HomeMy WebLinkAboutFlood Vulnreability Study April 2019 CITY OF FRIENDSWOOD
FLOOD VULNERABILITY AND
MITIGATION STUDY
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TABLE OF CONTENTS
EXECUTIVE SUMMARY 1
I. PROJECT OBJECTIVES 2
II. BACKGROUND AND STUDY AREA 3
III. METHODOLOGY 4
Hydrologic Model(Vflo®®) 4
Hydraulic Model (HEC-RAS) 5
Data Sources 6
IV. FLOOD VULNERABILITY ANALYSIS 7
V. ASSESSMENT OF FLOOD MITIGATION SCENARIOS 12
Methods 14
Bridge Removal 14
De-snagging 15
Terracing 16
Diversion 17
Detention 20
Combination Scenarios 23
Results and Discussion 24
Bridge Removal 24
De-snagging 25
Terracing 25
Diversion 26
Detention 28
Combination Scenarios 29
Model Limitations 31
VI. CONCLUSIONS AND RECOMMENDATIONS 33
VII. APPENDICES 35
Appendix A: LIDAR Difference Map 35
Appendix B:Summary Tables by Storm 36
Appendix C: Profiles 39
Appendix D: Floodplain Maps 49
EXECUTIVE SUMMARY
This study evaluates the existing flood vulnerability of the City of Friendswood, TX in the
context of Hurricane Harvey (2017) and the 100-yr storms (HCFCD and NOAA ATLAS 14 -
2018 depths). A distributed hydrologic model (Vflo®®) was used to simulate watershed
response (i.e., stormwater runoff) for the various storms. Peak discharges were exported
into a 1D steady-state hydraulic model, HEC-RAS, to compute water surface elevations
throughout the main channel, Clear Creek. These results were then used to generate
floodplain maps focusing on the City of Friendswood. The baseline models representing
existing conditions of the watershed were validated against available stream gages and high
water marks for Harvey.
Next, the study investigates various mitigation options to address the city's flooding
problem: bridge removal, de-snagging, grass terracing, diversion and detention. Among the
five individual mitigation scenarios, the Terracing II option from Dixie Farm Rd to I45
provides the most flood reduction benefit, with an estimated peak water surface elevation
reduction in the range of-3-6 ft within Friendswood for the new (NOAA ATLAS 14) 100-yr
storm. However, it is important to note that there are limitations to the modeling method
which make it difficult to capture the altered hydrodynamics and conveyance of mitigation
options such as de-snagging and terracing. As a result, there remains uncertainty in the
downstream impacts of the mitigation strategies which require physical modifications to
large stretches of the main channel. In addition to the standalone mitigation scenarios, the
impacts of selected combination scenarios were assessed. The scenario with terracing
performed from Dixie Farm Rd to I45 combined with 50% flow diversion at Bay Area Blvd
showed the most flood reduction of all the scenarios analyzed. However, the caveats
regarding modeling limitations apply in this scenario as well.
1
I. PROJECT OBJECTIVES
The main objectives of this project are twofold: (1) perform a flood vulnerability analysis at
the City of Friendswood in the context of Hurricane Harvey(August 2017),the existing 100-
yr (HCFCD), and the new 100-yr (NOAA Atlas 14 - 2018) storms; and (2) evaluate the
effectiveness of various flood mitigation options proposed by the City of Friendswood.
1. Flood Vulnerability Analysis
This task is accomplished by developing a distributed hydrologic model (Vflo®®) and
using a 1D steady-state hydraulic model (HEC-RAS),focusing on the City of Friendswood.
Modeled results for Hurricane Harvey are validated against available observed data (i.e.,
USGS or HCFCD stream gages and official high water marks). In addition, the City's flood
vulnerability under the existing and new 100-yr 24-hr storms are also evaluated. The
three models serve as baseline for subsequent flood mitigation analysis.
2. Assessment of Flood Mitigation Scenarios
The effectiveness of the following mitigation options are compared against the baseline
scenarios:
■ Bridge removal
■ Bank de-snagging
■ Terracing
o I: Dixie Farm Rd to Bay Area Blvd
o II: Dixie Farm Rd to I45
■ Diversion
o I: Divert flow at Bay Area Blvd
O II: Divert Turkey Creek to Bay Area Blvd
■ Detention
■ Combination scenario
O I: FM 2351 bridge removal+ Terracing
O II: Terracing I + 50% Diversion at Bay Area Blvd
O III: Terracing II + 50% Diversion at Bay Area Blvd
2
II. BACKGROUND AND STUDY AREA
The City of Friendswood, TX (Figure II.1) is located approximately 18 miles southeast of
downtown Houston.The city limit covers an area of 21.2 mi2,predominantly within the Clear
Creek watershed. The city's land use mostly consists of residential and developed open /
green spaces. The main drainage channels in Friendswood include Clear Creek and its
tributaries (i.e. Mary's Creek, Coward's Creek, Chigger Creek, Mud Gully,and Turkey Creek).
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Figure II.1. City of Friendswood, TX
Friendswood is prone to flooding from heavy storms, as evidenced by the extensive flood
damage caused by Hurricane Harvey in August 2017 which caused flood impacts to over
2,000 single-family homes. Upper Clear Creek including Mud Gully and Turkey Creek
contributed about 55% of the flows into Friendswood while Mary's, Coward's and Chigger
3
Creek contributed the remaining 45% of flows combined. The heavy rainfall combined with
hydraulic properties of the region including low slopes and high imperviousness were likely
drivers of the flood event. Additionally, due to its flat topography and its proximity to the
coast, the city is also vulnerable to flooding from storm surge. To better understand and
address the various flood issues in Friendswood, this study aims to evaluate the existing
flood vulnerability of the city in the context of Hurricane Harvey and the 100-yr design
storms and also to investigate potential mitigation options for implementation.
III. METHODOLOGY
Hydrologic Model(Mee)
In this study, hydrologic analysis is conducted using a physics-based,distributed hydrologic
model,Vflo®®1.This model was developed by Dr. Baxter Vieux (Vieux&Associates),and has
been used in numerous floodplain and/or watershed studies throughout the Greater
Houston Region.The main advantage of using a distributed hydrologic model such as Vflo®®
over lumped parameter models (e.g., HEC-HMS) is its capability to better represent the
topography and land use conditions of a study area. A Vflo®® model domain (Figure III.1)
consists of a series of inter-connected grid cells, each having its own set of physical (e.g.,
slope,flow direction) and hydraulic (e.g.,roughness,hydraulic conductivity)properties.The
model employs the Green and Ampt equation to compute infiltration. Overland runoff and
channel flow are routed via the Kinematic Wave and Modified Puls methods. Rainfall input
options are uniform precipitation (e.g., design storms), rain gage interpolation, and radar
rainfall. For this project, a Vflo®® model that covers the entire Clear Creek and Armand
Bayou watersheds was developed and calibrated against historic storms.
1 https://www.vieuxinc.com/solutions/Vflo®distributed-hydrologic-model/
4
Channel cells
> Overland cells . J./ • - •
>
N .
—> Flow direction
)21) /
>. \\44>4(
Rainfall
Runon
Runoff
/r./.
Infiltration
A A
Figure III.1.Vflo® domain overview
Hydraulic Model(HEC-RAS)
The hydraulic model, HEC-RAS2, is developed by the Hydrologic Engineering Center (HEC)
of the U.S.Army Corps of Engineers.This model has been widely used in various channel and
floodplain studies throughout the U.S., and is also the basis for the generation of the official
FEMA floodplain maps. This study uses a 1D steady-state HEC-RAS model of Clear
Creek/Armand Bayou watershed developed by the HCFCD, obtained through the Model and
Map Management (M3) system 3. As shown in Figure III.2, the 1D steady-state model
represents the channel by a number of cross-sections (i.e., River Stations), and iteratively
computes the water surface elevations within each cross-section based on head difference
using the Energy Balance and Manning's equations. Some limitations of the 1D steady-state
HEC-RAS model include time independent results,no hydraulic routing,and 1D flow.For this
study, the original M3 HEC-RAS model (Version 3.1) was converted to and run in Version
5.0.6. This was done due to the new capabilities of the software, such as the ability to import
terrain data and generate floodplain maps within the model.
2 https://www.hec.usace.army.mil/software/hec-ras/
3 http://www.m3models.org/
5
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41011111i 1‘1 . _ 10-
efAre
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Figure III.2.HEC-RAS domain of Clear Creek with cross-sections with terrain data
(2018 LIDAR)
Data Sources
To develop a hydrologic model that best represents the actual conditions of the study area,
high-resolution spatial and temporal datasets are required. For topography, 1-meter
resolution LIDAR 2008 (HCFCD) was used 4 . Land use and land cover (LULC) and
imperviousness (30-meter) were obtained from the National Land Cover Databases (NLCD
2011). Soil data was obtained from the Natural Resources Conservation Service6 (NRCS). To
simulate Hurricane Harvey (2017), hourly gage-adjusted radar rainfall data was obtained
from NOAA's National Severe Storms Laboratory' (NSSL).This equates to approximately 40
inches of rain in 4 days. For the 100-yr storms, published values from HCFCD8 (i.e., 13.5
inches in 24 hrs) and NOAA Atlas 149 (i.e., 17.65 inches in 24 hrs) were used. For model
validation, stream gages from HCFCDlo and USGS11,as well as USGS high water marks12 were
used.
4 In February 2019,Rice University obtained access to LIDAR 2018 from HGAC (https://www.h-
ac.com/lidar-cost-share/default.aspx).This new dataset was compared to the 2008 LIDAR dataset(see
Appendix A),with results showing that the majority of the study area experienced no significant elevation
change.As such,LIDAR 2018 was not used to update the previously developed Vflo®model,however for
visualization purposes,LIDAR 2018 was used to generate floodplain maps in HEC-RAS
s https://www.mrlc.gov/data
6 https://websoilsurvey.sc.egov.usda.gov/App/HomePage.htm
https://www.nssl.noaa.gov/projects/mrms/
8 https://www.hcfcd.org/media/1999/hcfcd-hydrology-hydraulics-manual 03-2016.pdf
9 https://hdsc.nws.noaa.gov/hdsc/pfds/pfds_gis.html
io https://www.harriscountyfws.org/
11 https://waterdata.usgs.gov/tx/nwis/inventory/
12 https://stn.wim.usgs.gov/STNDataPortal/#
6
IV. FLOOD VULNERABILITY ANALYSIS
The peak water surface elevation (WSEL) results produced by the HEC-RAS model were
compared to Harris County stream gage data for Harvey and the HCFCD 100-yr storm for
model validation and to assess Friendswood's existing flood vulnerability. Floodplain maps
for the storms were also included in this analysis. For all modeled baseline scenarios,
downstream boundary conditions were set to normal depth. The stream gages used for
model comparison are shown in Figure IV.1.
N -
�'� Pry Lind B
Gage 135
Fnen wood
Gage 120
USGS 08077600
Gage 110 L`
40
Gage 130
0.5 1 2
Miles ,
Figure IV.1. HCFCD stream gages for model validation
For Hurricane Harvey, an available USGS highwater mark located at Gage 135 was also used
for comparison. The validation results for Harvey are shown in Table IV.1. Overall, the
modeled peak WSELs matched well with the reported peak WSELs with less than a foot
difference.At Gage 135,the stream gage peak WSEL differed by the USGS highwater mark by
about 1.7 ft. Since the modeled peak WSEL at this location lies between the range reported
by the stream gage and the highwater mark, the model results here were considered
satisfactory.
7
Table IV.1. Model validation at Friendswood for Harvey; all values are shown in ft
Measurement Observed USGS High Modeled Peak Difference from
Location (HCFCD) Water Mark WSEL Observed
WSEL
Gage 135 27.70 29.43 28.97 -0.46 (from HWM)
Gage 120 24.20 - 24.50 0.30
Gage 130 21.10 - 21.79 0.69
Gage 110 16.60 - 16.64 0.04
Figure IV.2 shows a floodplain depth map for Harvey created using HEC-RAS. Although the
floodplain map does not delineate actual flood impact from Harvey, it highlights
Friendswood's vulnerability for extensive flood impact during large storms.
JtM HOUSTON PKWY Harvey- 2018 Lidar
j •) r Y A
afkk;r .
.
•
•
i
,�, •� Areal Extent:28,577 Acres
Legend
Depth (ft.) Harvey
0 -2.8
A �2.9 - 7-4
] 7.5 - 12-.9
0 0.5 1 2 3 4 13 - 18.4
CCCi Miles ME 18.5 -45.1
Figure IV.2. Harvey floodplain map with updated 2018 LIDAR
8
For the HCFCD 100-yr storm, the model comparison results are shown in Table IV.2. Results
for the 100-yr storm using the M3 model are also shown. There are a number of
discrepancies between the reported peak WSEL from the HCFCD FWS site and the results
from the M3 model also developed by HCFCD. At the stream gage locations, although our
RAS model showed discrepancies larger than half a foot compared to what HCFCD reported
online, our RAS model matched with the M3 model by less than half a foot difference. Since
our model better corresponds with the M3 model, which is considered the official model
released by HCFCD, the model results obtained for the 100-yr storm are considered
satisfactory.
Table IV.2. Model comparison at Friendswood for the HCFCD 100-yr storm; all values
are shown in ft
Measurement Reported Modeled Peak Modeled- M3 Peak WSEL1 Modeled -M3
Location WSEL(HCFCD) WSEL Reported
(HFCD)
Gage 135 25.90 25.32 -0.58 25.83 -0.51
Gage 120 21.00 20.09 -0.91 20.53 -0.44
Gage 130 17.20 16.74 -0.46 16.68 0.06
Gage 110 12.40 11.30 -1.10 11.15 0.15
9
LAM HOUSTON PKWY 100-Year HCFCD- 2018 Lidar
•
•
• r
s .
•
tri
Areal Extent 18,258 Acres
Legend
Depth (ft.) 100yr HCFCD
AI 10 -28
I 129- 74
=75- 129
0051 2 3 4 = 13- 184
Miles - 185 -451
Figure IV 3 City of Fnendswood, TX, 100-yr (HCFCD) floodplain map with updated
2018 LIDAR
For the NOAA Atlas 14 (2018) 100-yr storm, there are no available stream gage peak
comparisons available yet from HCFCD However, the modeled peak WSELs are shown in
Table IV 3 for reference
Table IV 3 Modeled peak WSEL for the NOAA Atlas 14 - 2018 100-yr storm, all values
are shown in ft
Measurement Modeled Peak
Location WSEL
Gage 135 28 17
Gage 120 23 14
Gage 130 20 15
Gage 110 13 75
10
M HOUSTON PKWY 100-Year NOAA Atlas 14- 2018 Lidar
tt,
t. ' r, i>,>
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i Ire
at
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1 I Areal Extent 24,652 Acres
• Legend
Depth (ft.) 100yrAtlas 14
N
A0 -28
Li29-74
MI75-129
0051 2 3 4 =13-184
Miles =18 5-45 1
Figure IV 4 City of Fnendswood,TX, 100-yr(NOAA Atlas 14 2018) floodplain map
with updated 2018 lidar
The results shown for Harvey and the 100-yr storms provide a baseline for the flood
vulnerability of Fnendswood In the proceeding sections, the impacts of the various flood
mitigation strategies are presented as changes in peak WSEL from the baseline condition to
show the flood reduction potential expected with each strategy
11
V. ASSESSMENT OF FLOOD MITIGATION SCENARIOS
For the flood mitigation scenarios discussed in the next sections, results from HEC-RAS were
obtained for the 9 watchpoints shown in Figure V.1. The supplementary Excel workbook
provided with this report contains data tables for all mitigation scenarios at all 9
watchpoints.The pre-mitigation (i.e.,existing) peak WSEL at these watchpoints for the three
storms analyzed are shown in Table V.1.
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moos (Thailand).NGCC,®OpenStreetMap contributors,and th 3 IS User
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Figure V.1. Location of HEC-RAS watchpoints. US = upstream and DS= downstream
Table V.1. Pre-mitigation Peak WSEL at each watchpoint for all 3 modeled storms
Watchpoint Location Storm 1 Peak WSEL (ft)
100yr (HCFCD) 24.72
1 US FM 2351 100yr (Atlas 14) 27.72
Harvey 28.03
2 DS FM 2351 100yr (HCFCD) 23.3
12
100yr (Atlas 14) 26.27
Harvey 27.16
100yr (HCFCD) 20.8
3 US Whispering Pines 100yr (Atlas 14) 23.78
Harvey 24.79
100yr (HCFCD) 19.79
4 DS Whispering Pines 100yr (Atlas 14) 23.05
Harvey 24.2
100yr (HCFCD) 19.18
5 US FM 528 100yr (Atlas 14) 22.42
Harvey 23.64
100yr (HCFCD) 18.51
6 DS FM 528 100yr (Atlas 14) 21.63
Harvey 23.12
100yr (HCFCD) 16.68
7 DS Bay Area Blvd. 100yr (Atlas 14) 20.1
Harvey 21.79
100yr (HCFCD) 11.13
8 I-45 100yr (Atlas 14) 13.62
Harvey 16.49
100yr (HCFCD) 6.18
9 Clear Lake 100yr (Atlas 14) 8.59
Harvey 10.38
13
Methods
The modeling of the mitigation options required separate approaches. Bridge removal, de-
snagging,terracing, and bridge removal+ terracing were implemented in HEC-RAS. In these
scenarios, the same Vflo® peak flows obtained in the pre-mitigation scenario were applied
to the modified HEC-RAS geometries; this limitation prevents channel hydrodynamics from
being accurately resolved, and the implications are further explained in the Model
Limitations section of the report. Diversion and detention scenarios were implemented in
Vflo® and supplied to HEC-RAS with no geometry modifications. Terracing+ diversion was
implemented using Vflo® for the diversion and HEC-RAS for the terracing. The following
describes the setup and implementation of each mitigation option in more detail.
Bridge Removal
The bridge removal scenario focused on removing three major bridges within Friendswood:
the bridge located at FM 2351, the bridge at Whispering Pines and the bridge at FM 528
(Figure V.2).
� T
gik
FM 2351
Nassau
In.1 Bay
Whispering Pines
Friel'
'O FM 528
I
0.5 1. .__ 2 /—\.5-/--C-/
o Miles
Figure V.2.Bridges removed in bridge removal mitigation scenario; (1) FM 2351, (2)
Whispering Pines (3) FM 528.
Four modeling scenarios were analyzed: three individual bridge removals and one combined
removal of all three bridges. To simulate the removal of the bridges (see below for example
of bridge cross-section), the selected bridge cross-sections and the corresponding upstream
14
and downstream ineffective flow areas were removed from the HEC-RAS geometry file. For
each bridge removal scenario, Harvey and the 100-yr storms were simulated using peak
flows obtained from Vflo®.
40
SO
zo
�•. Div {'~ t. . __
10
7000 7500 8000 8500 9000
Station(ft)
Figure V.3. Cross-section view of bridge at FM 2351
De-snagging
De-snag was modeled by changing the overbank surface roughness coefficient of the cross-
sections from Dixie Farm Rd to Bay Area Blvd. in HEC-RAS. The original roughness
coefficients in this region range from 0.12 to 0.15,which typify vegetative environments.The
de-snagging performed here simulates the clearing of trees and vegetation that contribute
to increased friction, and assumes a uniform short grass surface of roughness coefficient
0.04.
This de-snagging(Figure V.4)was performed up to a distance of 200 ft from the left and right
banks of the main channel. The meanders of the natural channel were not altered. Harvey
and the 100-yr storms were simulated using peak flows from Vflo®.
15
Legend
�rl� Ground
/J Y •
Bank Sta
4
200 ft \ 200 ft
/
Roughness=0.04 1 / Roughness=0.04
i
\ )
VJ
v
Figure V.4. De-snagging sample configuration
Terracing
The following design for grass terracing was applied to channel cross-sections in HEC-RAS.
For Terracing I,the template was applied from Dixie Farm Rd to Bay Area Blvd. For Terracing
II, the template was applied from Dixie Farm Rd to I45.
30-
Legend
25: \ 1
Template
0 20: •
Bank Sta
0, 15 Daylight Cut
u,
10:
5:
0'
-400 -300 -200 -100 0 100 200 300 400
Station 200
19.-40
Figure V.5. Terracing channel geometry template
A channel template was generated in HEC-RAS's`Channel Design/Modification'module.The
template (Figure V.5) consists of 200 ft terraces (10 ft deep) on both banks of the main
16
channel, and a trapezoidal channel with a depth of 14 ft and a bottom width of 60 ft. Side
slopes for both the terraces and the channel are 1:4. A uniform roughness coefficient of 0.04
is applied to both the terraced banks and the channel to represent grass lining.The following
figure shows an example of how the terracing is implemented at the cross-section
representing Watchpoint 1.
07 :Ila 04n�-s-� ala 07 1
5H Legend
20 Modified XS
Ground
------i--- —
0
10
Ineff
w Bank Sta
0- \ / Template
Bank Sta
8500 9000 9500 10000
Station
Figure V.6.RAS Cross-section showing terracing implementation at Watchpoint 1
Diversion
Two modeling techniques were used to simulate the diversion scenarios presented in this
report.
Diversion I:Divert flow at Bay Area Blvd
Three diversion scenarios were modeled to represent the diversion of flows at Bay Area
Blvd.by 10%, 25%, and 50%. The diversions were modeled by reducing the existing volume
of flow at Bay Area Blvd.,which coincides with the location of Gage 130 (Figure V.7),by 10%,
25% and 50% in Vflo®. These reductions in flow were then propagated downstream of the
diversions. Note that no actual diversion structures/channels were modeled using this
method.A reduced flow hydrograph was simply supplied at the Gage 130 cell in Vflo® as a
forcing parameter. New peak flow values were then obtained from Vflo®for Harvey and the
100-yr storms and exported to HEC-RAS.
17
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t �Q aPye o
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Bay Area Blvd/
\ / N" '
' Gage 130
.
A 1
•
1
0 0.5 1 2 ♦- -
Miles
s
Figure V.7. Location of proposed diversions
Diversion II:Divert Turkey Creek to Bay Area Blvd
A diversion scenario where 50% of the flows introduced by Turkey Creek into the main
channel were diverted to Bay Area Blvd. Figure V.8 shows the drainage area of Turkey Creek
that was selected for the diversion. Vflo® was used to simulate the diversion, and the
corresponding peak flows were provided to HEC-RAS. The HEC-RAS geometry was not
modified to represent the implementation of the diversion channel. Thus, results of the
diversion are only reported within the main channel in the next section.
18
Drainage Area = 6494 acres (10 sq. mi)
Turkey Creek
' l
Clear Cree
Diversion
' Mary's Creek
Coward's Creek\ �� +
f
\higger Creek j
Figure V.8. Drainage area of Turkey Creek for Diversion (shown in orange)
The location and route of the diversion, shown in Figure V.9,was chosen to avoid developed
areas and take advantage of available green space. Unlike the Diversion I scenario, the
Turkey Creek diversion was simulated by modifying the directions of the Vflo® cells
underlying the diversion route toward Bay Area Blvd and re-enter the main channel. The
cells along the diversion were modified to trapezoidal cross-sections with a bottom width of
60 ft with a side slope of 1:4. The lining of the diversion channel was modeled as grass
(manning's roughness = 0.04). For the 100 yr storms and Harvey, a flow hydrograph at the
location of the diversion at Turkey Creek was scaled by 50% and supplied to the two cells
immediately downstream (one to feed into the diversion channel and one to feed into the
main channel). These reduced hydrographs served to simulate 50% of Turkey Creek's flow
being diverted to Bay Area Blvd and 50% still allowed to enter the main channel.
19
Legend//41 •
Turkey Creek Diversion
1 "S
♦
♦ Friendswood
f. i.i
♦ 1
♦ ;;�
* •
♦ *�
4
44
• 1
, e •
'1
/ 1
1 1
r' �' */ Bay Area Blvd'
7' ,91t
1(4'
wit
1.›'.\ .
Figure V.9. Location of Turkey Creek Diversion
Detention
Proposed detention ponds (Figure V.10) were incorporated into Vflo® using shapefile
provided by the City of Friendswood as reference.
20
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Figure V.10.Location of proposed detention ponds
Design rating curves were not available for the modeling team, and thus the ponds were
assumed to have a 10-ft depth and resemble theoretical"box-shaped" detention ponds (see
Figure V.11)with acreage equal to the aerial extent of the shapefiles (Table V.2).
TI
JN$JEMERGED ORIFICE
Figure V.11.Detention pond schematic
21
Table V.2.Acreage of proposed detention ponds calculated using GIS
Pond# Designed Acreage
1 73.0
2 36.5
3 64.2
4 53.1
5 21.2
6 33.3
The modeling team acknowledges the intended nature of the ponds for capturing riverine
overflow,however, due to the limitation of the finite difference implementation of Vflo®and
the 1-D steady-state nature of the HEC-RAS model, the modeled detention ponds were
modeled to capture drainage areas that are approximately 3 times the size of the pond and
release flows directly into the main channel. The stage-discharge relationship assumes a 3 ft
diameter orifice located at the 8 ft stage for all ponds and an emergency outflow structure
represented by a 50 ft broad-crested weir above the 10 ft stage. The respective rating curves
are shown in Figure V.12.
Stage-Storage Curve
1.5E+07 1
I
1.0E+07 5.0E+06
0.0E+00
0 2 4 6 8 10
Stage (ft)
22
Stage-Discharge Curve
50
4' 40
U
a 30
t 20
0 10
0
0 2 4 6 8 10
Stage (ft)
Figure V.12. Rating curves used for detention ponds in Vflo®. (above) stage-storage
curve for pond#2; (below) stage-discharge curve for all ponds
Combination Scenarios
Bridge Removal + Terracing to Bay Area Blvd
For this scenario, in addition to the grass terracing being applied from Dixie Farm Rd. to Bay
Area Blvd. as discussed in the previous section, the bridge at FM 2351 was also removed to
model the combined effects of the scenarios. A RAS geometry file was developed with these
combined mitigation options implemented. Harvey and the 100-yr storms were simulated
using peak flows from Vflo®.
Terracing +Diversion
To model the combination of terracing and diversion,peak flows computed by Vflo®which
reflect Diversion I at Bay Area Blvd are supplied to HEC-RAS where the terraced geometry is
used. For the first terracing and diversion scenario, the peak flows from the 50% diversion
scenario were used with the terraced geometry to Bay Area Blvd. In the second terracing and
diversion scenario, the peaks flows from the 50% diversion scenario were used with the
terraced geometry to I45. Harvey and 100-yr storms simulated for both combination
scenarios.
23
Results and Discussion
The effects of each mitigation scenario are summarized in this section for the Atlas 14 100-
yr storm. Results at each of the 9 watchpoints for each storm are provided in Appendix B.
Bridge Removal
Table V.3 shows the change in peak WSEL for the Atlas 14 100yr storm.
Table V.3. Changes in peak WSEL under the 1OOyr(Atlas 14) storm for bridge
removal scenarios
WSEL Change (ft)
FM 2351 Whispering FM 528 3 bridges
Watchpoint Location Pines
Removed Removed Removed Removed
1 US FM 2351 -1.22 -0.03 -0.03 -1.37
2 DS FM 2351 0.00 -0.08 -0.09 -0.17
3 US Whispering Pines 0.00 -0.16 -0.19 -0.36
4 DS Whispering Pines 0.00 0.00 -0.26 -0.26
5 US FM 528 0.00 0.00 -0.31 -0.31
6 DS FM 528 0.00 0.00 0.01 0.01
7 Bay Area Blvd. 0.00 0.00 0.00 0.00
8 I-45 0.00 0.00 0.00 0.00
9 Clear Lake 0.00 0.00 0.00 0.00
For the individual bridge removal scenarios, the greatest flood reduction impacts occurred
immediately upstream of the location of the removed bridge due to the elimination of
backwater effects caused by the obstruction. Among individual bridge removal scenarios,
removing the bridge at FM 2351 resulted in the largest reduction in WSEL upstream (-1.2
ft). Interestingly, removing all three bridges did not show significant changes from the FM
2351-only removal scenario upstream of FM 2351. This is likely partially attributed to the
varying elevations of the bridges which can either interfere with or have minimal impact on
water levels.The FM 2351 bridge deck is at an elevation of 22-30 ft whereas the Whispering
Pines and the FM 528 bridge decks are at an elevation of 17-22, and 20-27 ft, respectively.
For higher peak WSELs especially observed in Harvey and the Atlas 14 100-yr storm,bridges
at lower elevations create minimal obstructions as compared to bridges at higher elevations.
Generally,the impacts to flood levels are muted the farther one moves away from the bridge
24
removal location. No noticeable changes in peak water levels were observed downstream of
Friendswood toward I-45 and Clear Lake due to the 1D steady-state modeling methodology.
In actuality, there might be slight changes in downstream water levels due to improved
conveyance upstream.
De-snagging
Table V.4 shows the change in peak WSEL for the Atlas 14 100yr storm.
Table V.4. Changes in peak WSEL under the 100yr (Atlas 14) storm for de-snagging
Watchpoint Location WSEL change (ft)
1 US FM 2351 -1.23
2 DS FM 2351 -1.57
3 US Whispering Pines -0.96
4 DS Whispering Pines -0.92
5 US FM 528 -0.57
6 DS FM 528 -0.41
7 Bay Area Blvd. *0.00
8 I-45 *0.00
9 Clear Lake *0.00
De-snagging reduces peak WSEL throughout Friendswood by approximately 0.4-1.5 ft for
the Atlas 14 100yr storm. The clearing of vegetation across a distance of 200 ft from the
banks reduces friction in the channel and improves overall channel conveyance. This in turn
reduces peak WSELs for the Friendswood area with greater impact upstream and decreasing
impact moving downstream.
*We would expect de-snagging to have some impacts past the de-snagged segment,however
due to modeling limitations (i.e., 1D steady-state),these impacts are not well-captured.
Terracing
Table V.5 shows the change in peak WSEL for the Atlas 14 100yr storm.
Table V.5. Changes in peak WSEL under the 100yr (Atlas 14) storm for grass
terracing
WSEL change (ft)
Watchpoint Location Terracing to BAB Terracing to I45
1 US FM 2351 -4.83 -6.23
2 DS FM 2351 -3.96 -5.44
3 US Whispering Pines -2.37 -4.29
25
4 DS Whispering Pines -2.05 -4.13
5 US FM 528 -1.53 -3.65
6 DS FM 528 -1.09 -3.30
7 Bay Area Blvd. *-0.11 -2.68
8 I-45 *0.00 *0.00
9 Clear Lake *0.00 *0.00
For the Atlas 14 100-yr storm, grass terracing to Bay Area Blvd decreases flood depth in the
range of 0.1 to 4.8 ft within Friendswood and terracing to I45 decreases flood depth from 2.7
to 6.2 ft within Friendswood. The large overbank setbacks provide additional flood storage
capacity.The additional channel capacity with terracing extended to I-45 causes water levels
to decrease throughout Friendswood as compared to the terracing to Bay Area Blvd.
It is important to note that since the hydraulic analysis was conducted using a 1D steady-
state approach, potential detention and/or storage responses (which are time-dependent)
of the modified channel and terraces could not be accounted for. In other words, the grass
terracing results presented in this study only accounts for the effect of conveyance
improvement due to the modification of channel configurations at the applied cross-sections.
*We would expect terracing to have more notable impacts at locations past the terraced
segment, however due to modeling limitations (i.e., 1D steady-state), these impacts are not
well-captured.
Diversion
Table V.6 shows the change in peak WSEL for the Atlas 14 100-yr storm for Diversion I at
Bay Area Blvd.
Table V.6. Changes in peak WSEL under the 1OOyr (Atlas 14) storm for the proposed
diversions.
WSEL Change (ft)
Watchpoint Location 10% Div 25% Div 50% Div
1 US FM 2351 -0.03 -0.05 -0.09
2 DS FM 2351 -0.08 -0.15 -0.24
3 US Whispering Pines -0.16 -0.33 -0.52
4 DS Whispering Pines -0.21 -0.48 -0.78
5 US FM 528 -0.26 -0.58 -0.97
6 DS FM 528 -0.37 -0.84 -1.39
7 Bay Area Blvd. -0.65 -1.54 -2.77
8 I-45 -0.68 -1.63 -3.03
9 Clear Lake -0.40 -0.65 -0.67
26
Results from diversion scenarios showed some change in water surface elevation within
Friendswood (in the range of—0.1 to 1 ft for the 50% diversion). Generally,more significant
reduction in both water surface elevation and peak flow was observed downstream of the
diversion, at I-45 and Clear Lake (in the range of 1 to 3 ft). The impact of the diversions
decreases with increasing distance upstream. Recall, that the modeling method used here
represents the diversion as a "black hole"and does not re-introduce the diverted water into
the main channel/system.
Table V.7 shows the change in peak WSEL for the Atlas 14 100-yr storm for Diversion II from
Turkey Creek toward Bay Area Blvd.
Table V.7. Changes in peak WSEL under the 100-yr (Atlas 14) storm for the Turkey
Creek Diversion
Watchpoint Location WSEL change (ft)
1 US FM 2351 -0.89
2 DS FM 2351 -0.92
3 US Whispering Pines -0.71
4 DS Whispering Pines -0.71
5 US FM 528 -0.63
6 DS FM 528 -0.54
7 Bay Area Blvd. -0.34
8 I-45 0.04
9 Clear Lake 0.08
The Turkey Creek Diversion reduces water levels within Friendswood by about 0.3 - 0.9 ft.
There are minor increases in water levels (less than 0.1 ft) downstream of Bay Area Blvd
with the flow re-routing.As shown in Table V.8, Turkey Creek's drainage area accounts for
only —8% of the combined drainage area of the reaches that enter the main channel at
Friendswood.Therefore,it is not surprising that diverting 50% of Turkey Creek's flows does
not have a significant impact in reducing water levels throughout Friendswood, especially
approaching the location where the diverted water re-enters the main channel. Because the
Turkey Creek diversion was not physically implemented in the HEC-RAS geometry, a
floodplain map and WSEL data within the diversion channel could not be obtained.
Table V.8.Drainage area comparison of reaches feeding into Friendswood
Reach Contributing Drainage Area
(sq. mi)
Upper Clear Creek+ Mud Gully 61
Mary's Creek 16
Coward's Creek 12
Chigger Creek 32
27
10
Turkey Creek
(8/o of total contributing area)
Detention
Table V.9 shows the change in peak WSEL for the Atlas 14 100yr storm.
Table V.9. Changes in peak WSEL under the 100yr (Atlas 14) storm for the proposed
detention ponds
Watchpoint Location WSEL change (ft)
1 US FM 2351 -0.01
2 DS FM 2351 -0.02
3 US Whispering Pines -0.04
4 DS Whispering Pines -0.05
5 US FM 528 -0.06
6 DS FM 528 -0.08
7 Bay Area Blvd. -0.13
8 I-45 -0.19
9 Clear Lake -0.15
Results of this preliminary modeling effort showed minimal effect in reduction of water
surface elevation throughout Friendswood with the additional detention. Peak flows were
reduced throughout the channel, on the order of 1-3%, for both Harvey and the 100-yr
rainfalls. Within Friendswood, WSEL was reduced on the order of hundredths of a foot for
the Atlas 14 100-yr, with elevation falling in greater increments moving downstream. The
minimal impact of the ponds is not surprising due to the small,regionally-distributed nature
of the ponds.Additionally,as noted in a presentation to the City of Friendswood in Dec 2018,
the impacts of hypothetical ponds located at Mary's Creek (Figure V.13) and Coward's Creek
were evaluated.These ponds had 3-4 times the capacity of the ponds modeled in this report,
and also showed little flood reduction during Harvey and 100-yr storms.
28
.. .1
S
W
7i�'
. •MI 2 1 Iti•• .• ..,..
.a-`'' Potential detention - 4(1:► t`.,
, . '0
• , 'v -,
.
=w '
-< •. r
•
• i 14 14
i , , ' . t '
• IF t'
Figure V.13.Location of modeled detention pond designed to capture Mary's Creek
drainage area; theoretical box pond capacity was 12,500 acre-ft
Table V.10.Change in peak WSEL at the middle of Friendswood with detention pond
at Mary's Creek
Storm WSEL Change (ft)
100-yr -0.81
100-yr,Atlas 14 -0.69
Harvey 0.00
Combination Scenarios
Bridge Removal+ Terracing to Bay Area Blvd
Table V.11 shows the change in peak WSEL for the Atlas 14 100yr storm.
Table V.11. Changes in peak WSEL under the 100yr (Atlas 14) storm for bridge
removal + grass terracing
Watchpoint Location WSEL change (ft)
1 US FM 2351 -5.24
2 DS FM 2351 -3.96
29
3 US Whispering Pines -2.37
4 DS Whispering Pines -2.05
5 US FM 528 -1.53
6 DS FM 528 -1.09
7 Bay Area Blvd. *-0.11
8 I-45 *0.00
9 Clear Lake *0.00
For the Atlas 14 100-yr storm, the combination of FM2351 bridge removal and grass
terracing decreases water surface elevation in Friendswood in the range of 0.1 to 5.2 ft.Other
than at the site of the bridge, the differences between the results of the combined scenario
versus grass terracing-only are minimal.
*We would expect terracing to have more notable impacts at locations past the terraced
segment, however due to modeling limitations (i.e., 1D steady-state), these impacts are not
well-captured.
Terracing +Diversion
Table V.12 shows the change in peak WSEL for the Atlas 14 100-yr storm.
Table V.12. Changes in peak WSEL under the 100yr (Atlas 14) storm for the
terracing+ 50% diversion scenarios
WSEL change (ft)
Watchpoint Location Terracing to BAB Terracing to I45 +
+ 50%Diversion 50% Diversion
1 US FM 2351 -6.37 -7.74
2 DS FM 2351 -5.60 -7.12
3 US Whispering Pines -4.51 -6.91
4 DS Whispering Pines -4.39 -7.02
5 US FM 528 -3.93 -6.68
6 DS FM 528 -3.60 -6.63
7 Bay Area Blvd. *-3.06 *-7.32
8 I-45 *-3.03 *-3.03
9 Clear Lake *-0.67 *-0.67
The combination scenario with terracing to Bay Area Blvd is expected to decrease water
surface elevation by between 3 and 6.4 ft within Friendswood.The flow diversion in addition
to the terracing reduces water levels past Bay Area Blvd and enhances flood reduction within
Friendswood. The combination scenario with terracing extended to I45 decreases water
30
levels in Friendswood by about 6.6-7.7 ft. The extended terracing adds channel capacity and
increases flood reduction impacts.
*Due to modeling limitations (i.e., 1D steady-state), the impacts of this mitigation strategy,
especially in the area where the terraced channel meets the diversion location, are not well-
captured.
Model Limitations
This section serves to summarize and reiterate the limitations and assumptions of the
modeling method for each of the mitigation scenarios performed in this study.
Bridge Removal: Because Vflo® does not account for the presence of bridges or channel
obstructions, the peak flows throughout the main channel before and after the bridge
removals were assumed to remain the same in the HEC-RAS model.Additionally,beyond the
deletion of bridges and associated ineffective flow areas from the HEC-RAS geometry, no
other channel modifications were performed.These assumptions,although minor,should be
noted when reviewing the modeling results.
De-Snagging: The Vflo® peak flows throughout the main channel before and after the de-
snagging were assumed to remain the same in the HEC-RAS model. Therefore the
hydrodynamics of the de-snagging are not well-captured especially past the location of the
de-snagged segment. Therefore there is uncertainty in the modeling results obtained at the
watchpoints past Bay Area Blvd.
Terracing:The Vflo®peak flows throughout the main channel before and after the terracing
were assumed to remain the same in the HEC-RAS model. In reality, the drastic increase in
channel capacity and added storage during a flood event would alter the peak flows observed
along and past the terraced segment. Since Vflo® is not currently capable of capturing these
impacts, the assumption that the peak flows remain the same is overly simplifying. In other
words, the current modeling method could not account for the actual channel
hydrodynamics (i.e., hydraulic routing).
Diversion: For Diversion 1 at Bay Area Blvd, the diverted water is assumed to leave the
system without any physical re-routing back into the main channel. This assumption
introduces some uncertainty in the modeling results if the water were to be routed back into
the main channel in reality. For Diversion 2 from Turkey Creek to Bay Area Blvd, it was out
of the scope of the project to alter the HEC-RAS geometry to account for the presence of the
diversion channel. This does not allow for assessing flood levels around the diversion
channel- only in the main channel.
Detention: No design rating curves were available for incorporating into Vflo®. Instead,
theoretical rating curves were developed,which might not capture the actual behavior of the
31
ponds. Additionally, Vflo® is currently not suitable for modeling inline detention and
capturing riverine overflow. Similarly, the 1-D nature of the HEC-RAS model does not
capture the physics of the inline detention ponds.
Bridge Removal + Terracing: The same limitations and assumptions remain for the bridge
removal-only and terracing-only scenarios.
Terracing +Diversion at Bay Area Blvd:The same limitations and assumptions remain for the
terracing-only and diversion-only scenarios. Importantly, assuming that the flows in the
diversion scenario can then be applied to the HEC-RAS model where the terracing is applied
does not capture the interaction between the two mitigation strategies where we expect
peak flows would be impacted especially in the downstream areas. The use of a 1-D steady
state model in this scenario is not the most suitable approach for representing the highly
time-dependent nature of this scenario.
32
VI. CONCLUSIONS AND RECOMMENDATIONS
A summary of the results for each mitigation scenario for the Atlas 14 100-yr storm is shown
in the table below.
Table VI.1. Summary of peak WSEL reduction (in feet) under the 100yr (Atlas 14);
values indicate approximate range of changes in peak WSEL within Friendswood
Mitigation Scenarios
3 Bridge De-snagging Terracing Diversion at Turkey Creek Detention FM 5321+ Terracing to
Removal to 145 BAB(50%) Diversion grass I45 +50%
terracing Diversion
-0.3 to 1.4 -0.4 to 1.6 -2.7 to 6.2 -0.1 to 1.0 -0.5 to 0.9 Negligible -1.1 to 5.2 -6.6 to 7.7
The specific implications of each mitigation scenario are as follows:
Bridge Removal
Removing the bridge at FM 2351 has the greatest effect on lowering water levels at locations
that are immediately upstream of the bridge.
De-Snagging
De-snagging has the greatest flood reduction impact at the first few watchpoints and
decreases in effectiveness progressively downstream.
Terracing
Terracing reduces flood levels the most at the first watchpoints and decreases in
effectiveness progressively downstream, however significant flood reduction is observed
throughout Watchpoints 1-7.
Diversion Scenarios at Outlet
Greatest overall impact from the diversion scenarios was seen at I-45 and Clear Lake while
imparting minimal change on WSEL (less than 1 ft)within Friendswood.Generally,reduction
in WSEL increased with closer proximity to the diversion location.
Detention Ponds
There was minimal change in WSEL throughout the reach due to the small, regionally
distributed ponds.
Bridge Removal + Terracing
This combination scenario does not show significant differences in flood reduction impact
as compared to the terracing-only scenario.There are slight peak WSEL reductions upstream
33
of the FM 2351 bridge, but the results are largely controlled by the significantly increased
channel capacity due to the terracing.
Terracing +50% Diversion
This combination scenario showed large flood reduction impacts at all watchpoints in
Friendswood.
Additional Considerations
Although it is not within the current scope of the project, future improvements to the
modeling method might require the use of unsteady and/or 2D approaches to better
represent the responses and quantify the performances of specific mitigation options. For
example, the impacts of diversions could potentially be better quantified by modeling the
physical structure in a 2D environment. The evaluation of other mitigation options such as
detention and terracing could also benefit from using an unsteady modeling approach. Due
to the inherent limitations discussed previously, the results presented in this study are not
meant to serve as the basis for the actual engineering design and/or construction of specific
mitigation scenarios. Instead,this study's main purpose is to provide a better understanding
of the existing flood vulnerability of the City of Friendswood and also to provide valuable
insights regarding the potential impacts and benefits of a wide range of mitigation strategies.
34
VII. APPENDICES
Appendix A: LIDAR Difference Map
Legend
Difference (ft.) #,:
-H.2h--10 _• J
. •
9.99 --1
-a.99 - 1
I 10.U1 - 105.26 .lam ;$ •l4•rr J.,. 7.
-.L.:, ySx.' :� ,' ....411-: t,_
1 rt�' -1"- •.�-4 .:�4J M1 r r_i5•: .: .rM .N, “f _ _• M: -,,•t nri
16
'j.'-.1c- '. aryf 'qµ t'-•.y :—.1'•. 1Y'_r''$ •, -- ;y:,�, '..rr'v ,.
If
Map shows elevation difference in ft between the 2018 and 2008 LIDAR
35
Appendix B: Summary Tables by Storm
Table VII.1. Summary table for Harvey
Change in Peak WSEL(ft)
Watch Point 3-Bridge I De-Snag Terracing Terracing 500/0 Turkey Detention FM 2351 + Terracing Terracing
Location Removal to BAB to I45 Diversion Creek Terracing to BAB+ to I45 +
at BAB Diversion 50% 50%
Diversion Diversion
1: US FM -0.79 -0.86 -3.78 -4.64 -0.11 -0.35 -0.03 -4.11 -5.24 -6.67
2351
2: DS FM -0.09 -1.47 -3.38 -4.38 -0.15 -0.40 -0.04 -3.37 -5.00 -6.54
2351
3: US
Whispering -0.20 -0.87 -1.83 -3.07 -0.38 -0.31 -0.04 -1.83 -3.87 -6.11
Pines
4: DS
Whispering -0.19 -0.75 -1.56 -3.00 -0.47 -0.29 -0.04 -1.56 -3.93 -6.37
Pines
5: US FM 528 -0.19 -0.44 -1.10 -2.57 -0.74 -0.26 -0.05 -1.10 -3.52 -6.07
6: DS FM 528 0.00 -0.37 -0.90 -2.51 -1.03 -0.25 -0.06 -0.90 -3.48 -6.27
7: DS Bay 0.00 0.00 -0.09 -1.94 -2.9 -0.16 -0.09 -0.09 -3.13 -6.88
Area Blvd
8: DS I-45 0.00 0.00 0.00 0.00 -3.62 0.01 -0.06 0.00 -3.62 -3.62
9: Clear Lake 0.00 0.00 0.00 0.00 -1.52 0.02 -0.04 0.00 -1.52 -1.52
36
Table VII.2. Summary table for 100-yr (HCFCD)
Change in Peak WSEL(ft)
Watch Point 3-Bridge De-Snag Terracing to 50% Turkey Detention FM 2351 + Terracing to
Location Removal I45 Diversion at Creek Terracing I45 + 50%
BAB Diversion Diversion
Watch Point 1: -1.25 -1.77 -7.34 -0.07 -1.24 0.03 -6.10 -8.20
US FM 2351
Watch Point 2: -0.20 -1.50 -6.47 -0.10 -0.74 0.01 -4.86 -7.43
DS FM 2351
Watch Point 3:
US Whispering -0.55 -1.09 -5.36 -0.25 -0.62 -0.07 -3.22 -6.91
Pines
Watch Point 4:
DS Whispering -0.15 -1.08 -4.73 -0.33 -0.47 -0.10 -2.47 -6.42
Pines
Watch Point 5: -0.18 -0.73 -4.26 -0.44 -0.37 -0.16 -1.94 -6.03
US FM 528
Watch Point 6: 0.01 -0.55 -3.90 -0.48 -0.25 -0.16 -1.48 -5.82
DS FM 528
Watchpoint 7:
DS Bay Area 0.00 0.00 -2.94 -1.93 -0.11 -0.19 -0.14 -5.51
Blvd
Watch Point 8: 0.00 0.00 0.00 -2.54 0.07 -0.19 0.00 -2.54
DS I-45
Watch Point 9: 0.00 0.00 0.00 -0.31 0.13 -0.14 0.00 -0.31
Clear Lake
37
Table VII.3. Summary table for 100-yr Atlas 14
Change in Peak WSEL(ft)
Watch Point 3-Bridge De-Snag Terracing to 50% Turkey Detention FM 2351 + Terracing to
Location Removal I45 Diversion at Creek Terracing I45 +50%
BAB Diversion Diversion
Watch Point 1: -137 -1.23 -6.23 -0.07 -0.89 -0.01 -5.24 -7.74
US FM 2351
Watch Point 2: -0.17 -1.57 -5.44 -0.10 -0.92 -0.01 -3.96 -7.12
DS FM 2351
Watch Point 3:
US Whispering -0.36 -0.96 -4.29 -0.31 -0.71 -0.02 -2.37 I -6.91
Pines
Watch Point 4:
DS Whispering -0.26 -0.92 -4.13 -0.41 -0.71 -0.03 -2.05 -7.02
Pines
Watch Point 5: -0.31 -0.57 -3.65 -0.64 -0.63 -0.05 -1.53 -6.68
US FM 528
Watch Point 6: 0.01 -0.41 -3.30 -0.96 -0.54 -0.06 -1.09 -6.63
DS FM 528
Watchpoint 7:
DS Bay Area 0.00 0.00 -2.68 -2.77 -0.34 -0.14 -0.11 -7.32
Blvd
Watch Point 8: 0.00 0.00 0.00 -3.03 0.04 -0.20 0.00 -3.03
DS I-45
Watch Point 9: 0.00 0.00 0.00 -0.67 0.08 -0.15 0.00 -0.67
Clear Lake
38
Appendix C: Profiles
The following figures are channel profile plots for each mitigation scenario under the 100-yr Atlas 14 storm.
Three Bridge Removal
60 Legend
Existing
3 Bridge Removal
Ground
40-
_ 6
e 20-
O _ X
0 0
W �
_ N
U1
0- v
co
-20-
60000 80000 100000 120000 140000
Main Channel Distance(ft)
39
De-Snagging
60 Legend
Existing
De-snagging
•
Ground
40-
r L
r
LL
2 20 X
a o
o
co -cs
a>
W m
Q
C6
CO
-20-
60000 80000 100000 120000 140000
Main Channel Distance(ft)
40
Terracing to Bay Area Blvd
60- Legend
Terracing to BAB
Existing
Terraced Ground
40- Existing Ground
20-
�f� E
x
•
Q> - m
N
0
f9
m �
_20-
60000 80000 100000 120000 140000
Main Channel Distance(ft)
41
Terracing to I45
60- Legend
Existing
_ Terracing to 145
- Terraced Ground
40 Existing Ground
cc
_ I E
2 20 p��1----~~ w
x
p
C
O
d - a _ _/
IT ElLf1 N
0- 1.
— a
T
CO
00
_20-
60000 80000 100000 120000 140000
Main Channel Distance(ft)
42
50%Diversion at Bay Area Blvd
60- Legend
Existing
50% Diversion
40- Ground
20
� _ x
O
w _
0- Y
— m
-20-
60000 80000 100000 120000 140000
Main Channel Distance(ft)
43
Turkey Creek Diversion to Bay Area Blvd
60- Legend
Turkey Crk Diversion
Existing
- Ground
40-
f r cc
——
_
r
-----
20- w
p
c
O _ff,
a) m
W m
0 — >
co
m
7\11\ri
-20-
60000 80000 100000 120000 140000
Main Channel Distance(ft)
44
Detention Ponds
60- Legend
Existing
Proposed Detention
40 Ground
- s
20- LL
v
� _ x
o °
> - m
w
0- a
-20-
60000 80000 100000 120000 140000
Main Channel Distance(ft)
45
Bridge Removal+Terracing
60- Legend
Remove FM 2351 +Terracing
Existing
Terraced Ground
40- Existing Ground
- 20-
p
O
N a
W 00
0- v -
m
-20-
60000 80000 100000 120000 140000
Main Channel Distance(ft)
46
Terracing to Bay Area Blvd+ 50% Diversion at Bay Area Blvd
60- Legend
Existing
Terracing to BAB+50%Diversion
- Terraced Ground
40 Existing Ground
oc
r
20- L r~� —~—�
x
is --�
>
W m
0- v a
co
m
r
-20-
60000 80000 100000 120000 140000
Main Channel Distance(ft)
47
Terracing to IDS+ 50% Diversion at Bay Area Blvd
60- Legend
Existing
_ Terracing to 145+50%Diversion
- Terraced Ground
40- Existing Ground
f--
LL
20- �� r x
0
= _
_��0 L—l~
__
CO
co
W
0- Q
r
-20-
60000 80000 100000 120000 140000
Main Channel Distance(ft)
48
Appendix D: Floodplain Maps
3 Bridge Removal
*;$ [3 Bridge Removal
,,,,:.- , , Jr. • / ,,
,.. 6.. V.
4 ..) j•-. --..
) , !_ • r _
i i ft 1 , i ' , ' ,;(` .1 assau \i
'tek / , /Bay
.,-
d•
t ..: 91;5* "
P ear land .,,,,,..
..
- --
, .
Webster
Fn., ;:oo. S.- —
- „ -
t t
/
L - 1111-
•
4*
t 1
br
ce
44
,2 \ Depth (ft)
* ,
100-yr Atlas 14
N
0-2 8
A1 I 2 9 -7 4
= 7.5 - 12.9
0 0.5 1 2 = 13.0- 18.4
Miles Areal Extent: 23,312 Acres = 18.5-45.1
49
De-Snag
De-Snag
.
2
< • r
3
Frier. woo
•
_ .. League
Vl
4•
8
r Depth (ft)
N 100-yr Atlas 14
0-2.8
2.9 - 7.4
7.5 - 12.9
0 0.5 1 2 = 13- 18.4
Miles Areal Extent: 23,010 Acres = 18.5 -45.1
50
Terracing to Bay Area Blvd
r Terracing to Bay Area Blvd
,ij 0
y
� .r, �,,,ry�. Nassau��
/Vtt Bay
Webster '.-,
Fii.. , .. -,
r.
i - . L I-
x
Depth (ft)
100-yr Atlas 14
N = 0 - 2.8
A =2.9 - 7.4
= 7.5 - 12.9
0 0.5 1 2 = 13.0 - 18.4
Miles Areal Extent: 21,776 Acres I= 18.5 - 45.1
51
Terracing to 145
/ Terracing to 1-45
N
r ' ' 4 i.
11".,
2 I 4 i' ` .1-..,
♦, ` i
i
,u � N i---n I
�� E r,
Fe:a Lail - _ 1 s �-
d'
1
Fll ell SO:. —
\ L-- , Ili-
rP
d
r Depth (ft)
N \ 100-yr Atlas 14
A — MO-
n 2.9 - 7.4
7.5 - 12.9
0 0.5 1 2 = 13.0 - 18.4
Miles Areal Extent: 21,158 Acres 111. 18.5-45.1
52
50% Diversion at Bay Area Blvd
50% Diversion at Bay Area Blvd
_ i l�,\ ¢ « \ .e rV
. 4 '.. Sp aC ��I Q a,rm
I 4 ' w �,-, �.� J Lrrvion El /
�< _ 0\a Johnsen
CV c like Cent. 4_
�.`1 assausas),,
a /V Bay
ly Feai l.aiio s
,e Webster < - ,
,- - _
a .. ,�� - Lea hi-
, \ \ I
in
Depth (ft)
L N 100-yr Atlas 14
An 2.9 - 7.4
7.5- 12.9
0 0.5 1 2 = 13.0 - 18.4
Miles Areal Extent: 21,977 Acres 111. 18.5 -45.1
53
Turkey Creek Diversion
Turkey Creek Diversion
A
I/ ts 1
, 4,4, _
itt Pe:ir uii 1 ,) is
9f .
il
a .
ri
•
ter L- 101'`'.
u A A i
If
Depth (ft)
L N 100-yr Atlas 14
0-2.8
A2.9 - 7.4
7.5 - 12.9
0 0.5 1 2 = 13.0 - 18.4
Miles Areal Extent: 23,203 Acres I= 18.5-45.1
54
Detention Ponds
C
Detention Ponds
/ -
_ e o
"
a
«
Jr
cber%ter
2
4 '.. Q ��
4 _
a
at Bay
1. Pe Huai l s
FrL -
A
t
er
\ L- ill]-
- Depth (ft)
N 100-yr Atlas 14
A n0-2.8
n 2.9 - 7.4
7.5 - 12.9
0 0.5 1 2 = 13- 18.4
Miles Areal Extent: 23,412 Acres 111. 18.5 -45.1
55
Bridge Removal+ Terracing
FM 2351 Removal + Terracing to BAB
1 y1�� 7
;
r
Feaa Lai i - ♦ s
ly
? e
7
—
9 �e/ •
-c
\ L- 1111'
Cr
If
Depth (ft)
L N 100-yr Atlas 14
A 0-2.8
n 2.9 - 7.4
7.5 - 12.9
0 0.5 1 2 = 13- 18.4
Miles Areal Extent: 21,749 Acres = 18.5 -45.1
56
Terracing to Bay Area Blvd+ 50% Diversion at Bay Area Blvd
Terracing to BAB + 50% Diversion at BAB Reu.
/ '
z )
1. ;. 3 (fr..
• r
.17 Bay
J'
Fi-I t-1
FII ell
t
.1
ap
ate
N Depth (ft)
N ` 100-yr Atlas 14
n0-2.8
n 2.9 - T4
7.5- 12.9
0 0.5 1 2 13.0 - 18.4
Miles Areal Extent: 20,087 Acres 111. 18.5 -45.1
57
Terracing to 145+50% Diversion at Bay Area Blvd
•
t Terracing to 145 + 50% Diversion at BAB
��` l i ' .«,
\j 4 e
a ,Q eay
7
Fri 1, -
}
tr _ Ledglle i.
hp
- Depth (ft)
L N 528 100-yrAtlas 14
— 0-2.8
2.9 - 7.4
7.5 - 12.9
0 0.5 1 2 = 13.0 - 18.4
Miles Areal Extent: 19,706 Acres 111. 18.5 -45.1
58