Low impact development (LID) is an alternative comprehensive approach to urban stormwater management. It can be used to address a wide range of water quality issues, erosion control problems, and reduce capital cost of a storm drainage system. The LID cascading planes layout has been widely applied on new community developments, and the Level Spreaders are commonly used in combination with riparian buffers as a stormwater Best Management Practice (BMP) in many parts of the developments. These systems have not been extensively studied in urban environments to determine the impact on the flow path, nor do level spreaders have a complete detailed design guideline. This paper provides the Kinematics wave cascading model and runoff volume analysis numerical techniques to model a low impact development layout system for the purpose of comparison between effective imperviousness and traditional area weight method imperviousness.
In stormwater management, watershed imperviousness is a primal parameter in urban hydrology to evaluate the storm runoff rate and volume. This study uses a cascading-plane model to represent the physical landscaping layout and use the runoff volume to evaluate the effective imperviousness. The cascading model for the level spreader provides a new methodology to analyze the basin imperviousness. Under the traditional area weight method concept, the impervious rate for the case study should remain constant at 50% since the impervious area is equal to pervious area. With the cascading model however, the effective imperviousness can be represented from 14% to 81%, based on different rainfall depth and different soil infiltration rates. For a high frequency but low intensity rainfall event, the traditional area weight method over- estimates the storm runoff volume. The main reason that the traditional area weight method overestimates storm runoff is that the method does not consider the basin’s flow path and ignores the additional soil infiltration volume when overflow passes over the pervious surface area.
This study introduces the effective imperviousness rate, and provides the cascading model was successful at representing the physical behaviors in terms of runoff and infiltration of the level spreader system.