The water speed of rivers on the plain is usually half a meter to one meter per second, with a low drop, which is pushed away by the water head behind.

If the mountainous areas have a large drop, it cannot be generalized. For example, the average flow rate of the Yangtze River in the Three Gorges section can exceed three meters per second. The Han River in the Qinling Mountains and Daba Mountains is almost the same as that.

According to this standard, the straight line distance from the Han River Path excavation point in the northeast of Hechi County to Chencang City is only 160 miles away. Including the twists and turns of the mountain area, it is only 200 miles away, and the water flows in one day.

However, when the Western Han Dynasty waters broke, the amount of water to the north was not large, and the river had dried up for a full 380 years. The amount of water in the first day and a half was basically absorbed by the grasslands and forests along the way.

The river water arched north little by little, and basically until the 29th day, a obviously continuous flow of water was formed before Dasan Pass, and the water depth was only more than a foot and less than ten steps wide.

Later, Li Su found that this was not possible, and the estimate was wrong. From the 29th, Liu Bei sent troops to carry out another wave of homework urgently-

The existing river flowing south is filled with a cofferdam to pile up soil, reduce the flow to the south, and force more water to the north. In this way, the water volume from Hechi County to Wuxing County is sharply reduced, which is considered to be the water volume and water level of the northern branch of the Western Han River in a short time, so that small boats and rafts with draft no more than three feet deep can pass safely. (70 cm)

The vanguard troops marched north along the river from the 29th day of the first lunar month, and the fleet went north on the first day of the February 2nd, two days later than the infantry, and everything was in order.

...

On the 30th day of the first lunar month, in the afternoon, Zhang Ji, the guardian in Chencang City, patrolled the whole city every day as usual.

When he walked to the southeast corner of the city, he saw a strange scene: a large area of ​​water spread under the base of the city wall in front of him, at least a few miles in diameter, but it didn't look deep, just a big puddle.

"What's going on? Why is there so much water accumulated at the base of the city wall? There won't be any trouble in the city? Is it because the moat has increased water?" Zhang Ji caught a subordinate named Zhang Xian to question him. Zhang Xian was his follower, responsible for this section of the city defense.

As a military important place, Chencang City certainly has moats, but it has been flooded in moats every year, and it is often just a moat. This is related to the terrain of Chencang, because it is not suitable for building a fully enclosed water-bearing moat all year round.

The terrain of Chencang City is high in the west and low in the east, and close to the Longshan Pass to the west, so the terrain is naturally uplifting.

There is Qinling Mountains on the south side and the Weihe River on the north side. In theory, it should be "high in the south and low in the north", but this cannot be generalized. Because the southeast corner of Chencang City is the former path of the Western Han River 380 years ago, and it is the point where the Western Han River flows into the Weihe River.

After the old waters of the Western Han Dynasty were exhausted, the fertile riverbed gradually accumulated due to the eroded fertile riverbed. Chencang City gradually expanded eastward over the past three hundred years, and the rammed earth wall was also broken and rebuilt several times. Therefore, the urban area gradually grew larger, including the flat river site in the city.

When choosing a city site, we have always wanted to get as close to water as possible, and to try to cover the flat alluvial plains of the river valley. Chen Cang is already on the edge of the Guanzhong Plain, so the city expansion must of course expand to the flatter east.

But it is precisely because the east side is very low that if you fill the moat with water all year round, the east side will overflow, and even if it is not well controlled, it will easily flood the base of the city wall.

Therefore, in peacetime, there were at least dozens of feet between the two vertices of Chencang's U-shaped moat and the Wei River to the north. Only when the city defense is in danger will it be digging through the broken hole and pouring the Wei River in, and the amount of water in it must be controlled at any time.

This is also the common sense of geographical knowledge that Zhang Ji learned slowly in the past two years since he arrived at the garrison. Today, when he saw that the southeast corner of the city wall was flooded, he felt a little anxious.

The commander Zhang Xian didn't understand it either. After hesitating for a while, he replied in a speculative tone: "General, it's almost February now, and it's about to plow spring. The spring rain a few days ago, maybe it's just that the melting snow on the northern slope of the Qinling Mountains flows out of the Sanguan Valley, gathering with the spring rain, with a little water accumulation. I've tried it just now. The water depth at the base of the wall is only more than a foot, so it shouldn't be a problem."

Zhang Ji thought about it and it was the same.

The dragon raises its head on the second day of the second month, which is the solar term for spring plowing. Now there are two days left, so it is normal for spring rain to continue.

Zhang Ji nodded and ordered: "I hope I'm too worried. Come and see it tomorrow. If the water level declines a little, it's not worth making a fuss. Keep a little more eye on it."

"As for order!" Zhang Xian agreed.

Zhang Ji went to the west and the north again to inspect the two sides. He found that there was no other problem and his worries faded a little. He went back to his house to rest in the evening and said nothing all night.

However, when he inspected the city defense again the next day, on the morning of the first day of the second month, he found that the problem was a little more than one foot deep under the wall in the southeast corner had risen to three feet, and the worst was that it had already spread to the east gate of Chencang City.

Although the city gate can be closed, the cracks of the door are not sealed and waterproof at all, so the water flows into the city, flooding several blocks of the southeast corner. After lunch, the entire east gate south of the city gate to the southeast corner of the city wall was flooded.

In the afternoon, the water level was measured, and the water depth of the outer wall was three feet and seven inches, while the inner wall was only one foot and two inches.

Zhang Ji did not understand geography and hydrology, nor did he read history, so of course he did not know that this was because after the Western Han Dynasty returned to the old road to the north, only the rivers in the Qinling Mountains were still blocked by the Qinling Mountains and could only flow along the deepest position at the bottom of the valley. However, after flowing out of the Qinling Mountains, it spread out because it lost the constraints of the mountain.

From leaving the Qinling Mountains north of Sanguan to finally entering the Wei River, the forty-mile river channel has disappeared due to 380 years of human activities and turned into flat ground. The river water has formed a lake here, where the terrain is low. It takes several more days to accumulate, and the water level rises several feet, and finally level with the northeast corner of the city and the bank of the Wei River. The river water will no longer rise.

At this point, Zhang Ji knew that the matter must be a big deal, so he urgently summoned the civil servants in the city, especially the households and Tian Cao, who were in charge of civil affairs and occasionally organized private self-study water conservancy to discuss what to do.

Cao Cao, who was accused of the world, also knew how to do it. After spending an afternoon carefully observing the water situation, he reported to Zhang Ji: "General, this matter is indeed abnormal, but now it has come to this point. Bei Zhi suggested digging the Dongcheng Moat and the Wei River, so that part of the accumulated water in the east of the city can be introduced into the Wei River."

Zhang Ji was still worried about backflowing, and said worriedly: "Isn't that a method used only when the war is urgent? I remember you said that if you dig the East Trenches and the Wei River, the Wei River will flood some city walls."

Tian Caoshou: "No, the water level in the east of the city is now higher than that of the Wei River. In the past, even if the Wei River is finally submerged, it would not be as deep as the Wei River."

Zhang Ji looked at the sky. It was already evening, and it was not easy to organize the construction of civilians and soldiers. He said, "Then make preparations first and calculate how many manual materials and equipment are needed. I will approve it for you.

Construction will start tomorrow morning to dig the gap between the East Trench and the Wei River to drain water. However, you must always pay attention to the water level. If the spring rain and the ice flood pass, the water level will drop to lower than the Wei River, you can block it immediately."

Tian Cao: "Abide by the order!"

...

Unfortunately, the next day, early on the second day of the second month, Zhang Ji had no chance to send Tian Cao to take his men out of the city to construct.

Because overnight, the water level rose by more than a foot, and several troops rushed out of the Dasanguan Valley Road.

Zhao Yun had 15,000 cavalrymen, and the first five thousand men rushed straight into Weibin that night.

Then, relying on a batch of wooden rafts that were laid down from the former path of the Western Han River, it flowed to the northernmost end of the Chencang East Trench. The cavalry untied the wooden rafts, carried the wood over the last dozens to more than a hundred feet away, threw it into the Wei River and tied it again, so that there was a simple boat raft that could cross the river in the Wei River.

This method is similar to Mohammed II's "Golden Horn Bay Dry Land Boat", but the project volume is much smaller, because the water-abundant lake formed by the Western Han River in the east of Chencang City was only a river embankment dozens of feet wide between the Wei River, and the height drop is not large.

In this way, Liu Bei's army almost obtained a batch of ships on the Wei River on the first night of sending troops to attack, which was almost impossible for other Northern Expeditioners in history to imagine. Because whenever the Northern Expedition encountered in Guanzhong, the "river control" of the Wei River was basically in the hands of local troops, and the ships were collected by local warlords in advance. Such suddenness led to the emergence of Zhao Yun being unable to be wary of.

When Zhang Ji woke up, Zhao Yun had already crossed the river and set up camp on the north bank of the Wei River, blocking the contact between Zhang Ji and the northern counties.

Zhao Yun had a total of 15,000 cavalrymen, and the remaining 10,000 people were also pouring out of the Sanguan Road.

The second five thousand people were stationed in the east of the city, more than ten miles away from the city, avoiding the newly formed large water hole in the east of the city, in order to block Zhang Ji from asking for help from Mei County and Chang'an, so that Chang'an could get the information that Chen Cang was surrounded by sneak attacks and try to be slow to avoid Li Jue's reaction.

The last five thousand people waited for the boats on the Wei River to be idle and crossed the river north on the morning of February 2nd.

Their purpose is to advance along the tributary of the Wei River on the north bank of the Wei River. It is best to be able to defraud the city or force the submission of Yumi County and Wei County along the coast of the Wei River. It is best to be able to defraud the city and surrender. It doesn’t matter if they can’t defraud the city and surrender. They can leave troops to monitor the county, and then continue to advance north along the Wei River, and go to Jieting, the key to Longshan on the west bank of the end of the Wei River, and grab the Jieting.

The purpose of seizing Jieting was also mentioned earlier, because I knew that Guo Si's 70,000 army was in Longxi now, so I wanted to block the two passes that Longxi retreated to Guanzhong, namely the Weishui River Valley and Jieting, so that Guo Si could not turn back to Longshan to reinforce Zhang Ji.

Of course, we cannot expect Zhao Yun to be responsible for the subsequent defense work. We use cavalry at the beginning to act quickly and unexpectedly. Once we have firmly established our foothold, Guan Yu will naturally have a steady stream of infantry troops to follow up, be responsible for the defense war to block the entrance and take over these defense areas.

For example, on the second day of the second month, just the first day after the army left the valley, Guan Yu sent Yan Yan to lead 5,000 infantry, including an 800-man camp, stationed on both sides of the Weishui Valley entrance in the west of Chencang City, blocking the way south of Guo Si first.

Then Zhang Ren was sent to bring 5,000 infantry, with the same equipment configuration. He marched on foot to Jieting, which took about three days of rushing marching, and then took over the Jieting Pass snatched by Zhao Yun's cavalry and was responsible for subsequent defense.

Guan Yu has been practicing troops in Shu for more than two years and has also understood the temperament of the generals in Shu.

Guan Yu knew that the specialty of the surrendered generals such as Yan Yan and Zhang Ren was mountain defense warfare, rather than a decisive battle on the plains or a tough battle. In particular, Yan Yan and Zhang Ren had no combat experience in fighting against the heavy cavalry group on the plains or being harassed by archers and cavalrymen. Therefore, they could not be expected to fight against the main force of Li Jue and Guo Si, and they could only lead the troops personally.
Chapter completed!