Floodplain influences on sediment dynamics of the urban creek Case study: Johnson creek, Portland, U.S.A.

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Sangaralingam Ahilan, Mingfu Guan, Nigel Wright, Andrew Sleigh

Tuesday 30 june 2015

11:30 - 11:45h at Oceania (level 0)

Themes: (T) Sediment management and morphodynamics, (ST) River morphodynamics

Parallel session: 5B. Sediment - River

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This study explores the influence of floodplains on the sediment dynamics of Johnson Creek, a highly urbanised stream known for frequent flooding and which contains sections that do not meet water quality standards under the U.S. Federal Clean Water Act. Understanding sediment dynamics in an urban watershed is an important aspect in a stream’s water quality as most pollutants generated from the industrial and densely populated urban areas are attached to the fine sediment particles. These can have adverse effects on people and habitats for fish and wildlife. The study area is focused on the downstream of Johnson Creek, the East Lent reach, where the bank of the creek has been reconfigured to reconnect the river to a restored floodplain on a 28 ha site to provide more space for the river to flow and be stored. The aim of this study is to investigate whether sediments from the watershed accumulate in the restored floodplain over a period of time or flushes out to the main Willamette River. A layer-based morphodynamic model is used in this study to model suspended and bed-load sediment dynamics between the main channel and the floodplain. The model encompasses three modules: a hydrodynamic module governed by 2D Shallow Water equations, a sediment transport module and a bed deformation module for updating the bed elevation due to erosion and deposition. The flood events and sediment sizes that are derived from the historical data sets are used as the upstream boundary of the model. The model provides a simulated bed topography, erosion and deposition in the floodplain, a velocity profile and water level over the simulated time. This study illustrates a sustainable way to minimise the impact of urbanisation on flood and river water quality through green intervention.