DELAWARE RIVER BASIN COMMISSION

Scour

Sound design of stream encroachment infrastructure uses the theoretical and empirical principles of fluvial geomorphology. Preventing the damaging scour of bridge abutments is one such concern. The Standard Handbook of Civil Engineering (1983) stresses locating stream crossings at stable sites and minimizing disturbance. This objective is not always achieved, often to minimize initial capital cost. However, the maintenance costs for such low capital cost structures may result in higher life-cycle costs.

Pidcock Creek, old abandoned bridge pier, Washington Crossing State Park, Solebury Township, Bucks County, PA. Fortunately the bridge, with its footings being undermined by scour, is no longer functional.

An undersized culvert at a stream crossing causes backup during high runoff. The resultant increased hydraulic head forces high velocity flow through the culvert. This tends to scour a relatively deep pool at its outlet. This condition dislodges considerable sediment, which is deposited downstream. This negative aspect might be somewhat offset by the unique habitat of the created pool. However, another undesirable result of undersizing of the culvert is that, when clogged, it creates a temporary upstream swamp. The temporary swamp, when drained to maintain the culvert, becomes an extensive mudflat. Such a condition exists at the Reeders Creek culvert near the Commission offices at State Police Drive, Ewing Township, Mercer County, NJ.

Reeders Creek culvert, upstream temporary swamp, April 1999.

Reeders Creek culvert, downstream pool, April 1999.

Reeders Creek, portion of the drained former swamp, August 5, 1999.

Masonry bridge over Brandywine Creek and its floodplain, connecting Pocopson and Birmingham Townships, Chester County, PA.

Some jurisdictions in the U.S. have upgraded design standards for stream crossing structures to minimize the undesirable impacts to streams. Such designs provide correct cross-section dimensions to minimize change to the upstream and downstream stream channel cross-section dimensions. Flood plain flows pass through separate openings with inverts at the higher, flood plain elevations.

Recent three-barrel culvert design in Maryland. Note the higher elevation inverts of the two side culverts designed to pass the floodplain flow.

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