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以往設計涵洞时,涵身允許最大坡度,除地形限制者外,常以允許流速控制,即以允許流速为均流流速,求其摩擦坡度(即发生該均流流速之坡度)if=V~2/(C~2R)作为涵身最大限制坡度。例如涵洞出入口为一般标准鋪砌形式时,涵内設計流速常以5米/秒为限,涵身最大坡度就是水流在涵内发生V=5米/秒的均流状态的坡度;又如陡坡涵洞,出口作緩流設备,涵内允許流速为8米/秒,涵身最大坡度就是水流在涵内发生V=8米/秒的均流状态的坡度。上述办法并未考虑涵内水流是变速流的情况。如果涵洞入口处水流是寬顶堰水流状态,涵身坡度大于临界坡,則水流流入涵洞,水位落低,經过临界水深后水深继續变小,流速继續增大,假使涵身很长,水流将逐漸趋近于与涵身坡度相适应的均流水深与流速,假使涵身不长,則水流可能尙未变到均流的水深即已流出涵洞,出口流速尙未达到均流流速值,所以涵内最大流
In the past when the culvert was designed, the maximum gradient was allowed for the culvert. In addition to the topographic restrictions, it was often allowed to control the flow rate, that is, to allow the flow rate to be the mean flow rate, and to find the friction slope (that is, the gradient at which the flow occurred) if = 2 / (C ~ 2R) as the culvert limit slope. For example, the culvert entrances and exits are generally paved, the design flow rate in the culvert is usually limited to 5m / s, and the maximum slope of the culvert is the slope of the current sharing state where V = 5m / s. Another example is the steep slope culvert , Exports for slow-flow equipment, culvert allows the flow rate of 8 m / s, culvert maximum slope is the flow in the culmination of V = 8 m / s average current gradient. The above method does not consider the situation where the flow in the culvert is variable speed. If the water flow at the culvert entrance is a wide weir flow state and the culvert slope is greater than the critical slope, the water flows into the culvert and the water level goes down. After the critical water depth, the water depth continues to decrease and the flow rate continues to increase. If the culvert is long , The water flow will gradually approach the mean flow depth and flow rate corresponding to the slope of the culvert. If the culvert is not long, the water flow may not change to the mean flow depth, ie, the culvert has already been discharged, and the exit flow velocity 尙 does not reach the mean flow velocity Value, so culvert maximum flow