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Abstract: Tsunami deposits contain information about the flow that created them which can be interpreted to estimate tsunami magnitude. Sandy deposits near Sendai Airport are modeled using an inverse sediment transport model to explore the spatial and temporal variation of tsunami flow speed in the 11 March 2011 Tohoku-oki tsunami. The 2011 deposits contain grain-size signatures of both sediment transport convergences and sediment settling out of suspension. Sediment transport convergences form massive or inversely graded intervals. Sediment falling out of suspension forms a specific type of normal grading, termed suspension grading. The Jaffe and Gelfenbaum (2007) inverse sediment transport model, which relates the thickness and bulk grain size of suspension-graded intervals to tsunami flow speed, was applied to 24 suspension-graded intervals, ranging in thickness from 2 to 4cm, at 7 trenches located from about 250 to 1350m inland from the shoreline. Modeled tsunami flow speeds range from 2.2 to 9.0m/s, and were strongly dependent on the choice of Manning''s n roughness parameterization. Flow speeds were highest from about 75–300m landward of the forested sand dunes where the tsunami encountered lower roughness in a low-lying area as it traveled downslope, and decreased by 16% as the tsunami moved inland over the subsequent 596m. As many as 5 suspension-graded intervals, interpreted as forming during the onshore flow of up to 5 waves, were identified at each location, with a tendency for an inland decrease in the number of suspension-graded intervals suggesting fewer waves inland. Tsunami flows tended to be fastest in the lower two intervals (earlier waves) and slowest in the uppermost interval (last wave) of the deposits. Modeled reconstructions of the suspension-graded intervals at a vertical resolution of 1-cm reproduced observed upward fining of the distributions reasonably well, supporting the use of the inverse model. Estimates of the time required to form 99% of the thickness of the suspension-graded intervals ranged 2 to 5min, allowing that sediment falling from suspension could have formed these portions of the deposits in the time between waves. Application of the inverse model to determine paleotsunami flow speeds from deposits, although now possible, is complicated by uncertainty in roughness. As inverse approaches to estimating roughness and tsunami flow speed improve, the magnitude of paleotsunamis will be better understood and the ability to assess tsunami hazard from paleotsunami deposits will improve. [Copyright &y& Elsevier]

Abstract: This paper presents a simple model for tsunami sedimentation that can be applied to calculate tsunami flow speed from the thickness and grain size of a tsunami deposit (the inverse problem). For sandy tsunami deposits where grain size and thickness vary gradually in the direction of transport, tsunami sediment transport is modeled as a steady, spatially uniform process. The amount of sediment in suspension is assumed to be in equilibrium with the steady portion of the long period, slowing varying uprush portion of the tsunami. Spatial flow deceleration is assumed to be small and not to contribute significantly to the tsunami deposit. Tsunami deposits are formed from sediment settling from the water column when flow speeds on land go to zero everywhere at the time of maximum tsunami inundation. There is little erosion of the deposit by return flow because it is a slow flow and is concentrated in topographic lows. Variations in grain size of the deposit are found to have more effect on calculated tsunami flow speed than deposit thickness. The model is tested using field data collected at Arop, Papua New Guinea soon after the 1998 tsunami. Speed estimates of 14 m/s at 200 m inland from the shoreline compare favorably with those from a 1-D inundation model and from application of Bernoulli''s principle to water levels on buildings left standing after the tsunami. As evidence that the model is applicable to some sandy tsunami deposits, the model reproduces the observed normal grading and vertical variation in sorting and skewness of a deposit formed by the 1998 tsunami. [Copyright &y& Elsevier]

Abstract: Modern subaerial sand beds deposited by major tsunamis and hurricanes were compared at trench, transect, and sub-regional spatial scales to evaluate which attributes are most useful for distinguishing the two types of deposits. Physical criteria that may be diagnostic include: sediment composition, textures and grading, types and organization of stratification, thickness, geometry, and landscape conformity. Published reports of Pacific Ocean tsunami impacts and our field observations suggest that sandy tsunami deposits are generally <25 cm thick, extend hundreds of meters inland from the beach, and fill microtopography but generally conform to the antecedent landscape. They commonly are a single homogeneous bed that is normally graded overall, or that consists of only a few thin layers. Mud intraclasts and mud laminae within the deposit are strong evidence of tsunami deposition. Twig orientation or other indicators of return flow during bed aggradation are also diagnostic of tsunami deposits. Sandy storm deposits tend to be >30 cm thick, generally extend <300 m from the beach, and will not advance beyond the antecedent macrotopography they are able to fill. They typically are composed of numerous subhorizontal planar laminae organized into multiple laminasets that are normally or inversely graded, they do not contain internal mud laminae and rarely contain mud intraclasts. Application of these distinguishing characteristics depends on their preservation potential and any deposit modifications that accompany burial. The distinctions between tsunami and storm deposits are related to differences in the hydrodynamics and sediment-sorting processes during transport. Tsunami deposition results from a few high-velocity, long-period waves that entrain sediment from the shoreface, beach, and landward erosion zone. Tsunamis can have flow depths greater than 10 m, transport sediment primarily in suspension, and distribute the load over a broad region where sediment falls out of suspension when flow decelerates. In contrast, storm inundation generally is gradual and prolonged, consisting of many waves that erode beaches and dunes with no significant overland return flow until after the main flooding. Storm flow depths are commonly <3 m, sediment is transported primarily as bed load by traction, and the load is deposited within a zone relatively close to the beach. [Copyright &y& Elsevier]