Algal bloom prevention based on the hydrological process regulation in
the middle and lower Han River
Abstract
Han River, the largest tributary of the Yangtze River in China, has
experienced at least 10 large-scale algal blooms since 1992 because of
eutrophication and hydrological/hydraulic regime alteration.
Eutrophication control is a long-term process, but the regulation of
hydrological process is a direct and effective way to prevent algal
blooms. In this study, we attempted to find the difference of
hydrological process between the years with and without algal blooms.
Hydrological statistics and numerical simulation results show that the
key high-flow processes were missing in early spring when algal blooms
occur. The hydrological processes in the middle and lower Han River have
undergone major changes because of the impoundment of the middle route
of the South-to-North Water Diversion Project. The construction of
cascade reservoirs in the middle and lower Han River, such as in
Xinglong, have weakened the hydrodynamic conditions in the damming river
sections, thereby providing favourable hydrological conditions for the
occurrence of algal blooms. During the critical period for algal growth
from January to March, the cross-sectional average velocity and
discharge in the years with algal blooms were significantly lower than
those years without algal blooms. The threshold levels of flow rate for
algal bloom prevention at the Shayang to Xiantao River section were 1100
m3 s−1 (0.24 m s−1) and 810 m3 s−1 (0.74 m s−1), respectively. We
developed the hydrological process regulation schemes on the basis of
the hydrological/hydraulic thresholds according to the different
sections where algal blooms occur from two aspects: a) single reservoir
operations, such as full-emptying and water level fluctuation
operations; and b) joint operation of cascade reservoirs and the Water
Diversion Project from Yangtze to Han River.