ARS | Publication request: IMAGE-BASED RIPRAP SIZE DETERMINATION

Submitted to: American Society of Agricultural Engineers Meetings Papers
Publication Type: Proceedings
Publication Acceptance Date: July 9, 2000
Publication Date: July 9, 2000

Interpretive Summary: Large quantities of loose rock or riprap, as it is commonly known, are used every year to protect the earth's surface against damage by flowing water. Engineering designs often dictate the use of a certain size and gradation of rock material. Traditional methods of determining rock size require each rock in a sample to be passed through a sieve and/or weighed. This is a very labor-intensive task. This research tested a new method of measuring rock size that uses a digital photograph as the sample. By tracing around each rock in the photograph, this commercially available software product determines the rock size and gradation. The results obtained by processing photographs agreed very well with the traditional sieve sampling technique. The results were best when the camera was held perpendicular to the rock surface. Three different users processed the same photographs and generated very similar results. These results should increase confidence in the software performance. Water resources managers and engineers should be interested in this alternative method to quickly and accurately characterize rock size and gradation. This method saves time, requires less equipment, and reduces handling hazards.

Technical Abstract: Loose stone riprap is commonly used to stabilize earth surfaces in water resources projects. Existing methods of determining rock size and gradation, such as weighing and/or sieving individual rocks in a sample, can be very labor intensive. This paper describes and evaluates a commercially available, image-based method of determining stone size and gradation. This product uses a digital photograph as the sample input. Two identical scaling disks of known size are positioned in each photograph. By manipulating this digital image, the size of each visible rock is determined. Six different rock samples ranging in size from 52- to 278-mm median diameter were evaluated by sampling and sieving a representative amount of material. This traditional sieving method was then compared with the image-based method. Image-based results compared very favorably with sieve results. The angle that the photograph was taken relative to the stone surface is an important parameter. Image-based techniques represent quick and practical solution to the problem of accurately determining rock size and gradation.