Q: When dealing with temporary stream crossings, I have been told several things about where to put waterbars to prevent road sediment from entering the stream. I have heard that you are supposed to put a waterbar on each side of the streambank at the crossing to prevent any water flowing down the road from getting into the stream. I have also been told to never put any waterbars inside the SMZ. Would you please clear this up for me?
A: It is certainly easy to see how both recommendations could seem valid, but let’s see if we can’t shed some light on this debate.
The thought behind closing out a temporary stream crossing by constructing a waterbar at the edge of a stream is that it will provide a place to stop or redirect any water entering the SMZ just before it gets into the stream. However, experience has shown that waterbars at the streamside typically serve more as a source of sediment. These waterbars will often slough off into the stream with a heavy rain. If rains are heavy enough to cause the stream to overflow its banks, the waterbar may be completely washed away.
Therefore, as a general rule, waterbars should be kept at least 50 feet away from the stream channel (i.e., outside the SMZ). A properly constructed waterbar at the edge of the SMZ will divert water off the road as it approaches the SMZ and give plenty of distance for the water to slow down, spread out, and drop any sediment before it reaches the stream. Just be sure you don’t construct a long narrow wing ditch off that water bar into the SMZ that will channelize the water and direct it into the stream.
Of course, as with any rule, there are exceptions. If your SMZ is wider than the minimum 50-foot recommendation then it would be alright to have a waterbar within the SMZ. If you are dealing with steep approaches and erosive soils then it may be necessary to construct an additional waterbar with in the 50-foot buffer. However, you should try to keep it as far back from the stream bank as possible and be sure that it does not discharge directly into the stream channel.
It is important that you also re-establish the original slope and condition of the streambank as best as you can when you pull out your temporary stream crossing. An effort should be made to stabilize the streambanks where the crossing and approaches to the crossing were. Stabilizing these areas becomes even more important for crossings with more erodible soils or steeper slopes. This can be accomplished using grass seed or logging slash; however, you want to make sure that any logging slash you put next to the stream is well incorporated into the soil and above flood level so that it will not wash into the channel during high flows. If this is done, there should be little or no erosion at the crossing.
As always, the very best way to prevent sediment from entering a stream at a crossing is to avoid putting in the crossing in the first place.
For more information on stream crossing BMPs visit the Texas Forest Service webpage at http://txforestservice.tamu.edu/water or contact me at (936) 639-8180.
* This article was published in the June 2011 issue of the Texas Logger