Richard L. Watson, Ph.D.
1/13/97

CRITICAL ERRORS IN
INLET STABILITY
CALCULATIONS in

Packery Channel Feasibility Study:
Inlet Functional Design and Sand-Management
Report 1 of a two-part series
Draft final report
by
Nicholas C. Kraus, Daniel J. Heilman
December 10, 1996




SUMMARY STATEMENT

An error in division by the authors of the feasibility study totally invalidates the report’s conclusion on inlet stability. The Bruun parameter (P/Mt) 65,160,000 divided by 5,400,000 is 12, not 120 as calculated by the authors. This forces the conclusion that Packery Channel will be unstable and unsafe for navigation ("there will be formation of a wide and high bar, and navigation becomes difficult to very difficult").

INTRODUCTION

The feasibility study uses several computations to estimate inlet stability, or the ability of the tidal flow through the inlet to keep it open. The authors choose to use inlet stability computations by Per Bruun. Basically, these stability calculations compare the flow velocities or the volume of water flowing through the inlet due to tides and other forces to the volume of sediment which must be removed from the inlet to keep it open. If the tidal flow velocities and quantities are high relative to the amount of sediment supplied by the longshore sediment transport system in the surf, then the inlet will stay open. If the flow velocities and volume are low, the inlet will fill with sand bars, be navigationally extremely hazardous and probably close. The methods of Bruun have been widely accepted by coastal engineers and others since the early 1960’s. Bruun’s methods have been incorrectly applied in the feasibility study.


MAXIMUM FLOW SPEED

Kraus and Heilman state: "A rule of thumb, as described by Bruun (1990, 1991) and others, states that the maximum flow speed in an inlet entrance should be about 3 ft./sec for promoting if not assuring inlet stability. As described in the previous section and by numerical simulations in Report 2 (Brown and Militello, 1996) the depth-averaged flow speed at the entrance will rarely exceed 2.5 ft/sec. Because the littoral drift rate is relatively small on the Gulf Coast as compared to the Atlantic Ocean and Pacific Ocean coasts, a weaker entrance current may be capable of maintaining an entrance" (Kraus and Heilman, p. 81).

(Note by RLW. In fact, the one dimensional model showed peak flow velocities to be below 1 ft/sec for all conditions. Report 2 indicates that the one dimensional model was used to investigate currents in the entrance channel and along the length of Packery Channel. "The one-dimensional model was applied to investigate the currents in the entrance channel and along the length of Packery Channel" (Brown and Militello, p. 6). "During neap tide, the calculated currents have ebb velocities under 0.5 ft/sec (15 cm/sec) and over some intervals the current only flowed in the flood direction. During the spring portion of the tidal cycle, peak ebb velocity ranges from about 0.5 ft/sec to 0.8 ft/sec" (p. 30-31, Brown and Militello, Report 2). "The two dimensional model was applied to quantify changes in circulation and water level in the adjacent bay region as well as currents and discharge through Packery Channel" (p. 6, Report 2). "The 2D model is expected to over predict the currents at the Gulf entrance, because the model does not include transition losses at the entrance associated with turbulence "(Brown and Militello, report 2, p. 61.)

Even the calculated current speed through Packery Channel entrance for Hurricane Dolly was less than 1 ft/sec (Fig. 20., Brown and Militello report 2). In spite of the fact that the bulk of the evidence in Report 2 shows that the maximum flow velocities in the entrance will be less than 1 ft/sec, the Dr. Kraus reports that the flow speed in the entrance will rarely exceed 2.5 ft/ sec. This is true, but misleading, because the data shows that it will rarely exceed 1 ft/sec! This is like stating that senior citizens are over 21. The report should state that the flow velocity, and in particular the ebb flow velocity will rarely exceed 1 ft/sec. This is far below the 3 ft/sec needed for inlet stability according to Bruun. This is lower than the 1.5 ft/sec velocities in the Fish Pass (Corpus Christi Water Exchange Pass) (Watson and Behrens, 1976, Behrens, Watson, and Mason, 1977). "The current velocity in the Fish Pass was measured by Behrens, et al. (1977) and Watson and Behrens (1976), with a representative value of 1.5 ft/sec." (Kraus and Heilman, p. 78).

Note that with the assumed gross longshore sediment transport of 200,000 cubic yards and the low flow velocities in the pass (lower than in the Fish Pass) , the authors conclude that the inlet MAY be able to maintain an entrance. If the higher rates of gross longshore sediment transport reported throughout the scientific and engineering literature are correct, the inlet will have little chance of maintaining a navigable entrance. Correct use of the flow velocities computed in the model studies of Brown and Militello, (Report 2 of the feasibility study), show that the inlet will have little chance of success even with the too low gross longshore sediment transport calculated by Kraus and Heilman.


TIDAL PRISM VERSUS SEDIMENT TO BE FLUSHED

This inlet stability computation compares the tidal prism, or the quantity of water that flows in and out of the inlet on an average tidal cycle to the quantity of sand that must be flushed from the inlet in a year. P is the tidal prism (the quantity of water flowing in a tidal cycle) and Mt is the volume of sand that must be removed from the inlet annually. This stability measure divides the tidal prism by the amount of sediment to be flushed out of the inlet in one year (P/Mt). Note that if the tidal prism (P) is large (a lot of water flowing to flush sediment) and the sediment to be flushed (Mt) is small, then the division will result in a large number. If the tidal prism is small, or the amount of sediment that must be removed is large, then the calculation will result in a small number. Big numbers mean that the inlet will be successful and small numbers mean that the inlet will be a dangerous failure.

The authors apply this inlet stability computation as follows: "Bruun (1991) discusses inlet stability with focus on a parameter P/Mt introduced by himself and coworkers in the 1960s, where Mt is the total amount of material carried to the entrance in a year. In the present situation, Mt includes longshore transport and wind-blown sand transport. The P/Mt parameter describes a balance between deposition by sediment and the transporting capacity of the inlet, represented by P. (Note by RLW, P is the tidal prism, the volume of water that flows in and out of the channel in a tidal cycle) According to Bruun (1991), for P/Mt>300, there will be little sediment shoaling or bar seaward of the entrance; for 150<P/Mt<300, there will be limited shoaling and a minor bar; for 100<P/Mt<150 there will be a small entrance bar and only minor navigation problems; for 100<P/Mt<50 (should be 50<P/Mt<100, RLW) there will be a wider and higher bar and increasing navigation problems, and for P/Mt<50, there will be formation of a wide and high bar, and navigation becomes difficult to very difficult (Emphasis by RLW). In the present situation, assuming Mt = 200,000 cu yd = 5,400,000 cu ft, we have P/Mt = 120, which indicates a bar would tend to form and navigation problems would be minor, suggesting that corrective measures should be taken for assuring safe navigation, as discussed in the next section" (Kraus and Heilman, p. 81).

Note that a few pages earlier, P, the tidal prism, was presented as 65,160,000 cu ft (Kraus and Heilman, p. 79).

Note that 65,160,000 divided by 5,400,000 is 12 and is not 120!!! P/Mt is less than 50. P/Mt is 12! This indicates that there will be formation of a wide and high bar and navigation becomes difficult to very difficult. Bruun (1991, p. 848) actually has an additional class for P/Mt <20, (very shallow ocean bar, navigation very difficult.) Note further that if the gross sediment transport is much greater than 200,000 cubic yards per year as most of the literature indicates, and that the surf zone is wider such that the jetty ends will be exposed to surf many times per year as ordinary observation indicates, then the result will be a number much smaller than 12 and the prognosis for inlet stability and navigation will be even worse. The fact that Packery Channel, like the Fish Pass, will be flood dominated; that is, more water flows in than out, will further enhance development of a bar in the entrance and rapid shoaling.


CONCLUSIONS

The authors of the feasibility study misplaced a decimal point. Their own numbers and chosen estimate for inlet stability indicate that the inlet will be both unstable and very dangerous! Use of published data which indicates a more vigorous wave regime than presented in the feasibility study and higher rates of sediment transport indicates an even less viable situation. Packery Channel, as designed with 1400 ft jetties, will have no chance of success, even with the too low rates of sand transport used by the feasibility study authors. It is very unlikely that a navigable channel can be maintained without dredging at least two times per year and perhaps more often than that. There are no navigable inlets on the Texas Coast with jetties less than 2300 ft. and those are only marginally successful.

REFERENCES

Behrens, E.W., Watson, R.L., and Mason, C., 1977. Hydraulics and Dynamics of New Corpus Christi Pass, Texas: A Case History, 1972-73. GITI Report 8, U.S. Army Coastal Engineering Research Center, 126p.

Behrens, E.W., and R.L. Watson, 1977. Corpus Christi water exchange pass 1972 to 1976, in: Coastal Sediments '77, 5th Symposium of the Waterway, Port, Coastal and Ocean Division, Amer. Soc. Civ. Engrs., 790-796.

Brown, Cheyrl A., and Militello, Adele, 1996. Packery Channel Feasibility Study: Bay Circulation and Water Level, Report 2 of a Two-Part Series, Draft Final Report, Conrad Blucher Institute for Surveying and Science, Texas A&M University-Corpus Christi, Technical Report TAMU-CC-CBI-96-07.

Bruun, P., 1990. Tidal Inlets on Alluvial Shores. In Port Engineering, Fourth Edition, Volume 2, Harbor Transportation, Fishing Ports, Sediment Transport, Geomorphology, Inlets, and Dredging. Gulf Publishing Company, Houston, Texas.

Bruun, P., 1991. Coastal Inlets. In Herbich, J.B., editor, Handbook of Coastal and Ocean Engineering, Volume 2, Offshore Structures, Marine Foundations, Sediment Processes, and Modeling. Gulf Publishing Company, Houston, Texas.

Kraus, N. C. and Heilman, D.J., 1996. Packery Channel Feasibility Study: Inlet Functional Design and Sand-Management, Report 1 of a Two-Part Series, Draft Final Report, Conrad Blucher Institute for Surveying and Science, Texas A&M University-Corpus Christi, Technical Report TAMU-CC-CBI-96-06.

Watson, R.L., 1975. The littoral drift system of the Texas coast, Final report on contract research with the Corpus Christi Office of Marine Geology, U.S.G.S.

Watson, R.L., 1975. The relationship between littoral drift rate and the longshore component of wave energy flux, Ph.D. dissertation, The University of Texas at Austin, 106p.

Watson, Richard L., 1975. The Longshore Transport System of the Texas Coast. Report Prepared for the Corpus Christi Office of Marine Geology of the U.S. Geological Survey, 29p. unpub.

Watson, R.L., and Behrens, E.W., 1976. Hydraulics and Dynamics of New Corpus Christi Pass, Texas: A Case History, 1973-75. GITI Report 9, U.S. Army Coastal Engineering Research Center. 175p.