North I-25 EIS, ROD 2...11 CDOT Drainage Design Manual ... Culvert Description Flow Direction Marcy...

May 2017

2002 SOUTH I-25 CORRIDOR AND US 85 CORRIDOR RECORD OF DECISION 1 REEVALUATION AND SECTION 4(f) EVALUATION 2

US 85 Highlands Ranch Parkway to C-470 3 4

Floodplain Technical Report 5

6 7 8 9 10 11 Prepared for: 12

13 14 15 16 17 18 19 20 21 22 Prepared by: 23

24 25 26 27 28 29 30 31 32 33 34 May 2017 35 36

Floodplain Technical Report

May 2017

REEVALUATION AND SECTION 4(f) EVALUATION

TOC-i

CONTENTS 1

Page No. 2

1.0 Introduction/Background ........................................................................................ 1 3 1.1 History ............................................................................................................................ 1 4 1.2 Study Area ..................................................................................................................... 2 5 1.3 Purpose for Reevaluation ............................................................................................... 2 6

2.0 Affected Environment ............................................................................................ 5 7 2.1 Summary of Resource from FEIS/ROD .......................................................................... 5 8 2.2 Changes in Laws, Regulations or Guidance since FEIS/ROD ........................................ 5 9

Douglas County Storm Drainage Design and Technical Criteria Manual ........................ 5 10 CDOT Drainage Design Manual ..................................................................................... 6 11 CDOT Erosion Control and Stormwater Quality Guide ................................................... 6 12 US Army Corps of Engineers Section 408 ...................................................................... 6 13 Executive Order 11988: Floodplain Management ........................................................... 6 14

2.3 Change in Resource Base since FEIS/ROD ................................................................... 6 15

3.0 Description of the Alternatives ............................................................................... 7 16 3.1 No-Action Alternative ...................................................................................................... 7 17 3.2 Refined Selected Alternative .......................................................................................... 7 18

Design Changes Included in the Refined Selected Alternative ....................................... 8 19

4.0 Environmental Consequences ............................................................................. 15 20 4.1 Impacts ..........................................................................................................................15 21

Summary of Impacts from FEIS/ROD ............................................................................15 22 Changes in Impacts since FEIS/ROD ............................................................................16 23 Significance of Change ..................................................................................................16 24

4.2 Mitigation .......................................................................................................................16 25 Summary of Mitigation from FEIS/ROD .........................................................................16 26 Changes in Mitigation since FEIS/ROD .........................................................................17 27

5.0 Agency Coordination Conducted ......................................................................... 17 28

6.0 References .......................................................................................................... 17 29 30

Figures 31

Figure 1. Status of US 85 Corridor Segments ........................................................................... 3 32 Figure 2. US 85 Reevaluation Study Area ................................................................................. 4 33 Figure 3. Floodplain Areas in the Reevaluation Study Area....................................................... 7 34 Figure 4. Refined Selected Alternative Typical Section ............................................................. 8 35 Figure 5. US 85 Highlands Ranch Parkway to C-470 Refined Selected Alternative .................. 9 36 Figure 6. Changes in Access and Turning Movements ............................................................10 37 Figure 7. Highlands Ranch Parkway Continuous Flow Intersection ..........................................11 38 Figure 8. Town Center Drive Continuous Flow Intersection ......................................................11 39 Figure 9. Highlands Ranch Parkway Continuous Flow Intersection U-Turn Access .................12 40 Figure 10. Town Center Drive Continuous Flow Intersection U-Turn Access ........................12 41 Figure 11. Combined Access for Spring Gulch Equestrian Facility and Grace Presbyterian 42

Church ......................................................................................................................13 43


May 2017

2002 South I-25 Corridor and US 85 Corridor Record of Decision Reevaluation and Section 4(f) Evaluation

US 85 Highlands Ranch Parkway to C-470 TOC-ii

Figure 12. High Line Canal Trail Grade-separated Crossing: Existing Condition and Future 1 Condition Simulation .................................................................................................15 2

3

Tables 4

Table 1. FEMA Information ...................................................................................................... 5 5 Table 2. Peak Flows ................................................................................................................ 5 6 7


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Acronyms-i

Acronyms and Abbreviations 1

ADA Americans with Disabilities Act of 1990 2

BFE base flood elevation 3

BMP best management practices 4

cfs cubic feet per second. 5

DFIRM Digital Flood Insurance Rate Map 6

FEMA Federal Emergency Management Agency 7

USACE U.S. Army Corps of Engineers 8

C-470 Colorado State Highway 470 9

CDOT Colorado Department of Transportation 10

CFR Code of Federal Regulations 11

CUHP Colorado Urban Hydrograph Procedure 12

DRCOG Denver Regional Council of Governments 13

FEIS Final Environmental Impact Statement 14

FEMA Federal Emergency Management Agency 15

FHWA Federal Highway Administration 16

I-25 Interstate 25 17

MAP-21 Moving Ahead for Progress in the 21st Century Act 18

NEPA National Environmental Policy Act 19

PEL Planning and Environmental Linkages 20

ROD Record of Decision 21

SH 67 State Highway 67 22

URA Uniform Relocation Assistance and Real Property Acquisition Policies Act 23

US 85 United States Highway 85 24

USACE United States Army Corps of Engineers 25

26


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1

1.0 Introduction/Background 1

1.1 History 2

The United States Highway 85 (US 85) South Corridor extends 25.5 miles from Interstate 25 3

(I-25) in Denver to the Town of Castle Rock in Douglas County. From a regional perspective, 4

this corridor is a multimodal major arterial for longer-distance, regional trips. The corridor also 5

provides access to numerous commercial and residential developments that are crucial to 6

Douglas County’s economy. 7

In May 2001, the Colorado Department of Transportation (CDOT) and the Federal Highway 8

Administration (FHWA) completed the South I-25/US 85 Final Environmental Impact Statement 9

(FEIS) (CDOT 2001a). A Record of Decision (ROD) was signed in August 2001 that selected 10

the Preferred Alternative from the FEIS, referred to as the Selected Alternative. A Revised 11

Record of Decision was signed in 2002. There were no changes to the Selected Alternative in 12

the 2001 ROD in the 2002 Revised ROD (CDOT 2002). 13

The FEIS/ROD outlined a set of improvements to address transportation needs for a 2020 14

horizon year along US 85 from approximately Meadows Parkway to Blakeland Drive. Since 15

then, Douglas County has helped provide funding to CDOT to combine with their own funding to 16

design and construct six segments of the Selected Alternative from the FEIS/ROD. In addition, 17

funding has been provided to improve sections of I-25. 18

The US 85 Corridor segments and their status are shown in Figure 1. 19

As additional residential and commercial growth occurs in the northwest portion of the county, 20

further studies have been conducted to identify what transportation improvements are 21

necessary to support the development. Douglas County is conducting two separate but 22

coordinated studies of US 85. 23

The US 85 Corridor Improvements Planning and Environmental Linkages (PEL) Study 24

Report (Douglas County 2016) updated the 2002 FEIS/ROD recommendations for 25

transportation improvements to US 85 from approximately State Highway 67 (SH 67) in 26

Sedalia to 0.5 mile north of County Line Road. The PEL study identified the long-term 27

transportation needs beyond 2040. It was done primarily to determine what improvements 28

are needed in addition to those selected in the FEIS/ROD. The PEL study defined a 29

Purpose and Need, developed and evaluated a set of alternatives, and recommended 30

improvements for the study area. Near-term improvements to 2020 include providing six 31

through lanes with continuous flow intersections between Highlands Ranch Parkway and 32

Colorado State Highway 470 (C-470) (which includes a multiuse path on the east side of US 33

85) and providing six through lanes from C-470 to 1,200 feet north of County Line Road 34

(including a new bridge over C-470, a grade-separated Centennial Trail, and a flyover ramp 35

for northbound to westbound traffic). More details about the recommendations and 36

improvements beyond 2020 are in the PEL study document. 37


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US 85 Highlands Ranch Parkway to C-470 2

The Highlands Ranch Parkway to C-470 Project is another segment of the FEIS/ROD 1

Selected Alternative. Douglas County obtained funding from the Denver Regional Council of 2

Governments (DRCOG) to construct this project beginning in 2019. Before design can 3

proceed, the 2002 FEIS/ROD needs to be reevaluated to reflect current conditions. This 4

National Environmental Policy Act (NEPA) Reevaluation determines if the findings from the 5

FEIS/ROD remain valid, so that this segment of the FEIS/ROD Selected Alternative can 6

proceed to final design and construction. 7

1.2 Study Area 8

The approximate 312-acre environmental resource study area for this NEPA Reevaluation is 9

located in Douglas County along two miles of US 85, from Highlands Ranch Parkway to C-470. 10

The study area begins approximately 1,900 feet south of the intersection of US 85 and 11

Highlands Ranch Parkway and extends north to C-470, as shown in Figure 2. The eastern and 12

western boundaries vary along the length of the study area but extend an average of 500 feet to 13

700 feet in either direction of US 85. The boundaries were set to encompass areas on either 14

side of US 85 associated with the Refined Selected Alternative improvements to be evaluated 15

for direct and indirect impacts. 16

1.3 Purpose for Reevaluation 17

The purpose for this Reevaluation is to reevaluate the 2002 FEIS/ROD to address changes to 18

conditions that have occurred since it was issued by FHWA, and to reanalyze impacts of 19

recommended improvements for the Highlands Ranch Parkway to C-470 project. The 20

Reevaluation uses data from the most recent fiscally constrained 2040 Regional Transportation 21

Plan. The FEIS/ROD used data from the 2020 Regional Transportation Plan. The Reevaluation 22

identifies changed existing and future conditions; identifies a refined Selected Alternative for 23

improvements to improve capacity, operational performance and safety for traffic volumes in 24

2040; identifies changes in legislation, regulations, and guidance related to the improvements; 25

reanalyzes impacts; and develops needed changes to the mitigation measures identified in the 26

FEIS/ROD. 27

28


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Figure 1. Status of US 85 Corridor Segments

Source: HDR 2016.

1


May 2017



Figure 2. US 85 Reevaluation Study Area

Source: HDR 2016.

1


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2.0 Affected Environment 1

2.1 Summary of Resource from FEIS/ROD 2

The two floodplain resources from the FEIS/ROD include Marcy Gulch and Spring Gulch. Both 3

are delineated as Special Flood Hazard Areas by the Federal Emergency Management Agency 4

(FEMA). Applicable information for these two resources is presented below in Table 1 and Table 5

2. 6

Table 1. FEMA Information

Stream Name FIRM Panel Date Zone

Marcy Gulch 08035C0016F September 30, 2005 A

Spring Gulch 08035C0016F September 30, 2005 A

Source: FEMA 2016.

7

Table 2. Peak Flows

Basin ID

Drainage Area

(acres)

10-year Peak Flow (cfs)

50-year Peak Flow (cfs)

100-Year Peak Flow (cfs)

Cross-Drain

Location

Existing Cross-Culvert

Description

Flow Direction

Marcy Gulch

2,534 N/A N/A 1,145 MP 199.8 15x20-ft Concrete

Box Culvert West

Spring Gulch

1,274 N/A N/A 212 MP 198.95 6x6-ft Concrete

Box Culvert West

Sources: FEMA 2016; CDOT 2002.

cfs = cubic feet per second.

2.2 Changes in Laws, Regulations or Guidance since FEIS/ROD 8

There have been several changes to regulations and guidance since the Floodplain and 9

Drainage Assessment Technical Report South I-25 Corridor and US 85 Corridor EIS (CDOT 10

2000) was prepared. These changes and their documents are summarized below. 11

Douglas County Storm Drainage Design and Technical Criteria Manual 12

Douglas County’s Storm Drainage Design and Technical Criteria Manual (Douglas County 13

2008) was amended in July 2008. In May 2013, specific chapters were updated. Any applicable 14

Douglas County criteria will adhere to the updated manual. For this Reevaluation, segment 15

rainfall input values for the Colorado Urban Hydrograph Procedure (CUHP) analysis will adhere 16

to the current manual. 17


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CDOT Drainage Design Manual 1

The CDOT Drainage Design Manual was released in 2004 (CDOT 2004a). Chapter 10, Bridges, 2

was revised in September 2009. All drainage design follows this updated CDOT manual. 3

CDOT Erosion Control and Stormwater Quality Guide 4

CDOT’s Erosion Control and Stormwater Quality Guide was released in 2004 (CDOT 2004b). 5

Chapter 5, Construction Best Management Practices, was revised in July 2014. Construction 6

erosion control design follows this updated CDOT guide. 7

US Army Corps of Engineers Section 408 8

On September 30, 2015, the U.S. Army Corps of Engineers released policy and procedural 9

guidance pursuant to Section 408 to incorporate changes as a result of Section 1006 of the 10

Water Resources Reform and Development Act of 2014. This guidance requires non-federal 11

sponsors provide assurances that the USACE project being impacted can be operated and 12

maintained in accordance with requirements prescribed by USACE. This guidance and its 13

requirements were followed for this Reevaluation. 14

Executive Order 11988: Floodplain Management 15

On October 2003, the 44 Code of Federal Regulations, Part 9 was last updated. This code of 16

regulations contains Executive Order 11988, which “requires federal agencies to avoid to the 17

extent possible the long and short-term adverse impacts associated with the occupancy and 18

modification of flood plains and to avoid direct and indirect support of floodplain development 19

wherever there is a practicable alternative.” This guidance and its requirements were followed 20

for this Reevaluation. 21

2.3 Change in Resource Base since FEIS/ROD 22

In 2005, Douglas County updated its floodplain mapping to meet the new Digital Flood 23

Insurance Rate Map (DFIRM) requirements. The FIRM for the updated area has an effective 24

date of September 30, 2005. Both Spring Gulch and Marcy Gulch are mapped as approximate 25

FEMA Zone A floodplains (Figure 3). Within these zones, increases to the base flood elevation 26

(BFE) are allowed within 1 foot. 27

The Spring Gulch floodplain is contained behind the U.S. Army Corps of Engineers (USACE) 28

dam. The Marcy Gulch floodplain contains most of the area of Marcy Gulch and is only 29

separated by roadway and rail embankments. 30

The FEIS/ROD did not address the on-site stormwater network design. In the current conditions, 31

runoff from US 85 is primarily conveyed in ditches to major outfalls—Spring Gulch, High Line 32

Canal, and Marcy Gulch. These ditches also accept controlled runoff from some of the adjacent 33

properties, and there is a series of driveway culverts that convey runoff under the property 34

accesses. 35

36


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Figure 3. Floodplain Areas in the Reevaluation Study Area

Source: FEMA 2016.

1

3.0 Description of the Alternatives 2

3.1 No-Action Alternative 3

The No-Action Alternative consists of leaving US 85 in its current condition between Highlands 4

Ranch Parkway and C-470, with two general purpose lanes in each direction. Improvements to 5

other sections of US 85 and to portions of I-25 as adopted in the 2002 ROD have already been 6

implemented and are assumed as part of the No-Action Alternative network. The No-Action 7

Alternative also includes improvements to C-470 as defined in the recent Finding of No 8

Significant Impact. 9

3.2 Refined Selected Alternative 10

The 2002 FEIS/ROD Selected Alternative included widening both US 85 and I-25. The I-25 11

recommendations included widening to eight lanes between C-470 and Meadows Parkway and 12

six lanes between Meadows Parkway and Douglas Lane. An east side frontage road was 13


May 2017



included between Schweiger Interchange and Castle Pines Parkway. Interchange modifications 1

were included at Schweiger, Surrey Ridge Road, Castle Pines Parkway and Plum Creek 2

Parkway. All improvements on I-25 that were in the Revised ROD have been completed, except 3

for the widening of the Happy Canyon Road bridge. 4

For US 85, widening to six lanes between Highlands Ranch Parkway and C-470 and four lanes 5

south to Meadows Parkway was recommended. The SH 67 interchange was to be reconfigured, 6

a frontage road was recommended at Sedalia, and a minor realignment was recommended at 7

Cook Ranch. Bicycle and pedestrian facilities were to be included all along US 85, a grade 8

separation at the High Line Canal trail was included, and enhanced wildlife crossings were 9

recommended. 10

Improvements in the 2002 FEIS/ROD Selected Alternative between Highlands Ranch Parkway 11

and C-470 included a six-through-lane section (eight lanes including the auxiliary lanes) with a 12

total width that ranges from 106 to 131 feet. The travel lanes are 12 feet wide. The alternative 13

includes a raised median, inside curb and gutter, outside curb and gutter, inside shoulders, 14

continuous auxiliary lanes, and a shared-use path. It also includes improvements to the High 15

Line Canal Trail by changing the existing at-grade crossing to a grade-separated crossing under 16

US 85. Access consolidation includes modification to right-in/right-out accesses, based on the 17

Final US 85 Access Management Plan, South I-25 Corridor and US 85 Corridor EIS (CDOT 18

2001). 19

The Refined Selected Alternative includes all of the features described above, most of which are 20

illustrated in the cross-section in Figure 4. 21

Figure 4. Refined Selected Alternative Typical Section

Source: HDR 2016.

22

Design Changes Included in the Refined Selected Alternative 23

Changes in the Refined Selected Alternative design compared to the Selected Alternative 24

include continuous flow intersections at Town Center Drive and Highlands Ranch Parkway and 25

minor changes to access and some elements of the cross-section, culvert sizes, bus stop 26

enhancements, and retaining walls (Figure 5). All of these changes are minor refinements to the 27

same basic alternative. 28


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Figure 5. US 85 Highlands Ranch Parkway to C-470 Refined Selected Alternative

Source: HDR 2016.


May 2017



US 85 Mainline. The width of the auxiliary lane increased 10 feet to 12 feet. In some 1

locations, to minimize impacts, the auxiliary lane may be 11 feet. The FEIS/ROD design had 2

included an alignment shift to the west. This is no longer a part of the Refined Selected 3

Alternative. It also includes a wider raised median (30 feet compared to 10 feet) and no 4

inside shoulders at the continuous flow intersections. 5

Intersection and Access Improvements. Changes in access and turning movements are 6

described in Figure 6. There are notable changes at the intersections below. They are 7

described and illustrated on the following pages. 8

o Highlands Ranch Parkway and Town Center Drive. 9

o Norwood Drive, Carder Court, and Brandon Drive. 10

o Spring Gulch Equestrian Facility and Grace Presbyterian Church. 11

Figure 6. Changes in Access and Turning Movements

Source: HDR 2016.

Location Existing Conditions2002 Selected

Alternative

Refined Selected

Alternative

Change from

FEIS/ROD

Midway - Town

Center Drive

No U-turn

southbound to

northbound

Grace Presbyterian

ChurchNo change

Highlands Ranch

Parkway - Dumont

Way

No U-turn

southbound to

northbound

Note: N/S through movements assumed for all intersections

No Access Documented

(Church constructed 2012)

Left turns (except

southbound) and

east/west through

movements

displaced

Brandon Drive -

Spring Gulch

Equestrian Facility

Combined access to

Spring Gulch

Equestrian Facility

and Grace

Presbyterian Church

NEPA Reevalution AccessFEIS/ROD Access

Left turns and

east/west through

movements

displaced

Norwood Drive

Left turns and

east/west through

movements

displaced

Carder Court


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At Highlands Ranch Parkway and 1

Town Center Drive, there are 2

continuous flow intersections. This 3

innovative intersection design improves 4

operations for intersections with a high 5

number of left-turn movements. This 6

type of traffic pattern exists on US 85 7

within the study area, and the Refined 8

Selected Alternative incorporates this 9

design modification at the Highlands 10

Ranch Parkway and Town Center Drive 11

intersections. When compared to a 12

traditional signal-controlled intersection, 13

the primary differentiating feature of the 14

continuous flow intersection is the 15

relocation of left-turn movements on an 16

approach to the other side of the 17

opposing traffic flow. Figure 9 and 18

Figure 10 display the continuous flow 19

intersection layouts at Highlands Ranch 20

Parkway and Town Center Drive with 21

the relocated left-turn movement 22

highlighted. 23

24

Figure 7. Highlands Ranch Parkway Continuous Flow Intersection

Source: HDR 2016.

Figure 8. Town Center Drive Continuous Flow Intersection

Source: HDR 2016.


May 2017



At Norwood Drive, Carder Court, and Brandon Drive, the intersections are right-in/right-1

out. Left-turning traffic is relocated to adjacent intersections, as shown in Figure 9 and Figure 2

10. 3

Figure 9. Highlands Ranch Parkway Continuous Flow Intersection U-Turn Access

Source: HDR 2016.

4

Figure 10. Town Center Drive Continuous Flow Intersection U-Turn Access

Source: HDR 2016.

5

Access to the Spring Gulch Equestrian Facility (owned by the U.S. Army Corps of 6

Engineers) is combined with access to Grace Presbyterian Church. This is a 3/4 7

movement; however, the southbound left turn movement may be eliminated at CDOT’s 8

discretion if safety issues materialize. Traffic destined to southbound US 85 from this access 9

would make a U-turn at Town Center Drive. This change includes paving of the driving entrance 10

and relocating the entrance 120 feet south (Figure 11). The Grace Presbyterian Church was not 11

in this location in 2002, so the project setting has changed. 12


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1

Figure 11. Combined Access for Spring Gulch Equestrian Facility and Grace Presbyterian Church

Source: HDR 2016.


May 2017



Retaining Walls. There are more retaining walls (approximately 80,000 square feet) to 1

avoid or minimize parking or property impacts, minimize riparian vegetation impacts, 2

minimize impacts to water quality treatment and drainage features, minimize impacts to 3

Section 4(f) historic and recreation properties, and minimize impacts to the railroad bridge 4

substructure and foundations. 5

Improved Bus Stops and Connections. The design now includes improvements to the 6

existing RTD 402L bus stops: 7

The stop on US 85 north of Highlands Ranch Parkway is being eliminated. The existing 8

stop on the north side of Highlands Ranch Parkway east of US 85 is being moved and 9

enhanced with a bench, shelter, and bike racks. 10

The stop on US 85 north of Town Center Drive is being moved to south of Town Center 11

Drive and enhanced with bench, shelter, bike racks, and bike lockers. 12

For both southbound and northbound stops, the Refined Selected Alternative includes 13

sidewalk connections from the bus stop to the adjacent side street. 14

Shared-use Path. There are minor changes in the width of shared-use path and the width 15

of separation between the roadway and path. 16

Highlands Ranch Parkway to Blakeland Drive. Rather than a consistent 5-foot 17

landscaped buffer, the path has 2-foot gravel shoulders, and its distance from the 18

roadway generally varies from between 2 and 22 feet with landscaping in the buffer 19

where there is adequate room. At the railroad crossings south of Blakeland Drive, the 20

path is detached and set back from the roadway by 14 feet. 21

Blakeland Drive to C-470. The Refined Selected Alternative has a wider path (10 feet 22

instead of 8 feet) that is detached on the east side of US 85 with landscaping in the 23

buffer where there is adequate room. On the west side, it is also 10 feet wide but 24

attached. 25

C-470 Trail (also called the Centennial Trail). The shared-use path is connected to the 26

C-470 Trail with an improved at-grade crossing of US 85. The at-grade crossing 27

enhancements for the C-470 Trail include restriping the crosswalks, adding new 28

Americans with Disabilities Act of 1990 (ADA) ramps, reconfiguring the existing median 29

island, and providing better wayfinding through the intersection. A future grade-30

separated crossing will be constructed in a later project when funded. 31

Water Quality Treatment. To meet current municipal separate storm sewer systems (MS4) 32

requirements, the Refined Selected Alternative assumes conversion of an existing parcel 33

owned by Douglas County (north of Brandon Drive) by the High Line Canal to a water 34

quality facility. This location has been tentatively selected at this phase of design, but 35


May 2017



specific details may change during the final design process. If the changes result in 1

additional environmental impacts, those will be documented in a reevaluation. 2

High Line Canal Trail Grade-separated Crossing. The culvert for the High Line Canal 3

Trail underpass at US 85 (Figure 12) is now 2 feet higher and wider than the Selected 4

Alternative—12 feet high and 14 feet wide. 5

Figure 12. High Line Canal Trail Grade-separated Crossing: Existing Condition and Future Condition Simulation

Source: CDOT 2002.

4.0 Environmental Consequences 6

4.1 Impacts 7

Summary of Impacts from FEIS/ROD 8

The FEIS/ROD identified impacts of 0.86 acre to the Marcy Gulch floodplain. It stated that 100-9

year flood surface elevations would not be impacted; however, other uses (such as aesthetics 10

and wildlife habitat) would be impacted. No impacts to the Spring Gulch floodplain were 11

identified. 12


May 2017



Changes in Impacts since FEIS/ROD 1

The Refined Selected Alternative would impact 0.94 acre of the Marcy Gulch floodplain because 2

of inaccuracies in the existing FEMA floodplain mapping, which shows the floodplain on top of 3

the existing roadway. The calculated water surface elevations may be impacted immediately 4

upstream of the US 85 culvert, but no change in base flood elevation is anticipated. The minor 5

loss of natural floodplain value would occur within the limits of the proposed culvert extension. 6

The Refined Selected Alternative would not impact the floodplain at Spring Gulch. 7

In existing conditions, there are portions of US 85 that outfall directly into the High Line Canal. In 8

the Refined Selected Alternative, the storm outfalls would reduce the direct discharges to High 9

Line Canal. 10

The Refined Selected Alternative may also include temporary construction impacts to the 11

floodplain and drainage systems. During construction, additional sediment and other pollutants 12

may be created within the study area and could drain toward Marcy Gulch or Spring Gulch. 13

Discharge of sediment or other pollutants could impact the floodplain and existing 14

drainageways. Additionally, construction activities within the floodplain would be at risk of 15

flooding in the event of severe runoff. 16

Significance of Change 17

There are no significant changes in floodplain impact from the Selected Alternative to the 18

Refined Selected Alternative. Extension of the Marcy Gulch culvert and subsequent impacts to 19

the Zone A floodplain in the area are expected to have little or no impact to the environment in 20

the area. A negligible rise in BFE is expected. Any impacts to wetlands or riparian area in the 21

vicinity of the culvert extension at Marcy Gulch are addressed in the Waters of the U.S. 22

Technical Report, US 85 Corridor Improvements, Highlands Ranch Parkway to C-470 23

Reevaluation (HDR 2016). 24

The on-site stormwater and urban section design of US 85 represents a departure from the 25

FEIS/ROD because it was never addressed in the previous documents. The outfall locations in 26

the Refined Selected Alternative are planned to follow existing drainage patterns, and there is 27

no intra-basin transfer of flows. Stormwater design includes erosion control measures to 28

minimize downstream impacts. 29

These minor changes in impacts do not represent a new significant impact that was not 30

addressed in the FEIS/ROD. 31

4.2 Mitigation 32

Summary of Mitigation from FEIS/ROD 33

The FEIS/ROD stated all practicable measures to minimize harm to floodplains were 34

incorporated in the Preferred Alternative (Selected Alternative). Little to no change to historic 35

drainage patterns would be expected. Standard practices for bridge, culvert, and highway 36


May 2017



construction, including construction erosion and sediment control, will mitigate floodplain 1

impacts. 2

Changes in Mitigation since FEIS/ROD 3

Additional floodplain impact mitigation is included in the Refined Selected Alternative. Erosion 4

protection is planned at the Marcy Gulch outfall to mitigate any scour or erosion that could result 5

from a concentrated outfall. The urban section storm system will mitigate the numerous erosion 6

issues from the existing ditches along the corridor. 7

Construction activities will include construction erosion and sediment control best management 8

practices (BMP). A Stormwater Management Plan will be required to control construction 9

impacts and mitigate sediment and other pollutants generated within the project area. 10

Impact to the Marcy Gulch floodplain will require coordination with and permitting by Douglas 11

County. 12

5.0 Agency Coordination Conducted 13 No agency coordination was conducted specific to the floodplain resource. 14

6.0 References 15 Colorado Department of Transportation (CDOT). 2000. Floodplain and Drainage Assessment 16

Technical Report South I-25 Corridor and US 85 Corridor EIS. November. 17

— — —. 2001a. South I-25 Corridor and US 85 Corridor FEIS. Final Environmental Impact 18

Statement, Section 4(f) Evaluation. May. 19

— — —. 2001b. Final US 85 Access Management Plan, South I-25 Corridor and US 85 Corridor 20

EIS. March. 21

— — —.2002. South I-25 Corridor and US 85 Corridor Record of Decision (ROD). October 2002. 22

— — —.2004a. Drainage Design Manual. 23

https://www.codot.gov/programs/environmental/water-quality/documents/drainage-design-24

manual. 25

— — —. 2004b. Erosion Control and Stormwater Quality Guide. 26

https://www.codot.gov/programs/environmental/landscape-architecture/erosion-storm-quality 27

Douglas County. 2008. Storm Drainage Design and Technical Criteria Manual. July. 28

— — —. 2016. US 85 Corridor Improvements Planning and Environmental Linkages (PEL) 29

Study Report. 30

Federal Emergency Management Agency (FEMA). 2016. Douglas County Colorado and 31

Incorporated Areas Flood Insurance Study (FIS). March 16. 32

https://www.codot.gov/programs/environmental/water-quality/documents/drainage-design-manualhttps://www.codot.gov/programs/environmental/water-quality/documents/drainage-design-manualhttps://www.codot.gov/programs/environmental/landscape-architecture/erosion-storm-quality


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HDR. 2016. Waters of the U.S. Technical Report, US 85 Corridor Improvements, Highlands 1

Ranch Parkway to C-470 Reevaluation. 2

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