DRAINAGE IMPACT ANAYLSIS FOR THE LITTLE AMERICA DEVELOPMENT

DRAINAGE IMPACT ANAYLSIS FOR THE

LITTLE AMERICA DEVELOPMENT

In Support of a Major Plan Amendment

June 26, 2013

Prepared by:

JE Fuller/Hydrology & Geomorphology, Inc.

1 W. Deer Valley Rd., Ste. 101 Phoenix, AZ 85027

Prepared for:

Little America Hotels & Resorts, Inc. 2515 E Butler Ave

Flagstaff, AZ 86004

Drainage Impact Analysis For the Little America Development

Executive Summary

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Executive Summary

The Drainage Impact Analysis documents the hydrologic and hydraulic conditions resulting from the proposed change in land-use for the Little America Development. It should be noted that the hydrologic and hydraulic modeling, computational approaches and drainage concepts presented in this report will require further refinement as development proceeds.

Based on coordination with the City of Flagstaff stormwater staff, the Drainage Impact Analysis focused on the following four objectives:

Objective No. 1. Determine the minimum level of on-site Low Impact Development (LID) stormwater storage required for the site.

Outcome: Minimum level of LID storage for the proposed development was calculated to be 316,384 cu. ft. Discussion regarding land-use and required LID stormwater storage is included in Section 2.2.

Objective No. 2. Based on HEC-1 hydrologic modeling, demonstrate that there is no increase (0.00 ft.) in the ponding elevation at “Lake Continental” (HEC-I ID “Outlet”).

Outcome: To demonstrate that there is a no-rise condition at “Lake Continental”, the Little America Development will need to detain a minimum of 18 acre-feet of on-site storage. Discussion regarding on-site detention volume is included in Section 2.3.2.

Objective No. 3. Using the HEC-1 discharges, assess the project impacts at the E. Butler Avenue crossing of the Rio de Flag (HEC-1 ID “EFG”) assuming a fully developed site with no on-site storage.

Outcome: A hydrologic model was prepared assuming a fully developed site with no on-site storage. Resultant peak discharges were used to evaluate the road crossing at E. Butler Avenue. The hydraulic model results show an increase at the E. Butler Avenue crossing of 0.02 ft., which is less than the 0.1 ft. allowed by the City. Discussion regarding the E. Butler Avenue crossing of the Rio de Flag is provided in Section 3.1.

Objective No. 4. Assess project impact on the Rio de Flag floodplain.

Outcome: A hydraulic model (HEC-RAS model) was prepared to compare the Rio de Flag’s existing and proposed 100-year floodplain boundaries. The proposed condition model included preliminary Golf Course grading, minor golf cart structures and proposed future road crossing at the southeast property boundary. Minor impacts to the floodplain occur within and adjacent to the Little America Development, primarily upstream of proposed structures crossing the Rio de Flag. Discussion regarding the Rio de Flag floodplain analysis is included in Section 3.2.


Table of Contents

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Table of Contents

1 Introduction ......................................................................................................................... 1-1

1.1 Report Purpose ............................................................................................................. 1-1

1.2 Proposed Land-Use Plan Description ............................................................................ 1-1

1.2.1 Resort/Conference Center ..................................................................................... 1-1

1.2.2 Low/Medium/High Density Residential ................................................................. 1-1

1.2.3 Commercial ............................................................................................................ 1-2

1.2.4 Golf Course Amenities ........................................................................................... 1-2

1.3 Drainage Impact Analysis Objectives ............................................................................ 1-2

2 Simplified Stormwater Storage Evaluation .......................................................................... 2-1

2.1 Hydrologic Modeling ..................................................................................................... 2-1

2.2 Land Use Changes and LID Volumes ............................................................................. 2-1

2.3 Hydrologic Modeling Results......................................................................................... 2-4

2.3.1 Proposed Land Use Change - Without On-Site Stormwater Storage .................... 2-4

2.3.2 Proposed Land Use Change – With Required On-Site Stormwater Storage ......... 2-5

2.4 On-Site Stormwater Storage Summary ......................................................................... 2-6

2.4.1 LID Storage Volume................................................................................................ 2-6

2.4.2 General Stormwater Storage Volume.................................................................... 2-6

3 Hydraulic Analysis ................................................................................................................ 3-1

3.1 E. Butler Avenue crossing of the Rio de Flag ................................................................. 3-1

3.1.1 E. Butler Avenue Hydraulic Evaluation .................................................................. 3-1

3.1.2 E. Butler Avenue Hydraulic Evaluation Results ...................................................... 3-1

3.2 Rio de Flag Floodplain ................................................................................................... 3-3

3.2.1 Topography ............................................................................................................ 3-3

3.2.2 Steady Flow Data ................................................................................................... 3-3

3.2.3 Downstream Boundary Conditions ........................................................................ 3-3

3.2.4 Flow Regime ........................................................................................................... 3-3

3.2.5 Development of HEC-RAS Model Input Data ......................................................... 3-4

3.2.5.1 Stream Centerline ........................................................................................... 3-4

3.2.5.2 Cross-Sectional Cut Lines ................................................................................ 3-4

3.2.5.3 Bank Stations .................................................................................................. 3-4

3.2.5.4 Flow Path Centerlines ..................................................................................... 3-4

3.2.5.5 Cross-Section Roughness Coefficients (Manning’s n-Value) .......................... 3-4

3.2.5.6 Proposed Conditions Hydraulic Structures..................................................... 3-5

3.2.6 HEC-RAS Modeling Results .................................................................................... 3-5

3.2.7 Limitations of Modeling Results ............................................................................ 3-6

3.2.8 Hydraulic Modeling Conclusions ............................................................................ 3-7


List of Tables

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List of Tables

Table 2–1 Percent Impervious for Proposed Land Uses ............................................................. 2-1

Table 2–2 Percent Impervious and LID Volume Requirement per Land-Use Plan Area and USACE HEC-1 Sub-Area ............................................................................................................................ 2-3

Table 2–3 HEC-1 Changes in RTIMP for Sub-Areas F3 and G ...................................................... 2-4

Table 2–4 USACE Existing Condition HEC-1 Model ..................................................................... 2-4

Table 2–5 Proposed Condition HEC-1 Model.............................................................................. 2-5

Table 2–6 USACE HEC-1 Model (With Adjusted Percent Impervious) ........................................ 2-5

Table 3–1 E. Butler Avenue Crossing WSE Change ..................................................................... 3-1

Table 3–2 Rio de Flag Proposed Structures ................................................................................ 3-5

Table 3–3 Floodplain Elevation Changes .................................................................................... 3-6

List of Figures

Figure 1-1: Site Location and Overview ...................................................................................... 1-3

Figure 2-1: Proposed Land-Use Plan Area .................................................................................. 2-2

Figure 3-1: E. Butler Avenue Crossing ......................................................................................... 3-2

List of Appendices

Appendix A. Hydrology Supporting Data

Appendix B. Hydraulics Supporting Data

Appendix C. Project Data Disk


Introduction

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1.1 Report Purpose

In support of the Little America major land-use plan amendment, the Drainage Impact Analysis for the Little America Development (Drainage Impact Analysis) is being submitted to the City of Flagstaff (City) for review and approval. The Drainage Impact Analysis documents the hydrologic and hydraulic conditions resulting from the proposed change in land-use. It should be noted that the hydrologic and hydraulic modeling, computational approaches and drainage concepts presented in this report will require further refinement as development proceeds.

A site location and overview map is provided as Figure 1-1, shown below.

1.2 Proposed Land-Use Plan Description

As discussed above, the Drainage Impact Analysis has been prepared in support of a proposed change in land-use. Four primary land-use types are being proposed within the Little America Development: resort/conference center, low/medium/high density residential, park/open space, commercial and golf course amenities.

The vision for the Little America property is a mixed-use, resort development with hotel and conference facilities that will expand and complement the current facilities at the Little America Flagstaff Hotel. The conceptual master plan capitalizes on the opportunities present on the property by creating a vibrant mixed-use neighborhood where residents can live, work and play at virtually the center of the Flagstaff City limits. The proposed plan includes retail and office opportunities adjacent to I-40 and E. Butler Avenue with high density residential within walking distance. The gently rolling terrain combined with a large area of relatively flat terrain provides the opportunity for a wide range of recreational activities including golf, hiking, running, biking trails connected within the property and to the Flagstaff Urban Trail System (FUTS). The diverse density within the residential areas provides for a mix of housing types and price ranges with proximity to the retail and office space as well as the resort and recreation amenities.

A brief description of each land-use category is provided below. Proposed land-use hydrology is discussed in Section 2. The conceptual land-use plan for the Little America Development is shown on Figure 2-1.

1.2.1 Resort/Conference Center

The new hotel and conference center is anticipated to include approximately 200 hotels rooms, 40,000 square feet of conference space and resort amenities that include pool, spa, wedding/event space, tennis, etc.

1.2.2 Low/Medium/High Density Residential

The concept plan supports mixed-uses and a significant range of residential types within the 1,391 units noted on the plan. These residential types include: for rent/leasable apartments, workforce housing, stacked condo, brownstones, townhomes, duplex, detached patio homes,

1 Introduction


Introduction

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mix of single family detached homes, and high-end resort residential (wrapping the resort parcel and branded by the proposed hotel).

1.2.3 Commercial

The commercial/mixed-use area may include neighborhood retail such as a local market, gas station, movie theater, restaurants and other goods and services for residents, visitors and commuters along with the potential of office suites above retail.

1.2.4 Golf Course Amenities

Recreational opportunities include public parks, an 18-hole resort/public golf course and clubhouse as well as an integrated multi-use trail system connecting residents and visitors to homes, resort and recreational opportunities within the site and to the regional FUTS trail system in Flagstaff.

1.3 Drainage Impact Analysis Objectives

Based on coordination with the City of Flagstaff stormwater staff, the Drainage Impact Analysis focus is on the following 4 objectives:

Objective No. 1. Determine the minimum level of on-site Low Impact Development (LID) stormwater storage required for the site.

Objective No. 2. Based on HEC-1 hydrologic modeling, demonstrate that there is no increase (0.00 ft.) in the ponding elevation at “Lake Continental” (HEC-I ID “Outlet”).

Objective No. 3. Using the HEC-1 discharges, assess the project impacts at the E. Butler Avenue crossing of the Rio de Flag (HEC-1 ID “EFG”) assuming a fully developed site with no on-site storage.

Objective No. 4. Assess project impact on the Rio de Flag floodplain.


Introduction

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Figure 1-1: Site Location and Overview


Simplified Stormwater

Storage Evaluation

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2.1 Hydrologic Modeling

Hydrologic modeling for this project was based upon the existing condition HEC-1 model (Rd100byr.dat) provided by the City from the U.S. Army Corps of Engineers (USACE) Feasibility Report and Final Environmental Impact Statement, which is dated September 2000. The Technical Appendices Volume 1 of the USACE report can be referenced for additional details on the hydrology model used for this report. The USACE existing condition was used for the following:

Based on proposed development, determine if a water surface elevation (WSE) increase would occur at “Lake Continental”.

Based on proposed development, determine if a WSE increase greater than 0.1 feet would occur at the Rio de Flag crossing of E. Butler Avenue near Fox Glenn Park.

Modification of the USACE existing condition HEC-1 model was performed to determine hydrologic and hydraulic impacts resulting from the proposed development at “Lake Continental” and E. Butler Avenue.

2.2 Land Use Changes and LID Volumes

The proposed land-use conditions were modeled by adjusting the percent impervious (RTIMP) for HEC-1 Sub-Areas F3 and G. No other changes were made for the exponential loss rate data (LE), base flow characteristics (BF), or the given unit graph (UI). This modeling approach is in-line with the approach taken by the USACE during development of their Future Land Use HEC-1 model, i.e. USACE only modified each Sub-Area impervious percent to reflect future conditions.

The applied impervious percentages for the proposed land-uses are consistent with values found in Table 2-2a of the NRCS TR-55 and Figure 2-3 of the ADOT Hydrology Manual. See Figure 2-1 for Future Land Use and Figure A- 1 for Future Land Use with USACE basin overlay. Average percent impervious area per proposed land-use category are shown in Table 2–1. In addition, included in Table 2–2 are the percent impervious area and LID stormwater storage volume requirement per Land Use Plan Area and USACE HEC-1 Sub-Area [Objective No. 1 – Section 1.3]. As seen in Table 2–2, several of the proposed land-use areas can be found in both Sub-Area F3 and G.

Table 2–1 Percent Impervious for Proposed Land Uses

Proposed Land Use Categories Average % Impervious

Commercial 85

Resort/Hotel/High Density Residential 70

Lower Density Residential 40

Golf Course No Change in imperviousness anticipated

2 Simplified Stormwater Storage Evaluation



Storage Evaluation

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Figure 2-1: Proposed Land-Use Plan Area



Storage Evaluation

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Table 2–2 Percent Impervious and LID Volume Requirement per Land-Use

Plan Area and USACE HEC-1 Sub-Area

Land

Use

Plan

Area

Acres Units/Acre 1Applied %

Impervious

Impervious

Area

(Acres)

UASCE

HEC-1

Sub-Area

3LID Volume

(cu ft.)

Area A 5.81 20 70% 4.07 G 14,763

Area B 4.77 14 70% 3.34 F3 12,121

Area C 10.15 14 70% 7.11 F3 25,791

Area D 4.55 16 70% 3.19 F3 11,562

Area E 6.94 14 70% 4.86 F3 17,635

Area F 2.93 18 70% 2.05 F3 7,445

Area G 3.42 18 70% 2.39 G 8,690

Area H 4.10 20 70% 2.87 G 10,418

Area I 2.92 12 70% 2.04 G 7,420

Area J 4.10 16 70% 2.87 F3 10,418

Area K 4.09 12 70% 2.86 F3 10,393

Area L 4.69 14 70% 3.28 F3 11,917

Area M 3.34 12 70% 2.34 F3 8,487

Area N 6.07 12 70% 4.25 F3 15,424

Area O 7.55 12 70% 5.29 F3 19,185

Area P 8.84 12 70% 6.19 F3 22,462

Area Q 4.00 4 40% 1.60 F3 5,808

Area R 3.60 4 40% 1.44 F3 5,227

Area S 5.50 4 40% 2.20 F3 7,986

Area T 7.80 4 40% 3.12 F3 11,326

Area U 3.20 8 40% 1.28 F3 4,646

Area V 5.59 8 40% 2.24 F3 8,117

Area W 4.10 8 40% 1.64 F3 5,953

Area X 3.00 4 40% 1.20 F3 4,356 2Area A1 4.56 NA 85% 3.88 G 0

2Area A2 8.64 NA 85% 7.34 G 0

Area A3 5.78 NA 85% 4.91 G 17,834

Area B-1 9.70 NA 70% 6.79 F3 24,648

Area B-2 2.50 NA 70% 1.75 F3 6,353

152.24 98.38 316,384 1Impervious percentages to the proposed land-uses that were consistent with values found in Table 2-2a of the

NRCS TR-55 and Figure 2-3 of the ADOT Hydrology Manual 2Area contains existing impervious surfaces that are consistent with the proposed % imperviousness. These

areas were removed from the computations. 3LID Volume = (Impervious Area)(1-Inch Runoff)



Storage Evaluation

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2.3 Hydrologic Modeling Results

2.3.1 Proposed Land Use Change - Without On-Site Stormwater Storage

The proposed condition HEC-1 model includes an increase in the percent impervious of Sub-areas F3 and G by 2.4% and 0.4%, respectively. See Table 2–3.

Table 2–3 HEC-1 Changes in RTIMP for Sub-Areas F3 and G

HEC-1 Sub Area F3 G

Area

(sq. mi.) 4.68 6.16

USACE Model RTIMP Percent

(Rd100byr.dat) 25 24

Existing Impervious Area

(sq. mi.) 1.17 1.48

Increase in Impervious Area

(sq. mi.) 0.1107 0.0255

Total Percent Impervious Area

(sq. mi.) 1.28 1.50

Proposed Conditions

HEC-1 Model RTIMP Percent

(100.dat)

27.4 24.4

Percent Change (Existing to Proposed) 2.4 0.4

Adjusted Proposed Conditions

HEC-1 Model RTIMP Percent

(100adj.dat)

25.31 24.2

Table 2–4 shows the existing condition (no changes to land-use) HEC-1 modeling results (100-year peak discharge, storage, and WSE) at E. Butler Avenue and “Lake Continental”. Table 2–5 shows the proposed land-use change (reflected by an increased impervious area) HEC-1 modeling results (100-year peak discharge, storage volume, and WSE) at E. Butler Avenue and “Lake Continental”.

Table 2–4 USACE Existing Condition HEC-1 Model

Model Location

100-Year

Peak Flow

(cfs)

100-Year Peak

Storage

(acre-ft.)

100-Year Peak WSE

(ft.)

1Near E. Butler Ave. (CP-5) 4,041 N/A N/A 2Lake Continental (CP-7) 302 3,117 6,764.81

1CP-5 seen in Figure A- 1 listed as “EFG” in HEC-1 model.

2CP-7 seen in Figure A- 1 listed as “Outlet” in HEC-1 model.

Note: Existing conditions HEC-1 model (Rd100byr.dat/.out) on data disk.



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Table 2–5 Proposed Condition HEC-1 Model

Model Location

100-Year Peak

Flow

(cfs)

100-Year Peak

Storage

(acre-ft.)

100-Year Peak WSE

(ft.)

Near E. Butler Ave. (CP-5) 4,055 N/A N/A

Lake Continental (CP-7) 302 3,139 6,764.85 Note: Proposed conditions HEC-1 model (100.dat/.out) on data disk.

As seen from Table 2–4 and Table 2–5, changes in land-use (increased impervious area) without mitigation storage volume in place, results in a WSE increase of 0.04 feet at “Lake Continental”. This increase violates the “no rise” requirement for WSE requested by the City. A discussion in regards to meeting the “no rise” requirement with the use of on-site stormwater storage is provided below in Section 2.3.2.

2.3.2 Proposed Land Use Change – With Required On-Site Stormwater Storage

[Objective No. 2 – Section 1.3]

Based on the above discussed WSE increase of 0.04 feet at “Lake Continental” (Section 2.3.1), the City requires the proposed development to retain a portion of the stormwater runoff volume created by the increase in impervious area.

To meet the “no rise” criteria at “Lake Continental”, the proposed conditions percent impervious was incrementally decreased for Sub-Areas F3 and G until a “no rise” condition was met at “Lake Continental”. The resulting allowable runoff volume, associated with this modeled difference in proposed condition percent imperviousness is 18 acre-feet (see Table 2–6).

Table 2–6 USACE HEC-1 Model (With Adjusted Percent Impervious)

Model Location

100-Year

Peak Flow

(cfs)

100-Year

Runoff Volume

(acre-ft.)

100-Year Peak

WSE

(ft.) 1Lake Continental (CP-7)

Proposed Condition 302 3,139 6,764.85

Lake Continental (CP-7) Proposed Condition - Adjusted 302 3,121 26,764.81

Difference 18 1Sub-Areas F-3 and G percent impervious area changed as shown in Table 2–3. (HEC-1 Model 100adj.out)

2WSE matches the UASCE existing conditions WSE at “Lake Continental” (See Table 2–4)

Note: Adjusted conditions HEC-1 model (100adj.dat/.out) on data disk.

The runoff volume will to be detained on-site through the use of stormwater storage basins and or other LID related facilities. Stormwater storage basins will be further assessed during subsequent, more detailed design phases of the Little America Development.



Storage Evaluation

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2.4 On-Site Stormwater Storage Summary

On-site storage will be composed of Low Impact Development (LID) storage volume and general stormwater storage volume.

2.4.1 LID Storage Volume

LID storage volume is intended to retain the initial 1-inch of rainfall runoff. Across the proposed development, this 1-inch of rainfall runoff equates to 7.3 acre-feet (316,384 cu. ft. ÷ 43,560 ft./acre) of required LID storage volume (see Table 2–2).

During the design phase of the project, implementation of LID storage will be based on Best Management Practices (BMPs) as outlined in the City’s Low Impact Development, Guidance Manual for Site Design and Implementation. A Master Stormwater LID Plan for the entire site must be prepared to account for phased development. At minimum, each phase of the development will include the LID facilities from the Master Stormwater LID Plan. Interim temporary LID facilities may be required for project phasing.

2.4.2 General Stormwater Storage Volume

General stormwater storage volume will be implemented in order to ensure a “no rise” condition at “Lake Continental”. As discussed in Section 2.3, the required on-site retention volume to achieve this is 18 acre-feet, which will be phased with development.

General stormwater storage facilities will be placed to detain runoff volume through the use of in-line storage basins. In-line storage basins are located within significant drainages at the following strategic locations:

1. Within and adjacent to proposed golf course.

2. Upstream of roadway crossings.

3. Just upstream of development boundaries.

4. Underground storage within commercial and high density residential parcels along E. Butler Avenue if space constraints warrant.

Stormwater storage basins are not shown on figures; however, stormwater storage basins will be phased as development progresses. It should be noted that stormwater storage basins will not be located within the Rio de Flag 100-year floodplain.


Hydraulic Analysis

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3.1 E. Butler Avenue crossing of the Rio de Flag


3.1.1 E. Butler Avenue Hydraulic Evaluation

The City provided JEF with a HEC-RAS model (foxglenndivided.prj) for the Rio de Flag, which included cross-section in the vicinity of E. Butler Avenue. This model was prepared as part of the Floxglenn Development LOMR (April 2004). The following procedure was used to evaluate the hydraulic impact at E. Butler Avenue that is associated with the nominal increase in 100-year peak discharge for the proposed Little America Development land-use changes (discussed in Section 2.3);

1) Discharges used for the E. Butler Avenue hydraulic evaluation are 4,041 cfs and 4,055 cfs for existing and proposed conditions land-uses, respectively. (See Table 2–4 and Table 2–5.

2) The Foxglenn HEC-RAS model was developed using the Effective FEMA 100-year discharge of 3,250 cfs. The downstream boundary condition for the Effective model was a fixed water surface elevation of 6,773.9 feet at model cross-section 11.32. Per this study, the existing and proposed 100-year discharges are 4,041 cfs and 4,055 cfs, respectively. Given this increase in 100-year discharge, for this study, the downstream boundary condition was changed to normal depth with a slope of 0.00375 ft./ft.

3.1.2 E. Butler Avenue Hydraulic Evaluation Results

Existing and proposed WSE at HEC-RAS Cross-Section 11.455 (upstream cross-section at E. Butler Avenue, FEMA identified Cross-Section AT, See Figure 3-1) were compared to determine magnitude of the change. The results show a minor increase in water surface elevation, but within the 0.1 foot increase allowable by the City (see Table 3–1).

Table 3–1 E. Butler Avenue Crossing WSE Change

Cross-

Section ID

WSE using Existing

USACE Peak Discharge (ft.)

WSE using Developed

USACE Peak Discharge (ft.)

Difference (ft.)

11.455 6779.10 6779.12 0.02

3 Hydraulic Analysis


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Figure 3-1: E. Butler Avenue Crossing

HEC-RAS Cross section 11.455


Hydraulic Analysis

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3.2 Rio de Flag Floodplain


The purpose of the hydraulic modeling was to compare the Rio de Flag’s existing and proposed 100-year floodplain boundaries. The proposed condition floodplain modeling includes future golf course grading and associated structures. It is the intent of the Little America Development to provided assurance that for subsequent design of golf course grading, a low impact approach will be implemented and maintained within the Rio de Flag floodplain. Furthermore, the intent of the golf course grading is to remain out of the delineated floodway.

Existing and proposed HEC-RAS models were developed by JE Fuller (RiodeFlag.prj). Hydraulic modeling was performed using the U.S. Army Corps of Engineers HEC-RAS software, version 4.1. Computations are in accordance with the guidelines presented in the City’s Stormwater Management Design Manual (July 2000). In addition, HEC-RAS models were developed using HEC-GeoRAS software, which is an extension for use with ArcGIS and was developed through a Cooperative Research Agreement between the USACE Hydrologic Engineering Center and ESRI Inc.

HEC-RAS model output tables and cross-section plots are provided in Appendix B and the electronic model is included on the Data Disk in Appendix C. Location of model cross-sections are shown on the Existing Condition and Proposed Condition floodplain exhibits (Figure B- 1 and Figure B- 2, respectively), located in Appendix B.

3.2.1 Topography

Existing condition, 2-foot contour topography (NAVD 88) was used for development of a digital terrain model (DTM), which was then used to cut HEC-RAS cross-sections with HEC-GeoRAS. It should be noted that points and break lines were not provided to JEF with the topography.

3.2.2 Steady Flow Data

HEC-RAS models were developed for existing and proposed conditions using the Effective FEMA Discharge of 3,150 cfs.

3.2.3 Downstream Boundary Conditions

HEC-RAS model downstream boundary conditions were established from cross-sections 13.01 (FEMA Lettered Cross section BF). The starting water surface elevation at cross-section 13.01 is 6793.8 feet.

3.2.4 Flow Regime

As typical with FEMA most flood studies, a sub-critical flow regime was used for HEC-RAS modeling of the existing and proposed conditions. A sub-critical flow regime was selected for computation of “conservative” water surface elevations.


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3.2.5 Development of HEC-RAS Model Input Data

HEC-GeoRAS is an extension for use with ArcGIS and was developed through a Cooperative Research Agreement between the USACE Hydrologic Engineering Center and ESRI Inc. This tool has been designed to process geospatial data for HEC-RAS model development based on a DTM and complementary data sets. Conversely, this tool is also capable of exporting model results for use in ArcGIS processing. The HEC-GeoRAS import files applicable for this project include:

Stream centerline. Cross-sectional cut lines. Flow path centerlines.

3.2.5.1 Stream Centerline

The stream centerline defines the predominant center of flow during an event. Typically this line is identified by a stream’s thalweg. For HEC-RAS modeling of existing and proposed conditions, the stream centerline was taken from the Preliminary DFIRM database for the Rio de Flag. The stream centerline was not modified between models.

3.2.5.2 Cross-Sectional Cut Lines

Based on the stream centerline and a project DTM, a cross-section cut line defines a section’s geometry, length, location (River Station), and alignment to the flow. This cross-section information is processed by HEC-GeoRAS and converted to HEC-RAS format. Cross-section river stations are based on river-feet distances and start at the downstream cross-section, increasing in numeric value based upon the upstream length along the stream centerline.

For HEC-RAS modeling of existing and proposed conditions, a sufficient number of cross-sectional cut lines were placed to reflect changes in channel geometry, profile and roughness. Given the project’s intent to understand any impacts from golf course grading within the Rio de Flag, cross-section locations, alignments and geometries are coincidental for existing and proposed conditions.

3.2.5.3 Bank Stations

For HEC-RAS modeling of existing and proposed conditions, bank station locations are consistent with those in the FEMA-effective Rio de Flag HEC-2 model.

3.2.5.4 Flow Path Centerlines

Flow path centerlines are used to define the hydraulic flow path in the left overbank, main channel, and right overbank by identifying the center-of-mass of flow within these areas. Downstream reach lengths are calculated for each cross-sectional cut line based on the distance between cut lines along the flow path centerlines.

3.2.5.5 Cross-Section Roughness Coefficients (Manning’s n-Value)

Roughness coefficients represent the resistance to flow in channel and overbank areas. HEC-RAS accounts for this roughness by use of the Manning’s n-value (n-value). The n-value is


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determined from the values of the factors that affect the roughness, such as vegetation type and density, bed material and channel irregularities. Manning’s n-values were consistent with the Effective FEMA HEC-2 model. The n-values for the cross-sections are as follows:

0.032/0.035 for channel/overbank for cross-sections 13.01 to 14.02 0.032/0.040 for channel/overbank for cross-sections 14.09 to 14.65 0.034/0.040 for channel/overbank for cross-sections 14.84 to 14.94

3.2.5.6 Proposed Conditions Hydraulic Structures

There are five hydraulic structures included in the proposed condition HEC-RAS model. The structures added to the model are concept size and type only. The structures are located to facilitate cart movement between golf holes and for the crossing of the Rio de Flag. Table 3–2 lists the structures modeled in the proposed condition HEC-RAS.

Table 3–2 Rio de Flag Proposed Structures

Structure ID Structure

Cross-Section

Structure Type

Road Bridge #1 13.5 Bridge with 50-foot spans at the proposed

E.J.W Powell Boulevard extension. Trail Bridge #1 13.67 Wooden Golf Cart Bridge

Trail Bridge #2 14.01 Wooden Golf Cart Bridge



3.2.6 HEC-RAS Modeling Results

As discussed above, the hydraulic analyses documented in this report are for existing and proposed conditions. The purpose of the hydraulic modeling was to delineate floodplain boundary changes due to the development of the golf course within the floodplain fringe (outside of delineated floodway). The changes in water surface elevations are provided in Table 3–3. Floodplain boundaries are shown on the Existing Condition and Proposed Condition Floodplain workmaps (Figure B- 1 and Figure B- 2, respectively), located in Appendix B. In addition, HEC-RAS model cross-section output is provided in Appendix B. HEC-RAS modeling is provided electronically on the attached Data Disk in Appendix C.


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Table 3–3 Floodplain Elevation Changes

Cross-

section

ID

Existing Water

Surface

Elevation (ft.)

Proposed Water

Surface

Elevation (ft.)

Net Change1

(ft.)

13.01 6793.80 6793.80 0.00

13.34 6794.08 6794.08 0.00

13.44 6794.17 6794.17 0.00

13.46 - Bridge -

13.47 6794.18 6794.19 0.01

13.53 6794.20 6794.22 0.02

13.62 6794.30 6794.31 0.01

13.66 6794.34 6794.36 0.02

13.67 - Bridge -

13.68 6794.35 6794.40 0.05

13.77 6794.41 6794.46 0.05

13.91 6794.63 6794.73 0.10

14 6794.98 6795.08 0.10

14.01 - Bridge -

14.02 6795.10 6796.56 1.46

14.09 6796.89 6797.22 0.33

14.096 - Bridge -

14.1 6797.30 6798.63 1.33

14.28 6799.54 6799.76 0.22

14.39 6800.27 6800.42 0.15

14.397 - Bridge -

14.4 6800.35 6800.82 0.47

14.51 6801.01 6801.31 0.30

14.65 6801.95 6802.02 0.07

14.84 6804.12 6804.13 0.01

14.9 6804.25 6804.26 0.01

14.94 6808.99 6808.99 0.00 1Positive value indicates an increase in floodplain elevation

3.2.7 Limitations of Modeling Results

Due to the limitations in the topographic data, specifically a lack of detailed feature data (points and breaklines), the Rio de Flag profile in HEC-RAS shows a stepped stream profile. While not ideal, the modeling and subsequent floodplain mapping is adequate for preliminary evaluation of the potential impacts caused by the addition of minor grading and hydraulic structures within the floodplain. Further refinement of the hydraulic data will be required for final floodplain modeling.


Hydraulic Analysis

June 2013 Page: 3-7

3.2.8 Hydraulic Modeling Conclusions

As seen in Table 3–3, the addition of structures and minor grading within the floodplain fringe - associated with the proposed golf course - causes minor floodplain impacts. These impacts are primarily seen as a water surface increase directly upstream of each structure. At the upstream (northern) limit of the Little America Development (Cross-Section 14.51) the net difference in WSE between the existing and the proposed conditions is approximately 0.3 ft. (3.6 inches).


Appendix A:

Hydrology Supporting Data

June 2013

Appendix A. Hydrology Supporting Data

Figure A- 1: General HEC-1 Hydrologic Elements

CP 5

CP 6

CP 7

Subarea H

Subarea G

Subarea F3Subarea F2

Subarea F1Rio

De Fl

ag

General HEC-1 Hydrologic ElementsFigure A-1:

0 2,500 5,0001,250

Feet I

Notes

LegendUSACE_FlowPathsUSACE_BasinsUSACE_CPs

Site boundary

Little America DevelopmentFlagstaff, AZ

1) USACE basins, concentration points andflowpaths were digitized a from partiallyrectified exhibits (Plate 2A and Plate 2C).


Appendix B:

Hydraulics Supporting Data

June 2013

Appendix B. Hydraulics Supporting Data

Figure B- 1: Existing Conditions Floodplain

Figure B- 2: Proposed Conditions Floodplain

Rio de Flag Cross-section Plots

13.44

13.47

13.62

13.77

13.91

13.68

13.53

14.00

14.02

13.34

14.84

13.01

14.51

14.65

14.28

14.10

14.3914.40

14.94

6840

6830

6820

6810

6850

6800

6860

6870

6880 6890

6790

6900

6910

6920

69306940

6950

6960

6970

6980

6910

6890

6870

6850

6850

6870

6880

6800

6870

6860

6880

6880

68506880

6880

6880

6860

6830

6860

6880

6900

6790

6930

6930

6790

6860

6980

6860

6790

6920

6850

6870

6850

6910

6870

6870

6920

6870

6870

6870

6840

6850

6850

6880

6850

6820

6870

6870

6860

6860

6870

6880

6870

6860

6860

6880

6870

6870

6850

6880

6910

Rio de

Flag

13.66

14.9

14.09

Existing Conditions FloodplainFigure B-1:

0 800 1,600400

Feet I

Notes

LegendCross-sectionsRio de FlagSite boundaryExisting Conditions FloodplainSection Lines


1) Vertical Datum NAVD 882) Two foot contours not dispalyed for clarity

13.44

13.47

13.62

13.77

13.91

13.68

13.53

14.00

14.02

13.34

14.84

13.01

14.51

14.65

14.28

14.10

14.3914.40

14.94

6840

6830

6820

6810

6850

6800

6860

6870

6880 6890

6790

6900

6910

6920

69306940

6950

6960

6970

6980

6910

6890

6870

6850

6850

6870

6880

6800

6870

6860

6880

6880

68506880

6880

6880

6860

6830

6860

6880

6900

6790

6930

6930

6790

6860

6980

6860

6790

6920

6850

6870

6850

6910

6870

6870

6920

6870

6870

6870

6840

6850

6850

6880

6850

6820

6870

6870

6860

6860

6870

6880

6870

6860

6860

6880

6870

6870

6850

6880

6910

Rio de

Flag

13.66

14.9

14.09

Proposed Conditions FloodplainFigure B-2:

0 800 1,600400

Feet I

Notes

LegendCross-sectionsBridgesRio de FlagSite boundaryExisting Conditions FloodplainProposed Conditions FloodplainSection Lines


1) Vertical Datum NAVD 882) Two foot contours not dispalyed for clarity

6810 6800

6820

6830

6840

6850

Rio de Flag

Typical Floodplain Impacts

50 0 5025 Feet

Golf Cart Crossing

Golf Cart Crossing

Golf Cart Crossing

Golf Cart Crossing

Road Crossing

0 50 100 150 200 250 300 3506800

6810

6820

6830

6840

6850

LittleAmerica_RiodeFlag Plan: 1) Ex 2) Proposed River = Rio de Flag Reach = Reach 1 RS = 14.94

Station (ft)

Ele

vatio

n (f

t)

Legend

WS 100-Year - Proposed

WS 100-Year - Ex

Ground - Proposed

Bank Sta - Proposed

Ground - Ex

Bank Sta - Ex

0 50 100 150 200 250 300 350 4006800

6805

6810

6815

6820

6825

6830

6835

6840


Station (ft)

Ele

vatio

n (f

t)

Legend


WS 100-Year - Ex

Ground - Proposed

Ineff - Proposed

Bank Sta - Proposed

Ground - Ex

Ineff - Ex

Bank Sta - Ex

1

0 100 200 300 400 5006790

6800

6810

6820

6830

6840

6850

6860


Station (ft)

Ele

vatio

n (f

t)

Legend


WS 100-Year - Ex

Ground - Proposed

Bank Sta - Proposed

Ground - Ex

Bank Sta - Ex

0 100 200 300 400 5006790

6800

6810

6820

6830

6840

6850


Station (ft)

Ele

vatio

n (f

t)

Legend


WS 100-Year - Ex

Ground - Proposed

Bank Sta - Proposed

Ground - Ex

Bank Sta - Ex

2

0 100 200 300 400 5006790

6800

6810

6820

6830

6840

6850

6860


Station (ft)

Ele

vatio

n (f

t)

Legend


WS 100-Year - Ex

Ground - Proposed

Bank Sta - Proposed

Ground - Ex

Bank Sta - Ex

0 50 100 150 200 250 300 350 4006790

6800

6810

6820

6830

6840


Station (ft)

Ele

vatio

n (f

t)

Legend


WS 100-Year - Ex

Ground - Proposed

Bank Sta - Proposed

Ground - Ex

Bank Sta - Ex

3

0 50 100 150 200 250 300 350 4006790

6800

6810

6820

6830

6840

6850


Station (ft)

Ele

vatio

n (f

t)

Legend


WS 100-Year - Ex

Ground - Proposed

Bank Sta - Proposed

Ground - Ex

Bank Sta - Ex

0 100 200 300 400 5006790

6800

6810

6820

6830

6840

6850


Station (ft)

Ele

vatio

n (f

t)

Legend


WS 100-Year - Ex

Ground - Proposed

Bank Sta - Proposed

Ground - Ex

Bank Sta - Ex

4

0 100 200 300 4006794

6796

6798

6800

6802

6804

6806

6808

6810


Station (ft)

Ele

vatio

n (f

t)

Legend


WS 100-Year - Ex

Ground - Proposed

Bank Sta - Proposed

Ground - Ex

Bank Sta - Ex

0 50 100 150 200 250 300 350 4006790

6795

6800

6805

6810

6815

6820

6825


Station (ft)

Ele

vatio

n (f

t)

Legend


WS 100-Year - Ex

Ground - Proposed

Bank Sta - Proposed

Ground - Ex

Bank Sta - Ex

5

0 100 200 300 400 500 600 7006790

6800

6810

6820

6830

6840

6850

6860


Station (ft)

Ele

vatio

n (f

t)

Legend


WS 100-Year - Ex

Ground - Proposed

Bank Sta - Proposed

Ground - Ex

Bank Sta - Ex

0 100 200 300 400 500 600 7006790

6800

6810

6820

6830

6840

6850


Station (ft)

Ele

vatio

n (f

t)

Legend


WS 100-Year - Ex

Ground - Proposed

Bank Sta - Proposed

Ground - Ex

Bank Sta - Ex

6

0 200 400 600 800 10006790

6795

6800

6805

6810

6815

6820


Station (ft)

Ele

vatio

n (f

t)

Legend


WS 100-Year - Ex

- Proposed

Ground - Proposed

Ineff - Proposed

Bank Sta - Proposed

- Ex

Ground - Ex

Ineff - Ex

Bank Sta - Ex

0 200 400 600 800 10006790

6800

6810

6820

6830

6840


Station (ft)

Ele

vatio

n (f

t)

Legend


WS 100-Year - Ex

Ground - Proposed

Bank Sta - Proposed

Ground - Ex

Bank Sta - Ex

7

0 200 400 600 800 10006780

6785

6790

6795

6800

6805

6810


Station (ft)

Ele

vatio

n (f

t)

Legend


WS 100-Year - Ex

- Proposed

Ground - Proposed

Bank Sta - Proposed

Ground - Ex

Bank Sta - Ex

0 200 400 600 800 10006780

6785

6790

6795

6800

6805

6810

6815

6820

6825


Station (ft)

Ele

vatio

n (f

t)

Legend


WS 100-Year - Ex

- Proposed

Ground - Proposed

Bank Sta - Proposed

Ground - Ex

Bank Sta - Ex

8

0 200 400 600 800 10006785

6790

6795

6800

6805

6810

6815

6820

6825


Station (ft)

Ele

vatio

n (f

t)

Legend


WS 100-Year - Ex

Ground - Proposed

Bank Sta - Proposed

Ground - Ex

Bank Sta - Ex

0 200 400 600 800 10006785

6790

6795

6800

6805

6810

6815

6820


Station (ft)

Ele

vatio

n (f

t)

Legend


WS 100-Year - Ex

Ground - Proposed

Bank Sta - Proposed

Ground - Ex

Bank Sta - Ex

9

0 200 400 600 800 1000 1200 1400 16006785

6790

6795

6800

6805

6810

6815


Station (ft)

Ele

vatio

n (f

t)

Legend


WS 100-Year - Ex

- Proposed

Ground - Proposed

Ineff - Proposed

Bank Sta - Proposed

- Ex

Ground - Ex

Ineff - Ex

Bank Sta - Ex

0 500 1000 1500 2000 25006788

6790

6792

6794

6796

6798

6800

6802

6804

6806


Station (ft)

Ele

vatio

n (f

t)

Legend


WS 100-Year - Ex

- Proposed

Ground - Proposed

Ineff - Proposed

Bank Sta - Proposed

- Ex

Ground - Ex

Ineff - Ex

Bank Sta - Ex

10

0 100 200 300 400 500 6006785

6790

6795

6800

6805

6810

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6820


Station (ft)

Ele

vatio

n (f

t)

Legend


WS 100-Year - Ex

Ground - Proposed

Bank Sta - Proposed

Ground - Ex

Bank Sta - Ex

0 100 200 300 400 5006780

6790

6800

6810

6820

6830

6840


Station (ft)

Ele

vatio

n (f

t)

Legend


WS 100-Year - Ex

Ground - Proposed

Bank Sta - Proposed

Ground - Ex

Bank Sta - Ex

11


Appendix C:

Project Data Disk

June 2013

Appendix C. Project Data Disk

USACE Existing HEC-1 Model (Rd100byr.dat/.out)

USACE Updated HEC-1 Model with fully developed conditions (100.dat/.out)

USACE Updated HEC-1 Model adjusted for “Lake Continental” evaluation (100adj.dat/.out)

HEC-RAS Model

Rio de Flag (RiodeFlag.prj) Fox Glenn LOMR (Foxglenndivided.prj)

DRAINAGE IMPACT ANAYLSIS FOR THE LITTLE AMERICA DEVELOPMENT

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Transcript of DRAINAGE IMPACT ANAYLSIS FOR THE LITTLE AMERICA DEVELOPMENT