FLOOD00925 - Laserfiche WebLink

Home Browse Search

KINEMATIC WAVE 1. BACKGROUND THEORY. The application of the kinematic wave method differs from the unit hydrograph technique in the following manner. First, the method takes a distributed view of the subbasin rather than a lumped view, like the unit hydrograph approach. The distributed view point allows the model to capture the different responses from both perviOUS and impervious areas in a single urban subbasin. Second, the kinematic wave technique produces a non-linear response to rainfall excess as opposed to the linear response of the unit hydrograph. When applying the kinematic wave approach to modeling subbasin runoff, the various physical processes of water movement over the basin surface, infiltration, flow into stream channels, and flow through channel networks is considered. Parameters, such as roughness, slope, area, overland lengths, and channel dimensions are used to ' define the process. The various features of the irregular surface geometry of the basin are generally approximated by either of two types of basic flow elements: (1) an overland flow element, and (2) a stream or channel flow element. In the modeling process, overland flow elements are combined with channel flow elements to represent a subbasin. The entire basin is modeled by linking the various subbasins together. In a typical urban system, rain falls on two types of surfaces: (1) those that are essentially impervious, such as roofs, driveways, sidewalks, road, and parking lots; and (2) perviOUS surfaces, most of which are covered with vegetation and have numerous small depressions which produce local storage of rainfall. The contribution to the flood hydrograph of open areas (pervious surfaces) is characteristically different than that from impervious areas. An obvious difference is that the open areas can infiltrate whereas the impervious areas do not infiltrate significant amounts. A less obvious difference is that the open areas are not seweredas heavily as' impervious areas, making for longer overland flow paths to major conveyances such as open channels and storm sewers. Furthermore, the open areas have hydraulically rougher surfaces which impedes the flow to a greater extent than the relatively smoother surface of the paved areas. The overall impact of these differences is to cause the runoff from the impervious areas to have significantly shorter times of concentration, larger peak discharges and volumes per unit area than from open (pervious) areas. The lumped approach to modeling this type of basin would average the runoff characteristics of both the open and imperious areas into one unit hydrograph. In performing this averaging operation, the peak runoff response of the basin will normally be underestimated when the impervious area is the dominant contributor to the runoff 7-56