The district engineer must determine the compensatory mitigation to be required in a DA permit, based on what is practicable and capable of compensating for the aquatic resource functions that will be lost as a result of the permitted activity. The purpose of a functional assessment is to evaluate current wetland functions and predict potential changes to a wetland's functions that may result from proposed activities. A wetland is compared to similar wetlands that are relatively unaltered. The approach is based on combining variables that are typically structural measures or indicators that are associated with one or more ecosystem functions. Functions normally fall into one of three major categories: (1) physical (e.g., storage of surface water), (2) chemical (e.g., removal of elements and compounds), and (3) biological (e.g., topography, depth of water, number and size of trees).
The HGM approach to functional assessment estimates the change in functioning induced by alteration of a wetland, either positive or negative. Negative effects (i.e, reductions in sustainable levels of functioning) are normally determined in association with dredge-and-fill permits. The permit review process could use output from an assessment as one tool to determine if the project results in significant degradation. Output from HGM models can also be used to determine the amount of positive effects (i.e., increases in sustainable levels of functioning) associated with compensatory mitigation requirements, normally through restoration of previously altered wetlands of the same type.
For more information on the Galveston District's iHGM policy, please see our Standard Operating Procedure.