.jpg/:/rs=w:1209,h:605,cg:true,m/cr=w:1209,h:605)
SEDIMENTATION & EROSION ANALYSIS
Moving water carries sediment, and where it moves faster than the bed or bank can withstand, it takes material with it — undermining a bridge pier, cutting back an abutment, or stripping a watershed and filling the reservoir downstream. Waterway analyses that process at whatever scale the problem occurs. At a structure, that means two-dimensional hydraulic modelling to find the actual velocity and shear stress at the pier or bank rather than a cross-section average, comparing it against what the in-place material can tolerate, and sizing the riprap, gabion or armouring needed to hold. At the watershed scale, it means sediment yield and erosion modelling to identify where the material is coming from and which interventions would meaningfully reduce it. In both cases the point is the same: understand where the energy is concentrated before deciding what to build.
OUr PROJECTS
Cunningham Street Bridge Scour Mitigation
A one-dimensional model averages velocity across the full width of a channel. Where the water is flowing straight, that is fine. At Cunningham Street it was not — the creek bends hard immediately upstream of the bridge, so the flow does not spread evenly. It slams to the outside of the curve and arrives at the piers as a concentrated jet.
Waterway built a high-resolution two-dimensional model to see it. The mesh was constructed in RAS25 for its refined meshing, using aligned quadrilateral cells roughly two feet wide through the channel so the grid followed the flow direction rather than cutting across it. The bridge was modeled with its low chord and all eight piers.
The existing one-dimensional model gave a hundred-year velocity of 4.4 feet per second. The two-dimensional model gave 10.3 — more than double, in the same flood. We designed rip rap features to protect against this erosion.
