To study the backwater extent law of close mulit-parallel bridges,the backwater extent rule of a single bridge was analyzed and applied to the research of two parallel bridges analogously,getting the corresponding laws.The results showed that if the passive influenced bridges are all at the downstream of the active influenced bridges,the backwater of the active influenced bridges will be influenced by both its own water resistance and the positive effects of the passive influenced bridges.On the contrary,it will be influenced by the negative effects of the passive influenced bridges,and the influence coefficient decreases with the growth of the spacing between each bridge.It can be assumed that there will be a calculative backwater when the backwater of parallel bridges are influenced by each other,this backwater has a secondary influence on other bridges and then infinitely approach the real backwater.Based on this assumption,the empirical formula of each bridge backwater can be summarized when the multi-bridges are parallel.
