Tidal Hydrology, Hydraulics and Scour at Bridges

This manual draws extensively from the results of a Pooled Fund Project "Development of
Hydraulic Computer Models to Analyze Tidal and Coastal Stream Hydraulic Conditions at
Highway Structures." The authors gratefully acknowledge the special efforts of the lead
state, South Carolina Department of Transportation and William Hulbert (formerly SCDOT),
the Pooled Fund Project’s Technical Advisory Panel, and Johnny Morris (formerly FHWA) for
their support and guidance in completing the Pooled Fund Project.
The authors also wish to acknowledge the technical assistance, review, and guidance
provided by Larry Arneson and Joseph Krolak (FHWA), and Scott Douglass (University of
South Alabama) for their efforts in completing this First Edition of Hydraulic Engineering
Circular No. 25 – Tidal Hydrology, Hydraulics, and Scour at Bridges.
The purpose of this manual is to provide guidance on hydraulic analysis for bridges
over tidal waterways. This document includes descriptions of: (1) common physical features
that affect transportation projects in coastal areas, (2) tide causing astronomical and
hydrologic processes, (3) approaches for determining hydraulic conditions for bridges in tidal
waterways, (4) applying the hydraulic analysis results to provide scour estimates. This
document is not intended to provide guidance on coastal surge modeling (modeling that is
used to predict the magnitude of hurricane-produced storm surges based on direct simulation
of hurricane conditions). However, the information provided by other agencies (including
FEMA, NOAA, USACE, States Agencies) on surge conditions is used to estimate the
hydraulic conditions of tidally affected bridges.
By using the methods in this manual, better predictions of bridge hydraulics and scour
in tidal waterways will result. In many cases, simplified tidal hydraulic methods will provide
adequate results. However, when the simplified methods yield overly conservative results,
use of the recommended modeling approaches will provide more realistic predictions and
hydraulic variables and scour.
Location and hydraulic design studies for tidal bridges should be conducted in
accordance with 23 CFR 650A, when applicable. Since this document provides guidance on
the hydraulic analysis of bridges over tidal waterways, the methods described herein can
help assess potential impacts of proposed structures and encroachments on floodplains.
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