COL00721 – Wildfire Impacts on Peak Flows and Sediment Delivery: Implications for Irrigation Infrastructure and Management
Sponsoring Institution: National Institute of Food and Agriculture
Department of Civil and Environmental Engineering
Flooding, erosion, mass wasting, sedimentation and water quality impairment can all be major concerns after wildfire. Although considerable advances have been made in understanding post-fire runoff, erosion, and mass wasting at the plot to small watershed scale, many impacts of fires are felt further downstream through flooding and water quality problems. To improve understanding of wildfire effects at larger scales, we need studies that connect processes from precipitation to runoff generation, sediment erosion and transport, and water quality impairment and that connect across scales from hillslopes to watersheds. Such an understanding is increasingly important because it is anticipated that climate change will lead to more frequent and higher severity fires in the coming years.
We will conduct a case study of two watersheds in Northern Colorado that burned during the 2012 High Park Fire. Building on an existing monitoring network, we will establish nested monitoring of precipitation, runoff, erosion, channel change, and sediment delivery to quantify post-fire hydrologic and geomorphic response at scales from hillslopes to watersheds. The proposed case study will provide valuable insights on how regional climate patterns, topographic characteristics, and fire patterns interact to deliver peak flows and sediment downstream. On their own, these insights may help irrigation managers assess relative risks of fire on their operations and implement adaptive management strategies. Additionally, the multi-scale, multi-year dataset resulting from the proposed case study will provide a basis for developing a framework to assess the regional risk of wildfires on flooding, sediment production and delivery, and water quality impairment. Such a framework could be built into a tool for identifying the highest priority locations for pre-fire fuels treatments and post-fire restoration treatments. The field site and case study data will be incorporated into undergraduate and graduate science and engineering courses, and research results will be presented to the stakeholders through field tours and presentations at workshops and other meetings related to High Park Fire research and restoration.
Goals / Objectives:
The goal of the proposed work is to understand processes connecting burned upstream source areas with downstream sediment delivery, and is motivated by the following questions:
- How much sediment eroded from hillslopes after a wildfire is delivered to downstream locations?
- What controls these patterns of post-fire erosion, deposition, and sediment delivery?
- How long can we expect high peak flows and sediment loads to affect downstream infrastructure after a wildfire?
To answer these questions, we will conduct a case study with the following objectives:
- determine spatial and temporal patterns of erosion and deposition at scales from hillslopes to watersheds;
- relate these patterns of erosion and deposition to watershed topography, precipitation patterns, burn severity, and drainage network characteristics; and
- to determine how long elevated hillslope sediment production and downstream sediment delivery continue post-fire.
The results will help guide the most effective application of post-fire rehabilitation treatments, and users to better predict the downstream risks to agricultural and municipal water supply systems.
The basic design is to compare runoff, sediment production, and sediment delivery from two tributary watersheds to the Cache la Poudre River that burned in the 2012 High Park Fire, Hill Gulch and Skin Gulch. Precipitation over the study area will be characterized using rain gages and NEXRAD radar data. Runoff will be monitored with stage recorders at different points in the channel network. Erosion from hillslopes will be monitored using an existing network of sediment fences. Erosion and deposition within the channel network will be characterized using repeat surveys of channel cross sections and longitudinal profiles combined with topographic differencing of LiDAR digital elevation data available through a related research effort. Water quality and sediment delivery will be monitored at the outlets of the study watersheds using turbidimeters and suspended sediment samplers.
Data analyses will examine how the three primary response variables (peak flows, sediment transport, and turbidity) vary overtime and space. Over time, the measurement record will be separated into flow events, and statistical analyses will evaluate how the forcing variables (precipitation and snow melt) and response variables relate to one another for the series of events recorded. The temporal analysis will also examine how frequency distributions of peak flow magnitudes, sediment transport, and turbidity change over time since the fire. Over space, spatial data on watershed characteristics (size, shape, soils, elevations, slopes, burn severity, vegetation cover, and hydraulic connectivity of hillslopes) will be compiled, and statistical analyses will evaluate how these characteristics relate to runoff generation, sediment yields, and turbidity. Combining the temporal and spatial analyses, we will compile data collected from multiple scales in each study watershed and construct sediment budgets that illustrate how magnitudes of hillslope erosion change over time and how these relate to storage and transport of sediment through the channel network.
This project will provide valuable research experience for a Ph.D. student. The field sites will be used as a teaching laboratory for undergraduate and graduate science and engineering courses. The results of the project will be presented to the research and management community through a dissertation, presentations at scientific conferences, and publications in international journals.
- Undergraduate and graduate students in natural sciences and engineering (through formal classroom/field assignments and informal field tours);
- Regional forest and water supply managers at the USFS, NRCS, City of Fort Collins Utilities, City of Greeley, and Northern Colorado Water Conservancy District (through annual meetings, outreach programs, discussions and field tours);
- Land owners in the High Park Fire burn area (through outreach programs, informal discussions, and a public forum);
- Downstream water users affected by post-fire flooding and sedimentation (through outreach programs and a public forum).
Activities that will be achieved during the project include:
- Teaching: the field site and case study data will be used in undergraduate and graduate science and engineering courses. Students in CIVE 521 (Hydrometry) will use data collected during this project for applied laboratory projects. Kampf will present the project as a case-study on post-fire erosion and channel change in WR/GR 304 (Sustainable Watersheds). Students in WR416 (Land Use Hydrology) will visit the study area during a class field trip.
- Mentoring: A Ph.D. student will be responsible for conducting the case study and will receive mentoring throughout the project.
- Field work: The case study will include the collection and analysis of data describing the study sites’ topography, rainfall, runoff, and sediment production and delivery.
- Outreach: Public outreach will be an integral component of the project. The team maintains active communication with USFS, City of Fort Collins Utilities, and NRCS and will coordinate field visits and research plans with these groups and other stakeholders identified. Results will be presented to these and other stakeholder groups through field tours and presentations at workshops and other meetings related to High Park Fire research and restoration. Our team also collaborates with K-12 outreach programs developed at the Natural Resources Ecology Laboratory in collaboration with science teachers in residence at NREL. We will organize a public forum to present research findings to landowners and water users affected by the burn.
Products that will be achieved during the project include:
- Graduate a Ph.D. student in Civil and Environmental Engineering;
- The Ph.D. student’s dissertation;
- 2-3 presentations at international conferences, such as the American Geophysical Union Fall Meeting;
- 2-4 articles published in international scientific journals, describing scientific outcomes as well as new methods that will be developed (e.g. LIDAR differencing, modeling sediment delivery at watershed scale).
A major outcome of this project will be the improved scientific understanding of how wildfire affects peak flows and sediment delivery at the watershed scale (> 5 km2). The research will contribute to process-level understanding of post-fire geomorphic change on hillslopes and longitudinally through channel networks, and it will develop new methods for connecting hillslope and channel processes to watershed-scale predictions of sediment delivery.
Our results will help inform and increase the efficiency of post-fire mitigation treatments, indicating which parts of the landscape are at most at risk to erosion/sedimentation/water quality impairment.
Engagement with the NRCS and City of Fort Collins Utilities will help ensure that findings can be actively used for future post-fire mitigation plans. Engagement with landowners and water users will help identify key user needs for information about post-fire risks.
Keywords: Wildfire, Runoff, Sediment, Erosion, Precipitation, Watershed, Water Quality, Geomorphic response, Hydrologic Response, Fire Effects, Sediment Transport, Sediment Production, Sediment Delivery, Post-Fire Runoff, Post-Fire Erosion, Turbidity, Water Quality Impairment