HomeMy WebLinkAbout1.06 Exhibit F - Debris Flow Hazard ReviewHalo Development Partners, LLC
AMI’S ACRES MUDFLOW ANALYSIS
#100-WTR-T44776
March 18, 2026
Halo Development Partners, LLC
This page intentionally left blank
Halo Development Partners, LLC
iii
AMI’S ACRES MUDFLOW ANALYSIS
#100-WTR-T44776
March 18, 2026
PRESENTED TO: PRESENTED BY:
Halo Development Partners, LLC
1401 21st St
Sacramento, CA 95811
P +1 (703) 887-4114
Tetra Tech
2100 Osuna Road, Suite 100
Albuquerque, NM 87113
P +1 (505) 881-3188
Tetratech.com
Halo Development Partners, LLC
iv
This page intentionally left blank.
Halo Development Partners, LLC
v
CONTENTS
EXECUTIVE SUMMARY ............................................................................................................................. IX
INTRODUCTION ................................................................................................................................... 2
MUDFLOW ANALYSIS ........................................................................................................................ 5
MUDFLOW CHARACTERIZATION AND PROCESSES ................................................................. 5
FLO-2D MODEL ............................................................................................................................... 8
Model Extents and Topography .......................................................................................... 8
Overland Roughness Values ............................................................................................ 10
Infiltration ........................................................................................................................... 11
Channels ........................................................................................................................... 14
Reduction Factors and Walls ............................................................................................ 14
HYDROLOGY ................................................................................................................................ 14
MUDFLOW HYDROGRAPHS ....................................................................................................... 19
EXISTING CONDITIONS MODEL ................................................................................................. 24
Existing Conditions Model Results .................................................................................... 24
PROJECT CONDITIONS ............................................................................................................... 31
Project Condition Channels .............................................................................................. 35
Debris Basins .................................................................................................................... 36
Project Conditions Model Results ..................................................................................... 37
Debris Basin Maintenance ................................................................................................ 48
RECOMMENDATIONS ...................................................................................................................... 52
REFERENCES .................................................................................................................................... 53
TABLES
Table 2-1. Mudflow behavior as a function of sediment concentration (FLO-2D Manual, 2021)
6
Table 2-2. Manning's n Roughness Values .......................................................................... 10
Table 2-3. Horton’s Infiltration Parameters for different hydrologic soil groups .................... 12
Table 2-4. NRCS Soil types .................................................................................................. 12
Table 2-5. Peak rainfall for the 2-hour, 25 and 100-year events (inches) ............................. 14
Table 2-6. Summary of predicted peak clearwater flow and volume for the 25-, 100-, and 200-
year rainfall events. ..................................................................................................................... 18
Table 2-7. Summary of 2-hr 25-, 100-, and 200-yr peak clearwater flow and volume, bulked
volumes, and average sediment concentrations for the mudflow sources ................................. 23
Table 2-8. Predicted storage of each proposed debris basin for the 2-hour 25-year rainfall
event. This design quantities are conceptual in nature and require refinment. ........................... 40
Table 2-9. Predicted reduction in flow volume crossing onto the Frontage Road and I-70
between EC and PC models for the 25-, 100-, and 200-year events. ........................................ 40
Table 2-10. Predicted maximum velocity at DB1 outlet channel for Existing and ....................... 41
Halo Development Partners, LLC
vi
Table 3-1. Freeboard and Safety Factor Recommendations (copied from FEMA 2012) ..... 52
FIGURES
Figure 1-1. Existing site of the Ami’s Acres campground west of Glenwood Springs, CO with
cross-sections indicated for hydrgraph collection from clearwater model and identified inflow
points for mudflows. ...................................................................................................................... 3
Figure 1-2. Proposed housing development plans overlaid with property boundaries, existing
channels, and inflow locations. ..................................................................................................... 4
Figure 2-1. Classification of hyper-concentrated sediment flows (modified from FLO-2D Manual
2014). ............................................................................................................................................ 7
Figure 2-2. Extents of the FLO-2D model overlaid with basin elevations/contours and showing
the project extent boundary. ......................................................................................................... 9
Figure 2-3. Land-use zones used to assign Manning’s n-values to the Existing Conditions
FLO-2D model. ........................................................................................................................... 11
Figure 2-4. NRCS Soil Mapping. ........................................................................................... 13
Figure 2-5. Rainfall distribution for the 2-hour, 25-, 100-, and 200-year rainfall events. ........ 15
Figure 2-6. Predicted flow hydrographs for the 2-hour, 25-year rainfall event for for West,
Central, and East sub-basins. ..................................................................................................... 16
Figure 2-7. Predicted flow hydrographs for the 2-hour, 100-year rainfall event for West,
Central, and East sub-basins. ..................................................................................................... 17
Figure 2-8. Predicted flow hydrographs for the 2-hour, 200-year rainfall event for West,
Central, and East sub-basins. ..................................................................................................... 18
Figure 2-9. Clearwater hydrograph, sediment concentration (Cv) and bulked sediment
hydrographs for West Mudflow Source for the 2-hour, 25-year rainfall event. ............................ 20
Figure 2-10. Clearwater hydrograph, sediment concentration (Cv) and bulked sediment
hydrographs for Central mudflow source for the 2-hour, 25-year rainfall event. ......................... 21
Figure 2-11. Clearwater hydrograph, sediment concentration (Cv) and bulked sediment
hydrographs for East mudflow source for the 2-hour, 25-year rainfall event. ............................. 22
Figure 2-12. Maximum mudflow depth for the 2-hour 25-year rainfall event under Existing
Conditions. The design plan is shown for reference purposes only. ........................................... 25
Figure 2-13. Maximum mudflow velocity for the 2-hour 25-year rainfall event under Existing
Conditions. The design plan is shown for reference purposes only. ........................................... 26
Figure 2-14. Maximum mudflow depth for the 2-hour 100-year rainfall event under Existing
Conditions. The design plan is shown for reference purposes only. ........................................... 27
Figure 2-15. Maximum mudflow velocity for the 2-hour 100-year rainfall event under Existing
Conditions. The design plan is shown for reference purposes only. ........................................... 28
Figure 2-16. Maximum mudflow depth for the 2-hour 200-year rainfall event under Existing
Conditions. The design plan is shown for reference purposes only. ........................................... 29
Halo Development Partners, LLC
vii
Figure 2-17. Maximum mudflow velocity for the 2-hour 200-year rainfall event under Existing
Conditions. The design plan is shown for reference purposes only. ........................................... 30
Figure 2-18. Proposed FLO-2D terrain mods to contain mudflows at the west, central, and
eastern mudflow sources. ........................................................................................................... 32
Figure 2-19 Proposed project grading provided by client merged with terrain modifications and
Garfield County 2016 LiDAR. ...................................................................................................... 33
Figure 2-20. Land-use zones used to assign Manning’s n-values to the Project Conditions
FLO-2D model. 34
Figure 2-21. Channel profile extending northwestern debris basin (DB1) down to the
southwestern debris basin (DB2). The main channel averaged approximately 6 feet of depth. . 35
Figure 2-22. Cross section of the terrain modifications for the proposed channel out of the
northwest debris basin (DB1). The orange line shows the Existing conditions (unmodified) terrain,
while the blue line shows the proposed channel and the graded terrain. ................................... 36
Figure 2-23. Profile showing the proposed embankment and overflow section for DB1. The
orange line shows the Existing conditions (unmodified) terrain, while the blue represents the top
elevation of the embankment with the weir. ................................................................................ 37
Figure 2-24. Flood plain cross sections used to compute flow hydrographs. .......................... 38
Figure 2-25. Comparison of discharge (cfs) vs time (hrs) from Cross Section 14 in the Existing
and Project Condtions for the 25-year mudflow. ......................................................................... 39
Figure 2-26. Comparison of total volume through each cross section for the Existing and Project
Conditions in the 25-year mudflow. ............................................................................................. 41
Figure 2-27. Predicted maximum mudflow depth for the 2-hour 25-year rainfall event under
Project Conditions. ...................................................................................................................... 42
Figure 2-28. Predicted maximum mudflow velocity for the 2-hour 25-year rainfall event under
Project Conditions. ...................................................................................................................... 43
Figure 2-29. Predicted maximum mudflow depth for the 2-hour 100-year rainfall event under
Project Conditions. ...................................................................................................................... 44
Figure 2-30. Predicted maximum mudflow velocities for the 2-hour 100-year rainfall event under
Project conditions. ....................................................................................................................... 45
Figure 2-31. Predicted maximum mudflow depths for the 2-hour 200-year rainfall event under
Project Conditions. ...................................................................................................................... 46
Figure 2-32. Predicted maximum mudflow velocities for the 2-hour 200-year rainfall event under
Project Conditions ....................................................................................................................... 47
Figure 2-33. Predicted maximum mudflow depths for the 2-hour 25-year rainfall event under
Project Conditions with DB1 filled. ............................................................................................. 49
Figure 2-34. Predicted maximum mudflow depths for the 2-hour 100-year rainfall event under
Project Conditions with DB1 filled. ............................................................................................. 50
Figure 2-35. Predicted maximum mudflow depths for the 2-hour 200-year rainfall event under
Project Conditions with DB1 filled. ............................................................................................. 51
Halo Development Partners, LLC
viii
This page intentionally left blank.
Halo Development Partners, LLC
ix
EXECUTIVE SUMMARY
Halo Development Partners, LLC is proposing to construct a residential housing development at
the site of an existing campground located west of Glenwood Springs, Colorado and north of
Interstate 70 (I-70). The Project is located near the base of Storm King Mountain on a series of
alluvial fans. Tetra Tech was contracted by Halo Development Partners to perform a mudflow 1
analysis to characterize the mudflow risk and develop mitigation recommendations.
A site investigation was carried out in November 2025 by Dr. Dai Thomas of Tetra Tech. Three
main flow paths were identified. The largest flow path is located near the western boundary of the
property, and there is evidence of historic debris flows. Debris flows from the western channel
flows onto the Ami’s Acres property and is predicted to flow onto the neighboring property to the
west. The other two drainages are relatively small and are identified as the Central and Eastern
sources on Figure 1-1.
This analysis was performed in advance of the final grading and drainage plans for the project
site. The ‘Project Conditions’ FLO-2D model is based on assumptions regarding sizes, locations
and maintenance of swales and debris basins aligned to route and attenuate mudflows to levels
within acceptable standards of the industry as established by FEMA, without increasing risks on
offsite properties, including onto I-70. Conclusions, calculations and results of this analysis are
subject to modification pending final design of drainage facilities and a final execution of the
mudflow analysis.
The development (Project) includes: (1) re-grading the ground, (2) construction of debris flow
basins and channels, (3) construction of roads, (4) and residential dwellings.
Tetra Tech developed FLO-2D models of the Existing conditions (EC) and Project conditions (PC)
to simulate the mudflows. The FLO-2D domain covers the basin of the Project Site, extending
roughly 66 acres from I-70 up the south-eastern side of Storm King Mountain. The model has a
5-foot grid size and contains 114,324 grid cells. The Project Site is at an elevation of
approximately 5,890 feet and the elevation at the top of the basin is approximately 6,847 feet,
with slopes greater than 35-40 degrees in some areas, particularly on the northern side of the
basin.
Two-hour hyetographs were developed to represent the short-duration thunderstorm events for
the 25-, 100-, and 200-year peak rainfall events based on the Colorado Unit Hydrograph
Procedure (CUHP) developed by the Urban Drainage and Flood Control District (UDFCD, 2014.
The 2-hour 25-, 100-, and 200-year rainfalls are 1.22, 1.56, and 1.71 inches, respectively.
A digital terrain model (DTM) was developed to represent the Existing conditions based on 2016
Garfield County LiDAR mapping that was obtained from the USGS, and from topographic
mapping collected in November 2025 by SurvCo of Glenwood Springs. The 2016 DTM and 2025
survey data were merged into one DTM that represents the Existing site conditions. The DTM has
1 The terms mudflow and debris flow are commonly used interchangeably in engineering
literature.
Halo Development Partners, LLC
x
a 2-foot pixel resolution and was used to assign elevations to the FLO-2D grid elements. The
Project Condition DTM was a mosaic of the 2016 Garfield County LiDAR, the November 2025
Survey, and the proposed graded terrain provided by the client on March 3rd, 2026.
Manning’s n-values roughness polygons were delineated in QGIS, a Geographic Information
Software (GIS) program, to represent zones with similar roughness characteristics based on
aerial photography, field reconnaissance, and site plans. The specific Manning’s n-values were
assigned to the roughness polygon based on guidance in the FLO-2D manual (FLO-2D, 2021).
Soil infiltration losses in FLO-2D were estimated using Horton’s soil-infiltration method (Horton,
1933). Soil mapping was obtained from the Natural Resource Conservation Service (NRCS) and
Horton infiltration values were assigned to each soil group.
The Existing conditions model was initially run using the rainfall/infiltration capabilities for the 2-
hour, 25-, 100-, and 200-year rainfall events to predict the clearwater flow hydrographs at three
inflow locations that debris flow could potentially originate from. The clearwater hydrographs were
then bulked to represent mudflow using a variable sediment concentration hydrograph.
The FLO-2D model was run using the mudflow capabilities for the Existing and Project conditions
for the following hydrologic scenarios:
• 2-hour 25-year runoff hydrograph with sediment bulking up to 45-percent by volume
• 2-hour 100-year runoff hydrograph with sediment bulking up to 20-percent by volume
• 2-hour 200-year runoff hydrograph with sediment bulking up to 20-percent by volume
The Existing conditions model was developed to represent the current topography, including the
recent site development of the property to the west of Ami’s Acres. Grid elevations were assigned
to FLO-2D grid using the Existing conditions DTM. Area reduction factors were applied to the
model to reflect the loss of storage and blocking of flow from the buildings, and roughness values
were changed to reflect the development and land cover.
The Existing conditions DTM and FLO-2D model were modified to reflect the Project conditions.
The DTM was modified to represent the proposed grading for the project area and construction
of debris basins and channels to route the mudflow away from the proposed lots. The proposed
western channel will drain from the main northwestern detention basin and convey flows in an
approximately southerly direction along the property boundary and into a second smaller debris
basin. Flows from the second basin will exit into another constructed channel that flows
approximately southwest, then into an existing large riprapped lined located on the neighboring
property. For the other two smaller mudflow sources, detention basins are recommended on the
uphill side of the proposed development, with site grading. The central basin includes a short
channel to direct flow away from the property lot to the west. The Project conditions DTM was
used to assign elevations to the model, and the Manning’s n values applied to the model were
adjusted to represent the proposed design.
The EC and PC models were run over the same 25-year, 100-year, and 200-year peak flow
hydrographs, and the model outputs were used to develop inundation mapping, evaluate the
changes in mudflow depths and velocities, and evaluate the reduction in flow onto the Frontage
Road and I-70.
Halo Development Partners, LLC
xi
Following is a summary of the predicted model results:
• The proposed sediment basins are located to capture sediment and convey the flow into
constructed channels and away from the proposed development. It is anticipated that the
sediment basins will store the large sediment material, and flow leaving the sediment
basin will contain much lower sediment concentrations.
• The proposed sedimentation basins store sediment and reduce the volume of sediment
flowing off the south boundary of the property and onto the frontage road and I-70.
• The proposed project confines flooding on the neighboring property to the west to an
existing channel.
• It will be necessary to clean and maintain the debris basins to prevent mudflow onto
properties, particularly those near the Debris Basin 1 in the northwest corner of the
development.
Halo Development Partners, LLC
2
INTRODUCTION
Halo Development Partners is proposing to construct a residential housing development in
Garfield County, west of Glenwood Springs, Colorado and north of Interstate 70 (I-70) (Figure
1-1). The property is currently an existing privately owned and operated campground. The Project
site is located near the eastern side of Storm King Mountain at the base of a series of alluvial
fans. Per Garfield County Section 7-207, part E (Garfield County, 2013) Development shall only
be permitted to occur in an alluvial fan if the Applicant demonstrates that the development cannot
avoid such areas, and the development complies with the following minimum requirements and
standards, as certified by a qualified professional engineer, or qualified professional geologist,
and as approved by the County:
1. Development shall be protected using structures or other measures on the uphill side
of that channel, dam, or divert the potential mud or debris flow.
2. Disturbance shall be prohibited in the drainage basin above an alluvial fan, unless an
evaluation of the effect on Runoff and stability of the fan and on the ground water recharge
area shows that disturbance is not substantial or can be successfully mitigated.
As such, a mudflow analysis is required to analyze development risks. This report does not assess
avalanche, rockfall and landslide hazards. Stormwater and drainage designs are not included in
the Existing or Project condition models.
The proposed design includes:
• re-grading the ground
• construction of sediment (debris) basins
• construction of a new channel
Tetra Tech developed FLO-2D models (FLO-2D, 2021) of the Existing and Project conditions to
perform the mudflow modeling. The models were initially run using a rain-on-grid method using
the rainfall/infiltration capabilities for the 2-hour, 25-, 100-, and 200-year rainfall events to predict
the clearwater flow hydrographs at three locations that debris flow could potentially originate from.
The hydrograph inflow sources are identified as the Western, Central and Eastern sources on
Figure 1-1. The clearwater hydrographs were then bulked to represent mudflow using a variable
sediment concentration hydrograph. These bulked hydrographs were then applied as inflows to
the mudflow simulations (Figure 1-1). The inflow locations were moved a short distance upstream
from their computed point to a confined section of channel to better allow the model to predict the
spread of mudflow in the channel and on the fan.
The Existing and Project conditions mudflow models were run for the following hydrologic
scenarios:
• 2-hour 25-year runoff hydrograph with sediment bulking up to 50-percent by volume
• 2-hour 100-year runoff hydrograph with sediment bulking up to 20-percent by volume
• 2-hour 200-year runoff hydrograph with sediment bulking up to 20-percent by volume
The Project impacts were evaluated by comparing the differences in the Existing and Project
conditions model output, including changes in mudflow depths, velocities, and flow.
Halo Development Partners, LLC
3
Figure 1-1. Existing site of the Ami’s Acres campground west of Glenwood Springs, CO with
cross-sections indicated for hydrgraph collection from clearwater model and
identified inflow points for mudflows.
Halo Development Partners, LLC
4
Figure 1-2. Proposed housing development plans overlaid with property boundaries, existing
channels, and inflow locations.
Halo Development Partners, LLC
5
MUDFLOW ANALYSIS
The Existing conditions FLO-2D model was initially run using the rainfall/infiltration capabilities to
predict the clearwater flow hydrographs at three sub-basins that were identified as having the
potential to create mudflows. The sub-basin inflows are named West, Central, and East based on
location (Figure 1-1).
The model was run for the 2-hour, 25-, 100-, and 200-year rainfall events. The predicted flow
hydrographs were modified to represent the sediment bulking that could potentially occur during
mudflow events. The Existing and Project conditions models were then run for the 2-hour, 25-,
100-, and 200-year mudflow hydrographs and the model output were compared to evaluate the
impacts of the project.
MUDFLOW CHARACTERIZATION AND PROCESSES
Hyper-concentrated sediment flows (mudflows and mud floods) are part of a continuum in the
“physics of flowing water and sediment movement that ranges from clear water flow to mass
wasting processes (landslides)” (SLA and O’Brien 1989). In general, the sediment-transport
characteristics in the continuum range from suspended and bed load transport in water floods to
mass wasting in landslide events. The National Research Council Committee (NRC, 1982)
proposed four categories to delineate this continuum: water floods, mud floods, mudflows, and
landslides (Table 2-1). The bounds of each of these categories can be approximated based on
the fluid properties, and specifically by the sediment concentration (by volume) of the fluid (Figure
2-1).
The sediment concentration of fluid is defined as the ratio of the sediment volume to the water
volume and is given by:
CV = Volume of Sediment / (Volume of Water + Volume of Sediment)
The continuum indicates that water floods are mostly comprised of water with some sediment
(low concentration of sediment), whereas landslides are mostly comprised of bulk sediment with
some water (high concentration of sediment). The concentration of the sediment is an important
component in determining the physical processes that govern the behavior of the fluid-sediment
mixture in each of these categories. For example, the flow characteristics of a mud flood are
dominated by the turbulent and viscous forces within the fluid matrix, whereas movement of a
landslide is dominated by the dispersive stresses and particle friction.
This study focuses on the sediment-transport characteristics of the mud flood and mudflow
categories. However, a brief description of the sediment transport characteristics of the water
floods and land sliding events is presented for the purpose of describing the bounding categories.
Flood flows generally have sediment concentrations of less than 20 percent (by volume). They
are essentially water floods with high bed load and suspended loads where the bed load may be
affected by the high concentration of suspended load (i.e., fine sediment wash load). The
sediment-transport characteristics of water floods are modeled using conventional bed- and
suspended-load formulas and methodologies.
Halo Development Partners, LLC
6
Table 2-1. Mudflow behavior as a function of sediment concentration (FLO-2D Manual, 2021)
Event Sediment Concentration Flow Characteristics
by Volume by Weight
Landslide 0.65 - 0.80 0.83 - 0.91 Will not flow; failure by block sliding
0.55 - 0.65 0.76 - 0.83 Block sliding failure with internal deformation during
the slide; slow creep prior to failure
Mudflow 0.48 - 0.55 0.72 - 0.76 Flow evident; slow creep sustained mudflow; plastic
deformation under its own weight; cohesive; will not
spread on level surface
0.45 - 0.48 0.69 - 0.72 Flow spreading on level surface; cohesive flow;
some mixing
Mud Flood 0.40 - 0.45 0.65 - 0.69 Flow mixes easily; shows fluid properties in
deformation; spreads on horizontal surface but
maintains an inclined fluid surface; large particle
(boulder) setting; waves appear but dissipate rapidly
0.35 - 0.40 0.59 - 0.65 Marked settling of gravels and cobbles; spreading
nearly complete on horizontal surface; liquid surface
with two fluid phases appears; waves travel on
surface
0.30 - 0.35 0.54 - 0.59 Separation of water on surface; waves travel easily;
most sand and gravel has settled out and moves as
bed load
0.20 - 0.30 0.41 - 0.54 Distinct wave action; fluid surface; all particles
resting on bed in quiescent fluid condition
Water Flood < 0.20 < 0.41 Water flood with conventional suspended load and
bed load
Halo Development Partners, LLC
7
Landslides generally have sediment concentrations greater than 55 percent (by volume) and are
considered as bulk solid movement as opposed to fluid motion. Landslides may range from slow-
moving earth flow and creeping soil masses to rapid rotation or slippage failures. Hyper-
concentrated sediment flows are defined as flood events with sediment concentrations that range
between approximately 20 and 55 percent by volume. However, the sediment concentration for a
given event is generally considered to be between 20 and 50 percent (O’Brien, 2004). The fine
sediment concentration (silt, clay and fine sands in the fluid matrix) controls the properties of the
fluid, including, viscosity, density, and yield stress. Mudflows are non-Newtonian and they have
much higher viscosities and densities compared to water flows. These properties result in
mudflows having significantly slower velocities compared to water floods on the same slope. The
fine sediments increase the density of the fluid matrix, which increases the buoyancy of sediments
thereby creating conditions that allow gravel to boulder-sized material to be transported near the
flow surface by mudflows. The yield stress is a measure of the internal fluid resistance to flow and
affects both the initiation and cessation of flows. For the purposes of this report, all hyper-
concentrated sediment flows (mud floods and mudflows) will be referred to as mudflows.
The sediment matrix of a hyper-concentrated flow is non-homogeneous, and the sediment
properties change significantly as they flow down steep channels or across alluvial fans. As the
mudflow moves over the alluvial fan, dewatering of the fluid matrix can occur by infiltration and
Figure 2-1. Classification of hyper-concentrated sediment flows (modified
from FLO-2D Manual 2014).
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Co nce n tra tio n by Vo lu me (C V)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Co
n
ce
ntrati
o
n by Weig
ht (C
W)
Water Flow
Transition Zone
M ud Flood
M u d Flo w L a n d s li d e
Halo Development Partners, LLC
8
escapement to the surface. This may further increase the concentration of the hyper-concentrated
sediment flows and alter the transport characteristics of the flow. “Almost all hyper-concentrated
sediment flows are fully turbulent, unsteady and non-uniform, and are characterized by surging,
flow cessation, blockage, and roll waves” (SLA and O’Brien, 1989).
During a mudflow event, the average sediment concentration over the duration of the hydrograph
generally ranges between 20 and 35 percent by volume with peak concentrations approaching
50 percent (Figure 2-1 Table 2-1). Most basins with a history of mudflow events will eventually
develop a sediment supply in the channel bed such that even relatively small rainfall-runoff storms
may generate mudflow surges. In general, mudflows have a distinct pattern of flood evolution.
Initially, clear water flows from the basin rainfall-runoff may arrive at the fan apex. This may be
followed by a surge or frontal wave of mud and debris (40- to 50-percent concentration by
volume). When the peak arrives, the average sediment concentration generally decreases to the
range of 30 to 40 percent by volume. On the falling limb of the hydrograph, the sediment
concentration decreases due to the reduced availability of sediment; however, surges of higher
sediment concentration may occur.
FLO-2D MODEL
FLO-2D is a 2-dimensional (2-D) hydraulic model that was developed to perform both clearwater,
sediment-transport, and hyper-concentrated sediment flow-routing in channels and/or on alluvial
fans with an unconfined flow path. The model utilizes a volume conservation scheme to simulate
both sub- and super-critical flows in the channel or floodplains, as well as flows exiting from the
channel to the floodplain, and vice versa. Overland flow is modeled using a 2-D diffusive wave
approximation of the momentum equation. A central difference routing scheme with eight potential
flow directions is used to simulate the progression of the flood wave hydrograph over a system of
square grids.
The FLO-2D model contains several components that are used to represent and model the
complex topography and processes, including: channel-floodplain flow exchange, loss of storage
due to buildings, flow obstructions, simulation of hydraulic structures, simulation of street flow,
and simulation of hyper-concentrated sediment flows (mudflows). Hyper-concentrated sediment
flow is simulated by the FLO-2D model using a quadratic rheological model that includes viscous
stress, yield stress, turbulence, and dispersive stress terms as a function of sediment
concentration. FLO-2D does not have the ability to model unsteady phenomena such as surging.
It also assumes a rigid boundary, and therefore, does not model channel, overbank, or fan
degradation.
MODEL EXTENTS AND TOPOGRAPHY
The model domain spans the basin of the Project Site, extending roughly 670 acres from I-70 up
the southern side of Storm King Mountain (Figure 2-2). The model has a 5-foot grid size and
contains grid cells. The Project site is at an elevation of approximately 5,775 feet and the elevation
at the top of the basin is approximately 6,847 feet, with slopes greater than 30-40 degrees in
some areas, particularly on the western side of the basin.
A digital terrain model (DTM) was developed to represent the Existing conditions based on 2018
LiDAR mapping that was obtained from the USGS and from topographic mapping provided by
SurvCo (2025). The DTM has a 2-foot pixel resolution and was used to assign elevations to the
grid elements.
Halo Development Partners, LLC
9
Figure 2-2. Extents of the FLO-2D model overlaid with basin elevations/contours and
showing the project extent boundary.
Halo Development Partners, LLC
10
OVERLAND ROUGHNESS VALUES
Manning’s n-values roughness polygons were delineated in using a Geographic Information
Software (GIS) program, to represent zones with similar roughness characteristics based on
aerial photography, field reconnaissance, and site plans. The specific Manning’s n-values were
assigned based on guidance in the FLO-2D manual (FLO-2D, 2021).
The n-values assigned to these areas ranged from 0.02 for roads, which are relatively smooth, to
0.35 for medium forest. Land classifications and associated n-values are provided in Table 2-2
and the land use mapping is shown in Figure 2-3.
Table 2-2. Manning's n Roughness Values
Land Use Manning’s n Roughness Value
Urban/Structures 0.04
Debris Basin/
Channel 0.04
Roads/Streets 0.02
Medium Forest 0.35
Alluvial Fan 0.20
Scrub/Shrub 0.30
Campground 0.25
Graded Area 0.2
Halo Development Partners, LLC
11
Figure 2-3. Land-use zones used to assign Manning’s n-values to the Existing Conditions
FLO-2D model.
INFILTRATION
Soil infiltration losses in FLO-2D were modeled using Horton’s soil-infiltration (Horton 1933),
which consists of an initial infiltration rate (fi), a final infiltration rate (f0), and a decay coefficient
(a) (Table 2-3). The Colorado Urban Hydrograph Procedure (CUHP) program lists a range of
values for various Horton parameters for given NRCS Hydrologic Soil Groups (UDFCD, 2005). In
general, the values for each parameter vary with soil type (for example clay, loam, sand, bedrock)
and degree of saturation (dry, moist). The soil infiltration properties applied to each sub-basin
were based on Hydrologic Soil Groups provided with the NRCS soil mapping.
Halo Development Partners, LLC
12
The majority of the soils are classified as Hydrologic Soil Group (HSG) C (Table 2-4 and Table
2-5). Hydrologic Soil Group C is consists of soils that have a slow infiltration rate when thoroughly
wet.
Table 2-3. Horton’s Infiltration Parameters for different hydrologic soil groups
Hydrologic Soil
Group
Initial Rate (in/hour) Final
Rate
(in/hour)
Decay Coefficient
(1/second)
A 5.0 1.0 0.0007
B 4.5 0.6 0.0018
C 3.0 0.5 0.0018
D 3.0 0.5 0.0018
Table 2-4. NRCS Soil types
Map Unit Name HSG
Torriorthents-Rock outcrop complex, steep C
Atencio-Azeltine complex, 1 to 3 percent slopes C
Arle-Ansari-Rock outcrop complex, 12 to 65 percent slopes C
Halo Development Partners, LLC
13
Figure 2-4. NRCS Soil Mapping.
Halo Development Partners, LLC
14
CHANNELS
Channel elements were not used in the Existing conditions FLO-2D model.
REDUCTION FACTORS AND WALLS
Other model parameters typically applied to FLO-2D models include width reduction factors
(WRFs) and area reduction factors (ARFs). The WRFs and ARFs are assigned to the FLO-2D
grid elements to represent the blockage of flow paths and reduction in storage that mostly occur
due to the presence of buildings. Width reduction factors are applied to represent the blockage of
flow through the side of an element, while ARFs represent the loss of floodplain storage volume
due to the buildings. For example, a wall might obstruct 40 percent of the flow width of a grid
element side, and a building could cover 75 percent of the grid element.
Existing structures were digitized from aerial imagery (Google Earth) (Figure 1-1). A shapefile
was generated from these structures and was used to assign the ARFs to grid elements using
the model development tools in FLO-2D for the Existing Conditions. Houses that overlapped with
the graded area for the Project Conditions were removed from the PC model ARF/WRF layer.
HYDROLOGY
A hydrology analysis was conducted using the FLO-2D model to determine the peak flows at 3
sub-basins that were identified as having the potential to create mudflows (Figure 1-1). A 2-hour
hyetograph was developed to represent the short-duration thunderstorm events for the 25- ,100,
and 200-year peak rainfall events based on the Colorado Unit Hydrograph Procedure (CUHP)
developed by the Urban Drainage and Flood Control District (UDFCD, 2014). The 2-hour
hyetograph is developed based on the 1-hour point rainfall depth, similar to the design storm used
by UDFCD, but adjusted for Glenwood Springs precipitation frequency (Table 2-5 and Figure 2-5).
The process used for developing 2-hour hyetographs from one-hour point rainfall depths is
described in the Rainfall Chapter of the Aspen Urban Runoff Management Plan (City of Aspen,
2014). The 200-year rainfall was scaled up based on the 100-year rainfall hyetograph. The
adjusted 2-hour 25-, 100-, and 200-year rainfalls are 1.22, 1.56, and 1.71 inches, respectively
(Table 2-5). The FLO-2D model was run for the 2-hour, 25-, 100- and 200-year peak rainfall
events. Figure 2-6 through Figure 2-8 show the precited flow hydrographs and Table 2-6
summarizes the peak flow and volumes.
Table 2-5. Peak rainfall for the 2-hour, 25 and 100-year events (inches)
Event 1-Hour NOAA Atlas 14 Value 2-Hour CUHP Adjusted
25-Year 1.06 1.22
100-Year 1.35 1.56
200-Year 1.49 1.71
Halo Development Partners, LLC
15
Figure 2-5. Rainfall distribution for the 2-hour, 25-, 100-, and 200-year rainfall events.
Halo Development Partners, LLC
16
Figure 2-6. Predicted flow hydrographs for the 2-hour, 25-year rainfall event for for West,
Central, and East sub-basins.
Halo Development Partners, LLC
17
Figure 2-7. Predicted flow hydrographs for the 2-hour, 100-year rainfall event for West,
Central, and East sub-basins.
Halo Development Partners, LLC
18
Figure 2-8. Predicted flow hydrographs for the 2-hour, 200-year rainfall event for West,
Central, and East sub-basins.
Table 2-6. Summary of predicted peak clearwater flow and volume for the 25-, 100-, and
200-year rainfall events.
Inflow
25-Year Peak
Clearwater
Flow (cfs)
25-Year
Clearwater
Volume (ac-ft)
100-Year
Peak
Clearwater
Flow (cfs)
100-Year
Clearwater
Volume
(ac-ft)
200-Year
Peak
Clearwater
Flow (cfs)
200-Year
Clearwater
Volume
(ac-ft)
West
Inflow 20.1 0.59 30.9 0.88 37.1 1.08
Central
Inflow 2.1 0.06 3.2 0.08 3.8 0.1
East
Inflow 3.9 0.10 5.8 0.14 6.9 0.18
Halo Development Partners, LLC
19
MUDFLOW HYDROGRAPHS
The clearwater inflow hydrographs were bulked with sediment using a developed sediment
concentration (by volume, Cv) hydrograph to represent the mudflow source hydrograph. The total
volume of the water and sediment in a mudflow can be determined by multiplying the clearwater
volume by the bulking factor, where the bulking factor is defined by: 𝐵𝐵𝐵𝐵=1(1 −𝐶𝐶𝑣𝑣)
For example, a sediment concentration of 10 percent (Cv=0.10) creates a bulking factor of 1.11,
indicating the flood volume is 11 percent greater than if the flood was water only.
The sediment concentration hydrograph for the 25-year west mudflow source represents the likely
variation in sediment concentration throughout the storm hydrograph and has the following
characteristics:
• The initial rising limb and the last part of the recessional limbs of the hydrographs have a
sediment concentration of 20 percent, which corresponds to the minimum concentration
for a mudflow.
• The steep rising limb of the hydrograph is bulked to a maximum concentration of 45
percent to simulate the frontal wave of the mudflow. The peak of the sediment
concentration hydrograph occurs 3 minutes before the peak of the clearwater hydrograph.
• The sediment concentration at the peak of the clearwater hydrograph is less than the peak
sediment concentration to simulate water dilution.
• The average sediment concentration over the hydrograph is approximately 34 percent.
For the 100- and 200-year storms, a constant bulking factor of 0.2 was set throughout the inflow
time series. The higher volumes of water are more likely to dilute the flows below a “mudflow”
concentration. Figure 2-9 through Figure 2-11 show the representative clearwater, sediment and
bulked flow hydrographs for the three mudflow sources, and Table 2-7 summarizes the bulked
mudflow hydrographs for the 25-, 100-, and 200-year events, respectively.
Within FLO-2D, multiple defined mudflow characteristics are offered based on literature. For this
project, the Aspen Pit 1 mudflow option was selected. The Aspen Pit 1 mudflow typically results
in greater depths and less runout (inundation extents).
Halo Development Partners, LLC
20
Figure 2-9. Clearwater hydrograph, sediment concentration (Cv) and bulked sediment
hydrographs for West Mudflow Source for the 2-hour, 25-year rainfall event.
Halo Development Partners, LLC
21
Figure 2-10. Clearwater hydrograph, sediment concentration (Cv) and bulked sediment
hydrographs for Central mudflow source for the 2-hour, 25-year rainfall event.
Halo Development Partners, LLC
22
Figure 2-11. Clearwater hydrograph, sediment concentration (Cv) and bulked sediment
hydrographs for East mudflow source for the 2-hour, 25-year rainfall event.
Halo Development Partners, LLC
23
Table 2-7. Summary of 2-hr 25-, 100-, and 200-yr peak clearwater flow and volume, bulked volumes, and average sediment
concentrations for the mudflow sources
Rainfall Event Peak Clearwater
Flow (cfs)
Peak Clearwater
Volume (ac-ft)
Peak Bulked
Flow (cfs)
Bulked Volume
(ac-ft)
Total Sediment
Volume (ac-ft) Ave Conc.
2-Hour 25-Year, West 20.1 0.59 36 0.9 0.3 0.32
2-Hour 25-Year, Central 2.1 0.06 3 0.1 0.0 0.20
2-Hour 25-Year, East 3.9 0.10 5 1.0 0.9 0.20
2-Hour 100-Year, West 30.9 0.88 39 1.1 0.2 0.20
2-Hour 100-Year, Central 3.2 0.08 4 0.1 0.0 0.20
2-Hour 100-Year, East 5.8 0.15 7 0.2 0.0 0.20
2-Hour 200-Year, West 37.1 1.1 46.3 1.3 0.3 0.20
2-Hour 200-Year, Central 3.8 0.10 5 0.1 0.0 0.20
2-Hour 200-Year, East 6.9 0.18 9 0.2 0.0 0.20
Halo Development Partners, LLC
24
EXISTING CONDITIONS MODEL
The FLO-2D model was run for the 2-hr, 25-, 100-, and 200-year peak flow events and the model
output was used to develop depth and velocity inundation mapping. Locations where mudflows
originate are referred to as “mudflow sources” or “sources” to distinguish them from the previously
discussed locations where clearwater hydrograph inflows were determined.
EXISTING CONDITIONS MODEL RESULTS
For the 25-year event at the West Mudflow Source, the flow is confined in the gulley, and the max
depths range up to 3 feet (Figure 2-12). Downstream of the gully, the flow spreads out onto a fan
and branches in three directions, southwest, south, and southeast. The southwest and south
paths flow from the Ami’s Acres property onto the neighboring property to the west and into an
existing debris basin and average approximately 0.5 ft in depth. The southeast portion flows over
the campground and continues south towards the Frontage Road. This flow poses a threat to the
proposed developments due to its combined velocities, flow volume, and depths (Figure 2-12,
Figure 2-13). For the 25-year storm the flood depths range between 1 to 3 feet in depth across
the fan and velocities range from 3 to 4 ft/s. These flows impact property lots along the western
flank (Figure 2-12).
The Central Source and East Sources are minor and show max depths of up to 1.2 and 0.8 feet
in the channel, respectively. The Central Source remains confined until it intersects the existing
roadway, however its flow depths are relatively low once it exits the natural channel (less than 1
foot at Lewis Loop), and therefore its impact is likely minor. The East Source is approximately 14-
percent of the West Source’s flow (5 cfs versus 36 cfs), and discharges into a much less confined
area (Table 2-7). The flow travels south until it intersects the existing road, continuing south and,
across the Frontage Road and I-70.
Maximum velocities for Existing conditions within the site exceed 4 ft/s only in the north corner of
the flow extent for the West Source for the 200- year event, where the terrain is steep, and flows
are less viscous due to the dilution from higher rainfall (Figure 2-17). Velocities are generally low
for the 25-year event due to the higher concentration and the viscous nature of debris flows.
Velocities from the West mudflow source are less than 2.5 ft/s. Average max velocities for the
Central and East mudflow sources are less than 1 ft/s.
Halo Development Partners, LLC
25
Figure 2-12. Maximum mudflow depth for the 2-hour 25-year rainfall event under Existing
Conditions. The design plan is shown for reference purposes only.
Halo Development Partners, LLC
26
Figure 2-13. Maximum mudflow velocity for the 2-hour 25-year rainfall event under Existing
Conditions. The design plan is shown for reference purposes only.
Halo Development Partners, LLC
27
Figure 2-14. Maximum mudflow depth for the 2-hour 100-year rainfall event under Existing
Conditions. The design plan is shown for reference purposes only.
Halo Development Partners, LLC
28
Figure 2-15. Maximum mudflow velocity for the 2-hour 100-year rainfall event under Existing
Conditions. The design plan is shown for reference purposes only.
Halo Development Partners, LLC
29
Figure 2-16. Maximum mudflow depth for the 2-hour 200-year rainfall event under Existing
Conditions. The design plan is shown for reference purposes only.
Halo Development Partners, LLC
30
Figure 2-17. Maximum mudflow velocity for the 2-hour 200-year rainfall event under Existing
Conditions. The design plan is shown for reference purposes only.
Halo Development Partners, LLC
31
PROJECT CONDITIONS
This analysis is being prepared in advance of the development of a grading and drainage plan for
the project site. The ‘proposed conditions’ FLO-2D model is based on assumptions regarding
sizes, locations and maintenance of swales and debris basins design to route and attenuate
mudflows to levels within acceptable standards of the industry as established by FEMA, without
increasing risks on offsite properties. Conclusions, calculations and results of this analysis are
subject to modification pending final design of drainage facilities and a final execution of the
mudflow analysis.
The proposed Project Conditions model was developed by making terrain modifications to the
preliminary grading plan to represent proposed mudflow mitigation structures (Figure 2-18). The
intent for all features is to capture and redirect the mudflow away from vulnerable properties,
reduce flow across and onto the frontage road and I-70, and ensure no adverse impacts on the
property to the west. The base terrain was modified using the RAS Mapper tool in Version 6.7
Beta 5 of HEC-RAS (USACE, 2025). These terrain modifications were accomplished by using
polygons and polylines to denote locations of channels, low/high ground, and debris basin
excavations. The locations of these modifications can be seen overlaid with the proposed project
lot outlines in Figure 2-18.
The design features for each mudflow source include:
• West Mudflow Source – Excavation of the northwest debris basin (DB1) to an elevation of
5,845 ft, a 10-foot bottom width channel runs 300 ft south out of DB1 along the western
boundary of the property line. This channel leads into the southwestern debris basin (DB2)
with a bottom elevation of 5,778 ft. An additional 10-foot bottom width channel extends 70
feet from DB2 south onto the neighboring property and into an existing riprap channel.
DB1 and DB2 both have embankments which help contain and direct flow into the
channels. The embankment for DB1 is set to 5,865 ft elevation. The embankment for DB2
was set to an elevation of 5,790 ft elevation. Each embankment contains an overflow weir
section which is lower than the embankment top height. For DB1 the weir elevation is
5,852 ft, and for DB2 the elevation is 5,785 ft elevation. Two berms were added on the
east side of the main channel as it exits DB1 and DB2 to provide addition grade and
prevent flow from overtopping the channel.
• Central Mudflow Source – Excavation of a central debris basin (DB3) with a bottom
elevation of 5,844 ft and an embankment with overflow weir at 5,849 ft and 5,847 ft
elevation, respectively. A 10-foot bottom width channel runs 25 ft out of this debris basin
and conveys flow south, onto the road below.
• East Mudflow Source – Excavation of the east debris basin (DB4) with a bottom elevation
of 5,845 ft, embankment, and overflow weir. The elevation of the embankment and
overflow weir was set to 5,849 ft and 5,847 ft, respectively.
The terrain modifications were burnt into the DTM, and the DTM was used to update the grid
elevations in the FLO-2D model (Figure 2-19). Manning’s N values were updated to reflect the
added channels and debris basins (Figure 2-20). The Project conditions model was run over the
same 25-,100-, and 200-year peak mudflow hydrographs as the Existing conditions model and
the model output was used to map and evaluate the changes in mudflow depths and velocities.
Halo Development Partners, LLC
32
Mudflow depths represent the maximum depth during the simulation and not the water-surface or
mudflow elevation at the end of the simulation.
The Manning’s N values were adjusted to account for Project conditions. The areas with added
channels and detention basins had their Manning’s N values set to 0.04 to represent the
excavated channel (Figure 2-20).
Figure 2-18. Proposed FLO-2D terrain mods to contain mudflows at the west, central, and
eastern mudflow sources.
Halo Development Partners, LLC
33
Figure 2-19 Proposed project grading provided by client merged with terrain modifications and
Garfield County 2016 LiDAR.
Halo Development Partners, LLC
34
Figure 2-20. Land-use zones used to assign Manning’s n-values to the Project Conditions
FLO-2D model.
Halo Development Partners, LLC
35
PROJECT CONDITION CHANNELS
The model cell size was set to 5 feet to capture channels that were burned into the terrain using
the HEC-RAS Mapper tool. Channel depths were an average of 6 ft below existing terrain and a
10 ft bottom width (Figure 2-21). Side slopes were set at 1.5H:1V (Figure 2-22). Alignments and
depths will need to be modified to account for a finished channel design with appropriate side
slopes.
Figure 2-21. Channel profile extending northwestern debris basin (DB1) down to the
southwestern debris basin (DB2). The main channel averaged approximately 6
feet of depth.
Halo Development Partners, LLC
36
Figure 2-22. Cross section of the terrain modifications for the proposed channel out of the
northwest debris basin (DB1). The orange line shows the Existing conditions
(unmodified) terrain, while the blue line shows the proposed channel and the
graded terrain.
DEBRIS BASINS
The proposed design (Figure 2-18) contains four sediment basins for storing water and sediment
and reducing the downstream flow. These basins are intended to store the larger sediments that
are typically transported on the rising limb of the hydrograph, and conveying the lower sediment
concentration flows into the receiving channels (Figure 2-23). Storage for each debris basin was
estimated based on the acre-feet of discharge for each inflow location.
Halo Development Partners, LLC
37
Figure 2-23. Profile showing the proposed embankment and overflow section for DB1. The
orange line shows the Existing conditions (unmodified) terrain, while the blue
represents the top elevation of the embankment with the weir.
PROJECT CONDITIONS MODEL RESULTS
Under project conditions, the model predicts mudflow inudation of the northwest lot for the 2-hour,
25-, 100-, and 200-year events (Figure 2-27 to Figure 2-32). Mudflow from the central source
impacts some downhill lots. The peak discharge for these uncontrolled flows for the 100-year
event is 4 cfs, which is relatively low and will be mitigated by onsite drainage. Importantly, the
model results indicate a reduction in flow volume onto the Frontage Road and I-70. The following
section will reference floodplain cross sections (CS) for comparison between the Existing and
Project conditions models (Figure 2-24).
Halo Development Partners, LLC
38
Figure 2-24. Flood plain cross sections used to compute flow hydrographs.
Halo Development Partners, LLC
39
Basin DB1, which is first to receive flow from the West mudflow source, captures a significant
amount of the debris flow. The estimated volume of material retained in DB1 is approximately
0.31 acre-feet (480 yd3). Debris basin storage quantities were calculated by subtracting total
inflow volume and total outflow volume for each debris basin (Table 2-8). DB1 attenuates the flow
compared to the EC model (Figure 2-25).
Figure 2-25. Comparison of discharge (cfs) vs time (hrs) from Cross Section 14 in the Existing
and Project Condtions for the 25-year mudflow.
The remaining mudflow is fully contained by the design channel and downstream debris is
subsequently stored in DB2 before discharging into the existing riprap channel on the neighboring
property (Figure 2-27 to Figure 2-32). Maximum flow depths were evaluated at several cross
sections along the outlet channel from DB1 to determine available freeboard. The average
freeboard depth across sections for the 25-year mudflow (worst-case) is 3ft, which meets FEMA
freeboard criteria.
Table 2-8 shows the estimated volume excavated to create each basin and the volume stored
after a 25-year mudflow by all 4 debris basins. These values are similar for the 100- and 200-year
events, however the mud content of the volume retained is likely reduced due to dilution. The
Central Source still shows flooding exiting DB3 and heading south. Additional stormwater
drainage infrastructure is recommended to mitigate any excess flow that is not contained within
the basins. Excess flow is shallow (<0.5ft depth) and has typically lower concentrations of
sediment. For the East Source, the entirety of the flooding was captured by DB4 for the 25-year
event.
Halo Development Partners, LLC
40
Table 2-8. Predicted storage of each proposed debris basin for the 2-hour 25-year rainfall
event. This design quantities are conceptual in nature and require refinment.
Basin Name Final Depth Storage (ac-ft)
Northwest Debris Basin (1) 0.31
Southwest Debris Basin (2) 0.17
Central Debris Basin (3) 0.02
East Debris Basin (4) 0.06
At the 100- and 200-year events, the flow patterns are similar to the 25-year event. While the
Central mudflow source still shows shallow flooding (less than 0.5 ft), the flow is diluted from larger
material being stored within the debris basin (DB3) and additional rainfall. At the East Source,
flooding occurs for the 100- and 200-year events. The flooding is shallow (<0.5 ft) and diluted by
the increased rainfall and storage of larger material inside DB4.
Quantities were calculated for the total volume of sediment passing through all floodplain cross
sections for each run and compared between the EC and PC projects (Figure 2-26). A comparison
of the Existing conditions flows to the Project condition flows indicates how the overall floodplain
storage is changed with the implementation of the debris basin and channel concept. The
confinement of the flow into the designed channel does increase volumes through specific cross
sections (i.e. CS-17 onto neighboring property), however the overall volume of flow moving south
onto the Frontage Road and Interstate 70 (CS-12/CS-15) is reduced for all mudflow events. The
debris basins also reduce the total flow for the Central (CS-3) and East (CS-7) Mudflow Sources.
Table 2-9 is a comparison of total flow volume across CS-12 and CS-15, which combined capture
all the flow exiting the property boundaries and onto the Frontage Road and I-70. The total volume
in acre-feet is calculated for each event in both EC and PC models, and a comparison is made to
determine whether the volume of flow leaving the property is higher or lower in the PC model.
The results indicate that there is a general reduction of flows off the property boundary in PC
model. Negative values indicate a reduction in flow onto the Frontage Road in the Project
Conditions.
Table 2-9. Predicted reduction in flow volume crossing onto the Frontage Road and I-70
between EC and PC models for the 25-, 100-, and 200-year events.
Event Volume Change at CS-12 (ac-ft) Volume Change at CS-15 (ac-ft)
25-Year Mudflow -0.04 0.00
100-Year Mudflow -0.01 -0.10
200-Year Mudflow 0.11 -0.16
Halo Development Partners, LLC
41
Figure 2-26. Comparison of total volume through each cross section for the Existing and
Project Conditions in the 25-year mudflow.
Like existing conditions, debris flow velocities are relatively low under Project conditions (Figure
2-28). Main channel velocities from the West, Central, and East mudflow sources decrease under
Project Conditions at the 25-year event compared to the Existing conditions. The maximum
velocity in the channel through CS-14 for the EC run is approximately 1.9 ft/s, whereas in the PC
run it is 1.6 ft/s. This is due to the flow being slowed down as it exits the steep valley to the north
and is captured by DB1. Flows at the Central and East Sources show a similar reduction. For the
100- and 200-year flows, the lower viscosity and higher volumes increase the velocities in the
channelized flow compared to the unconfined channels under Existing Conditions (Table 2-10).
Table 2-10. Predicted maximum velocity at DB1 outlet channel for Existing and
Project condtions (ft/s).
Event Existing Conditions (ft/s) Project Conditions (ft/s)
Q25 1.9 1.6
Q100 3.7 7.1
Q200 4.4 8.0
Halo Development Partners, LLC
42
Figure 2-27. Predicted maximum mudflow depth for the 2-hour 25-year rainfall event under
Project Conditions.
Halo Development Partners, LLC
43
Figure 2-28. Predicted maximum mudflow velocity for the 2-hour 25-year rainfall event under
Project Conditions.
Halo Development Partners, LLC
44
Figure 2-29. Predicted maximum mudflow depth for the 2-hour 100-year rainfall event under
Project Conditions.
Halo Development Partners, LLC
45
Figure 2-30. Predicted maximum mudflow velocities for the 2-hour 100-year rainfall event
under Project conditions.
Halo Development Partners, LLC
46
Figure 2-31. Predicted maximum mudflow depths for the 2-hour 200-year rainfall event under
Project Conditions.
Halo Development Partners, LLC
47
Figure 2-32. Predicted maximum mudflow velocities for the 2-hour 200-year rainfall event
under Project Conditions
Halo Development Partners, LLC
48
DEBRIS BASIN MAINTENANCE
As an example of the need to regularly maintain and clean the northwestern debris basin (DB1),
it was filled to the level of the weir overflow to predict the mudflow impacts if the sediment basin
was full, and a second debris flow event were to occur. The predicted mudflow extents for the 25,
100-, and 200-year storms are shown in Figure 2-33 through Figure 2-35.
The housing lots on the northernmost section of the proposed design is most heavily impacted by
the overflow. The channel still manages to a significant portion of the flow away from the
development. These results highlight the need for maintenance of the debris basin and keeping
its capacity available for storing mudflows.
Halo Development Partners, LLC
49
Figure 2-33. Predicted maximum mudflow depths for the 2-hour 25-year rainfall event under
Project Conditions with DB1 filled.
Halo Development Partners, LLC
50
Figure 2-34. Predicted maximum mudflow depths for the 2-hour 100-year rainfall event under
Project Conditions with DB1 filled.
Halo Development Partners, LLC
51
Figure 2-35. Predicted maximum mudflow depths for the 2-hour 200-year rainfall event under
Project Conditions with DB1 filled.
Halo Development Partners, LLC
52
RECOMMENDATIONS
Proposed conceptual designs should be reviewed and refined by the design engineer. Grading
plans are required to be developed and an addition FLO-2D model ran with the proposed site
grading including any debris flow mitigation features. The modeling presented here represents
conceptual designs showing the feasibility of debris flow management.
Based on the results of this analysis, debris flow is most likely to affect the north western most
proposed lots. It is recommended that the northwestern most lot be removed from development
to make way for the proposed channel and required grading to raise the elevation of the ground
flanking the proposed channel. Additional storm water infrastructure may be needed for the
Central and East Mudflow Source to manage the clearwater overflow that leaves the debris basin.
For the lower-frequency events. Frequent inspection and cleaning of the debris basins is
recommended to maintain capacity.
The following are general recommendations for mudflows. Building penetrations such as windows
or doors should be set with freeboard above the design mudflow depth. A minimum of 3 feet of
freeboard is typically recommended by FEMA for surging waves which would include all walls that
will have direct impact forces from mudflow and debris (Table 3-1). A minimum of 2 feet of
freeboard is typically recommended for all walls with minor surging such as walls impacted by
mud flows moving laterally, or that are in the shadow of up-fan buildings (Table 3-1).
Table 3-1. Freeboard and Safety Factor Recommendations (copied from FEMA 2012)
If the design depth cannot be achieved, then impact resistant glass could be considered for
windows with the capabilities to withstand the required static and dynamic forces as discussed
below. In the case of doors and garage doors, two conditions should be considered if the design
depth cannot be achieved. First, the door should be designed for static and dynamic loading and
include provisions for seepage; secondly, there should be an alternative exit for people to leave
the building.
Halo Development Partners, LLC
53
REFERENCES
City of Aspen, 2014. Urban Runoff Management Plan. November.
Federal Emergency Management Agency (FEMA), 2012. Engineering Principals and Practices
for Retrofitting Flood-Prone Residential Structures. Appendix D – Alluvial Fan Flooding.
FLO-2D, Pro, 2021. FLO-2D Software, Inc. Nutrioso, Arizona.
Garfield County, 2013. Land Use and Development Code. Garfield County, CO.
https://www.garfieldcountyco.gov/community-development/wp
content/uploads/sites/12/Complete-Land-Use-and-Development-Code.pdf. Accessed March,
2025.
United States Army Corps of Engineers (USACE), October 2025. HEC-RAS Mapper User's
Manual, Version 6.7 Beta 5.
National Research Council, 1982. “Selecting a methodology for delineating mudslide hazard
areas for the National Flood Insurance Program.” National Academy of Sciences report by the
advisory Board on the Built Environment, Washington, D.C.
Natural Resources Conservation Service (NRCS), 2012. National Engineering Handbook Part
630 Hydrology. Chapter 14 Stage Discharge Relations. United States Department of Agriculture,
Washington, D.C.
O’Brien, J.S., 2004. Simulating Mudflow Guidelines. Guidelines provided with FLO-2D model.
Simons, Li and Associates, Inc. (SLA) and O’Brien, J.S., 1989. Flood Hazard Delineation for
Cornet Creek, Telluride, Colorado. Submitted to the Federal Emergency Management Agency,
Region VIII, March.
SurvCo, Inc., Glenwood Springs, CO, Survey Data and Site Drawings, 2025-11-07