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Contour Sandbags Fact Sheet

What are Contour Sandbags?

Biodegradable bags are filled with on site soil and bedded in a shallow trench forming a continuous barrier along the contour (across the slope) to intercept water running down the slope.

When are Contour Sandbags Used?

Contour Sandbags are used on burned slopes that have less than 30% of the original ground cover remaining and are at risk for increased erosion. They can be installed on slopes up to 70 percent, however their effect diminishes greatly on slopes steeper than 50 percent. Soils can be shallow, but not less than about 6 inches. Contour Sandbags increase infiltration, add roughness, reduce erosion, and help retain eroded soil on the slope. Contour Sandbags should be effective for a period up to one year, providing short term protection on slopes where permanent vegetation will be established to provide long term erosion control. Contour Sandbags accomplish the same treatment as Log Terraces, but require less skilled labor to install and can be placed on the slope more effectively. Sandbags should not be placed across drainage swales and channels with more than 1 acre of contributing drainage area because they are not sturdy enough to resist the forces of concentrated flows.

What Materials are Needed?

  • Sandbags
  • Hand tools -shovels, polaskis

How are Contour Sandbags Installed?

Installation of Contour Sand Bags is straight forward and is an easy practice for untrained laborers, landowners and volunteer groups to complete.

  • Layout a contour line on the slope with a hand level and wire flags.
  • Dig a shallow depression, about 2 to 3 inches deep along the flag line
  • Use the soil from the trench excavation to fill bags ½ to ¾ full.
  • Fold the top over and lay the filled bags end to end in the trench.
  • Seat the bags with foot tamped backfill on the upstream side such that water flowing down the slope will not run under them.

How Many Sandbags Are Required?

The horizontal spacing of Contour Sandbags is determined with consideration for normal rainfall intensity, slope steepness, soil characteristics, and the extent of surface cover remaining after the fire. Figures 1 depicts the placement of sandbags on the slope. Table 1A and 1B show recommended spacings for treating burn areas within the June 2000 HI MEADOWS and BOBCAT Fires in Colorado

Contour sandbag construction detail.


FIGURE 1 - Typical Contour Sandbag Installation


Tables 1A, 2A, 3A - Recommended Spacing for Contour Sandbags based on Burn Severity - Hi Meadows Fire Area

Table 1A, Burn Severity Low Intensity

Land Slope
(percent)

Spacing
(feet)

Quantity
(bags/acre)

< 5 %

250

135

5 - 10 %

200

164

10 - 20 %

120

272

20 - 50 %

60

544

> 50 %

40

818


Table 2A, Burn Severity Moderate Intensity

Land Slope
(percent)

Spacing
(feet)

Quantity
(bags/acre)

< 5 %

160

204

5 - 10 %

120

272

10 - 20 %

60

544

20 - 50 %

30

1088

> 50 %

20

1634


Table 3A, Burn Severity Severe Intensity

Land Slope
(percent)

Spacing
(feet)

Quantity
(bags/acre)

< 5 %

130

250

5 - 10 %

90

364

10 - 20 %

40

818

20 - 50 %

20

1634

> 50 %

20

1634

 

Tables 1B, 2B, 3B - Recommended Spacing for Contour Sandbags based on Burn Severity - Bobcat Fire Area

Table 3B, Burn Severity Low Intensity

Land Slope
(percent)

Spacing
(feet)

Quantity
(bags/acre)

< 5 %

350

94

5 - 10 %

300

108

10 - 20 %

200

164

20 - 50 %

100

328

> 50 %

50

656


Table 3B, Burn Severity Moderate Intensity

Land Slope
(percent)

Spacing
(feet)

Quantity
(bags/acre)

< 5 %

200

164

5 - 10 %

160

204

10 - 20 %

100

328

20 - 50 %

50

656

> 50 %

20

1634


Table 3C, Burn Severity Severe Intensity

Land Slope
(percent)

Spacing
(feet)

Quantity
(bags/acre)

< 5 %

150

218

5 - 10 %

100

328

10 - 20 %

50

656

20 - 50 %

20

1634

> 50 %

20

1634

NOTE: After a fire many trees are weakened from burning around the base of the trunk. The trees can fall over or blow down without warning. Shallow rooted trees can also fall. Therefore be extremely alert when working around burned trees.