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Over one million simulations were conducted using the Hec-Ras4b (US Army Corps of Engineers, 2004) model to evaluate the sensitivity of model predictions to field data accuracy, density and estimation techniques and provide guidance towards balancing human resource allocation with model accuracy. Notable differences were identified in model accuracy if a project is concerned with river processes occurring within the limits of the bankfull channel versus floodplain regions. Increased cross section discretization, bankfull channel detail and main channel roughness were of greatest field survey and measurement importance when processes relevant to the bankfull channel are of concern (i.e. geomorphic processes or sediment transport). Conversely, where flood conditions are of highest consideration, estimates of floodplain roughness dominate the accuracy of the results of computed water surface elevations. Results for this case study also demonstrate that higher orders of total station field surveys provide little additional accuracy in final predicted water surface elevations, relative to proper estimates of in-channel and floodplain roughness. As long as drift in field surveys has been accounted for during or subsequent to total station surveys, survey techniques such as hangers can be readily employed with very little increase in final model prediction error, while improving field data acquisition efficiency.

Over one million simulations were conducted using the Hec-Ras4b (US Army Corps of Engineers, 2004) model to evaluate the sensitivity of model predictions to field data accuracy, density and estimation techniques and provide guidance towards balancing human resource allocation with model accuracy. Notable differences were identified in model accuracy if a project is concerned with river processes occurring within the limits of the bankfull channel versus floodplain regions. Increased cross section discretization, bankfull channel detail and main channel roughness were of greatest field survey and measurement importance when processes relevant to the bankfull channel are of concern (i.e. geomorphic processes or sediment transport). Conversely, where flood conditions are of highest consideration, estimates of floodplain roughness dominate the accuracy of the results of computed water surface elevations. Results for this case study also demonstrate that higher orders of total station field surveys provide little additional accuracy in final predicted water surface elevations, relative to proper estimates of in-channel and floodplain roughness. As long as drift in field surveys has been accounted for during or subsequent to total station surveys, survey techniques such as hangers can be readily employed with very little increase in final model prediction error, while improving field data acquisition efficiency.

Altered flow regimes following river regulation can result in significant changes in river bed geomorphology and subsequent negative ecological impacts caused by re-suspended sediments deposited on the riverbed. This study aimed to evaluate the consequences of implementing an ecological flow regime on sediments accumulated within the regulated river Juktån. Sediments were sampled and analysed for particle size distribution to estimate sediment stability. Flow alteration following the ecological flow regime was analysed with HEC-RAS unsteady flow simulation serving as a basis for calculations of forces acting to erode or retain deposited sediments. Additional analyses regarding critical flow were made with HEC-RAS steady flow simulation. Results show that 4 out of 15 cross-sections analysed would have the potential to erode and re-suspend sediments. The estimated average critical flow for when sediments become unstable with potential to re-suspend is 17 m3/s. The total sediment inventory of the studied reach is ~25000 ton, with ~3000-ton sediments potentially eroding into re-suspension. This is approximately 3% of river Umeälvens annual 100 000 ton suspended sediments before being regulated. Results indicate that river bed heterogeneity in river Juktån could benefit from implementing the ecological flow regime while not mobilizing such amounts of fine sediments that would cause clogging effects downstream the site of interest. The study also introduces the erosion rate equation which compares the annual erosion between two different flow regimes.