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

Chapter 5 Tidal Scour

5.1 Bridge Scour Analysis for Tidal Waterways

5.1.1 Overview

In the coastal region, scour at bridges over tidal waterways that are subjected to the effects of astronomical tides and storm surges is a combination of long-term degradation, contraction scour, local scour, and waterway instability. These are the same scour mechanisms that affect non-tidal (riverine) streams. Although many of the flow conditions are different in tidal waterways, the equations used to determine riverine scour are applicable if the hydraulic conditions (depth, discharge, velocity, etc.) are carefully evaluated.

Analysis of bridge scour in tidal waterways is very complex. The hydraulic analysis must consider the magnitude of the 100- and 500-year storm surge, the characteristics (geometry) of the tidal inlet, estuary, bay or tidal stream and the effect of any constriction of the flow due to the bridge. In addition, the analysis must consider the long-term effects of the normal tidal cycles on long-term aggradation or degradation, contraction scour, local scour, and stream instability. Coastal analyses require a synthesis of complex meteorological, bathymetric, geographical, statistical, and hydraulic disciplines and knowledge.

The astronomical tidal cycle with reversal in flow direction can increase long-term degradation, contraction scour, and local scour. If sediment is being moved on the flood and ebb tide, there may be no net loss of sediment in a bridge reach because sediments are being moved back and forth. Consequently, no net long-term degradation may occur. However, local scour at piers and abutments can occur at both the inland and ocean side of the piers and abutments and will alternate with the reversal in flow direction. If, however, there is a loss of sediment in one or both flow directions, there will then be long-term degradation in addition to local scour. Also, the tidal cycles may increase bank erosion, migration of the channel, and thus, increase stream instability.

The complexity of the hydraulic analysis increases if the tidal inlet or the bridge constrict the flow and affect the amplitude of the storm surge (storm tide) in the bay or estuary so that there is a large change in elevation between the ocean and the estuary or bay. A constriction in the tidal inlet can increase the velocities in the constricted waterway opening, decrease interior wave heights and tidal range, and increase the phase difference (time lag) between exterior and interior water levels. Analysis of a constricted inlet or waterway may require the use of an orifice equation rather than tidal relationships.

For the analysis of bridge crossings of tidal waterways, a three-level analysis approach similar to the approach outlined in HEC-20 is suggested. Level 1 includes a qualitative evaluation of the stability of the inlet or estuary, estimating the magnitude of the tides, storm surges, and flow in the tidal waterway, and attempting to determine whether the hydraulic analysis depends on tidal or river conditions, or both. Level 2 represents the engineering analysis necessary to obtain the velocity, depths, and discharge for tidal waterways to be used in determining long-term aggradation, degradation, contraction scour, and local scour. The hydraulic variables obtained from the Level 2 analysis are used in the riverine equations presented in HEC-18 to obtain total scour. Using these riverine scour equations, which are for steady-state equilibrium conditions for unsteady, dynamic tidal flow may result in estimating deeper scour depths than will actually occur (conservative estimate), but this represents the state of knowledge at this time for this level of analysis. For complex tidal situations, Level 3 analysis using physical and 2-dimensional computer models may be required.

The steady-state equilibrium scour equations given in HEC-18 are suitable for use to determine scour depths in tidal flows. As mentioned earlier, tidal flows resulting from storm surges are unsteady but no more so than most unsteady riverine flows. For both cases, scour depths are conservative.

5.1.2 Level 1 Analysis

The objectives of a Level 1 qualitative analysis are to determine the magnitude of the tidal effects on the crossing, the overall long-term stability of the crossing (vertical and lateral stability) and the potential for waterway response to change.

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. 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 are 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.

For the Level 1 analysis, it is important to evaluate whether the tidal fluctuations will significantly affect the hydraulics at the bridge crossing. If the influence of tidal fluctuations is considered to be negligible, then the bridge crossing can be evaluated based on the procedures outlined for inland river crossings presented in HEC-18. If not, then the hydraulic flow variables must be determined using dynamic tidal flow relationships. This evaluation should include extreme events such as the influence of storm surges and inland floods.

From historical records of the stream at the highway crossing, determine whether the worst-case conditions of discharge, depths and velocity at the bridge are the 100- and 500-year return period tide and storm surge, or the 100- and 500-year inland flood or a combination of the two. Historical records could consist of tidal and stream flow data from Federal Emergency Management Agency (FEMA), National Oceanic and Atmospheric Administration (NOAA), USACE, and USGS records; aerial photographs of the area; maintenance records for the bridge or bridges in the area; newspaper accounts of previous high tides and/or flood flows; and interviews in the local area.

If the primary hazard to the bridge crossing is from inland flood events, then scour can be evaluated using the methods given in HEC-18 and HEC-20. If the primary hazard to the bridge is from tide and storm surge or tide, storm surge and inland flood runoff, then use the approach outlined in the following sections on tidal waterways. If it is unclear whether the worst hazard to the bridge will result from a storm surge, maximum tide, or from an inland flood, it may be necessary to evaluate scour considering each of these scenarios and compare the results.

Tidal Inlets, Bays, and Estuaries. For tidal inlets, bays and estuaries, the goal of the Level 1 analysis is to determine the stability of the inlet and identify and evaluate long-term trends at the location of the highway crossing. This can be accomplished by careful evaluation of present and historical conditions of the tidal waterway and anticipating future conditions or trends.

Existing cross-sectional and sounding data can be used to evaluate the stability of the tidal waterway at the highway crossing and to determine whether the inlet, bay or estuary is increasing or decreasing in size, or is relatively stable. For this analysis it is important to evaluate these data based on past and current trends. The data for this analysis could consist of aerial photographs, cross section soundings, location of bars and shoals on both the ocean and bay sides of an inlet, magnitude and direction of littoral drift, and longitudinal elevations through the waterway. It is also important to consider the possible impacts (either past or future) of the construction of jetties, breakwaters, or dredging of navigation channels.

Sources of data would be USACE, FEMA, USGS, U.S. Coast Guard (USCG), NOAA, local Universities, oceanographic institutions, and publications in local libraries. For example, a publication by Bruun (1966), "Tidal Inlets and Littoral Drift" contains information on many tidal inlets on the east coast for the United States.

A site visit is recommended to gather such data as the conditions of the beaches (ocean and bay side); location and size of any shoals or bars; direction of ocean waves; magnitude of the currents in the bridge reach at mean water level (midway between high and low tides); and size of the sediments. Sounding the channel both longitudinally and in cross section using a conventional "fish finder" sonic fathometer is usually sufficiently accurate for this purpose.

Observation of the tidal inlet to identify whether the inlet restricts the flow of either the incoming or outgoing tide is also recommended. If the inlet or bridge restricts the flow, there will be a noticeable drop in head (change in water surface elevation) in the channel during either the ebb or flood tide. If the tidal inlet or bridge restricts the flow, an orifice equation may need to be used to determine the maximum discharge, velocities and depths (see Chapter 3).

Velocity measurements in the tidal inlet channel along several cross sections, several positions in the cross section and several locations in the vertical can also provide useful information for verifying computed velocities. Velocity measurements should be made at maximum discharge. Maximum discharge usually occurs around the midpoint in the tidal cycle between high and low tide, although constricted inlets usually cause peak discharge to occur closer to high and low tides.

The velocity measurements can be made from a boat or from a bridge located near the site of a new or replacement bridge. If a bridge exists over the channel, a recording velocity meter could be installed to obtain measurements over several tidal cycles. Currently, there are instruments available that make velocity data collection easier. For example, broad-band acoustic Doppler current profiles and other emerging technologies will greatly improve the ability to obtain and use velocity data.

In order to develop adequate hydraulic data for the evaluation of scour, it is recommended that recording water level gages located at the inlet, at the proposed bridge site and in the bay or estuary upstream of the bridge be installed to record tide elevations at 15-minute intervals for several full tidal cycles. This measurement should be conducted during one of the spring tides where the amplitude of the tidal cycle will be largest. The gages should be referenced to the same datum and synchronized. The data from these recording gages are necessary for calibration of tidal hydraulic models.

The data and evaluations suggested above can be used to estimate whether present conditions are likely to continue into the foreseeable future and as a basis for evaluating the hydraulics and total scour for the Level 2 analysis. A stable inlet could change to one which is degrading if the channel is dredged or jetties are constructed on the ocean side to improve the entrance, since dredging or jetties could modify the supply of sediment to the inlet. In addition, plans or projects which might interrupt existing conditions of littoral sediment transport should be evaluated.

It should be noted that in contrast to an inland river crossing, the discharge at a tidal inlet is not fixed. In inland rivers, the design discharge is fixed by the runoff and is virtually unaffected by the waterway opening. In contrast, the discharge at a tidal inlet can increase as the area of the tidal inlet increases, thus increasing long-term aggradation or degradation and local scour. Also, as Neill points out, constriction of the natural waterway opening may modify the tidal regime and associated tidal discharge (Neill 1973).

5.1.3 Level 2 Analysis

Level 2 analysis involves the basic engineering assessment of scour problems at highway crossings. Scour equations developed for inland rivers are recommended for use in estimating and evaluating scour for tidal flows. However, in contrast to the evaluation of scour at inland river crossings, the evaluation of the hydraulic conditions at the bridge crossing using either WSPRO or HEC-RAS is only suitable for tidally affected crossings where tidal fluctuations result in a variable tailwater control without flow reversal. Other methods, described in this manual, are recommended for tidally affected and tidally controlled crossings where the tidal fluctuation has a significant influence on the tidal hydraulics.

Evaluation of Hydraulic Characteristics. Several methods to obtain hydraulic characteristics of tidal flows at the bridge crossing are available. These range from simple procedures to more complex 1-dimensional and 2-dimensional unsteady flow models. The use of the simpler hydraulic procedures is discussed in Chapter 3. An overview of the unsteady flow models which are suitable for modeling tidal hydraulics at bridge crossings is presented in Chapter 4. The use of the simpler hydraulic procedures given in Chapter 3 can give large values if their underlying assumptions are violated. In these cases, 1- and 2-dimensional computer models can give more realistic values.

The velocity, depth and discharge at the bridge waterway are the most significant variables for evaluating bridge scour in tidal waterways. Direct measurements of the value of these variables for the design storm are seldom available. Therefore, it is usually necessary to develop the hydraulic and hydrographic characteristics of the tidal waterway, estuary or bay, and calculate the discharge, velocities, and depths in the crossing using coastal engineering equations. These values can then be used in the scour equations given in HEC-18 to calculate long-term aggradation or degradation, contraction scour, and local scour.

Scour Evaluation Concepts. The total scour at a bridge crossing can be evaluated using the scour equations recommended for inland rivers and the hydraulic characteristics determined using the procedures outlined in the previous sections. However, it should be emphasized that the scour equations and subsequent results need to be carefully evaluated considering other (Level 1) information from the existing site, other bridge crossings, or comparable tidal waterways or tidally affected streams in the area.

Evaluation of long-term aggradation or degradation at tidal highway crossings, as with inland river crossings, relies on a careful evaluation of the past, existing and possible future condition of the site. This evaluation is outlined under Level 1 and should consider the principles of sediment continuity. A longitudinal sonic sounder survey of a tide inlet is useful to determine if bed material sediments can be supplied to the tidal waterway from the bay, estuary or ocean. When available, historical sounding data should also be used in this evaluation. Factors which could limit the availability of sediment should also be considered.

Over the long-term in a stable tidal waterway, the quantity of sediment being supplied to the waterway by ocean currents, littoral transport and inland flows and being transported out of the tidal waterway are nearly the same. If the supply of sediment is reduced either from the ocean or from the bay or estuary, a stable waterway can be transformed into a degrading waterway. In some cases, the rate of long-term degradation has been observed to be large and deep. An estimate of the maximum depth that this long-term degradation can achieve can be made by employing the HEC-18 clear-water contraction scour equation to the inlet. For this computation the flow hydraulics should be developed based on the range of mean tide. It should be noted that the use of this equation would provide an estimate of the worst case long-term degradation which could be expected assuming no sediments were available to be transported to the tidal waterway from the ocean or inland bay or estuary. As the waterway degrades, the flow conditions and storage of sediments in shoals will change, ultimately developing a new equilibrium. The presence of scour resistant rock would also limit the maximum long-term degradation.

Potential contraction scour for tidal waterways also needs to be carefully evaluated using hydraulic characteristics associated with the 100- and 500-year storm surge or inland flood as described in the previous section. For highway crossings of estuaries or inlets to bays, where either the channel narrows naturally or where the channel is narrowed by the encroachment of the highway embankments, the live-bed or clear-water contraction scour equations can be utilized to estimate contraction scour.

Soil boring or sediment data are needed in the waterway upstream, downstream, and at the bridge crossing in order to determine if the scour is clear-water or live-bed and to support scour calculations if clear-water contraction scour equations are used. The HEC-18 critical velocity equation and the ratio of V_*/T can be used to assess whether scour would be clear-water or live-bed.

A mitigating factor which could limit contraction scour concerns sediment delivery to the inlet or estuary from the ocean due to the storm surge and inland flood. A surge may transport large quantities of sediment into the inlet or estuary during the flood tide. Likewise, inland floods can also transport sediment to an estuary during extreme floods. Thus, contraction scour during extreme events may be classified as live-bed because of the sediment being delivered to the inlet or estuary from the combined effects of the storm surge and inland flood. The magnitude of contraction scour must be carefully evaluated using engineering judgment which considers the geometry of the crossing, estuary or bay, the magnitude and duration of the discharge associated with the storm surge or inland flood, the basic assumptions for which the contraction scour equations were developed, and mitigating factors which would tend to limit contraction scour.

Evaluation of local scour at piers can be made by using the HEC-18 equations as recommended for inland river crossings. These equations can be applied to piers in tidal flows in the same manner as given for inland bridge crossings. However, the flow velocity and depth will need to be determined considering the design flow event and hydraulic characteristics for tidal flows including flow reversal.

5.1.4 Scour Equations

The HEC-18 (Richardson and Davis 2001) contraction and local scour equations can be applied to bridges in tidal waterways when the design hydraulic conditions are determined based on appropriate tidal hydrodynamic methods. The most recent edition of HEC-18 should be used. Specific HEC-18 equations are not included in this section in order to limit any inconsistencies between this manual and future editions of HEC-18. The overall procedures outlined in HEC-18 should be followed for tidal applications. Contraction scour should be computed based on the live-bed or clear-water equations depending on the velocity of flow approaching the bridge in the unconstricted waterway. The location of the approach flow will depend on whether worst case conditions occur during the flood or ebb tide.

Local scour can also be computed using the HEC-18 equations. HEC-18 includes pier scour for standard and complex pier geometry. The HEC-18 equations include wide pier correction factors that may be applicable to bascule piers when the pier is wide in comparison to the flow depth. The complex pier equation is applicable to piers that include waterline pile caps supported by a group of piles. Other local scour equations are presented in Melville and Coleman (2000), Hoffman and Verheij (1997), and Sheppard (2003).

If astronomical tide currents have high velocities, scour should be computed for these conditions in addition to design velocities produced by hurricane or storm surge conditions. Surges can produce extreme velocities that could produce very deep scour. The HEC-18 equations may be overly conservative for surge conditions because these equations were developed for ultimate scour conditions. While the surge may produce extreme velocity, the high velocity condition may persist for such a short duration that ultimate scour cannot be reached. Additional sediment transport analysis and judgment may be necessary for computing scour in tidal waterways.

5.2 Time Dependent Contraction Scour

Hurricane storm surges often produce extreme flow conditions for time periods of only a few hours. Computing ultimate contraction scour amounts for these conditions may not be reasonable. Ultimate contraction scour is reached when the sediment supply from upstream is matched by the sediment transport capacity in the scoured bridge opening. Equating sediment transport capacity to upstream supply results in the HEC-18 live-bed contraction scour equation, which uses a simplification of the Laursen sediment transport equation. Sediment transport relationships could also be used directly to compute ultimate contraction scour. Applying sediment transport formulas to contraction scour is recommended in HEC-18 for more complex situations. Specifically, HEC-18 states:

Both the live-bed and clear-water contraction scour equations are the best that are available and should be regarded as a first level of analysis. If more detailed analysis is warranted, a sediment transport model should be used.

A sediment transport model, such as the USACE HEC-6 could be used to compute ultimate contraction scour conditions for a constant flow rate as long as a sufficient simulation duration was used. It could also be used for unsteady conditions and/or for shorter durations. Similarly, sediment transport relationships could be used directly to make predictions of ultimate scour and, since the sediment transport equation produces a rate of sediment transport, also the rate of contraction scour.

Generally, sediment transport modeling is beyond the scope of most scour studies. However, a method could be developed using the same data required by the standard HEC-18 contraction scour equations and a suitable sediment transport equation within a spreadsheet application. The required data would be channel width, discharge and average flow depth within the bridge opening and at an approach section, median bed material size, fall velocity and an estimate of Manning n. Two other parameters that would need to be estimated are the scour hole entrance and exit slopes and sediment void space or porosity. The steeper the upstream and downstream scour hole slopes, the faster that scour will occur because a smaller volume of material is eroded. A 1V:1H could be assumed for upstream and downstream slopes for conservative results. The volume of erosion is greater than the computed sediment transport rate by a factor or 1/(1-h), where h is the void ratio. A value for h of 0.4 (40 percent void space) is reasonable for sand.

The other variable that must be input is the duration used for computation. The most extreme hydraulic condition is generally used to compute scour, even though this condition is brief. Figure 5.1 shows typical storm surge hydraulics. The most extreme condition occurs during the flood tide. The entire flood tide lasts for less than five hours. By applying the peak condition for half the duration of the flood tide, the total flow through the bridge would be approximately maintained. Because scour conditions do not occur during the entire flow, this approach is also conservative (i.e., using the most extreme hydraulic condition with half of the flow duration). For the conditions of Figure 5.1 a duration of three hours would be appropriate.

Figure 5.2 shows the results from a time dependent scour analysis using the approach described above. It shows the scour development through the time required to reach ultimate conditions. It also shows the ultimate scour estimates from HEC-18 (Laursen) and a sediment transport function, and the intermediate value of scour for a 3-hour duration. No specific time is associated with the HEC-18 result as it is for "ultimate" conditions.

Figure 5.1. Typical surge hydraulic conditions.

Figure 5.2. Time dependent scour results.

Figure 5.2 shows that 5.1 feet of contraction scour can occur in 3 hours and that it would require approximately 400 hours to reach ultimate contraction scour conditions. The sediment transport function predicts 13.3 feet of ultimate scour compared with 12.9 feet using the HEC-18 equation. Contraction scour for live-bed conditions is generally less extreme than equivalent clear-water conditions. However, live-bed scour reaches ultimate conditions in less time than equivalent clear-water conditions. For relatively small amounts of live-bed scour, three hours can be sufficient to reach the ultimate scour.

This approach of applying sediment transport calculations can result in a prediction of considerably less scour than the HEC-18 equation in some situations. By using the peak hydraulic conditions and steep upstream and downstream scour hole slopes, the method should produce conservative results. This level of conservatism is warranted due to the rapidly varied flow in a bridge constriction. Based on a surge hydrograph or flashy upland stream flow hydrograph, the engineer should select a reasonable time interval for the scour prediction. For hurricane storm surges, the time interval would typically range from two to five hours. It is recommended that the discharge and velocity hydrographs be reviewed to establish a reasonable time interval to apply the peak hydraulic conditions.

5.3 Time Dependent Local Scour

Several methods exist for predicting rates of pier and abutment scour. Gosselin and Sheppard (1998) concluded that more research is needed before meaningful relationships can be developed for time dependent local sour. This is because most of the research has been conducted on clear-water conditions (approach velocity less than the critical velocity for sediment transport) and at small laboratory versus prototype scales. It is generally accepted that local scour in live-bed conditions occurs much more rapidly than for clear-water conditions. As this area of research evolves there may be benefits to computing time dependent local scour amounts. One additional complication is that the time dependent local scour amounts would have to be added to ultimate local scour amounts produced by daily tides.

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