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BLM/CO/ST-04/006

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Cover photo: A Townsend’s bigeared bat, which is a special status bat species. Photo courtesy

of Kirk Navo, Colorado

Division of Wildlife.
By:

Dr. Robert H. King

Professor of Engineering

Colorado School of Mines

Division of Engineering

Golden, Colorado 80401-1887

303-273-3305

rking@mines.colorado.edu

Prepared for:

Bureau of Land Management

Colorado State Office

2850 Youngfield Street

Lakewood, Colorado 80215

U.S. Department of the Interior • Bureau of Land Management

October 2004
Suggested citation:

King, R.H. 2004. Microclimate effects from closing abandoned mines with culvert bat gates. Technical Note

416. U.S. Department of the Interior, Bureau of Land Management, Colorado State Office, Denver, CO.

BLM/CO/ST-04/006. 13pp.
i

Contents

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .iii

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1

Climate Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3

Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3

Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3

Computer Modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7

Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7

Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7

Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11

Cited References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13

iii

Abstract

Numerous bat species, including some threatened

species, inhabit thousands of abandoned mines

across the United States. Some of their bat habitat

has been lost as mines are closed to reduce hazards

to the public. Bat gates have been used to close

some mines because they restrict public access while

still allowing the bats to fly in and out. However,

they are not appropriate for mines with trench

portals. The Bureau of Land Management (BLM)

proposed using culverts in the trenches and closing

the culverts with bat gates. BLM worked with the

Colorado School of Mines (CSM) to study the

effects of this type of closure on the microclimate of

the mines, which in turn could affect bats’ fidelity

to the habitat. CSM developed a process that

integrated continuous climate measurements with

data from a computer model and tested it at three

abandoned uranium mines in southwestern

Colorado. Measurements were taken for 1 year

prior to and 1 year after closing the mines with

culvert gates. The results showed that the culvert

gate closure slightly modified the microclimate by

essentially extending the length of the mine, but

indicated that bats should be able to locate the

same climatic conditions a few feet away from their

original roosting locations. The computer model

predictions compared favorably with the measurements, and the model provided relevant insight

regarding the qualitative trends reflected by the

field measurements.

1

Introduction

There are over 300,000 abandoned underground

mines in the United States, many of which provide

important habitat for a variety of bat species.

Currently, the mines are being closed to reduce hazards to the general public. Approximately 33,000

abandoned mines have been closed in the United

States thus far and, of these, about 1,200 have been

closed with special gates to preserve bat roosting

habitat (Meier and Garcia 2000). These gates allow

bats to pass in and out of the mines, while keeping

the public out (Figure 1).

Some abandoned mines have an open trench that

declines below the ground surface to the mine portal

(Figure 2). These trenches typically are 10 to 20 meters

(33 to 66 feet) long, declining into the ground at

6 to 10 degrees. At the bottom end of the trench,

the mine portals vary in cross section from 1.2 meters

(4 feet) wide by 1.8 meters (6 feet) high up to 6 meters

(20 feet) wide by 2.4 meters (8 feet) high. Generally,

the trenches are too large to cover effectively with

grating and bat gates and are backfilled. However,

backfilling is a poor solution when it is important

to maintain habitat for bats, particularly special

status bat species.

In an attempt to preserve bat habitat on public lands,

the Bureau of Land Management (BLM) proposed

using culvert gates for the trench mine openings

(Figure 3). With this method, a culvert is placed in

the trench from the surface so that it extends part-way

into the mine portal. A bat gate is welded onto the

surface end of the culvert. Finally, the trench is backfilled, covering the original mine portal and culvert,

with the exception of the gated end. However, the

effect of culvert gates on the microclimate of the

mine, and consequently on bats’ fidelity to the mine

habitat, was unknown. Microclimate variations are

critical since temperature changes of only a few

degrees may cause bats to abandon maternity and

hibernation roosts (Tuttle and Taylor 1994).

Figure 1. A typical bat gate.

Figure 2. A typical trench portal leading to a uranium

mine in western Colorado.

Figure 3. A culvert gate prior to backfilling the trench.
2

Bat habitat in abandoned mines and caves was lost

when the surface openings were restricted in a

number of cases (Tuttle and Taylor 1994). Most of

these restrictions were more severe than culvert gate

closures. Other reports have hypothesized about the

causes of such habitat losses (Tuttle and Stevenson

1978). To determine whether the culvert gate closures would significantly impact bat habitat, BLM

selected three mines with trench portals for study.

They are designated as mines 31, 59, and 65 and

are located on Calamity Mesa, southwest of the

town of Grand Junction, Colorado (Figure 4). The

mines are less than 30 meters (98 feet) deep and are

excavated in the sandstone of the Salt Wash

Member of the Jurassic Morrison Formation.

For this study, biologists from the Colorado

Division of Wildlife surveyed the mines and

confirmed bat habitation. An expert in mine safety

from the National Park Service assessed mine safety

conditions and mapped the layout of the mines.

Then mine ventilation experts from the Colorado

School of Mines (CSM) evaluated the effect of

culvert gates on the mine microclimates. They

developed a process that involved integrating

measurements of temperature, humidity, airflow,

and radiation from the mines with data from a

computer model. 

This study was the first attempt to scientifically

evaluate underground climate changes from

restricting airflow with culvert gates. The results of

the study are summarized in this Technical Note.

Detailed results of CSM’s research are available

online at www.blm.gov/aml/technologyframeset.htm

or from the abandoned mine lands coordinator at

BLM’s Colorado State Office.

Figure 4. Location of study mines.
3

Climate Measurement 

Method

The microclimate in a mine varies temporally and

spatially because of complex interactions among the

surface climate, rock temperature, and mine airflow,

humidity, and geometry. For this study, the temporal

data was measured continuously with miniature

data loggers at a number of critical locations throughout the mines and on the surface. Measurements

were taken from September 1998 to November

2000, approximately 1 year prior to and 1 year after

closing the mines with culvert gates. Measurement

points were located near the underground air/rock

interface as well as in the center of mine passages.

Obtaining measurements was challenging because

the instruments were subject to vandalism and

removal prior to closure of the mines. Those that

didn’t have armored cables were susceptible to

rodent damage. In addition, all of the instruments

had to be powered with long-lasting batteries, as

access was impossible in winter and difficult,

expensive, and hazardous at other times.

Surface temperature fluctuations create thermal

gradients in abandoned mines resulting in a

nonuniform air density distribution. Under the

action of gravity, this nonuniform air density

induces airflow called natural convection. Though

active mines have fans that produce large volumes

of airflow, the study mines have low airflow rates,

around 0.05 meters per second (0.16 feet per second). Reasonably priced commercial devices cannot

measure air velocity lower than about 0.3 meters

per second (1 foot per second), and they are too

fragile to deploy for long periods in an abandoned

mine. CSM developed a $300 ultrasonic transducer

that measured air velocities ranging from 0–0.06

meters per second (0–0.2 feet per second), with a

small number of readings above 0.12 meters per

second (0.4 feet per second) in the study mines.

This velocity range compares with measurements

of 0.05–0.25 meters per second (0.16–0.82 feet per

second) from smoke tubes, in which smoke is used as

a tracer for slow-moving air, and model predictions

of about 0.06 meters per second (0.2 feet per second).

The study mines are abandoned uranium mines

that contain radiation. An instant working level and

gamma meter (provided by the National Park Service)

measured discrete radiation ranging from 0.94 to

11.84 working levels 1 during two mine visits. In

addition, dosimeters measured accumulated exposure

from combined x-ray, beta, and gamma radiation,

and alpha cups measured alpha radiation. Currently,

it is assumed that the relatively short-lived bats are

not impacted by radiation and no radiation effects

have been observed by CDOW. Nevertheless, the

data gathered at the study mines could be useful

for much-needed future research on radiation

effects on bats.

Results

When examining temperature and humidity data

over the period of the study, it was only necessary

to differentiate between the changes caused by the

culvert gate closure and those caused by surface

climate variations because the rock mass and the

mine geometry did not change during the study. To

isolate the culvert gate impacts, CSM had to locate

data for short periods when the surface climate was

approximately the same before and after mine closure.

1 A working level is a concentration of radon daughters that will deliver 1.3 x 105 MeV of alpha enrgy per liter of air.

At equilibrium, 1WL = 100pCi/l.
4

Therefore, the pre- and postclosure temperature

difference is probably caused by climate, not the

culvert gate closures.

In the winter, much of the surface data was lost due

to instrument failure, so definitive in-mine climate

analysis was not possible. Nevertheless, the in-mine

climate measurements seemed to be less variable in

winter than in summer in both pre- and postclosure

periods. In addition, the in-mine temperatures

seemed to increase slightly after closing the mine

with culvert gates.

Closure reduced the temperature variance at every

data logger and the variance decreased as distance

from the portal increased (Figure 6). Since the

surface temperature was warmer than the mine

Figure 5. Pre- and postclosure surface temperature for a subset of the data with similar surface conditions.

CSM found one 600-sample subset of postclosure

data for 11 days in July that was similar to an

equal-sized subset of the preclosure data (Figure 5).

(Preclosure dates were shifted to match postclosure

dates so data for both subsets could be plotted on

the same graph.) Mean temperature increased by

0.3 to 1.9 Kelvin (0.6 to 3.4 °F) from the preclosure climate for all but 2 of the 11 data loggers in

the 3 study mines.2 The pre- and postclosure data

subsets, while similar, were not exactly the same.

The surface temperature of 295 Kelvin (71 °F) at

the beginning of the preclosure data window was

lower than the surface temperature of 296 Kelvin

(73 °F) at the beginning of the postclosure data

window. Since air movement in the study mines

was minimal, the in-mine air temperatures were

strongly affected by preclosure conditions.

0

0 50 100 150 200 250

y=173.37x-1.3485

R2=0.7444

y=134.75x-1.6961

R2=0.862

300

Distance from portal (feet)

Variance in mine temperature ( ̊F2

) 50

100

150

200

250

Preclosure Postclosure

Figure 6. Pre- and postclosure variance in temperature versus distance from the portal.

2 The measurement error was +/– 0.4 to 0.7 Kelvin.
5

temperature during these periods (summer

months), the air was more affected by conduction

from the surrounding rock than by convection from

the surface except close to the portal (Figure 7). 

This declining climate variance with distance into

the mine leads to the conclusion that culvert gate

closures have the same effect on the microclimate

as if the mine were simply lengthened. Reduced

variance, or attenuation, is caused by thermal inertia;

i.e., the effect of the surrounding rock mass and

delayed reaction to changes in the surface temperature. Surface climate changes are not immediately

felt in a mine, and they have a reduced effect with

mine depth. Consequently, a mine may not react

completely to a surface temperature rise or fall

before the surface temperature trends in the opposite direction. Of course, the mine microclimate

reacts more quickly to surface climate changes when

the temperature difference between surface and

mine air increases, causing stronger convection

airflows. 

0

0 50 100 150 200 250

Preclosure Postclosure

y=73.197x-0.0898

R2=0.7623

300

Distance from portal (feet)

Average mine temperature ( ̊F)

10

20

30

40

50

60

70

80

y=70.937x-0.0844

R2=0.7457

Figure 7. Pre- and postclosure mean temperature versus distance from the portal in summer.

7

Computer Modeling 

Method

Commercially available software, CFD-ACE (CFD

Research Corporation, Huntsville, Alabama), was

used to numerically solve coupled, nonlinear, secondorder partial differential Navier-Stokes equations that

represent the natural convection airflow (Dodds

2001). The following assumptions and simplifications

were made in developing a model for this application:

• Mine Geometry – The model assumes that the

mine has a simple in-line opening that declines

6 degrees and dead-ends (Figure 8). The back of

the original mine portal is 2 meters (6.6 feet) below

the ground surface. This geometry is the same as

the simplest of the three study mines. It is also the

most restrictive for airflow and therefore is most

impacted by any changes, such as installation of a

culvert gate closure. Other geometries, such as

numerous mine openings and underground

workings, offer less resistance to airflow and

therefore less change in mine air temperatures from

any restrictions on airflow. The model also assumes

there are no changes in the mine cross section and

that the mine has smooth walls. These variables are

relevant in mine ventilation, but they are secondorder effects compared to the nonalignment of

the mine opening and the acceleration of gravity.

These simplifications and assumptions reduce the

model to a two-dimensional, planar slice of the mine.

They significantly reduce the computational effort,

while capturing the salient physical mechanisms.

• Rock Temperature – Mine air temperatures are

affected by heat conduction from the surrounding rock mass. The model assumes that the rock

temperature is constant and equal to the mean

annual surface temperature, which is about

284 Kelvin (52 °F) at approximately 1.83 meters 

(6 feet) into the rock mass. This rock temperature

is a rule of thumb for shallow mines. Generally,

rock temperatures do increase with depth by about

0.5 Kelvin (1 °F) per 30 meters (98 feet); however,

the study mines are too shallow for this effect.

Rock

Rock

Rock

Rock

T=Twall(=284 K)

T=Twall(=284 K)

Mine Drift

u=v=0

u=v=0

u=v=0

Rock

Culvert

P=Patm

T=Tout

0

-10

0

10

20

30

meters

5

10

meters

Figure 8. Vertical cross section of CFD model mine with typical boundary conditions.

Results

The surface and in-mine temperature and airflow

data collected helped calibrate and verify the computer model of the natural convection airflow in

the mines.

The computer model predicted two convection cells

that were previously unreported. One cell is near

the mine portal and the other is throughout the

remainder of the mine (Figure 9). In-mine climate

measurements support the two-cell model prediction,

as pre- and postclosure average temperatures are
higher in the cell near the mine portal than they

are in the rest of the mine (Figure 7). Furthermore,

temperature variance is much greater in the cell

near the mine portal (Figure 6).

The model responded to changes in surface temperatures and showed that, characteristically, it should

take about 1 hour for a change in outside temperature

to completely take effect inside the mine, dependent

upon the location and magnitude of the temperature

change. Actual changes took about 2 hours. The

model results attenuate with depth into the mine,

similar to the in-mine climate measurements. The

in-mine climate data show that when surface temperature fluctuates about 12 Kelvin (22 °F), mine

temperatures fluctuate about half as much. Actual

attenuation within the test mines was more dramatic

than in the model mine.

Figure 9. Simulated mine temperature (Kelvin) profile.

Figure 10. Simulated mine temperature (Kelvin) profile with culvert.

8

The researchers varied the computer model to study

the effects of several scenarios that would have been

costly to test in the field:

• The first change to the model was to add a culvert

gate onto the original mine portal, where the

culvert is 10 meters (33 feet) long, 4 meters 

(13 feet) in diameter, and laid in the trench

bottom at a 6-degree incline. The surface end of

the culvert became the new mine portal. The

microclimate temperature profile and two 

convection cells did not change in shape, and

the portal cell remained close to the new portal

(compare Figures 9 and 10).

• The next change was to increase the culvert

inclination to 27 degrees. The temperature profile
9

Figure 11. Simulated mine temperature (Kelvin) profile with culvert at steep angle.

and convection cells were similar in shape and

magnitude to the 6-degree culvert model (Figure 11).

The model did not simulate all of the complexities

inherent to a real mine. Real mine openings are

three-dimensional, have rough rock surfaces, contain turns and bends, have variable dimensions,

and contain abrupt changes in perimeters caused

by roof falls.

11

Conclusions

Abandoned mines with trench portal type openings

pose a special challenge for land managers who are

trying to preserve ecosystems by maintaining habitat

for a diverse set of bat species. This study found

that using the culvert gate technique for closing

these mines resulted in only small temperature

changes on the order of 1–2 Kelvin (2–4 °F), with

a measurement error of +/– 0.4 to 0.7 Kelvin. The

increased temperatures occurred during the summer

maternity roosting season, so even though the

increases were small, they could be beneficial to

the bats. Although the study was not conclusive, it

indicated there was also little in-mine temperature

change from the culvert gate closure in winter.

While bats may be sensitive to these small temperature changes, they do not have to vacate the habitat.

Guano spots in the mines studied indicated bats

were not congregated at either the portal or the end

of the mine. Any roosting bats would experience

similar or greater temperature ranges on a frequent

basis. The changes are small enough that bats can

easily find roosts slightly deeper in a mine or

toward the portal where the temperature regime is

the same as before. Based on these results, CSM

found no reason for managers to discontinue the

use of culvert gates, especially if the alternative is to

backfill the mines and eliminate the habitat.

Though a culvert gate backfill is more expensive

(about $3,000 per opening) than a simple bat gate

(about $1,000 per opening), which is an important

consideration when closing thousands of mines,

small-diameter culverts installed at a high angle can

minimize costs.

CDOW surveys indicated reduced bat activity in

the mines in the first season after closure with culvert gates (Navo et al. 2000). The reduced activity

might not be the result of microclimate changes

from adding the culvert gates; it could instead be

the result of frequent visits by researchers, instrumentation installed in the mines, closure activity by

construction contractors, increased recreational

activity in the area, or surface climatic changes.

CSM recommends that bat surveys continue and

that other causes for the reduced bat activity be

investigated. CSM also recommends that new mines

be identified for additional pre- and postclosure

monitoring and computer modeling. Future models

should study more extensive mine complexes, more

closely evaluate effects of actual conditions, and

analyze detailed features of bat gate designs.

The model of airflow and heat transfer in mines

developed by CSM can be used by land managers

to predict the effects of modifications to caves and

mines. While the model requires further development and modifications for specific sites, using it is

more cost-effective than conducting physical

experiments in the field. Combining model analysis

with baseline monitoring and analysis of temperature, humidity, and airflow gives land managers a

comprehensive procedure to assess the effect of

remedial actions on cave and abandoned mine

microclimates and avoid any future unintended

loss of bat habitat. 

13

Cited References

Dodds, J.A. 2001. Microclimate modeling of bat

habitat in abandoned mines, Master of Engineering

Report T-5513. Colorado School of Mines,

Golden, Colorado.

Meier, L. and J. Garcia. 2000. Importance of mines

for bat conservation. In Vories, K.C. and D.

Throgmorton (eds.). Proceedings of bat conservation and mining: A technical interactive forum.

Office of Surface Mining, Alton, Illinois, and

Southern Illinois University, Carbondale, Illinois.

pp. 17-28.

Navo, K.W., T.E. Ingersoll, L.R. Bonewell, N.

LaMantia-Olson, and A.J. Piaggio. 2000. 2000

bat/mine evaluations. Colorado Division of

Wildlife, Monte Vista, Colorado. 

Tuttle, M.D. and D.A.R. Taylor. 1994. Bats and

mines. Bat Conservation International Resource

Publication No. 3. 50 pp.

Tuttle, M.D. and D.E. Stevenson. 1978. Variation

in the cave environment and its biological

implications. In Zuber, R. (ed.). Proceedings of

the National Cave Management Symposium–

1977. Big Sky, Montana. pp. 108-121.

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