Bridges

Definition A Bridge is defined in Minnesota as a structure over 10’ in length, sized hydraulically to convey water under a highway, railroad or other embankment Road grade, geometrics, site-distance, vertical curvature or ecological considerations may also influence the size of the waterway opening

Basic Criteria For DNR- no stage increase over 0.5’ for new crossings. Check flood damage potential upstream Velocities no more than 8-9 fps Match the channel width up and down stream

Case Studies Br 20558- ADT < 50, minimum flood is 5 year Design speed: 30 mph Sag Vertical Curve Match the natural channel bottom Used a 100 year design event as road profile drives the design Used single span structure with deep beam to save on structure size.

Case Studies Br 85560 25 year design was used ADT <50 (Dead End), minimum flood is 5 year Inplace 4- 36” CMP with Slab Resident was cut off during floods, township wanted dry crossing Road design 30 mph, design speed set the road profile Max velocities between 8-9 fps Stage increase < 0.5’ Designated trout stream 25 year design was used

Case Studies Br 58552 Used a 100 year design event ADT < 50, minimum flood is 5 year Sag Vertical Curve, design speed 30 mph Old bridge was constricting the river Channel Bottom Upstream and Downstream ~ 80’ wide High debris stream Vertical curve drives the design Used a 100 year design event

Case Studies Br 08552 100 year design was used ADT 510 minimum flood is 25 year event Road profile is driving the design Inplace road does not overtop Stage increase was to be held at a minimum 0.6’ for the inplace 0.5’ for the proposed 100 year design was used

Case Studies Br 30515 100 year design was used ADT 925 minimum flood is 25 year ARRA Project- Federal Funds were used 55 mph design speed was required as ARRA funds were used Sag Curve was designed for the design speed Bridge sized to match natural channel bottom 100 year design was used

Case Studies Br 22605 ADT < 50, 5 year event Road Profile is driving the design Matched the waterway opening of inplace bridge Br 55583 ADT 2200, 50 year minimum event Velocities are high, 8.5 fps Match existing bridge lengths 100 year design was used on both bridges

Deck Drains Federal Requirement: Army Corps of Engineers General Permit does not allow bridges with over 500 heavy vehicles per day to drain directly to a trib to a navigable water No State Regulation, but DNR has conditions in the General Permit that will not allow direct drainage to a stream When ever possible, keep the deck drainage from being discharged directly to a stream