Pulsed Flow Wave Attenuation on a Regulated Montane River

This study investigated the effects of hydrograph shape on attenuation of regulated pulsed flow events below a hydropower dam by categorizing and modelling the downstream movement of representative pulses on the upper Tuolumne River in the Sierra Nevadas of California, USA. We extracted underlying hydrograph shape patterns using principal component analysis on individual pulsed flow events released from 1988 to 2012 (n = 4439). From principal component loadings, six shape categories were determined: rectangular, front‐step, back‐step, goalpost, centred tower, and other. Attenuation of representative pulses from each shape type was then modelled using a one‐dimensional hydraulic model of 42 river km. Model results demonstrated a durational threshold for representative pulses (~3–5 h) over which the degree of attenuation of ramping rates and peak discharge approached a limit. Simulations of front and back‐step representative pulses showed trade‐offs between attenuation of peak magnitudes and steepness of rising ramping rates. Reshaping pulses to reduce the adverse ecological effects of rapid changes in stage and velocity downstream was infeasible if the system was required to maintain current electricity production and recreational service levels. Copyright © 2015 John Wiley & Sons, Ltd.