Scour is the process and result of flowing water eroding the bed and banks of a stream. Scour at nearly 14,300 bridges 1 spanning water, and the stability of river and stream channels in Pennsylvania, are being assessed by the U.S. Geological Survey (USGS) in cooperation with the Pennsylvania Department of Transportation (PennDOT). Procedures for bridge-scour assessments have been established to address the needs of PennDOT in meeting a 1988 Federal Highway Administration mandate requiring states to establish a program to assess all public bridges over water for their vulnerability to scour. The procedures also have been established to help develop an understanding of the local and regional factors that affect scour and channel stability. This report describes procedures for the assessment of scour at all bridges that are 20 feet or greater in length that span water in Pennsylvania. There are two basic types of assessment: field-viewed bridge site assessments, for which USGS personnel visit the bridge site, and office-reviewed bridge site assessments, for which USGS personnel compile PennDOT data and do not visit the bridge site. Both types of assessments are primarily focused at assisting PennDOT in meeting the requirements of the Federal Highway Administration mandate; however, both assessments include procedures for the collection and processing of ancillary data for subsequent analysis. Date of bridge construction and the accessibility of the bridge substructure units for inspection determine which type of assessment a bridge receives. A Scour-Critical Bridge Indicator Code and a Scour Assessment Rating are computed from selected collected and compiled data. PennDOT personnel assign the final Scour-Critical Bridge Indicator Code and a Scour Assessment Rating on the basis of their review of all data. Data for scour assessments are incorporated from various sources, including (but not limited to) PennDOT; private contractors; various federal, state, and municipal government agencies; and the USGS. All data from sources outside of the USGS are received through and accepted as correct by PennDOT (Bryan Spangler, P.E., Pennsylvania Department of Transportation, oral commun., 1999). For the purposes of this project, PennDOT quality-control procedures are considered to be acceptable.
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Streamflow hydrographs of 309 streamflow stations in Pennsylvania were analyzed by using three computer-assisted empirical methods-local-minimum , fixed-interval, and sliding-interval-to separate the groundwater and surface-runoff components. The 2-, 5-, 10-, and 25-year base-flow-recurrence intervals were determined for each station. The 50-and 100-year recurrence intervals were determined for stations with minimums of 25 and 50 years of record, respectively. All active and inactive continuous-recording Pennsylvania streamflow stations with 10 or more years of record through water year 1986 were analyzed. Stations on the Delaware River in New York and New Jersey also were included. Hydrograph analysis by computer provides consistency of results by removing the subjectivity inherent in manual analyses and considerably reduces the time required to make the analyses. Estimates of the 25-year base-flow-recurrence interval made by using the local-minimum technique, which gives the most conservative estimate, range from 0.069 to 0.888 million gallons per day per square mile. Base flows estimated by using the three computer-assisted techniques compared well with published base flows estimated by using manual methods for three studies in southeastern Pennsylvania. Examples of the effects of geology, climate, regulation, and urbanization on base flow are provided. Hydrograph analysis for base flow divides streamflow into its two component parts: overland runoff and groundwater discharge (base flow). Base-flow-hydrograph analysis techniques commonly are used to estimate groundwater contributions to streamflow and to define ground-water/surface-water relations. They also are used to calculate hydrologic budgets and to estimate recharge rates.
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