August BMP Q&A

By: Chuck Coup, BMP Forester (Ret.), Texas Forest Service

Q:  There are all sorts of difficult situations that can arise when my guys are attempting to construct a functioning wing ditch. Sometimes they work and sometimes they don’t. Can you tell me some of the most common problems you come across with wing ditches?

A:  Isn’t it amazing how a structure as simple as a wing ditch can sometimes be so troublesome! The primary function of a wing ditch is to collect runoff water from the road surface and roadside ditches and disperse it into stable areas away from water bodies or other sensitive areas. They are typically most effective when used in conjunction with a waterbar that intercepts, diverts, and drains runoff water from the road surface and the roadside ditch on the opposite side. It is really nothing more than a water outlet for the road, but knowing where and how to construct them in certain situations can be very tricky!

One of the most common problems I see with wing ditches is that they are longer than necessary. It is generally not effective to construct a wing ditch that carries runoff water long distances away from the road. This practice unnecessarily exposes additional soil to erosion and increases the distance that the runoff water has to flow before reaching stable, vegetated ground cover. Long wing ditches also run the risk of discharging polluted water into or near water bodies or other sensitive areas. Keep your wing ditches only as long as necessary to encourage the water to flow away from the road. One exception to this may be in extremely flat areas where it is difficult to get the water to drain away from the road. Typically though, if you have a little topography, gravity will do the trick. If you think you need to construct a long wing ditch in order to deal with a large volume of water being carried down your road, you should instead consider increasing the frequency of your wing ditches (and waterbars) by putting them closer together. That will divide the amount of water you are trying to manage between wing ditches.

Another problem I often come across is wing ditches that are constructed as narrow channels using the corner of the skidder or dozer blade. I frequently see these V-shaped channels in combination with wing ditches that are too long, resulting in a turnout that erodes and carries sediments excessive distances – completely the opposite of what we want. A better approach is to keep the dozer or skidder blade level with the ground and make a wide flat outlet that disperses the water over a broad area. This kind of outlet promotes sheet flow versus channel flow which spreads the water out and reduces its speed. Slow moving water cannot carry sediments as efficiently as fast moving channelized water can.

Other problems are typically related to where the wing ditch discharges. I occasionally come across waterbars that are correctly constructed on the edge of an SMZ but have a wing ditch that carries the polluted water through the SMZ and discharges it either directly into or within feet of the stream channel. That’s bad news for the fish and other aquatic wildlife. Another situation I repeatedly see are wing ditches constructed down steep slopes which eventually leads to excessive erosion. One way that you might avoid these situations is by putting a gentle uphill turn in your wing ditch outlet. “J-hooking” your wing ditches, as it is called, can help to divert the water away from sensitive areas like stream channels or steep slopes and also helps to slow the water’s speed. However, don’t confuse this practice with the futile attempt to carry water uphill by putting wing ditches on the high side of the road or against the slope of the land. The wing ditch should still have a slight down grade and follow the natural contour of the site (and be flat and only as long as necessary). That’s one approach that I have found to be effective, but I am sure that you guys have come up with more creative solutions to these problems. If you have, drop me a line as I would love to hear them!

Finally, if you get a chance, I would recommend that you go back and take a look at some of the wing ditches you installed in the past, especially the ones that you were uncertain about. See if they worked and if they didn’t what you might need to do differently in the future.

For more information on wing ditches and other BMPs visit the Texas Forest Service webpage at http://txforestservice.tamu.edu/water or contact me by phone at (936) 639-8180. 

* This article was published in the August 2011 issue of the Texas Logger