Predicting Hydromodification Impacts Using a Four Factor Approach

摘要

Hydromodification is defined as changes in runoff characteristics and in-stream processes caused by altered land use. The impact of hydromodification can manifest itself through adjustment of stream morphology via channel incision, widening, planform alteration, or coarsening of the bed material. The state of the practice for hydromodification management in California this past decade has been to mimic pre-development site hydrology. A popular method of mimicking the pre-development site hydrology is by maintaining the pre-development frequency distribution of runoff, known as flow duration control. The theory is that if the pre-development distribution of in-stream flows is maintained, then the baseline capacity to transport sediment, a proxy for the geomorphic condition, will be maintained as well. One issue with focusing only on hydrology, however, is that this approach assumes there are no changes in sediment supply, bed/bank material composition, and channel geometry due to land development. Changes to these additional three factors may impact the geomorphic stability of streams and should be recognized in future hydromodification-related regulatory, planning, and stream enhancement efforts. This paper presents a step-by-step approach for modeling geomorphic impacts due to changes in land use by: (1) properly characterizing the four primary factors controlling channel adjustment for both the pre- and post-development condition, (2) incorporating these characterizations as inputs to a long-term continuous sediment transport model, and (3) predicting the likelihood of channel form adjustment based on the ratio of sediment transport capacity in the pre- and post-disturbance conditions. Additionally, example results are provided to illustrate the basic theory, logic, and practical application of this four factor approach in order to predict geomorphic impacts caused by land use changes.