19 - Drainage Presentation - Sandy Creek Upper Tributaries 4 and 6Item No. 19
Memorandum
TO: Mayor
Mayor Pro Tem
City Council
FROM: Todd Mittge, City Engineer
SUBJECT: Drainage Analysis Presentation — Upper Tributaries 4 and 6, Big Sandy Creek
DATE: August 14, 2023
BACKGROUND: The City of Paris has drainage challenges throughout the city. This analysis
combines 5 months of first-hand observations by the City Engineer and other staff from the Winter
of 2023, along with historical data from the last major flood events since 2016 and anecdotal data
going back to the 1970's for Tributaries 4 and 6 of Big Sandy Creek. This analysis and its
associated recommendations complement the Cobb-Fendley Engineering report from 2016. Please
contact the City Engineering Department for a copy of that report.
Tributaries 4 and 6 drain parts of the Northeast and Southeast Quadrants of the City of Paris, Texas.
Drainage pathways include rooftops, surface drains on city streets, some enclosed drainage
systems, grass/dirt lined open channels, concrete lined V-shaped channels, concrete lined
trapezoidal channels, and concrete lined rectangular channels. All of the channelized pathways
vary in capacity, and may be inconsistent with the amount of flows required of them. A visual
illustration of these are shown below.
Upon my initial review of the situation, I recommend that the city:
1) Further Explore expanding the sizes of some channels in strategic locations.
2) Clean out the channels of significant obstacles and debris.
3) Build detention ponds in key locations.
4) Utilize city staff to perform this work as soon as possible, contracting out where needed.
5) Continue proper enforcement of drainage detention through our subdivision ordinances.
Improve ordinance language where necessary.
6) Conduct Lidar laser survey citywide to capture existing topography and aid in future
studies.
There are some city properties adjacent to the tributaries where flood overflow detention facilities
could be built which will help alleviate flooding downstream. There is also the opportunity to
partner with a private property owner to build a larger detention facility near Lamar Avenue.
Access is difficult in many areas, but using city crews and strategic access points, we can increase
capacity and help reduce the risk of flooding of area streets, residences and businesses.
STATUS OF ISSUE: Engineering staff has studied available data and compiled a comprehensive
report of the main drainage issues on Tributaries 4 and 6 as described above. We will be presenting
this to the council and be available for any questions and requests for further information.
BUDGET: This item will not have an impact on the current budget, but has an item included in
the FY23/24 budget currently under consideration by the council (e.g. The citywide Lidar Survey).
Memorandum
TO: Grayson Path, City Manager
Robert Vine, Assistant City Manager
FROM: Todd Mittge, P.E., City Engineer
SUBJECT: Drainage Analysis — Upper Basin of Tributaries 4 and 6, Big Sandy Creek
DATE: July 11, 2023
Executive Summary:
The City of Paris has drainage challenges throughout the city. This analysis combines 5 months
of first-hand observations by the City Engineer and other staff from the Winter of 2023, along with
historical data from the last major flood events since 2016 and anecdotal data going back to the
1970's for Tributaries 4 and 6 of Big Sandy Creek. This analysis and its associated
recommendations complement the Cobb-Fendley Engineering report from 2016. Please contact
the City Engineering Department for a copy of that report.
Tributaries 4 and 6 drain parts of the Northeast and Southeast Quadrants of the City of Paris, Texas.
Drainage pathways include rooftops, surface drains on city streets, some enclosed drainage
systems, grass/dirt lined open channels, concrete lined V-shaped channels, concrete lined
trapezoidal channels, and concrete lined rectangular channels. All of the channelized pathways
vary in capacity, and may be inconsistent with the amount of flows required of them. A visual
illustration of these are shown below.
Upon my initial review of the situation, I recommend that the city:
1) Further Explore expanding the sizes of some channels in strategic locations.
2) Clean out the channels of significant obstacles and debris.
3) Build detention ponds in key locations.
4) Utilize city staff to perform this work as soon as possible, contracting out where needed.
5) Continue proper enforcement of drainage detention through our subdivision ordinances.
Improve ordinance language where necessary.
6) Conduct Lidar laser survey citywide to capture existing topography and aid in future
studies.
There are some city properties adjacent to the tributaries where flood overflow detention facilities
could be built which will help alleviate flooding downstream. There is also the opportunity to
partner with a private property owner to build a larger detention facility near Lamar Avenue.
Access is difficult in many areas, but using city crews and strategic access points, we can increase
capacity and help reduce the risk of flooding of area streets, residences and businesses.
Analysis:
Drainage Area: Tributary 4 of Big Sandy Creek receives drainage from the Northeast Quadrant of
Paris, from approximately East Booth Street and 19th St NE over to 25th St NE and Highland
Road. The urban environment from which it comes drains south past East Price Street, through the
Bellcrest Apartments to Lamar Avenue, and connects with Tributary 6 in the vicinity of 26th St
SE and Simpson Street. Tributary 6 receives drainage from the area of North Collegiate Drive
near Pine Mill Road and flows South along Collegiate to Culbertson Street, and connects with
Tributary 4. See Exhibit A after the conclusion of the report for Drainage Analysis Map.
A pictorial tour of the various types of existing drainage improvements is shown below.
Image 1: Tributary 4: Example of a surface drainage flume on 21St Street NE.
Image 2: Tributary 4: Dirt/concrete debris lined channel south of East Cherry Street flowing
toward Wade Park.
Image 3: Tributary 4: Debris Filled channel upstream from Wade Park
Image 4: Tributary 4: Channel transition to Wade Park. Location of proposed Overflow basin on
Left side of channel.
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Image 5: Tributary 4: Smaller Concrete lined channel near the Bellcrest Apartments.
Image 6: Tributary 4: Channel behind Paris Family Dental Office on Lamar Avenue.
Image 7: Tributary 4: TXDOT Box Culvert under Lamar Avenue, which has been overtopped at
least twice in 2016, once in April 2022, and twice so far in 2023. March 91` and June 14th,
4
Image 8: Tributary 4: Old Masonry structure on South side of Lamar Avenue after flows pass
through box culvert as shown above.
Image 9: Tributary 6: Concrete Channel North of Culbertson Street, reconstructed in the early
1990's.
gal% :%piioi
/riy e
Image 10: Tributary 6: Smaller, original concrete lined channel South of Culbertson Street. It has
been proposed to replace this section of channel with a wider, rectangular section with city crews
to remove the constriction and alleviate flooding here. This must be evaluated further to ensure
that more homes are not flooded downstream.
Image
6
Image 12: Tributary 6: Channel widens slightly as it flows to the southwest toward the confluence
of both Tributaries.
Image 13: Confluence of Tributaries 4 and 6.
Image 14: Combined Tributary: Large grass lined channel as it crosses under South Collegiate
Drive. It is important to keep this maintained and flowing, as this acts as the main floodway (a
channel for an overflow of water caused by flooding) for the creek downstream of the residential
neighborhoods. Where we have the opportunity to widen this channel and place concrete blocks,
it will help hydraulic functionality of the basin.
2016 Flood Event,: A large flood event occurred in the city on the week of April 30, 2016.
According to the Cobb-Fendley Engineering Report, sponsored by the City Council after the event
transpired, rain amounts over a narrow time period contributed to flooding. "On April 30, 2016,
the City of Paris received 5.99 inches of rain. Over May 2 and May 3, the City received an
additional 0.5 inches of rain. On May 9 and May 10, the City received an additional 5.04 inches
of rain. This data was taken at Paris Cox Field airport and retrieved from the National Oceanic
and Atmospheric Administration (NOAA) Global Historical Climatology Network... Significant
flooding occurred after the April 30 rain event and again after the May 9 event. A total of 11.53
inches of rain fell over an 11 day period. "
This has been characterized in media and local discussion as a 500 year flood recurrence interval.
Each year, there is a 0.2% chance that this could happen. For reference a 100 year recurrence
interval has a I% chance of happening each year. This event has been anecdotally conveyed from
residents and business owners that areas that typically do not flood were flooded at least 2 times
in 2016.
Cobb-FendleyReort SummLiw
The Engineering Firm Cobb-Fendley from Frisco, Texas developed a report on Tributaries 4 and
6 in between August 2017 and April 2018. The following is a series of excerpts from their
executive summary and report.
"Cobb Fendley performed an on -the -ground survey of Tributaries 4 and 6, collecting drainage
culvert dimensions and channel cross-section data. The survey data was merged with surrounding
lidar data for use in the hydraulic analysis. The existing floodplain model was obtained from the
Federal Emergency Management Agency (FEMA) and updated with the survey data. Using the
updated survey data, the model produced a large reduction from the existing Special Flood Hazard
Area (100 year floodplain) and Floodway.
It appears that the City owns drainage easements for the existing drainage channel. Before design
and construction of any improvements to Tributaries 4 and 6, boundaries and easements should
be resolved to ensure the channels are within the easement. Temporary construction easements
will likely be necessary for construction access.
A combination of channel improvements and culvert improvements will maximize the decrease in
flooding impacts. The channel improvements maintain the top width of the existing channel and
create a more uniform grade from Clarksville Street to Lamar Avenue along Tributary 4. Tributary
6 would be deepened similarly from the Tributary 4 confluence upstream to Hubbard Street
without widening the channel.
It is recommended that the improvements occur in three phases. Phase 1 would consist of channel
and culvert improvements from the downstream end of the study up to the confluence of the two
tributaries between Simpson Street and Culbertson Street. Phase 2 continues the channel and
culvert improvements upstream along Tributary 4 to E. Price Street. Phase 3 replaces the channel
and culverts along Tributary 6. The total cost is $10.2m. "
Existing Conditions were recorded on both Tributaries as follows: Box culvert at Clark lane with
a utility line in the middle of the culvert:
Existing Condit:ions
Tributary ooith off Clarksville Street is a 15 -foot Mde eoinc etew,fine , veadea� waR channel. Duooat earn
of Clarksville Street, time charuoel is earthen aiiiii indemterrotoate .I"he culvert at Clark Lane has a ut'ihtti� line
CUt &rectly to to the iniddle of the cLAIvert (ttgeiume .f 1.
k'rnua, i— r1 ,,;1p/(frf,%' 1-0'fe f rcnOjg
At 24t` SE near Lamar:
At t°tutrfreer Street, the vertweel-rmvaH chaiir°iiia°mel changes from tm feet rote down tii,eeirn to 1.01 feet ide
e etreor°irr, There we several rropter les near temver Avenue on 24th Street SE that elllppear to sully lre
eeeeeeEbllGe from existing bridges eeroee t.re channO, "rhere to e steel NicAge under eorrretructioo as of
Noveerber 30, 2016 to access three diirteleerre eoro e the ehrer°roel (Figure 2). At this pltohftp Tributary 4 is 10
feet wide. Upstrearrt of tarner��� � venue, the eMnnde tapers gradually to e , .foot tope width tee eeoi el
channel.
, ., I ilrt ,,,a 1'45 e ,, 1 i a"
The old Masonry structure on the Downstream end of Lamar (shown earlier in the report):
The ex[sting culvert oirrr er Lanier. Avenue is e ooefot source of 1liydeee@te mrueff derrert. At e,rr iindet:errotu°iiete
point under taineir ttwnue, two concrete box culverts converge into e single, niasonry well eoodielt, There
appears to be e concrete floor, There are several other igrregii,ilar, culverts along t"etfrrrteet t,
I q"wre , ;r Sli fig , till','i`
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The location where Tributaries 4 and 6 meet near 26t' Street SE and Simpson:
1`rNftary 6 is a 25 -foot wide cup ( rete -lined, trapezoidal channel Yvith a,.unafl curve at the cnfiUence With
m° irmwtar rt (flgure 4The ecrmilrauenciek is e wide area of flooding In time FOAA effective flood rnap as wwweV
as the re0semf existing condition model el (see Appen&x ).
Cobb Fendley's analysis included combining city survey and lidar data with FEMA's hydrologic
and hydraulic models from the 1970's. Most of FEMA's data was on microfiche. This old
microfiche data was converted for use in HEC -RAS Version 4. 1.0 (Hydraulic Engineering Center
— River Analysis System), the industry standard in flood modeling. They applied the hydraulic
models to the existing Flood Insurance Rate Map (FIRMs), which had been last updated in 2011.
The effective FEMA FIRM Map was modified using the data that Cobb Fendley modeled, and the
"Special Flood Hazard Area (Commonly referred to as the "100 -year floodplain") shrank
significantly".
They recommended several projects, and ran a proposed model through HEC -RAS, resulting in
the 100 year floodplain shrinking significantly. This however came with an approximate price tag
of $10.2 Million. The report did discuss an immediate and inexpensive step that would potentially
reduce home owner's insurance need. Cobb Fendley discussed the process of revising the Special
Flood Hazard Area to current conditions based on their combination of survey, lidar, and
microfiche data, and how "Letter of Map" Amendments (LOMAs) could be sent to FEMA to
revise the floodplain to no longer require flood insurance for some properties. I contacted Cobb
Fendley about an updated fee and scope of services to provide what is needed to proceed with
these LOMAs. Their estimate was $10,000 to update their data with current conditions and work
with FEMA to amend the flood map. This would help give homeowners more options with their
future insurance, and/or lower existing insurance rates.
Going back to the two 2016 flood events, Cobb Fendley discussed the following interesting facts
about flood gauges on the Sulphur River downstream:
"The United States Geological Survey (USGS) has 2 stream gages in the same drainage
basin (Big Cypress — Sulphur Basin) as Big Sandy Creek Tributaries 4 and 6. Gage
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07343000 near Cooper, Texas (Cooper Gage herein) on the North Sulphur River is 10.9
miles upstream of the confluence of Big Sandy Creek and the North Sulphur River. Gage
07343200 near Talco, Texas (Talco Gage herein) on the Sulphur River is 22 miles
downstream of the confluence of Big Sandy Creek and the North Sulphur River. Both gages
measure discharge (flow rate) and gage height (water surface elevation).
The Cooper Gage was in flood stage (gage height of 30 feet or more) from 22:00 CDT on
April 29, 2016 until 02:45 CDT on April 30, 2016 The Talco Gage was in flood stage
(gage height of 20 feet or more) from 23:30 CDT on April 17, 2016 until 20:45 CDT on
May 14, 2016 It is outside of the scope of this analysis to determine backwater effects in
Paris due to downstream conditions in the Sulphur River, but the stream gage data
confirms that the greater Paris region received extraordinary amounts of rain. "
With the limited flood gauges in the immediate vicinity of Paris, this information is important. It
shows that as the storm was occurring in Paris, the river was already at or above flood stage. With
so much water contributing to the larger drainage basin, and the water from Paris also waiting in
the queue to drain, there was nowhere for the water to go. It was forced to back up and use the
floodplain as nature intended, which had a large impact on anyone living in those areas. I would
compare the situation to a bathtub that is filling up and trying to drain, but the sewer line is
undersized as it is leaving the house, and the sewer main at the street is already full from all of the
other houses, so the water backs up in to the house.
Cobb Fendley offered some Alternatives and a recommendation. The Appendices referenced
below are not included in this summary but can be found by reviewing the original Cobb Fendley
report:
"Alternatives
CobbFendley investigated several alternatives to mitigate flooding in the two tributaries.
The 100 year rainfall event can be contained completely within the limits of the existing
channel by deepening the entire channel and replacing all culverts. Some culverts will need
to be replaced with bridges unless additional right-of-way is acquired.
Culvert replacement (Alternative 1, shown in Appendix F) yielded little to no improvement
in the flooded areas. This indicates that channel conveyance capacity is the controlling
problem, not strictly culvert capacity. For the purpose of this analysis, culverts were
replaced with bridges and the existing stream cross section was maintained through the
bridge area. During the design phase, culverts should be sized and designed where right-
of-way permits to save costs.
Alternative 2, shown in Appendix G, expands on Alternative 1 and includes some channel
improvements to further reduce the water surface elevation. The channel improvements
include a v -shaped ditch that is concrete lined for most of the channel. The v -shaped ditch
option is cheaper than a vertical wall concrete channel, but the over top width of the
channel would be wider than the existing channel. Widening the channel would require
additional easement and/or right-of-way, which is inadvisable in an established residential
neighborhood.
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Alternative 3, shown in Appendix H, replaces the existing concrete -lined, vertical wall
channel with a deeper channel of similar construction. Deepening the channel must begin
at the downstream end as to not impede flow during construction. The overall width of the
channel is maintained so acquiring additional right-of-way or easements is not necessary.
Most of the channel is immediately adjacent to residences. Appendix H includes detailed
cost estimates and improvements for this alternative. "
"Recommendations
The Alternative 3 improvements should be constructed in 3 phases.
Phase 1: downstream of Clarksville, the existing earthen channel will be regraded to a 25 foot wide
grasslined channel with 4:1 side slopes at the existing depth. The channel upstream of Clarksville
Avenue up to and including the confluence with Tributary 6 will be replaced with a 15 foot wide
concrete channel with vertical walls, cut three to four feet (3'— 4') deeper. All existing culverts will
be replaced to match the proposed deeper channel flowline. A detailed breakdown of these costs
is shown in:
Lower Tributary 4 Channel improvements $ 2,947,438.89
Lower Tributary 4 Culvert Improvements $ 564,550.00
Phase 1 - Lower Tributary 4 Subtotal $ 3,511,988.89
Phase 2: the upper portion of Tributary 4 will be improved. The channel will be improved from the
confluence upstream to Lamar Avenue with a 15 foot wide concrete channel with vertical walls,
with a cut ranging from one to four feet (1' — 4'). Upstream of Lamar, no channel improvements
need to be made. All culverts along the upper reach of Tributary 4 need to be replaced, including
the culvert under Lamar Avenue and the culvert at Price Street.
Upper Tributary 4 Channel Improvements $ 2,403,489.64
Upper Tributary 4 Culvert Improvements $ 458,782.50
Phase 2 - Upper Tributary 4 Subtotal $ 2,862,272.14
Phase 3: Tributary 6 will be improved. The existing trapezoidal channel will be replaced with a 25 -
foot wide, vertical wall concrete channel. The culverts along Tributary 6 will need to be replaced
with bridges unless additional right-of-way is acquired.
Tributary 6 Channel improvements $ 640,092.88
Tributary 6 Culvert improvements $ 741,275.00
Phase 3 - Tributary 6 Subtotal $1,381,367.88
These costs are construction estimates only and do not include design survey or engineering fees.
These costs also do not account for inflation or any escalation in construction prices. These costs
were estimated using recent bid tabulations and Texas Department of Transportation average bid
prices.
Construction Subtotal $ 7,755,628.91
Contingency $1,551,125.78
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Fngineering, Surveying (10YQ .$ 930,675.47
ESTI TFC GRA ND TO TA L $ 10,237,430.16
In order to independently interpret the Cobb Fendley Report, City leadership hired Hayter
Engineering, Inc. to evaluate and summarize the report in 2018. Hayter prepared a memorandum.
to the city dated November 7, 201&
"During rainfall events in 20.15, 20.16, and 2017 residents living along Big Sandy Creek
Tributaries 4 and 6 experienced flood damages to their homes (see attached reap)... load damages
were most prevalent south of Lamar Avenue along 26th Street between Hubbard Street and
Collegiate Drive. the City of .Maris (City) retained Cobb-Fendley (CF) to perjbrm an analysis of
BigSandy Creek Tributaries 4 and 6 to determine improvements that could be made to minimize
flooding during rain events, CFYs report recommends as three -phased improvement plan,.,/roin
.Lamar Avenue south to the South Collegiate Drive 7"Mil.flead, totaling $10.2 million.
Hayter E, ngineering (HEI) reviewed CF, 's report and modeling and concluded that the biggest
factor in the observed flooding is the channel section between Simpson Street to ClarkLane. As
such, HEI hags focused their attention on this area in developing a preliminary solution that would
mitigate flooding along this section of channel, The table belowprovides some insight into why
HE7 believes the channel section from Simpson Street to Clark Lane plays a large role in the
observedflooding in this area. In particular, please note the floww coming to and the floww area of
each crossing. "
Crossing
Tributary
Crossing Size i
Flow
Flow Area
(span x rise)
(CFS)
Culbertson
Trib. 4
2 - 5..5x 4.5'
400
49.5
20h SE
"rib. 6
2 - s -..O' x.3,5''
399
56.0
sinipsoll
`nb.4 - D/S Confluence
2 - 9ff x 2.5"
876
45,O
Clei k
�i'SCq�ntluet�ice,4
Lj�jb-A - D
�-6-
76
J:
-84.0,
With theflows passing through the undersized existing culverts, HEIsurtnised that necessary
flows would not fit within existing channels, nor the existing channels easements if they were :an
place. "Upstream and downstream channel modifications would be required to accommodate
the increased span and rise of any proposed culverts. SinceTIE1 does not believe that lowering
the existing channel 'sflowline is afeasible alternative, EI focused on an alternative that
includes homelproperty buyouts along the west side qf'the channel. However, rather than buying
out these homes1properties to increase the capacity of the concrete channel, HEI is suggesting a
more cost-efficient,?ption that involves increasing the capacity of thefloodplain along the west
side of the existing channel. The existing concrete channel would be utilizedfor lowflows and
larger raiqfi711 events would spill out into an expandedfloodplain (a result of homelproperty
bqyouts along the west side of the reach) and be stored there until able to pass downstream.
They also recommended increasing the sizes of the culverts at Simpson Street and Clark.Lane,
M
"While this solution mitigates a major cause offlooding in this area and is significantly less than
the $10.2M improvement plan proposed by CF, the cost (monetarily and politically) would still
be high. Based on our experience, culvert improvement projects can be expected to range from
$175k to $225kper crossing. Add to that the home/property buyouts at $80k to $115kper (six
total) and the cost to demolish and increase (excavation) the floodplain capacity along the west
side of the channel and the City would be looking at a project in the $1.75M to $2.OMprice
range. "
HEI pointed out that their options did not address the flooding of Tributary 6 (or 4) due to the
undersized channels there.
"The Tributary 6 channel, immediately downstream of Culbertson, decreases in size. Upstream
of Culbertson, the concrete lined trapezoidal channel has a 5 -foot bottom width and 15 -foot top
width; however downstream of the Culbertson Street crossing the bottom width decreases to 2.5 -
feet and the top width decreases to 9 -feet. While the proposed solution indirectly addresses this
issue by reducing the backwater effect at Simpson Street, the flooding in this area could be
further mitigated with channel modifications downstream of Culbertson. "
This is the section shown in Images 10 and 11 earlier in this report. While widening this smaller
channel is necessary, the flooding downstream of it will likely worsen, and should be further
studied prior to excavation.
HEI also proposed adding storm drain pipes and inlets along 26th Street between Simpson and
Collegiate Drive. These pipes would probably need to be 4 feet in diameter and require half of
the street to be excavated to be installed. I estimate construction costs to install 4 foot diameter
storm systems and inlets along 2601 at approximately $300,000. More studies and precise
drainage calculations would need to be performed before implementing any of these strategies. A
stormwater system in this location would likely only make sense if the channel was widened
from the Confluence of Tributaries 4 and 6 (shown in Image 13) up to Culbertson Street (Image
10).
City Action following the Cobb Fendley Report and Hayter Memorandums:
Following the studies and analysis, City Leadership asked the Public Works Department to
reinforce their efforts to clean sections of drainage ditches throughout the city. Some of the areas
where they cleaned and improved the side channels of the ditches and tributaries are as follows:
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Image 15: 1260 241 Street SE. This ditch was reinforced with concrete blocks to hold back the
slope from the apartments, which was being eroded away. Organic material and sediment build-
up is removed periodically to better accommodate flow.
Image 16: 901 Soulli Collegiate Drive, This section of the tribe ary was excavated, with
reinforcing blocks placed to help provide more space for water flow and prevent erosion.
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Image 17: Soutli Collegiate facfi,ig North to Clarksville Streel. This section of the tributary was
excaval
Image 18: Love Civic Center Entrance. This section of tributary suffered from excessive scour
and was excavated to install concrete blocks and reinforced with rip rap to prevent further scour.
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Image 19: Love Civic Center Entrance flowing at capacity on June 14, 2023.
Public Works has done an excellent job with this type of channel improvement. This method could
be used throughout the city if staffing were available. A dedicated drainage crew has been
proposed for Public Works, and the Engineering Department fu11y supports that,
Recommendations:
Upon my initial review of the situation, I recommend that the city:
1) Further explore expanding the sizes of some channels in strategic locations.
2) Clean out the channels of significant obstacles and debris.
3) Build detention ponds in key locations.
4) Utilize city staff to perform this work as soon as possible, contracting out where needed.
5) Continue proper enforcement of drainage detention through our subdivision ordinances.
Improve ordinance language where necessary.
6) Conduct Lidar laser survey citywide to capture existing topography and aid in future
studies.
Regarding #1: Channel reconstruction and widening from the downstream end up toward the top
of the basin should be pursued. While this will take significant financial resources, there is some
work that we believe in-house crews can perform to help alleviate the situation. Care should be
taken to not make flooding worse downstream as each channel is analyzed for possible widening.
Regarding #2: There is some work that we believe in-house crews can perform to help alleviate
the situation.
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Regarding #3: There are opportunities where concentrated efforts could improve certain areas of
the floodplain. For example, the large open area on the western edge of Wade Park, which used to
be the location of the wading pool, could be used for overflow storage in a flood event. It would
be designed to fill up when needed, and then slowly empty as the flood event wanes, reducing the
intensity of flood events downstream. Exhibits showing this location as well as a location
immediately north of the Paris Family Dental are provided below.
Regarding #4: When possible, given the City has existing crews in the budget, we would
recommend utilizing these crews to perform some of this work as the workload allows.
Regarding # 5: As properties develop in this and any drainage area around the city, I recommend
each developer perform an engineering analysis of pre -development and post -development flows.
This is normally done on a 25 year recurrence interval (4% chance of this storm happening in any
given year). The water leaving the site needs to be metered to pre -development levels after the
build. In the case of large impervious surface area increase (i.e. parking lot, added roof space,
patios, etc.), this could involve vaults, storm water catchment areas, swales, and/or ponds. Flow
control devices such as weirs, orifice plates, baffles, and other methods of metering are very
effective for 25 year and lower storms. When storms exceed the 25 year recurrence interval, the
devices have overflow openings to allow free flow of water downstream.
The current ordinances relating to stormwater improvements during development do not currently
require what is discussed in the previous paragraph. The current drainage requirements located in
Exhibit 9A Subdivision Ordinance, Section IX. Design of Improvements, Subsection E: Drainage
reads as follows:
"E. Draingge.
Storm sewers, bridges, culverts, inlets, and drainage channels or ditches of a permanent design
adequate to carry off such rainfall shall be installed by the subdivider throughout the entire length
of the drainage area where the surface drainage traverses the subdivision. The subdivider may
construct in lieu of storm sewers suitable drainage channels which shall be of sufficient width,
depth and shape to carry off the surface drainage and allow for maintenance operations in keeping
such ditches open. All drainage ditches so constructed shall have their sides sloped so as to retard
erosions; the flow line of said ditches shall be permanently established by small low flow concrete
sections, and the banks of all curves or bends shall be permanently protected to prevent erosion.
No open drainage channels shall be constructed within the areas dedicated as public streets and
alleys in said subdivision provided however, that in cases where utilization o the land ol the
im_ roved development warrants otherwise, such ditches ma ,v be located in the center :, ortion o '
the street_ (except at street intersections where culverts or bridges of a permanent design shall be
constructed) where a right-of-way not less than eighty (80) feet in width, plus width, for drainage
channel, is provided.
Computations to determine whether such lots or land will be flooded by rainfall and the size of
drainage facilities adequate to prevent flooding shall be based upon the "Rational Method" using
the Texas State Highway Department's Frequency of Rainfall for District 1, and a five-year cycle
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for residential areas, Class I & 2, ten-year cycle for apartment areas and mass housing projects,
and twenty -five-year cycle for areas of lower zoning classification than residential.
No street shall be designed or allowed in any subdivision to carry more surface water than can be
carried by the street when the water is running curb deep.
All drainage easements over or across adjacent tracts of land that are necessitated by the
subdividing of a tract of land shall be secured and furnished to the City by the subdivider. "
The City Engineering Department would recommend an overhaul of this section to be included in
the Paris Development Code as it is developed. I would recommend that our Code of Ordinance
base most zoning at a minimum of the 50 year recurrence interval storm to store more water onsite
before discharging downstream. Requiring a larger recurrence interval storm (I.E. 100 year) for
each development would be better, but may prove cost prohibitive. Requiring stormwater detention
and a slower discharge rate would help in any rain event because there would be metering of water
off the site, as opposed to now where in many cases, there is no flood storage whatsoever. I also
highly recommend �.,etting away from automatically draining all of the stormwater to the public
streets. Enclosing the stormwater into sufficiently sized pipes after their private detention facility,
and draining directly to an established and appropriately sized tributary would be the best option.
I believe strong, two-way communication with private developers and detailed and enforceable
maintenance plans for private drainage facilities will be important moving forward.
I discussed some of these issues with the City of Sherman, Texas, and they have the following
statement in their Stormwater Manual, "A detention facility must be able to hold a 100 year (2
hour rainfall event) for the site without exceeding the pre -developed Discharge Rate. " The City
of McKinney also requires the 100 year event to be detained. Since our current requirement is not
defined in clear terms like Sherman and McKinney, I believe 50 year storm detention and discharge
is a reasonable requirement.
Each site is different, and an individually engineered solution needs to be evaluated each time. As
we move forward, the City Engineering Department will emphasize with each engineer working
on these projects that this is of utmost importance to the city.
Regarding #6: The Engineering Department would like to conduct an aerial survey of the entire
city to enhance understanding of existing drainage pathways. Lidar stands for Laser imaging,
detection, and ranging. This would be conducted by a plane flight, with a laser shooting every
surface throughout the city at 4-8 shots per square meter. This will help immensely as we look to
improve drainage capacity, understand flowpaths, revise floodplains, measure existing impervious
areas (e.g. rooftops, sidewalks, streets), etc. Prices include flights and data processing. Once
processed, we will have 1 foot contours throughout the city, and that will reduce survey costs for
individuals wanting to develop in Paris (or any users of the Platting process). The data would also
be ready and available for special consultants and will reduce the need for special surveys. Costs
for this work to be conducted is approximately $34,000.
c
Stormwater Detention O tions:
Wade Park Overflow Basin
Approximate Storage Capacity for a 5' deep pond: 158,312 Cubic Feet = 3.634 Acre Feet
Opinion of Probable Construction cost at $0.40/CF* = $63,300.
21
*Estimated cost to construct dry detention ponds from "Naturally Resilient Communities" website:
ht s:Hnresolutions.or °floodwater -detention/
Paris Family Dental Overflow Basin
Approximate Storage Capacity for a 3' deep pond: 65,335 Cubic Feet = 1.50 Acre Feet
Opinion of Probable Construction cost at $0.40/CF = $26,100.
22
Although not located in Tributary 4 or 6, another possible area for an Overflow Basin: 245 East
Polk Street. The owner of the Lot highlighted in Orange would like to donate the property to the
city, since it is unsuitable for building. The Lot highlighted in dashed purple will likely be a part
of a future year's tax sale and if obtained by the city, an Overflow basin could be built straddling
both lots. A basin of 85'x 125' would be recommended.
10,625 Square Feet.
Approximate Storage Capacity for a 3' deep pond: 31,875 Cubic Feet = 0.732 Acre Feet
Opinion of Probable Construction cost at $0.40/CF = $12,750.
23
Image 17: 645 & 655 East Polk Street. Vacant Lots which could be converted to overflow
basins.
There are likely many more lots like this around the city. Planning & Community Development
and other departments are on the look -out for other property owners wishing to donate parcels.
These proposed overflow detention basins are not permanent fixes to the serious problems that our
city faces. But placed in strategic places, and of sufficient size to detain a large amount of water,
they would help alleviate flooding problems downstream.
Existing Stormwater Pi a Re air O tions:
During the course of this drainage analysis, I discovered that an existing concrete stormwater pipe
system is having structural issues and is allowing sink holes to develop above the pipe. This pipe
is approximately 460 feet long, and is along the northern curb -line of East Cherry Street from 20th
Street NE to the Blue Sky Self Storage past 21St Street SE. To prevent additional sink hole
development and provide long-term maintenance on this pipe, I recommend lining this pipe section
with Insituform cured -in-place lining. This is a fiberglass -type liner which will reinforce the pipe
from the inside, without having to dig it up, and likely not require maintenance other than sediment
cleaning for decades. To dig up this section would require shoring for East Cherry Street, and
possible removal of utility lines immediately north of the pipe. Replacement costs are estimated
at approximately $500,000. Insituform has quoted $210,000 to line this pipe.
24
Image 18: Area above existing 48" storm pipe along the north side of East Cherry Street. Sink
holes are developing near the base of the utility pole shown. Additional sink holes appear
approximately 150 feet further East toward 21 sc Street NE.
Image 19: View at the end of the pipe in question. (shown on right).
25
Conclusion:
Long term solutions will involve hiring of a focused engineering team to design large scale
improvements such as constructing new sections of concrete channels. The City should consider
pursuing this, however it should be considered city-wide and not just for Tributary #4 and #6. In
the meantime, keeping existing channels free of debris and overgrowth, updating & enforcing our
subdivision ordinances for drainage retention, using the latest technology to digitally survey our
city, and creating new overflow basins in strategic, city -owned lots will be the best immediate
projects in my opinion as we move forward. City forces should be able to perform much of these
smaller tasks and will do a great job to keep the costs down. Strong, two-way communication with
the development community will also be key to successful future growth.
As former Council Member Clifford stated at a City Council meeting on 4/24/17, "it sounded like
with a 500 year event that no matter how much money they spent, they would not be able to stop
the flood. " He also said the improvements the City had already done seemed to be making a
difference. He is correct, and my goal is to work as hard as we can with the resources we have to
make things better where we can. We cannot guarantee that flooding will never occur, particularly
in the floodplain, but concentrated efforts where we can make small changes with significant
impacts will be the best way to move forward in the near term to try to alleviate the issue in a
financially feasible manner.
26
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