Determining the amount of scour that will occur with the proposed abutment foundations and the adjoining tidal channel is important for assessing the long-term stability and safety of the proposed bridge over Muddy Creek. Scour is the movement or removal of sediment from the streambed by the erosive motion of water moving and entraining sediment particles into the tidal estuary, typically induced by storm surges. The degree of scour along the streambed is significantly influenced by the velocity magnitude of the current; the higher the flow velocity, the greater capacity the flow has to move sediment. Complex hydrodynamic interactions associated with estuarine tidal flow, as well as freshwater entering the system at the head of the system and through groundwater entering along the length of the system, often induce sediment movement and exacerbate scour around a structure constructed within the stream bed. The extent of scour resulting from movement of water along the stream bed and the related structure interaction with the current is important to quantify for the design of bridge structures, as scour can lead to structural failure of these structures.
The flow under the proposed bridge and the adjoining causeway is very complex and predicting the extreme flooding events which cause scour and transport of bed material is complicated in nature and hence contains inherent limitations. Consequently, engineering judgment is required to evaluate the long-term scour effects relative to the assumptions that are included in the overall scour analysis approach. The physical characteristics of Muddy Creek in conjunction with the proposed bridge have resulted in a unique condition relative to the hydraulic and scour analysis; the results differ from the customary engineering guidance from MassDOT, FHWA, and USACE which assumes each successive return period storm to generate a more severe condition for the bridge structure.
It is recommended that scour counter-measures be utilized along the abutment surfaces, as the stability of the abutment structures become a concern at the levels of scour predicted for all simulated surge events. In addition, the channel, bridge abutments, and causeway overlie erodible sandy material. It is critical to the safety and long-term viability of the structure that scour counter-measures be appropriately designed to protect the channel immediately upstream, through the bridge crossing, and immediately downstream. The existing stone box culverts have produced scour holes on the downstream side of the causeway that are approximately 4-6 feet below the stream bed. The new bridge will allow substantially more tidal flow through the channel, and in severe cases of elevated water levels associated with storm surges, the new channel will experience significantly higher current velocities than the present channel. We recommend that during and periodically after construction the bridge should be monitored at regular intervals to assess local scour depths and whether additional scour protection is warranted.