Flood inundation maps and water-surface profiles for tropical storm Irene and selected annual exceedance probability floods for Flint Brook and the Third Branch White River in Roxbury, Vermont

Abstract

Flint Brook, a tributary to the Third Branch White River in Roxbury, Vermont, has a history of flooding the Vermont Fish and Wildlife Department’s Roxbury Fish Culture Station (the hatchery) and surrounding infrastructure. Flooding resulting from tropical storm Irene on August 28–29, 2011, caused widespread destruction in the region, including extensive and costly damages to the State-owned hatchery and the transportation infrastructure in the Town of Roxbury, Vermont. Sections of State Route 12A were washed out, and several bridges and culverts on Oxbow Road, Thurston Hill Road, and the New England Central Railroad in Roxbury were heavily damaged. Record high peak-discharge estimates of 2,140 cubic feet per second (ft 3 /s) and 4,320 ft 3 /s were calculated for Flint Brook at its confluence with the Third Branch White River and for the Third Branch White River at about 350 feet (ft) downstream from the hatchery, respectively. The annual exceedance probabilities (AEPs) of the peak discharges for Flint Brook and the Third Branch White River were less than 0.2 percent (less than a one in 500 chance of occurring in a given year). Hydrologic and hydraulic analyses of Flint Brook and the Third Branch White River were done to investigate flooding at the hatchery in Roxbury and support efforts by the Federal Emergency Management Agency to assist State and local mitigation and reconstruction efforts. During the August 2011 flood, the majority of flow from Flint Brook (97 percent or 2,070 ft 3 /s) diverged from its primary watercourse due to a retaining wall failure immediately upstream of Oxbow Road and inundated the hatchery. Although a minor amount of flow from the Third Branch White River could have overtopped State Route 12A and spilled into the hatchery, the Third Branch White River did not cause flood damages or exacerbate flooding at the hatchery during the August 2011 flood. The Third Branch White River which flows adjacent to the hatchery does not flood the hatchery for the 10-, 2-, 1, or 0.2-percent annual exceedance probabilities. The simulated water-surface elevations for August 2011 flood equal the elevations of State Route 12A about 500 ft downstream of Thurston Hill Road adjacent to the troughs between the rearing ponds. Four flood mitigation alternatives being considered by the Vermont Agency of Transportation to improve the hydraulic performance of Flint Brook and reduce the risk of flooding at the hatchery include: (A) no changes to the infrastructure or existing alignment of Flint Brook (existing conditions [2014]), (B) structural changes to the bridges and the existing retaining wall along Flint Brook, (C) realignment of Flint Brook to flow along the south side of Oxbow Road to accommodate larger stream discharges, and (D) a diversion channel for flows greater than 1-percent annual exceedance probability. Although the 10-, 2-, and 1-percent AEP floods do not flood the hatchery under alternative A (no changes to the infrastructure), the 0.2-percent AEP flow still poses a flooding threat to the hatchery because flow will continue to overtop the existing retaining wall and flood the hatchery. Under the other mitigation alternatives (B, C, and D) that include some variation of structural changes to bridges, a retaining wall, and (or) channel, the peak discharges for the 10-, 2-, 1-, and 0.2-percent annual exceedance probabilities do not flood the hatchery. Water-surface profiles and flood inundation maps of the August 2011 flood and the 10-, 2-, 1-, and 0.2-percent AEPs for four mitigation alternatives were developed for Flint Brook and the Third Branch White River in the vicinity of the hatchery and can be used by the Federal, State, and local agencies to better understand the potential for future flooding at the hatchery.