The hydraulic analysis will satisfy requirements of the FEDERAL-AID POLICY GUIDE, 23 CFR 650A (Location and Hydraulic Design of Encroachments on Flood Plains) and Federal Highway Administration Publication No. FHWA-IP-90-017 (Hydraulic Engineering Circular No. 18, Evaluating Scour at Bridges).
The hydraulic Engineer is required to define the most practical design for all hydraulic related issues. The analysis process evaluates, assesses and documents the impacts and consequences an encroachment has on the floodplain environment. The proposed design, countermeasure design and hydraulic related design parameters are defined by considering hydraulic constraints, cost, risks, regulatory requirements, channel behavior, environmental impacts, engineering requirements and social concerns.
Structure sizing is based on hydraulic requirements for floods up to Q100. High Risk sites may require sizing based on Q500 floods. The proposed structure is selected by hydraulically assessing NATURAL, EXISTING and ALTERNATE conditions.
Bridge foundation designs are checked for scour to verify the structure has a reasonable chance of surviving a Q500 flood.
The hydraulic study is based on surveyed cross-sections that define the 100-year floodplain. Normally, a minimum of one upstream cross-section, one downstream cross-section and one encroachment (bridge opening/road grade) cross-section is required. For minor action projects with minimal hydraulic risks, such as culvert extensions, approximate hydraulic calculations based on preliminary road survey data is acceptable. High risk, very complex sites may require the use of additional floodplain cross-sections and/or total station survey data.