By Gilbert Shibley

Some culverts are small enough so that installation can be a do-it-yourself project. I have done a few like that, in the 4 to 12 inch diameter range and up to 20 feet long. They were simple enough, and hardly worth mentioning. But now that I have done my third larger one, in the 24 to 72 inch range and up to 40 feet long, here are a few things I have learned. It may help some of our readers to think about similar projects of their own.

Crossing a stream by using a large culvert is one option, a bridge is another. While any of us might do the planning for a big culvert, installation itself is beyond what most of us can do with our own equipment. So you might get bids and contract the job to someone with the equipment and experience to tackle such a big project. Or maybe your logger will have what it takes, as mine did. His track-hoe (excavator) was to be on site anyway for parts of the road-building and logging. He also has considerable experience installing culverts. I found other kinds of help, too, but was in charge of this project myself last summer when we put in our latest, a 6' x 35' culvert.

First, one must determine whether the stream is fish-bearing. Ours is, both by observation and by listing on maps to be seen at the Molalla office of the Oregon Department of Forestry (ODF). A fish-friendly culvert should allow passage for juveniles in the under-6 inch class and perhaps even for mature spawners. Printed guides are available on how to achieve this, from either ODF or Oregon Department of Fish and Wildlife (ODFW). They specify maximum slope, depth if countersinking, and flow conditions at the ends, such as maximum height of outfall to stream surface.

How big does the culvert need to be? It must be able to handle the peak flows expected once every 50 years. How big a flow is that? Our Forest Practices Forester (FPF) helped with this one. It obviously depends on the size of the stream. That depends, in turn, on how much land area is drained by the stream above your culvert and on the average annual rainfall in your area.

My FPF checked my numbers on land area (from a topographic map) and rainfall. Then he plugged those into a table of values predicting 50-yr. flows, and then into one which tells culvert size needed for that flow. Since we were embedding the culvert into the stream-bed to 40% of the culvert's height, we needed a bigger diameter culvert to still carry the flow. I rounded up from the recommended size to 72", the next larger size, and ended up buying one 20 foot and one 15 foot section of that, plus a "belt" to clamp around at their junction.

My logger hauled them to our site and unloaded them with the track-hoe. Then he started digging the bed. It stirred up a little mud and colored the water. The authorities expect this, asking only that it be as little as possible and at the right time of year. (By the way, when I began planning the Forest Practices Act held one big surprise for me. It requires that work in our stream be within the time-frame of July 15 through August 30. We missed it by a week the first time and had to wait a year. I had known a Written Plan was needed as part of the permit process with ODF, as it is for this and any other work within 100 ft. of a stream. They have a form to follow, so it is not quite like writing a plan from scratch.)

When the "hoe" had dug down enough and we could see (not easy in muddy water) that the whole bed lay at the right slope and depth, we fastened the sections together, lowered them in place and began to back-fill. This culvert ended up six feet under! The fill material we used was the same clay-based soil taken from approach slopes on both sides that needed cutting down anyway, to ease the grade for log trucks. This went in at a medium moisture content, and packed well (layer by layer) with the logger's dozer.

Our final steps were to make the whole thing as fish-friendly as we could. I conned some able-bodied relatives into helping me put a pickup load of basketball-sized rocks into the stream-bed within the culvert, to give it extra texture. The last task done with the track-hoe was to take the big, old logs that had been the "log culvert" put down for this crossing 50 years ago and place them carefully back into the stream just below the culvert. A biologist from ODFW told us what angles would be best, and OK'd adding a couple of big boulders.

I put a liberal sprinkling of grass seed mix on the fill-slopes and scattered several bales of moldy hay as a mulch. Since it was so dry last September I went the extra mile and twice wet down my new seeding — while testing out my fire pump! Later I put some little water bars on the road surface with my hand shovel, to divert the fall rains toward ditches below. Monthly inspections through the winter showed me that all is well. Stream, culvert, and fill-slopes all in the same places as in October. The scene is, of course getting greener as time goes along-and so, perhaps, am I.

Master Woodland Manager Gilbert Shibley is a CCFFA Board Member.