Date of Award

4-1977

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Geology

Supervisor

Professor Gerard V. Middleton

Abstract

The Avon River estuary is in central Nova Scotia and is part of the northeastern arm of the Bay of Fundy. Three major rivers discharge into the estuary which is approximately 16 km long and up to 2 km wide. There are three sand bodies at the estuary mouth and another three within the estuary.

The Avon River estuary is macrotidal; tidal range is 15.6 m at lunar perigee. Tidal currents represent the only hydraulic process that significantly affects sediment transport and distribution. Maximum bottom current velocities increase from 0.6 m/s at the estuary mouth to 1.7 m/s at the head of the system. Every location in the estuary is either flood current or ebb current dominant.

Mean grain size forms a different pattern on the surface of each sand body, but the overall trend is that grain size decreases sharply from the mouth of the estuary to the estuary head; thus, there is an inverse relationship between mean grain size and maximum current velocity. Cumulative curve analysis indicates that sediment samples are composed of three log-normal grain populations. A large, coarse population that is present at the estuary mouth is absent at the estuary head.

Grain size distribution is a function of maximum flow conditions; Shields' criterion indicates that the maximum grain size present in each location can be transported by the local hydraulic regime. Each grain population can be related to a sediment transport mechanism; the coarse population is transported by traction, the intermediate by intermittent suspension, and the fine by suspension. The transition from traction to intermittent suspension occurs when shear velocity approximately equals settling velocity; the intermittent suspension - suspension transition reflects the condition that shear velocity is approximately equal to 5 times settling velocity. Hydraulic sorting, produced by different transport rates for each transport mechanism and by local physiography, limits penetration of coarse sediment into the Avon River, and causes a deficiency of coarse sediment at the estuary head. This, in turn, produces the observed inverse relationship between mean grain size and current velocity.

There are three major classes of bedforms in the system including ripples, megaripples, and sand waves; transverse bars exist in one small area. Surface bed configuration reflects maximum flow conditions; each type of bedform occupies a different stability field on plots of water depth versus current speed and current speed versus mean grain size. Flow regime increases from the estuary mouth to the head, and internal structures reflect surface bedforms.

Sediment transport paths define net flood and net ebb sediment transport zones; transport zone position is a function of hydraulics and local physiography. Sand bodies lie at the junction of two or more transport zones. The three sand bodies at the estuary mouth form an ebb tidal delta. At the estuary head, one sand body is part of an ebb tidal delta, another is a tidal point bar, and the third is a flood tidal delta. Glacial till deposited in the estuary and tidal current transport of offshore sediment probably are the most important sediment sources; shoreline erosion also contributes a substantial amount of sediment. Sediment transport rates, aerial photographs, and surveying, echo sounding, and chart data indicate that the Avon River estuary has been stable since at least 1865 and that the system is in approximate dynamic equilibrium.

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