River Flooding Can Lead to Accelerated Bridge Scouring and Foundation Deterioration which Requires Periodic Monitoring by Highway Agencies in the United States:
Contributed by Dr. Mohiuddin Ali Khan, Moorestown, New Jersey, USA
The major damage to transportation structures in the U.S. is from excessive flooding in local rivers. Unlike earthquakes, floods seem to hit at regular intervals in the northeast and parts of the U.S. The U.S. Geological Survey (USGS) and other federal agencies installed monitoring gauges on selected rivers which are providing data for flood risk assessment studies.
Bridge scouring has been a major threat to the structural integrity of bridge over rivers. There are 165 scour critical bridges in New Jersey and a similar number for review in Pennsylvania. Field inspections for scour have been a regular feature. Following the collapse of Schoarie Creek multispan bridge in New York State, the criteria of flood discharge here has now been upgraded for designing new bridges, from 100 years probability to 500 years. The peak 500-year flood discharge is approximately 1.7 times 100-year peak flood discharge.
The founder and President of Ali Khan & Associates, Dr. Mohiuddin Ali Khan, PE, has over 30 years of experience with government agencies and private entities in Washington D.C, New York, Virginia, Pennsylvania, Maryland, and New Jersey. Dr. Khan is Chairman of Structures Group for ASCE, Philadelphia Section and author of Bridge and Highway Structure Rehabilitation and Repair, 2010 (656 pages), McGraw-Hill, ISBN 9780071545914.
Dr. Uddin’s note: It has been estimated that 60% of all bridge failures result from scour and other hydraulic related causes. Bridge scour is one of the three main causes of bridge failure and the most common cause of highway bridge failure in the United States during 1961-1976.
Factors that affect the magnitude of local scour depth at piers and abutments include:
• Velocity of the approach flow
• Depth of flow
• Width of the pier
• Length of the pier
• Size and gradation of bed material
• Angle of attack of the approach flow to the pier
• Shape of the pier
• Bed configuration
• Ice formation or jams and debris
Remote sensing monitoring of scour depth and damage at piers and abutments include the following methods:
• Underwater photography
• Acoustic mapping with hydrophones (underwater sonar system)
• Green band LIDAR, such as SHOALS (Scanning Hydrographic Operational Airborne Lidar Survey)
Among these modern acoustic mapping systems, operated from a boat, have been used successfully to generate 3D images.
NCHRP REPORT 593 “Countermeasures to Protect Bridge Piers from Scour,” published by the National Academies, evaluated the effectiveness of five scour countermeasure systems:
• Partially Grouted and Geocontainers
• Articulating Concrete Block Systems
• Gabion Mattresses
• Grout-Filled Mattresses
As a part of highway asset management, bridges in the U.S. are periodically monitored for condition appraisal as a part of National Bridge Inventory System (NBIS). A deficiency is indicated when the ratings drops from a 5 (fair) to a 4 (poor) or less on a scale of 9-0. General Appraisal condition rating of a bridge is the lowest condition rating of the superstructure and the substructure of a bridge.
The 2007 bridge inventory and condition rating data shows: Total Bridges – 599,766 (25% Urban), structurally deficient – 72,520, functionally deficient- 79,804, and total deficient- 152,316 (or 25.4% of total national bridge inventory). Bridge and other infrastructure asset and maintenance management topics are discussed in detail by Hudson, Haas, and Uddin in Infrastructure Management, published by McGraw-Hill, 1997 (Japanese edition 2001 and Chinese edition 2005)