An ecologically dispatch strategy using environmental flows for a cascade multi-sluice system: A case study of the Yongjiang River Basin, China

Abstract

The construction of estuary sluice has profound and adverse biological consequences through its effect on the hydrodynamic process and salinity gradient. Environmental flow plays an essential role in the conservation of the ecological environment, but its calculation methods remain biased towards the traditional single-target fish species. Consequently, these flows ignore the ecological requirements of different biological communities and therefore disregard the long-term perspective of the riverine and estuarine ecosystem. Actually, we found that once the sluice was built, the available habitat throughout the river would show a degradation by 81.55% for fish that produce floating eggs and 14.25% for fish that produce sticky eggs. In addition, changes in the distribution of salinity in estuaries and coastal waters caused by the closure of sluice indicated that the habitats of fish living in low-salinity areas would disappear. This study proposes an ecologically oriented multi-sluice system operation strategy that integrates the demands of fish with different living habits on environmental flow into the joint operation of a multi-sluice system to maintain different ecological functions throughout the Yongjiang River basin. The spawning habits of fish in the study area were investigated, and the differences in their adaptability to various hydrological factors were summarized. MIKE 21 model and PHABSIM were used to ascertain the environmental flow through the data of velocity, water depth, and salinity. Model predictions of the effective habitat were supplemented by the optimum selection, and the discharge of the Fenghua River was suggested to be 105 m3/s. Likewise, the survival salinity requirements revealed that the discharge of Yongjiang sluice should reach 250–300 m3/s. Results were used to design the joint ecological scheduling schemes, with the aiming of ensuring that the flow velocity of the entire migration channel can meet the requirements of facilitating the upstream movement of fish whilst keeping eggs floating. In the Considering the upstream ability of fish, the tidal law of the estuary and the planned discarded water of estuary sluice, it is suggested that during the spawning time, seven consecutive days of the ebb tide should be selected to carry out the joint operation of Fenghua, Yaojiang and Yongjiang River, and the flow constraint factors are 105, 100 and 250 m3/s, respectively.