Sediment Source and Storage Study for Disaster Planning
Proactive planning for wildfire and flood sediment in the North, Middle, and South Boulder Creek Watersheds
Introduction
Wildfires change soil characteristics and vegetative cover resulting in profound changes to hillslope and channel erosion and sedimentation processes. After a wildfire, runoff and erosion can become several orders of magnitude higher than in pre-fire conditions - leading to extreme flooding, erosion, and sediment deposition that can damage infrastructure and may endanger human lives. Even if not extreme, increased erosion and sedimentation following a wildfire can impact water quality and quantity, reduce the capacity of reservoirs, affect water delivery infrastructure, impair transportation infrastructure, clog culverts, and degrade aquatic habitat.
Anticipating and planning for where the effects of sediment can be mitigated is important information for pre- and post-wildfire planning and response.
This project's purpose is to identify locations--the "pockets" and alluvial fans--in the Boulder Creek Watersheds that are (or could be with the help of floodplain restoration) natural sediment deposition areas.
Physically, these areas act as energy and sediment "sponges" during both floods and after rain on burn scars.
Knowing where these areas are is important for three reasons:
1) We can target the areas for protection or stream and meadow restoration to further enhance their sponge characteristics.
2) These are inherently hazardous locations for development and providing incentives to direct investment and infrastructure into safer areas will provide for community resiliency and reduce damage from future disturbances.
3) We can identify watersheds and sub-watersheds that have very few or limited amounts of these natural sponges between the potential burn areas and sensitive infrastructure and target those for forest health/wildfire mitigation practices.
Some of these depositional areas are functional in their current state and could be targeted for protection via local land-use practices such as easements , Fluvial Hazard Zones (FHZ) , floodplain regulations , or other conservation strategies. Others are in need of rehabilitation and should be considered as candidates for watershed-based restoration and hazard mitigation funding. See the Next Steps section for more information.
Sediment in a Watershed
Watersheds produce and route sediment and debris from sources to temporary or long-term storage at a variety of temporal and spatial scales. Wildfires impact both the sediment source characteristics of the watershed and the sediment transport characteristics of the channel. Sediment that enters into a stream corridor is either stored in the channel or on the floodplain, or transported downstream where it may cause problems as it encounters homes, bridges, roadways, and water storage and supply systems.
Watershed Sediment Sources
Sediment inputs are shown in red on the figure to the right and can be approximated as follows: 1) sediment may come from upstream reaches and from tributaries via fluvial transport; 2) it may be recruited (eroded) from the channel, banks, or floodplain within the reach; 3) it may be recruited or added via debris flows or landslides or 4) it can be added to the stream system through surface erosion of the hillslope (e.g., rilling and gullying).
Generally, wildfire impacts on sediment supply can be generalized as: 1) increased surface erosion (i.e., rilling and gullying); 2) increased likelihood or size of debris flows; and 3) increased likelihood of sediment inputs from the channel banks, bed, and floodplain due to erosion and scour caused by excess runoff.
Generally, research has found that approximately 30% of the total sediment eroded in a post-wildfire flood comes from rilling and surface erosion while the remaining 70% comes from debris flows and channel erosion. Of that 70%, approximately half is from the debris flow and half is from channel and floodplain scour (Ellett, 2019; Wicherski, 2017).
The size of the sediment that enters into the stream corridor also matters. Sediment supplied by surface erosion is likely to be fine-grained such as sands, silts, clays, and organic matter while the material supplied by debris flows is also likely to include gravels, cobbles, boulders, and even car- or house-sized rocks.
Additionally, ash, trees, and other debris can be moved into stream systems though these same processes.
There are many tools that scientists and planners use to estimate the quantities of sediment that originate in watersheds before and after a wildfire. The following slides discuss some of these.
Figure from the Colorado Fluvial Hazard Zone (FHZ) Mapping Protocol Version 1.0 (Blazewicz et. al., 2020).
Rilling and Surface Erosion
Rilling and surface erosion can be roughly estimated using tools such as the Wildfire Erosion and Sediment Transport Tool (WESTT). These tools quantify the sediment loading and delivery to streams that originate on hillslopes outside of established channels.
The most useful aspect of these models, and the way that they are used in this study, is not the quantity of sediment that is estimated, but rather the identification of the areas, or sub-watersheds, that are likely to produce the most sediment if a burn were to occur. Locating these areas and linking them to any downstream sediment sinks can provide the beginnings of a prioritization structure for protection and restoration projects.
Because this source of sediment is usually fine-grained--and subsequently stays moving in water at lower velocities than coarser gravels and cobbles--deposition and storage of this material in valley bottoms likely requires stream reaches with very torturous flow paths, ponded or open water, heavily vegetated valley bottoms, large quantities of woody material, and multiple flowpaths throughout the floodplain.
Above: Example of estimated surface erosion quantities for the Calwood fire produced by Colorado Forest Restoration Institute (CFRI) in 2020.
Right: Photo of surface erosion and rilling from the Buffalo Creek fire, June 2018.
Debris Flows
A debris flow (commonly called a mud slide) is a moving mass of loose mud, sand, soil, rock, water and air that travels down a slope. To be considered a debris flow, the moving material must be loose and capable of “flow”, and more than half of the solids in the mass must be larger than sand grains, i.e., gravel-, pebble-, cobble- and boulder-sized material. The consistency of a debris flow is like that of pancake batter.
A mud flow is a mass of water and fine-grained earth materials that flows down a stream, ravine, canyon, arroyo, or gulch. To be considered a mud flow, more than half of the particles must be sand-sized or smaller and can flow very rapidly. A mud flow is the sandy, more water-saturated analog of a debris flow.
For more info on debris flows, check out the Colorado Geologic Survey's video here.
Initially, the material that is evacuated in a debfis flow is colluvium that has accumulated over hundreds of years in the hollows, nooks, and depressions on steeper hillside slopes. But when debris flows encounter established channels further down the slope, they can cause large amounts of scour in them as they erode and pick up material that has, over time, accumulated on the valley floor.
Debris flows typically occur during sizable precipitation events and while their likelihood is exacerbated by wildfire, a wildfire is not necessary for a debris flow to occur. Debris flow paths and debris flow probability in Boulder County are identified by the Colorado Geologic Survey (CGS) and the United States Geological Survey (USGS).
It is also possible to link areas susceptible to debris flows to downstream sediment sinks. Because debris flows can have very coarse material, the runout zone for the boulders and large cobbles is typically proximal to the debris flow path and limited in extent once the debris flow enters into larger river valleys. However, the finer materials (sands, silts, clays, gravels, and smaller cobbles) can move significant distances through the system. Similar to the hillslope erosion models, the debris flow maps can add to a prioritization structure for protection or potential meadow restoration projects by identifying sub-watersheds that have the most potential to contribute this type of sediment to the stream system.
Conceptually, the mixed composition of the debris flow sediment means that deposition and storage of the coarser source material in valley bottoms may not require the same degree of physical connectivity and roughness provided by vegetation and other biota as the sediment generated on hillslopes. But, because there is the potential for half of the debris flow to be fine-grained, trapping of this finer component likely still requires stream reaches with complex flow paths, high floodplain connectivity, ponded or open water, heavily vegetated valley bottoms, and large quantities of woody material.
Above: the debris flow susceptibility plate for Boulder County produced by the Colorado Geological Survey (CGS) in 2014 (Morgan, et al., 2014).
Above: An example debris flow probability map for the Calwood Fire produced by the United States Geological Survey (USGS) in 2020 .
Above: The USGS debris flow probability map with the CGS debris flow paths and runouts overlain for the Calwood Fire.
Right: The remnants of a large debris flow in the Left Hand Creek watershed (not associated with a wildfire).
Channel and Floodplain Erosion
Sediment is also recruited from the bed, banks, and floodplain of the stream system when higher energy flows pulse through the system. This erosion can be triggered by intense rainfall, high or fast snowmelt, and can be exacerbated by a wildfire in the watershed that changes the runoff characteristics of the land.
There are multiple ways of estimating sediment transport in a river system but there are few that are easy to deploy quickly and give reasonable estimates of channel and floodplain erosion in a deterministic manner. It is possible, however, to identify the land susceptible to erosion even if the exact quantities of eroded sediment are unknown.
Most sediment that is eroded from the bed, banks, and floodplain is sediment that was placed there by the same stream. This means that generally, the sediment is mobile and has the ability to move into downstream reaches.
Right: Photo of channel erosion in Fourmile Creek, September 2013.
Sediment Transport
Sediment and debris move through stream systems fed by the energy of the water. Conceptually, sediment transport is a function of: 1) the amount of water moving; 2) the slope of the valley or channel; 3) the width of the flow, which is either the width of the channel or valley; and 4) the amount of resistance to flow the landscape is providing through vegetation, surface roughness, woody debris, and channel shape.
Sediment and debris transport is based on thresholds: there is either enough energy in the flow to move a piece of material, or there is not.
Video: Post-fire flashflood in Huerfano County on August 3, 2020, two years after the Spring Creek Fire (Video Credit: Nate Bolin).
Sediment Deposition
Sediment drops out of transport when the energy of the flow decreases. These reductions in energy can be caused by dense vegetation, localized obstructions like culverts or diversion dams, changes in flow depth and direction, and changes in channel or valley width and slope.
Major sediment deposition occurs in "pockets" where valley or canyon widths increase and slopes decrease (Brogan, et al., 2019; Sholtes, et al., 2018; Yochum, et al., 2017) or on alluvial fans, which occur where steeper tributaries enter larger valley bottoms, or where the main stream channels exit the canyons of the Front Range and splay onto the piedmont and plains.
Sediment deposition after rain over the Hayden Pass burn scar in 2019.
Depositional Areas: Planning for Sediment in Valley Bottoms
The map below shows the mapped Active Stream Corridors (in pink) and the locations of large-scale, natural depositional areas throughout the Boulder Creek Watershed (shaded green).
Figure 1. Mapped Active Stream Corridors and Depositional Pockets.
Depositional Areas: Alluvial & Debris Fans
The map below shows fans throughout the Boulder Creek Watershed. The areas that are shaded yellow are major alluvial fans that have the potential to store large amounts of sediment. The orange diamonds mark smaller alluvial or debris fans which may still be possible to use for sediment storage. Boundaries and locations of the fans are approximate.
Figure 2. Mapped Fans.
Fans
Fans can either store or supply sediment to a stream corridor. As a channel incises or cuts into an existing fan, it will erode the bed and banks and provide sediment to downstream reaches. If flow spreads over all or part of the surface of the fan, routinely changes its preferential channel, or has the space to expand into the valley, the fan will become a sediment sink. Additionally, the coarser materials from debris flows will typically deposit in fans that exist or form where high-gradient flow paths enter into larger river valleys.
There are several alluvial fans in the upper reaches of the Boulder Creek watershed. Some of these fans are relicts of glacial melt and some have more contemporary origins. Today, most have a single channel incising into the fan making them overall sediment sources to the stream system.
It is possible that certain restoration activities could be taken to convert these fans from sediment sources to sediment sinks.
Managment Strategies
In addition to the Active Stream Corridor and depositional pockets, the maps below show two sediment sources in order to illustrate the sediment supply to these areas: 1) The red, orange, and yellow shading represents potential surficial erosion based on the Wildfire Erosion and Sediment Transport Tool Study completed by Colorado Forest Restoration Institute (CFRI) in collaboration with the City of Boulder in 2019 (Gannon et al., 2019) (note: this is a large data set and may take some time to load and only areas with potential for sediment yields greater than 20 metric tons per hectare delivered to the stream scenario are shown. For details on the model runs and assumed fire characteristics including wind speed, fuel moisture, ignition, duration, etc. please contact us) and 2) The brown areas are locations identified by the Colorado Geologic Survey as susceptible to debris flows (Morgan et al., 2014). The purple areas are the extents of fires that have already occured. A legend can be opened by clicking on the icon in the lower-left corner of any of these maps.
The shaded dark green areas are upstream of the largest depositional pockets in the Boulder Creek watersheds. Generally speaking, these are at/around the 8500' elevation contour, west of the Peak-to-Peak Highway, and are on the large, glacially-formed valley floors. On South and Middle Boulder Creek, especially, these areas would benefit from restoration actions that focus on establishing large-scale floodplain connectivity and riparian revegetation.
Figure 3. Higher elevation depositional pockets and locations for large-scale meadow restoration.
These depositional areas west of the Peak-to-Peak highway are particularly important. They have the ability to capture sediment from areas that, according to the CFRI erosion modeling, are especially apt to deliver large amounts of sediment to the stream corridors while also being upstream of: 1) Barker Reservoir, 2) Gross Reservoir, 3) Lakewood Reservoir, 4) The Town of Nederland's Water Intake, 5) The residential and commercial areas in the Town of Nederland and 6) The residential areas of Pactolis and Pinecliffe.
The map below shows the area (in orange) that drains directly to Barker Reservoir without first passing through an identified depositional pocket. We could consider the sediment generated in this zone to be unmitigated by natural floodplain and riparian process and if other factors aligned, prioritizing forest treatments in these areas--along with the protection, restoration, and rehabilitation of the North Beaver Creek and Middle Boulder Creek meadows--may give a holistic and multi-faceted wildfire sediment management strategy.
Lakewood Reservoir and the Town of Nederland water intake are downstream of at least one major depositional area.
Figure 4. The orange area denotes the land that drains to Barker Reservoir without first passing through a large, depositional pocket. The southern portions of this area have already had large-scale forest restoration work performed by both Boulder County and USFS, though the prioritization and implementation of this work was completed via an entirely different process.
Similarly, the map below shows the same "unmitigated" area upstream of Gross Reservoir. Note that this study did not examine the tributaries that drain into South Boulder Creek from the south (i.e., Gamble Gulch, Lump Gulch, and South Beaver Creek) for depositional pockets. Also, note that debris flow modeling was not completed for Gilpin County so the absence of mapped debris flow paths in the Gilpin County portions of the South Boulder Creek watershed does not denote an absence of debris flows.
Figure 5. The orange area denotes the land that drains to Gross Reservoir without first passing through a large, depositional pocket.
Capturing sediment in the canyon reaches (between the elevations of approximately 8400' and 6000') of these streams will be decidedly more difficult. There are many small, depositional pockets throughout the canyon reaches, however, these fluvially-formed valleys are narrower and steeper than their higher-elevation, glacial counterparts. While their ability to capture sediment from upstream areas is more limited (though notably, still beneficial), meadow and riparian restoration activities in these canyon pockets may be most valuable as a practice to prevent additional sediment from entering the stream through channel and floodplain erosion triggered by high-energy, post-wildfire flows and for capturing the coarser sediment provided by debris flows.
Generally, the depositional pockets in the canyon reaches are located in the upper two-thirds of the canyons or above elevations of about 6500'-7000'. This generally coincides with the elevations above the geologic knickpoints throughout these watersheds. No depositional pockets exist below 6000 feet; sediment generated in these lower canyon reaches will not encounter natural depositional areas until the creeks leave the canyons and enter into the City of Boulder.
Figure 6. Example of depositional pockets in the canyon reaches, Switzerland Park on North Boulder Creek and Roger's Park on Middle Boulder Creek.
The major assets that are at risk in and downstream of the canyon reaches are the highways (SH119, SH72, Fourmile Canyon Drive), the homes in the canyons, the water delivery infrastructure in Fourmile Canyon, and, notably, downtown Boulder and the water diversions on Middle Boulder Creek at and west of Broadway. It may be more beneficial to prioritize forest treatments in these lower reaches of the canyons in order to protect these assets.
There is an outlier to these discussions about the characteristics of the mid-elevation drainages. The Forsythe Canyon Watershed is a mid-elevation watershed that enters directly into the northwest portion of Gross Reservoir. Unlike most of the other smaller drainages east of the Peak-to-Peak Highway, there is potential for meaningful sediment deposition within the valleys of this smaller watershed.
Figure 7. Forsythe Canyon Watershed.
Next Steps
The next phase of the study should be to assess which depositional pockets have characteristics that will facilitate sediment deposition in their current states and what protection mechanisms are available to keep them that way and which are candidates for meadow and/or floodplain restoration. Evaluations of existing and potential connectivity as well as vegetation and biotic influence, along with land ownership, are needed to conclude which areas could or should be prioritized for protection, rehabilitation, and restoration. Some of this work can be informed through remote sensing but it should be supported by field evaluations.
Generalized Pre-Fire Sediment Study Arc. The steps in the blue box have been completed during this study. The management actions are conceptual, there may be areas where both forest health practices and floodplain restoration are recommended. And there may be many other reasons to implement forest management practices.
This study could also be expanded geographically to include the southern tributaries to South Boulder Creek and the reaches downstream of Gross Reservoir, and/or the streams in the St. Vrain or Big Thompson Watersheds.
Middle Boulder Creek
References
References and additional background information available upon request.
Blazewicz, M., Jagt, K. and Sholtes, J. (2020). Colorado Fluvial Hazard Zone Delineation Protocol Version 1.0. Colorado Water Conservation Board.
Brogan, D., Nelson, P., and MacDonald, L. (2019). Spatial and temporal patterns of sediment storage and erosion following a wildfire and extreme flood. Earth Surface Dynamics Discussions 7(2): 563–590. doi.org/10.5194/esurf-7-563-2019
Ellett, N. (2019). Partitioned by Process: Measuring Post-Fire Debris Flow and Rill Erosion with Structure From Motion. Thesis for Master of Science in Geoscience. Boise State University. Boise.
Gannon, B., Wei, Y., and Nelson, P. (2019). Wildfire Erosion and Sediment Transport Tool Analysis Report Version IV (for Boulder County). Colorado Forest Restoration Institute, Colorado State University. Fort Collins.
Morgan, M., White, J., Fitzgerald, F. S., Berry, K., and Hart, S. (2014). Foothill and Mountainous Regions in Boulder County, Colorado That May Be Susceptible to Earth And Debris/Mud Flows During Extreme Precipitation Events. Colorado Geologic Survey Open-file Report 14-02.
Sholtes, J., Bledsoe, B., Yochum, S., and Scott, J. (2018). Longitudinal Variability of Geomorphic Response to Floods. Earth Surface Processes and Landforms 43(15): 3099–3113. doi.org/10.1002/esp.4472
Wicherski, W., Dethier., D., and Ouimet, W. (2017). Erosion and channel changes due to extreme flooding in the Fourmile Creek catchment, Colorado. Geomorphology (294): 87-98. doi.org/10.1016/j.geomorph.2017.03.030
Yochum, S., Sholtes, J. Scott, J., and Bledsoe, B. (2017). Stream Power Framework for Predicting Geomorphic Change: The 2013 Colorado Front Range Flood. Geomorphology (292): 178–192.
Disclaimer
While this study relies on some of the methods outlined in the Colorado Fluvial Hazard Zone Protocol to determine the Active Stream Corridors, the information presented here IS NOT a Fluvial Hazard Zone delineation nor a comprehensive hazard study. During a flood event, it is expected that damage WILL OCCUR beyond the boundaries of the Active Stream Corridor and Fans shown herein.
