Post-Fire Hydrology

Hillslope fire above the Stumpy Meadows reservoir during the King Fire (2014).

Wildfires alter the physical landscape of watersheds, potentially causing erosion, increased sedimentation, debris flows, and flooding. These physical, water quality, and ecological hazards need to be identified and quantified to effectively communicate risks to resource managers, law enforcement, local governments, and the public. The devastating effects of a fire generate many questions from resource managers:

    • Where do we need to communicate the risk of increased flooding and sedimentation?
    • What priority areas or critical infrastructure are threatened and how can these risks be mitigated?
    • What residential areas are at risk, and what magnitude of storm could potentially cause flooding?
    • What multi-benefit ecosystem restoration objectives can we achieve while stabilizing impaired areas?

Modeling Post-Fire Changes in Hydrology

Modeling processes have been developed to rapidly incorporate burn severity mapping into hydrologic models. This allows determination of relative changes in expected peak flow and sediment yield following a fire. cbec staff have expertise in many hydrologic and hydraulic models, and can leverage existing models to evaluate post-fire scenarios. cbec routinely applies physically based modeling approaches to understand and quantify relative changes in hydrology and sediment loading following a fire. Model outputs are useful to identify and prioritize locations requiring immediate mitigation resources or further analysis.

Section image credit: “King Fire – Stumpy Meadows area” by Pacific Southwest Forest Service, USDA is licensed under the CC BY license.

Post-Fire Assessment Services

Post fire debris flow

cbec employs professionals in hydrology, hydraulics, and geomorphology who have worked extensively throughout the western states’ fire-affected areas. cbec offers a suite of services that can be tailored and scaled to meet the needs of our clients.

Services include the specialties below, explore them in depth on the sections on the left:

    • Rapid Post-Fire Hydrologic Risk Assessment
      to understand changes in runoff and rapidly prioritize mitigation resources immediately following a fire
    • Risk Assessments of the Wildland-Urban Interface
      to understand and prioritize management impacts in areas that are at future risk of burning
    • Field-Based Watershed Assessment in Unburned or Burned Areas
      to gather baseline and post-fire conditions of ecological health, geomorphological setting, and critical infrastructure
    • Hydraulic Modeling
      to evaluate changes in flood risks to infrastructure in targeted locations

Rapid Post-Fire Hydrologic Risk Assessment

Example outputs from rapid post-fire hydrologic risk assessment

The rapid post-fire hydrologic risk assessment can simulate unburned and burned conditions across any watershed of interest. The burn severity output is integrated into the model to physically represent post-fire changes, and results indicate relative changes in runoff and sediment loading between unburned and post-fire conditions. Simulation results include peak flow, peak sediment transport rate, and total sediment yield, resulting from common design storms. Results can be viewed spatially and in a tabular comparison for select locations (as shown on right). These results will help in understanding how fires can alter runoff and sedimentation regimes across burned areas and help prioritize resources to locations where the largest changes are predicted.

Risk Assessments of the Wildland-Urban Interface

The wildland-urban interface (WUI) is the area where houses and wildland vegetation meet or intermingle, and where wildfire problems are most pronounced

Resource managers can take proactive steps to plan for wildfires. The rapid hydrologic risk assessment methodology can be completed to assess potential vulnerabilities within a watershed prior to the occurrence of a wildfire. This knowledge can be invaluable to assess whether mitigation actions can be taken prior to the occurrence of a wildfire to avoid or reduce negative outcomes, and what hazards may arise when an area burns. The link between watershed management of fuel treatments on fire response can allow managers to compare the costs and benefits of various management scenarios. This type of modeling effort has the ability to provide a quantitative basis to inform ecologically minded forest management.

Section image credit: “Friday road Fire_2006” by U.S. Fish and Wildlife Service Southeast Region is licensed under the CC BY license.

Field-Based Watershed Assessment in Unburned or Burned Areas

cbec staff conducting a field-based watershed assessment

While models are useful to rapidly assess physical impacts of a fire, field-based efforts are essential to understand potential watershed vulnerabilities. cbec staff regularly conduct interdisciplinary watershed assessments focused on both characterizing watershed health and disturbances, and to identify enhancement opportunities within our water dependent ecosystems. cbec is well-versed in customizing its process-based Fluvial Audit methodology to meet a client’s goals and available funding. Fluvial Audits are typically conducted to gather fluvial geomorphology and habitat data. The process can also capture infrastructure locations, flow conveyance capacities, and conditions to inform resources management decision making. If Fluvial Audit data are collected pre event, a baseline condition can also be documented as a comparison tool against the post-fire landscape. This methodology is an optimal way to understand current channel conditions and associated ecological habitats, and how they might change following a fire.

Hydraulic Modeling

2D hydraulic model used to evaluate flood extent and velocities at a bridge crossing

In areas identified with post-fire hazards, focused hydraulic modeling can be employed to show changes in flood inundation extents for unburned or post-fire conditions, determine risk to specific infrastructure, or design post-fire mitigation strategies. cbec is experienced at incorporating past modeling actions, in addition to developing new models to address specific resource concerns. These analyses provide site-specific answers that a hydrologic model is not capable of generating (including the spatial extents of flood impacts, and flow characteristics including depths, velocities, and shear stresses near critical infrastructure). Hydraulic models are also excellent tools to evaluate whether mitigation efforts, such as the re-sizing of culverts and other infrastructure, are adequate to avoid flood impacts.

How can cbec help?

Many agencies such as the Department of the Interior BAER teams, CalFire, or the USGS, will provide some level of post-fire risk assessment within their jurisdiction. These services can be a helpful way to understand immediate threats to infrastructure and human life. However, such efforts can be limited due to
jurisdictional boundaries or limited resources following a fire. cbec aims to provide the long-term services that fill the critical data gaps needed for ecosystem recovery following a fire. With many projects directly within or downstream of recently burned areas, cbec is familiar with fire related impacts on the unique and diverse water dependent ecosystems throughout California and the western states. While many agencies may need to finish mitigation efforts prior to redeployment to the next fire, cbec can tailor an approach to satisfy specific client needs, especially as they relate to the longer-term goal of restoring ecosystem health and function.