HEC 25 - Tidal Hydrology, Hydraulics, and Scour at Bridges

Acknowledgments

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.

Glossary

Apparent Ocean Level Rise The amount of long term rise in sea levels. The amount of rise is a combination of actual sea level rise or fall and regional subsidence, uplift or rebound.

Backshore The portion of the beach between the foreshore and the dunes or landward extent of the storm beach. The backshore is generally only acted upon by waves during storms.

Barycenter The center of mass of the earth moon system. This is the center of rotation of this two-body system.

Bay A body of water connected to the ocean with an inlet.

Celerity See wave speed.

Coastal Zone The line forming the boundary between the land and water is commonly called the coastline or shoreline. The strip of land of indefinite width that extends inland to the first major change in terrain features is commonly referred to as the coast of coastal zone. The coastal zone may be several kilometers wide.

Datum A reference point for vertical (elevation) measurements. See tidal datums and fixed datums.

Diurnal tide Tides with an approximate tidal period of 25 hours.

Ebb or ebb tide Flow of water from the bay or estuary to the ocean.

Embayment An indentation in the shoreline.

Estuary Tidal reach at the mouth of a river.

External Boundary Conditions Upstream and downstream (ocean) constraining variables at the model limits. These include upstream flow hydrographs and downstream stage hydrographs.

Fetch The area over water where the wind is unobstructed with fairly uniform speed and direction.

Fixed Datums A reference level surface that has a constant elevation over a large geographical area.

Flood or flood tide Flow of water from the ocean to the bay or estuary.

Foreshore The portion of the beach between the backshore and the low tide elevation. The foreshore is the active beach where wave uprush and backwash occurs over the range of the tide.

Hurricane An intense type of tropical cyclone with well defined circulation and maximum sustained winds of 74 mph or higher.

Ineffective flow areas Portions of cross sections that provide storage but do not convey flow.

Internal boundary conditions Flow Controls at interior portions of a numerical model. These include structures (culverts, weirs, and bridges) and lateral inflow hydrographs.

Intracoastal waterway A series of natural and dredged channels within the U.S. coastline intended for small craft.

Junction confluence A Location where channel reaches (main channel and tributaries) connect.

Littoral transport or drift Transport of beach material along a shoreline by wave action. Also, longshore sediment transport.

Littoral zone The region that extends seaward from the coastline to just beyond the beginning of the breaking waves. Within this zone, waves and currents transport sediments. A current is generated by the incident waves within the littoral zone

Mean high water (MHW) The average of all high tides over a tidal epoch.

Mean higher high water (MHHW) The average of all daily higher high tides (for semidiurnal tides) over a tidal epoch.

Mean low water (MLW) The average of all low tides over a tidal epoch.

Mean lower low water (MLLW) The average of all daily lower low tides (for semidiurnal tides) over a tidal epoch.

Mean sea level The average of hourly tide heights over a tidal epoch.

Mean tide level The average of all high and low tides.

Mixed tide Semidiurnal tides that exhibit significant differences between the two high and two low tides.

National Tidal Datum Epoch (NTDE) The specific 19-year period adopted by the National Ocean Service (NOS) as the official time segment over which tide observations are taken and reduced to obtain mean values for tidal datums.

Neap tide Smaller than normal tides that occur approximately twice per month at the first and third quarter moon phase when the sun and moon are at right angles to the earth and the tidal forces counteract each other.

Network An assembly of channel reaches, junctions and storage areas that make up the numerical model geometric description of the waterway.

Numerical stability The ability of the program to converge to a solution at a time step.

Ocean Level Rise See apparent ocean level rise.

Passage A tidal waterway between two islands or between the mainland and an island.

Reaches Segments of the waterway between tributary confluences and between confluences and the upper and lower model limits.

Run-up, wave Height to which water rises above still-water elevation when waves meet a beach, wall, etc.

Saffir-Simpson hurricane scale A scale of one through five (called categories) based on the hurricane intensity.

Semidiurnal tide Tides with an approximate tidal period of 12.5 hours.

Set-up, wave Height to which water rises above still-water elevation as a result of storm wind effects.

Significant wave height The average height of the one-third largest waves for a specific set of wind, fetch and water depth conditions.

Spring tide Larger than normal tides that occur approximately twice per month at new and full moon when the sun and moon are aligned and the tidal forces are reinforced. See syzygy.

Still-water elevation Flood height to which water rises as a result of barometric pressure and wind occurring during a storm event without including waves heights. The water level without the effects of waves.

Storage areas Network features where the model performs simple storage routing assuming a level water surface.

Storm surge Coastal flooding phenomenon resulting from wind and barometric changes. The storm surge is measured by subtracting the astronomical tide elevation from the total flood elevation (Hurricane surge).

Storm tide Coastal flooding resulting from combination of storm surge and astronomical tide (often referred to as storm surge)

Sustained winds Wind speeds that persist for duration of one minute.

Syzygy The time when the moon and sun are aligned such that the tide forces of the bodies act to reinforce each other. This occurs at new and full moon.

Tidal amplitude Generally, half of tidal range.

Tidal current Flow in a tidal waterway that is caused by the rise and fall of tides. See ebb tide and flood tide.

Tidal cycle One complete rise and fall of the tide.

Tidal day Time of rotation of the earth with respect to the moon. Assumed to equal approximately 24.84 solar hours in length.

Tidal datum A local reference elevation relative to a tidal level, such a mean lower low water (MLLW).

Tidal epoch A cycle of approximately 18.6 years of the principle tide producing forces.

Tidal inlet A channel connecting a bay or estuary to the ocean.

Tidal passage A tidal channel connected with the ocean at both ends.

Tidal period Duration of one complete tidal cycle. When the tidal period equals the tidal day (24.84 hours), the tide exhibits diurnal behavior. Should two complete tidal periods occur during the tidal day, the tide exhibits semidiurnal behavior.

Tidal prism Volume of water contained in a tidal bay, inlet or estuary between low and high tide levels.

Tidal range Vertical distance between specified low and high tide levels.

Tidal waterways A generic term which includes tidal inlets, estuaries, bridge crossings to islands or between islands, inlets to bays, crossings between bays, tidally affected streams, etc.

Tides, astronomical Periodic diurnal or semidiurnal variations in sea level that result from centrifugal and gravitational forces between the earth, moon, sun and other astronomical bodies acting on the rotating Earth.

Tropical depression An organized system of clouds and thunderstorms with a defined circulation and maximum sustained winds of 38 mph.

Tropical storm An organized system of strong thunderstorms with a defined circulation and maximum sustained winds of 39 to 73 mph.

Tsunami Long-period ocean wave resulting from earthquake, other seismic disturbances or submarine land slides.

Waterway opening Width or area of bridge opening at a specific elevation, measured normal to principal direction of flow.

Wave height The vertical difference between successive wave crests and troughs.

Wave length The horizontal difference between two successive wave crests or two successive wave troughs.

Wave period Time interval between arrivals of successive wave crests at a point.

Wave speed or celerity The travel speed of a wave equal to the wave length divided by the wave period.

Chapter 1 Introduction

1.1 PURPOSE

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.

1.2 BACKGROUND

1.2.1 Previous Studies

This manual incorporates the results of the Pooled Fund Project "Development of Hydraulic Computer Models to Analyze Tidal and Coastal Stream Hydraulic Conditions at Highway Structures" (Ayres Associates 1994, 1997, 2002a, 2002b). The project was initiated in recognition of the need for more accurate approaches to determine hydraulic conditions at bridges in tidal waterways. The objectives of the Pooled Fund Project were to improve methods for determining hurricane storm tide hydrographs, to determine which computer models are well suited for bridge hydraulic applications in tidal waterways, and to develop training for these methods and models.

HEC-18 (Richardson and Davis 2001) contains several simplified methods for tidal hydraulic analysis. These methods are applicable to many bridge crossings in tidal waterways. As part of the Pooled Fund Study, computer models were investigated for more advanced tidal hydraulic analyses including 1- and 2-dimensional hydrodynamic models. Hydrodynamic models are capable of accurate hydraulic simulation of the situations when simplified methods yield unacceptably conservative results or are not applicable due to flow complexity. The Pooled Fund study supplemented the users manuals associated with specific hydrodynamic computer models by providing specific guidance on using these models for tide and hurricane surge conditions.

The tidal hydrology portion of the Pooled Fund study included estimates of peak hurricane surge elevations for 100- and 500-year hurricanes for the East and Gulf coasts and data of historic hurricane storm surges. Other agencies (including FEMA, NOAA, USACE, and State Agencies) have also developed estimates of hurricane surge elevations that can be used as part of the analysis described in this manual. The peak elevation and shape of the surge hydrograph affect the hydraulic conditions in a tidal waterway. Therefore, the Pooled Fund study provided guidance on developing a surge hydrograph from a known peak elevation, other hurricane characteristics and astronomical tide conditions. Guidance was also provided on included upland runoff and wind in tidal simulations.

1.2.2 Tidal Waterways

The first step in evaluation of highway crossings is to determine whether the bridge crosses a river which is influenced by tidal fluctuations (tidally affected river crossing) or whether the bridge crosses a tidal inlet, bay or estuary (tidally controlled). The flow in tidal inlets, bays and estuaries is predominantly driven by tidal fluctuations (with flow reversal), whereas, the flow in tidally affected river crossings is driven by a combination of river flow and tidal fluctuations. Therefore, tidally affected river crossings are not subject to flow reversal but the downstream tidal fluctuation acts as a cyclic downstream control. Tidally controlled river crossings will exhibit flow reversal.

Tidally affected river crossings are characterized by both river flow and tidal fluctuations. From a hydraulic standpoint, the flow in the river is influenced by tidal fluctuations which result in a cyclic variation in the downstream control of the tail water in the river estuary. The degree to which tidal fluctuations influence the discharge at the river crossing depends on such factors as the relative distance from the ocean to the crossing, riverbed slope, cross-sectional area, storage volume, and hydraulic resistance. Although other factors are involved, relative distance of the river crossing from the ocean can be used as a qualitative indicator of tidal influence. At one extreme, where the crossing is located far upstream, the flow in the river may only be affected to a minor degree by changes in tailwater control due to tidal fluctuations. As such, the tidal fluctuation downstream will result in only minor fluctuations in the depth, velocity, and discharge through the bridge crossing.

As the distance from the crossing to the ocean is reduced, again assuming all other factors as equal, the influence of the tidal fluctuations increases. Consequently, the degree of tail water influence on flow hydraulics at the crossing increases. A limiting case occurs when the magnitude of the tidal fluctuations is large enough to reduce the discharge through the bridge crossing to zero at high tide. River crossings located closer to the ocean than this limiting case have two directional flows at the bridge crossing, and because of the storage of the river flow at high tide, the ebb tide will have a larger discharge and velocities than the flood tide.

Tidal waterways are defined as any waterway either dominated or influenced by tides and hurricane storm surges. Several types of tidal waterways are depicted in Figures 1.1 through 1.4. These include estuaries, inlets, bays, and passages. An estuary (Figure 1.1) is the tidally influenced portion of a river. Estuaries may have a significant upland flow component or very little upland flow. The size of the channel often bears little relation to the amount of upland flow. Even for large rivers, the amount of daily tidal flow often far exceeds upland flows. Similarly, discharges associated with storm surges often greatly exceed upland flood flows many miles inland.

Figure 1.1 Estuary (after Neill 1973).

Figure 1.2. Bay and inlet (after Neill 1973).

Figure 1.3. Passages (after Neill 1973).

Figure 1.4 Barrier Islands forming complex tidal systems.

Bays are inland bodies of water connected to the ocean by inlets (Figure 1.2). Upland flows into bays are often negligible. Because the inlet constricts the tidal flows between the bay and ocean, the tidal range within a bay is typically much smaller than the ocean tidal range. The flow in the inlet can be related to the head differential between the bay and ocean and the amount of head loss created by the inlet constriction and channel. If the head differential is significant, then the velocity of flow in the inlet can be quite large. The large velocities can cause inlet instability, but may not necessarily result in discharges large enough to fill the bay during the tidal cycle. Flow within shallow bays is often significantly influenced by wind.

Passages between an island and mainland or between two islands are another type of tidal waterway (Figure 1.3). Significant tidal flow can occur in a passage if there is even a small shift in the timing of the tide from one side of passage to the other. The flow through passages may not be significantly influenced by tides but can be dominated by ocean currents and by wind. Wind can cause the water level to set up on the upstream side of a causeway crossing a passage and set down on the downstream side. The resulting head differential can produce significant amounts of flow through the causeway bridge. Causeways can also cause constrictions in bays and can exhibit similar wind effects.

In many areas, barrier islands can form complex tidal systems. Figure 1.4 shows that the hydraulic conditions at bridges in tidal waterways can be controlled by the interaction of ocean shoreline tide levels at multiple inlets propagating through an interconnected system of bays, channels, estuaries and rivers. The stability of the inlets and other tidal waterways will depend on sediment supplies and transport from inland and near shore sources.

The intracoastal waterway is a series of natural and dredged channels intended for small craft navigation within the U.S. coastline. The dredged channels may be within shallow bays or canals cut to connect interior waterways. The flow in the canal sections may be controlled by the tidal action at the ends of the cut sections and may be influenced by wind and upland runoff.

Bridge hydraulics in tidal waterways must account for the various sources of flow. These include upland floods, normal upland daily flows, tides, currents, storm surges, and winds. The stability of the tidal waterways must also be considered. Inlets can deepen, shift laterally and close. New inlets can form where barrier islands are breached. If littoral transport (sediment transported along the shore by waves and currents) is interrupted, waterway instability can result. Contraction scour can occur at constrictions to flow and local scour can occur at obstructions to flow. All these processes must be considered in the design and evaluation of bridge crossings in tidal waterways.

1.3 MANUAL ORGANIZATION

This manual is organized to:

• Provide background on tides, hurricanes, waves and other related topics (Chapter 2).
  Definitions are included in a Glossary
• Discuss general background of the simple approaches, including the fact that these approaches have limited application. These include tidal prism, orifice approach, routing approach, and method selection (Chapter 3).
• Describe dynamic computer modeling in general terms, including model extents, calibration, and troubleshooting (Chapter 4). One- and two-dimensional modeling, physical modeling, and model selection are also discussed in this chapter.
• Describe the use of the FHWA HEC-18 relationships for tidal scour computations and introduce special considerations for tidal bridges, including time dependent contraction and local scour, as well as lateral and vertical stability (Chapter 5).
• Discuss data requirements and sources (Chapter 6).
• Discuss and provide guidance on other considerations such as, dynamic beach processes, wave analysis countermeasures, and apparent ocean level rise (Chapter 7).
• Provide selected references (Chapter 8).
  
  

The internet is an extremely valuable source of information and data for tidal studies. These data include tide conditions, tide benchmarks, bathymetric and topographic surveys, mapping, aerial photography, and agency publications. Webpage addresses are provided for a number of these data sources. Given the fact that these addresses change, in some cases frequently, the user should be prepared to perform searches in order to locate the pertinent information. This practice is recommended not only to determine the current web address of previously useful information, but also because it will lead to new sources of information. In this manual, webpage addresses are accompanied by a group of search terms that should identify the specific site and other sites containing similar information.

1.4 DUAL SYSTEM OF UNITS

This edition of HEC-25 uses dual units (SI metric and English). The "English" system of units as used throughout this manual refers to U.S. customary units. In Appendix A, the metric (SI) unit of measurement is explained. The conversion factors, physical properties of water in the SI and English systems of units, sediment particle size grade scale, and some common equivalent hydraulic units are also given. This edition uses for the unit of length the meter (m) or foot (ft); of mass the kilogram (kg) or slug; of weight/force the newton (N) or pound (lb); of pressure the Pascal (Pa, N/m²) or (lb/ft²); and of temperature the degree centigrade (EC) or Fahrenheit (EF). The unit of time is the same in SI as in English system (seconds, s). Sediment particle size is given in millimeters (mm), but in calculations the decimal equivalent of millimeters in meters is used (1 mm = 0.001 m) or for the English system feet (ft).

It should be noted that the density and specific weight of sea water is approximately 3 percent greater than fresh water, the dynamic viscosity of sea water is approximately 7 percent greater than fresh water, and the kinematic viscosity of sea water is approximately 4 percent greater than fresh water. These comparisons of sea water and fresh water do not include the effects of suspended sediment, which can cause additional changes in density, specific gravity, and viscosity.

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