Great Changes on the Lower Yellow River Channel since 2000 and Future Prospective

With the construction of large hydropower projects on the upper and middle reaches, the development of soilwater conservation and irrigation projects, the probability that a big flood event will occur and flood peak discharges have been greatly reduced. It is unnecessary to widen the lower-reach river to allow floodplain inundation for flood peaks reduction. In recent years, there is a major breakthrough in the understandings of the mechanism and capacity of the sediment transport in the narrow and deep channel of the Yellow River. Lower-reach channel has huge flood discharge and sediment transport capacity, which points out the future direction of harnessing the river. After the reconstruction of Sanmenxia Reservoir and the operational mode of "storing clear water and releasing muddy water"to reduce deposition, non-siltation has already been achieved for the channel upstream of Huayuankou. After the operation of Xiaolangdi Reservoir for 13 years, great changes have taken place in the lower-reach channel with maximum longitudinal water surface elevation reduction of 1. 0- 2. 2 meters, and the bankfull discharge has been increased dramatically. But the wandering reaches are still wide,shallow, scatter and ill-conditioned, and they need to be regulated at both banks to form a stable, deep and narrow channel. Through multi-year sediment regulation of valley type of reservoirs like Xiaolangdi the combinations of flow and sediment entering the lower-reach can be optimized, and the reservoir can be used for a long time. Sediment should be managed to be released when the discharge is greater than 3 000 m3/ s, and it should be transported through the regulated new channel to sea. By using this approach, the river bed will not be elevated by deposition, and the beneficial use of the reservoir will be significantly increased too.

NASA's satellite image of Yellow River's mouth:

https://earthobservatory.nasa.gov/Features/WorldOfChange/yellow_river.php

Oroville Dam Crisis: Potential Emergency Spillway Breach (February 2017) - Part I

The Oroville Dam in Northern California is the tallest dam in the United States, rising ~770 feet (235 m) high. It was built on the upper Feather River in 1968, serving mainly for water supply, hydro-electric power generation and flood control. The dam impounds Lake Oroville, the 2^nd largest man-made lake in California, capable of storing more than 3.5 million acre-feet (4.4 km³), almost 1.1 trillion gallons of water for supplying farms and cities across the state. It’s a vital piece of infrastructure located in the Sierra Nevada foothills east of the Sacramento Valley.

Storms in early February, 2017 caused significant damage to the primary spillway when flood was released at 50,000 cubic feet per second (cfs), hindering the safe release of floodwater. The threat of uncontrolled release of water from the dam threatened more than 180,000 people living downstream along the Feather River. To alleviate the flood risk, there are four intended routes that engineers can use to allow water to pass downstream, from most to least preferred:

1.   Through the hydro-electric generators, which allow a maximum flow rate of 16,950 cfs.

2.   Through a river outlet or bypass valve, when operational it has a capacity of 5,400 cfs. The outlet was damaged during an accident in 2009 and has not been used since.

3.   Through the main (primary) spillway, which is used to quickly release large amounts of excess water down a concrete channel and control the height of the reservoir. It is controlled by gates and has a maximum capacity of 150,000 cfs. Based on the dam operator's contracted agreement for flood control, reservoir engineers lower the height of the reservoir before storms arrive. The practice allows the dam to capture storm runoff, reduces spillway flows, and lessens the likelihood of flooding below the dam.

4.   Over the top of an ungated emergency spillway, a 1,700-foot long and 30 ft high weir built several feet below the height of the main dam. Once the lake reaches an elevation of 901 ft, this emergency spillway naturally flows over the weir and down the unprotected natural hillside, which is used as the spillway channel.