4. Design Criteria for Basins and Dockage Structures. 



a. Water Area Perimeter Stabilization. Within the protected basins of a marina, or along 

 a lake or river shore in front of which boats are to be berthed, some type of bank 

 stabilization is usually desirable. Because the shore areas in these locations are not exposed 

 to large waves or strong currents, the degree of stabilization required against external forces 

 is usually minimal. Many marinas have no bank protection other than that provided by the 

 natural ground cover of the region. Some banks present a rather pleasing appearance if 

 properly cared for; others detract from the esthetics of an otherwise orderly appearing 

 installation. 



Armoring is usually required unless a natural bank already is a sand or shingle beach with 

 a flattened slope and adjusted to the action of water exposure. Protection may be needed 

 against scour by boat-wake waves (Sorensen, 1973), impact by floating debris or boats out 

 of control, or gouging by ice. Moreover, the amplitude of periodic water surface fluctuations 

 may make natural ground cover impractical. The most common slope armorings for marinas 

 are riprap, precast concrete block units (usually overlapping or interlocking as shown in 

 Fig. 33), and concrete slope paving. While availability of materials and economic 

 considerations usually dictate the selection, internal forces due to active soil pressures (often 

 intensified by surcharge loading at the top of the bank) must also be considered. These 

 forces usually determine the steepness upon which the slope will safely stand. 



Factors in determining the slope to which a bank should be trimmed are: (a) grain size 

 for a beach or an armored slope in uncohesive material, (b) soil tests for cohesive soils, 

 (c) anticipated dry -wet cycles that may affect the shde plane characteristics, such as water 

 level fluctuations and saturation by landscape watering, and (d) surcharge loading at the top 

 of the bank. A sand beach will usually be stable with a 1 on 8 slope in the tide and 

 wave-runup zone and a gravel or shingle beach with a 1 on 6 slope. Finer uncohesive soils 

 such as silty sands should normally be armored in the tide and wave -runup zone and tend to 

 vary in allowable slope steepness above the wave-runup zone from about 1 on 3 to 1 on 2 

 depending on grading characteristics and median diameter of soil particles. A poorly sorted 

 soil (with a steep grading curve) and small median diameter will require a flatter slope than a 

 well-graded soil (with a flat grading curve) and large median diameter. Cohesive soils such as 

 loams and clays will take steeper slopes but may be subject to plastic distortion and sliding 

 when saturated, especially during extreme low tides and under high surcharge loading. 

 Unless prior experience has already indicated the safe slope Umit for the particular soil 

 formation of the site, a soil mechanics engineer should be employed to make the slope limit 

 determination. Below extreme low water where scouring currents (over 3 feet per second) 

 are not present and where wave action is minimal, most soil will be stable on a slope c 

 about 1 on 3 to 1 on 4. 



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