed then the ditch is exposed to the forces of erosion. Another poor ditch protection strategy is to add rock, rip-rap, or concrete to protect the ditch. Water can easily work its way under and around rocks and concrete - particularly during large storms - to cause erosion and back-cutting. Speaking of storms, ditches should be constructed and maintained in anticipation of the largest potential storm - in our case hurricanes. If a ditch survives intact all year but is blown out by a single large storm, the ditch has still failed.
As our communities continue to grow and develop stormwater becomes an increasingly important issue. As we add impervious surfaces like roofs, parking lots, and roads, stormwater loads increase and flash flooding becomes a serious problem. One strategy to combat the negative consequences of development is to use Low Impact Development or LID. LID emphasizes use of many environmentally friendly technologies, like green roofs, cisterns, bioretention and swales. Traditional curb and gutter is replaced by broad flat grassy swales designed to slow and treat stormwater leaving impervious surfaces. Deep ditches, which can concentrate stormwater, are avoided as much a possible.
Think about how you build and maintain ditches. Perhaps a swale is in your future?
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