St. Johns/New Madrid RSEIS - Endangered Species & Wetlands ...

permanent waterbody HSI gained for every acre of mitigation, since lands are inundated
continuously, including the April 1 to May 15 timeframe (i.e., duration equals 100%).
Comments received by the Corps since the 2002 RSEIS express the opinion that the
proposed reforested mitigation tracts will generally be flooded only about a third (33%)
of the mid-season period, or for 15 days. However, it is clear that site-specific analyses
once tracts are acquired and on-site mitigation techniques are employed are needed to
determine actual inundation duration. However, if that general assumption of 33%
duration were used for planning purposes and mitigation was based solely on
reforestation, three times the amount of mitigation proposed in the 2002 RSEIS would be
required, plus the additional mitigation necessary to account for the transition period.
Reforesting cropland with bottomland hardwoods, in particular with 70% red oaks, is not
always feasible if the land is flooded for long periods during the growing season. The
flooding could cause excessive mortality of the trees. Tree species for specific mitigation
sites would be selected during the development of site-specific mitigation plans with
input from the interagency mitigation team.
2.3.4 Compensatory Mitigation Calculation
Determining unavoidable fish and wildlife habitat impacts is based on HUs, as originally
calculated in the 2002 RSEIS, and refined to AAHUs in this RSEIS 2. The goal of HEP
is to determine the functional value of landscapes expressed as AAHUs. Impacts and
mitigation are enumerated as AAHUs, and the difference between pre- and post-project
AAHUs is defined as the impact of the project. Therefore, mitigation must compensate
for lost AAHUs, and the amount of mitigation required to fully compensate impacts
depends on the techniques used and their associated habitat value (i.e., HSI value) in the
mitigation plan. AAHUs, not ADFAs, are the key unit used to determine mitigation
requirements. Benefits to mid-season fish rearing habitat from mitigation measures
would be calculated by the following equations:
Habitat Gains = AAHUs per tract with mitigation – AAHUs per tract without mitigation
Where AAHUs are calculated by (50-year project life),
AAHUs = Cumulative HUs/50 years
and Cumulative HUs are calculated by,
⎡ HSI + HSI2
⎤
Cumulative HUs = Σ [(T2 – T1) * (ADFA) * ⎢
⎣ 2 ⎥
⎦
where
Draft RSEIS 2
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