Q: I have installed several culverts that have blown out this winter due to high rain events. Last month you mentioned a workshop for county commissioners and road crews that talked about proper culvert sizing and installation? I would like to know how I can prevent my culverts from blowing out, but question whether they should be installed to withstand a 5 inch rain. Is this reasonable? What can I do?
A: Sorry to hear about your culverts blowing out. Culverts can be used as effective stream crossings, but when they fail, they can be very costly to replace, not mentioning the time it takes to reinstall them, but also the extra sediment that reaches the stream channel. This is why it is so important that proper culvert sizing and installation techniques are used.
It may seem as if a 5 inch rain is not a common occurrence in East Texas and would be hard to plan for, but in all actuality this is not as rare as you might think. This type of rain event does occur, especially in the last couple of years. Installing best management practices (culverts, waterbars, wing ditches, etc.) to safeguard against larger storm events would not be economically feasible and even the best structures would fail during natural disasters.
Before installing culverts, it is important to know the soil type, slope, and how big your drainage area is so you can properly size your culvert. Soils information can easily be found using a soil survey. Topographic maps can help you determine the slope and drainage area of the tract that you are working. To calculate the drainage area, use a topographic map to locate hill tops and decide which way runoff water will drain through a particular point (stream crossing). On a standard 1: 24000 USGS topographic map, one square inch equals 90 acres.
Once you have determined the soil type, slope, and drainage area, you can use a culvert sizing chart to assist you with selecting the right culvert. This chart is found on page 50 of the Texas Forestry Best Management Practices Handbook. Make sure that you buy a long enough pipe so that at least 1 foot extends on each side past the fill material.
Place the culvert in a firm, straight section of the stream channel on a 1-2% downgrade to prevent clogging. There should always be at least one foot of cover to help anchor culverts in and protect them from being damaged by traffic. A general rule of thumb to follow is one foot of fill per one foot of culvert diameter. It may also be necessary to install rip rap, geo-textile cloth, or large stone at the inlet and outlet to reduce erosion and washouts. The approaches to the crossing should be straight and at right angles to the streambed.
Frequent inspections and proper maintenance needs to be performed on all culvert crossings to ensure that they are safe and functional. A clogged or damaged culvert is very susceptible to blowing out under normal rainfall. If you have a question regarding BMPs, please call me at (936) 639-8180.
* This article was published in the April 2002 issue of the Texas Logger