Is It Worth Repairing Floodwater Retarding Structure No. 18A?
Introduction
The purpose of this project is to establish a cost ratio associated with a flood
retarding dam, known as floodwater retarding structure 18A, or dam 18A from hence
forth, north of the City of Celina within the northwest portion of Collin County, Texas.
Damn 18A, of the Little Elm and Laterals Watershed on an unnamed westerly
tributary of Little Elm Creek, is of concern due to its existing and concurrent eroding
action taking place producing areas of mass wasting along portions of the earthen dam.
The eroding sections of the dam can be seen here.
Dam 18A was built in 1970 by the Collin County Soil and Water Conservation
District with assistance from the National Resources Conservation Service and designed
for watershed protection and agricultural flood reduction with at least a 50 year lifespan.
The dam is now under the sole responsibility of Collin County and specifically the Collin
County Soil and Water Conservation District. Recently, because of the increased
destabilization of the dam, a 24 inch diameter pipe has been installed at the lowest point
of the dam to drain the pond and release the flood waters during heavy precipitation
events.
With the present erosion of the dam taking place, a cost ratio needs to be
established to determine of what cost Collin County would incur if the dam were to burst
during a heavy saturation period creating a full basin behind the dam. Once this cost is
determined in case of dam failure, does this cost exceed the money saved by allowing the
slow erosion of the dam to continue and possibly fail at a later time or possibly never
fail?
Questions to be Answered by Analysis
1. How much water does dam hold at maximum and how much precipitation is
needed for it to fill?
2. What is amount of water released by different breach sizes or by complete
collapse of dam and where does it go?
3. Are there people living in the possible flood zone and is there a possibility of cars
or trains passing by at the time of a breach?
4. What is the combined value of the parcels within the potential flood zone?
5. What are the costs of all possible damage and loss of life that could occur from a
dam breach?
6. Should Collin County elect to repair Flood Retarding Structure 18A?
Problem Objective
The main objective of this project is to receive a clear and concise “yes” or “no”
in regards to the question of whether or not to repair flood retarding structure 18A. The
yes or no received at the end of the analysis will be determined by examining the costs
associated with a possible dam burst compared to the costs associated with repair of the
dam. The costs associated with dam failure or repair will be determined by examining a
large amount of spatial data centrally available because of the geographic information
system. The spatial data easily seen and accessed with the GIS includes features such as
roads and city boundaries along with elevation, watershed, and parcel data.
In order to more easily understand all of the circumstances with respect to the
main question asked of the analysis, several points are to be addressed. When discussing
a “dam burst or failure”, an event such as this refers to a significant portion of the dam
collapsing during a time when the maximum amount of water possible is held back by the
dam.
The point of performing this spatial analysis is to understand all of the spatial
properties involved with a dam when it’s constructed because a dam’s purpose is to hold
back flood waters, and if the dam falls into a point of such disrepair to the point of being
unable to perform the original task of its construction, dire consequences could arise.
The analysis performed in this project could help build or add to further research on
additional dams located in rural Collin County to determine their impact on local streams,
watersheds, land values, and cost to the local government.
This project is of relevance due to the nearly 100 aging dams found across Collin
County. Designed with a 50 year life span, these dams are actually aging prematurely
and there may need to be a massive effort by the federal government and by Collin
County to solve the problems associated with these aging dams. This project could help
set a precedent by determining the costs associated with a dam burst event, and whether
these costs outweigh the option of leaving an aging dam “as is.”
This study of dam 18A is also being done because of its location on the private
land of Marvin and Renetta Wilson. The owners of the land have asked the county to
take the responsibility to repair the dam and have threatened to file suit if the government
of Collin County does not take action.16
Question 1: How much does dam hold at maximum capacity and how
much precipitation is needed for it to fill?
Dam 18A holds back water from a drainage basin area of approximately 685
acres1 and holds approximately 1,820,404,678 gallons of water2 if the pond is at
maximum capacity. A 16 acre surface area is the average pond size created by Dam 18A
after a heavy downpour, but a pond with a surface area of approximately 37.3 acres could
be potential held back by the dam should heavy rain event produce 15 inches within 24
hours. This amount of rain could fill the dam because of the dam’s large local watershed
of 685 acres. The surface area of the pond at maximum capacity along with contour lines
used to find average depth for water volume calculations is shown here.
The soil of Collin County lies within the Blackland Prairie4 and is well suited for
rapid surface drainage because of high clay content of the local alfisol and vertisol soils.
With so much clay in this soil, a heavy rain event will have lots of overland water flow
because of clay’s ability to quickly swell and prevent soil percolation with the addition of
water.6,7 The area of the Blackland Prairie is found stretching from south Texas up
through north Texas and can be seen here.8
Question 2: What is the amount of water released by different breach sizes or by
complete collapse of the dam and what lies in the path?
As found in the previous question, the dam at maximum capacity holds
approximately 1,820,404,678 gallons of water. If a complete collapse of the dam
occurred, all 1,820,404,678 gallons of water could possibly overtop state highway 289 by
3.7 feet and overtop the BN-SF railroad by 2.2 feet and move at a maximum speed of 4.1
feet per second.4 A total collapse of the dam results in a possible flooding of a .473
square mile area downstream, which could also possibly flood several structures within
the 100 year floodplain.9 The structures, railroad, and roadways within the potential flood
zone are shown here.
If there are one or several partial collapses, then the amount of water released
varies. There are several areas on the dam currently eroding away. Several situations
could occur in terms of different breach sizes if one or several of these particular areas
fail. There are two slides on the west side of the dam of similar size and there are five
slides on the east side of the dam of similar size with one exception. All of these eroding
parts of the dam can be seen here.
1) The two eroding portions on the west side of the dam have an approximate
surface area of 2290 ft2, and their vertical slice through the dam at its peak is
approximately 1080 ft2. All of the surface area has been estimated using a
combination of the ArcMap measuring tool and contour lines to determine height.
The amount of water released is found by using the basic formula10
Flow Rate = area x velocity x friction(the roughness of the channel)
I. Step 1 – Area = average width x depth = ((54ft + 22ft)/2) x 20ft = 1480 ft2
II. Step2 – Velocity = 4.1 ft/second found by consultants using expensive
hydrology software
III. Step 3 – Friction/Roughness of channel ~= 0.8 from friction with sides and
quicker moving water moving through center of channel
IV. Step 4 – Flow Rate = area x velocity x friction = 1480 ft2 x 4.1ft/sec x 0.8
= 4854.4 ft3 / second
3
At the rate of 4854.4 ft / second, it would take 83 minutes for the pond to drain
out. If both of the eroding spots on the west side of the dam were to fail, the time to drain
out would be roughly cut in half to around 42 minutes. If either of these situations did
occur, then the water released could potentially be channeled through the current
streambed and culverts without any overtopping of state highway 289 and only require
minimal repair.
If the larger slides on the east side of the dam were to give way, then the amount
of water released per second would be greater and it would take significantly less time for
the pond to drain, possibly resulting in an overtopping of S.H. 289 causing more damage
and more funds for repair. The slides on the east side of the dam are in more disrepair
than the slides on the west side of the dam therefore they are more likely to give way and
cause damage to S.H. 289 and other land further down the stream channel.
Question 3: Are there people living in the possible flood zone and is there a
possibility of cars or trains passing by at the time of a breach?
There are three houses lying within the actual 100 year floodplain of the Little Elm
creek downstream from Dam 18A, but none of these structures actually fall within the
possible modeled flood extent of a complete dam collapse. Although none of these
structures actually fall within the possible flood extent found by Wilson and Company,
their presence in the floodplain could lead to flooding in a dam collapse. A complete
dam collapse could lead to death due to drowning, especially if the breach occurred
during the night hours. The structures, along with the flood plain and flood extent from
question 2, can be seen here.
State Highway 289 lies 530 from Dam 18A and has a local 2003 vehicle count per
day of roughly 4,100.4 Over the period of a day, an average of 170.8 vehicles pass by the
dam every hour. 171 vehicles passing by per hour translates to 3 cars passing by nearly
every minute. The traffic count has surely increased since then with the rapid growth of
Celina and northern Collin County. A complete collapse of the dam, and the resulting
torrent of water, would have a very high chance of sweeping one or multiple vehicles off
the road, leading to a potential loss of life. Also, the time of day of the collapse would
make a large difference on how many vehicles could be swept away because of heavier
traffic volumes during the morning and evening hours as people travel to and from their
place of employment.
The Burlington Northern-Santa Fe rail line lies about 3000 feet from Dam 18A
with 12 trains passing by per day. 12 trains passing by this local location averages to 1
train every 2 hours.4 The average number of trains passing by per hour should be
relatively consistent because trains operate 24 hours a day consistently to optimize
delivery of goods. With the number of trains utilizing the lines downstream from the
dam being relatively low, lives lost and trains damaged by flooding from a dam collapse
is very minimal. A dam breach would most likely result in damage incurred to the rail
lines and bridge with resulting train delays and cost of repair.
Question 4: What is the combined value of the parcels within the potential flood
zone?
There are a total of 35 parcels that fall within or touch the potential flood zone
from a maximum release of water with a total combined appraised value of
$2,234,191.11.11 An image of the parcels is seen here. The area of the flood plain only
takes up about 40% of the area of the parcels12, so since a flood caused by a burst of the
dam will not completely destroy all parcels touching the floodplain downstream of Dam
18A, the estimated cost of the loss of land is $893,676.13
Question 5: What are the costs of all possible damage and loss of life that could
occur from a dam breach?
Should Collin County decide to repair the dam and bring it up to current safety
standards, the dam’s elevation will be risen 1.8 feet at the peak height and the slope of
each side will be brought to a 3:1 ratio.4 In addition to the previous work, an impact
basin will be installed below the principle spillway. The cost of repairing Dam 18A to
current safety standards is projected to cost $916,400.4 Although $667, 400 of the cost to
repair the dam would come from the federal government, with the remaining $249,000
coming from the county, the county can no longer receive the federal grant money as of
Tuesday, June 28, 2005.16
1. If a vehicle or vehicles were to be washed away while passing by the dam on S.H.
289, the driver or drivers could sue Collin County for negligence because of the
county’s failure to repair the dam when there were known problems. Also, the county
could lose more money if a driver died from being swept away after the dam burst
from the family of the driver. All of this could result in hundreds of thousands to
millions of dollars.
2. Houses lying within the floodplain may be destroyed. The houses within the
floodplain have a combined appraisal value is $217,531. In addition to the appraised
value, there would be the problem of the owners possibly suing. The suing owners
could possibly once again result in a payout of hundreds of thousands to millions of
dollars depending on loss of life.
3. The landowners of the other 32 parcels would want reimbursement for their portions
of land washed away during the flood. The value of the other 32 parcels is $676,145
after the cost of the other 3 parcels is subtracted out.
4. With a dam burst, a section of S.H. 289 about 490 feet long would also be damaged
or destroyed and would have to be repaired. An estimated cost to repair this section
of roadway is $50,000 to $100,000 according the similar figures for road update and
repair from the Collin County Capital Improvement Program.14 Repair work would
also have to be done on Business 289 further downstream. The length of the repair
work to be done on Business 289 is about 1050 feet and an estimated cost of repair is
$100,000 to $150,000 depending on damage.14
5. Damage would also be taken by the BNSF rail line and bridge because this line is
located just over half a mile from the dam. Wilson and Company estimated water
would overtop the rails by 2.2 feet in a dam burst-flood event.9 This damage alone
could cost more than $62,000 for the quarter mile of rail running through the
floodplain and flood areas.15 Another problem with damage to the rails is the loss of
profit to BNSF because of delays and rerouting that would take place. Therefore, a
total loss from damage to the rails could reach into the millions of dollars.
Question 6: Should Collin County elect to repair Flood Retarding Structure 18A?
According to the National Climatic Data Center, the North Texas region where
Collin County is found, averages 37.05 inches of precipitation per year.17 The highest
average precipitation per month for North Texas is the month of May with 5.3 inches.17
Although there are times when a heavy rainfall will fill the dam to its 16 acre surface, the
15 inches of precipitation required in a 24 hour period to fill the pond to the peak of the
dam is a highly unlikely event. Also, considering the highest ever recorded one day
precipitation event for Dallas/Fort Worth is 8.56 inches, set on April 25, 1922,18 a 15 inch
rain in 24 hours is even more unlikely.
Even with the unlikely chance of a 15 inch rain in 24 hours in Collin County, the
repairing of the dam should be an obvious and easily made decision by the government of
Collin County. Should Dam 18A ever burst with a full basin, the actual cost of the total
damage is as follows.
1) Should a vehicle be washed off S.H. 289, the range of the cost to the county could
vary from $50,000 to $1,000,000 depending on number of vehicles and possible
death.
2) If the 3 houses within the floodplain are damaged or destroyed, the damage would
total at least $271,531. The damage to the residential structures along with the
damage to land and possible loss of life could total $1,000,000 or more.
3) The cost of the other 32 parcels within the flood zone totals $676,145. The land
value for this area of Collin County is on the rise with growth occurring here at an
amazing pace, so the value of $676,145 may be much less than actual value.
4) The cost of repair to the roadways range from $150,000 to $250,000 when
compared to similar roadway and length in the Collin County Capital
Improvement Program.
5) The cost of repair to the BNSF rail line is at least $62,00015 and will almost
certainly be higher when the cost of possible bridge repair for the line is added.
Should BNSF decide to sue for loss of profits from train delays and rerouting, the
whole rail situation could cost $500,000 or more.
The grand total of all possible costs associated with the burst of Dam 18A at
maximum capacity is $1,155,676 on the very lowest end, with the high total being
possibly $3,426,145. These values have been found by researching values and costs
associated with each of the five different scenarios incurred, but in actuality, the costs
could be significantly higher depending on the payout of potential lawsuits.
The $916,400 cost estimate of dam repair is lower than the low estimate of cost
should the dam breach. Also, should Collin County decide to repair the dam with no help
from the federal government, there will be no 100 year contract forcing Collin County to
pay for upkeep and maintenance of Dam 18A. With a one-time cost to repair Dam 18A,
the government of Collin County will then have no fear of any upcoming dam breach and
all the upcoming costs associated with such an event.
When considering whether or not the money should be spent on the repair of
Flood Retarding Structure 18A, the government of Collin County should vote a
definitive “yes”.
Conclusion
There is so much damage involved with the break of a dam from the resulting
flood that no matter the situation, if there is any dam, whether large or small, the decision
for repair should always be made if in a deteriorating state. A dam is made to hold back
large amounts of water, and if a dam no longer has the capacity to hold back a large
amount of water without a danger, repairs should be made.
Dam 18A does not typically hold back a large basin of water presently from the
installation of a large outlet for the water, but on occasion, the amount of precipitation in
the local watershed far outweighs the amount of water released by the 24 inch pipe
installed to release the flood waters in a controlled manner. Since Dam 18A is in a very
deteriorated state and still has the potential to fill during extended heavy precipitation,
repair should be made immediately to insure the well being of local residents and to
prevent the huge cost incurred in case of dam failure.
Literature Review
Literature for this analysis includes research on waterflow through soils, impacts
of dam failures, weather extremes, and water modeling.
Data
All digital data collected from the Collin County ArcSDE geodatabase and the
Collin County Appraisal District.
References
Works Cited
Notes
1) Insert Celina annexations and come up with percentage increase in city per year to
back up conversion from rural to urban
2) 16 acre surface area or 696,500 sq.ft
3) With average depth of 7 ft., then there’s gallons 36,463,864.5 gallons 
(696500*7)*7.479  computations from
http://www.srh.noaa.gov/wgrfc/convert.html; (1,622,681*15)*7.479 =
1,820,404,678 gallons.; 1,622,681ft2*15ft = 24,340,215 ft3 of water.
4) Traffic count for SH 289 just north of dam in 1999 is 2630 in 24 hours, from
dfwinfo, http://ims.dfwinfo.com/tcins/viewer.htm…see what increase in Celina is
since then and also perhaps subdivisions around there to make a projection
5) Make excel of land values and possible impacts of dam burst and create map of
parcel value ranges and possibly create graph
6) Create map of Celina increase in size for as many years as possible with possible
accompanying percentage graph
7) Do calculation of rainfall in local shed to see how long it would take for pond to
fill up and how much water is leavings the 24 inch pipe per hour, talk to BJ; also
possibly create contours with elevation data and calculate the slope or steepness
of the shed
8) Find out how much it costs to create a highway and railroad; can look at DOT and
railroad data
9) When mapping polygons, sometimes use crosshatching and dashed borders
instead of filling and fading
10) Use slope and other 3D Analyst tools and then reclassify to easily see classes
11) Possibly land values over time and plot them
12) Research to see if anyone has done a dam burst study
13) Find equation from spatial analysis in powerpoint final presentation and find out
how much rain it takes to fill dam. Area of dam shed is 29,869,696 sq. ft.
14) See if flood data or hydrology software is available in the library for free…
15) How much is a new locomotive?