FLOOD00925 - Laserfiche WebLink

Home Browse Search

RESERVOIR ROUTING 1. BACKGROUND THEORY. Routing a hydrograph through a reservoir normally is accomplished by level-pool reservoir routing which assumes a level water surface behind the reservoir. This hydrologic routing technique uses the principle of conservation of mass which can be described as inflow minus outflow is equal to the rate of change of storage within the reservoir. Mathematically the conservation of mass equation can be expressed as: L,-=t.J2 - Q.., + 0 2 :::: 2 2 ~2~1 At (1 ) Where: o = Outflow I = Inflow S = Storage At = Computation time interval Subscripts 1 and 2 denote the beginning and end of the routing interval, respectively. This equation is the basic equation employed in the Modified Puis routing technique which can be applied to either reservoir or channel routing problems. The basic difference between the reservoir and channel routing applications is that a channel is subdivided into several subreaches in which each subreach is analyzed individually as a level pool reservoir. In applying equation (1) to a reservoir, a storage-outflow relationship is required. To develop a storage-outflow relationship, an elevation-storage relationship and an elevation-outflow relationship should consider flow through the outlet works, the emergency spillway, and over the top of the embankment if there is potential for overtopping. During the routing process, as water enters storage, the outflow capability increases because the pool level rises which increases outflow. This increasing outflow, with increasing water in storage, continues until the reservoir reaches a maximum level which will occur at the moment that the outflow equals the inflow on the recession side of the inflow hydrograph. Once the outflow becomes greater than the inflow, the storage level will start decreasing. The difference between the outflow and the inflow hydrograph on the recession side reflects water withdrawn from storage. 7-70