NV FLAP 39(1) Hydraulics Memoradum.pdf

Subject: NV FLAP 39(1), Kings Canyon Road Final Hydraulics Memo

From: Megan Frye Hydraulic Engineer

FHWA-Central Federal Lands Lakewood, CO

To: Emilio Burgos Project Manager Lakewood, CO

This memorandum presents the field site observations, design analyses, and hydraulic recommendations for the proposed project along Kings Canyon Road, located 2 miles southwest of downtown Carson City, NV. Kings Canyon Road is primarily used as a recreational route providing access to the Lake Tahoe-Nevada State Park. Figure 1 provides a project location map for reference.

Figure 1. Kings Canyon Road

Carson City has expressed a desire to improve roadway safety and increase parking capacity at a high-use trailhead along Kings Canyon Road. The roadway is narrow, ranging from 21 – 23 feet wide and the asphalt is in poor condition. The drainage structures are undersized and the road experiences annual flooding which contributes to non-point source pollution and further degradation of the road and trailhead. The existing parking area is routinely overflowing with vehicles, forcing visitors to park along the narrow stretch of road leading to the trailhead.

Memorandum

Date: January 28, 2020

Kings Canyon Road

The Central Federal Lands Highway Division, in cooperation with Carson City County and the Humboldt- Toiyabe National Forest, plans to widen 0.73 miles of Kings Canyon Road to a paved width of 28 feet.

Adequate drainage structures will be implemented at stream crossings and road intersections. Trailhead improvements will be completed on the existing parking area and include the construction of a new parking area with bathroom facilities.

This memo addresses the hydraulic recommendations associated with the proposed project. The following items will be addressed:

Hydrologic and hydraulic criteria and computational methods

Hydraulic analysis and recommendations for drainage structures.

DESIGN CRITERIA

The current (2019) and projected (2039) average daily traffic counts (ADT) on Kings Canyon Road are 600 and 892, respectively. The design criteria for Kings Canyon Road is based on the criteria defined for a low standard road by the Project Development and Delivery Manual. As such, the following design criteria apply:

Culvert Design (48” or greater)

Capacity: 25-year flood

Headwater/Depth Ratio: 1.5 (< 48” culverts), 1.2 (≥48” culverts)

Roadside Ditch Capacity/Stability Design

Capacity: 10-year flood

Stability: 10-year flood

HYDROLOGIC ANALYSIS

A detailed analysis of the hydrology that covers the project area was completed by HDR as part of a restudy for portions of Kings Canyon Creek, Kings Split, Ash Canyon Creek, Vicee Canyon Creek, and Combs Canyon Creek. The study is referred to as the Carson City Restudy and was completed in 2010.

The parameters developed in this study were used to develop the hydrologic analysis for the project.

The two major drainage crossings within the project area are the Kings Split and the Kings Canyon Creek.

The Watershed Modeling System (WMS v. 10.1) was used to delineate both drainage basins, identified in Figure 2.

Figure 2. Kings Canyon Road Drainage Basins.

Table 1 details the drainage areas and existing hydraulic control structure for each basin.

Existing Crossing Station

Drainage Area

(sq. miles)

Existing Structure

108+66 1.78 36” CMP

141+16 5.01 4’ x 8’ RCBC, 24” CMP, 12” CMP

Table 1. Major Drainage Basins

The peak discharges for each drainage basin were calculated using HEC-1 through WMS. The SCS Curve Number Method was used to compute rainfall excess and losses. The SCS Unit Hydrograph Method was used as the rainfall runoff transformation within each basin. Basin variables such as lag time and the curve number (CN) were pulled from the Carson City Restudy report.

The peak flows calculated for both basins are presented in Table 2.

Curve

Number Lag Time

(hr.)

10-yr 25-yr 50-yr

108+66 56 1.1 90 185 285

141+16 65 5.9 275 420 545

Table 2. Drainage Basin Peak Flows

Kings Canyon

141+16

Kings Split

108+66

HYDRAULICS ANALYSIS - CULVERTS

EXISTING CONDITIONS

Culverts were analyzed using HY-8 culvert analysis software published by the Federal Highway Administration. The existing conditions at the culvert locations were analyzed to determine if the existing structures have adequate capacity.

Table 3 details the existing capacity check at each culvert crossing for the 25- and 50-year discharge.

The results from the HY-8 analysis are located in Appendix A.

25-year 50-year

Existing Culvert

Headwater Depth (ft.)

Adequate Capacity?

Headwater Depth (ft.)

Adequate Capacity?

108+66 36” CMP 5.3 Overtopping 5.5 Overtopping

141+16 4’ x 8’ RCBC, 24” CMP, 12”

CMP

2.7 Overtopping 3.1 Overtopping

Table 3. Existing Culvert Capacity Check

Based on the analysis of the existing conditions at each site, neither crossing meets the capacity requirements for a low standard road.

PROPOSED CONDITIONS

The HY-8 analysis performed under this report strongly suggests culvert replacements at both stream crossing on the project.

The proposed structures at each crossing were analyzed again using HY-8. Table 4 details the proposed capacity check at each culvert crossing for the 25- and 50-year discharge. The results from the HY-8 analysis are located in Appendix B.

25-year 50-year

Station Proposed

Culvert Headwater Depth (ft.)

HW/D

Ratio

Capacity Satisfied

Headwater Depth (ft.)

HW/D

Ratio

Capacity Satisfied

108+66 4’ x 6’ RCBC 4.7 1.2 Yes 5.1 1.3 No

141+16 4’ x 8’ RCBC, 36” CMP, 18”

CMP

2.5 0.8 Yes 3.0 Overtopping

Table 4. Proposed Culvert Capacity Check

The culvert crossing at 141+16 falls within a FEMA-regulated special flood hazard area classified as a Zone AO with a base flood depth reported as 2 feet. To ensure the proposed recommendations do not adversely impact the 100-year floodplain, the existing and proposed headwater elevations for the 100-year discharge were compared using the 100-year peak flow value from the Carson City Restudy report.

Table 5 details the existing and proposed headwater elevations at the culverts within the regulated floodplain.

Station 100-year Peak Flow

100-year Headwater Elevation (ft.)

Existing Proposed

141+16 909 5066.27 5066.19

Table 5. Comparison of 100-year Headwater Elevations

HYDRAULICS ANALYSIS – DITCHES

The Watershed Modeling System, WMS v. 11.0 was used to delineate the watershed and calculate the basin drainage area. Soil and land cover data was gathered from the NRCS Web Soil Survey application.

For the watersheds contributing to the ditch drainage, peak discharges were calculated using the rational method.

Ditch capacity and stability were analyzed using the Hydraulic Toolbox v. 4.2 Channel Analysis and Channel Lining Design Analysis. Results are shown in Table 6 below. The results from the Hydraulic Toolbox analysis are located in Appendix C.

Peak Flow Flow Depth Top Width Velocity

Start End Side Q10 Q25 Q10 Q25 Q10 Q25 Q10 Q25

105+00 109+00 LT 0.3 0.4 0.2 0.2 1.1 1.3 2.8 3.0

113+00 120+00 LT 0.8 1.1 0.3 0.3 1.6 1.8 3.6 3.9

113+00 140+00 RT 1.7 2.2 0.4 0.4 2.2 2.4 4.3 4.6

120+00 135+00 LT 28.2 37.0 1.0 1.2 6.2 6.9 8.7 9.3

Table 6. Ditch Capacity Analysis

Based on the capacity analysis, a standard 1-foot deep V ditch is adequate for ditches 1 through 3. Ditch 4 requires additional ditch capacity. It is recommended that the V ditch depth be increased from 1 foot to 1.5 feet for Ditch 4.

REFERENCES

FEMA (2016) “Flood Insurance Study for Carson City, Nevada.” Study Number 320001V000C

FEMA (2010) “Federal Emergency Management Agency Task Order #36, Draft Hydrologic Analyses and Results for the Carson City Flood Insurance Study.” Project Number 200543- 91913-141

FHWA (2007) “Project Development and Design Manual for Central Federal Lands Highway Division” Chp. 7. Federal Highway Administration (FHWA).

FHWA HY-8 v. 7.5 (June 2016).

FHWA Hydraulic Toolbox v. 4.3 (December 2015).

APPENDIX A – HY-8 EXISTING CONDITIONS

108+66 – EXISTING 36” CMP

141+16 – MULTIPLE EXISTING CULVERTS

EXISTING 12” CMP

EXISTING 24” CMP

EXISTING 8’X4’ RCBC

APPENDIX B – HY-8 PROPOSED CONDITIONS

108+66 – PROPOSED 4’X6’ RCBC

141+16 – MULTIPLE EXISTING/PROPOSED CULVERTS

PROPOSED 18” CMP

PROPOSED 36” CMP

EXISTING 8’X4’ RCBC

APPENDIX C – HYDRAULIC TOOLBOX ANALYSIS

DITCH 1

10-YEAR

25-YEAR

DITCH 2

DITCH 3

DITCH 4