Dispersal and capture of sediment at the land/sea interface plays a critical role in the response of a continental margin to variations in fluvial discharge and climate because they determine sediment exchange between neighboring terrestrial and shelf environments. Prograding shorelines record the signature of these processes, but dispersal and deposition that leads to progradation operates through short-lived storms and floods, while the resulting stratigraphy evolves over centuries and longer. Over the past ~7 kyr the Waipaoa Sedimentary System, New Zealand, has
experienced shoreline progradation and expansion of the coastal plain in response to a large natural sediment supply and, more recently, anthropogenic disturbances. This project seeks to develop an understanding of shoreline behavior and the evolution of associated coastal environments for a range of timescales that span modern-day, observable events to Holocene progradation by combining field observations, available data, and numerical models. Our overarching objective is to investigate how the interplay of fluvial input, coastal hydrodynamics, and sediment dispersal control shoreline progradation on event and stratigraphic time scales.

Specific questions include: 1) How shoreline progradation rates scale with the volume and nature
of sediment delivered from uplands? 2) Does fluvial discharge mode dictate sediment exchange
between nearshore, inner shelf, and outer shelf? and 3) To what extent do variations in sediment
volume and grain size control exchange across the land/sea interface?

Observations of the capture and exchange of sediment across the boundary between land and
sea will be integrated into numerical models that predict short-term sediment dispersal and longterm stratigraphic development under variable flood and climate forcing. Efforts will focus on
two periods: the past 50 years; and the late Holocene, 50 y B.P. to 7 kyr. These intervals capture a range of progradation rates and both natural and anthropogenic disturbances to fluvial sediment
delivery, including deforestation within the past century, changes in climate, and relative sea level
during the late Holocene. Field experiments within Poverty Bay offshore of the Waiapoa River
mouth will be used to quantify the modern sediment budget and transport rates. Results will be
used to develop a sediment transport model based on a three-dimensional hydrodynamic model.
Transport predictions for individual storms and floods, made using this model, will be integrated
into a stratigraphic model that reflects a prograding land/sea interface under variable forcing
parameters. The stratigraphic model will use results from the event-scale model to predict
shoreline progradation and stratigraphic development over centuries and longer. Both short-term
and stratigraphic models will be calibrated using the preserved sedimentary record of the focus
time periods.

Intellectual Merit: A quantitative understanding of the physical processes that control the
exchange of sediment across the land/sea interface on a range of time scales is crucial to linking
terrestrial and marine environments. A processed-based understanding of this dynamic interface
between land and sea is fundamental to interpreting and predicting the sedimentary record of the
coastal-plain, shelf, and slope on a global scale. Integration of event-scale processes within a
stratigraphic model enables this work to be transportable to many settings worldwide. Moreover,
developing and testing modeling approaches that bridge the gap between events and the
stratigraphic record remains an area of fundamental research in sediment-transport modeling that
must be addressed before the Earth Sciences Community’s goal of a seamless, source to sink
model can be realized.

Broad Impacts: This study focuses on how physical processes in both the terrestrial and
inner-shelf environments control the dispersal and segregation of sediment, with emphasis on the
evolution of coastal environments on event, century, and millennial time scales. By employing
physically based numerical models that span time scales, the findings of this project can, in
principle, be generalized to coastal settings worldwide to provide predictions of how changes—
both natural and anthropogenic—to watersheds affect adjacent coastal communities.