Ayres Knowledge Center Scour Evaluation: Bridges with Unknown Foundations | Page 4

500-year discharge estimate for this bridge is Q 500 = 92,770 cfs based on the unsteady flow hydraulic model, and
corresponds to a water surface elevation of ±9.6 feet NAVD at the bridge.3.0 HYDRAULIC ANALYSIS
3.0 HYDRAULIC ANALYSIS
The hydraulic analysis consisted of an unsteady-state simulation of the 100-year event conditions using
HEC-RAS. A model based on survey data, aerial photographs, and the NOAA Daytona Beach DEM (2009) was
developed to predict hydraulic conditions at the bridge crossing for the storm event discharge. HEC-RAS
input/output are included in Attachment B, Hydraulic Analysis.
4.0 SCOUR ANALYSIS
The scour analysis was performed using procedures outlined in the Reclassify Unknown Foundation Bridges
Procedural Manual and the FDOT Bridge Scour Manual. A median sediment particle size (D 50 ) of 0.2 mm was
assumed based on the Phase 1 soil description. Scour calculations assume an erodible material with a
homogeneous particle size distribution.
Several Soundings were available from the various inspection reports. No trend towards vertical aggradation or
degradation was evident; therefore, no degradation was predicted as part of this analysis. However, evaluation of
the soundings and survey taken (Ayres, 2011) indicates that some local scour may already be taking place at the
bascule piers.
Live bed scour mode is anticipated based on contraction scour computations performed using HEC-18 (FHWA,
2001) methodology.
The velocities used for the 500-year storm surge event pier scour calculations were derived conservatively
assuming that all flow passes under the bridge. The velocities reported in HEC-RAS were adjusted up by a ratio
of the HEC-RAS reported water depth at each pier to the height of the bridge opening before scour at each pier
(velocities used are 1.0 to 2.5 time greater than those reported by HEC-RAS for the 500-year storm surge event.
No abutment scour calculations were performed for this analysis. The abutments are protected by seawalls on
both sides of the channel. The seawalls do not act like abutments and thus local abutment scour is not relevant for
this bridge. The seawalls, however, would be subject to the computed contraction scour for both the 100͈year and
500͈ year storm surge events. The stability of these seawalls to withstand the contraction scour is unknown and
should be evaluated. A schedule for inspection of the stability of these seawalls after major storm events and
during routine bridge inspections should be developed.
Sheppard scour equation was used to compute local scour at all interior bents. The effective width of these
battered pile bents was conservatively taken to be 1.5 times the pile width for bents where the pile cap was
anticipated to be impinging on the 100͈year event flows. The effective width for all other battered pile interior bents
was taken as the width from out to out of the battered piles at the FEMA still͈water elevations.
5.0 MATERIAL AND DOCUMENTATION
- Phase 1 Scour Evaluation Report (September 2011)
- Daytona Beach Digital Elevation Model, NOAA dated March 2009
- Volusia County Flood Insurance Study, dated September 29, 2011
- Bridge Inspection Report dated 4/28/2010
- Bridge and bathymetric survey (2011)
- Volusia County 1-ft Contours
- Attachments:
A. Hydrologic Analysis
B. Hydraulic Analysis
C. Scour Analysis
D. FIS Extract
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