Ecologically Aware Design of Waterway-Encapsulating Structures

Aquatic organism passage (AOP) in waterways-encapsulating structures, particularly culverts, is of growing concern to environmental regulatory agencies, and the Indiana Department of Transportation (INDOT) is seeking systematic responses to this concern in the hydraulic design of such structures. This study reviews design approaches to enhance or accommodate aquatic organism passage through culverts, and proposes a simplified design procedure that requires less data input and analysis, and yet results in a structure complying with the current regional general permit (RGP) conditions. It also makes as much use of already existing INDOT standard specifications for riprap and coarse aggregates that would be as backfill material to form a stable bed within the culvert. The simplified procedure is intended for new larger structures for which a culvert bed needs to be installed, and for expected Indiana conditions of low-gradient ( Because of their larger size compared to traditional culverts, AOP-designed structures are associated with higher installation and material costs, which may however be compensated partially or wholly by lower costs over their operational lifetime. Previous work on life-cycle cost (including social/ecological costs) analysis of AOP-designed culverts was reviewed, but it is concluded that reliable data and methodologies for an adequate analysis are not yet available. The study then explores the consequences of alternative regulatory schemes formulated on the basis of habitat or biotic integrity indices. These may permit simple yet more flexible schemes with the same or even better ecological outcomes