Attachment D - Drainage Letter.pdf

May 8, 2024

Updated August 15, 2024

Drainage Supervisor Development Review

Aurora Water

26791 E. Quincy Avenue

Aurora, CO 80016

RE: Final Drainage Le�er for the Sand Creek Trail Improvements –

MBT Filing No. 1 and Morris Heights Filing No. 3

Aurora PROS Project No: 5962A

To the Drainage Supervisor - Development Review

Herein is a summary of the drainage study performed as part of the Sand Creek Trail Improvement project, which is an Aurora Parks Recrea3on and Open Space (PROS) project.

I. PROJECT BACKGROUND

Project Loca3on:

The proposed Sand Creek Trail Improvement project is in the City of Aurora, north of Fitzsimons Parkway and

Peoria Street. (Figure 1). The project extends from approximately the City and County of Denver limits on the west to approximately 750 feet west of the city and county limits.

Figure 1: Project Vicinity Map (Google Aerial Imagery Date October 2020)

08.15.2024

FACSIMILE

This electronic plan is a facsimile of the signed and sealed pdf set

Elysa M. Loewen Date

Drainage ReviewAPPROVED FOR ONE YEAR

By: Richard Ommert on Aug 16,2024

224143LTR2-APPREXT1

1958-3004

02E, 01F, 02F, 03F06/04/2025

Project Descrip4on:

The exis4ng Sand Creek Regional Greenway Trail in this loca4on is composed of approximately 1,140 linear feet of

12-foot-wide crusher fines trail. The purpose of the project is to convert the exis4ng crusher fines trail to a 10-foot-wide concrete trail with 3-foot-wide crusher fines shoulder to match the exis4ng paved trail that con4nues east to the Colorado Front Range Trail and west where the Sand Creek Regional Greenway Trail/Colorado Front Range Trail con4nues into the City and County of Denver. This sec4on of trail was included in a previous conceptual plan in 2020 prepared by Stream for the Mile High Flood District (MHFD) and City and County of Denver (CCOD) but not paved as part of the project.

II. HISTORIC AND PROPOSED CONDITIONS

Historic Drainage

The project area or limits of construc4on are 0.98 acres total (including all access and staging areas) the project area drains to the north ul4mately entering Sand Creek and con4nuing east within Sand Creek channel. This Major Basin in this area is the Sand Creek Basin. The sub-basin defined as part of this project is approximately 6.66 acres in size (Basin A) includes the limits of construc4on for this project, a por4on of

Peoria street, the RTD bridge, Sand Creek, and open space containing the exis4ng crusher fines trail and some exis4ng concrete trail. The high eleva4on onsite is 5342’ and the low point of the basin is 5290’ with an average slope across the basin of approximately 7.6%. A summary of the exis4ng drainage sub-basin characteris4cs is shown below:

Historic Drainage Characteris2cs Summary

Basin A

Basin Size 6.66 Acres

Imperviousness (%) 24.6%

Historic Minor Storm Runoff Q5 2.01 cfs

Historic Major Storm Runoff Q100 9.12 cfs

Proposed Drainage

The proposed trail improvements will generally maintain the historic drainage paEerns on the site; some modifica4ons will be made to make improvements to local drainage adjacent to the trail, but minor modifica4ons will be made to the exis4ng trail grade and overall basin size, basin overall slope and design point will remain unchanged. Impervious area will increase with the replacement of the exis4ng crusher fines trail with concrete pavement. The overall high point, low point and overall slope across the basin will remain unchanged. A summary of the proposed basin characteris4cs is summarized below with a comparison to the change from historic/exis4ng condi4ons.

Proposed Drainage Characteris2cs Summary

Basin A

Δ

From

Exis2ng/Historic

Condi2ons

Basin Size 6.66 Acres 0 Acres

Imperviousness (%) 26.9% 2.3%

Proposed Minor Storm Runoff Q5 2.28 cfs 0.27 cfs

Proposed Major Storm Runoff Q100 9.75 cfs 0.63 cfs

III. STORMWATER QUALITY

Stormwater Quality

Stormwater quality facili4es or volumetric structures are not considered feasible for a linear trail project such as the proposed project where collec4on of a concentrated runoff would require significant modifica4ons to the grades and floodplain. However, the site naturally provides water quality treatment by runoff reduc4ons methods u4lizing exis4ng na4ve vegeta4on buffers also known as minimizing directly connected impervious areas (MDCIA) as defined by the Mile High Flood Districts (MHFD) Urban Storm Drainage Criteria Manual

(USDCM). The added impervious area associated with the newly paved trail is considered unconnected impervious area (UIA) and the adjacent na4ve vegeta4on buffer is considered Receiving pervious area (RPA).

Per USDCM guidance runoff reduc4on may be used as a standalone water quality treatment if the UIA: RPA rela4onship results in a reduc4on of the WQCV of 60%, as summarized below, the rela4onship between the site UIA: RPA results in a runoff reduc4on of 100% which exhibits adequate water quality treatment. A summary of the calcula4ons is shown below, and full calcula4ons are including in Appendix A:

Unconnected impervious area (UIA) = 14,753 K2

Receiving Pervious Area (RPA) = 64,342 K2

WQCV (Associated with new impervious area) = 615 K3

WQCV Reduc4on = 100%

Untreated WQCV = 0 <3

IV. FLOODPLAIN IMPACT

The proposed trail improvement project is located adjacent to Sand Creek and is within the Sand Creek Zone

AE floodplain with floodway. The trail alignment is generally matching the exis4ng trail alignment or includes minor fill in ineffec4ve flow areas (IEFAs); hydraulic analysis was completed to show no impact to the exis4ng

Sand Creek floodplain or floodway is an4cipated as part of the proposed project (See Appendix B for Floodplain

Impact informa4on). A statement of no impact to the Sand Creek floodplain and floodplain applica4on has been submiEed and approved on July 19, 2024.

V. VARIANCES

1. Stormwater Deten4on

A waiver/variance to stormwater deten4on requirements is requested due to the minimal increase in impervious areas and due to the project being a linear trail project. The project meets the requirements in sec4on 10.3 of the revised storm design and technical criteria sec4on for obtaining a variance for deten4on for a linear trail project. The new impervious area will drain to exis4ng vegetated RPA areas with a ra4o greater than 1:2 (UIA: RPA); the vegeta4on in this area is exis4ng and mature and disturbance to this area will be kept to a minimum or restored. The trail is designed to sheet flow to RPA areas except for a small por4on of the trail near the underpass in which flows will enter a grass swale prior to crossing the channel via pipe. The UIA:

RPA connec4on meets the requirements per MHFD USDCM Volume 3 for a runoff reduc4on (100% WQCV as men4oned previously), and the grass swale is also designed to meet the requirements in the USDCM Volume

3 (see calcula4ons in Appendix A) and has capacity to convey up to 100-year storm flows. In summary the project meets the requirements listed for the deten4on variance for linear trail projects.

In addi4on, the increase in runoff from exis4ng to proposed is 0.27cfs and 0.63cfs in the Minor (5-yr) and Major

(100-yr) storm events respec4vely. The minor increase in impervious area is not an4cipated to increase the peak runoff in this loca4on; in addi4on, since much of the improvements will occur in the floodplain, construc4on of deten4on is not recommended in prac4ce.

2. Grass Swale Longitudinal Slope < 2.0%

A variance is requested for a longitudinal slope of less than 2.0% along the east side of the proposed trail. The swale has a minimal required depth of 10-inches and generally needs to match the average slope of the adjacent channel grade as trail grades were specifically designed to result in matching or reducing exis4ng grade in a “no-fill” condi4on to the exis4ng floodplain. To keep the side slopes (3H:1V max with ECB protected slopes) the longitudinal grade needs to be less than 2.0% (~1.7%).

Further the grass swale is not intended to be a treatment BMP of the proposed impervious area and merely exists to route minor storms from offsite or upland flows around and below the proposed trail via culvert. The swale does not include an under drain.

VI. TRAIL CULVERT CROSSING

The plan includes removal and replacement of an exis4ng 18-inch RCP Culvert crossing located at one of the low points of the trail. There is currently not a safety grate located on the upstream end of the culvert crossing.

The plan does not include the installa4on of safety grate since the culvert is a rela4vely short straight segment of pipe (with the ability to see through to the downstream opening). Furthermore, the flow rates during a major storm (100-year) event are projected to be less than 2.0 CFS. Addi4onal guidance by MHFD is expected to be released in a technical memorandum in May 2024 to support not including safety gra4ng on a culvert of this nature.

VII. CONCLUSION

The proposed improvements to the Sand Creek Regional Greenway trail proposed will have negligible impact to exis4ng drainage on the site. As summarized in this report the overall drainage paEerns will be maintained and the impact to runoff quan4ty will be insignificant, and calcula4ons show the exis4ng vegeta4on that will be maintained and/or restored on site will provide stormwater quality via runoff reduc4on from the new impervious surfaces. The proposed development will not have any adverse effects on downstream proper4es/developments.

Please do not hesitate to contact me directly at (720) 667-2063 or eloewen@loeweneng.com. if there are any ques4ons or concerns regarding this leEer

Sincerely, Elysa M. Loewen, P.E.

Vice President

Loewen Engineering, Inc.

HORIZ. SCALE: 1" = 40'

80'40'0

Project Number:

File Name:

Revisions:

Date:

Reviewed by:

Designed by:

5962A

KJW

EML

JULY 2024

BEFORE YOU DIG, GRADE, OR EXCAVATE

FOR THE MARKING OF UNDERGROUND

MEMBER UTILITIES

CALL UTILITY NOTIFICATION

1-800-922-1987

CALL 2-BUSINESS DAYS IN ADVANCE

CENTER OF COLORADO

OR

M O

R R

IS

H

E

IG

H T

S F

LG

M B

T

Parks, Recreation & Open Space

Know what's below.

Call before you dig.

S A

N D

C R

E E

K T

R A

IL

I

M P

R O

V E

M E

N T

S

Drainage Plan.dwg

D R

AI

N

AG

E

PL

AN

08.15.2024

FACSIMILE

This electronic plan is a facsimile of the signed and sealed pdf set

Elysa M. Loewen Date

Appendix A Calcula2ons

Hydrologic Soil Group—Adams County Area, Parts of Adams and Denver Counties, Colorado; and Denver County Area, Colorado

Natural Resources Conservation Service

Web Soil Survey National Cooperative Soil Survey

9/20/2023

511900 512200 512500 512800 513100 513400 513700 514000 514300 514600

511900 512200 512500 512800 513100 513400 513700 514000 514300 514600

39° 45' 51'' N

4°

1'

6' ' W

39° 45' 51'' N

4°

9'

3'

' W

39° 44' 50'' N

4°

1'

6'

' W

39° 44' 50'' N

4°

9'

3'

' W

N

Map projection: Web Mercator Corner coordinates: WGS84 Edge tics: UTM Zone 13N WGS84 0 500 1000 2000 3000

Feet 0 150 300 600 900

Meters Map Scale: 1:13,400 if printed on A landscape (11" x 8.5") sheet.

Hydrologic Soil Information

MAP LEGEND MAP INFORMATION

Area of Interest (AOI) Area of Interest (AOI)

Soils Soil Rating Polygons

A

A/D

B

B/D

C

C/D

D

Not rated or not available

Soil Rating Lines A

A/D

B

B/D

C

C/D

D

Not rated or not available

Soil Rating Points A

A/D

B

B/D

C

C/D

D

Not rated or not available

Water Features Streams and Canals

Transportation Rails

Interstate Highways

US Routes

Major Roads

Local Roads

Background Aerial Photography

The soil surveys that comprise your AOI were mapped at scales ranging from 1:20,000 to 1:24,000.

Please rely on the bar scale on each map sheet for map measurements.

Source of Map: Natural Resources Conservation Service Web Soil Survey URL:

Coordinate System: Web Mercator (EPSG:3857)

Maps from the Web Soil Survey are based on the Web Mercator projection, which preserves direction and shape but distorts distance and area. A projection that preserves area, such as the Albers equal-area conic projection, should be used if more accurate calculations of distance or area are required.

This product is generated from the USDA-NRCS certified data as of the version date(s) listed below.

Soil Survey Area: Adams County Area, Parts of Adams and Denver Counties, Colorado Survey Area Data: Version 19, Sep 1, 2022

Soil Survey Area: Denver County Area, Colorado Survey Area Data: Version 4, Sep 1, 2022

Your area of interest (AOI) includes more than one soil survey area. These survey areas may have been mapped at different scales, with a different land use in mind, at different times, or at different levels of detail. This may result in map unit symbols, soil properties, and interpretations that do not completely agree across soil survey area boundaries.

Soil map units are labeled (as space allows) for map scales 1:50,000 or larger.

Date(s) aerial images were photographed: Jun 9, 2021—Jun 12, The orthophoto or other base map on which the soil lines were compiled and digitized probably differs from the background imagery displayed on these maps. As a result, some minor shifting of map unit boundaries may be evident.

Hydrologic Soil Group—Adams County Area, Parts of Adams and Denver Counties, Colorado; and Denver County Area, Colorado

Natural Resources Conservation Service

Web Soil Survey National Cooperative Soil Survey

9/20/2023

Hydrologic Soil Group

Map unit symbol Map unit name Rating Acres in AOI Percent of AOI

AsB Ascalon sandy loam, 0 to 3 percent slopes

B 128.9 15.1%

EgA Ellicott-Glenberg complex, 0 to 3 percent slopes, occasionally flooded

A 166.4 19.5%

Lu Loamy alluvial land B 219.8 25.8%

MISLD Gravel pits A 0.8 0.1%

Tc Terrace escarpments A 5.7 0.7%

TtB Truckton loamy sand, 0 to 3 percent slopes

A 12.8 1.5%

TtD Truckton loamy sand, 3 to 9 percent slopes

A 74.7 8.8%

TuB Truckton sandy loam, 0 to 3 percent slopes

A 3.5 0.4%

WmB Weld loam, 1 to 3 percent slopes

C 145.7 17.1%

Subtotals for Soil Survey Area 758.5 89.0%

Totals for Area of Interest 851.9 100.0%

Map unit symbol Map unit name Rating Acres in AOI Percent of AOI

NOTCOM No Digital Data Available

93.4 11.0%

Subtotals for Soil Survey Area 93.4 11.0%

Totals for Area of Interest 851.9 100.0%

Hydrologic Soil Group—Adams County Area, Parts of Adams and Denver Counties, Colorado;

and Denver County Area, Colorado

Natural Resources Conservation Service

Web Soil Survey National Cooperative Soil Survey

Description

Hydrologic soil groups are based on estimates of runoff potential. Soils are assigned to one of four groups according to the rate of water infiltration when the soils are not protected by vegetation, are thoroughly wet, and receive precipitation from long-duration storms.

The soils in the United States are assigned to four groups (A, B, C, and D) and three dual classes (A/D, B/D, and C/D). The groups are defined as follows:

Group A. Soils having a high infiltration rate (low runoff potential) when thoroughly wet. These consist mainly of deep, well drained to excessively drained sands or gravelly sands. These soils have a high rate of water transmission.

Group B. Soils having a moderate infiltration rate when thoroughly wet. These consist chiefly of moderately deep or deep, moderately well drained or well drained soils that have moderately fine texture to moderately coarse texture.

These soils have a moderate rate of water transmission.

Group C. Soils having a slow infiltration rate when thoroughly wet. These consist chiefly of soils having a layer that impedes the downward movement of water or soils of moderately fine texture or fine texture. These soils have a slow rate of water transmission.

Group D. Soils having a very slow infiltration rate (high runoff potential) when thoroughly wet. These consist chiefly of clays that have a high shrink-swell potential, soils that have a high water table, soils that have a claypan or clay layer at or near the surface, and soils that are shallow over nearly impervious material. These soils have a very slow rate of water transmission.

If a soil is assigned to a dual hydrologic group (A/D, B/D, or C/D), the first letter is for drained areas and the second is for undrained areas. Only the soils that in their natural condition are in group D are assigned to dual classes.

Rating Options

Aggregation Method: Dominant Condition

Component Percent Cutoff: None Specified

Tie-break Rule: Higher

Hydrologic Soil Group—Adams County Area, Parts of Adams and Denver Counties, Colorado;

and Denver County Area, Colorado

Natural Resources Conservation Service

Web Soil Survey National Cooperative Soil Survey

Project: 23-011 Sand Creek Trail

Date: Jan-24

By: EML

Area (sf): 289907.70 Area (acres): 6.66 Area (sf): 289907.70 Area (acres): 6.66

Surface Area (sf) Fraction of Basin % Imperviousness Weighted %

Imperviousness Surface Area (sf)

Fraction of Basin % Imperviousness

Weighted %

Imperviousness

Crusher Fines/Gravel/Rock 14495.00 0.05 40 Crusher Fines/Gravel/Rock 4368.00 0.02 40

Water 16292.20 0.06 100 Water 16292.20 0.06 100

Concrete 4198.60 0.01 95 Concrete 15520.70 0.05 95

Road/Bridge Deck 35983.21 0.12 95 Road/Bridge Deck 35983.21 0.12 95

Native Vegetation 218938.69 0.76 5 Native Vegetation 217743.59 0.75 5

Total Area 6.66 Total Area 6.66

Weighted Imperviousness 24.6% Weighted Imperviousness 26.9%

Basin Overall Imperviousness Calculations

Existing Proposed

Basin A Basin A

24.6 26.9

Undeveloped native vegetation

From CoA Storm Drainage

Criteria, Nov. 2023

Crusher Fines Trail

Concrete, Roadway, RTD Bridge

Imperviousness Calculations

9/22/23, 9:58 AM Precipitation Frequency Data Server https://hdsc.nws.noaa.gov/pfds/pfds_printpage.html?lat=39.7552&lon=-104.8472&data=depth&units=english&series=pds 1/4

NOAA Atlas 14, Volume 8, Version 2 Location name: Aurora, Colorado, USA* Latitude: 39.7552°, Longitude: -104.8472°

Elevation: 5336 ft**

* source: ESRI Maps

** source: USGS

POINT PRECIPITATION FREQUENCY ESTIMATES

Sanja Perica, Deborah Martin, Sandra Pavlovic, Ishani Roy, Michael St. Laurent, Carl Trypaluk, Dale Unruh, Michael Yekta, Geoffery Bonnin

NOAA, National Weather Service, Silver Spring, Maryland

PF_tabular | PF_graphical | Maps_&_aerials

PF tabular PDS-based point precipitation frequency estimates with 90% confidence intervals (in inches)1

Duration Average recurrence interval (years)

1 2 5 10 25 50 100 200 500 1000

5-min 0.222 (0.181‑0.274)

0.273 (0.222‑0.337)

0.365 (0.296‑0.452)

0.449 (0.362‑0.558)

0.576 (0.453‑0.751)

0.683 (0.522‑0.897)

0.797 (0.588‑1.07)

0.922 (0.651‑1.27)

1.10 (0.745‑1.54)

1.24 (0.816‑1.76)

10-min 0.325 (0.265‑0.401)

0.400 (0.325‑0.494)

0.534 (0.433‑0.661)

0.657 (0.530‑0.816)

0.843 (0.663‑1.10)

1.00 (0.764‑1.31)

1.17 (0.861‑1.57)

1.35 (0.953‑1.85)

1.61 (1.09‑2.26)

1.82 (1.19‑2.57)

15-min 0.396 (0.323‑0.489)

0.488 (0.397‑0.602)

0.652 (0.528‑0.806)

0.802 (0.646‑0.996)

1.03 (0.809‑1.34)

1.22 (0.932‑1.60)

1.42 (1.05‑1.91)

1.65 (1.16‑2.26)

1.96 (1.33‑2.76)

2.22 (1.46‑3.14)

30-min 0.558 (0.454‑0.688)

0.684 (0.556‑0.844)

0.909 (0.737‑1.12)

1.11 (0.898‑1.38)

1.42 (1.12‑1.85)

1.68 (1.29‑2.21)

1.96 (1.44‑2.63)

2.26 (1.60‑3.10)

2.69 (1.82‑3.78)

3.03 (1.99‑4.29)

60-min 0.699 (0.570‑0.863)

0.851 (0.692‑1.05)

1.12 (0.911‑1.39)

1.37 (1.11‑1.71)

1.75 (1.38‑2.28)

2.07 (1.58‑2.72)

2.42 (1.78‑3.24)

2.79 (1.97‑3.83)

3.32 (2.25‑4.67)

3.75 (2.46‑5.30)

2-hr 0.841 (0.689‑1.03)

1.02 (0.833‑1.25)

1.34 (1.09‑1.64)

1.63 (1.32‑2.01)

2.08 (1.65‑2.70)

2.46 (1.90‑3.21)

2.87 (2.13‑3.83)

3.32 (2.36‑4.52)

3.95 (2.70‑5.52)

4.47 (2.96‑6.27)

3-hr 0.921 (0.757‑1.12)

1.11 (0.910‑1.35)

1.45 (1.18‑1.77)

1.76 (1.44‑2.17)

2.25 (1.79‑2.90)

2.66 (2.05‑3.45)

3.10 (2.31‑4.11)

3.58 (2.56‑4.86)

4.27 (2.93‑5.93)

4.83 (3.21‑6.74)

6-hr 1.10 (0.905‑1.33)

1.31 (1.08‑1.59)

1.70 (1.40‑2.07)

2.06 (1.69‑2.52)

2.61 (2.08‑3.34)

3.07 (2.38‑3.95)

3.56 (2.67‑4.69)

4.10 (2.95‑5.52)

4.86 (3.36‑6.70)

5.48 (3.67‑7.60)

12-hr 1.34 (1.11‑1.61)

1.60 (1.33‑1.93)

2.07 (1.72‑2.50)

2.49 (2.05‑3.02)

3.11 (2.49‑3.93)

3.63 (2.83‑4.62)

4.17 (3.14‑5.43)

4.76 (3.44‑6.34)

5.58 (3.87‑7.61)

6.23 (4.20‑8.57)

24-hr 1.62 (1.36‑1.94)

1.94 (1.62‑2.32)

2.49 (2.07‑2.98)

2.96 (2.45‑3.57)

3.66 (2.94‑4.56)

4.22 (3.30‑5.31)

4.80 (3.63‑6.18)

5.41 (3.93‑7.13)

6.26 (4.37‑8.46)

6.94 (4.71‑9.46)

2-day 1.91 (1.60‑2.26)

2.27 (1.90‑2.69)

2.87 (2.40‑3.42)

3.39 (2.82‑4.05)

4.13 (3.33‑5.10)

4.72 (3.72‑5.89)

5.33 (4.05‑6.79)

5.96 (4.35‑7.78)

6.83 (4.80‑9.13)

7.50 (5.13‑10.1)

3-day 2.06 (1.73‑2.43)

2.44 (2.05‑2.88)

3.08 (2.58‑3.64)

3.62 (3.02‑4.31)

4.40 (3.56‑5.40)

5.01 (3.96‑6.22)

5.64 (4.31‑7.15)

6.29 (4.61‑8.17)

7.18 (5.06‑9.55)

7.87 (5.41‑10.6)

4-day 2.17 (1.83‑2.56)

2.57 (2.17‑3.03)

3.23 (2.72‑3.82)

3.80 (3.18‑4.50)

4.60 (3.72‑5.62)

5.23 (4.14‑6.47)

5.88 (4.50‑7.42)

6.55 (4.81‑8.47)

7.46 (5.27‑9.88)

8.16 (5.62‑11.0)

7-day 2.48 (2.11‑2.90)

2.90 (2.46‑3.40)

3.61 (3.05‑4.24)

4.20 (3.53‑4.95)

5.05 (4.11‑6.13)

5.71 (4.54‑7.02)

6.39 (4.92‑8.02)

7.09 (5.24‑9.11)

8.05 (5.72‑10.6)

8.78 (6.09‑11.7)

10-day 2.77 (2.36‑3.23)

3.20 (2.73‑3.74)

3.93 (3.33‑4.60)

4.55 (3.83‑5.34)

5.42 (4.42‑6.54)

6.10 (4.87‑7.46)

6.79 (5.25‑8.49)

7.51 (5.57‑9.61)

8.49 (6.06‑11.1)

9.24 (6.43‑12.3)

20-day 3.59 (3.08‑4.16)

4.08 (3.49‑4.73)

4.88 (4.16‑5.67)

5.56 (4.71‑6.47)

6.49 (5.33‑7.77)

7.22 (5.80‑8.74)

7.95 (6.18‑9.84)

8.71 (6.50‑11.0)

9.71 (6.98‑12.6)

10.5 (7.35‑13.8)

30-day 4.25 (3.65‑4.90)

4.81 (4.13‑5.55)

5.73 (4.90‑6.62)

6.49 (5.52‑7.52)

7.52 (6.19‑8.94)

8.31 (6.69‑10.0)

9.10 (7.09‑11.2)

9.89 (7.41‑12.5)

10.9 (7.89‑14.1)

11.7 (8.26‑15.4)

45-day 5.04 (4.34‑5.78)

5.74 (4.94‑6.59)

6.87 (5.89‑7.90)

7.78 (6.64‑8.98)

8.99 (7.41‑10.6)

9.90 (7.99‑11.8)

10.8 (8.43‑13.2)

11.7 (8.76‑14.6)

12.8 (9.24‑16.4)

13.6 (9.62‑17.7)

60-day 5.68 (4.91‑6.50)

6.53 (5.64‑7.48)

7.88 (6.78‑9.04)

8.96 (7.66‑10.3)

10.4 (8.55‑12.2)

11.4 (9.21‑13.6)

12.4 (9.70‑15.1)

13.4 (10.0‑16.6)

14.6 (10.6‑18.5)

15.4 (10.9‑20.0)

1 Precipitation frequency (PF) estimates in this table are based on frequency analysis of partial duration series (PDS).

Numbers in parenthesis are PF estimates at lower and upper bounds of the 90% confidence interval. The probability that precipitation frequency estimates (for a given duration and average recurrence interval) will be greater than the upper bound (or less than the lower bound) is 5%. Estimates at upper bounds are not checked against probable maximum precipitation (PMP) estimates and may be higher than currently valid PMP values.

Please refer to NOAA Atlas 14 document for more information.

Back to Top

PF graphical

Precipitation https://www.commerce.gov/ https://www.commerce.gov/ http://www.noaa.gov/ http://www.noaa.gov/ https://hdsc.nws.noaa.gov/pfds/pfds_printpage.html?lat=39.7552&lon=-104.8472&data=depth&units=english&series=pds 2/4

Back to Top

Maps & aerials

Small scale terrain https://hdsc.nws.noaa.gov/pfds/pfds_printpage.html?lat=39.7552&lon=-104.8472&data=depth&units=english&series=pds 3/4

Large scale terrain

Large scale map

Large scale aerial

3km

2mi

100km

60mi

100km

60mi https://hdsc.nws.noaa.gov/pfds/pfds_printpage.html?lat=39.7552&lon=-104.8472&data=depth&units=english&series=pds 4/4

Back to Top

US Department of Commerce National Oceanic and Atmospheric Administration

National Weather Service National Water Center

1325 East West Highway Silver Spring, MD 20910

Questions?: HDSC.Questions@noaa.gov

Disclaimer

100km

60mi https://www.commerce.gov/ http://www.noaa.gov/ http://www.weather.gov/ https://www.weather.gov/owp/oh mailto:HDSC.Questions@noaa.gov https://www.weather.gov/disclaimer

Designer:

Company: 2-yr 5-yr 10-yr 25-yr 50-yr 100-yr 500-yr

Date: 1-hour rainfall depth, P1 (in) = 1.12 2.42

Project: a b c

Location: Rainfall Intensity Equation Coefficients = 28.50 10.00 0.786

2-yr 5-yr 10-yr 25-yr 50-yr 100-yr 500-yr

Overland

Flow Length

Li (ft)

U/S Elevation

(ft)

(Optional)

D/S Elevation

(ft)

(Optional)

Overland

Flow Slope

Si (ft/ft)

Overland

Flow Time ti (min)

Channelized

Flow Length

Lt (ft)

U/S Elevation

(ft)

(Optional)

D/S Elevation

(ft)

(Optional)

Channelized

Flow Slope

St (ft/ft)

NRCS

Conveyance

Factor K

Channelized

Flow

Velocity

Vt (ft/sec)

Channelized

Flow Time tt (min)

Computed tc (min)

Regional tc (min)

Selected tc (min) 2-yr 5-yr 10-yr 25-yr 50-yr 100-yr 500-yr 2-yr 5-yr 10-yr 25-yr 50-yr 100-yr 500-yr

0.14 0.14 0.16 0.18 0.24 0.30 0.41 18.37 21.85 21.85 2.10 4.54 2.01 9.12

0.15 0.16 0.17 0.20 0.25 0.32 0.43 18.03 21.52 21.52 2.12 4.58 2.28 9.75

0.044300.00 5314.14 5301.00

Rainfall Intensity, I (in/hr)

2.28 3.48 26.02150.0235290.005301.00476.50

Peak Flow, Q (cfs)

Calculation of Peak Runoff using Rational Method

Overland (Initial) Flow Time Channelized (Travel) Flow Time Time of ConcentrationRunoff Coefficient, C

Subcatchment

Name

Area

(ac)

NRCS

Hydrologic

Soil Group

Percent

Imperviousnes s

Basin A - EX 6.66 A 24.6

Select UDFCD location for NOAA Atlas 14 Rainfall Depths from the pulldown list OR enter your own depths obtained from the NOAA website (click this link)

Cells of this color are for required user-input

Cells of this color are for optional override values

Cells of this color are for calculated results based on overrides

Kyli Whittle

Loewen Eng

1/8/2024

Sand Creek Trail Project

Aurora, CO

Version 2.00 released May 2017

Basin A - PRO 6.66 A 26.9 300.00 5314.14 5301.00 0.044 476.50 5301.00 5290.00 0.023 15 2.28 3.48 25.52

I ��/ℎ� = a ∗ P� b + t� t� =

0.395 1.1 − C� L�

S� t� = L�

60K S�

L�

60V�

Computed t� = t� + t�

Regional t� = 26 − 17i + L�

60 14i + 9 S�

Selected t� = max t3�4�353 , min Computed t� , Regional t� t3�4�353= 5 (urban) t3�4�353= 10 (non-urban)

Q 89: = CIA

Runoff Calculations

Worksheet Unprotected

Sheet 1 of 1

Designer:

Company:

Date:

Project:

Location:

SITE INFORMATION (User Input in Blue Cells)

WQCV Rainfall Depth 0.60 inches

Depth of Average Runoff Producing Storm, d6 = 0.43 inches (for Watersheds Outside of the Denver Region, Figure 3-1 in USDCM Vol. 3)

Area Type UIA:RPA

Area ID A

Downstream Design Point ID A

Downstream BMP Type None

DCIA (ft

UIA (ft ) 14,753

RPA (ft ) 64,342

SPA (ft

HSG A (%) 100%

HSG B (%) 0%

HSG C/D (%) 0%

Average Slope of RPA (ft/ft) 0.020

UIA:RPA Interface Width (ft) 1100.00

CALCULATED RUNOFF RESULTS

Area ID A

UIA:RPA Area (ft ) 79,095

L / W Ratio 0.07

UIA / Area 0.1865

Runoff (in) 0.00

Runoff (ft ) 0

Runoff Reduction (ft ) 615

CALCULATED WQCV RESULTS

Area ID A

WQCV (ft ) 615

WQCV Reduction (ft ) 615

WQCV Reduction (%) 100%

Untreated WQCV (ft ) 0

CALCULATED DESIGN POINT RESULTS (sums results from all columns with the same Downstream Design Point ID)

Downstream Design Point ID A

DCIA (ft ) 0

UIA (ft ) 14,753

RPA (ft ) 64,342

SPA (ft ) 0

Total Area (ft ) 79,095

Total Impervious Area (ft ) 14,753

WQCV (ft ) 615

WQCV Reduction (ft ) 615

WQCV Reduction (%) 100%

Untreated WQCV (ft ) 0

CALCULATED SITE RESULTS (sums results from all columns in worksheet)

Total Area (ft ) 79,095

Total Impervious Area (ft ) 14,753

WQCV (ft ) 615

WQCV Reduction (ft ) 615

WQCV Reduction (%) 100%

Untreated WQCV (ft ) 0

City of Aurora

Design Procedure Form: Runoff Reduction

Elysa M. Loewen

Loewen Engineering, Inc.

January 8, 2024

Sand Creek Trail Improvement Project

UD-BMP (Version 3.07, March 2018)

Water Quality - Runoff Reduction Calculations

Worksheet Protected

Area Type

DCIA

UIA:RPA

SPA

BMP Type

None

RG

EDB

SF

RP

CWP

UIA:RPA Runoff Equation based on Multivariable SWMM Analysis

(Technical Memorandum - March 15, 2018)

Constant P2 (in) Area (ft ) L/W Ratio Slope (ft/ft) UIA/Area (UIA/Area)

C0 C1 C2 C3 C4 C5 C6

A Soils 5.81E-01 -7.79E-01 -3.34E-07 -1.93E-03 7.03E-02 -2.49E+00 2.64E+00

B Soils -7.77E-02 -9.25E-01 -2.45E-07 -1.45E-03 5.02E-02 -1.36E-02 9.24E-01

C/D Soils -1.13E-02 -8.99E-01 -2.68E-07 -1.57E-03 5.45E-02 3.55E-01 4.64E-01

Where: P2 (in) = 2-hour WQCV Rainfall Depth

Area (ft ) = UIA:RPA area (sum of UIA and RPA)

L/W Ratio = Ratio of total flow length to width for UIA:RPA area

Slope (ft/ft) = average overland flow slope of RPA

UIA / Area = UIA / (UIA + RPA)

Equation Variable Constraints:

0.60 ≤ (UIA / Area) ≤ 1.0 (Type A Soils)

0.30 ≤ (UIA / Area) ≤ 1.0 (Type B Soils)

0.00 ≤ (UIA / Area) ≤ 1.0 (Type C/D Soils)

0.25 in ≤ P2 ≤ 0.95 in

1,000 ft ≤ Area ≤ 80,000 ft

0.0625 ≤ L/W Ratio ≤ 16.0

0.005 ft/ft ≤ Slope ≤ 0.333 ft/ft

Default Hydrologic Parameters used in SWMM Modeling

Pervious Impervious Pervious Impervious

A 5.0 2.52 1.0

B 4.5 6.48 0.6

C/D 3.0 6.48 0.5

0.10

Depression Storage

(inches)

Horton Infiltration

Initial Rate

(in/hr)

Decay Coeff

(hour -1

Final Rate

(in/hr)

Manning's n for

Overland Flow Hydrologic

Soil Group

0.10 0.01 0.10

Soil

Type

Runoff (in) = C0+(C1*(0.95 -P2(in)))+(C2*Area(ft ))+(C3*L/W Ratio)+(C4*Slope(ft/ft))+(C5*UIA/Area)+(C6*(UIA/Area)^2) a 0.91�� − 1.19�' + 0.78�

Designer:

Company: 2-yr 5-yr 10-yr 25-yr 50-yr 100-yr 500-yr

Date: 1-hour rainfall depth, P1 (in) = 0.84 1.12 1.36 1.74 2.05 2.39 3.28

Project: a b c

Location: Rainfall Intensity Equation Coefficients = 28.50 10.00 0.786

2-yr 5-yr 10-yr 25-yr 50-yr 100-yr 500-yr

Overland

Flow Length

Li (ft)

U/S Elevation

(ft)

(Optional)

D/S Elevation

(ft)

(Optional)

Overland

Flow Slope

Si (ft/ft)

Overland

Flow Time ti (min)

Channelized

Flow Length

Lt (ft)

U/S Elevation

(ft)

(Optional)

D/S Elevation

(ft)

(Optional)

Channelized

Flow Slope

St (ft/ft)

NRCS

Conveyance

Factor K

Channelized

Flow

Velocity

Vt (ft/sec)

Channelized

Flow Time tt (min)

Computed tc (min)

Regional tc (min)

Selected tc (min) 2-yr 5-yr 10-yr 25-yr 50-yr 100-yr 500-yr 2-yr 5-yr 10-yr 25-yr 50-yr 100-yr 500-yr

0.06 0.07 0.14 0.31 0.38 0.47 0.57 11.12 15.21 15.21 1.90 2.53 3.07 3.92 4.62 5.39 7.40 0.15 0.25 0.59 1.71 2.48 3.57 5.93

0.06 0.07 0.14 0.31 0.38 0.47 0.57 14.59 15.64 15.64 1.88 2.49 3.03 3.87 4.56 5.32 7.30 0.10 0.17 0.41 1.18 1.72 2.46 4.10

0.06 0.07 0.14 0.31 0.38 0.47 0.57 11.12 13.85 13.85 1.99 2.64 3.20 4.10 4.83 5.63 7.73 0.05 0.08 0.19 0.54 0.78 1.12 1.86

0.03 0.03 0.10 0.28 0.36 0.45 0.55 4.86 5.19 10.00 2.28 3.03 3.68 4.71 5.55 6.47 8.87 0.03 0.04 0.16 0.59 0.89 1.31 2.21

0.130194.00 5336.15 5311.00

Rainfall Intensity, I (in/hr)

1.30 4.09 28.23100.0175305.565311.00320.00

Peak Flow, Q (cfs)

Calculation of Peak Runoff using Rational Method

Overland (Initial) Flow Time Channelized (Travel) Flow Time Time of ConcentrationRunoff Coefficient, C

Subcatchment

Name

Area

(ac)

NRCS

Hydrologic

Soil Group

Percent

Imperviousnes s

12" HDPE 1.40 B 10.0

Select UDFCD location for NOAA Atlas 14 Rainfall Depths from the pulldown list OR enter your own depths obtained from the NOAA website (click this link)

Cells of this color are for required user-input

Cells of this color are for optional override values

Cells of this color are for calculated results based on overrides

Kyli Whittle

Loewen Engineering

1/8/2024

Sand Creek Trail

Sand Creek at Peoria Street

Version 2.00 released May 2017

15 2.33 1.06

(Combined

Swale A & B)

25.82 Swale A, West

(Max Flow)

0.98 B 10.0 231.41 5326.36 5309.14 0.074 147.92 5309.14 5305.56 0.024

28.235305.56 0.017 15 1.96 2.735336.15 5311.00 0.130 320.00 5311.00Swale B, East 0.42 B 10.0 194.00

25.49 Existing 18"

RCP

0.45 B 5.0 84.34 5354.01 5311.74 0.501 28.53 5311.74 5311.13 0.021 10 1.46 0.33

I ��/ℎ� = a ∗ P� b + t� t� =

0.395 1.1 − C� L�

S� t� = L�

60K S�

L�

60V�

Computed t� = t� + t�

Regional t� = 26 − 17i + L�

60 14i + 9 S�

Selected t� = max t3�4�353 , min Computed t� , Regional t� t3�4�353= 5 (urban) t3�4�353= 10 (non-urban)

Q 89: = CIA

Culvert and Swale Sizing

*The swale capacity was computed with the maximum drainage area (0.98 acres) into the west side of the swale (A), draining to the proposed area drain and HDPE pipe under the Peoria Street bridge. The swale was designed to reroute nuisance level flows. During a 100-yr storm, the swale will become inundated by Sand Creek since it is located within the 100-yr floodplain.

Freeboard is not required due to no adjacent structures near the swale.

Channel Report

Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Wednesday, Mar 13 2024

Trail Swale 5yr Capacity Check

Triangular Side Slopes (z:1) = 3.00, 3.00 Total Depth (ft) = 0.83

Invert Elev (ft) = 1.00 Slope (%) = 1.70 N-Value = 0.075

Calculations Compute by: Known Q Known Q (cfs) = 0.17

Highlighted Depth (ft) = 0.29 Q (cfs) = 0.170 Area (sqft) = 0.25 Velocity (ft/s) = 0.67 Wetted Perim (ft) = 1.83 Crit Depth, Yc (ft) = 0.19 Top Width (ft) = 1.74 EGL (ft) = 0.30

0 1 2 3 4 5 6 7

Elev (ft) Depth (ft) Section

0.75 -0.25

1.00 0.00

1.25 0.25

1.50 0.50

1.75 0.75

2.00 1.00

Reach (ft)

Trail Swale 10yr Capacity Check

Triangular Side Slopes (z:1) = 3.00, 3.00 Total Depth (ft) = 0.83

Invert Elev (ft) = 1.00 Slope (%) = 1.70 N-Value = 0.075

Calculations Compute by: Known Q Known Q (cfs) = 0.41

Highlighted Depth (ft) = 0.41 Q (cfs) = 0.410 Area (sqft) = 0.50 Velocity (ft/s) = 0.81 Wetted Perim (ft) = 2.59 Crit Depth, Yc (ft) = 0.26 Top Width (ft) = 2.46 EGL (ft) = 0.42

0 1 2 3 4 5 6 7

Elev (ft) Depth (ft) Section

0.75 -0.25

1.00 0.00

1.25 0.25

1.50 0.50

1.75 0.75

Trail Swale 50-yr Capacity Check

Triangular Side Slopes (z:1) = 3.00, 3.00 Total Depth (ft) = 0.83

Invert Elev (ft) = 1.00 Slope (%) = 1.70 N-Value = 0.075

Calculations Compute by: Known Q Known Q (cfs) = 1.72

Highlighted Depth (ft) = 0.69 Q (cfs) = 1.720 Area (sqft) = 1.43 Velocity (ft/s) = 1.20 Wetted Perim (ft) = 4.36 Crit Depth, Yc (ft) = 0.46 Top Width (ft) = 4.14 EGL (ft) = 0.71

0 1 2 3 4 5 6 7

Elev (ft) Depth (ft) Section

0.75 -0.25

1.00 0.00

1.25 0.25

1.50 0.50

1.75 0.75

Trail Swale 100-yr Capacity Check

Triangular Side Slopes (z:1) = 3.00, 3.00 Total Depth (ft) = 0.83

Invert Elev (ft) = 1.00 Slope (%) = 1.70 N-Value = 0.075

Calculations Compute by: Known Q Known Q (cfs) = 2.46

Highlighted Depth (ft) = 0.79 Q (cfs) = 2.460 Area (sqft) = 1.87 Velocity (ft/s) = 1.31 Wetted Perim (ft) = 5.00 Crit Depth, Yc (ft) = 0.53 Top Width (ft) = 4.74 EGL (ft) = 0.82

0 1 2 3 4 5 6 7

Elev (ft) Depth (ft) Section

0.75 -0.25

1.00 0.00

1.25 0.25

1.50 0.50

1.75 0.75

Project:

Pipe ID:

Design Information (Input) Pipe Invert Slope So = 0.0050 ft/ft

Pipe Manning's n-value n = 0.0120

Pipe Diameter D = 12.00 inches

Design discharge Q = 0.59 cfs

Full-Flow Capacity (Calculated)

Full-flow area Af = 0.79 sq ft

Full-flow wetted perimeter Pf = 3.14 ft

Half Central Angle Theta = 3.14 radians

Full-flow capacity Qf = 2.74 cfs

Calculation of Normal Flow Condition

Half Central Angle (0<Theta<3.14) Theta = 1.19 radians

Flow area An = 0.21 sq ft

Top width Tn = 0.93 ft

Wetted perimeter Pn = 1.19 ft

Flow depth Yn = 0.32 ft

Flow velocity Vn = 2.78 fps

Discharge Qn = 0.59 cfs

Percent of Full Flow Flow = 21.6% of full flow

Normal Depth Froude Number Frn = 1.02 supercritical

Calculation of Critical Flow Condition Half Central Angle (0<Theta-c<3.14) Theta-c = 1.20 radians

Critical flow area Ac = 0.22 sq ft

Critical top width Tc = 0.93 ft

Critical flow depth Yc = 0.32 ft

Critical flow velocity Vc = 2.73 fps

Critical Depth Froude Number Frc = 1.00

CIRCULAR CONDUIT FLOW (Normal & Critical Depth Computation)

Sand Creek Trail Pipe Under Trail Under Bridge

MHFD-Culvert, Version 4.00 (May 2020)

Project:

Pipe ID:

Design Information (Input) Pipe Invert Slope So = 0.0030 ft/ft

Pipe Manning's n-value n = 0.0130

Pipe Diameter D = 18.00 inches

Design discharge Q = 1.31 cfs

Full-Flow Capacity (Calculated)

Full-flow area Af = 1.77 sq ft

Full-flow wetted perimeter Pf = 4.71 ft

Half Central Angle Theta = 3.14 radians

Full-flow capacity Qf = 5.77 cfs

Calculation of Normal Flow Condition

Half Central Angle (0<Theta<3.14) Theta = 1.21 radians

Flow area An = 0.50 sq ft

Top width Tn = 1.40 ft

Wetted perimeter Pn = 1.82 ft

Flow depth Yn = 0.49 ft

Flow velocity Vn = 2.64 fps

Discharge Qn = 1.31 cfs

Percent of Full Flow Flow = 22.7% of full flow

Normal Depth Froude Number Frn = 0.78 subcritical

Calculation of Critical Flow Condition Half Central Angle (0<Theta-c<3.14) Theta-c = 1.13 radians

Critical flow area Ac = 0.42 sq ft

Critical top width Tc = 1.36 ft

Critical flow depth Yc = 0.43 ft

Critical flow velocity Vc = 3.15 fps

Critical Depth Froude Number Frc = 1.00

CIRCULAR CONDUIT FLOW (Normal & Critical Depth Computation)

Sand Creek Trail Existing Pipe Under Trail

MHFD-Culvert, Version 4.00 (May 2020)

Appendix B Floodplain Impact Memorandum

1 | P a g e

MEMORANDUM

To: Jared Coleman P.E., CFM – Project Engineer, Aurora Water/ Development Review

Craig Perl, P.E., CFM – Floodplain Administrator, City of Aurora

CC: Jessica Schafer – Landscape Designer| Planning, Design and Construction, City of Aurora

From: Elysa Loewen, P.E., for and on behalf of Loewen Engineering Inc.

Date: July 11, 2024

RE: Sand Creek Trail – Floodplain No-Impact Memorandum

BACKGROUND

The City of Aurora Parks Recreation and Open Space (PROS) has retained Loewen Engineering, Inc. to prepare design plans for improvement to approximately 1,140 linear feet of existing 12’ wide crusher fines trail. The improvement included replacing the existing crusher fines trail with a 10’ wide concrete paved trail and 3’ wide crusher fines shoulder to match the trail to the east and west of this section.

The current trail is located within Zone AE with Floodway Special Flood Hazard Area (SFHA) of

Sand Creek. The effective floodplain is based FEMA LOMR (RTD) LOMR (18-08-0814P) at Peoria as well as a downstream FEMA LOMR (10-08-0937P) related to Baranmoor Ditch.

Per discussion with Aurora staff on June 27, 2024, it was determined that the effective floodplain model should be used to analyze the floodplain impacts of this project.

HYDRAULIC ANALYSIS

Existing Conditions Model

Hydraulic analysis was performed using HEC-RAS version 6.4.1 software by the US Army Corps of

Engineers (USACE). An existing conditions model was developed in HEC-RAS using the effective model from the FEMA LOMR (18-08-0814P) provided to us by the Mile High Flood District

(MHFD). The existing conditions model was developed using effective information while incorporating best available topography data in our project area (ground survey collected as part of the Sand Creek Trail Project by City of Aurora Survey Section). The existing conditions model utilized effective cross sections from the effective model (LOMR 18-08-0814P) (exhibited in the attached floodplain workmap). Manning’s roughness values and 100-year flow rates were held consistent with those in the effective model.

2 | P a g e

Proposed Conditions Model

A proposed conditions model was developed from the existing model with changes to the updated topography from proposed grades. A comparison of the existing to proposed WSEL is summarized in the following table.

Table 1: 100-year WSEL Comparison Table

Existing

Conditions

Proposed

Conditions

Existing

Conditions

Proposed

Conditions

Existing

Conditions

Proposed

Conditions

COMPARISON

XS Station XS Station 100-Year

Flow (cfs)

100-Year

Flow (cfs)

100-Year

WSEL

100-Year

WSEL

PROPOSED

MINUS EXISTING

WSEL

71 71 29000 29000 5318.38 5318.38 0

70.4 70.4 29000 29000 5316.94 5316.94 0

70.3 70.3 29000 29000 5316.94 5316.94 0

70.2 70.2 29000 29000 5317.15 5317.14 -0.01

70.157 70.157 Culvert

70.15 70.15 29000 29000 5316.76 5316.74 -0.02

70.1 70.1 29000 29000 5315.61 5315.59 -0.02

70.03 70.03 29000 29000 5314.83 5314.81 -0.02

70.01 70.01 29000 29000 5315.72 5315.72 0

70 70 29000 29000 5315.76 5315.76 0

69.05 69.05 Bridge

68.1 68.1 29000 29000 5314.52 5314.52 0

68 68 29000 29000 5314.73 5314.73 0

67.75 67.75 Bridge

67.7 67.7 29000 29000 5314.22 5314.22 0

67.6 67.6 29000 29000 5311.66 5311.66 0

67.5 67.5 29000 29000 5311.15 5311.12 -0.03

67.4 67.4 29000 29000 5309.65 5309.65 0

67.3 67.3 29000 29000 5308.79 5308.79 0

67.2 67.2 29000 29000 5309.18 5309.18 0

Comparison between the existing model WSEL and the proposed WSEL results in a no-rise (0.00’) to the existing conditions. As noted above, four cross sections result in a decrease in WSEL to a maximum of -0.03’.

Ineffective Flow Areas (IEFA’s)

The model utilizes ineffective flow areas (IEFA’s) consistent with the effective model at the Peoria

Street Bridge. Ineffective flow areas are areas that are not included as part of active flow areas

3 | P a g e due to the obstructions caused by a bridge. All project fill areas are located with the IEFAs for the bridge and the remaining portion of the project either match or reduce project elevations.

ADDITIONAL FLOODPLAIN CONSIDERATIONS

Our team received the Sand Creek flood hazard area delineation (FHAD) from the Mile High Flood

District on July 26, 2023, which is currently underway as a floodplain study. The FHAD has not been completed or made effective through FEMA, however we were informed that the Model through our project area was complete and the FHAD was under final review. In addition, we have been provided the most updated floodplain delineations for this area by the FHAD consultant (ICON Engineering, Inc.) which have been updated and reflected in our floodplain workmap. In addition to showing a no-rise from existing to proposed conditions in the current effective model, the design of the new trail also eliminates fill within the FHAD 100-year floodplain boundary.

The FHAD is currently under Final Review and is not considered to be the effective model or best available information yet, so the current effective model was used in all hydraulic modelling efforts. But was important for our team to consider the floodplain impact of this project.

4 | P a g e

Below is also an Engineering “No Impact” Certification based on the proposed design.

Engineering “No-Impact” Certification

This statement certifies that I am a duly qualified engineer licensed to practice in the state of

Colorado. Further, it is to certify that the attached data supports that the Sand Creek

Improvement Project will not impact the base flood elevations, floodplain elevations, and floodplain widths on Sand Creek. This statement also certifies that this project will not affect flood elevations, floodplain elevations, and floodplain widths on cross sections unpublished, but involved in the proposed improvements.

SIGNED: _______________________________________

DATE: _________________________________________

Contact Information: Loewen Engineering, Inc

7388 Revere Parkway

Suite 601

Centennial CO, 80112

(720) 667-2063

Enclosure:

Sand Creek Trail Project Floodplain Workmap

Model Output

Cross Sections

LOMR (18-08-0814P) Floodplain Workmap (For Reference)

07.11.2024

National Flood Hazard Layer FIRMette

0 500 1,000 1,500 2,000250 Feet

Ü

SEE FIS REPORT FOR DETAILED LEGEND AND INDEX MAP FOR FIRM PANEL LAYOUT

SPECIAL FLOOD

HAZARD AREAS

Without Base Flood Elevation (BFE) Zone A, V, A99

With BFE or DepthZone AE, AO, AH, VE, AR

Regulatory Floodway

0.2% Annual Chance Flood Hazard, Areas of 1% annual chance flood with average depth less than one foot or with drainage areas of less than one square mileZone X

Future Conditions 1% Annual Chance Flood HazardZone X

Area with Reduced Flood Risk due to Levee. See Notes.Zone X

Area with Flood Risk due to LeveeZone D

NO SCREENArea of Minimal Flood HazardZone X

Area of Undetermined Flood HazardZone D

Channel, Culvert, or Storm Sewer

Levee, Dike, or Floodwall

Cross Sections with 1% Annual Chance

17.5 Water Surface Elevation

Coastal Transect

Coastal Transect Baseline Profile Baseline Hydrographic Feature

Base Flood Elevation Line (BFE)

Effective LOMRs

Limit of Study Jurisdiction Boundary

Digital Data Available

No Digital Data Available

Unmapped

This map complies with FEMA's standards for the use of digital flood maps if it is not void as described below.

The basemap shown complies with FEMA's basemap accuracy standards

The flood hazard information is derived directly from the authoritative NFHL web services provided by FEMA. This map was exported on 11/16/2023 at 2:44 PM and does not reflect changes or amendments subsequent to this date and time. The NFHL and effective information may change or become superseded by new data over time.

This map image is void if the one or more of the following map elements do not appear: basemap imagery, flood zone labels, legend, scale bar, map creation date, community identifiers, FIRM panel number, and FIRM effective date. Map images for unmapped and unmodernized areas cannot be used for regulatory purposes.

Legend

OTHER AREAS OF

FLOOD HAZARD

OTHER AREAS

GENERAL

STRUCTURES

OTHER

FEATURES

MAP PANELS

B 20.2

The pin displayed on the map is an approximate point selected by the user and does not represent an authoritative property location.

1:6,000

104°51'10"W 39°45'36"N

104°50'32"W 39°45'8"N

Basemap Imagery Source: USGS National Map 2023

Existing Gravel Trail will be replaced with proposed concrete trail in same alignment

Project Number:

File Name:

Revisions:

Date:

Reviewed by:

Designed by:

5962A

KJW

EML

June 2024

BEFORE YOU DIG, GRADE, OR EXCAVATE

FOR THE MARKING OF UNDERGROUND

MEMBER UTILITIES

CALL UTILITY NOTIFICATION

1-800-922-1987

CALL 2-BUSINESS DAYS IN ADVANCE

CENTER OF COLORADO

OR

M O

R R

IS

H

E

IG

H T

S F

LG

M B

T

Parks, Recreation & Open Space

Know what's below.

Call before you dig.

S A

N D

C R

E E

K T

R A

IL

I

M P

R O

V E

M E

N T

S

HORIZ. SCALE: 1" = 40'

80'40'0

HEC-RAS River: Sand Creek Reach: Sand Creek Park Profile: 100 YR

Reach River Sta Profile Plan Q Total Min Ch El W.S. Elev Crit W.S. E.G. Elev E.G. Slope Vel Chnl Flow Area Top Width Froude # Chl

(cfs) (ft) (ft) (ft) (ft) (ft/ft) (ft/s) (sq ft) (ft)

Sand Creek Park 71 100 YR PRO 29000.00 5303.18 5318.38 5315.94 5320.13 0.001519 12.11 3653.29 840.12 0.59

Sand Creek Park 71 100 YR EX 29000.00 5303.18 5318.38 5315.94 5320.13 0.001519 12.11 3653.29 840.12 0.59

Sand Creek Park 70.4 100 YR PRO 29000.00 5303.44 5316.94 5316.94 5319.67 0.003148 16.52 3378.45 806.26 0.85

Sand Creek Park 70.4 100 YR EX 29000.00 5303.44 5316.94 5316.94 5319.67 0.003148 16.52 3378.45 806.26 0.85

Sand Creek Park 70.3 100 YR PRO 29000.00 5301.54 5316.94 5316.94 5319.61 0.002570 15.92 3466.83 808.22 0.78

Sand Creek Park 70.3 100 YR EX 29000.00 5301.54 5316.94 5316.94 5319.61 0.002570 15.92 3466.83 808.22 0.78

Sand Creek Park 70.2 100 YR PRO 29000.00 5300.93 5317.14 5315.54 5318.90 0.001340 12.00 3949.78 850.98 0.57

Sand Creek Park 70.2 100 YR EX 29000.00 5300.93 5317.15 5315.54 5318.90 0.001336 11.98 3957.72 853.35 0.57

Sand Creek Park 70.157 Culvert

Sand Creek Park 70.15 100 YR PRO 29000.00 5300.93 5316.74 5315.52 5318.75 0.001555 12.75 3615.12 797.25 0.61

Sand Creek Park 70.15 100 YR EX 29000.00 5300.93 5316.76 5315.52 5318.76 0.001542 12.71 3628.54 798.01 0.61

Sand Creek Park 70.1 100 YR PRO 29000.00 5300.84 5315.59 5315.59 5318.52 0.002305 15.03 2961.83 707.09 0.73

Sand Creek Park 70.1 100 YR EX 29000.00 5300.84 5315.61 5315.61 5318.53 0.002247 15.01 2982.83 708.11 0.73

Sand Creek Park 70.03 100 YR PRO 29000.00 5300.51 5314.81 5314.09 5317.88 0.002470 15.84 2719.98 615.56 0.76

Sand Creek Park 70.03 100 YR EX 29000.00 5300.51 5314.83 5314.09 5317.88 0.002455 15.81 2727.41 616.58 0.76

Sand Creek Park 70.01 100 YR PRO 29000.00 5300.27 5315.72 5311.86 5317.26 0.001157 10.64 3282.95 586.12 0.52

Sand Creek Park 70.01 100 YR EX 29000.00 5300.27 5315.72 5311.87 5317.26 0.001163 10.66 3272.42 2187.17 0.52

Sand Creek Park 70 100 YR PRO 29000.00 5300.18 5315.76 5311.04 5317.14 0.000952 10.01 3509.69 668.37 0.48

Sand Creek Park 70 100 YR EX 29000.00 5300.18 5315.76 5311.04 5317.14 0.000956 10.03 3500.13 1962.17 0.48

Sand Creek Park 69.05 Bridge

Sand Creek Park 68.1 100 YR PRO 29000.00 5299.93 5314.52 5310.44 5316.24 0.001192 11.36 3223.80 292.18 0.54

Sand Creek Park 68.1 100 YR EX 29000.00 5299.93 5314.52 5310.48 5316.25 0.001200 11.39 3210.50 292.16 0.54

Sand Creek Park 68 100 YR PRO 29000.00 5299.88 5314.73 5310.15 5315.98 0.000872 9.62 3771.63 409.97 0.46

Sand Creek Park 68 100 YR EX 29000.00 5299.88 5314.73 5310.14 5315.98 0.000874 9.63 3766.22 409.94 0.46

Sand Creek Park 67.75 Bridge

Sand Creek Park 67.7 100 YR PRO 29000.00 5299.88 5314.22 5310.42 5315.79 0.001125 10.58 3324.94 643.80 0.51

Sand Creek Park 67.7 100 YR EX 29000.00 5299.88 5314.22 5310.35 5315.79 0.001128 10.59 3320.68 643.76 0.51

Sand Creek Park 67.6 100 YR PRO 29000.00 5300.34 5311.66 5311.66 5315.20 0.004384 15.66 2132.73 567.59 0.95

Sand Creek Park 67.6 100 YR EX 29000.00 5300.34 5311.66 5311.66 5315.20 0.004383 15.66 2132.74 568.62 0.95

Sand Creek Park 67.5 100 YR PRO 29000.00 5296.34 5311.12 5311.12 5314.75 0.004249 15.80 2104.81 549.97 0.93

Sand Creek Park 67.5 100 YR EX 29000.00 5296.34 5311.15 5311.15 5314.76 0.004200 15.75 2112.44 550.42 0.93

Sand Creek Park 67.4 100 YR PRO 29000.00 5296.34 5309.65 5309.65 5313.37 0.004766 15.55 1926.06 288.10 0.98

Sand Creek Park 67.4 100 YR EX 29000.00 5296.34 5309.65 5309.65 5313.37 0.004766 15.55 1926.06 288.10 0.98

Sand Creek Park 67.3 100 YR PRO 29000.00 5292.24 5308.79 5308.79 5312.79 0.004864 16.12 1854.35 257.27 0.98

Sand Creek Park 67.3 100 YR EX 29000.00 5292.24 5308.79 5308.79 5312.79 0.004864 16.12 1854.35 257.27 0.98

Sand Creek Park 67.2 100 YR PRO 29000.00 5291.81 5309.18 5306.21 5311.46 0.001502 12.85 2857.19 292.43 0.60

Sand Creek Park 67.2 100 YR EX 29000.00 5291.81 5309.18 5306.21 5311.46 0.001502 12.85 2857.19 292.43 0.60

-200 0 200 400 600 800 1000 1200

SAND_CREEK_TRAIL Plan: 1) EX 2) PRO

RS 71 - Sta. 26+70

Station (ft)

E le v a ti o n ft

Legend

EG 100 YR - PRO

EG 100 YR - EX

WS 100 YR - EX

WS 100 YR - PRO

Crit 100 YR - PRO

Crit 100 YR - EX

- PRO

- PRO

Ground - PRO

Ineff - PRO

Bank Sta - PRO

- EX

- EX

Ground - EX

Ineff - EX

Bank Sta - EX

SAND_CREEK_TRAIL Plan: 1) EX 2) PRO

RS 70.4 - Sta. 24+97

Station (ft)

E le v a ti o n ft

Legend

EG 100 YR - PRO

EG 100 YR - EX

Crit 100 YR - PRO

WS 100 YR - EX

WS 100 YR - PRO

Crit 100 YR - EX

- PRO

- PRO

Ground - PRO

Ineff - PRO

Bank Sta - PRO

- EX

- EX

Ground - EX

SAND_CREEK_TRAIL Plan: 1) EX 2) PRO

RS 70.3 - Sta. 24+92

Station (ft)

E le v a ti o n ft

Legend

EG 100 YR - PRO

EG 100 YR - EX

Crit 100 YR - PRO

WS 100 YR - EX

WS 100 YR - PRO

Crit 100 YR - EX

- PRO

- PRO

Ground - PRO

Ineff - PRO

Bank Sta - PRO

- EX

- EX

Ground - EX

SAND_CREEK_TRAIL Plan: 1) EX 2) PRO

RS 70.2 - Sta. 23+95

Station (ft)

E le v a ti o n ft

Legend

EG 100 YR - EX

EG 100 YR - PRO

WS 100 YR - EX

WS 100 YR - PRO

Crit 100 YR - PRO

Crit 100 YR - EX

- PRO

- PRO

Ground - PRO

Ineff - PRO

Bank Sta - PRO

- EX

- EX

Ground - EX

SAND_CREEK_TRAIL Plan: 1) EX 2) PRO

Station (ft)

E le v a ti o n ft

Legend

EG 100 YR - EX

EG 100 YR - PRO

WS 100 YR - EX

WS 100 YR - PRO

Crit 100 YR - PRO

Crit 100 YR - EX

- PRO

- PRO

- PRO

- PRO

- PRO

Ground - PRO

Ineff - PRO

Bank Sta - PRO

- EX

- EX

- EX

- EX

- EX

Ground - EX

SAND_CREEK_TRAIL Plan: 1) EX 2) PRO

RS 70.15 - Sta. 23+85

Station (ft)

E le v a ti o n ft

Legend

EG 100 YR - EX

EG 100 YR - PRO

WS 100 YR - EX

WS 100 YR - PRO

Crit 100 YR - PRO

Crit 100 YR - EX

- PRO

- PRO

Ground - PRO

Ineff - PRO

Bank Sta - PRO

- EX

- EX

Ground - EX

-500 0 500 1000 1500 2000 2500

SAND_CREEK_TRAIL Plan: 1) EX 2) PRO

RS 70.1 - Sta. 23+10

Station (ft)

E le v a ti o n ft

Legend

EG 100 YR - EX

EG 100 YR - PRO

WS 100 YR - EX

Crit 100 YR - EX

WS 100 YR - PRO

Crit 100 YR - PRO

- PRO

- PRO

Sediment Fill - PRO

Ground - PRO

Ineff - PRO

Bank Sta - PRO

- EX

- EX

Sediment Fill - EX

Ground - EX

SAND_CREEK_TRAIL Plan: 1) EX 2) PRO

RS 70.03 - Sta. 21+60, New section u/s of Peoria for more gradu

Station (ft)

E le v a ti o n ft

Legend

EG 100 YR - EX

EG 100 YR - PRO

WS 100 YR - EX

WS 100 YR - PRO

Crit 100 YR - PRO

Crit 100 YR - EX

- PRO

- PRO

Sediment Fill - PRO

Ground - PRO

Ineff - PRO

Bank Sta - PRO

- EX

- EX

Sediment Fill - EX

Ground - EX

0 500 1000 1500 2000 2500

SAND_CREEK_TRAIL Plan: 1) EX 2) PRO

RS 70.01 - Sta 20+60

Station (ft)

E le v a ti o n ft

Legend

EG 100 YR - EX

EG 100 YR - PRO

WS 100 YR - PRO

WS 100 YR - EX

Crit 100 YR - EX

Crit 100 YR - PRO

- PRO

- PRO

Sediment Fill - PRO

Ground - PRO

Ineff - PRO

Bank Sta - PRO

- EX

- EX

Sediment Fill - EX

Ground - EX

SAND…