GEOHAZARD ASSESSMENT OF PROPOSED CONSTRUCTION EXCAVATION AT THE RESERVE AT BAY HARBOR, GALVESTON ISLAND, TEXAS

Any proposed coastal construction project on west Galveston Island seeking a Department of the Army permit must be subjected to a cumulative impact assessment (CIA) to quantify expected changes to the region's coastal resources. If the project includes construction excavation, the assessment must include a coastal hazards element (aka geohazard assessment) wherein the following are quantified: (1) The area in which the effects of the proposed project will be felt, (2) The impacts that are expected in the area as a result of the proposed project, (3) Other actions – past, present, and reasonably foreseeable future – that have had or are expected to have impacts in the same area, (4) The impacts or expected impacts from these other actions, and (5) The overall impact that can be expected if the individual impacts are allowed to accumulate.

The rationale for conducting a geohazard assessment is based upon the premise that construction-related excavation may weaken the structural integrity of west Galveston Island by increasing the potential for it to be overwashed, breached, or cut into two distinct island segments.

In this paper, the methods and results of a geohazard assessment performed in association with the permitting of an upscale resort subdivision are described. The development proposes excavation of a 1,500 ft by 100 ft (~3.5 acre) boat access channel. The site-specific assessment (see No. 1 & 2 above) addresses excavation effects on hazard potential by evaluating three coastal phenomena before and after construction: (a) relative sea-level rise, (b) storm surge and overwash, and (c) shoreline change. This is followed by a discussion of how one establishes the project area (aka same area; see No. 3 above) to which the CIA is applied. The overall or cumulative impact of all construction excavation “actions” (see No. 4 and 5 above) is then evaluated by time-series analysis of (a) historical aerial photography, (b) catastrophic event landfall, and (c) public shore-protection projects.