. The Earth beneath the sea : History. Ocean bottom; Marine geophysics. 532 INMAN AND BAGNOLD [chap. 21 Permeability also affects the slope of a swash beach of coarse material in another way. A loss of swash energy occurs over the iqiper part of the beach by a mass loss of water l)y downward gravity percolation. As a consequence the vahie of c in relation (18) decreases progressively towards the beach crest where it may actually become zero, there being here no return flow at all over the beach surface. Hence, on model pebble beaches the angle /S increases pro- gressively upwards, and may appr
. The Earth beneath the sea : History. Ocean bottom; Marine geophysics. 532 INMAN AND BAGNOLD [chap. 21 Permeability also affects the slope of a swash beach of coarse material in another way. A loss of swash energy occurs over the iqiper part of the beach by a mass loss of water l)y downward gravity percolation. As a consequence the vahie of c in relation (18) decreases progressively towards the beach crest where it may actually become zero, there being here no return flow at all over the beach surface. Hence, on model pebble beaches the angle /S increases pro- gressively upwards, and may approach the angle 0 of repose, just as it is found to do in nature. It has been shown (Bagnold, 1940) that the increase in beach slope towards the beach crest can be prevented in the model by the insertion of an impermeable plate just below the beach surface. Fig. 7a shows the profile of a typical model beach of small pebbles (diameter 250 200 150 cm 100 50 0 60 SLOPE 36° 40 - (0) /^ v^. 20 0 20 1- WATER LEVEL y' 1 SLOPE 15° y^ - 1 1 NO EXAG 1 1 1 1 5 4 DISTANCE- 3 FEET +2 + I. 5 4 DISTANCE- FEET Fip. 7. Comparison of model beach profiles formed by waves during: (a) a stationary water level; and (b) a falling tide. The model beach consisted of pebbles with a mean diameter of 7 mm and a natural angle of repose of 33|° ; breaker height at the plunge point was approximately 29 cm. (After Bagnold, 1940.) 7 mm). The profile and its slope reproduce those formed in nature very well. When sand is introduced below the beach the well known abrupt discontinuity in angle between sand and pebble is also reproduced. For waves of a given height, the profiles are but little affected by changes in the wavelength of the attacking waves. The effect of a slowly falling tide on the same pebble beach is to produce a nearly plane beach face with a uniform slope of 19°, as shown in Fig. 7b. This draw-down of the beach face by falling tides is similar to that observed to occur in nature where the tide
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