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VTrans Molly’s Falls Brook Relocation

 

The Cabot-Danville US Route 2 highway safety improvements project required 200-ft of Molly’s Falls Brook to be relocated. Extensive stakeholder coordination began at site inspection, with representatives from Vermont’s Agency of Transportation and Department of Fish & Wildlife. DuBois & King formulated a conceptual design and met with the Vermont River Management Program. Professional services included:

Data collection and analysis
Design of a new channel
Preparation of a topographic basemap
Documentation of the existing channel
HEC-RAS model to predict water surface elevations up to 100-yr level
Design of stone cross vanes to sustain aquatic species
Preservation of selected trees for immediate riparian cover

VTrans Brandon Brook Slope Stabilization/Stream Restoration

 

Design and construction services for emergency river and roadway reconstruction at several intersections of Brandon Brook and Route 73 following Tropical Storm Irene. Initial road reconstruction, as well as the flood itself, resulted in significant channel instability and complete loss of habitat in three locations, totaling approximately 2,900-ft. Channel stabilization design features included 13 stone weirs to create habitat. The design elevated the channel bottom to increase channel width. Channel stabilization also restored floodplain access and reduced the potential for undermining of the new riprap on the roadway embankment. Rehabilitation included stone veins to direct flow away from the roadway and to restore habitat. The channel was seeded with hundreds of randomly placed boulders to provide habitat shelter and structure for natural growth. Prepared simple plans for contractor use and provided on-site (in-river) direction.

River inspection
Geomorphic assessment
Field design
Restoration plans
Regulatory coordination
Construction direction

Sucker Brook Channel Stabilization and Natural Channel Design

 

In the mid 1980s, a high flow event re-routed Sucker Brook out of its channel, away from a 25-ft waterfall, through an abandoned sand and gravel pit and traversed exposed sand, silts, and clays. Over the next twenty years, the channel steadily eroded with distinct headcuts progressing upstream. Approximately 72,000 cubic yards of material were transported downstream, leaving a steep, unstable channel at the bottom of a 40-ft gully. Vermont River Management called the Sucker Brook Tributary the worst channel avulsion in the State.

Project objectives were to reduce sediment loading from the reach, minimize construction and maintenance costs, and restore aquatic and riparian habitat. Regulators from the Vermont River Management Program and the US Army Corps of Engineers were kept informed of design considerations.

An evaluation matrix compared alternatives in terms of reduction in sediment loading, long-term stability, construction and maintenance costs, in-stream and riparian habitat, and impact to adjacent landowners. Presentation of alternatives at public meetings facilitated input and the concurrence of stakeholders and adjacent landowners.

Final design included a step-pool reach with eight stone weirs and a detailed planting plan to restore the riparian zone. A two-year construction sequence was developed that recognized limited construction funding and included volunteer labor to accomplish the planting program.

Scribner Covered Bridge Flooding Remediation

 

The historic Scribner Covered Bridge and an adjacent roadway intersection and residence are subjected to frequent flooding by the Gihon River. The bridge is undersized, significantly constricting the channel width, and rests on ledge. DuBois & King identified alternatives to reduce flooding and advanced a preferred alternative to preliminary design. Services included hydraulic modeling of the river and the floodplain for the 10-, 25-, 50-, and 100-year flood events. The project team identified 8 distinct alternatives to reduce the frequency or severity of flood damages in the area and prepared accompanying cost estimates. Alternatives included the addition of culverts, drainage ditches, roadway realignment, riprap or articulated concrete blocks, and specific combinations of these options. The preferred alternative included grading changes to concentrate the over-topping flows and armoring of the surface to reduce damages. Floodplain access was maintained to promote river stability. Professional services included:

Topographic survey
Geomorphologic review
Hydrology to develop peak river flow estimates
Hydraulic modeling
Identification and evaluation of alternatives
Cost estimates
Preliminary plans for the preferred alternative

Oak Hill Stream Channel Restoration

 

DuBois & King conducted hydrologic modeling of the watershed above an incising stream channel that had been straightened and lined with rock during highway construction. Working for the Winooski Natural Resources Conservation District, D&K used LiDAR elevation data and ArcView GIS to delineate subwatersheds and define flow paths, and confirmed the information with a detailed watershed reconnaissance. The modeling showed that additional impervious surface in the watershed would have little impact on the total volume of water delivered to the stream. However, the roadways in the watershed altered natural flow paths and resulted in a significant increase in the volume of water delivered to the stream channel.

The results of the hydrologic analysis were used in the design of a new stable stream channel. The channel was sized to accommodate the predicted bankfull flow and access to the adjacent floodplain was maintained or restored where existing infrastructure allowed. The design blended hydraulic engineering principles and natural channel design principles to achieve a channel with dimensions, slope, sinuosity, and material that are appropriate for the setting and stable over time. Services included preparation of bid and contract documents, permitting assistance, and construction inspection.