How to Effectively Assess Bridge Shading Impacts

The effect of shade cast by new bridge construction is often considered a permanent impact, when it could be permitted as temporary. This discrepancy increases mitigation costs and unduly burdens county Public Works department budgets. Effective site assessment to determine expectation of in-situ recovery and persistence of self-sustaining habitat can justify temporary impacts, saving your project time and money.

Effectively Assessing Bridge Sites

To avoid incorrectly classifying an impact as permanent, a strong site assessment should consider a range of factors that will support native vegetation recovery, including:

  • Post-project hydraulic regime;
  • Channel position relative to infrastructure;
  • Channel geomorphology;
  • In-situ soils and soil disturbance;
  • Availability of existing native vegetation propagules; and
  • Solar exposure throughout the year.

Many native habitats found under or adjacent to bridges are adapted to periods of low-light conditions, and especially those plant species commonly found in the understory. Most wetland vegetation communities are highly tolerant of the dynamic hydrology that bridge structures span. Additionally, some native wetland vegetation communities can passively re-establish in areas that may be identified as permanent impacts.

Building Contingencies for Future Mitigation Actions

To allow areas to passively revegetate, “if/then” conditions should be proposed within negotiated permits. For example: If anticipated passive revegetation does not occur within a specified period, then adaptive measures can be implemented to promote successful on-site re-establishment.

Passive regrowth and ecological recovery can define success. To assess native vegetation community recovery, measures of seedling recruitment can be extrapolated to total recovery of pre-project vegetative cover. Although pre-project vegetative cover is the ultimate outcome, adaptive measures need not be implemented if vegetation is trending toward this goal.

Keep in mind that native habitat is surprisingly resilient within jurisdictional areas. However, if passive revegetation does not occur, underperformance can trigger on-site adaptive measures such as post-project planting and/or seeding.

Success with a Second Opinion

The Edinger Street Bridge replacement project in Huntington Beach required the Orange County Public Works Department to confront the prospect of mitigating for permanent impacts to tidal salt marsh habitat. The County’s first site assessment identified permanent shade impacts outside of the bridge footprint. After performing a second assessment of the site conditions, our habitat restoration experts concluded that natural regrowth was highly likely.

We successfully negotiated a mitigation plan with the California Coastal Commission that recognized temporary impacts previously thought to be permanent and allowed for passive restoration of temporary impact areas with appropriate monitoring and potential adaptive measures. Thus, the project significantly reduced the acreage of off-site wetlands establishment in the coastal zone.

Assessing the impacts of bridges requires the expertise of experienced habitat restoration professionals. Our experts understand riparian and coastal ecosystems and can inform the permitting process to avoid unnecessary project impacts and costs.

For more information on habitat restoration and mitigation, contact Principal and Senior Habitat Restoration Specialist Michael Sweesy (Southern California) at or Ed Armstrong (Northern California) at