SMC study suggests not all strategies to protect health of erosion-prone streams created equal

Posted May 9, 2020
Runoff flows into a bioswale in Marina del Rey in Los Angeles County. The Southern California Stormwater Monitoring Coalition (SMC) is examining whether bioswales and other low-impact development (LID) approaches for reducing erosion risk in streams could be more effective at protecting stream biological health than traditional approaches that rely on channel hardening.

The Southern California Stormwater Monitoring Coalition (SMC) has completed an analysis shedding light on whether next-generation management strategies for reducing erosion risk in streams could be more effective at protecting a stream’s ecological health than traditional approaches that rely on channel hardening.

The SCCWRP-led study, completed in April, found that the biological integrity of hydromodification-prone streams is much more likely to be degraded in streams lined with concrete, rocks and other armoring modifications than in non-armored streams, where low-impact development (LID) and other strategies are often deployed instead. LID strategies, which are newer approaches to guarding against both flooding and erosion risk, are designed to minimize direct channel hardening that can be harmful to aquatic life.

The study underscores the value of continuing to investigate whether newer hydromodification management strategies could serve as a more effective management pathway for protecting stream biology than traditional channel hardening. Hydromodification is defined as any unwanted change to channel form caused by altered hydrological flow patterns.

In Southern California, hydromodification risk has historically been managed by armoring channels, especially with concrete lining. These hardened channels, however, tend to be associated with severe biological degradation, which has paved the way for development of alternative LID strategies that are intended to promote more natural movement of water.

In recent years, LID strategies have been codified in hydromodification management plans for streams across Southern California. LID strategies include bioretention, rain gardens and other solutions for minimizing alterations to how water flows through an area following development and redevelopment.

The SMC hydromodification analysis represents the first region-wide effort to explore the relationships between hydromodification, channel armoring and biological condition; previous efforts have looked at these issues in isolation only. Southern California stream managers are responsible for protecting the ecological health of streams, in addition to managing both hydromodification and flooding risk.

During the study, researchers analyzed five years of data collected as part of the SMC’s stream survey. The data sets are unique in that they include detailed information on both biological condition and hydromodification potential for nearly 300 stream sites across the South Coast region; these two types of data are typically not collected in tandem.

Researchers compared biological condition in hydromodification-prone channels (e.g., low-gradient sandy streams) to hydromodification-resistant streams (i.e., both hardened channels and channels naturally resistant to hydromodification).

The study found that biological condition tends to be worse in hydromodification-prone channels than in naturally hydromodification-resistant channels (e.g., streams with extensive cobbles or bedrock). Naturally hydromodification-resistant channels have coarse substrate such as gravel or boulders, or intact floodplains that allow stream energy to dissipate naturally.

The potential of LID strategies to better support stream biology has underscored the need for tools that will enable managers to systematically evaluate the effectiveness of LID-based hydromodification management approaches.

SCCWRP is continuing to lay a scientific foundation for this work, ensuring managers can take science-informed actions to optimally protect and restore stream biology – while simultaneously guarding against erosion and flooding.

The SMC hydromodification study is scheduled to be published as an SCCWRP technical report in the coming weeks.

For more information, contact Dr. Kris Taniguchi-Quan.

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