Research Areas > Stormwater > Assessment and Management of Hydromodification Effects

Project: Assessment and Management of Hydromodification Effects

Background and Objectives

Hydromodification can result in adverse effects to stream habitat, surface water quality, and water supply, while the associated stream erosion can threaten infrastructure, homes, and businesses. In response to these changes, state and local agencies create standards and management approaches to control and/or mitigate hydromodification effects on natural and semi-natural streams. Regulatory programs often rely on tools that use a scientific understanding of hydromodification processes to inform management decisions. Developing such tools is particularly challenging for hydromodification, since streams may respond very differently based on both their inherent properties (e.g., substrate type) and landscape settings.

The goal of this project was to develop a series of assessment tools and predictive models (applicable to a range of stream types) that support implementation of hydromodification management measures. This project produced tools to help answer the following questions:

1) Which streams are at the greatest risk of hydromodification effects?
2) What anticipated effects (in terms of increased erosion, sedimentation, or habitat loss) are associated with changes in land use that result in increased impervious cover?
3) What are some potential management measures that could be implemented to offset hydromodification effects? How effective are they likely to be?
4) How can the effectiveness of management actions be evaluated through monitoring and assessment?

Status

This project was initiated in 2007 and completed in 2013.

Methods

The project consisted of four technical tasks.

• The first task involved developing a mapping and classification system for streams based on their susceptibility to the effects of hydromodification. Susceptibility should be evaluated based on both properties of the stream and expected future increases in impervious cover. The relative susceptibility of different stream types can be classified based on the erodibility of different channel boundary materials, channel evolution stage, floodplain connectivity, geologic controls, and other factors. Such a system can help managers prioritize streams for protection and management.

• The second task establishes protocols for ongoing monitoring that are carefully designed to assess the effects of hydromodification and the effectiveness of management actions. Development of standard monitoring protocols will facilitate regional information sharing on project performance.

• The third task involved developing and refining modeling tools that couple hydrologic simulations, physical process models, and risk-based modeling to assess the effects of hydromodification on stream condition. This task also involved developing guidance for selecting appropriate modeling tools for specific circumstances to assess the likelihood of stream channel response to expected changes in hydrology associated with changes in land use patterns.

• The fourth task involved developing a series of tools that can easily be applied to make hydromodification management recommendations. These tools utilize the results of previous research to create a series of plots, nomographs, and checklists. Tool development was based on data from 85 project locations, representing a range of stream conditions throughout Southern California.

Findings

The wide variety of streams and watershed settings in Southern California (in terms of opportunities and constraints) means that a “one-size-fits-all” approach is not very effective. Instead, assessment tools and management approaches must be customized for the particular setting.

From a modeling perspective, suites of modeling tools should be used to answer increasingly specific questions as projects progress from planning to implementation. (The uncertainty associated with each tools should be clearly articulated when presenting output from any assessment tool.)

Similar to modeling and assessment, management actions should be customized based on the needs, constraints, and goals (desired endpoints) for a given site. In general, hydromodification management should focus on restoring watershed processes, such as:

• Runoff management,
• Maintaining coarse sediment supply,
• Maintaining and restoring hillslope coupling and sediment transport capacity, and
• Maintaining and restoring floodplain connections.

Hydromodification management should evolve from narrowly-scoped, project-based actions to solutions within an integrated watershed strategy that accounts for watershed processes. Finally, hydromodification control measures cannot be driven solely by new development and redevelopment; legacy effects must be remedied to ensure streams and watersheds recover fully.

Success of management actions should be evaluated through systematic monitoring (using hydrologic, physical, and biological endpoints). Monitoring should be question driven and consist of four basic elements:

1) Performance monitoring -- How do specific BMPs or facilities function relative to their designs?
2) Effectiveness monitoring -- How well do specific management actions or suites of actions protect the condition or beneficial use of receiving waters?
3) Spatial and temporal trends monitoring -- What is the spatial footprint of responses to management actions relative to discharge locations? How are conditions improving or declining over time?
4) Characterization monitoring -- What is the condition of target areas relative to specific benchmarks (e.g. standards, reference condition, ambient)?

In general, the first and second elements (performance and effectiveness monitoring) of hydromodification monitoring can be addressed by multi-year monitoring programs typically managed by municipalities and other local entities. In contrast, the third and fourth elements (trends and ambient condition) must be addressed over longer time scales (e.g., decadal) through cooperative regional monitoring that involves multiple entities, including state, regional, and local agencies and grant programs.

Partners

This project was conducted in coordination with Colorado State University, Fort Collins, and the Southern California Stormwater Monitoring Coalition.

Tools

• Modeling and Managing Hydromodification Effects: Summary of Available Tools and Decision-Making Approach. 2013. E Stein, B Bledsoe. Technical Report 753. Southern California Coastal Water Research Project. Costa Mesa, CA.

• Framework and tool for rapid assessment of stream susceptibility to hydromodification. 2011. BP Bledsoe, ED Stein, RJ Hawley, D Booth. pp. 25-47 in: Kenneth Schiff and Karlene Miller (eds.), Southern California Coastal Water Research Project 2011 Annual Report. Southern California Coastal Water Research Project. Costa Mesa, CA.

• Hydromodification Susceptibility Screening Tool - Data entry and calculation tool that assists the field-based assessment of channel condition.

• Hydromodification Screening Tools: Technical basis for development of a field screening tool for assessing channel susceptibility to hydromodification. 2010. BP Bledsoe, RJ Hawley, ED Stein, DB Booth. Technical Report 607. Southern California Coastal Water Research Project.

• Hydromodification Screening Tools: Field manual for assessing channel susceptibility. 2010. BP Bledsoe, RJ Hawley, ED Stein, DB Booth. Technical Report 606. Southern California Coastal Water Research Project. Costa Mesa, CA.

• Hydromodification Screening Tools: GIS-based catchment analyses of potential changes in runoff and sediment discharge. 2010. DB Booth, SR Dusterhoff, ED Stein, BP Bledsoe. Technical Report 605. Southern California Coastal Water Research Project. Costa Mesa, CA.

Presentations

• Hydromodification Management Tools (Video) - January 2011 presentation to SCCWRP member agencies on modeling to produce regime diagrams.

• Developing Tools for Hydromodification Management and Assessment (Video) - January 2010 presentation to SCCWRP member agencies on development of screening tools for assessing susceptibility to hydromodification.