Unintended Impacts of the CGP - Sediment Starved Streams
This session will examine Construction General Permit (CGP) methodology and compatibility with hydromodification and flood control requirements. The presenters will discuss how different project requirements interact and examine their competing interests.
The CGP requires projects with the highest risk of discharging sediment to meet a numeric standard. Not coincidentally, these projects are also likely to have the highest potential for naturally-occurring erosion. Currently, no effort is made to understand how much coarse-grained sediment is naturally discharged from a project site. This encourages a “bunker” mentality where every effort is made to keep all sediment onsite.
Discharge of coarse-grained sediment from natural landscape and existing onsite streams must be preserved to maintain the dynamic equilibrium of receiving waterbodies. Altering the amount of naturally discharged coarse-grained sediment fluxes to these waterbodies during construction and post-development may create unintended consequences, as waterbodies become sediment starved.
As the economy continues to recover we will see an increase in the number of large residential, commercial, and industrial development projects. Large projects often take many years to construct and are more likely to implement sediment basins and traps. This may starve receiving waterbodies of needed sediment which will lead to a greater chance of altering geomorphology.
The presenters will engage the audience with questions to encourage an interactive experience. Input from attendees will help the group take away real world applications. Some of these questions include: Should the Risk Assessment process take into account the characteristics of the downstream receiving waterbodies? Is it possible to be too successful at keeping sediment onsite? Is having 0 NTU, or close to it, discharge ever a good thing? Are the tools we have to work with good enough to produce the results we want? Are the current dials (BMPs) too big to allow us to polish discharge? How can the use of Passive Treatment help us improve erosion and sediment control at construction sites?
The presenters will provide elements of response based on existing construction projects in Southern California that are representative of these conditions.
Maintaining healthy stream function is growing in importance, as evidenced by the increase in hydromodification requirements throughout the state. Developers should identify streams in and around projects and determine how they will be affected during project planning. Site design elements should also be incorporated to maintain healthy stream function. During construction, the design and placement of Best Management Practices (BMPs) should consider downstream waterbody function so they do not restrict sediment flux. Project planning, design, and implementation must begin to consider all aspects of the regulatory environment in which we work.