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Stone Weeper

General

Stone weepers are outflow devices constructed of stone that reduce the flow velocity of runoff while minimizing channel erosion and promoting sediment deposition. Stormwater enters a swale or vegetated channel and, under normal circumstances, is ponded temporarily behind the weeper. Ponding allows sediment and other pollutants to settle out, while allowing some water to infiltrate and evaporate. The water that remains is slowly passed through the voids in the structure, continuing on towards the outfall. In the event of large storm conditions, runoff is conveyed safely over the top of the structure. As a result, stone weepers are best used in conjunction with other erosion control practices.

Stone weepers may be used as outlet structures in swales and vegetated channels. They are very versatile structures that conform to a wide variety of situations, making them one of the most widely used practices in Dane County.

Advantages

  • Cost-effective
  • Versatile
  • Relatively easy to construct
  • Reduces erosion and promote sediment deposition

Disadvantages

  • Requires periodic sediment removal
  • Ineffective with large storm events

Example of Stone Weeper

Adapted from Illinois Natural Resources Conservation Service (NRCS)

Design

The size of the structure will depend upon the site, but should be 1-5 feet in height; have a minimum top width of 2 feet; and should extend across the entire conveyance structure. Slopes should have a maximum ratio of 2:1, as greater slopes may become unstable and require excessive maintenance. In addition, stone weepers must be underlain with geotextile fabric to protect the structure from undercutting, which may cause the device to fail and result in channel erosion.

The size of stone selected for use in stone weepers will vary depending upon the individual needs of the site. However, they should consist of, at a minimum, a 1-foot layer of 1-inch washed stone over a 1-foot layer of clear stone, free of fines and sand, sized to meet the requirements of the design storm. The entire structure must be underlain with geotextile fabric to prevent the stone from settling.

Construction

  • To prevent the erosion of adjacent materials, stone should be placed at the toe and on the sides of the structure
  • Stone weepers must be underlain by geotextile fabric
  • Stone weepers shall be constructed immediately after grading is completed on the conveyance structure and be functional before the site upslope of the practice is disturbed

Maintenance

  • Stone weepers shall be inspected periodically and after all storm events for evidence of undercutting and erosion
  • Additional stone may be required to offset settlement and lost stones

Method to Determine Practice Efficiency

Stone weepers promote sedimentation by reducing the flow velocity of runoff. The efficiency for this practice may be found by using the equation below.

Flow Through Stone Weeper

Source: NAHB/NRC Designated Housing Research Center at Penn State University