Attachment_7_Dixie_NF_Supervisors_&_Cedar_City_RD_Office_Drainage_Miti....pdf

DIXIE NF SUPERVISOR’S

AND CEDAR CITY RANGER

DISTRICT OFFICE

DIXIE NATIONAL FOREST

DRAINAGE MITIGATION REPORT

March 8, 2019

Prepared for:

USDA Forest Service Intermountain Region (R4) Facilities

Engineering Design Group 324 25th Street

Ogden UT 84401

Prepared by:

Watson Engineering Company, Inc.

472 North 2150 West, Suite 7

Cedar City, UT 84721

In collaboration with:

Case, Lowe & Hart, Inc.

2484 Washington Boulevard, Suite 510

Ogden, UT 84401

472 North 2150 West, Suite 7 | Cedar City, Utah 84721 Office: 435.586.3004 Fax: 435.586.7480 Web: www.wecinc.com

Drainage Mitigation Report

PROJECT: Dixie National Forest DATE: March 8, 2019

Ranger District Office

Cedar City, Utah WEC Project #: 18-3302

TO: Case, Lowe & Hart, Inc.

2484 Washington Blvd, Ste. 510 PHONE: (801) 399-5821

Ogden, Utah 84401 FAX:

ATTN: Kevin J. Lewis EMAIL: kevinl@clhae.com

REMARKS:

Property Information:

Within the NW¼ & NE¼ of Section 11, Township 36 South, Range 11 West, Salt Lake Meridian, Cedar City, Iron County, Utah Project Address: Main Street & 900 North Street Parcel #: B-0067-0001-0000 Acct. #: 0047586 Land area: 5.42 acres (developing 3.73 acres) Flood Zone C; FIRM Map 490074 0004 B

This report is being submitted to you as part of the proposed development for the Dixie National Forest

Supervisor’s and Cedar City Ranger District Office to be constructed near the intersection of Main Street and 900

North Street in Cedar City, Utah. It is understood that the development will consist of a new building, picnic areas, drive lanes, parking lots and landscape areas. The new building is proposed to be an approximately 15,850 square foot single-story, wood-framed structure with a slab-on-grade foundation.

Drainage Mitigation Letter Page | 2

The development is proposed for 3.73 acres on the west portion of the 5.42 acre lot (see Figure 1). The lot is bounded on the west by Main Street, on the north and south by existing commercial and undeveloped properties, and on the east by a golf course. There are three (3) existing buildings on the property that will be removed for the new development. There are existing public utilities installed along with the concrete curb and gutter along the west side of the property on Main Street. There is a ninety-nine foot (99’) wide Right-of-Way (ROW) for Main Street.

The property is proposed to have an interior drive lanes, which will run east to west, to provide access to the parking lots and building in the development. The property will have street access from Main Street at the west of the site and from 100 East Street at the southeast of the site. The proposed site design includes two (2) site detention ponds located at the northwest and the southwest corners of the property that will collect the majority of the drainage runoff from the site.

Approximately ninety percent (95%) of the drainage runoff generated from the built-up, fully developed site is designed to be directed to the two (2) proposed detention ponds for the property. The majority of runoff, approximately seventy percent (70%), will be coming from the east of the site and will be directed to valley gutters that are designed to direct the runoff to the detention pond at the southwest of the property. This includes the runoff coming from the undeveloped, east portion of the site that has an area of 1.69 acres (see Figure 1). The southwest pond will collect the runoff and channel it to the pond at the northwest of the property via a six inch

(6”) storm drain pipe. At the northwest portion of the site, the runoff, approximately twenty-five percent (25%), will be directed to a valley gutter, running to the north, directly to the northwest pond. From the northwest pond, the runoff will be directed to the curb and gutter system along Main Street via a six inch (6”) storm drain pipe. The remaining five percent (5%) of runoff, being generated at the area near the site access, is designed to go directly to the curb and gutter system along Main Street.

The southwest pond, the six inch (6”) storm drain pipe connecting the ponds, and the northwest pond, as a system, will be designed to detain the drainage runoff for a 25 year, 24-hr storm event. Emergency overflow systems will be designed to detain and discharge the drainage runoff for a 100 year, 24-hr storm event. A larger diameter storm drain pipe will be designed to discharge runoff to Main Street from the northwest pond as part of the emergency overflow system.

Page | 3

The following is the storm drain pipe calculation using the Chezy-Manning’s Equation to determine the capacity of the proposed 6” PVC storm drain pipe discharging runoff to Main Street from the northwest pond:

6-inch Diameter Pipe:

𝑄

𝐴𝑅 √𝑆 = 0.99 cfs n = 0.009 (PVC) A = 0.196 sft R = 0.125; D/4 (Hydraulic Radius)

S = 1.5% (0.015)

The USDA Urban Hydrology for Small Watersheds TR-55 manual and design method was used to calculate the design values for the Existing Conditions for the entire property and the Existing Conditions for the area to be developed, and the Developed, Built-up Conditions for the entire property and the area to be developed. The design point precipitation frequency estimates used for these calculations was obtained from Cedar City

Engineering Department Rainfall Data. The following values were used for this design (calculations attached):

Property Condition

Hydrologic Soil Group

Runoff Curve

Number

Average Recurrence Interval (24-hr Duration) (Inches)

10-Year 25-Year 100-Year Existing/Present

(3.73 acres) C 87.9 1.41 1.75 2.33

Existing/Present (5.42 acres)

C 89.1 1.41 1.75 2.33

Built-Up/Fully Developed (3.73 acres)

C 89.9 1.41 1.75 2.33

Built-Up/Fully Developed (5.42 acres)

C 89.7 1.41 1.75 2.33

The Existing Conditions runoff calculations utilized conditions including areas for the existing buildings, pavement, dirt roads and open areas. The Built-Up Conditions runoff calculations averaged the land use areas for the new building, parking lot, drive lanes and landscaping. The Runoff Curve Numbers listed are from the TR-55 manual.

Page | 4

Property Condition

Peak Discharge (24-hr Duration) (cfs)

10-Year 25-Year 100-Year Existing/Present

(3.73 acres)

2.3 3.5 5.8

Existing/Present (5.42 acres)

3.1 4.7 7.8

Built-Up/Fully Developed (3.73 acres)

2.9 4.2 6.8

Built-Up/Fully Developed (5.42 acres)

3.5 5.1 8.2

The following table lists the final drainage runoff and runoff volumes for the fully developed property within the

3.73 acre area:

Volume Type Built-Up Runoff (24-hr Duration)

10-Year 25-Year 100-Year Runoff, Q (in) 0.607 0.876 1.370

Runoff Vol. (cft.) 8,217 11,866 18,555 Runoff Vol. (Ac-ft) 0.189 0.272 0.426

The following table lists the final drainage runoff and runoff volumes for the fully developed property within the

3.73 acre area and the 1.69 area that is going to remain undeveloped, for a total of 5.42 acres:

Volume Type Built-Up Runoff (24-hr Duration)

10-Year 25-Year 100-Year Runoff, Q (in) 0.600 0.868 1.360

Runoff Vol. (cft.) 11,796 17,071 26,753 Runoff Vol. (Ac-ft) 0.271 0.392 0.614

Page | 5

In conclusion, the proposed development of this property will produce an increase drainage runoff of approximately 0.4 cfs for a 25 year, 24-hr storm event for the entire 5.42 acre area. The total built-up runoff volume for the 3.73 acre area has been calculated to be 11,866 cft. (0.272 Ac-ft) for a 25 year, 24-hr storm event. The total built-up runoff volume for the entire property (5.42 acres) has been calculated to be 17,071 cft. (0.392 Ac-ft) for a

25 year, 24-hr storm event.

Watson Engineering Company, Inc. appreciates the opportunity to provide this information to you for this project.

If you have any additional questions, please do not hesitate to contact us (435-586-3004).

Signed:

Date: March 8, 2019

Kyle T. Kump, E.I.T., M.S.I., C.W.I.

Project Engineer / Special Inspector

Reviewed:

Date:

Tim G. Watson, S.E., P.E., P.L.S.

President / Principal

March 8, 2019

Mar. 8, 2019

Page | 6

Figure 1 – View showing the property and the area being developed and the area remaining undeveloped.

DATE : 3/8/2019

3.73 16.2

0.023

TABLE 2-2 FIGURE 2-3

C 98 15% C 87 10% C 86 75%

RUNOFF CURVE NUMBER, CN : 87.9

0.20 (SCS LAG) 8644.82437

II

FREQUENCY (YR) 10 25 100

RAINFALL (24-HR), P (IN) 1.41 1.75 2.33

INITIAL ABSTRACTION, Ia (IN) = 0.299 0.299 0.299 Ia/P = 0.212 0.171 0.128

UNIT PEAK DISCHARGE, qu (CSM/IN) = 775 795 810

RUNOFF, Q (IN) = 0.513 0.763 1.230

POND & SWAMP ADJ., Fp (%) = 0.0 0.0 0.0 PEAK DISCHARGE, qp (CFS) = 2.3 3.5 5.8

FREQUENCY (YR) 10 25 100

RUNOFF VOLUME (CF) 6,942 10,327 16,659

LENGTH (FT) :

COVER DESCRIPTION

URBAN HYDROLOGY FOR SMALL WATERSHEDS (TR-55)

GRAPHICAL PEAK DISCHARGE

CONDITION : PRESENT

CNHYDR. SOIL

GROUP

ORIGINAL PROPERTY CONDITION - 3.73 ACRES

WATERSHED DATA

DESCRIPTION :

AREA

IMPERVIOUS AREA (DIRT ROADS)

IMPERVIOUS AREA (ROOFS & PVMNT)

SLOPE (FT/FT) :

FOREST SERVICE

S11, T36S, R11W, SLM

CEDAR CITY, IRON COUNTY, UTAH

RAINFALL DISTRIBUTION :

TIME OF CONCENTRATION, TC (HR) =

ELEV. DIFF (FT) :

OPEN AREA (<50% GRASS)

DRAINAGE AREA (Ac) :

PROJECT :

LOCATION :

HydrologyTR55082806 - Forest Service, Cedar City © WATSON ENGINEERING COMPANY, INC.

5.42 20.5

0.019

TABLE 2-2 FIGURE 2-3

C 98 25% C 87 5% C 86 70%

RUNOFF CURVE NUMBER, CN : 89.1

0.30 (SCS LAG) 14909.5745

II

FREQUENCY (YR) 10 25 100

RAINFALL (24-HR), P (IN) 1.41 1.75 2.33

INITIAL ABSTRACTION, Ia (IN) = 0.247 0.247 0.247 Ia/P = 0.175 0.141 0.106

UNIT PEAK DISCHARGE, qu (CSM/IN) = 650 675 700

RUNOFF, Q (IN) = 0.566 0.828 1.311

POND & SWAMP ADJ., Fp (%) = 0.0 0.0 0.0 PEAK DISCHARGE, qp (CFS) = 3.1 4.7 7.8

FREQUENCY (YR) 10 25 100

RUNOFF VOLUME (CF) 11,138 16,281 25,788

TIME OF CONCENTRATION, TC (HR) =

RAINFALL DISTRIBUTION :

IMPERVIOUS AREA (ROOFS & PVMNT)

IMPERVIOUS AREA (DIRT ROADS)

OPEN AREA (<50% GRASS)

LENGTH (FT) :

SLOPE (FT/FT) :

HYDR. SOIL

GROUP

COVER DESCRIPTION

CN

AREA

CEDAR CITY, IRON COUNTY, UTAH

WATERSHED DATA

DESCRIPTION : ORIGINAL PROPERTY CONDITION - 5.42 ACRES DRAINAGE AREA (Ac) :

CONDITION : PRESENT ELEV. DIFF (FT) :

URBAN HYDROLOGY FOR SMALL WATERSHEDS (TR-55)

GRAPHICAL PEAK DISCHARGE

PROJECT : FOREST SERVICE

LOCATION : S11, T36S, R11W, SLM

3.73 17.0

0.022

TABLE 2-2 FIGURE 2-3

C 98 57.2% C 79 42.8%

RUNOFF CURVE NUMBER, CN : 89.9

0.20 (SCS LAG)

II

FREQUENCY (YR) 10 25 100

RAINFALL (24-HR), P (IN) 1.41 1.75 2.33

INITIAL ABSTRACTION, Ia (IN) = 0.247 0.247 0.247 Ia/P = 0.175 0.141 0.106

UNIT PEAK DISCHARGE, qu (CSM/IN) = 810 825 850

RUNOFF, Q (IN) = 0.607 0.876 1.370

POND & SWAMP ADJ., Fp (%) = 0.0 0.0 0.0 PEAK DISCHARGE, qp (CFS) = 2.9 4.2 6.8

FREQUENCY (YR) 10 25 100

RUNOFF VOLUME (CF) 8,217 11,866 18,555

RAINFALL DISTRIBUTION :

IMPERVIOUS AREAS (PARKING LOT, ROOF)

OPEN SPACE, FAIR (50-75% GRASS)

TIME OF CONCENTRATION, TC (HR) =

LENGTH (FT) :

HYDR. SOIL

GROUP

COVER DESCRIPTION

CN

AREA

SLOPE (FT/FT) :

CEDAR CITY, IRON COUNTY, UTAH

WATERSHED DATA

DESCRIPTION : PROPOSED DEVELOPMENT - 3.73 ACRES DRAINAGE AREA (Ac) :

CONDITION : ON-SITE BUILT UP ELEV. DIFF (FT) :

URBAN HYDROLOGY FOR SMALL WATERSHEDS (TR-55)

GRAPHICAL PEAK DISCHARGE

PROJECT : FOREST SERVICE

LOCATION : S11, T36S, R11W, SLM

0 484.676 1294.085

P e a k

D is c h a rg e c fs

Time (sec.)

Triangular Hydrograph

10 Yr

25 Yr

100 Yr

5.42 21.3

0.018

TABLE 2-2 FIGURE 2-3

C 98 45.5% C 79 25.5% C 87 5.0% C 86 24%

RUNOFF CURVE NUMBER, CN : 89.7

0.31 (SCS LAG)

II

FREQUENCY (YR) 10 25 100

RAINFALL (24-HR), P (IN) 1.41 1.75 2.33

INITIAL ABSTRACTION, Ia (IN) = 0.247 0.247 0.247 Ia/P = 0.175 0.141 0.106

UNIT PEAK DISCHARGE, qu (CSM/IN) = 680 695 710

RUNOFF, Q (IN) = 0.600 0.868 1.360

POND & SWAMP ADJ., Fp (%) = 0.0 0.0 0.0 PEAK DISCHARGE, qp (CFS) = 3.5 5.1 8.2

FREQUENCY (YR) 10 25 100

RUNOFF VOLUME (CF) 11,796 17,071 26,753

TIME OF CONCENTRATION, TC (HR) =

RAINFALL DISTRIBUTION :

IMPERVIOUS AREAS (PARKING LOT, ROOF)

OPEN SPACE, FAIR (50-75% GRASS)

IMPERVIOUS AREA (DIRT ROADS)

OPEN AREA (<50% GRASS)

LENGTH (FT) :

SLOPE (FT/FT) :

HYDR. SOIL

GROUP

COVER DESCRIPTION

CN

AREA

CEDAR CITY, IRON COUNTY, UTAH

WATERSHED DATA

DESCRIPTION : PROPOSED DEVELOPMENT - 5.42 ACRES DRAINAGE AREA (Ac) :

CONDITION : ON-SITE BUILT UP ELEV. DIFF (FT) :

URBAN HYDROLOGY FOR SMALL WATERSHEDS (TR-55)

GRAPHICAL PEAK DISCHARGE

PROJECT : FOREST SERVICE

LOCATION : S11, T36S, R11W, SLM

0 747.460 1995.717

P e a k

D is c h a rg e c fs

Time (sec.)

Triangular Hydrograph

10 Yr

25 Yr

100 Yr