Catalog of worldwide tidal bore occurrences and characteristics

By SUSAN BARTSCH-WINKLER and DAVID K. LYNCH

U.S. GEOLOGICAL SURVEY CIRCULAR 1022 DEPARTMENT OF THE INTERIOR DONALD PAUL HODEL, Secretary

Dallas L. Peck, Director

Free on application to the Books and Open-File Reports Section, U.S. Geological Survey, Federal Center, Box 25425, Denver, CO 80225

CONTENTS

Abstract Introduction Field methods and data collection Survey and literature search Acknowledgments Catalog of tidal bores Tidal bore characteristics Observations and experiments in Turnagain Arm, Alaska Conclusions References cited

1. Photograph of tidal bore, Turnagain Arm, Alaska

2-4. Maps showing:

5-6. Photographs showing:

1. Responses to questionnaires inquiring about existence of tidal bores
2. Known tidal bore locations in the world and their characteristics
3. Bore-producing regions classified according to tidal type
4. Distribution of tidal types
5. Distribution of tidal range
6. Distribution of tidal bores
7. Undular bore and whelps near the mouth of Araguari River, Brazil
8. Complex wavefront of the Turnagain Arm, Alaska, bore

INTRODUCTION

A tidal bore is a tidally generated wave whose amplitude, in some regions, may exceed 6 m (Tricker, 1965; Rowbotham, 1964; Lynch, 1982). Tidal bores occur both in estuarine and freshwater environments. A bore is a solitary wave that typically propagates up a slowly flowing estuary with the incoming tide (fig. 1). Those that ascend the estuary a greater distance than the width of the estuary are contained in this listing, with the exception of the Araguari River bore which forms at sea. Exposed mudflats and broad, beach-like washes which experience a bore-like incoming tide, such as occurs at Mont St. Michel in France, are not included.

Only a few bores have been described in detail (Champion and Corkan, 1936; Waters, 1947; Dalton, 1951; Destriau, 1951; Barnes, 1952; Chitale, 1954; Abbott, 1956; Rowbotham, 1964; Tricker, 1965; Roy, 1972; Jouanneau and Latouche, 1981). Early explorers made accounts of bores that are now primarily of historical interest (Martius, ca. 1837; Moore, 1888, 1893; Branner, 1884; Beaver, 1914). Sykes (1937, 1945) studied the now-rare bore on the Colorado River in Mexico. Many bores are mentioned incidentally in scientific papers and unpublished field reports which discuss other subjects (for example, Maxwell, 1968; Komori, 1979; Amos and Long, 1980; Bartsch-Winkler, 1982; Murphy, 1983; Bartsch-Winkler and Ovenshine, 1984).

Worldwide tidal bore localities document the characteristics of known tidal bores, existing now and in the past, and facilitate locating additional areas where undocumented bores might exist. Unreported bores undoubtedly occur in inaccessible and unpopulated regions and have never been witnessed by scientists;

FIELD METHODS AND DATA COLLECTION

sonian Institution, and the Library of Congress. In addition, a request was made for information on tidal bores from attending scientists at the XXI International Congress on Sedimentology, held in Hamilton, Ontario, Canada (Lynch and Bartsch-Winkler, 1982). Certain institutions thought to have such knowledge were contacted for information on specific tidal bores.

ACKNOWLEDGMENTS

This survey could not have been accomplished without the cooperation of the many respondents to the questionnaire and that of colleagues with whom we conferred during the data-gathering phase of the study. We are indebted to The Cousteau Society for providing valuable photographs and films of the Araguari River bore. We especially wish to thank those whose extra effort contributed much to the catalog: J. Baldwin (U.S.A.), John Boon (England), G. Boss (Canada), C. Brossard (France), J. Byrne (U.S.A.), P.N. Cornish (Ireland), C. Desplanque (Canada), B.W. Flemming (South Africa), Stephen G. Gassaway (U.S.A.), A.M. Haigh (England), R.M. Hillman (Australia), V. Josanto (India), C.G. Kershaw (England), Lung-fa Ku (Canada), J.A. Lawrie (Australia), D. Luo (China), Y. Mailvaganam (Malaysia), John McManus (Scotland), Afranio Mesquita (Brazil), A. Mitchell (Australia), Richard Murphy (U.S.A.), P.P. Periera (Brazil), G.S. Quraishee (Pakistan), Yan Quinshang (China), K.S. Richards (England), J. Richardson (England), J. Rottman (England), Ir. A. Shahrizaila (Malaysia), P. Sheehan (Australia), J. Simpson (England), M.A. Sweeney (England), R.B. Thorne (England), S. Tovey (Australia), R.A.R. Tricker (England), P. Valls (France), J. Vasdev (India), H.J. Walker (U.S.A.), G. White (Australia), Michael Woodward (Canada), Shao Xusheng (China), Brian Zaitlin (Canada), D. Zeheng (China), and H. Zengcui (China). We thank J. Simpson and J. Rottman for valuable theoretical discussions on tidal bores, and T .D. Hamilton, J.S. Kelley, and H.S. Schmoll who carefully reviewed the manuscript.

CATALOG OF TIDAL BORES

Figures 2 and 3, respectively, show the worldwide types and ranges of tides (Davies, 1977). Table 2lists the tidal data collected in the survey arranged alphabetically by river name in each country; figure 4 locates each occurrence on the world map. Tidal information in table 2 includes data on the major bodies of water into which the estuaries flow.

Approximately half the world's coasts have mixed diurnal and semidiurnal tides, or diurnal tides; the rest have semidiurnal tides. Because semidiurnal tides that occur nearly every 12 hours rise approximately twice as fast for the same tide range as diurnal tides that occur nearly every 24 hours, bores are restricted to regions with semidiurnal or nearly semidiurnal tides with ranges in excess of 4 m (figs. 2, 3, 4). Nearly all tidal bores occur in regions of high amplitude (greater than 4 m) semidiurnal tides (table 3). Thus, tidal bores form in regions where the tide influx is rapid. In addition, the fluvial discharge must be relatively slow moving. A requirement for the formation of the hydraulic jump (a sudden change in water height) represented by the bore is that fluvial discharge in the estuary must flow more slowly than the shallow water wave velocity (Tricker, 1965; Lynch, 1982). Therefore, bores typically form on gently sloping riverbeds commonly identified as meandering and having large deltas. Typically, bores occur in settings where the estuary crosses broad lowland regions along the coast, but this characteristic is not limiting. One exception to this is the bore or bores that occur in Turnagain Arm of Upper Cook Inlet, Alaska. Turnagain Arm estuary is surrounded by glaciated peaks of the Chugach Range which exceed 1,200 m within 2 km of tidewater, but it is also a Late Holocene fiord that has been infilled with unconsolidated intertidal sediment. Bore-bearing riverbeds are typically composed of unconsolidated clay, silt, or sand that is easily transported and deposited contemporaneously in relatively broad intertidal zones.

Most bores occur within about 100 km of the estuary mouth even though the tidal effects are evident much farther inland. The deepest inland penetration of bores is apparently in the Amazon Basin where bores on the Capim, Guajara, and Moju Rivers, which flow into the Amazon, occur more than 150 km inland. Due to the large size and great width of the Para River, into which the Capin, Guama, and Moju Rivers flow, and the Amazon River, into which the Guajara River flows, bores may form 100 km inland. Conversely, due to the vast Araguari River delta built into the Atlantic Ocean, an undular bore occurs as much as 10 km offshore from the river mouth (fig. 5).

Tidal bores apparently form at all latitudes, although reports in areas from about lat 60° to 90° N. and 60° to 90° S. are missing. The most northerly region reported in this survey is in upper Cook Inlet, Alaska, at lat 61° N. However, bores are suspected to occur in the Baffin Island region, Canada, at lat 65° N.

This survey is intended to be ·a general guide to tidal bore occurrence in the world and not an all-inclusive or final documentation. Undoubtedly more tidal bore locations exist than are listed here, but either they occur in remote settings from which no information is available, they are ephemeral, or they are insignificant and go unnoticed by commercial interests. Our information, in some cases, was limited to older or rare published Angola Argentina Australia Bangladesh Belgium Benin Brazil Burma Cameroon Canada Chile China, People's Rep. Colombia Congo Costa Rica Cuba Denmark Djibouti Dominican Republic Ecuador Egypt El Salvador England Ethiopia Finland France French Guiana Gabon Gambia Germany, Fed. Rep. Germany, Dem. Rep. Ghana Greenland Guadeloupe Guatemala Guinea Guinea-Bissau

papers, which may be inaccurate today because of changes in the estuary concerned. Much of the data comes from unpublished government documents, obscure scientific publications, popular articles, and reports from knowledgeable lay persons and scientists who took an interest in, and sporadically observed, particular bores. Some tidal bores are known locally by special names unfamiliar to scientists ("pororoca" is the name of the bore on the Amazon River, and "eagre" is the name given to bores in England). In some cases, only one bore in a series of bores that occur along many waterways in a region is well known and publicized, and the others receive no recognition. For example, in Canada the Petitcodiac bore is well reported, though bores on the Maccan, Shubenacadie, Hebert, and Salmon Rivers also occur (all draining to Bay of Fundy; table 2). (-)

(-) Guyana Haiti Honduras Hong Kong Iceland India Indonesia Iran Iraq Ireland Israel Ivory Coast Jamaica Japan Kenya Korea, North Korea, South Kuwait Liberia Madagascar Malaysia Martinique Mauritania Mauritius Mexico Morocco Mozambique Netherlands New Caledonia New Guinea New Zealand Nicaragua Nigeria Norway Oman Several countries consider information on tidal bores as classified, limiting knowledge, in these cases, to older publications. It is also possible that some bores (for example, the Orinoco) do not exist at all, the reports being erroneous or misinterpretations.

The best data on tidal bores result from detailed studies performed as part of a systematic engineering effort to eliminate them. Bores occasionally cause damage to river commerce and, as a result, have become targets for control (Komori, 1979; Zeheng, 1982). Also, tidal bores may have once occurred in rivers that have been dammed for irrigation or flood control, and have, thus, been eliminated. The large bores on the Seine (France), Colorado (Mexico), and Qiantang Jiang (China) have decreased to relative insignificance or have been eliminated by engineering and hydrologic projects. (-) (-)

(-) Pakistan Panama Peru Philippines Poland Portugal Qatar Reunion Saudi Arabia Scotland Senegal Sierra Leone Solomon Islands Somalia South Africa Southwest Africa Spain Sri Lanka Sudan Surinam Sweden Taiwan Tanzania Thailand Togo Tonga Trinidad/Tobago

u.s.A.

U.S.S.R. Uruguay Vanuatu Venezuela Viet Nam Western Samoa Yemen Zaire (-)

... ·•>~.:~ 2·~~1JA:.pYJJllijl~ 1 5?~ 1~ 11~00 2~00 MILEs ,'\ijjQUIJJ!lillJJ~ . /

Figure 2. Worldwide distribution of tidal types (modified from Davies, 1980, fig. 32, p. 49).

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a:~ a: :t

m ---

m ---

m --- m --- --- ---

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Figure 4. Worldwide distribution of tidal bores. Numbers are keyed to locations in table 2. A-G show regions where additional or undocumented bores are likely to occur.

1. u.s.A.------

The data display a large variation in tidal bore characteristics, in part due to the diverse sources and to the various estuarine settings. In some cases, estuary mouths are not well defined and tidal stations are at a distance from them, resulting in only approximate tidal information. Properties of tidal bores may change in any given location from one year, season, month, or week, to the next. Certain tidal bores may form only during specific times of the lunar monthly cycle (for example, during spring tides, when tide range is the highest). Deposition and erosion by the river may result in a change in channel configuration. River outflow may increase at certain times of the year, changing the ratio of outflow to inflow. Thus, tidal bore occurrence is unpredictable, in some cases, because the formation of the wave depends on such variable properties as tide range, amount and velocity of runoff and inflow, and channel depth and configuration.

TIDAL BORE CHARACTERISTICS

Tidal rhythms in shallow estuaries are asymmetric in time due to deceleration of flood current speeds caused by bottom friction and the adverse outflow current; that is, the length of time required for the flood cycle is less than the time required for the ebb cycle. At points further up the estuary this asymmetry becomes progressively pronounced; flood cycle duration becomes shorter and ebb cycle duration grows longer. The result of this fact is that the water rises faster than it falls, inferring faster flood current speed. In extreme cases, the first flood stage (at low tide) occurs as a bore. Bores typically occur in series, moving simultaneously up closely spaced river systems or intertidal channels within an estuary with the incoming tide. They also may occur in succession within a tidal channel, marking tidal pulses or plateaus following the initial pulse, each separated by a bore.

Initially, the bore may be an impulsive influx of brackish or fresh water depending on the accumulation of fresh water at the estuary mouth prior to the turn of the tide, but rapid increase in salinity occurs at any given location shortly after the bore passes (for example, Roy, 1972; Bartsch-Winkler and others, 1985). The bore also causes an abrupt increase in turbidity, temperature, surface structure, and bottom pressure at any given location, and, of course, flow direction is reversed after passage of the bore. The kinetic energy of bores is partially dissipated within the channel and at the shoreline, causing rapid erosion and consequent increase in suspended load.

A tidal bore, a hydraulic jump, forms in response to the increased shallowing in the estuary upstream from the mouth, producing disequilibrium in the opposing energy levels of the incoming tide and the river outflow. In high tide range areas with the proper characteristics, a bore is the leading edge of the incoming tide and, at various locations in the estuary, the bore may be either turbulent (breaking, fig. 1) or undular (nonbreaking, fig. 5). The form of the bore depends on the ratio of the water depth on either side of the bore (downstream or oceanward depth/upstream or landward depth). If the ratio is small or close to unity, the form is nonbreaking; as it nears a ratio of 1.4, the form becomes breaking (Tricker, 1965; Lynch, 1982). Most bores change form and decrease in height as they move upstream into shallower water due to increase in friction and due to change in the bottom configuration of the estuary.

Numerous smaller waves (whelps) may form immediately behind a bore (fig. 5). Where the bottom configuration and river depth remain relatively constant, whelps increase in number and propagate in time downstream (oceanward) relative to the leading edge (Favre, 1935; Benjamin and Lighthill, 1954). Whelps, because they have less height, do not propagate as rapidly as the larger leading wave and are left behind to eventually dissipate.

The surface transverse (shore-to-shore) profile of a bore depends on the depth profile of the channel and the flow velocity in the river, with those portions of the bore in deeper water propagating faster than those in shallower water (fig. 6). Where the bottom configuration changes (shallows) upstream, a secondary wave with greater height may form behind the leading wave (because it is in relatively deeper water). This secondarily formed wave may temporarily propagate faster than the leading wave. Also, since the bore propagates perpendicular to the leading edge, the near-shore parts

OBSERVATIONS AND EXPERIMENTS IN TURNAGAIN ARM, ALASKA

In addition to a change in direction and a rapid rise in surface level of the water in the tidal channels after passage of the bore, the water also showed a marked increase in suspended sediment. The turbulent wave scours into the channel bottoms, and is a major force in sediment redistribution. The saline wedge, a feature typical of most estuaries caused by freshwater flow atop saltwater, is diffuse in this dynamic hydrologic system. Near the mouth of Turnagain Arm, salinity increased markedly after the change in tide, but near the head of the arm, the salinity patterns were more complex (Bartsch-Winkler and others, 1985). At the head, the arrival of saline water occurred on the flood-dominant south side as much as 30 min after arrival of the bore. Although most streams in the area are glacially derived and frigid, on some days shallowing outflow water had a temperature that was higher due to solar heating than incoming tidewater temperature. Thus, though the freshwater temperature varied with the weather, the temperature of the deeper marine water was more constant. Upon passage of the bore, surface water temperature typically changed by several degrees.

CONCLUSIONS

Settings in which tidal bores occur are generally meandering river systems having gentle gradients, where discharge is relatively slow moving with respect to the tidal flow. The mouths of the rivers typically are large deltas. Tidal bores generally occur within the estuary less than 100 km from the mouth, although the Araguari bore forms offshore. Formation of tidal bores is dependent on the rapid rate in the rise of tide level, so they occur in regions with high tides where the range exceeds 4 m and the tides are semidiurnal or nearly semidiurnal. Tidal bores in this catalog propagate up the tidal estuary a greater distance than the width of the estuary. In some places in the world, tidal bores have been purposely eradicated because they have caused havoc in port areas.

Tidal bores are solitary, tidally generated, naturally occurring, moving waves that range from 0.2 to 6.0 min height. They have a greater amplitude than wind- or turbulence-caused waves. The wave is undular if the ratio of downstream to upstream depth is less than about 1.4; greater than that, the wave is breaking. The transverse profile of a tidal bore changes with the depth configuration of the channel up which it moves and with the depth and velocity of incoming tidewater and river outflow. Subtle variations due to the effects of refraction and reflection along the shore also may take place. Bores may form as initial waves of the flood tide, and may occur in several channels simultaneously, or as successive waves that identify tidal pulses or plateaus. The speed of a bore is faster in deeper water than in shallower water. Refraction and reflection of the wave at the channel edges transfer energy from deeper to shallower water. Whelps, slower moving, undular waves, may form behind and follow the bore.

Tidal bores occur or have occurred throughout the world in at least 67 locations in 16 countries. Areas (see fig. 4) favorable for tidal bore occurrence but where no occurrences are documented, or areas where there may be additional occurrences to the ones reported, include: (A) Argentina from Montevideo to Tierra del Fuego, (B) northern Canada in the region of lower Baffin Island and upper Hudson Bay, (C) Central America along the Pacific Coast from Guatemala to Colombia, (D) southeastern Africa and western Madagascar, (E) western Iceland, the United Kingdom, and Northern Europe, (F) northeastern U.S.S.R. in the Sea of Okhotsk west of Kamchatka Peninsula, and (G) North and South Korea in Korea Bay and the Yellow Sea. Such areas are suspect to have bores because they have high tide ranges and semidiurnal tidal characteristics. If bores are never reported from these areas, it will probably be due to the presence of either swiftly moving rivers with steep gradients or ephemeral river systems, both of which generally are associated with mountainous coastlines.

In Turnagain Arm, Alaska, bores occur daily with each incoming tide. They cause an increase in salinity, suspended sediment, surface character, and bottom pressure, a decrease in water illumination due to turbidity, and a change in temperature. Studies of bores show that their behavior corresponds to that modeled for turbulence and diffusion in naturally occurring waves, as has been hypothesized by others.

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