draft - 2014-02 hydrologic and hydraulic analysis for 610-59 interchange - north

Prepared for: Prepared by: Texas Department of Transportation AECOM Houston, Texas Houston, Texas CSJ 0217-17-145 60275246 February 2014

Water TBPE Reg. No. F-3580

Hydrologic & Hydraulic Analysis for IH 610 W / US 59 S Interchange North Drainage Elements

Prepared for: Prepared by: Texas Department of Transportation AECOM Houston, Texas Houston, Texas CSJ 0217-17-145 60275246 February 2014

Water TBPE Reg. No. F-3580


Anthony W. Holder, Senior Specialist Engineer, PE, CFM Name as Registered, Title

96751 Texas Serial No.

December 28, 2005 Date Registered

TBPE Firm Registration No. F-3580


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Contents

Executive Summary ............................................................................................................ 1

1.0 Introduction .................................................................................................................. 3 1.1 Project Name and Purpose .............................................................................................3 1.2 Project Limits .................................................................................................................3 1.3 Project Objectives ..........................................................................................................4 1.4 Assumptions and Constraints .........................................................................................4 1.5 Prior Studies ..................................................................................................................5 1.6 Coordination ..................................................................................................................5

2.0 Methodology ................................................................................................................ 7 2.1 General Hydrologic and Hydraulic Analysis .....................................................................7 2.2 Available Data................................................................................................................7 2.3 Hydrologic Methodology .................................................................................................7

2.3.1 Drainage Area Delineation ................................................................................7 2.3.2 Peak Flow Estimation .......................................................................................8 2.3.3 Time of Concentration ......................................................................................9 2.3.4 Land-Use ....................................................................................................... 10 2.3.5 Hydrograph Development ............................................................................... 11

2.4 Hydraulic Methodology ................................................................................................. 11

3.0 Existing Conditions .................................................................................................... 13 3.1 Location and Topography ............................................................................................. 13 3.2 SWMM Model Collaboration with LJA/H&H Resources .................................................. 13 3.3 Existing Conditions Drainage Areas .............................................................................. 13 3.4 Land Use ..................................................................................................................... 13 3.5 Existing Storm Sewer Network...................................................................................... 14 3.6 HCFCD Unit Numbers .................................................................................................. 14 3.7 HCFCD Channel Improvements ................................................................................... 14 3.8 Existing Conditions Results and Analysis of Problem Areas ........................................... 15

3.8.1 US 59 Southbound Frontage Road ................................................................. 15 3.8.2 US 59 Northbound Frontage Road .................................................................. 16 3.8.3 Post Oak Boulevard/Richmond Drive Intersection ............................................ 16 3.8.4 Effects of Removing Silt from W129-00-00 and Westpark Drive Box Storm

Sewer ............................................................................................................ 16


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3.8.5 Effects of W129-00-00 Stage 3 Improvements ................................................. 17

4.0 Proposed Improvements ........................................................................................... 18 4.1 LJA/H&H Resources Model Area .................................................................................. 18 4.2 Proposed Drainage Area Changes ............................................................................... 18 4.3 Proposed Land Use Changes ....................................................................................... 18 4.4 Right-of Way ................................................................................................................ 18 4.5 Recommended Drainage Improvements ....................................................................... 19

4.5.1 Detention Under IH 610 .................................................................................. 19 4.5.2 IH 610 NBFR and US 59 NBFR Improvements ................................................ 19 4.5.3 US 59 SBFR Improvements ............................................................................ 20

5.0 Recommended Alternative Analysis Results ............................................................ 21 5.1 Analysis Results........................................................................................................... 21

6.0 Entity Participation ..................................................................................................... 24

7.0 Conclusions and Recommendations ........................................................................ 26

List of Appendices Appendix 1 Field Visit Photographs

Appendix 2 Hydrologic Calculations

Appendix 3 SWMM Model Layouts

Appendix 4 Water Surface Elevation Comparisons 100 Year Storm




Appendix 8 Hydrographs

Appendix 9 Electronic Data


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List of Tables Table 1: Summary of Recommended Solutions ...................................................................................2 Table 1-1: Segment Station Limits .......................................................................................................4 Table 2-1: Intensity-Duration-Frequency Coefficients for Harris County .................................................9 Table 2-2: Flow Velocity by Flow Type ............................................................................................... 10 Table 2-3: Runoff Coefficient (C) Values for Various Land Use Types ................................................. 10 Table 2-4: Green and Ampt Loss Parameters for All Watersheds........................................................ 11 Table 5-1: Storm Sewer Improvements in Recommended Alternative ................................................. 22 Table 5-2: Flow and Volume Reductions at Selected Locations (W129 Stage 3) ................................. 23 Table 6-1: Percentage of flow by Entity .............................................................................................. 24 Table 6-2: Summary of Potential Costs by Entity ................................................................................ 25

List of Exhibits Exhibit 1 Vicinity Map

Exhibit 2 Overview of Proposed Project

Exhibit 3 Flood Insurance Rate Map

Exhibit 4 Topography and Overland Drainage Patterns

Exhibit 5 Existing Conditions Drainage Areas

Exhibit 6 Existing Conditions Land Use

Exhibit 7 Existing Conditions, W129-00-00 Stage 2, 100-Year Inundation

Exhibit 8 Existing Conditions, W129-00-00 Stage 2, Inundation Comparison

Exhibit 9 Existing Conditions, W129-00-00 Stage 3, 100-Year Inundation

Exhibit 10 Existing Conditions, W129-00-00 Stage 3, Inundation Comparison

Exhibit 11 Proposed Drainage Area Changes

Exhibit 12 Proposed Land Use Changes

Exhibit 13 Recommended Solutions East

Exhibit 14 Recommended Solutions West

Exhibit 15 Recommended Alternative, W129-00-00 Stage 2, 100-Year Inundation

Exhibit 16 Recommended Alternative, W129-00-00 Stage 2, Inundation Comparison

Exhibit 17 Recommended Alternative, W129-00-00 Stage 3, 100-YR Inundation

Exhibit 18 Recommended Alternative, W129-00-00 Stage 3, Inundation Comparison

Exhibit 19 Proposed Contributing Drainage Areas By Agency


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List of Abbreviations

AC-FT Acre-feet AECOM AECOM Technical Services, Inc. AEP Annual Exceedance Probability BFE Base Flood Elevation CFS Cubic Feet Per Second DC Direct Connector DEM Digital Elevation Model FEMA Federal Emergency Management Agency FIS Flood Insurance Study FM Farm-to-Market FT Feet FT/S Feet Per Second GIMS Geographic Information & Management System (City of Houston) GIS Geographic Information System HCAD Harris County Appraisal District HCFCD Harris County Flood Control District HCTRA Harris County Toll Road Authority HGAC Houston-Galveston Area Council HGL Hydraulic Grade Line H&H Resources H&H Resources, Inc. IN/HR Inches Per Hour LF Linear Feet LiDAR Light Detection and Ranging LJA LJA Engineering, Inc. NAVD North American Vertical Datum NBFR Northbound Frontage Road NRCS Natural Resources Conservation Service (Formerly SCS) PGL Profile Grade Line PS&E Plans, Specifications, and Estimates R Clark Unit Hydrograph Storage Coefficient RCB Reinforced Concrete Box RCP Reinforced Concrete Pipe ROW Right-Of-Way RRC Railroad Commission of Texas SBFR Southbound Frontage Road


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List of Abbreviations (continued)

SBNB IH 610 southbound to US 59 northbound SCS Soil Conservation Service (Currently NRCS) tc Time of Concentration TSARP Tropical Storm Allison Recovery Project TxDOT Texas Department of Transportation UPRR Union Pacific Railroad USGS United States Geological Survey WSE Water Surface Elevation W129 HCFCD Channel with Unit Number W129-00-00


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Executive Summary

This study includes hydrologic and hydraulic analyses for proposed revisions to the IH 610/US 59 interchange in southwest Houston, to be reflected on the Plans, Specifications, and Estimates. Exhibit 1 shows the general project vicinity. The purpose of the study was to identify existing drainage conditions including problem flooding areas, and to propose alternatives to the drainage design to alleviate existing problems, as well as to accommodate the proposed paving improvements. The project is divided between two firms. AECOM Technical Services, Inc. (AECOM) is designing and analyzing the northern drainage elements, which refers to those elements along IH 610 generally north of Richmond Avenue and along US 59 generally east of IH 610. LJA Engineering, Inc. (LJA) and their drainage sub-consultant H&H Resources, Inc. (H&H Resources) are designing and analyzing the southern drainage elements. The extents of the AECOM and H&H Resources pavement and drainage analysis areas are shown in Exhibit 2 and Exhibit 3. This report describes the drainage for the AECOM portion of the system.

The planned paving improvements for the AECOM portion of the system include four distinct sub-projects. First is a direct connector (DC) from IH 610 southbound to US 59 northbound (SBNB DC). Second is a new ramp taking southbound traffic from the Westheimer Road entrance ramp to US 59 southbound (Westheimer Ramp). Third is a realignment of Post Oak Boulevard at the Richmond Avenue intersection (Post Oak Realignment). Fourth is a realignment of the US 59 northbound frontage road (US 59 NBFR), under the proposed DC. Exhibit 2 shows the project area and the proposed pavement revisions for each of the sub-projects.

In addition to the paving improvements, the Recommended Alternative includes five distinct drainage improvements that are solutions to identified drainage issues. The recommended solutions are identified by letters in a list that is consistent with the drainage solutions listed in the H&H Resources Report, and are summarized in Table 1. Solution D consists of adding detention within the center of the interchange, south of Richmond Avenue and north of the US 59 main lanes.

Solution L is rerouting flow from the US 59 main lanes directly to the Westpark Drive box culverts and preventing the flow from entering the IH 610 NBFR depressed section. Solution Q is rerouting flow along the US 59 NBFR to W129-00-00 (W129) by adding a new trunk system along the US 59 northbound frontage road and increasing the size of the storm sewer crossing beneath US 59 at the Newcastle Road intersection. Solution Q also includes increasing the inlet capacity along the US 59 NBFR by replacing the inlets with type C-1 inlets. Solution R is increasing the grade of the US 59 NBFR in the curve at the upstream end of the IH 610 NBFR depressed section to reduce the amount of overland flow entering the depressed section. Solution S is eliminating an 18 connection between two storm sewer systems along the US 59 southbound frontage road. In addition, Solution Q also reduces frequent flooding along the US 59 NBFR west of Newcastle Drive.

The proposed drainage improvements are shown on Exhibit 13 and Exhibit 14.

The drainage improvements in the Recommended Alternative (Solution Q) will provide substantive relief of street flooding along the US 59 NBFR, in the, however the full benefit of the improvements will not be realized until and unless the W129 Stage 3 improvements are completed (see Section 3.7 for details about past and planned W129 improvements). At the same time, Solutions L, Q, R, and S reduce the flows into the depressed section of the IH 610 northbound frontage road, where it passes under US 59. These flow reductions will reduce the stresses on the pump station at that location. The


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pump station analysis is being handled by the LJA/H&H Resources, Inc. team, who also proposed Solution L.

Within the northern portion of the project area, the proposed drainage improvements in the Recommended Alternative will increase water surface elevations along portions of the US 59 southbound frontage road, due to the elimination of the 18 RCP connecting the eastern portion of the drainage system to the western portion that flows to the IH 610 northbound frontage road depressed section (Solution S). This increase in water surface elevation is not considered to be an impact, because it was part of a previous design, and should have been part of the existing conditions.

Solution D will provide some street flooding reduction in the Post Oak Realignment area, and will reduce the overland flow into the depressed section of the Westpark Tollway exit/IH 610 southbound frontage road. The storm sewer elements associated with the realignment of Post Oak Boulevard are not specifically designed to reduce flooding, but work with the increased detention volume in Solution D to improve drainage in the intersection.

The paving and drainage improvements described in this report, if constructed in accordance with the analysis described herein, will result in no adverse impact to flooding risk during storms up to and including the 100-year storm event for the area encompassed by the north drainage elements.

Table 1: Summary of Recommended Solutions

ID Report Reference Issue Solution

D 4.5.1 (4.3.2) Existing flooding of Post Oak/Richmond/IH 610 SBFR interchange

Add W18 Detention

L 4.5.2 (4.4.8) Existing flooding of IH 610 NBFR Reroute storm sewer line (42 RCP) to Westpark Drive.

Q 4.5.2 Existing flooding of US 59 NBFR and IH 610 NBFR Reroute storm drainage (48 RCP) to W129-00-00, add C-1 Inlets

R 4.5.2 Existing flooding of IH 610 NBFR Raise grade of US 59 NBFR

S 4.5.3 Existing flooding of IH 610 NBFR Plug existing 18 interconnect along US 59 SBFR

Report References in parentheses are locations in the H&H Resource Report that discuss the same solution.


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1.0 Introduction

1.1 Project Name and Purpose This study includes hydrologic and hydraulic analyses for proposed revisions to the IH 610/US 59 interchange in southwest Houston. Exhibit 1 shows the general project vicinity. The purpose of the study was to identify existing drainage conditions including problem flooding areas, and to propose alternatives to the drainage design to alleviate existing problems, as well as to accommodate the proposed paving improvements. The project is divided between two firms. AECOM Technical Services, Inc. (AECOM) is designing and analyzing the northern drainage elements. LJA Engineering, Inc. (LJA) and their drainage sub-consultant H&H Resources, Inc. (H&H Resources) are designing and analyzing the southern drainage elements. This report describes the drainage for the northern portion of the system which outfalls though the IH 610 trunk line north of the project to Buffalo Bayou and the US 59 trunk line which outfalls east of the project to Harris County Flood Control District (HCFCD) Unit W129-00-00 (W129).

The planned paving improvements for the AECOM portion of the project include four distinct sub-projects. First is a direct connector (DC) from IH 610 southbound to US 59 northbound (SBNB DC). Second is a new ramp taking southbound traffic from the Westheimer Road entrance ramp to US 59 southbound (Westheimer Ramp). Third is a realignment of Post Oak Boulevard at the Richmond Avenue intersection (Post Oak Realignment). Fourth is a realignment of the US 59 northbound frontage road (US 59 NBFR), under the proposed DC. Exhibit 2 shows the project area and the proposed pavement revisions for each of the sub-projects, as well as the starting and ending alignment stations for the sub-projects.

This drainage analysis represents five distinct drainage improvement solutions that will be reflected in the construction plans for the northern portion of the IH 610 / US 59 interchange project (CSJ 0217-17-145). The five drainage improvements solutions are shown on Exhibit 13 and Exhibit 14. These improvements will be reflected in the Plans, Specifications, and Estimates.

1.2 Project Limits The proposed pavement changes follow several different roadway alignments. There are new alignments for the IH 610/US 59 DC and the new Westheimer Ramp. Stationing along the various alignments is given in Table 1-1. The SBNB DC stationing starts at the combined exit from IH 610 and southbound entrance from Westheimer Road and ends at the merge with US 59 near Newcastle Road. Stationing along the Westheimer Ramp begins at the start of new pavement on the south end and extends to the end of new pavement on the north end. Stationing along the Post Oak realignment extends from the south end to the north end. Stationing along the US 59 NBFR extends from the west end to the east end. Stationing along IH 610 extends from south to north and is provided for Solution L, which does not involve pavement revisions.


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Table 1-1: Segment Station Limits

Roadway Description Baseline Begin Station End Station

SBNB DC SBNB DC 97+05.50 148+37.80

Westheimer Ramp Westheimer Ramp 571+97.56 592+42.60

Post Oak Realignment IH 610 SBFR 214+74.94 228+11.75

US 59 NBFR US 59 NBFR 181+14 209+60

IH 610 NBFR/US 59 NBFR IH 610 202+00 212+00

1.3 Project Objectives The work to be performed by the Engineer is the preparation of a drainage study to determine existing flooding characteristics, evaluate hydrologic and hydraulic impacts associated with the proposed project, determine proposed improvements to reduce flooding in the project area, and develop hydrologic and hydraulic data for use in plans, specifications, and estimates (PS&E) for the proposed IH 610 / US 59 interchange modifications.

1.4 Assumptions and Constraints As a part of this study the following assumptions and constraints should be noted:

1. The drainage system within the area of the intersection is complex. Overland flow directions differ from subsurface flow directions. There are man-made obstructions to flow due to raised roadways and railroads and man-made flow paths due to depressed roadways, storm sewers, and open channels.

2. The analysis and design have been divided between two teams, AECOM and LJA/H&H Resources. Each team developed a model for its project area, then the teams merged the models and communicated revisions, so that this report and the H&H Resources Report refer to the same model. AECOM is responsible for the model features in the northern portion of the IH 610 / US 59 interchange as shown on Exhibit 3.

3. The proposed system depths and allowable downstream flows are constrained by the existing storm and open channel systems along IH 610 and W129.

4. One major outfall for the drainage system is HCFCD Unit Number W129-00-00. HCFCD has a master plan for improvements to this system, which consists of three Stages. Stage 1 is complete. Stage 2 is under construction, Stage 3 is currently scheduled for construction in early 2016. The portion of the system that will be upgraded in Stage 3 is currently open channel, and has significant siltation issues. These siltation issues extend upstream into the dual 12 ft x 8 ft RCB system that drains Westpark Drive. This analysis considers the situation at the end of Stage 2, in which the W129 system will include silt in the open channel portion and the dual 12 ft x 8 ft RCB portion, the situation where the silt has been removed, but none of the Stage 3 improvements have been made, and the situation at the end of Stage 3, in which the improvements have been made, and the silt has been removed.


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1.5 Prior Studies Prior studies referenced during the analysis and design process include the plans for the proposed improvements to W129-00-00, as-built construction plans for the various pieces of the IH 610 / US 59 interchange and surrounding streets, and the Flood Insurance Study models for Buffalo Bayou. A list of prior study reference materials utilized is provided below.

Southwest Freeway, from Alief Road to McCue Road (CSJ 0027-13-010 and 011) Southwest Freeway, from Lancashire Road to Kirby Drive (CSJ 0027-13-013 and 014) Southwest Freeway (Segment III) Frontage Street Relocation & Widening including AVL in

Freeway Median, from South of Westpark to East of IH-610 US 59 Interchange (CSJ 0027-13-123)

Southwest Freeway / Transitway - Segment IV From west of Newcastle to east of Shepherd (CSJ 0027-13-133)

West Loop Freeway Embankments at Southwest Freeway Interchange (CSJ 0271-17-001) West Loop Freeway Interchange (Stage II) with Southwest Freeway (US 59) (CSJ 0271-

17-002) West Loop Freeway Interchange (Stage III) with Southwest Freeway (US 59) (CSJ 0271-

17-007) West Loop Freeway From US 59 Interchange to Woodway Drive (CSJ 0271-17-008) West Loop Freeway From Beechnut Ave. to Alief Rd. (CSJ 0271-17-009) IH 610 from South of Westpark Dr. to North of Richmond Ave. (CSJ 0271-17-125) IH 610 from North of Richmond Ave. to South of Post Oak (CSJ 0271-17-126) Harris County Frontage Roads and Tollway Connector (CSJ 0271-17-138) Harris County Flood Control District Project W129-00-00-C001. Conveyance

Improvements Enclosure from W130-00-00 to San Felipe. March 17, 2004. Harris County Flood Control District Project W129-00-00-C002. Conveyance

Improvements Stage 2 from San Felipe to Richmond Ave. Feb 3, 2011. Preliminary Engineering Report: Proposed Channel Improvements Unit W129-00-00,

Harris County Flood Control District, July 1979. Engineering Report First Stage Investigation and Evaluation of HCFC Unit W129-00-00.

August 1992.

1.6 Coordination Throughout the completion of this study significant coordination was performed with Texas Department of Transportation (TxDOT) and with the firms preparing the adjoining project, LJA and H&H Resources.

Coordination with H&H Resources was a major part of performing the drainage study. Each team is modeling a separate drainage system, however surface flooding during the larger storm events creates complex flow transfers between the models. H&H Resources prepared a hydraulic model of the southern portion of the drainage area, while AECOM prepared a model of the northern portion. Because there are several interconnection points between the two model areas, the models were combined to create a single model for this analysis.

AECOM met with HCFCD on September 26, 2012 to discuss the W129 channel improvements and this project. HCFCD provided information about the status of the W129 channel improvements, and the associated modeling and survey.


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H&H Resources submitted a report dated December 2013 and titled Hydrologic & Hydraulic Report IH 610 W/ US 59 S Interchange South Drainage Elements [H&H Resources Report] that is a companion to this report and describes the drainage elements on the south side of the interchange that were the focus of the H&H Resources drainage analysis.


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2.0 Methodology

2.1 General Hydrologic and Hydraulic Analysis The Storm Water Management Model (SWMM) was selected for this impact analysis. The version used on this project is XPSWMM 2012, Service Pack 1, by XPSolutions.

The hydrologic and hydraulic methodologies used in this analysis are described below. The hydrology was a combination of the Rational Method and the Clark Unit Hydrograph method, with hydrographs developed within SWMM to match Rational Method peak flows. Hydraulic computations were performed within SWMM.

2.2 Available Data Project related data were collected and used in the study. The following is a list of data collected and/or used:

x 2008 Light Detection and Ranging (LiDAR) data x 2001 LiDAR Flown in 2008 Non-Uniform Spatial Adjustment x 2010 and 2012 aerial maps from Houston-Galveston Area Council (HGAC) x Proposed schematic roadway profile x HCFCD M3 effective models downloaded from HCFCD website x Digital land use type data created from aerials x Land parcel and ownership data x Tropical Storm Allison Report and Project (TSARP) watershed/sub-watershed boundaries x Pipeline data from the Railroad Commission of Texas (RRC) x HCFCD Right-of-Way (ROW) x HCFCD channel network x Surveyed storm sewer locations, sizes, and elevations x Subsurface Utility Engineering (SUE) data

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 which serves as the basis for Federal Emergency Management Agency (FEMA) flood insurance study (FIS) and Flood Insurance Rate Map (FIRM) elevations.

2.3 Hydrologic Methodology 2.3.1 Drainage Area Delineation LiDAR topographic data was used in conjunction with a geographic information system (GIS) to delineate subbasins and offsite drainage areas. City of Houston Geographic Information & Management System (GIMS) storm sewer data was used in developing offsite drainage areas.


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2.3.2 Peak Flow Estimation Peak flow estimates were calculated for a preliminary impact analysis, using trunkline-level drainage areas ranging in size from 0.8 ac to 283.5 ac. The storms calculated included annual exceedance probabilities (AEPs) of 50%, 10%, 4%, and 1% (i.e., recurrence interval of 2-year, 10-year, 25-year, and 100-year storm event, respectively). While the annual exceedance probability is a more accurate term, as two 100-year storms could occur in back-to-back years, this report will refer to the more commonly used recurrence interval to describe the storms.

2.3.2.1 Rational Method The Rational Method was used to estimate peak flows for all drainage areas, including the one drainage area where the contributing drainage area was greater than 200 acres. The utilization of the Rational Method follows the guidelines described in Chapter 4, Section 12 of the TxDOT Hydraulic Design Manual (March 2011). The Rational Method equation is expressed as

Q = CIA,

where

Q = maximum rate of runoff (cfs),

C = runoff coefficient,

I = average rainfall intensity (in/hr), and

A = drainage area (ac).

A runoff coefficient (C) was assigned to each land-use type in this study. A composite C-value was then calculated for each drainage area using weighted C-values of various land-uses within the drainage area. For the details on the determination of area-weighted composite C-values, please refer to Section 2.3.4, Land-Use.

The rainfall intensity was determined using the equation

I =(),

where

I = rainfall intensity (in/hr), and

tc = time of concentration (min).

b, d, e = coefficients for specific frequencies listed by county in the rainfall Intensity-Duration-Frequency coefficients provided by TxDOT.

For the details on the estimation of tc, please refer to Section 2.3.3 Time of Concentration. Table 2-1 presents the b, d, and e values used in this study for Harris County as provided online by TxDOT at: http://onlinemanuals.txdot.gov/txdotmanuals/hyd/ the_rational_method.htm#i999837.


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Table 2-1: Intensity-Duration-Frequency Coefficients for Harris County

Annual Exceedance Probability Return Period e b d

50% 2-Year 0.800 68 7.9

10% 10-Year 0.753 81 7.7

4% 25-Year 0.724 81 7.7

2% 50-Year 0.728 91 7.7

1% 100-Year 0.706 91 7.9

Once runoff coefficient (C), average rainfall intensity (I) and drainage area acreage (A) were determined, they were substituted into the Rational Method equation to calculate peak flows.

2.3.3 Time of Concentration A travel time-based tc was computed for each drainage area where the Rational Method was applied. The tc is the time at which the entire watershed contributes to runoff and is calculated based on the time it takes for flow to travel from the most hydraulically remote point in the drainage area to the mouth of the drainage area.

The tc is required in the Rational Method for estimating the design average rainfall intensity. In order to determine tc for the Rational Method, the runoff flow path that requires the longest travel time was identified in GIS based on LiDAR data and aerial photos. The flow path was then divided into segments for overland flow and concentrated flow. Overland flow is a combination of sheet flow and shallow gutter flow to the inlet. Beyond this limit, concentrated flow was assumed, representing either storm sewer or HCFCD channels. The travel time for each segment of flow type along flow path was then calculated as

t = L/(v*60),

where

t = travel time (minute),

L = flow path length (feet), and

v = flow velocity (ft/s).

For sizing storm sewer and mitigation basins, a flow velocity of 1.5 ft/s was used for overland flow to calculate peak flows generated in the ROW areas. For storm sewer and HCFCD channel flow, a velocity was assumed based on the typical storm sewer design capacity. Table 2-2 summarizes the velocities for various channel and conduit flow types used in the analysis.


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Table 2-2: Flow Velocity by Flow Type

Non-Sheet Flow Type Flow Velocity (ft/s)

Sheet Flow and Gutter Flow 1.5

Storm Sewer or HCFCD Channel 3.0

Using these flow velocity values, the travel time for flow through each segment was calculated. The total tc for a drainage area was then determined as the sum of travel times for the runoff to flow through segments of both flow types along the entire flow path within the drainage area.

2.3.4 Land-Use Peak flow estimations were calculated using a runoff coefficient (C) which is a function of land-use. Land-use was determined using a combination of aerial photography (2012) and Harris County Appraisal District (HCAD) land-use designation. Table 2-3 presents C-values for various land-use types in the study. The land-use data reflects the study areas existing land use as of year 2012.

Table 2-3: Runoff Coefficient (C) Values for Various Land Use Types

Land Use Category Runoff Coefficient Percent Impervious

Cover Dense Commercial 0.8 85

Dense Commercial/ Residential MF 0.8 85

Dense Commercial/Industrial 0.8 85

Residential - Medium Lot 0.4 40

Residential Small Lot 0.6 60

Roadway 0.9 100

The existing and proposed land-use data was intersected with existing and proposed drainage areas, respectively, to allow computation of the area weighted C for each drainage area. The intersected land-uses and associated areas for each drainage area were identified. Using the following equation, an area weighted C-value was calculated for each drainage area.

=

,

where

Ci = Land-use C-values for intersected portion of drainage area,

Ai = Area associated with land-use for intersected portion of drainage area, and


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= Area weighted C for the entire drainage area.

The area weighted C-values for each drainage area were used to calculate existing and proposed peak flow estimates for the Rational Method and the percentage of impervious cover was used to develop hydrographs from the drainage areas.

2.3.5 Hydrograph Development Once the area and existing/proposed weighted C-value, weighted percent impervious cover, and time of concentration were developed for each drainage area, peak flows for the existing/proposed storms were calculated using the Rational Method. The subbasin parameters were entered into SWMM, which was then utilized to calculate hydrographs for the impact analysis. A runoff node was created for each drainage area, and assigned the appropriate area, tc, and percent impervious cover.

The Green and Ampt Loss Method was used to account for precipitation loss, while the Clark Unit Hydrograph was selected for transforming the direct runoff response. Watershed-specific parameters, including drainage area, percent imperviousness, and tc, and storage coefficient (R) were inserted into the SWMM model. The general hydrologic parameters, including initial loss, moisture deficit, suction, and conductivity used in loss method were taken from the TSARP technical white paper for the Buffalo Bayou watershed. These parameters are summarized in Table 2-4.

Table 2-4: Green and Ampt Loss Parameters for All Watersheds

Parameter Initial Loss (in) Moisture Deficit Suction

(in) Conductivity

(in/hr)

Value Used 0.1 0.385 12.45 0.024 Along with the subbasin parameters described above, a spreadsheet was used to vary the Clark Unit Hydrographs R until the peak flow from the subbasin matched the calculated value within 0.5%, where possible. A unique storage coefficient was developed for each storm. In some cases, the peak flows did not match to within 0.5%. In addition, once the parameters were entered into SWMM, the peak flows from the SWMM engine did not match the spreadsheets values. As a result, the peak flows within the SWMM model will not exactly match the Rational Method peak flows calculated for every drainage area. This issue will not significantly affect the impact analysis, as the timing of the hydrograph peak and the placement of the proper volume near the peak are the driving factors for the impact analysis, especially for the larger storms.

2.4 Hydraulic Methodology The impact analyses were performed to determine whether revisions to the inlets or trunkline system could be expected to ameliorate flooding conditions in the project area. Specifically, two areas with known flooding problems were analyzed by AECOM, and solutions are proposed to reduce the flooding problems in those areas. Those areas are at the Richmond Avenue/Post Oak Drive intersection, and along the US 59 northbound frontage road, west of Newcastle Road. A second goal of the AECOM portion of the project was to reduce the flows into the IH 610 frontage road depressed sections where possible, lessening the stresses on the pump stations. H&H Resources has proposed changes within the southern portion of the model area as appropriate.


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2.4.1.1 Storm Water Management Model (SWMM) Analysis The SWMM model was prepared from several sources. For IH 610, an existing SWMM model prepared by JF Thompson, Inc. was used (CSJ 0271-17-126). For the W129 channel, including recent, ongoing, and proposed channel improvements, a HEC-RAS model was used to extract information. The City of Houston GIMS database was used to determine which offsite areas contribute to the project area.

Overland storage and flow connectivity was developed using the 2001 Non Uniform Spatial Adjustment LiDAR, flown in 2008. From this dataset, storage versus elevation curve information was determined for each storage node, using the HEC-GeoRAS and ArcHydro tools within ArcGIS. Overland flow paths were entered as short trapezoidal channels or weirs representing the street cross-section and elevation. Overflow elevations were selected from the LiDAR, at the high point that would obstruct flow between the storage nodes.

Storage nodes were connected to the subsurface nodes in two ways, depending on whether the SWMM model already had inlet and lead data, or whether it was a lower resolution offsite area. In the areas with inlet and lead data, the storage nodes were connected to the subsurface system by weirs and orifices representing inlets. In offsite areas with lower resolution, the storage nodes were connected directly to the subsurface system. In the offsite areas, the assumption made by connecting the system in this manner is that inlet and lead losses are not significant to the proposed project. In reality, these inlets and leads do affect the drainage system, however because they are outside the project area, this simplification is considered reasonable.

Because the SWMM analysis is primarily interested in assessing the overland flow conditions, the comparisons of water surface elevation in this report will focus on the overland storage nodes. In addition, especially for the more frequent storms (e.g., 2-year), the approximate gutter elevations have been extracted from the LiDAR dataset. Any water surface elevation increases in areas where the proposed above ground water surface elevations are less than 0.5 ft above the gutter elevation will not be recorded as impacts, as this is an allowable driving head to force water through an inlet.


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3.0 Existing Conditions

3.1 Location and Topography The IH 610 / US 59 interchange is located in southwest Houston, near the Houston Galleria. The existing overland flow direction near the IH 610 / US 59 interchange is generally northwest to southeast, however the closed conduit system along IH 610, and HCFCD Unit Number W129-00-00 both flow from the south to the north toward Buffalo Bayou (HCFCD Unit Number W100-00-00). There are also diversions of flow into other drainage systems.

Exhibit 3 displays the proposed IH 610 / US 59 interchange improvements on the latest FIRM. The proposed interchange improvements are not located within a regulatory floodplain.

Exhibit 4 displays the general topographic features of the project area and its surroundings. In the exhibit, the LiDAR elevation data, storm sewers, and open channels are shown, and arrows show the general slope of the land.

3.2 SWMM Model Collaboration with LJA/H&H Resources AECOM and H&H Resources originally developed SWMM models independently, with coordination for the points where the models would connect. After the initial analyses were complete, and due to the large number of interconnection points between the models, the firms decided to merge the models into a single model that would contain all of the area for both existing and proposed conditions.

The AECOM and H&H Resources combined hydraulic model was thereafter jointly developed and jointly modified to represent all of the interconnected drainage area and all of the proposed modifications to the drainage system. At each stage of the design, AECOM and H&H Resources collaborated to maintain the integrity of the combined model.

All the model structure and parameters used within the LJA/H&H Resources model area (south drainage elements) are described in the H&H Resources Report. The structures and parameters associated with the north drainage elements are described in this report.

3.3 Existing Conditions Drainage Areas Existing conditions drainage areas are shown on Exhibit 5. The drainage areas include all of the area that outfalls to W129, as well as some area east of W129 that drains into an interconnected system and areas west of W129 that drain into the interconnected storm sewer system that serves IH 610 north of US 59. These drainage areas are delineated for a trunkline level analysis, and do not include inlet-level drainage areas. The drainage areas were also delineated with an overland flow analysis in mind. The drainage divides are generally at street high points.

3.4 Land Use The existing land use near the IH 610 / US 59 interchange is a mixture of roadway pavement and right-of-way, high density residential and commercial, and single family residential. Land use categories were assigned based on a combination of HGAC land use information, aerial photography, and knowledge of the project area.


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Exhibit 6 shows a map of existing land use, at a scale sufficient to cover the entire modeled area. Within the roadway ROW, the percent impervious cover was assumed to be 100 and the runoff coefficient at 0.9. This assumption will result in a conservative estimate of existing conditions flow rates and runoff volume, which is desirable since the area has known flooding problems. Appendix 2 contains tables of existing conditions land use data for each drainage area derived from the land use map.

3.5 Existing Storm Sewer Network The existing storm sewer network included in the SWMM model was developed from a variety of sources. Previous SWMM modeling of the IH 610 drainage system, from the IH 610 / US 59 interchange north to Buffalo Bayou was the source of the model subsurface structure in that area. That model was associated with recent construction activities (CSJ 0271-17-126).

Another major source for the storm sewer network was the HEC-RAS model for the HCFCD W129 improvements, which, along with the construction plans, provided lengths and elevations for developing the SWMM model.

The final major source for subsurface drainage information was the City of Houston GIMS database, which was used to determine connectivity and pipe sizes for areas outside of TxDOT right-of-way.

Inside TxDOT right-of-way, recent construction plans and survey were used to confirm and update the information from the previous SWMM modeling and the GIMS. Appendix 3 contains a layout of the full modeled storm sewer network, as well as the overland flow connections.

3.6 HCFCD Unit Numbers The ultimate outfall for the IH 610 / US 59 interchange is to Buffalo Bayou, HCFCD Unit W100-00-00. The primary drainage pathways are through a storm sewer system along IH 610 and through HCFCD Unit Number W129-00-00, which is currently being partially converted to a closed system by HCFCD. For this report, W129 is used to refer to HCFCD Unit Number W129-00-00 in all cases except where the formal name is more appropriate for clarity.

Two secondary drainage pathways to the east are available through interconnections with neighboring drainage systems. Under the Westpark bridge crossing the Union Pacific Railway (UPRR), there is an interconnection between the W129 system and the D111-00-00 drainage system that ultimately drains to Brays Bayou (D100-00-00). This interconnect consists of a series of culverts and open ditches between the W129 channel and a 6 ft x 6 ft RCB that drains to D111-00-00. These culverts and ditches were verified by a field visit. Appendix 1 contains field photos of this diversion.

The second interconnect to the east is a 90 RCP storm sewer that enters W129 from the east at Richmond Avenue that is also connected to a separate storm sewer outfall. This storm sewer allows water to flow both ways, depending on the driving head conditions within the interconnected system. In general, flow enters the W129 system through this interconnect, though at the end of storms, the flow direction can reverse, allowing water to leave the W129 system.

3.7 HCFCD Channel Improvements Until recently, W129 was an open channel system starting at the outfall of the Westpark Drive storm sewer system, just east of Newcastle Drive, and extending north to Buffalo Bayou. In 2004, plans were finalized to enclose the downstream portion of W129, converting it to a dual 12 ft x 10 ft RCB system north of San Felipe, and that section has been completed. Stage 2 of the project, to enclose


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the middle section between San Felipe and Richmond Drive with a dual 12 ft x 10 ft RCB, is currently under construction. The remaining section, Stage 3, between Richmond Avenue and the outfall of the dual 12 ft 8 ft RCB system that drains Westpark Drive from the west, is included in the overall W129 improvement analysis, but does not have finalized plans or a funding schedule for completion. Exhibit 4 shows the alignment of W129 and the extent of the three improvement stages. Because Stage 3 of the W129 improvements will provide a major benefit to the drainage in the IH 610 / US 59 interchange, the impact analysis study considers conditions with and without the Stage 3 improvements. The Stage 3 improvements considered in this analysis were extracted from a HEC-RAS model of the W129 channel that was part of the W129 improvement study. The Stage 3 improvements include a 30 ft wide, vertical walled open channel underneath the Westpark Drive bridge over the railroad track, which becomes a dual 12 ft x 10 ft RCB system when it turns northward to join the remainder of the W129 system.

Just south of US 59 and east of Newcastle Road, Westpark Drive crosses a railroad track. The dual 12 ft 8 ft RCB system that drains Westpark Drive outfalls into the headwaters of W129 under this bridge. Upstream of this point, the HCFCD channel designation shapefile has the RCB system designated as W129-01-00, and lists its type as historical.

The drainage system analysis was performed for each of three conditions for the W129 channel. The models were executed with the Stage 3 improvements complete, with just the Stage 2 improvements complete, and with the Stage 2 improvements, but without silt in the Stage 3 area and in the Westpark Drive storm sewer. For clarity, the W129 improvements Stage is listed with each of the alternatives, but for simplicity, the W129 or W129-00-00 moniker is not included in all the tables and discussions. Wherever Stage 2 Silt or Stage 2 With Silt is shown in a table or discussion, it refers to the conditions with the W129 Stage 2 improvements complete, including siltation along the open portions of the W129 channel as well as siltation within the dual 12 ft x 8 ft RCB system along Westpark Drive. Wherever Stage 2 Without Silt or Stage 2 No Silt is shown or mentioned, it refers to the W129 Stage 2 conditions, but with all silt removed. Similarly, wherever Stage 3 is mentioned, it refers to the conditions with both the W129 Stage 2 and the Stage 3 improvements complete, and also refers to a condition without siltation in the either the W129 improved system or within the dual 12 ft x 8 ft RCB system under Westpark Drive.

3.8 Existing Conditions Results and Analysis of Problem Areas Exhibits 7 through 10 include an overview of the entire model area and three zoomed in pages showing the areas of interest for this project. Exhibit 7 shows the expected inundation depths from 100-year storms, respectively, for existing conditions including the W129 Stage 2 improvements (Existing Stage 2). Exhibit 8 shows the relative inundation areas for the 2-, 10-, 25-, and 100-year storms for the Existing Stage 2 condition. These exhibits indicate that in the project area, there are street flooding problems along the northbound and southbound frontage roads of US 59, near Newcastle Road, and at the Post Oak Boulevard/Richmond Drive intersection. These problem areas shown in the models corroborate known flooding issues at those locations. Exhibits 9 and 10 show the same information for existing conditions, with the W129 Stage 3 improvements in place (Existing Stage 3). A third analysis for the Stage 2 configuration with silt removed was modeled to determine interim impacts. The water surface elevations from this analysis are included in the tabular results, but are not shown in any exhibits.

3.8.1 US 59 Southbound Frontage Road In the 2-year storm, the inlet capacity is inadequate but the trunkline system has sufficient capacity to carry the flows. Exhibit 8, sheets 2 and 3 show the inundation during the four modeled storms. During larger storms, the frontage road is inundated, which is an expected condition. The water surface


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elevation in the W129 system is the driving factor behind the water surface elevation during larger storms. Frontage road improvements would not significantly improve the drainage except for smaller, more frequent storms. At the far west end of the storm sewer system, the survey indicates that there is an 18 RCP connection between this storm sewer system and the neighboring system that flows into the depressed section. The most recent construction drawings show that this connection was supposed to have been severed when the depressed section was built, which would have resulted in higher water surface elevations along the US 59 SBFR. The model indicates that water will flow through this connection and into the depressed section.

3.8.2 US 59 Northbound Frontage Road This area is known to experience standing water during relatively frequent events. The existing conditions model confirms this condition, showing 2-year water surface elevations more than 0.5 ft above the gutter elevation. Appendix 7 shows water surface elevations in the 2-year event and compares them to estimated gutter elevations. MH_SA07 and MH_SA08 are storm sewer nodes in this area. The primary cause of flooding appears to be a combination of high tailwater in the W129 system and insufficient capacity for the storm sewer system. In the larger storms, the water surface elevation is primarily driven by the tailwater conditions in the W129 system. During the largest storms, water flowing overland leaves this area and travels west toward the depressed pavement in the area being analyzed by LJA/H&H Resources.

3.8.3 Post Oak Boulevard/Richmond Drive Intersection This area is known to experience standing water during relatively frequent events. There are multiple interconnections between the storm sewer systems east and west of IH 610. The existing conditions model confirms this condition, showing 10-year water surface elevations more than 0.5 ft above the gutter elevation. Appendix 6 shows water surface elevations in the 10-year event. Node 33 is a storm sewer node in this area. Node SA-45 is an overland node in this area. Appendix 8 shows the hydrograph of flow from this intersection entering the depressed portion of the IH 610 SBFR/Westpark Tollway exit. In the 100-year storm, the depressed section fills and the flow direction reverses, with water entering the intersection from the depressed section.

3.8.4 Effects of Removing Silt from W129-00-00 and Westpark Drive Box Storm Sewer

If the proposed improvements are implemented before the W129 improvements are implemented, part of the project will be to remove the silt from the Westpark Drive storm sewer and the open channel part of W129. It is understood that this is a temporary solution, and that until Stage 3 of the W129 improvements are implemented, it is very likely that silt will accumulate in the system.

Removing the silt from the system provides significant benefits to the upstream area, however one effect that it demonstrates in the analysis is the importance of phasing a drainage improvement project properly. The W129 system, being properly phased, was started at the downstream end. Because the upstream area is still significantly restricted by the presence of silt in the system, the improvements benefit the area near the upstream end of the Stage 2 improvements significantly. These benefits are temporary, however, and re-deposition of silt will likely result in increases in water surface elevation in that area until the Stage 3 improvements are completed. Because the models do not include a prior to Stage 1 version, it is impossible to confirm from the models (but readily apparent) that the water surface elevations in this area are lower after silt removal than they were prior to any of the W129 improvements.


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Appendix 4 through Appendix 7 show the water surface elevations differences between the Stage 2 With Silt and Stage 2 Without Silt model runs, under existing conditions. Removal of the silt increases the overland water surface elevations by up to 0.29 ft. Inside the storm sewer systems, removing the silt can increase the HGL more, however these increases do not cause flooding risk.

3.8.5 Effects of W129-00-00 Stage 3 Improvements The Stage 3 improvements proposed by others for W129 would alter the conditions on the US 59 frontage roads even further than removing the silt. As discussed above, these improvements are part of a larger project that has had its own impact analysis and phasing analysis. As such, the water surface elevation rises in some locations between Stage 2 Without Silt and Stage 3, however these increases are small and isolated. This is expected, as the staged project lowers water surface elevations for interim conditions, expecting them to go back up after the final stage.

Exhibits 7 and 9 show the overall 100-year inundation area and inset maps for the primary areas of interest for Stage 2 With Silt and Stage 3, respectively.

Appendix 4 through Appendix 7 show the water surface elevations differences between the Stage 2 With Silt/Stage 2 Without Silt model runs and the Stage 3 runs under existing conditions. The differences between the Stage 3 and Stage 2 With Silt are similar to the differences between Stage 2 with and without silt. The differences between Stage 3 and Stage 2 Without Silt, however, are relatively small in the northern part of the system. There are a few locations with increases in WSE, however, for the most part Stage 3 is a benefit, with reductions along the upper end of W129.


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4.0 Proposed Improvements

4.1 LJA/H&H Resources Model Area Proposed changes within the LJA/H&H Resources model area are described in the H&H Resources Report. Some of those changes (W18 detention and the re-routed 42 RCP) are also discussed in this report.

4.2 Proposed Drainage Area Changes Drainage area changes are anticipated as a result of the proposed pavement projects. The primary changes are where rainfall is intercepted on a bridge and routed into a different drainage area. AECOM developed drainage areas for the SBNB DC and the Westheimer Ramp and determined where those drainage areas would drain via slotted rails, where they would drain via inlets that would connect to conduits that follow bents, and where they would enter storm systems under or near embankments or retaining walls. AECOM coordinated with H&H Resources to determine which H&H Resources pavement would affect AECOM drainage areas and which AECOM pavement would affect H&H Resources drainage areas. Exhibit 11 shows the changes in the AECOM drainage areas associated with the proposed pavement revisions.

4.3 Proposed Land Use Changes For proposed conditions, the same land use was used except for the areas of revised pavement and detention. The existing land use was updated using the proposed pavement. It is determined that the entire proposed pavements of SBNB DC and Westheimer ramp are enclosed in the existing ROW (i.e., roadway as land use type). Therefore, addition of the two proposed pavements would not change the land use. However, Post Oak Boulevard near Richmond Avenue is proposed to be shifted westward that would affect the existing commercial plaza at the street intersection. This roadway shift would cause land use changes between dense commercial and roadway for proposed conditions in drainage areas of SA-45 and SA-46. In addition, changes in proposed drainage areas (discussed in Section 4.1) would add or remove land use associated with the areas changed. The area newly within the proposed drainage area was assigned an appropriate land use type. Exhibit 12 shows the changes in land use between existing and proposed conditions. Because the land fronting the existing IH 610 / US 59 interchange is fully developed, and all has an existing runoff coefficient higher than the typical 0.65 that is used for the 150 ft development strip, the 150 ft development strip is not separately identified in the land use characterization. In areas where the development within the 150 ft strip has a separate drainage system that drains away from the roadway, drainage areas extend only to the edge of the ROW.

4.4 Right-of Way This project primarily occurs within existing right-of-way. Additional right-of-way is required at the Post Oak Boulevard and Richmond Avenue intersection. The changes in proposed ROW are shown on the proposed conditions land use map, Exhibit 12 where the land use category is Dense Commercial to Roadway.

Along the US 59 NBFR, a 10 ft strip of additional right-of-way will be required in the section where the SBNB direct connector passes over the US 59 NBFR. In this area, the NBFR will be shifted to allow for construction of the piers for the direct connector (in the area of Solution Q).


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Additional right-of-way will also be required along the US 59 NBFR where the 42 parallel RCP trunkline (Solution L) will be placed outside the existing pavement along the frontage road. There is no new pavement in this area, so this will not affect the land use categories shown on Exhibit 12. The alignment of the 42 RCP and the additional right-of-way have not been fully defined, and the additional ROW is not shown on any of the exhibits within this report.

4.5 Recommended Drainage Improvements During the development of the project, AECOM analyzed several different possibilities for improving drainage conditions in the two primary areas of interest. These alternatives were described in an April 2013 draft of this document. Since that time, through coordination with the adjacent Engineer and TxDOT and review of those and other alternatives, a Recommended Alternative has been selected that includes improvements not described in the April 2013 draft report. This final report will only document the recommended alternative.

Exhibit 13 shows the location and layout for the portion of the Recommended Alternative along the US 59 NBFR, while Exhibit 14 shows the location and layout for the portion of the Recommended Alternative on the US 59 NBFR and under the IH 610 main lanes.

Where the various improvements are also discussed in the H&H Resources Report, identifying letters are the same as those used in the H&H Resources Report. Not all of the improvements discussed in the H&H Resources Report are described in this report. For any items not discussed in the H&H Resources Report, a new ID letter has been assigned that does not duplicate any of the items in the H&H Resources Report. Table 1 in the Executive summary lists all the improvements (solutions) and their ID letters described in this report.

The various parts of the Recommended Alternative are described below.

4.5.1 Detention Under IH 610 This improvement (D) consists of adding detention in the area under the IH 610 / US 59 interchange north of the US 59 main lanes (model node W18), without changing the interconnect between the storage area and the Richmond Avenue storm sewer. The detention volume and layout was developed by the H&H Resources team in coordination with the AECOM team, as the placement and connectivity falls within the H&H Resources analysis area, but the actual detention facility falls within the area to be covered by the AECOM PS&E sheets. The detention pond is described in Section 4.4.1 of the H&H Resources Report. This improvement is intended to lower the water surface elevations during frequent events in the Richmond Avenue/Post Oak Boulevard intersection and surrounding areas, as well as to mitigate any impacts associated with the project. The detention pond location is shown on Exhibit 14.

The existing hydraulic connection between the Richmond Avenue storm sewer system and what is now a ditch under the IH 610 main lanes, but will become the north end of the W18 detention will remain a 36 RCP, and will not be reconstructed. The lower tailwater within the W18 detention will result in reduced water surface elevations in the intersection and surrounding areas.

4.5.2 IH 610 NBFR and US 59 NBFR Improvements These improvements are intended to reduce the high-frequency street flooding along the US 59 NBFR and to reduce the burden on the pumping station that serves the IH 610 NBFR depressed section, from which it would be pumped into the Westpark Drive storm system.


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The improvements consist of (Q) adding a new trunkline along the US 59 NBFR to improve drainage in the area, (R) raising the pavement grade of the frontage road to reduce flow into the depressed section of the IH 610 NBFR, and (L) adding a parallel trunkline east of the depressed section that flows south to the Westpark Drive box culverts.

The new trunkline improvements (Q) consist of approximately 3000 ft of 30 to 48 RCP along the US 59 NBFR that are interconnected with the drainage system under the U-turn in the Newcastle Road intersection and with two existing 30 RCP systems that cross the US 59 main lanes, along with appropriately sized inlets and leads along the US 59 NBFR. The new trunkline is shown on Exhibit 13 and Exhibit 14. The new trunkline enters an existing 30 RCP at the turnaround near W129. There will be no work performed within the railroad or HCFCD ROW. The new trunkline removes the connection between model node 1-S24 and 1-S23 and connects model node 1-S24 through model nodes 1-S27, 1-S29-I, N-885, N-886, N-887, and N-887.1 to W129 at model node 96+45. Along this new trunkline, where inlets are required, they will be type C-1 inlets.

The US 59 NBFR pavement grade changes (R) will occur between two driveways along the frontage road, and will raise the gutter elevation at the high point of the pavement to elevation 55.6 ft. This will reduce, but not eliminate overland flow into the depressed section during more intense storm events. The approximate location of the pavement grade revisions is shown on Exhibit 14. This change raises the overland connection between model nodes 1-S29 and 4-N27.

The parallel trunkline (L) is described in the H&H Resources Report, Section 4.4.8, and its alignment is shown in their Appendix D. It is a 42 RCP that will run under the METRO HOV lane and the US 59 NBFR, then parallel the US 59 NBFR south toward Westpark Drive, and eventually cross the US 59 NBFR a second time and re-enter the TxDOT drainage system within TxDOT ROW. This parallel trunkline captures some existing US 59 mainlane drainage and routes it directly to the Westpark Drive box culverts, rather than first allowing it to flow into the wet well for the depressed section and requiring it to be pumped back out. The trunkline will pass at least two existing inlets on the US 59 NBFR, but will not be interconnected, as those inlets carry flow to the depressed section. The approximate layout of this parallel trunkline is shown in Exhibit 14. This parallel trunkline connects model node P4-J42 to model node 4-DB8-1. The existing storm system downstream of node P4-J42 will be plugged at that location.

4.5.3 US 59 SBFR Improvements This improvement (S) consists of removing or plugging the 18 RCP that connects the US 59 southbound frontage roads drainage system flowing east toward W129 from the storm sewer system that drains the IH 610 northbound frontage road depressed section that passes under US 59. The improvement removes the subsurface connection between model node 10 and model node 4-N3, as shown on Exhibit 14. This improvement is intended to reduce the burden on the pumping station that serves the IH 610 northbound frontage road depressed section, from which it would be pumped into the Westpark Drive storm system.

The previous construction drawings reviewed by AECOM do not show this 18 RCP connection, however the survey located the storm sewer. Removing this connection will allow the system to work as it was previously designed to work. Any increases in water surface elevation are not impacts, as they were accounted for in prior designs.


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5.0 Recommended Alternative Analysis Results

The Recommended Alternative was modeled under the three different W129 conditions described in Section 3.7, Stage 2 With Silt, Stage 2 Without Silt, and Stage 3.

5.1 Analysis Results Appendix 4 through Appendix 7 show the results for existing conditions and with the recommended alternative, for each of the three W129 conditions, and for each of the storms. There is one table for each storm, which combines the results from the Stage 2 With Silt, Stage 2 Without Silt, and Stage 3 models. Exhibits 13 and 14 show the improvements referenced in the following sections.

The following discussion of results focuses on the changes to flows and water surface elevations with the W129 Stage 3 improvements completed. The results with the other W129 scenarios are similar.

Table 5-1 shows the lengths and sizes of the proposed storm sewer improvements in the Recommended Alternative. The Recommended Alternative also includes four type C-1 inlets along the US 59 NBFR. Exhibit 13 shows the portions of the Recommended Alternative along the east part of the US 59 NBFR. Exhibit 14 shows the portions of the Recommended Alternative along the west part of the US 59 NBFR, the US 59 SBFR and under the IH 610 main lanes, as well as the W18 detention. SWMM Node IDs are shown on the Recommended Alternative SWMM model layout in Appendix 3.

The Recommended Alternative lowers the 2-year HGLs in the US 59 NBFR (1-S29, SA-09, SA-08, SA-07), but in the 100-year storm, that area sees no change in water surface elevation. The US 59 SBFR (SA-10 through SA-16) area also sees reductions in the 2-year water surface elevation, however in the 100-year storm, the US 59 SBFR sees higher water surface elevations as a result of plugging the 18 RCP that connects the system to the IH 610 NBFR depressed section. As discussed in Section 4.5.3, this is not an impact, because the 18 RCP was supposed to be removed/plugged in a previous project. The Recommended Alternative also reduces HGLs in the Post Oak Realignment area (SA-45) by up to 0.4 ft in the 100-year storm, but does not reduce the water surface elevation in the 2-year storm significantly.

Under existing and Recommended Alternative conditions with the Stage 3 improvements, flows into the IH 610 NBFR depressed section are reduced substantially and flows from Richmond Avenue into the IH 610 / US 59 infield where the proposed detention is placed are increased substantially, as are flows into W129, as shown in Table 5-2. Flows from the Richmond Avenue/IH 610/Post Oak Boulevard intersection into the IH 610 SBFR/Westpark Tollway exit depressed section are not significantly reduced, however the other improvements near that depressed section reduce the flooding in the depressed section. Appendix 8 shows hydrographs of the flows into the locations summarized in Table 5-2. Negative values indicate an increase in flow or volume in post-project conditions.

In essence, the Recommended Alternative shifts flows from the Post Oak Boulevard / Richmond Avenue intersection into the Infield, and from the IH 610 NBFR depressed section into W129, and offsets the impacts of these increased flows by adding detention volume to the system. Table 5-2 shows the changes in peak flow and in total flow volume for the four storms, and for the locations described above.


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All of these internal changes do not, however, result in any increases to combined flow entering Buffalo Bayou. The additional detention volume reduces pressure on W129 and lowers flows into Buffalo Bayou. Table 5-2 also includes the total flow and volume entering Buffalo Bayou at all three outfall locations (nodes 423, W129_Buf, and RiverOaks) plus overland flow to two locations to the east (OSys1 and OSys2). Appendix 8 includes combined Buffalo Bayou hydrographs for the three outfall locations. In addition to these outfalls, there are two other locations where water can leave the system heading south toward Brays Bayou. Flows to Brays Bayou are reduced in all storms.

Table 5-1: Storm Sewer Improvements in Recommended Alternative

US Node DS Node Length (ft) Diameter

(in) US Invert

(ft) DS Invert

(ft) Solution

1-S23 1-S24 69 30 50.6 50.4 Q

1-S24 1-S27 535 30 50.2 49.3 Q

1-S27 1-S29-I 36 30 49.3 49.2 Q

1-S29-I N-885 531 36 49.2 48.1 Q

N-885 N-886 400 42 48.1 47.6 Q

N-886 N-887 400 42 47.6 47.1 Q

N-887 N-887.1 614 48 47.1 45.9 Q

MH_SA09 N-885 32 36 50.1 50.0 Q

MH_SA08 N-886 50 36 48.9 48.8 Q

MH-SA07 N-887 92 30 47.1 47 Q

P4-J42 4-DB8-1 1273 42 49.81 47.06 L


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Table 5-2: Flow and Volume Reductions at Selected Locations (W129 Stage 3)

Location Reduction in Peak Flow (cfs) Reduction in Volume (ac-ft)

100-Year

25-Year

10-Year

2-Year

100-Year

25-Year

10-Year

2-Year

Westpark Exit / IH 610 SBFR 2.5 3.1 3.1 0.0 -11.4 0.1 0.0 0.0

IH 610 / US 59 Infield -47.2 -37.4 -34.3 -4.3 17.5 -0.1 -1.4 0.0

IH 610 NBFR 42.5 44.3 40.6 21.2 15.9 10.1 7.2 2.4

W129 From US 59 SBFR -60.6 -57.7 -56.6 -32.3 -15.9 -11.8 -8.6 -4.4

Combined Flow Direct to Buffalo 42.3 88.2 101.6 247.4 3.4 -3.1 -4.1 -4.9

Flow to Brays through Westpark 6x6 RCB 1.0 2.4 4.4 27.5 0.0 1.0 1.4 2.0

For the Stage 2 With Silt and Stage 3 inundation maps with the Recommended Alternative, see Exhibits 15 through 18. Exhibits 15 and 17 display the 100-yr inundation maps, while Exhibits 16 and 18 presents an inundation area comparison between the storm events, showing the inundation areas of the 2, 10, 25, and 100 year storm events. The Stage 2 Without Silt condition is not shown in an exhibit, as it has ponding levels between the other two W129 conditions.


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6.0 Entity Participation

Several entities contribute flow to the project area and are affected by the flooding problems that occur within the project area. These entities are the City of Houston, TxDOT, HCTRA, and METRO. Each of these entities will benefit from the proposed improvements. To estimate a potential cost-sharing percentage for each entity, the drainage areas were separated by entity (shown color coded on Exhibit 19) and the total inflow for these areas were calculated. For the area west of IH 610 and north of US 59 that is shown on Exhibit 19 as Partial Contribution, the flow contribution was estimated by estimating the capacity of the storm sewers connecting to the south as a fraction of the capacity of the storm sewers connecting directly to IH 610 to the east. The contribution was approximately 4% of the flow from that area. A summary of the total flow is shown in Using these percentages, potential costs for each solution can be distributed among the affected entities. Table 6-2 shows this breakdown of costs by entity. All the solutions described in this report are part of AECOMs design phase work under CSJ 0217-17-145. The H&H Resources Report includes solutions that are to be constructed under three current TxDOT contracts, and thus has three different groupings, plus a line for the HCFCD W129-00-00 Stage 3 improvements.

Table 6-1.

Using these percentages, potential costs for each solution can be distributed among the affected entities. Table 6-2 shows this breakdown of costs by entity. All the solutions described in this report are part of AECOMs design phase work under CSJ 0217-17-145. The H&H Resources Report includes solutions that are to be constructed under three current TxDOT contracts, and thus has three different groupings, plus a line for the HCFCD W129-00-00 Stage 3 improvements.

Table 6-1: Percentage of flow by Entity

Entity TXDOT City of Houston HCTRA METRO Total

Flow (cfs) 309 762 17 32 1120

% of Flow 27.6% 68.0% 1.5% 2.9% 100.0%


25

CSJ 0217-17-145 February 2014

Table 6-2: Summary of Potential Costs by Entity

Grp ID Issue Solution TxDOT City of Houston HCTRA METRO Total

2: J

ob 1

45

D Existing flooding of

Post Oak / Richmond / IH 610 SBFR interchange

Add W18 Detention Cost breakdown provided in H&H Resources Report, Table 6.2

L Existing flooding of IH 610 NBFR

Reroute storm sewer line (42 RCP) to Westpark Drive.

Cost breakdown provided in H&H Resources Report, Table 6.2

Q Existing flooding of US 59 NBFR and IH 610 NBFR

Reroute storm drainage (48 RCP) to

W129-00-00, Type C-1 Inlets

$79,580 $196,481 $4,422 $8,294 $288,777

R Existing flooding of IH 610 NBFR

Raise grade of US 59 NBFR $33,069 $81,647 $1,838 $3,447 $120,000

S Existing flooding of IH 610 NBFR

Plug existing 18 RCP interconnect along

US 59 SBFR $1,000 $0 $0 $0 $1,000


26

CSJ 0217-17-145 February 2014

7.0 Conclusions and Recommendations

The proposed drainage improvements in the Recommended Alternative will provide substantive street flooding reduction along the US 59 northbound frontage road, however the full benefit of the improvements will not be realized until and unless the W129 Stage 3 improvements are completed. At the same time, the proposed improvements reduce the flows into the depressed section of the IH 610 northbound frontage road, where it passes under US 59. These flow reductions will reduce the stresses on the pump station at that location.

The proposed drainage improvements in the Recommended Alternative will increase water surface elevations along portions of the US 59 southbound frontage road, due to the elimination of the 18 RCP connecting the eastern portion of the drainage system to the western portion that flows to the IH 610 northbound frontage road depressed section. This increase in water surface elevation is not considered to be an impact, because it was part of a previous design, and should have been part of the existing conditions. The drainage improvements included in the Recommended Alternative are shown on Exhibits 13 and 14.

The proposed drainage improvements in the Recommended Alternative will also provide some street flooding reduction in the Post Oak Realignment area, and will reduce the overland flow into the depressed section of the Westpark Tollway exit/IH 610 southbound frontage road.

Additional benefits of the proposed improvements are described in the H&H Resources Report.

The paving and drainage improvements described in this report, if constructed in accordance with the analysis described herein, will result in no adverse impact to flooding risk during storms up to and including the 100-year storm event for the area encompassed by the north drainage elements.

Project Location

COUNTY

59

59

45

FED. RD.DIV. NO. STATE PROJECT NO.

HIGHWAYNO.

EXHIBITNO.

JOBNO.

SECTIONNO.

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STATEDISTRICT

6 TEXAS

HOU HARRIS 0271 17 146 1

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IH 610/US 59 INTERCHANGENORTHERN DRAINAGE ELEMENTS

SHEET 1 OF 10 5,0002,500 Feet Vicinity MapAECOM Technical Services, Inc5444 Westheimer Rd, Suite 200Houston, Texas 77056www.aecom.comTBPE Reg. No. F-3580

Texas Departmentof Transportation

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US 59 NB Frontage RoadEnd Sta 209+60US 59 NB Frontage RoadStart Sta 181+14

SBNB DC Start Sta 97+05.50Westheimer Ramp End Sta 592+42.60

Post Oak RealignmentEnd Sta 228+11.75

Post Oak RealignmentStart Sta 214+74.94

Westheimer Ramp Start Sta 571+97.56

Re-Route Storm SewerFrom Approx IH 610 Sta 202+00To Approx IH 610 Sta 212+00

W129-00

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Vicinity Map

FED. RD.DIV. NO. STATE PROJECT NO. HIGHWAYNO.

EXHIBITNO.JOBNO.SECTIONNO.CONTROLNO.STATEDISTRICT

6 TEXASHOU HARRIS 0271 17 146 2

IH610/US59


SHEET 1 OF 1

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AECOM

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FEMA Flood Zones

Areas of 0.2% Annual Chance Flood Event

Areas of 1% Annual Chance Flood Event

Floodway Areas in Zone AE

Cross Section Line

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HOU HARRIS 0271 17 146 3

IH610/US59


SHEET 1 OF 1

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draft - 2014-02 hydrologic and hydraulic analysis for 610-59 interchange - north

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