Q What is the recommended spacing of cross drains for an existing 300-m long, 4-m wide road in the Boise National Forest that is within 10 m of a creek?
The slope of the buffer between the road and the creek is 10% and the road gradient is 4%. The nearest X-DRAIN climate station is Deadwood Dam. The site is on the Idaho Batholith, a coarse-grained soil most closely described as a sandy loam.
A From a single X-DRAIN run,
| Sediment Yield | |||
|---|---|---|---|
| Spacing (m) | Number of segments in 300 m | From 1 segment | For 300 m |
| 10 | 30 | 0.3 | 9 |
| 30 | 10 | 6 | 60 |
| 60 | 5 | 24 | 120 |
| 120 | 2.5 | 85 | 212 |
| 240 | 1.25 | 298 | 372 |
From inspection, it appears that a cross drain spacing of 60 m or longer greatly increases the sediment yield rate. An outsloping road spacing of 10 m, or a spacing of 30 m, will likely result in an acceptable level of sediment delivery.
Q A 60-m skid trail is believed to be a potential erosion source in a recently harvested site in West Virginia on a clay loam soil. The slope of the hillside is 10 percent, the distance to the nearest channel is 50 m, and the slope of the skid trail is 8 percent. The width of the trail is 3 m (10 ft).
What is the recommended spacing of water-bars to prevent soil from entering the stream?
A The Lewisburg, West Virginia climate and clay loam soil are selected, as are the buffer gradient and length. The erosion in year 1 is taken from the X-DRAIN results, and the erosion for each subsequent year is reduced by 20 percent per year the vegetation regrows. The total erosion for 5 years can then be determined.
Estimate of erosion for first 5 years of skid trail.
| Year | Sediment Yield (kg) | ||
|---|---|---|---|
| 10-m spacing | 30-m spacing | 60-m spacing | |
| 1 | 1.2 x 6 = 7.2 | 17 x 2 = 34 | 88 x 1 = 88 |
| 2 | 7.2 x 0.8 = 5.8 | 34 x 0.8 = 27 | 88 x 0.8 = 70 |
| 3 | 7.2 x 0.6 = 4.3 | 34 x 0.6 = 20 | 88 x 0.6 = 53 |
| 4 | 7.2 x 0.4 = 2.9 | 34 x 0.4 = 14 | 88 x 0.4 = 35 |
| 5 | 7.2 x 0.2 = 1.4 | 34 x 0.2 = 7 | 88 x 0.2 = 18 |
| 6 | 7.2 x 0.0 = 0.0 | 34 x 0.0 = 0 | 88 x 0.0 = 0 |
| Total | 22 | 102 | 264 |
The manager may decide that the sediment yield from the 30-m spacing is tolerable over a five-year period, but not the yield from the 60-m spacing.