Mill Creek
Low Water Crossing with Embedded Box Culverts

Project Summary

Designated as a “Wild Trout Management Area”, Mill Creek is a cold water stream with a self sustaining population of rainbow trout that were introduced in the 1880’s. Mill Creek is currently one of only five Missouri streams populated by wild, self-sustaining stream-bred rainbow trout. There have been several bank stabilization and trout habitat improvement projects completed along Mill Creek.

Vehicle access at the Mill Creek crossing is required year-round for private residences and recreation. The original concrete slab low water crossing structure at the Mill Creek site was replaced in 1976 with a low-water bridge, probably in response to fisheries concerns. However, the bridge was too low with closely spaced piers, which frequently trapped woody debris and required regular maintenance.

A replacement crossing was desired to reduce maintenance and improve vehicle safety, while providing fish passage. The adjacent channel dimensions were used to guide the design. Within the project reach Mill Creek is a pool-riffle gravel-bedded stream with a gradient of about 2%. Beyond the stream crossing the channel width is about 40 ft (12 m) and banks are 2 to 3 ft (0.6 to 0.9 m) high. The well-vegetated flood plain is about 3 ft (0.9 m) below the wooded terrace that forms the broader valley floor. The channel appears to be vertically stable, but has moderate potential to migrate laterally.

The replacement is a vented low water crossing, built in 1994, that consists of three cast-in-place concrete box culverts, each 14 ft (4.3 m) wide and 5 ft (1.5 m) tall. Standard State Highway triple box culvert design drawings were used. The culvert bottoms were embedded 1 ft (0.3 m) below stream grade to allow formation of a natural streambed within the culverts. The width and height of the box culvert openings approaches the bankfull cross sectional shape and area. The crossing has sloping wings extending upstream and downstream of each box culvert designed to sweep debris over the road deck.

The vented low water crossing designed to minimize constriction of the bankfull channel was the preferred structure type for the site. It allows for continuous passage of fish under the roadway, experiences relatively infrequent overtopping, improved public safety, is less susceptible to plugging and damage from large woody debris, reduces needed maintenance, and was less expensive than a channel spanning bridge.

Shortly after construction concerns regarding lateral migration of the stream and the potential for flanking of the crossing structure arose. A concrete headwall faced with riprap was constructed along the upstream and downstream edges of the crossing approaches to minimize the risk of flanking and protect the road from scour when overtopped by the stream.

Post Project Observations and Lessons Learned

The structure overtops annually, and sometimes more than once per year. Maintenance is required at least once a year, which is dramatically less than the previous structures at the site. In May 2002, the region received 18 in (46 cm) of rain in 18 days, causing the largest flood over the structure to date. Woody debris and sediment deposited on the deck and approach slabs had to be removed. The downstream channel appeared unaffected by the flood event which overtopped the crossing. Although the crossing structure sustained no significant damage from the flood, a 2003 bridge inspection identified tension cracks in the road deck between piers. Standard box culvert engineering designs may not account for the frequent debris loading experienced by low water crossings. When box culverts are used in low water crossings it may be necessary to add additional steel reinforcement to tension zones.

Gravel is accumulating on the bar and on the riffle upstream and in the right box culvert, when looking downstream. Flow constriction and the resulting increased water velocity have caused some bed scour in the box culvert furthest from the gravel bar. These changes do not appear to have reduced the structure’s effectiveness for aquatic organism passage, and in general, the structure appears to be working well in a challenging environment of flashy flows and high sediment loads.

This case study is featured in the 2006 US Forest Service publication:
Low-Water Crossings: Geomorphic, Biological, and Engineering Design Consideration.

Published 05/27/08