20 ft high dam and only 6 ft top width would scare me a little, even for a 1/2 acre pond. NCRS calls for a minimum of 12 feet, page 45, right hand side.
http://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs144p2_030362.pdf---------------
Top width and alignment—For dams less than 10
feet high, a conservative minimum top width is 6 feet.
As the height of the dam increases, increase the top
width. The recommended minimum top width for
earth embankments of various heights is:
Height of dam/Minimum top width
Under 10' = 6'
11' to 14' = 8'
15' to 19' = 10'
20' to 24' = 12'
25' to 34' = 14'
If the top of the embankment is to be used for a roadway,
provide for a shoulder on each side of the roadway
to prevent raveling. The top width should be at
least 16 feet.
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Something to look at, especially if you are having erosion problems.
