HomeMy WebLinkAbout1.47 North Hangs-Inflow Outflow Memo
North Hangs
October 2023 1
MEMO: North Hangs Mine – Inflow/Outflow Structure Designs
DATE: October 31, 2023
TO: Garfield County Department of Community Development
The North Hangs Mine, as proposed by Western Slope Materials, near Silt, CO, will disturb 72.8
acres of ground through sand and gravel mining. Mining of sand and gravel material at the site will
entail the excavation of substantial depth of alluvium within the floodplain of the Colorado River in
this area. A risk associated with mining within a river floodplain is that of pit capture due to erosion
of the barrier between the river and mining pit. In order to mitigate this risk and prevent pit capture,
the Colorado Division of Reclamation, Mining, and Safety has implemented flood water routing and
river embankment protection policies for gravel pits along rivers. For most gravel pits that come
within 150-feet of a river, inflow/outflow structures are installed. This will take place at North Hangs
Pit; this memo is intended to provide Garfield County with additional information about these
structures.
1. Need for Structures
Alluvium deposits along Colorado rivers are a primary source of sand and gravel needed by the
state for construction of infrastructure. In order to facilitate the safe development of these deposits,
sand and gravel mines are authorized to mine within a few hundred feet of rivers. However, during
floods, it is recognized that the embankment separating the river and the sand and gravel pit can be
eroded to the point that passing through the gravel pit becomes the new preferred river path. Gravel
pits can become these new paths because they lower the ground level below the river bottom
during mining. During normal river flow, the embankment between the river and the mine pit is
higher than both the pit and the river level, and the river stays within its main channel. During
flooding, the river level elevates above the embankment level, and flood waters leave the river
channel (i.e., flooding). Any existing low point along the river embankment becomes the flood
water’s “preferred” path for leaving the river channel. This preferred path can lead to erosion of the
pit slope in the direction of the river, a process known as head cutting. Head cutting can erode the
pit slope into the river, creating a new river channel, and leading to pit capture. Pit capture occurs
when the erosion of the river embankment is sufficient to cause the river to flow into and out of a
gravel pit groundwater lake as its new channel. Naturally, pit capture and head cutting are to be
prevented. Figure 1 shows the general process.
North Hangs
October 2023 2
Figure 1. River Flood Water Interaction with Gravel Pit
Colo. River
Colo. River (starting to flood)
Colo. River (flooding)
Gravel mine pit slope (erosion risk)
Eroded river embankment
creates new river channel
Colo. River (post‐flood)
North Hangs
October 2023 3
2. Structure to Protect the River Embankment
The Mile High Flood Control District that regulates stormwater structures in the Denver metro area
developed a set of technical criteria, design guidelines, and flood control structures to prevent head
cutting and pit capture. Implementation of these flood control structures, commonly known as
inflow/outflow structures, allows gravel mines to operate close to rivers without risking pit capture or
other damage to the river embankment. Inflow/outflow structures establish short, reinforced
channels in the river embankment through which flood waters can flow into and out of the gravel pit
during flooding. These reinforced channels provide a preferred path for flood water flow that will not
erode, thus preventing head cutting and pit capture. Flood water flows through the inflow/outflow
structure into and out of the pit during flooding, but does not create a new river channel. When flood
waters recede, the river is still in its main channel. This process is shown in Figure 3 with an
example of these structures at a reclaimed gravel pit near Craig, CO in Figure 2.
Figure 2. Inflow/Outflow Structure Example
Inflow/Outflow
Structure
Inflow/Outflow
Structure
North Hangs
October 2023 4
Figure 3. River Flood Water Interaction with Gravel Pit with Inflow/Outflow Structures
Colo. River
Colo. River (starting to flood)
Colo. River (flooding)
Water flows through structure during flood
River stays in main channel
Colo. River (post‐flood)
North Hangs
October 2023 5
3. Structure Designs
Inflow/outflow structures can be built in a variety of different ways. Mile High Flood Control District
has outlined design criteria and provided standardized templates to simplify the design,
construction, and approval of these structures. Examples of these templates are shown in Figures
2.8 through 2.10. Each of these templates show an inflow/outflow structure section. North Hangs
Mine will implement one of these structures’ designs or something similar when mining begins to
encroach on the Colorado River embankment. The specific design used will be approved by the
Colo. Division of Reclamation, Mining, and Safety before mining encroaches into the river
embankment.
North Hangs
October 2023 6
North Hangs
October 2023 7