By: Jake Donellan, BMP Forester (Ret.), Texas Forest Service
Q: I am a road contractor for a large timber company and I was looking at my bluebook the other day and I noticed that in Part II - Recommended Specifications, you make a distinction between Broad-Based Dips and Rolling Dips. I was under the impression that these two were basically the same thing. Can you explain the difference in these two and when you might use one over the other?
A: Don't feel bad about not knowing the difference between these two BMPs, we are asked this question quite often. At first glance these two practices are very similar and both are designed to accomplish basically the same goal: provide cross drainage on in-sloped roads. The differences between these practices are very subtle but despite these minor differences, the benefits of these BMPs can be a very valuable during road and skid trail construction and/or maintenance.
Both rolling dips (Pg. 45 in the blue book) and broad-based dips (Pg. 42 in the blue book) are dips and reverse slopes in the road surface with an outslope for natural cross drainage. Rolling dips are designed to be used on haul roads and heavily used skid trails. Broad-based dips differ in that they are designed for use primarily on heavily used haul roads. Rolling dips can be used on roads with up to a 15% grade while broad-based dips should be used on roads that do not exceed a 12% grade. The spacing on broad-based dips should change with every 2% change in gradient, while the spacing with rolling dips changes with every 5% change in gradient.
With broad-based dips, the reverse grade should always be 3% and near 20 feet in length.
Rolling dips are a little more flexible in that the reverse grade can be 3% to 8% and can range from 10 feet to 15 feet in length. These differences allow higher vehicle speeds on broadbased dips than on rolling dips.
Since broad-based dips are used on high traffic roads some other considerations may be necessary; on some soils the dip and reverse grade section may require bedding with crushed stone to avoid rutting the road surface, also, energy absorbers such as rip rap and, in some cases, a level area should be installed at the outfall of the dip to reduce water velocity thus assuring no erosion of cast materials.
With both types of dips, neither the dip nor the hump should have a sharp, angular break, but should be rounded to allow smooth flow of traffic. Properly constructed dips require minimal annual maintenance and continue to function years after abandonment.
You can get a copy of the blue book at yourlocal Texas Forest Service office or you can view it online at http://txforestservice.tamu.edu/water. If you have any questions regarding BMPs please contact me.
* This article was published in the February 2004 issue of the Texas Logger