Dam Removal Express Assessment Models (DREAM). Part 2: Sample runs/sensitivity tests

This paper presents sample runs of the Dam Removal Express Assessment Models (DREAM) presented in the companion paper, Cui et al. (2006): DREAM-1 for simulation of sediment transport following the removal of a dam behind which the reservoir deposit is composed primarily of noncohesive sand and silt, and DREAM-2 for simulation of sediment transport following the removal of a dam behind which the upper layer of the reservoir deposit is composed primarily of gravel. The primary purposes of the sample runs presented here are to validate some of the assumptions used in the model and to provide guidance as how accurately the field data should be collected. Sample runs indicate that grain size distribution of the reservoir sediment deposit is the most important piece of information needed during the field campaign. Other than the grain size distribution of the reservoir sediment deposit, errors within a reasonable range in other parameters do not result in significant variations in the predicted depositional patterns downstream of the dam, although different magnitudes of sediment deposition may result from such errors. Sample runs also indicate that when the reservoir deposit is composed primarily of gravel, sediment deposition downstream of the dam following dam removal may not propagate far downstream of the dam, and may be limited to isolated reaches where sediment transport capacity is low. Farther downstream sediment deposition becomes progressively smaller due to the attenuation of sediment transport and gravel abrasion. When the reservoir deposit is primarily fine sediment, however, there may be more extensive sediment deposition (both larger area and higher magnitude) downstream of the dam following dam removal. Dredging part of the sediment in advance reduces the downstream impact due to the reduced volume, and the extra distance provided by dredging allows for attenuation of sediment transport. Sample runs with staged dam removal indicate that it provides only limited benefit compared to a one-time removal in case the reservoir deposit is composed primarily of coarse sediment, but may provide significant benefits in case the reservoir deposit is composed primarily of fine sediment. The benefits of a staged removal for the latter case include reduced magnitude and area of deposition as well as reduced suspended sediment concentration downstream of the dam.