Saturday, March 28, 2015

Ground Water Recharge and Forestry

Hal Berton has an article on one of the actions post Hazel/Oso Landslide that has taken place: seattletimes.state-tackles-steep-challenges-to-step-up-logging-oversight.

Below are a couple of DEM images of the bench areas above the Town of Concrete that were mentioned in the article. There have been some slides around the perimeters of these terraces.

I was up on a valley side terrace in a different valley this week.

Nice level ground for logging operations with a stand of third growth forest.

The soil underlying the bench is sand and gravel. Hence, any rain water or snow melt will readily infiltrate into the ground and recharge the groundwater. The removal of trees will increase the amount of water being recharged into the ground.

The slopes along the edge of the terrace are very steep and based on topography the water entering the ground on the level area shown above would likely discharge somewhere on this slope.

About 400 feet down slope water was pouring out of the ground in a series of large springs. I was a bit surprised at the volume and suspect the source area of water is more extensive than the surface topography suggests. In this particular case the water appears to be flowing through bedrock joints along the top of a granitic body and no unstable slopes were identified. So in this case, lots of recharge potential but the groundwater discharge is not being directed towards any obvious unstable slopes.   

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