Paleotsunami Inundation of a Beach Ridge Plain: Cobble Ridge Overtopping and Interridge Valley Flooding in Seaside, Oregon, USA

Table 5

Bed shear stress estimated for paleotsunami flow down beach ridge slopes at selected overtopping localities.

Locality	Flow depth (m)	Slope gradient	Shear stress (N m^−2)	Shear stress (dyne cm^−2)
Area 3
Ave N	0.5	0.017	83	830
Ave N	1.0	0.017	167	1670
Ave N	1.5	0.017	250	2500
Ave N	2.0	0.017	333	3330
Area 5
3rd Ave	0.5	0.013	64	640
3rd Ave	1.0	0.013	127	1270
3rd Ave	1.5	0.013	191	1910
3rd Ave	2.0	0.013	255	2550
Area 6
Ave K	0.5	0.012	49	490
Ave K	1.0	0.012	98	980
Ave K	1.5	0.012	147	1470
Ave K	2.0	0.012	196	1960
Area 1
Palmrose	0.5	0.002	10	100
Palmrose	1.0	0.002	20	200
Palmrose	1.5	0.002	29	290
Palmrose	2.0	0.002	39	390

Tractive forces or bed shear stresses (Newtons m⁻³ and dyne cm⁻³) are calculated for four paleotsunami overtopping localities using a range of flow depths (1-2 m depth) and measured slopes (m:m gradients) in the modified DuBoys equation (Table 3). Ground surface gradients at the paleotsunami overtopping localities are shown in Figures 16 and 17. Larger flow depths (2.0 m) could represent paleotsunami events #3 and #5. Small flow depths (0.5–1.0 m) could represent paleotsunami event #1 at AD 1700.