PROCEDURES FOR PRELIMINARY ANALYSIS OF 

 TIDAL INLET HYDRAULICS AND STABILITY 



Robert M. Sorensen 



I . INTRODUCTION 



Preliminary design of proposed modifications (e.g., installation of 

 jetties) at an existing tidal inlet or of a new inlet that is to connect 

 an inland bay with the sea will require analysis of the hydraulic charac- 

 teristics of the sea-inlet-bay system and determination of the probable 

 stable dimensions of the inlet channel cross section. Although Section 

 5.73 of the Shore Protection Manual (SPM) (U.S. Army, Corps of Engineers, 

 Coastal Engineering Research Center, 1975) •'■ discusses the various factors 

 involved in inlet design, it does not provide guidance or specific tech- 

 niques for conducting these analyses. This report presents methods for 

 calculating the time-dependent average cross-sectional velocity in an 

 inlet channel, the bay tidal level range, and the phase lag between sea 

 and bay tides, as well as the expected stable channel cross-sectional 

 area. Required input data for these calculations include the ocean tidal 

 period and amplitude, the inlet channel length and hydraulic resistance, 

 and the bay surface area. An example is presented to demonstrate these 

 calculations for a hypothetical sea-inlet-bay system. 



II. DEFINITION OF TERMS 



Figure 1 shows an idealized sea-inlet-bay system. The jettied inlet 

 channel has a length, L, width, B, average depth, d, cross-sectional 

 area, k^, below mean sea level (MSL) , and instantaneous average veloc- 

 ity, V. Flow in the system is generated by a sea tide having a period, 

 T, and amplitude, ag , and results in a bay level response having the 

 same period and an amplitude, a^j. The time of high water in the bay 

 lags the sea high water by a phase lag, e, usually given in degrees. 

 kj-i is the bay surface area and ZAj^aj^, the volume of water that flows 

 into and then out of the bay on a tidal cycle, is commonly known as the 

 tidal prism, P. Parameters needed to define the inlet channel hydraulics 

 include entrance- and exit-loss coefficients, kgy^ and kg^, a resist- 

 ance coefficient, f (Darcy-Weisbach) or n (Manning), and the hydrau- 

 lic radius, R, which equals the cross-sectional area divided by the 

 wetted perimeter. The acceleration of gravity is g. 



III. TIDAL INLET HYDRAULICS 



King (1974)^ solved the basic equations of motion and continuity for 

 an inlet-bay system (Fig. 1). He presented curves for the dimensionless 



U.S. ARMY, CORPS OF ENGINEERS, COASTAL ENGINEERING RESEARCH CENTER, 



Shore Protection Manual , 2d ed.. Vols. I, II, and III, Stock No. 008-022- 

 00077-1, U.S. Government Printing Office, Washington, D.C., 1975, 

 1,160 pp. 



^KING, D.B., "The Dynamics of Inlets and Bays," Technical Report No. 22, 

 Coastal and Oceanographic Engineering Laboratory, University of Florida, 

 Gainesville, Fla. , Mar. 1974. 



