Environmental Assessment
Volume B-3.8
Mine
Hydrology
Ambatovy Project
96
January 2006
groundwater discharge could result in increased or decreased flows at
downstream locations depending on the season.
Changes in flows and water levels, in conjunction with changes in sediment
supply, may also affect the Torotorofotsy wetlands as well as downstream
channel morphology as these systems adapt to reach new equilibrium conditions.
The closure scenario involves breaching the pond embankments and allowing
surface runoff to drain to the natural receiving streams without being held for
settling. Runoff volumes may still be greater than under natural conditions due
to changes in vegetation and infiltration.
Changes in flows, water levels, and sediment levels in receiving water bodies
may also affect water quality, fish and aquatic resources, socio-economics and
land use. Water users are described in the socioeconomics and land use baseline
reports (Volume K, Appendices 1.1 and 3.1) and effects of hydrologic changes
on water users are described in the socioeconomic and land use EA reports
(Volume B, Sections 5.1 and 5.3).
3.8.5.2 Assessment Methods
Changes in flows at the pond outlet locations were evaluated based on expected
changes in watershed drainage area, land type, and related runoff characteristics.
Baseline, operational and post-closure flows were derived using a simple
hydrologic model that incorporates monthly rainfall and different runoff rates for
different land types. The runoff coefficients applied account for gross water
losses associated with evaporation, groundwater infiltration and local storage.
The baseline runoff coefficients are based on 2004-2005 monitoring data, while
coefficients for development conditions are based on professional judgment and
experience with similar land types and developments.
Assumptions for Hydrologic modeling are based on current project design. The
environmental team has worked closely with the engineering design team to
develop reasonable assumptions of sufficient detail to enable monthly water
balance simulation. Some aspects of engineering and therefore specific
assumptions may change as a result of detailed engineering, however, mitigation
will ensure that any impacts on flows, water levels, and sediment levels will be
within the levels presented in this assessment. Details of model inputs and
assumptions are provided in Volume I, Appendix 8-2.
