Based on an engineering assessment conducted by DuBois & King, Inc., the following is proposed to replace this culvert, based on Vermont Codes and Standards. Completed work included: An environmental assessment of bankful width, Streamstats hydrology report, and HY-8 hydraulic report was used to determine culvert sizing. The culvert needs to be upsized to meet codes and standards. Bankful width is 18.2 feet. Bankful width was determined by averaging 3 upstream and four downstream field measurements of the stream. Two additional measurements were not used in the calculations because they were not representative of the natural stream. The proposed culvert is sized in accordance with Chapter 4 and Chapter 6.4 of the VTrans Hydraulics Manual, dated May 28, 2015. Chapter 4 identifies the appropriate Annual Exceedance Probability (AEP) for the roadway classification and Chapter 4 identifies the appropriate hydrological methodologies. Chapter 6.4 identifies the appropriate Hydraulic design criteria for sizing the culvert. Criteria includes: 1. The headwater should be no higher than the roadway shoulder during the design event. 2. The headwater corresponding to the design event should not adversely affect property to a greater degree than what occurs under existing conditions. (In general, damage occurs when floodwaters enter the interior of buildings or other structures.) 3. For sites not covered by the FEMA NFIP, backwater increases from culvert crossings should not exceed 1.0 foot during the passage of the 1% AEP storm event. Sites that are covered by the NFIP should satisfy FEMA floodplain and floodway regulations for backwater. 4. From a hydraulic perspective, allowable headwater is dictated by the ratio of headwater depth to culvert diameter (or rise), which is 𝐻W/𝐷, where 𝐻W is headwater depth in feet, and 𝐷𝐷 is the rise or diameter of the culvert in feet. Table 6-3 presents hydraulic criteria for allowable headwater. 5. Evaluate the hydraulic conditions of the downstream channel to determine tailwater depths for a range of flood flows.
