The Six-D System for Effective Waterbars

Michael J. Fumiss

USDA - Forest Service

Six Rivers National Forest

To remember how to make effective waterbars, just remember the 6-D System.

The six D's are: DRAINAGE AREA, DISTANCE, DIAGONAL, DIVERT, DISCHARGE, and DISSIPATE.

The purpose of waterbars is to control erosion on'roads, skid trails, trails and firelines. To be effective, waterbars must break up drainage areas so that runoff during heavy storms can be handled by the soil with little or no erosion. Road and trail surfaces usually cause runoff during heavy rainfall and snowmelt. Waterbars break up runoff into small enough units so that it doesn't have enough energy to erode our soils. Waterbarring approximates the energy-dispersing drainage effect of an outsloped road or trail.

Effective waterbarring can make the difference between an eroding, degraded site or impassable road; and a stable, productive, and accessible natural system. Of course, locating roads and skid trails on higher, flatter, drier, and rockier ground and away from steep, wet, or unstable ground is the most important erosion control measure.

This brings us to the first D_DRAINAGE AREA. When deciding where to put waterbars, study the situation and get a good estimate of the DRAINAGE AREA. Waterbar spacing tables are based on a standard width of road or skid trail (the runoff area); about 12 feet. If the road or trail is about 12 feet wide then you're Ok using these tables. However, if the road or trail is wider, or several are comina toaether, or the cut bank "leaks" water, then you have to adjust-because the DRAINADE ARE.Cis different than the tables account for. The adjustment is simple; which brings us to the next D-DISTANCE.

DISTANCE means the distance between waterbars on a road or trail, or between a waterbar and the next upslope drainage break. Firstly though, if there is a "leaky" cut bank or a small stream crossing the road or trail, it's best to put a waterbar at that point so that the water can keep going downhill in the same place it is accustomed to. During the dry season, finding these can really test your ability to read the land. Missing these is a major cause of erosion.

Where two roads or trails come together or they are wider that 12 feet; simply figure the proportional difference between the surfaces you're considering and 12 feet. Then modify the DISTANCE in the table you're using by that proportion. For example, if a road is 15 feet wide instead of 12 feet, it is one-quarter wider; it's DRAINAGE AREA is one-quarter greater; and it's DISTANCE to the next upslope break should be one-quarter less than the table indicates. Another example: If two trails are coming together downslope and each is 12 feet wide, their combined DRAINAGE AREA is double so the DISTANCE should be halved. Keep in mind that waterbar spacing tables prescribe maximum DISTANCE between waterbars. Erosion is best checked when the DRAINAGE AREA is minimized so placing waterbars closer together than the maximum DISTANCE is often advisable.

After deciding where to put the waterbar, the next decision is how to construct it so that it does the job. An important principle in working with flowing water is that the more you oppose the water's energy, the tougher it is to get it to do what you want and keep it that way.

In other words, don't bully the flow, lead it. Waterbars built directly across a road oppose the water's energy and tend to fail. Waterbars built DIAGONAL to the road lead the water off and work much better. A DIAGONAL waterbar has a gentle slope along its base, and is easier to drive over. A simple rule is to add 5 to the gradient of the road and build the waterbar at that many degrees off perpendicular.

A good waterbar will DIVERT the water off the road or trail. To do this it must be sufficiently deep to handle all the flow and must also be durable so that it will do so for as long as it's needed (be sure to consider traffic). Excavation is much more effective than fill-in making a durable and effective waterbar (a ditch or a dip beats a dike).

Another feature of a good waterbar is that it will DISCHARGE the flow. If it is a dam, it is likely to fail. It must have an open outlet.

Finally, a good waterbar should DISSIPATE the flow just below the outlet to exhaust its eroding power and cause it to infilter the soil. This can be done by placing slash, rock or debris below the outlet, or modifying DISTANCE to take advantage of natural features that will DISSIPATE the water's erosive energy.

So remember, when locating and building waterbars; consider the DRAINAGE AREA, then place them the right DISTANCE apart, at a DIAGONAL to the road, so that they DIVERT, then DISCHARGE, then DISSIPATE the energy of the flowing water. And remember to make them deep enough so they'll be durable.

Recommended maximum* spacing for waterbars on temporary roads, trails, skid trails, and firelines**

When in doubt, reduce the spacing. Soils are non-renewable & walerbars are inexpensive.

From: UDSA-Forest Service, "Sale Administrator's Handbook'