hydrologic and hydraulic report txdot

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HYDROLOGIC &HYDRAULIC REPORT

IH 610 W/ US 59 S INTERCHANGESOUTH DRAINAGE ELEMENTS

CSJ: 0271-17-146

HARRIS COUNTY, TEXAS

Prepared for:

&

VOLUME 1

H & H RESOURCES, INC. (F-3678)

DECEMBER 2013


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Hydrologic & Hydraulic Analysis

IH 610 W/ US 59 S Interchange South Drainage Elements, CSJ: 0271-17-146

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TABLE OF CONTENTSVOLUME 1 OF 2

EXECUTIVE SUMMARY .......................................................................................................... 1

Table 1 Summary Cost Table ............................................................................................................................ 1

1 INTRODUCTION .................................................................................................................. 2

1.1 PROJECT DESCRIPTION AND LOCATION............................................................................ 2

1.2 PURPOSE............................................................................................................................. 3

1.3 SCOPE OF WORK................................................................................................................ 4

1.4 DATA SOURCES................................................................................................................... 5

1.5 OBSERVATIONS FROM AS-BUILT PLANS RESEARCH........................................................ 7

1.5.1 US 59/METRO Plans ................................................................................................................................... 7

1.5.2 Westpark Tollroad Detention Ponds ........................................................................................................... 8

1.5.3 Westpark Culvert Plans ............................................................................................................................... 9

1.5.4 Westpark Tollway Plans ............................................................................................................................ 10

1.5.5 Westheimer Connector plans ..................................................................................................................... 14

1.6 OBSERVATIONS FROM ANECDOTAL INFORMATION........................................................ 14

2 METHODOLOGY ............................................................................................................... 15

2.1 HYDROLOGIC APPROACH................................................................................................ 15

2.2

HYDRAULIC APPROACH................................................................................................... 17

3 HYDROLOGIC SUMMARY ............................................................................................. 19

3.1 OUTFALL IDENTIFICATION.............................................................................................. 19

3.2 OUTFALL SUMMARY........................................................................................................ 19

4 HYDRAULIC ANALYSIS .................................................................................................. 21

4.1 EXISTING CONDITIONS WITH SILT.................................................................................. 22

4.2 EXISTING CONDITIONS WITH NO SILT IN WESTPARK TRUNKLINE................................ 26

4.3 EXISTING CONDITIONS WITH W129STAGE 3 ................................................................. 28


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4.4 PROPOSED CONDITIONS WITH W129STAGE 3 ................................................................ 31

4.4.1 Increase Detention Pond at W16........................................................................................................ 33

4.4.2 Detention Pond at W18 ...................................................................................................................... 34

4.4.3 Detention Pond at 4-H3 ...................................................................................................................... 34

4.4.4 Oversized Storm Sewer at US 59 SBFR ............................................................................................. 35

4.4.5 Trench Drain along IH 610 SB/US 59 SB Direct Connector ............................................................ 36

4.4.6

Increase Detention Storage at Ponds DB6-1 and DB6-2 .................................................................. 36

4.4.7 Increase Height of Retaining Walls along IH 610 SBFR/Westpark Tollroad.................................. 37

4.4.8 Pump Station Modifications ............................................................................................................... 37

4.5 PROPOSED CONDITIONS WITH SILT................................................................................. 41

4.6 PROPOSED CONDITIONS WITH NO SILT IN WESTPARK TRUNKLINE.............................. 44

5 MITIGATION FOR THE IH 610 BRAYS BAYOU STORM SEWER ......................... 47

6 ENTITY PARTICIPATION ............................................................................................... 49

7

CONCLUSIONS AND RECOMMENDATIONS ............................................................. 51

Table 7 Summary Phase Table ........................................................................................................................ 52


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APPENDICES

APPENDIX A EXISTING CONDITIONS DRAINAGE AREA MAPS

PROPOSED CONDITIONS DRAINAGE AREA MAPS

APPENDIX B EXISTING HYDROLOGIC COMPUTATIONS

PROPOSED HYDROLOGIC COMPUTATIONS

APPENDIX C OVERALL EXISTING SWMM LAYOUT

EXISTING SUMMARIES OF RESULTS

VOLUME 2 OF 2

APPENDIX D OVERALL PROPOSED SWMM LAYOUT

PROPOSED SUMMARIES OF RESULTS

APPENDIX E DETENTION POND LAYOUTS

APPENDIX F BRAYS BAYOU STORM SEWER SYSTEM

SMALL WATERSHED VOLUME CALCULATIONSSTORM SEWER VOLUME COMPUTATIONS

APPENDIX G SEQUENCE OF CONSTRUCTION EXHIBITS

APPENDIX H CD WITH ELECTRONIC FILES


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EXECUTIVE SUMMARY

This is a comprehensive drainage study of IH 610 West at US 59 South interchange, in

Harris County, Texas. The purpose of the study is to identify and recommend solutions

for the existing drainage problems within the interchange and the US 59 corridor between

Chimney Rock and Newcastle. This report also presents mitigation measures

recommended to offset impacts due to proposed improvements.

The following is a summary of the identified issues along with recommended solutions.

These issues are located on Figure 1 in this section. The estimate of probable cost for

each solution is discussed in Section 6: Entity Participation.

Table 1 Summary of Recommended Solutions

I.D. Report Ref Issue Solution

A 1.5.1US 59 ML Ponding beneath METRO elevated

lane and beneath IH 610 MLTrench drains

B 1.5.3 Obs 2 Westpark storm sewer siltation Maintenance

C 4.3.1

Existing flooding of US 59 SBFR and adjacent

properties, proposed additional impervious

mitigation

Increase W16 Detention

D 4.3.2 Add W18 Detention

E 4.3.3Existing flooding of IH 610 SBFR/Westpark

Tollway

Add H3 Detention, 24 flex valve, and related

improvements

F 4.3.3 Add flex valve at Pond DB-7G 4.3.4 Raise grade of US 59 SBFR

H 4.3.4 Storm sewer and inlet improvements at US 59

SBFR

I 4.3.5 Increase Detention Storage at Ponds DB6-1 and

DB6-2

J 4.3.7 Adjust the pump timing for the IH 610 SBFR/

Westpark Tollroad & IH 610 NBFR pump stations

K 4.3.6 Increase wall height or add earthen berms at IH 610

SBFR/Westpark Tollroad.

L 1.5.4 Obs 5 Existing flooding of IH 610 NBFR Reroute storm sewer line (42 RCP)

M 1.5.4 Obs 4 Regrade abandoned pavement near Chimney Rock

Exit Ramp

N1 5

IH 610 Brays Bayou proposed additional

impervious mitigationStorm sewer improvements at IH 610 SBFR, NBFR

P1 1.5.3 Obs 2 Existing flooding of US 59 SBFR and Larchmont Complete Stage 3 of W129 channel improvements

1. Related improvements, not included on Figure 1


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1 INTRODUCTION

1.1 Project Description and Location

This project consists of a comprehensive drainage study of IH 610 West at US 59 South

interchange, in Harris County, Texas. The project is within the Cities of Houston and

Bellaire. The study includes an analysis of the existing drainage system and flooding

problems in and around the interchange, recommendations and analysis for the proposed

conditions, and mitigation of any impacts due to proposed improvements. The study is

being conducted as part of the reconstruction of the interchange, however, the study

includes mitigation of the entire proposed interchange improvements, which include

realignment of the direct connectors (DC), Post Oak Blvd, and the southbound (SB)

frontage road (FR) between IH 610 and Chimney Rock, a new Chimney Rock Rd exitramp, and reconstruction of the IH 610 main lanes (ML). The proposed direct connectors

constructed under CSJ 0271-17-145, -146, and -157 are Phase 1 of three phases.

The project is within two watersheds, Buffalo Bayou and Brays Bayou (see overall

drainage area maps in Appendix A). The boundary between the two is the old rail

corridor along Westpark Dr. The majority of the interchange drains to the east to Harrris

County Flood Control District (HCFCD) Ditch W129-00-00 through a large storm sewer

line along Westpark Dr. A portion of the area along the northbound (NB) and SB FRs

near Richmond Ave drains to the north through storm sewer along IH 610 to Buffalo

Bayou (HCFCD Unit W100-00-00). Part of the IH 610 NB to US 59 SB DC is within the

Brays Bayou watershed, as is the portion of IH 610 within the study area generally south

of Westpark Dr.to just south of Fournace Place.

This study and report is prepared by H & H Resources, Inc. (HHR), subconsultant to LJA

Engineering, Inc. (LJA), the prime consultant under TxDOT Contract 33-212P5016,

Work Authorization 1. HHRs role is to provide the drainage study as generally described

above, and to oversee drainage study implementation and improvements for the IH 610 /


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US 59 interchange. Klotz Associates (Klotz), another subconsultant to LJA under the

same TxDOT contract, is designing the IH 610 drainage system improvements south of

Westpark, based on mitigation recommendations in this study. A concurrent drainage

study and separate report is being prepared by AECOM, the prime consultant under a

separate TxDOT contract, CSJ 0271-17-145. The AECOM study area includes drainage

systems and contributing areas along IH 610 generally north of Richmond Ave., and

along US 59 generally east of the IH 610 NBFR.

As part of the overall interchange analysis, AECOM completed the W129-00-00 channel

analysis and determined the downstream boundary (tailwater) conditions for use in this

analysis for the Westpark storm sewer. Coordination with AECOM was also completed

at several locations where the systems between the W129-00-00 watershed and the IH

610N storm sewer are interconnected.

Aguirre & Fields, LP (AFLP), the prime consultant under yet another current TxDOT

contract (0271-17-157), is designing improvements for Phase 1 of the IH 610 / US 59

interchange, including improvements to existing detention facilities in the SE quadrant of

the interchange.

1.2 Purpose

The general purpose of this report is to identify the existing drainage problems within the

interchange and the US 59 corridor between Chimney Rock and Newcastle, and

recommend major drainage system improvements and mitigation. This study

incorporates all of the major drainage elements associated with the existing and proposed

roadway including the direct connectors and depressed sections. This report presents

recommendations for mitigation measures that will be required to offset calculated

impacts due to stated improvements.


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1.3 Scope of Work

The scope of work consists of performing an existing and proposed analysis that

incorporates all of the representative elements. The intent of the existing conditions

analysis is to establish a hydraulic baseline model for the existing storm sewer system

and identify any drainage problems. For the proposed conditions, the analysis was

completed to implement the proposed improvements due to the roadway construction,

recommend solutions for existing drainage problems and mitigate any impacts due to the

proposed changes. Additionally, the intent is to demonstrate that the proposed

improvements will not adversely impact the abutting properties and receiving streams

relative to the existing conditions. The drainage report includes hydrologic and hydraulic

computer models such as HEC-HMS and SWMM, drainage area maps, drainage outfall

descriptions, storm water storage requirements, and pump station recommendations.

Mitigation measures to reduce flooding in problem areas will be evaluated, and solutions

recommended. The report will present a comprehensive examination of the drainage

issues for the project area and will address the following items in detail:

Determine the existing drainage boundaries that contribute to the project area and

develop peak flows using rational method. Use Small Watershed Method and

Clark Unit Hydrograph method in HEC-HMS to develop hydrographs for import

into the hydraulic model. Develop an existing conditions hydraulic model, using SWMM (XP-Storm

Version 2012), from survey and as-built plans.

Identify drainage problems as shown in the existing hydraulic model and compare

to eyewitness information. Develop a list of the existing drainage problems

determined from the analysis and from research of the as-built plans.

Meet with TxDOT to discuss existing conditions analysis and existing problem

areas.

Determine proposed conditions drainage area boundaries based on proposed

roadway improvements for the ultimate interchange configuration within the


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project area. Develop proposed hydrology using same methodology as the

existing.

Develop a proposed conditions hydraulic model and implement design changes to

improve the existing drainage problems and mitigate potential impacts due to the

proposed changes.

Determine the impacts to the W129-00-00 and IH 610 South storm sewer system.

(The outfalls to Buffalo Bayou have been analyzed by AECOM and will be

discussed within their report.)

Coordinate with AECOM regarding all interconnections of the storm sewer and

tailwater conditions in the W129-00-00 channel.

Coordinate with Aguirre & Fields, LP, (AFLP) on proposed work in Phase 1 and

determine if any additional proposed work should be implemented with AFLPs

construction.

Limit the discharges to the receiving streams to existing capacity flows, as

determined by study of the existing conditions.

Provide a preliminary storm sewer trunk line design for the proposed conditions.

Coordinate with roadway and storm sewer PS&E designers to implement

proposed conditions as determined in the impact analysis.

Prepare a report of preliminary findings.

1.4 Data Sources

The following sources of information are used to develop the hydrologic and hydraulic

components: project topography/field surveys, aerial photography, as-built plans, City of

Houston Geographic Information & Management System (GIMS), and LiDAR

topography. All source data are either referenced to, or adjusted to the project vertical

datum: NAVD 88 (2001 adjustment). This datum is the same as the Harris County

benchmark network established by the Tropical Storm Allison Recovery Project

(TSARP), which serves as the basis for Federal Emergency Management Agency

(FEMA) Flood Insurance Rate Map (FIRM) effective flood study elevations.


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The project topography is provided by TxDOT and updated through field survey for the

project provided by RODS surveying. Subsurface utility survey is provided by RODS.

Aerial photography dated 2008 was obtained from LJA.

Several as-built plan sets were referenced for this study, dating back to the original

construction plans for US 59 and IH 610 in 1961. Five of the plan sets are determined to

be relevant for this study and are described below.

1. TxDOT/METRO 0027-13-123 (1987) - US 59 Southwest Freeway FR

relocation and freeway overlay widening from south of Westpark to east of IH610/US59 interchange.

2. Harris County Toll Road Authority (HCTRA) (2001) Westpark Tollway

from 0.1 mi west of Rice Ave. to 0.4 miles east of Rice Ave.

3.

Westpark Box Culverts (METRO) (1981) construction of Westpark

trunkline from Chimney Rock to W129-00-00

4. TxDOT 0271-17-138 (2001) - IH 610 depressed FRs and tollway connectorfrom Westpark Dr. to Richmond Ave.

5. TxDOT 0271-17-125 (2002) IH 610 from south of Westpark Dr. to north of

Richmond Ave, aka Westheimer Connectors.

The numbering system for the as-built plans was utilized to label the drainage areas and

nodes in the hydraulic model. The leading number refers to the plan set and the

following letter/number refers to the label as shown in the corresponding plan set. For

example, Node 1-F34 corresponds to MH-F34 as shown in Plan Set 1.

City of Houston GIMS is used to obtain data related to storm sewer systems outside of

the limits of the as-built plans. LiDAR is obtained from Harris County.


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1.5 Observations From As-built Plans Research

The following discussion is annotated to the plan source. Observations about existing

conditions and the implications for new drainage calculations and modeling are

described, as well as recommendations for constructed improvements if applicable.

1.5.1 US 59/METRO Plans

OBSERVATION 1

US 59 ML inlets are located beneath the METRO elevated lane east of IH 610 and behind

the slotted traffic rail. No inlet calculations for these inlets were found in the plan set.

Several existing inlets were removed in this area when this project was completed.

Contours indicate that the cross slope in this area is 0.2%. Plans show that the roadway is

in a superelevation transition and is also near the sag in the vertical profile.

Since inlet computations were not completed, the ponded width was not checked in this

area. Instead of a 2% cross slope, the 0.2% cross causes significantly more ponding than

expected or allowed. The superelevation transition occurs at sag in the profile which

further exacerbates the flooding condition.

Recommendation: Although the US 59 ML drainage system is outside the limits of the

proposed construction for this project, it is recommended that trench drains are placed

along the barrier on both sides of the roadway to reduce the ML flooding. (See Figure 1,

Location A.) The existing inlets could be utilized to connect to the existing drainage

system.

OBSERVATION 2

Another US 59 ML superelevation transition occurs beneath the IH 610 MLs. The

longitudinal slope in this area is almost 0%. This flat slope contributes to the lack of

conveyance.


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Recommendation: With the placement of the proposed Chimney Rock exit ramp, it is

recommended that trench drains are placed along the barrier between to reduce the ML

flooding and convey the flow under the proposed ramp. (See Figure 1, Location A.)

OBSERVATION 3

The construction of the METRO HOV lane through the interchange created a small swale

between the US 59 NBML and the HOV lane. The US 59 NBMLs prior to the HOV

drained directly into the grassy area under the IH 610 ML bridges and away from the US

59 NBML. Several columns for the existing direct connectors and IH 610 ML are within

the swale. These columns restrict the area in which water would be conveyed to the

inlets within the swale. Due to the restriction of the flow in the swale and the silting of

the openings in the slotted rail, the US 59 NBML has more ponding than it otherwise

would.

Recommendation: With the proposed roadway improvements to the US 59/IH 610 direct

connectors and the IH 610 MLs, several of the columns will be eliminated with the

removal of the existing bridges. In Phase 1, the IH 610NB/US 59SB and IH 610SB/US

59 NB direct connectors will be removed. For Phase 2 and 3, it is recommended that the

bent locations be modified to remove columns from the swale area if possible. These

changes should eliminate some of the ponding issues that are currently occurring. The

slotted rail and swale should also be maintained regularly to eliminate silting and debris

collection.

1.5.2 Westpark Tollroad Detention Ponds

OBSERVATION 1

The detention pond outlets in the plans do not represent what is now in the field. The

outlets were modified in the IH 610 FR and Westpark Tollroad depressed section plans

(Plan Set 4). Due to this change, it appears that the detention ponds may not be

functioning as originally designed.


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Recommendation: Modeling in this study will analyze outlets per current field survey.

1.5.3 Westpark Culvert Plans

OBSERVATION 1

The box culvert constructed in the early 1980s along the northern edge of Westpark

Drive replaced an open channel. The box was placed virtually flat, with almost no

longitudinal slope. West of Chimney Rock Road, the storm sewer is a single 10x5

RCB. It transitions to a 12x6 RCB through the intersection, then to a 2-10x7 RCB,

and finally to a 2-12x8 RCB. There are no junction boxes along this trunkline at

Westpark Drive, only openings in the wall between the boxes in various locations. All

of the trunklines from the north (those that drain US 59 and areas north of US 59)

connect to the north box culvert. Flow can only enter the south box through theseopenings. These openings vary in size from 2x2 to 6x6. The openings smaller than 6

in height are located 2 above the FL. The 6 openings match the FLs of the box culvert.

Since all of the storm sewer systems only tie to the north box (this also includes the

detention ponds along Westpark), the maximum capacity of the system may not be

realized because the flow is not effectively conveyed to the south box culverts.

Action item completed with modeling: The two storm sewer lines are modeled

separately and connected with orifices sized and located as shown in the plans to

determine the adequacy of the openings and operation of the system. The locations and

sizes were also verified by the surveyors.

OBSERVATION 2

AECOM mentioned during a coordination meeting that during construction of the

Westpark Tollroad, there were reports of the box culverts having a major silt problem

with the boxes filled as much as 60%. The silt problem seems to occur due to the very


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flat longitudinal slope of the storm sewer system and the presence of sand in the W129-

00-00 channel at the outfall of the Westpark System.

Action item completed: Since there are no manholes located on the Westpark storm

sewer line, the survey crew entered the storm sewer system and walked along the

northern RCB. The surveyors note the presence of silt and the depth of the silt along the

box culvert at each of the opening locations. Surveyors indicate that the silt depth

appears to be equal in each of the boxes.

Recommendation: The silt in the box culvert should be removed to allow for maximum

conveyance in the system. It is recommended that maintenance of the boxes should be

completed to keep the boxes clean of silt. Also, HCFCD completed a study to enclose

the W129-00-00 channel with box culverts from the confluence with Buffalo Bayou to

the Westpark storm sewer system. Construction of the HCFCD system was split into

three stages, with Stage 3 being the most upstream portion located between Richmond

and the Westpark system. The first two stages have been constructed with Stage 3

lacking current construction funding. Construction of Stage 3 should help minimize the

silting that occurs within the box and would allow for greater conveyance.

Action item completed with modeling: The Westpark storm sewer system is analyzed

with the silt in place in the existing and proposed condition to simulate the lost

conveyance within the system. The system is also analyzed in the existing and proposed

with the Phase 3 project in place and the silt removed from the Westpark storm sewer.

1.5.4 Westpark Tollway Plans

OBSERVATION 1

A 24 RCP gravity line from Inlet H3 (located between US 59 SBMLs and Connector B

(IH 610NB/US 59 SB) connects directly to detention pond DB-7; this line also connects

to storm sewer line between the METRO HOV lane and US 59 MLs. The retaining wall


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plans for Wall W-2 for the depressed IH 610 SBFR show that the low point in the wall is

at the corner of the US 59 SBML bridge and the separation area at elevation 56.20. Other

significant elevations:

The proposed pond design WSEL is shown as 57.00 with a flowline of 49.50.

The top of grate elevation at Inlet H3 is 54.94 with a flowline of 50.37.

The top of grate elevation at Inlet H2, located along the US 59 NB MLs, is 54.06

with a flowline of 49.86.

As the pond fills, the gravity line will fill with water and backup through the inlets. The

water will then spread out over the US 59 MLs near the HOV lane. Near inlet 4-H3, the

water will overflow the retaining wall and fall into the depressed IH 610 SBFR.

Action item completed with modeling: Overflow of this wall has been observed during a

flood event (See picture on following page). The elevations have been verified through

field survey. The system is modeled in SWMM and the observation should be verified

through the analysis.

Water overflowing the wall near Inlet 4-H3 (January 2012)

Photo Credit: Todd Thurber, LJA


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OBSERVATION 2

Several field changes to the detention pond at DB-7 may have created hydraulic

conditions different than intended by the design.

The plans indicate the 48 RCP between Ponds DB-7 and DB6-1 was placed at a

flatter slope than the original.

The contours indicate less volume in Pond DB-7 than was shown in the plans.

The capacity of the pipe is reduced from the as-built slope and requires more head to

meet same flow capacity as the design. Less exit flow would create higher WSEL in the

pond which could overflow into the depressed section.

Action item completed with modeling: During the last flood event in January 2012,

TxDOT personnel indicated that the wall was overtopped by water at this location and the

depressed section was flooded. It was also noted that Pond DB-7 was full of water but

Pond DB6-1 did not have water in it. The 48 outlet was blocked with debris and once

cleared, Pond DB-7 began to release water. According to rainfall data collected from the

Harris County Office of Emergency Management website, the rainfall recorded at the

closest locations to the interchange would be classified as less than a 2 year event.

Field survey has been completed and the elevations were utilized in the model. Although

the failure of the existing system last occurred due to clogging of the outfall pipe, thesystem should be modeled to determine if overtopping of the wall can occur at or before

the design event. The location is modeled in SWMM and the observation should be

verified through the analysis.

OBSERVATION 3

Inlets on the depressed section were designed for 25-yr storm frequency design capacity.

The elevation indicated for the maximum storage elevation in the proposed pump station

plans is higher than the elevation at the low point in the Westpark Tollway profile with

one lane ponded.


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Action item completed with modeling: This location is modeled in SWMM with the

pump station in place.

OBSERVATION 4

Water drains from the US 59 SBMLs through slotted traffic railing across an area of

abandoned temporary pavement. When the depressed section for Westpark Tollway and

IH 610 SBFR was built, the ditch along the pavement was not redesigned. The contours

indicate that the water is directed towards the wall and then overflows to the depressed

lanes below.

Action item completed with modeling: This area is modeled in SWMM and the

observation should be verified through the analysis. Regrading of this area will occur

with the proposed Chimney Rock exit ramp being placed.

OBSERVATION 5

In the design plans, the drainage area maps for the depressed section do not include the

overland flow from the direct connector side slopes. Also, overflow from the FRs along

the NW, NE, and SW quadrants appears to be possible if the HGL in these adjacent areas

becomes higher than the PGL entering into the depressions.

As indicated in the Westpark Tollway report (Brown & Gay Engineers, Inc. report dated

June 29, 2004) analyzing the flooding issues, the drainage areas to the pump stations are

larger than indicated in the original report. The design for pump stations should be

checked for the effect of the additional area. At the SE pump station, a large portion of

the US 59 ML drainage is directed to the pump station. The FR near the Houston

Chronicle building is also redirected to the pump station. The original plans connected

the FR storm sewer directly to the Westpark box culverts.


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At the NE quadrant, additional flow from the adjacent storm sewer systems can enter the

IH 610 NBFR depressed section. The storm sewer system for this area is split into three

segments. The north segment drains into the IH 610 north storm sewer system modeled

in the AECOM study. The middle segment at the curve in the FR is connected to the

depressed section system. The east segment is drained to W129-00-00 through a storm

sewer which is also modeled in the AECOM study. The southernmost inlet in the north

segment is an on-grade inlet that allows carryover to the middle segment, and this flow

was not accounted for in the depressed section computations. From the east segment, an

18 RCP was found in the field survey entering the middle segment storm sewer system.

According to the as-built plans, this line should have been removed and no connections

from the east should have occurred.

Action item completed with modeling: These areas are modeled in SWMM and the

observations should be verified through the analysis. Any changes to the pump station

design and/or timing at the IH 610 NBFR location could be done as part of CSJ 0271-17-

145, and the IH 610 SBFR station as part of Job 157 (Phase 1 construction plan sets).

1.5.5 Westheimer Connector plans

OBSERVATION 1

In the design plans, the 6x6 RCB downstream of the concrete lined channel beneath the

IH 610 MLs south of US 59 was realigned to avoid conflicts with the proposed

foundations for the connectors. During a field visit, it was noted that a flex valve was

present at the downstream end of the concrete lined channel. No field changes were

found in the as-built plans.

Action item completed with modeling: The flap gate is modeled in SWMM.

1.6 Observations from Anecdotal Information

TxDOT and the City of Houston have received several reports of flooding in the

Larchmont Subdivision located north of US 59 between Chimney Rock and South Rice,


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and in the US 59 SBFR at South Rice. According to contour information, the Larchmont

neighborhood, US 59 SBFR, and South Rice are significantly lower than the elevations at

Chimney Rock and Westpark. The Westpark storm sewer plans indicate that the top of

the box culvert is the pavement for Westpark Drive. Using the survey information

provided, the inside top of the Westpark storm sewer lines are higher than the ground

elevations along the US 59 FR and South Rice and within the Larchmont subdivision.

Considering that all of the outfalls tie to the Westpark system, these areas can flood due

to backwater from the Westpark boxes.

2 METHODOLOGY

The TxDOT Hydraulic Design Manual (HDM), October 2011, was utilized as the

primary methodological source for hydrologic and hydraulic analyses in this study.

Supporting methodologies were also obtained from the HCFCD Criteria Manual.

2.1 Hydrologic Approach

The Rational Method is used for peak flow calculations because the study area has small

watersheds unaffected by complex hydrologic features. The existing drainage areas were

delineated using the project topography, aerial photography, as-built plans, City of

Houston GIMS, LiDAR, and project survey data. The proposed drainage areas were

delineated based on the proposed roadway profiles and cross section data. The existing

and proposed drainage area maps are included in Appendix A.

Runoff coefficients (C-values) reflect the land use as indicated on the project aerial

photography and onsite observations. A composite C-value was determined for each of

the areas.

To compute Time of Concentration (Tc), the flow path was determined using available

topographic information. The travel time along the flow path was calculated using the


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following velocities: 0.5 fps for overland flow, 1.5 fps for gutter/channelized flow, and

3.0 fps for storm sewer flow. Tc is limited to >/= 10 minutes.

Rainfall data is taken from Recommendations for Rainfall Amounts in Harris County,

(TSARP White Papers (2002). The following are the Region 2 duration-depths for the

watersheds in this study:

Return Period (by Probability)

Duration 50% 10% 4% 1%

5 min 0.7 0.9 1.0 1.2

15 min 1.1 1.5 1.7 2.1

30 min 1.5 2.1 2.4 3.0

60 min 2.0 2.9 3.4 4.3

2 hr 2.3 3.6 4.3 5.7

3 hr 2.6 4.1 5.0 6.7

6 hr 3.1 5.1 6.4 8.912 hr 3.7 6.2 7.8 10.8

24 hr 4.4 7.6 9.6 13.2

Since the Rational Method produces only peak flow, additional methods are required to

develop hydrographs and runoff volume. Two methods are used, depending on the size

and location of the sub-area: Clark Unit Hydrograph Method, and the Small Watershed

Method.

The Clark Unit Hydrograph Method uses hydrologic parameters time of concentration

and Tc and R. Tc is computed as described above. The Storage Coefficient (R factor) is

adjusted to calibrate hydrograph peak to that computed by Rational Method. Rainfall

loss method is Green and Ampt which uses parameters Initial Loss, Moisture Deficit,

Suction, and Conductivity taken from Recommendation for Replacing HEC-1

Exponential Loss Function in HEC-HMS, (TSARP White Papers, 2002) as standard for

Buffalo Bayou and Brays Bayou. The following values are used:

Initial Loss = 0.1 inches

Volume Moisture Deficit = 0.385 (unitless)

Wetting Front Suction = 12.45 inchesHydraulic Conductivity = 0.024 in/hr


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The percent impervious ground cover is based on land use as indicated in project aerial

photography and field investigations. The computations using this unit hydrograph

method are made in HEC-HMS v3.5. For this study, the 2-, 10-, 25-, and 100-yr flows

were computed.

The (Malcom) Small Watershed Method is used for sub-areas BR-1 and BR-2 that drain

to Brays Bayou. This method is used to develop a curvilinear design hydrograph for

drainage areas < 640 acres, which peaks at a designated flow rate, and contains a runoff

volume consistent with the design rainfall. For more details on this method, see the

HCFCD Policy Criteria & Procedure Manual (2010), Section 3.6.

The HCFCD TSARP 24-hour rainfall and loss methods are used to determine the direct

runoff volume for the Small Watershed Method. Runoff volumes for the study event

(100-yr) were computed using excess rainfall based on percent impervious ground cover

and rainfall by frequency.

The Rational Method, HEC-HMS output, and hydrograph and volumecomputations are

included in Appendix B.

2.2 Hydraulic Approach

SWMM (XP-Storm 2012 V14.01) is used for hydraulic modeling because it accepts a

variety of drainage system components, such as externally-developed inflow hydrographs

for various frequency events, pipes, channels, culverts, weirs, inlets, and detention

facilities. The SWMM model uses a series of nodes and links to represent the physical

characteristics of the drainage system. The drainage system parameters are input into the

model through the Hydraulic mode of SWMM. The SWMM program offers several

methods for incorporating the runoff hydrographs into Hydraulic mode, including those

developed using the HEC-HMS and Small Watershed Method (SWM) at specific nodal

locations (Nodes) of the drainage network. The Hydraulic mode is a hydraulic flow


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3 HYDROLOGIC SUMMARY

3.1 Outfall Identification

The IH 610 / US 59 interchange ultimately drains to Buffalo Bayou and Brays Bayou

through several storm sewer connections. The focus of this report is on the following

outfalls.

Watershed Existing Outfall DescriptionApproximate B/L

Station

Existing Outfall

Structure

Buffalo Westpark box culverts to W129-00-00Chimney Rock to3050 E of IH 610

2/10x7 RCB/2-12x8 RCB

Brays IH 610 South (connection to SBFR) IH 610 ML 195+80 30 RCP

Brays IH 610 South (connection to NBFR) IH 610 ML 174+00 60 RCP

As indicated previously, the FRs in the NE and NW quadrants drain to the IH 610 North

storm sewer. This outfall as well as the hydraulic study of the outfall channel W129-00-

00 is being completed by AECOM.

3.2 Outfall Summary

The major outfall for the US 59/ IH 610 interchange is the Westpark box culverts. All of

the area between Westpark and IH 610 within the US 59 corridor drains to the Westpark

storm sewer through various connecting storm sewer systems. For this study, the

analysis of the Westpark storm sewer begins just west of the intersection with Chimney

Rock. The contributing drainage area upstream of this location is combined into one

drainage area and is applied at this location. Several storm sewer lines cross under US 59

and connect to the Westpark storm sewer. Some of the outfalls include drainage areas

with flow only from the US 59 corridor contributing; however, there are several

connections that carry offsite flow from north of the US 59 corridor to the Westpark

system. The lines carrying both on and offsite flow include a 48 RCP and 60 RCP at

Chimney Rock (Systems F and G), a 48 RCP at Barrington (System J), a 5x4 RCB and

48 RCP at Yorktown (System L), a 60 RCP at S. Rice (System M), and a 48 RCP at

McCue (System Q). According to the COH GIMS information, each of these lines was

built in the 1950s.


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Between S Rice and the US 59/IH 610 interchange, three detention ponds are included

with the construction of the Westpark Tollway and the depressed connection to Post Oak

Blvd. The two ponds along Westpark, DB6-1 to the east and DB6-2 to the west, are

directly connected to the Westpark storm sewer system with low flow orifice openings

and high flow overflow outlets. Although a small portion of the new Westpark Tollroad

storm sewer is connected to the detention ponds, these ponds act more as an offline

storage area for the Westpark storm sewer. The new storm sewer line along the southside

of the Westpark Tollroad is connected to the Westpark system near the outlet of Pond

DB6-1. The third detention pond, DB-7, is the outfall for the IH 610 SBFR and

Westpark Tollroad depressed section through the use of a pump station. The contributing

flow is pumped into Pond DB-7, and then conveyed through a 48 RCP into DB6-1.

Between the two depressed FRs, several changes have occurred since the original

construction to alter the flow paths; however, the only connection between the areas north

and south of the US 59 MLs has remained. Two 36 RCP (System W) convey flow from

the north to the south. The pipes discharge into a concrete lined channel where various

other storm sewer lines (part of System H and part of System S) also outfall. A 54 flex

valve is located at the downstream end of the concrete lined channel. A detention pond,

Node 1-W16, beneath the IH 610 MLs is also connected to the channel through 2-36

RCP. This acts as an offline storage area for the attenuated flow created by the flex

valve. Downstream of the flex valve, a 6x6 RCB outfalls into the Westpark storm

sewer system.

Another detention pond, DB-8, is located under the IH 610 MLs south of US 59. This is

the outfall for the depressed IH 610 NBFR and the US 59 MLs (System S) east of the

depressed section to approximately US 59 Station 191+00 along the northbound lanes

and 188+00 along the southbound lanes. This area also includes the at-grade US 59

NBFR east of the depressed section to station 189+00 and the contributing offsite area.


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A pump station conveys the flow to Pond DB-8 which is discharged to a 5x4 RCB to

the Westpark system.

Along Westpark, the roadway and adjacent properties between US 59 and Westpark

contribute directly to the Westpark storm sewer through roadway inlets and other storm

sewer lines. The flow for this area was placed in the Westpark system at various

locations within the hydraulic model.

South of Westpark, two major storm sewer lines run along each of the FRs to Cypress

Ditch (HCFCD T115-00-00) which outfalls into Brays Bayou. A 30 storm sewer line

crosses beneath IH 610 (Station 195+80) from the IH 610 NBFR to the IH 610 SBFR

storm sewer trunkline. The drainage area for the 30 line includes a portion of the IH 610

NB MLs from station 194+50 to 204+75, the IH 610 NBFR, and a portion of the

proposed IH 610 NB/ US 59 SB direct connector. Analysis of the IH 610 SBFR

trunkline is not part of this scope and is being completed as part of the Phase 1

construction, designed by AFLP. Mitigation (see Sect. 5) of the additional flow will be

completed upstream of the 30 RCP crossing.

South of the 30 RCP crossing, a large storm sewer system along the IH 610 NBFR

begins. The IH 610 South trunkline is located within the separation between the MLs and

NBFR. Due to the construction of the proposed direct connector, this storm sewer line

will need to be relocated between Station 175+00 and 188+00. Mitigation (see Sect. 5)

of additional flow due to the increase in impervious cover will be completed using inline

detention within the project limits.

4 HYDRAULIC ANALYSIS

Six major analyses were completed as part of this study with all of the frequencies

examined. The six analyses are Existing Conditions with silt, Existing Conditions with no

silt in Westpark Trunkline, Existing Conditions with W129 Stage 3, Proposed Conditions


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occur at the other frequencies. The US 59 SBFR, the Larchmont subdivision, and the

South Rice/ US 59 intersection are flooded in the 2-yr event. A summary of elevations

along the US 59 SBFR is shown below in Table 4.1a.

Table 4.1a Existing with Silt WSEL v Ground Elevations

Ex

Silt

WSEL

(ft)

Flood

Depth

(+/-)

Ex

Silt

WSEL

(ft)

Flood

Depth

(+/-)

Ex

Silt

WSEL

(ft)

Flood

Depth

(+/-)

Ex

Silt

WSEL

(ft)

Flood

Depth

(+/-)

4-A29 24.94 19.19 -5.75 40.31 15.37 49.89 24.95 56.44 31.50

4-J8 29.20 20.80 -8.40 40.93 11.73 49.94 20.74 56.46 27.26

4-JB2 34.32 30.37 -3.95 36.47 2.15 37.67 3.35 40.00 5.68

1-F34 55.00 56.17 1.17 57.35 2.35 57.66 2.66 58.00 3.00

1-G23 55.25 56.16 0.91 57.00 1.75 57.27 2.02 57.61 2.36

1-J8 53.78 55.60 1.82 56.56 2.78 56.99 3.21 57.49 3.71

1-J5 54.41 55.65 1.24 56.60 2.19 57.01 2.60 57.50 3.09

1-L14 53.60 55.88 2.28 56.97 3.37 57.44 3.84 57.95 4.35

1-L9a 54.91 55.74 0.83 56.63 1.72 57.02 2.11 57.49 2.58

1-L9 54.90 55.71 0.81 56.79 1.89 57.26 2.36 57.80 2.90

1-L11 54.28 55.75 1.47 56.63 2.35 57.02 2.74 57.48 3.20

1-M9 54.53 55.67 1.14 56.56 2.03 56.97 2.44 57.45 2.92

1-M7 53.62 55.67 2.05 56.56 2.94 56.97 3.35 57.45 3.83

1-M5 55.00 55.62 0.62 56.52 1.52 56.94 1.94 57.43 2.43

1-M12 54.00 55.63 1.63 56.51 2.51 56.92 2.92 57.40 3.40

1-M16 54.75 55.62 0.87 56.51 1.76 56.92 2.17 57.39 2.64

1-M19 55.00 55.63 0.63 56.49 1.49 56.89 1.89 57.36 2.36

1-Q5 54.52 55.65 1.13 56.46 1.94 56.85 2.33 57.32 2.80

1-Q4 54.52 55.80 1.28 56.39 1.87 56.78 2.26 57.25 2.72

1-Q3 54.00 55.83 1.83 56.38 2.38 56.77 2.77 57.23 3.23

1-Q7 54.50 55.19 0.69 55.74 1.24 56.13 1.63 56.58 2.08

4-B11 24.94 37.81 12.87 42.47 17.53 49.89 24.95 56.44 31.50

4-H3 54.35 54.70 0.35 54.84 0.48 54.88 0.53 56.44 2.09

1-W18 54.00 52.88 -1.12 54.71 0.71 55.14 1.14 56.36 2.36

1-W16 54.35 52.88 -1.47 54.63 0.28 55.17 0.82 56.40 2.05

Pond DB7 55.21 55.46 0.25 55.86 0.65 55.88 0.66 57.09 1.881-W14 55.00 52.88 -2.12 54.63 -0.37 55.17 0.17 56.44 1.44

Node

2-yr 10-yr 25-yr 100-yr

Ground

El

(ft)

The elevation at 1-L14 is the ground elevation in the adjacent street is 53.60. The flowline (FL) of the existing 2-5x2RCB is 51.68.


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The depressed sections do not flood at the 2-yr frequency although water does overtop

the retaining walls at 4-H3 and Pond DB-7. The retaining walls near 1-W18 and 1-W14

begin to overtop in the 10-yr frequency. A summary of the retaining wall elevations and

water surface elevations in this area is shown below.

Table 4.1b Existing WSEL v Retaining Wall Elevations

NodeRetaining

Wall ID

Retaining Wall

Overtopping

Elevation (ft)

2-yr

WSEL (ft)

10-yr

WSEL (ft)

25-yr

WSEL (ft)

100-yr

WSEL (ft)

4-H3 W-2 54.35 54.70 54.84 54.88 56.44

1-W18 W-4 54.00 52.88 54.71 55.14 56.36

1-W14 W-4 55.28 52.88 54.63 55.17 56.44

Pond DB7 W-2 55.21 55.46 55.86 55.88 57.09

As for the flooding witnessed in January of 2012, a preliminary analysis was completed

for the 2-yr frequency with the 48 RCP turned off in the model. The results indicate that

the IH 610 SBFR (node 4-J8) and the Westpark Tollway (node 4-A29) would be flooded.

The depth of water at node 4-A29 is 31.86 and 32.18 at 4-J8. The retaining walls near

inlet 4-H3 and at Pond DB-7 are also overtopped. Water does fill Pond DB6-1 but only

through backflow from the Westpark system. Pond DB6-1 empties as the tailwater on

the Westpark system recedes; however, Pond DB-7 and the depressed section remain full

of water throughout the duration of the analysis.

The Westpark storm sewer system appears to be balancing between the two boxes

through the openings along the length of the box. If the silt levels are greater than 2, the

orifices are also reduced. For this analysis, only a few of the orifice areas were reduced

by the silt. The flows and elevations within the two boxes are slightly different but not

significant.

For the 10-, 25-, and 100- yr frequencies, flooding in the IH 610 SBFR/ Westpark

Tollway occurs. A significant amount of flow from the McCue drainage area can flow

into the depressed section through overland flow. The high point in the gutter line


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between McCue and the depressed section is 54.79. The 10-, 25-, and 100- yr elevations

near McCue (node 1-Q7) are 55.74, 56.13, and 56.58, respectively. Flow begins to

overtop the high point in the gutter at STA 161+00 and drains to the low point in the US

59 SBFR (node 4-B11). The storm sewer in this area is connected to the IH 610 SBFR/

Westpark Tollway storm sewer system. Therefore, the additional flow is being conveyed

to the depressed section through storm sewer, and once the water surface elevation in this

portion of the US 59 SBFR exceeds 43.20, overland flow into the depressed section

occurs. The elevations at Nodes 4-A29 and 4-J8 are shown below.

Table 4.1c Existing WSEL v Ground Elevations in Depressed Sections

NodeGround El

(ft)

10-yr

WSEL (ft)

Flood Depth

(ft)

25-yr

WSEL (ft)

Flood Depth

(ft)

100-yr

WSEL (ft)

Flood Depth

(ft)

4-A29 24.94 40.31 15.37 49.89 24.95 56.44 31.50

4-J8 29.20 40.93 11.73 49.94 20.74 56.46 27.26

4-JB2 34.32 36.47 2.15 37.67 3.35 40.00 5.68

The IH 610 NBFR depressed section is flooded in the 10-, 25-, and 100-yr events.

Additional flow from the IH 610 NBFR at grade FR and the US 59 SBFR east of IH 610

drains toward the depressed section storm sewer system. Once the water surface

elevation in this quadrant reaches 53.00, overland flow into the depressed section occurs.

At the SE quadrant, additional overland flow from the east approaches the storm sewer

system near the Houston Chronicle building. At an elevation of 51.50, overland flow

into the depressed section occurs.

The pump stations for each of the depressed sections do not meet the 25-yr design storm.

For the west pump station, the adequacy of the pump station in the existing conditions

cannot be determined with the additional flow that overtops the retaining walls and

overland flow from the McCue area. The links representing the overtopping of theretaining walls and overland flow were turned off so the pump station could be more

accurately analyzed, assuming that unintended overflows are not present. For the 25-yr

event, the water surface elevation at node 4-A29 is 21.46, which is below the low point


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trunkline, and the box heights and orifices adjusted to as-built sizes. This analysis

demonstrates the results that regular maintenance of the Westpark Trunkline would have

on the overall system. A comparison between the existing with silt and existing without

silt at some of the critical locations is shown in Table 4.2a.

Table 4.2a Existing with Silt WSEL v Existing without Silt WSEL

Ex

Silt

WSEL

(ft)

Ex

No Silt

WSEL

(ft)

(+/-)

Ex

Silt

WSEL

(ft)

Ex

No Silt

WSEL

(ft)

(+/-)

Ex

Silt

WSEL

(ft)

Ex

No Silt

WSEL

(ft)

(+/-)

Ex

Silt

WSEL

(ft)

Ex

No Silt

WSEL

(ft)

(+/-)

4-A29 19.19 18.72 -0.48 40.31 35.78 -4.53 49.89 42.65 -7.24 56.44 55.99 -0.45

4-J8 20.80 20.63 -0.18 40.93 36.97 -3.96 49.94 42.68 -7.25 56.46 56.00 -0.46

4-JB2 30.37 30.32 -0.06 36.47 36.23 -0.24 37.67 37.41 -0.27 40.00 39.87 -0.121-J5 55.65 55.13 -0.52 56.60 56.42 -0.18 57.01 56.74 -0.27 57.50 57.26 -0.24

1-L9 55.71 55.11 -0.60 56.79 56.62 -0.17 57.26 56.97 -0.29 57.80 57.54 -0.26

1-M5 55.62 55.22 -0.40 56.52 56.38 -0.14 56.94 56.68 -0.26 57.43 57.19 -0.24

1-Q3 55.83 55.66 -0.17 56.38 56.21 -0.16 56.77 56.51 -0.26 57.23 57.00 -0.23

4-B11 40.58 37.49 -3.09 42.47 41.78 -0.69 49.89 42.71 -7.17 56.44 55.99 -0.45

4-H3 54.70 54.60 -0.10 54.84 54.81 -0.04 54.88 54.85 -0.03 56.44 55.99 -0.45

Pond DB7 55.46 53.80 -1.66 55.86 55.84 -0.01 55.88 55.87 -0.01 57.09 56.39 -0.70

1-W18 52.88 52.32 -0.56 54.71 54.31 -0.40 55.14 54.96 -0.17 56.36 55.89 -0.47

Node

2-yr 10-yr 25-yr 100-yr

Maintenance of the Westpark trunkline does reduce water surface elevations within thestudy limits; however, it does not eliminate the flooding problems along the US 59 SBFR

and adjacent properties or the flooding within the IH 610 SBFR/Westpark Tollway

depressed section. The IH 610 NBFR system is not affected by the changes at W129-00-

00. A comparison of the flood depths at some of the critical locations is shown in Table

4.2b.

Supplemental Tables 2.1 through 2.4 are available in Appendix C of the electronic file

included on the CD of the report.


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00-00 were provided by AECOM and include the HCFCD Stage 3 improvements to

W129-00-00. (See AECOMs report Hydrologic & Hydraulic Analysis: IH 610 W/US

59 S Interchange North Drainage Elements, CSJ: 0271-17-145 for details).

The analysis shows the tailwater along the Westpark storm sewer line is reduced, and in

turn, water surface elevations along the US 59 SBFR and in the IH 610 SBFR/Westpark

Tollway depressed sections are also reduced. A comparison between the existing with

silt and phase 3 at some of the critical locations is shown below.

Table 4.3a Existing with Silt WSEL v Existing with Stage 3 WSEL

Ex

Silt

WSEL(ft)

Ex

Stage 3

WSEL(ft)

(+/-)

Ex

Silt

WSEL(ft)

Ex

Stage 3

WSEL(ft)

(+/-)

Ex

Silt

WSEL(ft)

Ex

Stage 3

WSEL(ft)

(+/-)

Ex

Silt

WSEL(ft)

Ex

Stage 3

WSEL(ft) (+/-)

4-A29 19.19 18.72 -0.48 40.31 29.82 -10.49 49.89 40.65 -9.24 56.44 55.92 -0.51

4-J8 20.80 20.61 -0.18 40.93 30.15 -10.79 49.94 41.16 -8.78 56.46 56.03 -0.42

4-JB2 30.37 29.93 -0.44 36.47 35.74 -0.73 37.67 37.00 -0.67 40.00 39.77 -0.23

1-J5 55.65 54.67 -0.98 56.60 56.04 -0.56 57.01 56.57 -0.44 57.50 57.15 -0.35

1-L9 55.71 54.68 -1.03 56.79 56.14 -0.64 57.26 56.78 -0.48 57.80 57.39 -0.41

1-M5 55.62 54.92 -0.70 56.52 55.94 -0.58 56.94 56.51 -0.43 57.43 57.06 -0.37

1-Q3 55.83 55.56 -0.27 56.38 56.07 -0.31 56.77 56.33 -0.44 57.23 56.86 -0.38

4-B11 40.58 37.44 -3.14 42.47 39.54 -2.94 49.89 42.54 -7.35 56.44 55.92 -0.51

4-H3 54.70 54.59 -0.11 54.84 54.79 -0.05 54.88 54.83 -0.05 56.44 56.04 -0.40

Pond DB7 55.46 53.00 -2.46 55.86 55.74 -0.12 55.88 56.00 0.13 57.09 56.53 -0.56

1-W18 52.88 51.99 -0.89 54.71 54.59 -0.12 55.14 55.44 0.30 56.36 56.44 0.08

Node

2-yr 10-yr 25-yr 100-yr

Adding the Stage 3 improvements to W129-00-00 alone will not solve the flooding

problems along the US 59 SBFR and adjacent properties or the flooding within the IH

610 SBFR/Westpark Tollway depressed section. The IH 610 NBFR system is not

affected by the changes at W129-00-00. A comparison of the flood depths at some of the

critical locations is shown in Table 4.3b.


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Table 4.3b Existing with Stage 3 WSEL v Ground Elevations

Ex

Stage 3

WSEL

(ft)

(+/-)

Ex

Stage 3

WSEL

(ft)

(+/-)

Ex

Stage 3

WSEL

(ft)

(+/-)

Ex

Stage 3

WSEL

(ft)

(+/-)

4-A29 24.94 18.72 -6.23 29.82 4.88 40.65 15.71 55.92 30.98

4-J8 29.20 20.61 -8.59 30.15 0.95 41.16 11.96 56.03 26.83

4-JB2 34.32 29.93 -4.39 35.74 1.42 37.00 2.68 39.77 5.45

1-F34 55.00 55.19 0.19 56.76 1.76 57.25 2.25 57.73 2.73

1-G23 55.25 55.32 0.07 56.73 1.48 56.96 1.71 57.33 2.08

1-J8 53.78 54.64 0.86 55.99 2.21 56.55 2.77 57.11 3.33

1-J5 54.41 54.67 0.26 56.04 1.63 56.57 2.16 57.15 2.74

1-L14 53.60 54.76 1.16 56.33 2.73 56.95 3.35 57.56 3.96

1-L9a 54.91 54.66 -0.25 56.11 1.20 56.61 1.70 57.15 2.24

1-L9 54.90 54.68 -0.22 56.14 1.24 56.78 1.88 57.39 2.49

1-L11 54.28 54.77 0.49 56.10 1.82 56.60 2.32 57.15 2.87

1-M9 54.53 54.94 0.41 55.97 1.44 56.53 2.00 57.09 2.56

1-M7 53.62 54.94 1.32 55.97 2.35 56.53 2.91 57.09 3.47

1-M5 55.00 54.92 -0.08 55.94 0.94 56.51 1.51 57.06 2.06

1-M12 54.00 54.95 0.95 55.94 1.94 56.50 2.50 57.04 3.04

1-M16 54.75 54.97 0.22 55.94 1.19 56.49 1.74 57.03 2.28

1-M19 55.00 55.01 0.01 55.93 0.93 56.47 1.47 57.00 2.00

1-Q5 54.52 55.50 0.98 55.92 1.40 56.43 1.91 56.95 2.43

1-Q4 54.52 55.55 1.03 56.04 1.52 56.35 1.83 56.87 2.35

1-Q3 54.00 55.56 1.56 56.07 2.07 56.33 2.33 56.86 2.86

1-Q7 54.50 55.10 0.60 55.41 0.91 55.71 1.21 56.21 1.714-B11 24.94 37.44 12.50 39.54 14.60 42.54 17.60 55.92 30.98

4-H3 54.35 54.59 0.24 54.79 0.44 54.83 0.48 56.04 1.69

1-W18 54.00 51.99 -2.01 54.59 0.59 55.44 1.44 56.44 2.44

1-W16 54.35 51.98 -2.37 54.59 0.24 55.43 1.08 55.95 1.60

Pond DB7 55.21 53.00 -2.21 55.74 0.53 56.00 0.79 56.53 1.32

1-W14 55.00 51.98 -3.02 54.59 -0.41 55.41 0.41 55.93 0.93

Node

2-yr 10-yr 25-yr 100-yr

Ground

El

(ft)

The elevation at 1-L14 is the ground elevation in the adjacent street is 53.60. The flowline (FL) of the existing 2-5x2RCB is 51.68.

Summaries of the SWMM results are included in Appendix C Tables 3.1 through 3.4.


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4.4 Proposed Conditions with W129 Stage 3

The proposed conditions model with Stage 3 analysis includes the Westpark storm sewer

system with the as-built box and orifice heights as well as the tailwater for the completed

Stage 3 improvements to W129-00-00. It also contains several improvements within the

TxDOT ROW to reduce the flooding within the corridor and the adjacent properties. It

also mitigates the proposed increases in impervious cover for the ultimate US 59/IH 610

interchange.

As part of the realignment of the US 59 NB/IH 610 SB direct connector in Phase 1 Job

157, AFLP has proposed the removal of the embankment and 48 restrictor between

Ponds DB6-1 and DB-7. This change was implemented in this analysis prior to making

any other proposed updates. The new detention pond reduces the water surface elevation

in the vicinity of the retaining wall (wall elevation =55.21) for the 2-yr storm, but not

enough to avoid overtopping the wall. The 10- and 25-yr elevations are higher than the

existing elevations at Pond DB-7. This is due to the backwater from the Westpark storm

sewer system filling the pond which will allow more water to overflow to the depressed

section. A comparison of the elevations at existing Pond DB-7 and the AFLP proposed

Pond DB6-1 is shown in Table 4.4a. A comparison of elevations within the depressed

section at Node 4-A29 is also shown below in Table 4.4b. Supplemental Tables 4.1

through 4.4 are available in Appendix D of the electronic file included on the CD of the

report.

Table 4.4a Existing WSEL v Existing with AFLP Pond DB6-1

Location2-yr

WSEL (ft)

10-yr

WSEL (ft)

25-yr

WSEL (ft)

100-yr

WSEL (ft)

Existing Pond DB-7 55.46 55.86 55.88 57.09

Existing Pond DB6-1 55.39 56.35 56.80 57.35

Existing w/ AFLP Pond DB6-1 55.18 56.17 56.62 56.97

DB-7 Change in WSEL (ft) -0.27 0.31 0.74 -0.12

DB6-1 Change in WSEL (ft) -0.20 -0.18 -0.19 -0.38


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Table 4.4b Node 4-A29 Elevation Comparison

Location2-yr

WSEL (ft)

10-yr

WSEL (ft)

25-yr

WSEL (ft)

100-yr

WSEL (ft)

Existing Node 4-A29 19.19 40.31 49.89 56.44

Existing w/ AFLP Pond at Node 4-A29 18.90 41.13 50.95 56.24

4-A29 Change in WSEL (ft) -0.30 0.81 1.07 -0.20

It is recommended that the retaining wall adjacent to the existing Pond DB7 should be

raised to elevation 57 to avoid overtopping during the Phase 1 Job 157 construction.

Coordination with AFLP should be completed to determine the limits of the effected

retaining wall and proposed detention pond limits and volume modeled within this report.

At Node 4-H3, overflow of the adjacent retaining wall (wall elevation =54.35) will

continue to occur. Without additional storage volume at 4-H3 and a flex valve on the

existing 24 RCP outfall into Pond DB6-1 (see Section 4.4.3), the overtopping cannot be

stopped without causing the US 59 MLs to flood. The construction of the detention pond

as discussed below cannot be constructed until Phase 1 Job 146, which removes the

existing IH 610 NB/ US 59 SB direct connector. It appears that some excavation at 4-H3

could be completed during Phase 1 Job 157 construction without disturbing the

embankment; however, the effect of those improvements have not been analyzed. A

comparison of elevations at Node 4-H3 is shown in Table 4.4c.

Table 4.4c Node 4-H3 Elevation Comparison

Location2-yr

WSEL (ft)

10-yr

WSEL (ft)

25-yr

WSEL (ft)

100-yr

WSEL (ft)

Existing Node 4-H3 54.70 54.84 54.88 56.44

Existing w/ AFLP Pond at Node 4-H3 54.68 54.82 54.87 56.24

4-H3 Change in WSEL (ft) -0.02 -0.02 -0.01 -0.20

Reduction of the tailwater is critical in reducing the flooding within the US 59 corridor

and the adjacent properties. Two options were analyzed during the development of this

report. First, an analysis was completed that would meet the 10-yr storm event along the

US 59 SBFR. This option would require 2-10 x 10 RCB along the US 59 SB FR,


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modifications to the detention ponds DB6-1 and DB6-2 to include vertical walls and

excavation to elevation 30.00, and the construction of a new pump station to de-water

the ponds and storm sewer. This option is undesirable due to several reasons. First of all,

the water table in the interchange is very high. Seepage through the walls in the

depressed section has been noted. Therefore, excavation to an elevation approximately

15 deeper than the existing detention pond would not likely be feasible. Without the

possibility of deepening the detention pond, the large storm sewer boxes cannot be

placed. The flowline of the box culverts would need to be significantly lower to place

below the proposed pavement elevations. Another factor against this option is

constructability near the Rice Blvd overpass. A single 10 x 10 RCB can be placed

under the overpass; however, the depth of the RCB interferes with many utilities. Also,

the proposed routing of the box along the US 59 NB MLs cannot be done at the depth

needed. The width of the ROW is limited between a retaining wall and several

businesses. Through coordination with LJA, it was determined that the integrity of the

retaining wall could be compromised with the parallel placement of the 10 x 10 RCB.

The limited ROW would not allow room for the storm sewer to be placed at a distance

that would not have effects on the retaining wall.

The second option would also require larger trunklines along the US 59 SB FR, but

would maintain gravity drainage of detention ponds DB6-1 and DB6-2. Due to the slight

slope of the terrain and the minimal depth of the existing storm sewer system, a 10-yr

storm event cannot be obtained with this option. Therefore, a 2-yr storm sewer design for

the US 59 SB FR was developed, with the proposed recommendations that follow.

4.4.1 Increase Detention Pond at W16

First of all, Pond 1-W16 is expanded to include Pond DB-8 and the additional area

available due to the realignment of the IH 610 NB/ US 59 SB direct connector in Phase 1

Job 146. Approximately 22.86 ac-ft of storage volume is available at this location.

Additional volume could be realized by adding a retaining wall along the IH 610 NB/ US


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59 NB DC, however, the additional volume is not significant enough to have an impact

on the water surface elevations within the US 59 corridor or the adjacent properties. The

outlet from Pond 1-W16 will be the existing 5x4 RCB that currently serves as the outlet

from Pond DB-8. The current outfall for Pond 1-W16, 2-36 RCP that enter the concrete

lined channel, should be removed. This pond will now act as an offline storage area for

the Westpark storm sewer system. A detention pond layout is provided in Appendix E.

4.4.2 Detention Pond at W18

A detention pond north of US 59 is also planned at Node 1-W18. The maximum water

surface elevation of 55.00 is controlled by the existing US 59 MLs. The amount of

drainage area that can be directed to this detention pond is restricted by the two depressed

sections; therefore, additional flow from the south side of US 59 will be allowed to backup into the north pond through the existing 2-36 RCP. The outflow from the east pump

station will be redirected to the concrete lined channel just downstream of the 2-36 RCP.

A berm should be placed along the retaining wall on the west side so flow will not

overtop the retaining wall. For this analysis, it is assumed that retaining walls can be

placed along the DCs in the NE (Phase 2) and NW (Phase 1 Job 145) quadrants to

maximize the size of the detention pond. Approximately 38.19 ac-ft of storage volume is

available after Phase 2. This pond will also be utilized as mitigation volume for the

AECOM part of the study. A layout for this detention pond is also included in Appendix

E. An additional pond layout is also provided to show the limits of the detention pond

that can be constructed as part of Phase 1 Job 145. This alternate layout provides

approximately 28.68 ac-ft of detention.

4.4.3 Detention Pond at 4-H3

The gravity line between Inlet 4-H3 and the proposed Pond DB6-1 must remain in the

proposed conditions. A flex valve should be placed at the downstream end of the gravityline to eliminate backflow through the system. A small detention pond is proposed in the

separation between the US 59 MLs and the IH 610 SB/US 59 SB direct connector to

create storage volume for the runoff to this area. The inlet 4-H3 should be removed and


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US 59 at South Rice. The trunkline will then turn toward the east, parallel the US 59 NB

MLs, and discharge into Pond DB6-2.

4.4.5 Trench Drain along IH 610 SB/US 59 SB Direct Connector

Trench drain is proposed along the barrier of the IH 610 SB/US 59 SB direct connector to

capture mainlane flow and direct it toward the detention pond at 4-H3. In the existing

conditions, this area drains into a small concrete lined channel between the frontage road

and the direct connector. During the 10-yr storm event, the water overflows into the

depressed section along the US 59 SB FR and contributes to the flooding of the IH 610

SB FR/Westpark Tollroad depressed section. With the adjustment to the frontage road

profile, the edge of the direct connector cannot be drained to the concrete channel. The

low point in the direct connector edge of pavement is approximately 53.90. Trenchdrain is proposed between Station 360+00 and 363+50. This line should outfall into the

detention pond 4-H3. The results show that the water surface elevation in the 100-yr

storm will be 54.45, which allows 1 lane of the direct connector to be passable.

4.4.6 Increase Detention Storage at Ponds DB6-1 and DB6-2

The detention ponds DB6-1 and DB6-2 should be regraded and reshaped within the

available area to provide the maximum volume. The flowlines of Pond DB6-1 and DB6-

2 are proposed at elevations 47.64 and 47.50, respectively. The ultimate configuration

at Pond DB6-2 can be completed in Phase 1 Job 157. Pond DB6-1 will need to be phased

with a majority of the pond completed during Phase 1 Job 157. During Phase 2 of the

interchange reconstruction, Pond DB6-1 should excavated beyond the limits of the Phase

1 Job 157 definition to the edge of the existing HOV lane. This step assumes that the US

59 NB/ IH 610 NB DC can be reconfigured in the future construction to increase the

length of the bridge. (AECOM is investigating this proposed change.) The ultimate

storage available in each of the ponds is 49.26 ac-ft at Pond DB6-1 and 10.84 ac-ft atPond DB6-2. Pond layouts for Phase 1 Job 157 and Phase 2 conditions are included in

Appendix E.


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4.4.7 Increase Height of Retaining Walls along IH 610 SBFR/Westpark Tollroad

To eliminate the overflows of the retaining walls, it is recommended that the wall heights

are raised or protected by earthen berms. The wall along proposed Pond DB6-1 should

be raised to approximately elevation 57 at all locations south of US 59. North of US 59

adjacent to the proposed pond at 4-H3, the retaining wall should be protected up to

elevation 55. Along the east side of the Westpark depressed lanes, the walls both north

and south of US 59 should be raised or protected to elevation 56 as part of Job 146.

These changes will keep water from overflowing the walls at all frequencies analyzed.

4.4.8 Pump Station Modifications

The pump station on/off elevations should be adjusted to allow the pumps to perform

more effectively. With the overland flow and overtopping of the retaining walls

eliminated, the IH 610 SBFR/ Westpark Tollroad is not flooded in the 25-yr storm

frequency. A comparison of the on/off elevations for the existing and proposed

conditions at Pump Station PS-4 is shown in Table 4.4d.

Table 4.4d Pump Station PS-4 Existing V Proposed On/Off Elevations

On El (ft) Off El (ft) On El (ft) Off El (ft)

1 17.12 14.12 22.50 8.752 18.62 17.12 23.00 8.43

Sump 5.69 3.04 5.69 3.04

Existing ProposedPump

For the IH 610 NBFR, several changes to the on/off elevations were made to reduce the

flooding; however, the changes did not alleviate flooding in the 25-yr event. A change to

the storm sewer routing is proposed to reduce the drainage area to the pump station. First

of all, approximately 48% of the flow for P4-J42 will be redirected to a new storm sewer

line as proposed in the AECOM (Job 145) study. This storm sewer line will drain to the

east and directly into W129. This will eliminate several acres of offsite and US 59 ML

and FR flow from being directed towards the pump station. Also, a proposed 42 storm


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sewer line is proposed to disconnect the existing trunkline along the US 59 NB MLs from

the pump station system and reroute it directly to the Westpark trunkline. The routing of

this line is shown in Appendix D. The line will need to pass under the METRO HOV

lane and under the existing US 59 NB FR using jack and/or bore to avoid disruptions to

traffic. Approximately 0.05 ac of ROW will be needed for construction of this line.

With this adjustment, the depressed lanes will meet the 25-yr criteria. A comparison of

the on/off elevations for the existing and proposed conditions at Pump Station PS-5 is

shown in Table 4.4e.

Table 4.4e Pump Station PS-5 Existing V Proposed On/Off Elevations

On El (ft) Off El (ft) On El (ft) Off El (ft)1 22.45 20.70 27.50 18.75

2 22.70 21.95 30.00 19.00

3 24.70 22.95 32.00 18.65

4 26.70 23.70 32.50 18.55

Sump 16.64 15.64 16.64 15.64

Existing ProposedPump

A comparison of the water surface elevations to the ground levels within the depressed

section are shown in Table 4.4f. Water surface elevations between the existing and

proposed conditions are shown in Table 4.4g.

Table 4.4f Proposed WSEL v Ground Elevations in Depressed Sections

NodeGround El

(ft)

10-yr

WSEL (ft)

Flood

Depth (ft)

25-yr

WSEL (ft)

Flood

Depth (ft)

100-yr

WSEL (ft)

Flood

Depth (ft)

4-A29 24.94 24.68 -0.27 24.82 -0.12 37.91 12.97

4-J8 29.20 24.87 -4.34 25.03 -4.17 38.04 8.84

4-JB2 34.32 33.18 -1.14 33.50 -0.82 33.60 -0.73


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Table 4.4g Proposed WSEL v Existing WSEL in Depressed Sections

Node

10-yr

EX

WSEL

(ft)

10-yr

PR

WSEL

(ft)

Change

in

Depth

(ft)

25-yr

EX

WSEL

(ft)

25-yr

PR

WSEL

(ft)

Change

in

Depth

(ft)

100-yr

EX

WSEL

(ft)

100-yr

PR

WSEL

(ft)

Change

in

Depth

(ft)

4-A29 40.31 24.68 -15.64 49.89 24.82 -25.07 56.44 37.91 -18.53

4-J8 40.93 24.87 -16.07 49.94 25.03 -24.91 56.46 38.04 -18.41

4-JB2 36.47 33.18 -3.29 37.67 33.50 -4.17 40.00 33.60 -6.40

With these proposed changes, water surface elevations within the project limits are

greatly reduced when compared to existing conditions. Comparison of the Existing with

Silt and the Proposed with Phase 3 is shown below in Table 4.4h. The proposed

improvements also meet the 2-yr frequency for the frontage roads and the 25-yr in the

depressed sections. The water surface elevations for the improvements compared to the

ground elevations at critical locations are also shown below in Table 4.4i. Summaries of

the SWMM results are included in Appendix D Tables 5.1 through 5.4.


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Table 4.4h Existing with Silt WSEL v Proposed with Stage 3 WSEL

Ex

Silt

WSEL(ft)

Pr

Stage 3

WSEL(ft)

(+/-)

Ex

Silt

WSEL(ft)

Pr

Stage 3

WSEL(ft)

(+/-)

Ex

Silt

WSEL(ft)

Pr

Stage 3

WSEL(ft)

(+/-)

Ex

Silt

WSEL(ft)

Pr

Stage 3

WSEL(ft)

(+/-)

4-A29 19.19 23.05 3.86 40.31 24.68 -15.64 49.89 24.82 -25.07 56.44 37.91 -18.53

4-J8 20.80 23.13 2.33 40.93 24.87 -16.07 49.94 25.03 -24.91 56.46 38.04 -18.41

4-JB2 30.37 30.98 0.61 36.47 33.18 -3.29 37.67 33.50 -4.17 40.00 33.60 -6.40

1-J8 55.60 53.23 -2.37 56.56 55.61 -0.96 56.99 56.30 -0.70 57.49 57.06 -0.43

1-J5 55.65 53.07 -2.58 56.60 55.64 -0.96 57.01 56.36 -0.65 57.50 57.05 -0.44

1-L14 55.88 52.90 -2.99 56.97 55.82 -1.15 57.44 56.67 -0.77 57.95 57.38 -0.57

1-L6 55.74 52.92 -2.82 56.64 55.68 -0.96 57.02 56.37 -0.65 57.49 57.05 -0.44

1-L9 55.71 53.06 -2.65 56.79 55.72 -1.07 57.26 56.45 -0.81 57.80 57.12 -0.69

1-M9 55.67 52.88 -2.79 56.56 55.63 -0.93 56.97 56.33 -0.64 57.45 57.04 -0.40

1-M7 55.67 52.88 -2.79 56.56 55.60 -0.96 56.97 56.31 -0.66 57.45 57.04 -0.41

1-M5 55.62 52.85 -2.77 56.52 55.59 -0.93 56.94 56.29 -0.66 57.43 57.04 -0.391-M12 55.63 52.84 -2.78 56.51 55.59 -0.92 56.92 56.29 -0.64 57.40 57.04 -0.37

1-Q5 55.65 52.85 -2.80 56.46 55.57 -0.88 56.85 56.27 -0.58 57.32 57.02 -0.30

1-Q3 55.83 52.85 -2.98 56.38 55.57 -0.80 56.77 56.27 -0.50 57.23 57.02 -0.22

1-Q7 55.19 52.85 -2.34 55.74 55.57 -0.17 56.13 56.27 0.15 56.58 57.01 0.43

4-B11 37.81 37.15 -0.66 42.47 37.39 -5.09 49.89 37.47 -12.42 56.44 42.18 -14.26

4-H3 54.70 50.05 -4.65 54.84 51.72 -3.12 54.88 52.68 -2.20 56.44 54.36 -2.09

1-W18 52.88 50.36 -2.53 54.71 51.62 -3.09 55.14 52.73 -2.41 56.36 55.41 -0.95

1-W16 52.88 50.83 -2.05 54.63 51.92 -2.71 55.17 53.19 -1.98 56.40 55.55 -0.86

Pond W16 52.88 52.42 -0.47 54.64 55.09 0.44 55.17 55.76 0.58 56.40 56.53 0.12

Pond DB6-1 55.39 52.63 -2.76 56.35 55.31 -1.04 56.80 56.02 -0.79 57.35 56.80 -0.55

1-W14 52.88 50.86 -2.02 54.63 51.93 -2.70 55.17 53.28 -1.89 56.44 55.53 -0.91

Node

2-yr 10-yr 25-yr 100-yr


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Table 4.4i Proposed with Stage 3 WSEL V Ground Elevations

Pr

Stage 3

WSEL

(ft)

Flood

Depth

(+/-)

Pr

Stage 3

WSEL

(ft)

Flood

Depth

(+/-)

Pr

Stage 3

WSEL

(ft)

Flood

Depth

(+/-)

Pr

Stage 3

WSEL

(ft)

Flood

Depth

(+/-)

4-A29 24.94 23.05 -1.89 24.68 -0.27 24.82 -0.12 37.91 12.97

4-J8 29.20 23.13 -6.07 24.87 -4.34 25.03 -4.17 38.04 8.84

4-JB2 34.32 30.98 -3.34 33.18 -1.14 33.50 -0.82 33.60 -0.73

1-F34 55.00 55.14 0.13 56.45 1.45 57.05 2.05 57.66 2.66

1-G23 55.25 54.95 -0.30 56.36 1.11 56.50 1.25 57.08 1.83

1-J8 53.78 53.23 -0.55 55.61 1.83 56.30 2.52 57.06 3.28

1-J5 54.11 53.07 -1.04 55.64 1.53 56.36 2.25 57.05 2.94

1-L14 53.60 52.90 -0.70 55.82 2.22 56.67 3.07 57.38 3.78

1-L6 54.19 52.92 -1.27 55.68 1.49 56.37 2.18 57.05 2.86

1-L9 55.04 53.06 -1.98 55.72 0.68 56.45 1.41 57.12 2.08

1-L11 56.04 52.90 -3.14 55.67 -0.37 56.36 0.32 57.05 1.01

1-M9 54.77 52.88 -1.89 55.63 0.85 56.33 1.56 57.04 2.27

1-M7 54.54 52.88 -1.66 55.60 1.06 56.31 1.77 57.04 2.50

1-M5 54.94 52.85 -2.09 55.59 0.65 56.29 1.35 57.04 2.10

1-M12 54.36 52.84 -1.52 55.59 1.23 56.29 1.93 57.04 2.68

1-Q5 55.06 52.85 -2.21 55.57 0.51 56.27 1.21 57.02 1.96

1-Q3 55.48 52.85 -2.63 55.57 0.09 56.27 0.79 57.02 1.54

1-Q7 55.80 52.85 -2.95 55.57 -0.23 56.27 0.47 57.01 1.21

4-B11 40.58 37.15 -3.43 37.39 -3.19 37.47 -3.11 42.18 1.60

4-H3 55.00 50.05 -4.95 51.72 -3.28 52.68 -2.33 54.36 -0.64

1-W18 55.00 50.36 -4.65 51.62 -3.38 52.73 -2.27 55.41 0.411-W16 55.00 50.83 -4.17 51.92 -3.08 53.19 -1.81 55.55 0.55

Pond W16 58.00 52.42 -5.59 55.09 -2.92 55.76 -2.24 56.53 -1.47

Pond DB6-1 58.00 52.63 -5.37 55.31 -2.69 56.02 -1.99 56.80 -1.20

1-W14 57.00 50.86 -6.14 51.93 -5.07 53.28 -3.72 55.53 -1.47

Node

2-yr 10-yr 25-yr 100-yr

Ground

El

(ft)

The elevation at 1-L14 is the ground elevation in the adjacent street is 53.60. The flowline (FL) of the existing 2-5x2

RCB is 51.68.

4.5 Proposed Conditions with Silt

The proposed conditions model with silt analysis includes all of the proposed drainage

improvements discussed in the proposed conditions with Stage 3 analysis, but also

includes the silt levels within the Westpark storm sewer line and the existing W129-00-

00 conditions.


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IH 610 W/ US 59 S Interchange South Drainage Element

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