In 2018, the Maryland Transportation Authority’s (MDTA) I-895 bridge crossing Herring Run suffered scour that endangered the southbound bridge’s piers and abutments. In the late spring/early summer 2019, MDTA began corrective measures to mitigate this scour at the bridge. This included two-plus years of responding to multiple large storm events, more frequent stream bed/bridge monitoring, four separate authorizations from USACE and Maryland’s Department of the Environment for waterway construction permitting, scour countermeasure design, and several mobilizations for scour mitigation construction.
Herring Run at the I-895 bridge drains 18.7 square miles of Baltimore City and County. This drainage area is 16% of Baltimore City’s total land cover and 1% of Baltimore County’s. The project location is in the Coastal Plain Physiographic Province where the riverbed is dominated by sand and small gravels. Herring Run has a high sediment load and is considered “live-bed” scour condition in contrast to “clear-water”. The term “live-bed” indicates the river flow’s shear stress values generally exceed the riverbed material’s ability to resist movement. The bridge was originally constructed in 1956 with expansions in 1968 and 2007 to add travel/acceleration lanes for I-895. Even with regular bridge monitoring, the 2018 severe scour event occurred less than 30 days after a scheduled inspection.
With the accessibility to nearly 100 years of aerial imagery, more detailed imagery every six months from 2015 and later, bridge construction documents for all previous design/construction efforts, proximity of a USGS real time gage, 2-dimensional hydrodynamic modeling… a picture was painted that informed the final design and paths to protect the bridge infrastructure for years to come. The design team included structural/geotechnical/water resource engineers, environmental scientists, construction management experts and close communication with and leadership from MDTA’s managers that provided robust bridge infrastructure protection.
This presentation steps through the timeline and discoveries that led to a design/construction alleviating the scour threats while learning valuable lessons on ecological resiliency.