Hydraulic Assessment of Existing and Alternative Stream Crossings Providing Fish and Wildlife Passage at Seven Sites in Massachusetts

Abstract

Seven existing road crossing structures at streams in Massachusetts were evaluated hydraulically and compared to hypothetical alternative structures designed for Aquatic Organism Passage (AOP) using standards developed by the Massachusetts River Continuity Partnership. Hydraulic simulations made for flood flows ranging from 20- to 0.2-percent annual exceedance probability (AEP) indicate that the existing structures are at full capacity for many of the simulated AEP floods, causing appreciable backwater upstream from the structure, which exacerbates upstream flooding and causes road overflow in many cases. The existing structures also create an impediment to AOP by failing to meet standards for openness, height, span, and velocity.

Simulated hypothetical road crossing structures that provide for fish and wildlife passage by meeting or exceeding the AOP standards were able to convey most simulated AEP flood flows without causing appreciable backwater upstream from the structure. At sites where backwater was still present, it occurred only at the highest simulated flows and was compounded by the low downstream gradient that affected the water-surface elevation at the structure. The simulations of the alternative structures also indicate that, in addition to improved passage for fish and wildlife, the structures are more resilient to large floods and provide a greater buffer to uncertainties and potential changes in flood flows than the existing stream-crossing structures.

First posted August 28, 2014