QUAL2E Simulations of Pulse Loads

Abstract

QUAL2E is a riverine water‐quality program that has been used to study a wide range of environmental problems. Through numerous updates and improvements, it now contains some of the most sophisticated water‐quality kinetics of any water‐quality code in common use. However, due to significant numerical dispersion in its advective routine, QUAL2E may be inappropriate for certain types of simulations, such as the pulse‐input loads for combined sewer overflows. The existing scheme is an implicit backward difference scheme that produces positive numerical dispersion for all conditions. To reduce this limitation, QUAL2E was modified and the existing solution scheme replaced with an explicit backward‐difference scheme, enabling it to more accurately simulate this class of problem under certain simplifying conditions and time‐step controls. Numerical tests showed that the changes achieved this objective, and an application to a combined sewer overflow study on the Charles and Mystic Rivers in Boston is then described.