NV196(2)_Hydraulics_Memo.pdf

Subject: NV FLAP 196(2), Silver Peak Road

From: Megan Frye, PE 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 Silver Peak Road, located 34 miles southwest of Tonopah, Nevada. Figure 1 provides a project location map for reference.

Figure 1. Silver Peak Road - Project Location Map

The project purpose is to increase safety, reduce maintenance costs, and improve the drainage along the roadway. The current roadway is constructed of native soil. This material requires frequent maintenance year round. Additionally, the roadway frequently washes out in flash flooding events. The roadway is shut down for several days after flood events while crews reconstruct the washouts.

This project includes minor grading, drainage improvements, placing aggregate base and paving 2.45 miles of road. There are 2 drainages where reinforced concrete box culverts will be constructed.

Memorandum

Date: August 22, 2019

Silver Peak Road

Culverts will be designed and replaced at a few minor drainages. The entire 2.5-mile road will be paved at 26 feet wide with 12 foot lanes and 1 foot shoulders. Signing and striping will be added per MUTCD requirements.

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 and projected average daily traffic counts (ADT) on Silver Peak Road are 100 and 200, respectively. The design speed is greater than 45 miles per hour. The design criteria for this project are on a high standard road by the Project Development and Delivery Manual. As such, the following design criteria apply:

Culvert Design (48” or greater)

Capacity: 50-year flood

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

HYDROLOGIC ANALYSIS

A detailed analysis of the hydrology throughout the project was completed. Given the complex nature of the alluvial fan drainage, Google Earth imagery was used to best approximate the major drainage basins.

Figure 2 show the five major drainage basins coinciding with the major drainages in the area.

Figure 2. Silver Peak Road Major Drainage Basins

2 1 3

5 6 7 8

The existing roadway alignment falls on an active alluvial fan which makes identifying a clear flow path to each culvert difficult. The large basins were delineated and the percentage of the total flow to each culvert was varied when selecting the appropriate culvert size. Several sections of roadway were designed to act as large low water crossings during storm events. In these sections, the profile of the roadway was designed as close to existing ground as possible to promote flow across the roadway rather than impounding water on the upstream side of the roadway in ditches.

Table 1 details the drainage areas and major crossing associated with each basin.

Drainage Basin Drainage Area

(sq. miles) Proposed Culvert Stations

1 1.1 14+62

2 1.0 20+47 and 22+50 - 28+50 (LWC)

3 0.8 33+28

4 7.4 37+50 - 64+00 (LWC)

5 9.5 70+80 and 78+49

6 0.8 85+00 - 106+00 (LWC)

7 3.7 110+15 and 114+94

8 2.2 121+87 and 126+89

Table 1. Major Drainage Basins

No stream gages are located near the project site. The use of the USGS regression equations is the recommended method in the Nevada Department of Transportation (NDOT) Drainage Manual for determining project discharges when local recording stream gage data is not available. The basins fall within USGS Nevada Region 6. The results from the equations for the 5- through 100-year peak flows are summarized in Tables 2.

Drainage Basin

5-yr 10-yr 25-yr 50-yr 100-yr

1 12 50 125 200 545

2 12 45 105 175 475

3 10 30 65 105 285

4 50 130 290 495 1,015

5 70 195 485 840 1,725

6 10 40 95 155 430

7 35 115 285 485 1,140

8 20 60 135 230 550

Table 2. Major Drainage Basin Peak Flows

HYDRAULICS ANALYSIS

EXISTING CONDITIONS

The entire roadway falls within an alluvial fan. The flow paths to the roadway are highly variable and require significant ditching and channelization to direct flow through the existing culverts. The existing drainage along the roadway varies from 24” culverts to 60” culverts and most crossings have multiple culverts.

The two major areas of concern are at the beginning of the job and near the end of the job. The first (near the beginning) is a combination of a 60” pipe and a 48” pipe with significant degradation at the outlet and the potential for sedimentation in the pipes. To mitigate sedimentation, the maintenance crews have dug sediment basins at the inlets of all of the culverts to allow the sediment to settle out before it reaches the culvert. The proposed design at this crossing will be a combination of ditch grading and concrete box culverts to carry the large flows in this area. The second area of concern is near the end of the project. Maintenance has on-going issues with flow overtopping the roadway for around 1000 feet. The section of roadway appears to be in a sag and the flow from the alluvial flow disperses across this entire section.

HYDRAULIC RECOMMENDATIONS

There are three sections of the project that are designed to function as low water crossings. In these sections, the proposed roadway surface matches the existing ground elevation. The roadway is sloped from upstream to downstream to promote continuous flow across the roadway. A paved ditch is proposed on the upstream side of the roadway to mitigate the potential for subsurface flow below the pavement and undercutting of the pavement section. Figure 3 depicts the typical roadway section for the low water crossings.

Figure 3. Low water crossing typical section

For the defined drainages throughout the project, the proposed culverts were analyzed using HY-8 culvert analysis software published by the Federal Highway Administration. The proposed conditions at each location were analyzed to determine the appropriate structure size.

Table 3 details the proposed culvert size at each crossing.

Crossing Station Existing Culvert Proposed Culvert Notes

14+62 48”, 60” 8’x8’ RCBC Size requested by County for cleaning

20+47 n/a 42" HPDE

33+28 24”, 36” 48” HPDE

70+80 2-36” 8’x8’ RCBC Size requested by County for cleaning

78+49 2-24” 48" HDPE

110+15 n/a 36" HDPE

114+94 2-36” 8’x8’ RCBC Size requested by County for cleaning

121+87 n/a 36” HDPE

126+89 36”, 24”, 18” 8’x8’ RCBC Size requested by County for cleaning

Table 3. Summary of Proposed Culvert Sizes

The capacity of the cross culverts was analyzed for the 25- and 50-year storm events. Several of the crossings fall within the same major drainage basin as the proposed low water crossing. In those conditions, several flow split scenarios were analyzed. First, the flow was divided in half between the crossings in the basin. Second, the percentage of the drainage basin contributing to the crossing was approximated from aerial imagery and the flow divided. Table 4 summarizes the capacity analysis. The expected condition is shown in bold.

Station Culvert

Size

Flow

25-year 50-year

HW

Depth (ft)

HW/D

Overtopping

Depth (ft)

HW

Depth (ft)

HW/D

Overtopping Depth (ft)

14+62 8x8

RCBC

100 2.9 0.4 -- 4.0 0.5 --

20+47 42”

HDPE

50 3.7 1.0 -- 5.3 1.5 0.1

40 3.1 0.9 -- 4.8 1.4 --

33+28 48”

HDPE

100 3.3 0.8 -- 4.8 1.2 --

70+80 8x8

RCBC

50 4.6 0.6 -- 6.7 0.8 --

75 6.1 0.8 -- 9.1 1.1 --

78+49 48”

HDPE

50 6.5 1.6 0.5 6.9 1.7 0.9

25 5.5 1.4 -- 6.4 1.6 0.4

110+15 36”

HDPE

50 5.6 1.9 0.4 5.8 1.9 0.6

15 3.2 1.1 -- 5.1 1.7 --

114+95 8x8

RCBC

50 3.6 0.5 -- 5.1 0.6 --

85 5.1 0.6 -- 7.3 0.9 --

121+87 36”

HDPE

50 4.8 1.6 -- 5.4 1.8 0.3

25 2.8 0.9 -- 4.0 1.3 --

126+89 8x8

RCBC

50 2.2 0.3 -- 3.1 0.4 --

75 2.9 0.4 -- 4.1 0.5 --

Table 4. Cross Culvert Analysis

Under the expected conditions, both the culverts at 78+49 and 110+15 do not meet the minimum capacity requirements. The culvert at 78+49 will overtop the roadway somewhere between the 25-year and 50-year storm events. Because the roadway is designed to overtop at the low water crossings, this does not pose much of a concern. The culvert at 110+15 does not overtop the roadway at the 50-year storm event but does not meet the minimum headwater to depth ratio requirements. This means that more water than desirable will pond at the inlet during large storm events. Given the ditch slopes from 110+15 to 114+95, I expect the ponded water at the inlet to flow down the ditch to the concrete box culvert at 114+95. This box has sufficient capacity at the 50-year storm event to carry the additional ditch flow from 110+15.

The capacity of the box culverts was also analyzed for the 25-, 50-, and 100-year storm events with 100 percent of the basin flow through the boxes. In some cases, this scenario is unlikely but necessary to review in the analysis. Table 5 summarizes the capacity analysis.

Station Proposed

Culvert Size

Headwater Depth (ft.)

HW/D Ratio Outlet

Velocity (fps)

Minimum Riprap Class at Outlet

Proposed Riprap Class

25-yr 50-yr 100-yr 25-yr 50-yr 100-yr 25-yr 25-yr 25-yr

14+62 8’ x 8’ 2.9 4.0 8.0 0.4 0.5 1.0 14.4 1 2

70+80 8’ x 8’ 7.4 11.9 14.1 0.9 1.5 1.8 17.6 4 2

114+94 8’ x 8’ 5.7 8.2 10.3 0.7 1.0 1.3 13.7 2 2

126+89 8’ x 8’ 3.5 4.9 8.9 0.4 0.6 1.1 12.3 1 2

Table 5. Cross Culvert Analysis, Box Culverts

With the exception of the box culvert at 70+80, all of the box culverts have adequate capacity to convey 100 percent of the basin flow for the 50-year storm event. The box culverts at 14+62 and 126+89 have enough capacity to carry the full 50-year storm event without ponding at the inlet. This should help mitigate sediment deposition at the inlet of these boxes. The boxes at 70+80 and 114+94 have some ponding at the inlet for the 50-year event so some sediment deposition should be expected. The boxes were sized to allow the County maintenance to use a small loader to clean the sediment from the boxes.

Riprap protection was proposed to mitigate erosion at the outlet. The riprap was sized to protect against a 25-year storm event. The minimum classes are shown in Table 5. Given the remote location of the project and budgetary restrictions, it is unlikely that larger rock will be available in the general area.

Because of this, it is recommended that each box culvert have a riprap apron with class 2 riprap extending the full length of the wingwalls at the outlet.

The low water crossing sections were analyzed using the broad crested weir equation to determine the flow depth at the 25-, 50-, and 100-year storm events. For the safety reasons, the optimal flow depth for a low water crossing is less than one foot. The broad crested weir equation was manipulated to solve for the flow depth as follows:

𝑄

𝐶∗𝐿

= H, and H ≈ d

The weir coefficient is a constant that includes gravity and the discharge coefficient that accounts for hydraulic losses, C was assumed equal to 2.62 for this analysis. Table 6 shows the depths at each low water crossing for several storm events.

Flow Depth on Roadway (ft)

Section Length

(ft) 25-year 50-year 100-year

22+50 to 28+50 600 0.1 0.2 0.3

37+50 to 64+00 2650 0.1 0.2 0.3

85+00 to 106+00 2100 0.1 0.1 0.3

Table 6. Low water crossing depth

REFERENCES

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 (October 2017).

FHWA Hydraulic Toolbox v. 4.3 (October 2017).

APPENDIX A – HY-8 PROPOSED CONDITIONS

14+60 – CULVERT SUMMARY TABLE (100% FLOW TO RCBC)

20+47 – CULVERT SUMMARY TABLE (50% FLOW TO 42” HDPE)

20+47 – CULVERT SUMMARY TABLE (40% FLOW TO 42” HDPE)

33+50 – CULVERT SUMMARY TABLE (100% FLOW TO 48” HDPE)

70+80 – CULVERT SUMMARY TABLE (100% FLOW TO RCBC)

70+80 – CULVERT SUMMARY TABLE (75% FLOW TO RCBC)

70+80 – CULVERT SUMMARY TABLE (50% FLOW TO RCBC)

78+50 – CULVERT SUMMARY TABLE (50% FLOW TO 48” HDPE)

78+50 – CULVERT SUMMARY TABLE (25% FLOW TO 48” HDPE)

110+50 – CULVERT SUMMARY TABLE (50% FLOW TO 36” HDPE)

110+50 – CULVERT SUMMARY TABLE (15% FLOW TO 36” HDPE)

115+00 – CULVERT SUMMARY TABLE (100% FLOW TO RCBC)

115+00 – CULVERT SUMMARY TABLE (85% FLOW TO RCBC)

115+00 – CULVERT SUMMARY TABLE (50% FLOW TO RCBC)

121+87 – CULVERT SUMMARY TABLE (50% FLOW TO 36” HDPE)

121+87 – CULVERT SUMMARY TABLE (25% FLOW TO 36” HDPE)

126+89 – CULVERT SUMMARY TABLE (100% FLOW TO RCBC)

126+89 – CULVERT SUMMARY TABLE (75% FLOW TO RCBC)

126+89 – CULVERT SUMMARY TABLE (50% FLOW TO RCBC)

APPENDIX B – HYDRAULICS TOOLBOX - RIPRAP

14+60 (100% FLOW TO RCBC)

70+80 (100% FLOW TO RCBC)

115+00 (100% FLOW TO RCBC)

126+89 (100% FLOW TO RCBC)