Filter (Filter Zone) 2 : One or more layers of granular material graded (either naturally or by selection) so as to allow seepage through or within the layers while preventing the migration of material from adjacent zones
National Dam Safety Program, U.S. Department of Homeland Security Federal Emergency Management Agency Interagency Committee on Dam Safety, Federal Guidelines For Dam Safety: Glossary of Terms For Dam Safety (FEMA-148), Washington DC, USA 2004. http://www.damsafety.org/media/Documents/PDF/fema- 148.pdf, (Dam Sfty - Amer) {890}