Can Better Logging Control Reduce Flooding?

By John McFerrin and Frank Young

Some of us think it regrettable that it took an event like last summer's floods to focus attention on timbering and how logging practices relate to flooding. At the same time, we think that the floods may have helped create an opportunity to improve timber and logging laws.

The Coalition for Responsible Logging (CORL) has been working for years to pass bill(s) to amend the state’s Logging and Sediment Control Act. Up until now the goal has mostly been to reduce erosion and sedimentation. Recently, some of us have contemplated how our bill(s) would reduce flooding.

The general scientific principles that apply to timbering and flooding are so well established and so obvious that there is no reason to debate them here. Whether there is a flood depends upon what’s called "peak flow." It doesn't matter what the average flow or the total flow over time is. What matters is how much water gets to the stream at any one time. To prevent a flood, all we have to do is prevent so much of the water from reaching the streams at the same time.

Trees help tremendously to keep all of the water from reaching the streams at the same time. Some sticks on the leaves, evaporates, and never reaches the stream. By the time other raindrops meander their way through the leaves, over the ground, etc. the water that fell a while ago has already made its way downstream. We don't have large quantities reaching the stream at once, creating a flood. There are computer models which calculate runoff for different types of slope, soil type, vegetation, and rainfall. Those models will tell us what our common sense already does. For steep slopes with hard surfaces, most of the rain runs off and does so quickly. For gentle slopes with trees or absorbent surfaces, less runs off and what does run off runs off more slowly.

Newspapers have reported that the Governor's task force on flooding is doing computer modeling. We can assume they are feeding in data about the actual land use in watersheds, soil types in particular watersheds (available from the U.S. Geological Survey) and comparing the runoff with what it would have been (or the computer would have predicted) had there been no timbering, less timbering, better regulated timbering, etc.

So how do CORL’s bills help the problem of flooding?

(1.) Steep timber roads likely act as channels to direct water downhill faster than it could travel if it had to meander through the fallen leaves, around trees roots, etc. Our bills makes it more likely that there will be water bars installed on roads. Water bars are designed to retard the flow of water and re-direct it onto undisturbed or less disturbed areas. Anything that retards the flow reduces the peak flow of the stream.

(2.) Our bills would result in faster re-vegetation. Grass doesn't retard the water flow nearly as well as trees do, but it's better than bare dirt.

(3.) Our bill would help maintain the carrying capacity of the stream channels by reducing erosion of soil into streams, reducing their water carrying capacity. So often, when people talk about flooding, they always say that the stream was so full of sediment that it could no longer hold the runoff. To the extent that our bills reduce sediment, it maintains the channel capacity and the ability of the channel to carry the water without the stream leaving its banks.

(4.) By making it more likely that currently only nominally required Best Management Practices would be followed, our bills would reduce logging debris (mostly limbs, branches and leaves, but sometimes even whole logs) that clog culverts and streams.

Other measures, not included in CORL’s bills, but which may be introduced later, include restricting the percentage of a watershed that could be timbered over time, restricting slopes where timbering can occur, or both.

Some folks even suggest a general requirement that the Division of Forestry not approve a registration if it would cause or exacerbate flooding. This would require the agency to make specific findings about timbering jobs within specific watersheds.

We may pursue these issues as a part of supporting or otherwise commenting on the Governor's Task Force on Flooding reports.