Chapter 8 Floodplain Inundation Modeling and Forecasting for the Middle Fly

Abstract

Abstract This chapter describes how water levels are likely to increase on the Middle Fly in response to deposition of mine waste over the next 40 years or so. It also shows where floodplain inundation will increase and presents maps of flooded areas prior to mining, at mine closure and in 2050. The predictions are based on results from the 2003 sediment modeling study (Pickup and Cui, this volume) and an unsteady flow hydraulic model of the floodplain. The model incorporates backwater effects from the Strickland, downstream attenuation of flood waves and the influence of flows to and from off-river water bodies on the floodplain. The hydraulic model uses the unsteady version of the HEC-RAS model. Water surface profiles calculated in HEC-RAS are used with the GRADFLOW model to calculate the extent and location of floodplain inundation throughout the length of the Middle Fly. The floodplain inundation model uses topographic data collected as part of the Shuttle Radar Topography Mission (SRTM) in 2000. The data are not accurate enough for flood modeling and have been corrected using observed flood extents and water surface profiles from Middle Fly gauging stations. The flood extents were mapped from a variety of satellite images. Hydraulic model results show that increases in water level due to deposition of mine waste gradually pass down the river system and develop a fairly uniform water surface slope. This is a normal response to an increase in sediment load. Maps of the extent of inundation show that most of the floodplain will eventually be affected and large areas will experience a substantial increase in inundation frequency. More frequent flooding will also extend up floodplain tributaries but this effect will be limited because the floodplain is confined by Pleistocene terraces. Much of the impact will occur long after mine closure.