NV_US50(1)_Round Hill Pines_Final Hydraulics Memo.pdf

Subject: NV ‐ US50(1) – Round Hill Pines – Final Hydraulics Memo Date: January 31, 2022

From: Aaron Estep, PE Hydraulic Engineer FHWA Central Federal Lands Highway Division

To: Ryan Mathis, PE Project Manager FHWA Central Federal Lands Highway Division

This memorandum presents the hydrologic calculations, design analyses, and hydraulic recommendations for the proposed project along Lincoln Highway (US 50), located near the Round Hill Pines Resort along the Lake Tahoe shoreline. The proposed work will involve widening approximately 1,800 feet of US Route 50; incorporating northbound acceleration and turning lanes; and constructing a new entrance to the resort at safer location. As part of this work, the drainage basins to four drop inlets will be affected. The purpose of this memo is to evaluate these inlets and provide hydraulic recommendations for the proposed conditions. Figure 1 provides a regional project location for reference.

Figure 1: Regional Project Location Map

Memorandum

RECOMMENDATIONS

Based on the results of the hydrologic and hydraulic analysis outlined in this memorandum, the following recommendations are made for the four drop inlets and associated outlet pipes within the project extents along US 50.

Table 1. Summary of Recommendations

Station: Existing Configuration: Recommendation:

21+53

Inlet 2'x3' Equal‐Length Combo Functioning as required. Leave in place.

Outlet Pipe 18" CMP Extend pipe to accommodate road widening.

End Section Concrete headwall NDOT DS‐10 (Type I) or FHWA 602‐4 (Design A)

Outlet Protection

Discharges to forested slope. No outlet protection.

NDOT DS‐31 or FHWA C251‐50 (Riprap Apron);

Riprap ‐ NDOT Class 150 or FHWA Class 2

27+00

Inlet None Install new 2’x3’ Equal‐Length Combo

NDOT DS‐35 (Drop Inlet Type 3)

Outlet Pipe None Minimum 18” RCP

End Section None N/A – Tie into inlet at Station 27+67

Outlet Protection

None N/A – Tie into inlet at Station 27+67

27+67

Inlet 2.5'x4' Equal‐Length Combo Functioning as required. Leave in place.

Outlet Pipe 18” RCP Extend pipe to accommodate road widening.

End Section None ‐ Projecting pipe NDOT DS‐13 or FHWA 602‐4 (Design C)

Outlet Protection

Discharges to riprap. Outlet nearly completely buried.

NDOT DS‐31 or FHWA C251‐50 (Riprap Apron);

Riprap ‐ NDOT Class 300 or FHWA Class 2

30+42

Inlet 1'x14' Grate &

2'x10' Curved Vane Grate Replace with 2’x3’ Equal‐Length Combo

NDOT DS‐35 (Drop Inlet Type 3)

Outlet Pipe 12” CMP Minimum 15” CMP

End Section Unknown. Outlet nearly completely buried.

NDOT DS‐10 (Type I) or FHWA 602‐4 (Design A)

Outlet Protection

Unknown. Outlet nearly completely buried.

NDOT DS‐31 or FHWA C251‐50 (Riprap Apron);

Riprap ‐ NDOT Class 150 or FHWA Class 2

36+30

Inlet 1'x14' Grate &

2'x10' Curved Vane Grate Functioning as required. Leave in place.

Outlet Pipe 18" CMP Outside of construction footprint. Leave in place.

End Section Unknown Outside of construction footprint. Leave in place.

Outlet Protection

Unknown Outside of construction footprint. Leave in place.

PROJECT DESCRIPTION

The existing four lane highway is approximately 48 feet wide with variable shoulder width. The proposed work will include widening the road to 60 feet to allow for the addition of a northbound left turn lane as well as a northbound acceleration/merge lane. In addition to the proposed widening, a new proposed entrance to the Round Hill Pines Resort will be constructed. There are four drop inlets in the project area that drain this section of US 50 in addition to the adjacent hillslope to the east that is largely forested. The proposed work in this area will increase the drainage basins for these four inlets by approximately 12 feet to accommodate the new lane of travel along the western side of US 50. An existing aerial along with the general project extents are depicted in Figure 2.

Figure 2: Existing Aerial and General Project Extents Along US 50

This memo summarizes the hydrology and hydraulics components associated with the Round Hill Pines project and address the following items:

Hydraulic criteria and computational methods

Hydrologic evaluation of the existing and proposed US 50 roadway

Hydraulic analysis and recommendations for inlet locations along this portion of US 50

DESIGN CRITERIA

The design criteria used to evaluate the existing inlet locations are based the Nevada DOT Drainage Manual (NDOT, 2006). For the hydrologic analysis, the Rational Method was selected, and supported by the NDOT Drainage Manual, to determine the peak flow rates at these locations. Being that the drainage basins for the four drop inlets are all less than 200 acres, the Rational Method was appropriate.

For the hydraulic analysis, the classification of the US 50 roadway needed to be determined to identify the appropriate set of design criteria. Per the Douglas County Functional Classification Map provided by Nevada DOT, this section of US 50 is identified as “Other Principal Arterial”. Based on this classification, US 50 is subject to the following drainage requirements as specified in Table 3‐1 of the NDOT Drainage Manual:

Roadway Surface Drainage Facilities (Other Principal Arterial):

Minimum Design Return Frequency: 25 year

Maximum Allowable Spread Into Adjacent Travel Lane: ½ Lane

HYDROLOGIC ANALYSIS

EXISTING CONDITIONS

Within the project area, there are four drop inlets draining this portion of US 50 that will be affected by the proposed widening. The northern two inlets (approximately 36+30 and 30+42) are located along the western side of US 50 and consist nearly exclusively of roadway drainage. The southern two inlets (27+67 and 21+53) are located along the eastern side of US 50 and drain both portions of the roadway as well as the eastern hillslope. For each of these four inlets, the contributing drainage area was computed based on available surveyed topography and DEM. The computed drainage basins are displayed graphically in Figure 3.

Figure 3: Existing Project Drainage Basins

As mentioned previously, the Rational Method was selected to compute the flow rates to each of the four drop inlets located in the project area. For the runoff coefficient (C), the drainage areas were divided into areas of paved and forested land based on aerial imagery. A runoff coefficient of 0.9 was used for paved areas while a value of 0.2 was used for forested conditions. Time of concentrations were determined per NRCS methodology for sheet and shallow concentrated flows. Rainfall intensity‐duration‐frequency (IDF) curves were obtained from NOAA Atlas 14, Volume 1, Version 5 using the Precipitation Frequency Data Server. A rainfall intensity which corresponded with the time of concentration was used. A summary of existing basin parameters and rainfall intensities are provided below in Table 2.

Table 2. Existing Basin Parameters

Station:

Paved Surface (Asphalt)

Forested Weight Avg Runoff

Coefficient (C)

Time of Conc.

(min)1

Rainfall Intensity (i) (in/hr)

Drainage Area (A) (ac)

Area (ac)

C Area (ac)

C 25YR 100YR

21+53 0.95 0.9 5.55 0.2 0.30 8.9 2.9 4.2 6.5

27+67 1.22 0.9 9.60 0.2 0.28 7.9 3.0 4.4 10.82

30+42 0.28 0.9 0.00 0.2 0.90 5.0 3.7 5.3 0.28

36+30 0.99 0.9 0.00 0.2 0.90 5.9 3.5 5.1 0.99

1. Minimum Tc of 5 minutes was used.

The resulting existing peak discharge (Q), computed from the above parameters per the Rational Method, are listed below in Table 3.

Table 3. Existing Inlet Peak Discharges

Peak Discharge (cfs)

25YR 100YR

21+53 5.7 8.3

27+67 9.1 13.3

30+42 0.9 1.3

36+30 3.1 4.5

PROPOSED CONDITIONS

The proposed conditions of the project along US 50 are assumed to maintain the same drainage patterns and slopes with the exception of an increase in the paved roadway along the western side of the project area. Proposed basin parameters and peak flow rates are computed as described previously and are presented below in Tables 4 and 5, respectively.

Table 4. Proposed Basin Parameters

Paved Surface (Asphalt)

Forested Weight Avg Runoff

Coefficient (C)

Time of Conc.

(min)1

Rainfall Intensity (i) (in/hr)

Drainage Area (A) (ac)

Area (ac)

C Area (ac)

C 25YR 100YR

21+53 1.00 0.9 5.55 0.2 0.31 8.9 2.9 4.2 6.55

27+67 1.39 0.9 9.60 0.2 0.29 7.9 3.0 4.4 10.99

30+42 0.40 0.9 0.00 0.2 0.90 5.0 3.7 5.3 0.40

36+30 1.03 0.9 0.00 0.2 0.90 5.9 3.5 5.1 1.03

1. Minimum Tc of 5 minutes was used.

Table 5. Proposed Inlet Peak Flows

Peak Discharge (cfs)

25YR 100YR

21+53 5.8 8.4

27+67 9.5 14.0

30+42 1.3 1.9

36+30 3.2 4.7

HYDROLOGIC DISCUSSION

As part of this hydrologic analysis, the 25YR (Design Storm) and 100YR (Check Storm) were computed for all four drop inlets for both the existing and proposed conditions. The project work consists of widening a portion of US 50, and as such, increases the roadway area draining to each inlet. Correspondingly, the peak discharges in the proposed conditions are slightly elevated above existing. As a result of the proposed widening, the peak discharges for each of the four inlets experience an increase of between 0.1 and 0.4 cfs. These increases are fairly small for inlets 36+30, 27+67, and 21+53 and correspond to an increase of less than 5% over existing. The inlet at station 30+42 (located closest to the new entrance to the Round Hill Pines Resort), was the smallest drainage area but is located such that the entire western side of its drainage will be affected by the widening. For this inlet, an increase of 0.4 cfs is a nearly 45% increase over existing.

HYDRAULIC ANALYSIS

EXISTING CONDITIONS

Once the peak discharges were computed for the four drop inlets, the existing roadway hydraulics were evaluated against design criteria to determine the effectiveness of the current structures. As stated previously, the inlets should be designed for the 25‐year storm event and the maximum allowable spread is ½ of the adjacent travel lane (each travel lane is 12ft in width). With respect to the inlet efficiency, bypass flows were evaluated for downslope impacts as noted later in this section.

For these computations, the Federal Highway Administration’s Hydraulic Toolbox, Version 4.4, was used to compute the spread associated with the peak discharge as well as the intercepted flow for each inlet using the program’s Curb and Gutter Analysis tool. The existing inlet configurations used in this analysis are presented below in Table 6. For each inlet, the Manning’s Roughness value was assumed to be 0.015 and the cross slope of the gutter was assumed to equal the cross slope of the roadway. The values provided for the Inlet Configuration, Longitudinal Slope of the Road, Cross Slope of Pavement, and Gutter Width were determined based on the existing survey data. The Local Inlet Depression was assumed to be 3” for the inlets at stations 27+67 and 21+53 based on Google Street View imagery.

Table 6. Existing Inlet Configurations

Station: Inlet Configuration Longitudinal Slope of Road

(ft/ft)

Cross Slope of Pavement

(ft/ft)

Gutter Width (ft)

Local Inlet Depression

(in)

21+53 2'x3' Equal‐Length Combo Inlet 0.060 0.049 3.4 3

27+67 2.5'x4' Equal‐Length Combo Inlet 0.060 0.016 6.3 3

30+42 1'x14' Grate & 2'x10' Curved Vane Grate 0.030 0.024 4.0 ‐‐‐

36+30 1'x14' Grate & 2'x10' Curved Vane Grate 0.033 0.054 3.6 ‐‐‐

Based on these inlet configurations and the existing conditions peak flow associated with each drainage basin, the resulting intercepted flow, bypass flow, and total spread are presented in Table 7 for each inlet.

Table 7. Existing Inlet Calculations

Peak Flows (cfs)

Inlet Configuration

Design Storm (25‐Year)

Flows Spread

25YR

Intercepted

Flow (cfs)

Bypass Flow (cfs)

Computed Spread (ft)

Allowable Spread (ft)

21+53 5.7 2'x3' Equal‐Length Combo 3.9 1.8 5.5 9.4

27+67 10.91 2.5'x4' Equal‐Length Combo 4.5 6.4 14.2 12.3

30+42 0.9 1'x14' Grate & 2'x10' Curved Vane 0.9 0.0 5.6 10.0

36+30 3.1 1'x14' Grate & 2'x10' Curved Vane 3.1 0.0 4.6 9.6

1. Based on drainage patterns, bypass flow associated with the inlet at Station 21+53 ultimately discharges to Station 27+67 and therefore is incorporated into the peak flow at this station. The inlet peak flow (10.9 cfs) was computed as the basin peak flow (9.1 cfs) plus the bypass flow from Station 21+53 (1.8 cfs).

The results of the Hydraulic Toolbox calculations indicate that the three of the four existing inlets are adequately sized to handle the 25‐year storm event and not exceed the maximum allowable spread. Inlets 36+30 and 30+42 intercept all flow while stations 27+67 and 21+53 bypass 6.4 cfs and 1.8 cfs, respectively. Bypass flows from Station 21+53 are described in the table above. Bypass flows from Station 27+67 follow the eastern side of the roadway before ultimately entering a drainage ditch prior to the next inlet (Station 36+30). During the winter season, snowpack can clog the existing ditch and cause the bypass flows from 27+67 to drain across the roadway. For this reason, a new inlet is proposed upslope of the 27+67 inlet to reduce the bypass flows associated with the roadway widening. This new inlet will drain to the existing 27+67 drop structure to eliminate the need for an additional cross pipe. Flow rates to this new inlet will be based on an area weighted percentage of the original peak flow to the inlet at Station 27+67. Evaluation of this new inlet is provided in the following section.

PROPOSED CONDITIONS

Considering that the proposed widening is solely on the western side of the highway, it is determined that the inlets at Station 36+30, 27+67, and 21+53 can remain in place assuming construction can work around these inlets and they can handle the resulting flows from the widening. The inlet at Station 30+42 is required to be moved as the proposed widening will affect this inlet location. As previously mentioned, a new inlet will be placed at Station 27+00 to reduce the bypass flows associated with the existing inlet at Station 27+67. The recommended inlets will be determined based on the proposed flow rates. Table 8 below presents the results of the proposed flow rates on the five inlets. The same inlet configurations presented in Table 6 are used with the exception of a new recommended inlet for Station 30+42 and 27+00.

Table 8. Proposed Inlet Calculations

Peak Flows (cfs)

Inlet Configuration

Design Storm (25‐Year)

Flows Spread

25YR

Intercepted

Flow (cfs)

Bypass Flow (cfs)

Computed Spread (ft)

Allowable Spread (ft)

21+53 5.8 EX: 2'x3' Equal‐Length Combo 3.9 1.9 5.6 9.4

27+00 10.41 PR: 2’x3’ Equal‐Length Combo 4.0 6.4 11.7 12.0

27+67 7.42 EX: 2.5'x4' Equal‐Length Combo 3.4 4.0 12.2 12.3

30+42 1.3 PR: 2’x3’ Equal‐Length Combo 1.3 0.0 1.7 10.0

36+30 3.2 EX: 1'x14' Grate & 2'x10' Curved Vane 3.2 0.0 4.7 9.6

1. Based on drainage patterns, bypass flow associated with the inlet at Station 21+53 ultimately discharges to proposed Station 27+00 and therefore is incorporated into the peak flow at this station. The inlet peak flow (10.4 cfs) was computed as the basin peak flow for existing inlet at Station 27+00 (8.5 cfs) plus the bypass flow from Station 21+53 (1.9 cfs).

2. Based on drainage patterns, bypass flow associated with the proposed inlet at Station 27+00 ultimately discharges to the existing inlet at Station 27+67. The inlet peak flow (7.4 cfs) was computed as the basin peak flow for existing inlet at Station 27+67 (1.0 cfs) plus the bypass flow from Station 27+00 (6.4 cfs).

The three inlets at 36+30, 27+67, and 21+53 are recommended to remain in place. The inlet at Station 30+42 must be moved and therefore is suggested to be replaced with a 2’x3’ equal length combo drop inlet (NDOT Drop Inlet Type 3). This inlet will intercept all flow and prevent additional discharges from reaching the intersection with Sierra Sunset Lane. Additionally, the proposed inlet at Station 27+00 is suggested to be installed as a 2’x3’ equal length combo drop inlet (NDOT Drop Inlet Type 3) and will reduce bypass flows at the existing Station 27+67 inlet. The location for this inlet was determined as the best location to meet spread criteria for both the new inlet at 27+00 and existing inlet at 27+67. Should this inlet spacing conflict with utilities, it may be shifted closer to 27+67 and will continue to reduce bypass flows but will no longer meet spread criteria in the 25‐year event. If existing utilities prevent the installation of this new inlet, bypass flow at Station 27+67 will be 10.9‐cfs corresponding to an increase of 0.6‐cfs or 6% over the existing condition.

CHECK STORM

As a check, the 100‐year storm event was evaluated at all five inlets for both the existing and proposed conditions so as to determine the effect of the proposed widening at a larger storm event. While the check storm does not need to meet the requirements for allowable spread, it can indicate excessive safety risks during a larger storm event. Tables 9 and 10 below provide the intercepted flow, bypass flow, and total spread for the existing and proposed 100‐year peak flow as computed in the Hydrologic Analysis section.

Table 9. Existing Inlet Calculations

Flows (cfs)

Inlet Configuration

Check Storm (100‐Year)

Flows Spread

100YR

Intercepted

Flow (cfs)

Bypass Flow (cfs)

Computed Spread (ft)

Allowable Spread (ft)

21+53 8.3 2'x3' Equal‐Length Combo 5.2 3.1 6.4 9.4

27+67 16.41 2.5'x4' Equal‐Length Combo 6.1 10.3 16.5 12.3

30+42 1.3 1'x14' Grate & 2'x10' Curved Vane 1.3 0.0 5.6 10.0

36+30 4.5 1'x14' Grate & 2'x10' Curved Vane 4.5 0.0 5.3 9.6

1. Based on drainage patterns, bypass flow associated with the inlet at Station 21+53 ultimately discharges to Station 27+67 and therefore is incorporated into the peak flow at this station. The inlet peak flow (16.4 cfs) was computed as the basin peak flow (13.3 cfs) plus the bypass flow from Station 21+53 (3.1 cfs).

Table 10. Proposed Inlet Calculations

Flows (cfs)

Inlet Configuration

Check Storm (100‐Year)

Flows Spread

100YR

Intercepted

Flow (cfs)

Bypass Flow (cfs)

Computed Spread (ft)

Allowable Spread (ft)

21+53 8.4 EX: 2'x3' Equal‐Length Combo 5.2 3.2 6.4 9.4

27+00 15.71 PR: 2’x3’ Equal‐Length Combo 5.3 10.4 13.7 12.0

27+67 11.92 EX: 2.5'x4' Equal‐Length Combo 4.8 7.1 14.6 12.3

30+42 1.9 PR: 2’x3’ Equal‐Length Combo 1.9 0.0 1.9 10.0

36+30 4.7 EX: 1'x14' Grate & 2'x10' Curved Vane 4.7 0.0 5.4 9.6

1. Based on drainage patterns, bypass flow associated with the inlet at Station 21+53 ultimately discharges to proposed Station 27+00 and therefore is incorporated into the peak flow at this station. The inlet peak flow (15.7 cfs) was computed as the basin peak flow for existing inlet at Station 27+00 (12.5 cfs) plus the bypass flow from Station 21+53 (3.2 cfs).

2. Based on drainage patterns, bypass flow associated with the proposed inlet at Station 27+00 ultimately discharges to the existing inlet at Station 27+67. The inlet peak flow (11.9 cfs) was computed as the basin peak flow for existing inlet at Station 27+67 (1.5 cfs) plus the bypass flow from Station 27+00 (10.4 cfs).

The results of the check storm analysis indicate that there are slight increases in the computed spread between the existing and proposed 100‐year storm events but all inlets are close to meeting the allowable spread requirements of the design storm (25‐year event).

CULVERT ANALYSIS

As mentioned previously, the inlet at Station 30+42 will need to be relocated as a result of the proposed widening.

Based on the proposed grate size of the replacement inlet, the intercepted flow for the design storm (25‐year storm event) was computed to be 1.3 cfs. To accommodate this flow, a preliminary minimum recommendation for the outlet culvert was computed using the Manning’s Equation for full pipe flow (Equation 7‐1) as provided in the FHWA Hydraulic Engineering Circular No. 22, Third Edition. The equation is as follows:

𝑄 𝐾 𝑛

𝐷 . 𝑆 .

Where:

Q = Rate of Flow (cfs)

KQ = 0.46

n = Manning’s coefficient D = Storm Drain Diameter (ft) SO = Slope of the energy grade line (ft/ft) – Assumed to be the proposed pipe slope

By solving the Manning’s Equation for pipe diameter, the equation can be re‐written as follows:

𝐷 𝑛 ∙ 𝑄

𝐾 ∙ 𝑆 .

Assuming design flow (Q) to be 1.3 cfs, pipe slope (S) to be 0.01 (1%), and Manning’s n to be 0.024 (corresponding to a corrugated metal pipe), the resulting minimum diameter pipe is computed as:

𝐷

0.024 ∙ 1.3

0.46 ∙ 0.01 .

0.86 𝑓𝑡 10.3 𝑖𝑛

Therefore, based on the above calculations, its recommended that the outlet pipe associated with the relocated inlet at Station 30+42 be, at a minimum, a 12‐inch CMP with a slope of 1%. The existing outlet pipe size associated with this inlet is a 12‐inch CMP. The NDOT Drainage Manual, Section 3.3.2.2.3, specifies a minimum 15‐inch pipe size for lateral connectors, and therefore, it is recommended that a 15‐inch pipe be used with the proposed inlet as it meets the flow requirements shown above, meets the NDOT specifications, and allows for additional conveyance while reducing the flow velocities.

Similarly, the outlet pipe for the proposed inlet at 27+00 was computed to determine the minimum required diameter. Assuming design flow (Q) to be 6.4 cfs, pipe slope (S) to be 0.01 (1%), and Manning’s n to be 0.015 (corresponding to a reinforced concrete pipe), the resulting minimum diameter pipe is computed to be 1.3‐ft.

Therefore, the minimum required pipe size is an 18‐inch RCP.

OUTLET CONFIGURATION

Of the four inlets evaluated in this memorandum, three will be affected by the widening of US‐50 (30+42, 27+67, and 21+53). The inlet at Station 30+42 will need to be relocated entirely as a result of the proposed widening and as such will require a new outlet configuration. While the inlets at Stations 27+67 and 21+53 are proposed to remain in place, these inlets will require outlet pipe extensions to accommodate the additional roadway width and fill slope.

Additionally, the proposed inlet at Station 27+00 will drain to the existing inlet at Station 27+67 an therefore will result in a higher intercepted flow. Therefore, these two inlets will also require a new outlet configurations. For these three outlets, standard NDOT flared end sections (DS‐10 for metal pipes; DS‐13 for concrete pipes) along with standard NDOT riprap aprons (DS‐31) are proposed. The dimensions of the riprap aprons are based on the riprap size as well as the outlet pipe diameter. To determine appropriate riprap size, the methodology outlined in FHWA’s Hydraulic Engineering Circular No. 14, Third Edition (HEC‐14) was used. Appendix D (HEC‐14) provides equations for sizing riprap aprons. For this analysis, Equation D.2 (shown below) was used and directly relates riprap D50 with outlet velocity. Therefore, outlet velocity must first be determined.

𝐷 𝛼𝑉

Where:

D50 = riprap size (ft) α = 0.0126 V = culvert exit velocity (ft/s)

As described in HEC‐14, Chapter 3, Section 3.1.2, the outlet velocity is determined using the flow area at the outlet that is calculated using the higher of the tailwater or critical depth. Being that these three outlets do not discharge to a waterway, it is assumed that tailwater depth is equal to zero and therefore critical depth determines the flow area. To determine the critical depth, Figure B.2 (HEC‐14) graphs critical depth against pipe diameter and discharge for circular pipes. Using the 25‐year intercepted flow (Table 8 above) along with the outlet pipe size at each inlet, the critical depth (yc) was determined. Table B.2 (HEC‐14) further provides an indirect comparison of flow depth (assumed to be critical depth) to flow area for uniform flow in circular sections flowing partly full. The left most column in this table (y/D) is computed by dividing flow depth by the pipe diameter. Once computed, this value is used to locate the flow area to pipe diameter ratio (A/D2) in the second column. Using the determined flow area, outlet velocity can then be computed (V=Q/A). These calculations are summarized in Table 11 below.

Table 11. Outlet Velocity Calculations

Station

25‐Year Intercepted Flow (Q) (cfs)

Pipe Diameter

(D) (in)

Pipe Diameter

(D) (ft)

Critical Depth (yc) (ft) y/D A/D2

Flow Area (A) (ft2)

Outlet Velocity

(V) (ft/s)

21+53 3.9 18 1.5 0.70 0.47 0.3627 0.82 4.8

27+67 10.8 18 1.5 1.25 0.83 0.6969 1.57 6.9

30+42 1.3 15 1.3 0.45 0.36 0.2546 0.40 3.3

Based on the outlet velocities, the required minimum riprap size associated with the riprap apron is computed as shown in Table 12 below.

Table 12. Minimum NDOT Riprap Sizes

Station

Outlet Velocity

(V) (ft/s)

Riprap Size (D50) (ft)

Riprap Size (D50) (in)

Minimum NDOT Riprap Size1

21+53 4.8 0.29 3.45 Class 150 (D50 = 150mm or 6in)

27+67 6.9 0.60 7.17 Class 300 (D50 = 300mm or 12in)

30+42 3.2 0.13 1.61 Class 150 (D50 = 150mm or 6in)

1. NDOT Standard Specifications for Road and Bridge Construction, 2014, Section 719.03.01

STORMWATER MANAGEMENT

The Tahoe Regional Planning Agency (TRPA) Code of Ordinances Section 60.4.6 specifies standard BMP requirements to meet the water quality goals of the Lake Tahoe TMDL. In the existing condition, the four drop inlets discharge to either a riprap apron or directly to the natural ground surface. Flows from these outfalls then naturally infiltrate within the wooded area of Round Hill Pines Resort prior to reaching Lake Tahoe. The proposed road widening will result in a minor increase of approximately 0.38 acres of impervious roadway draining to the four drop inlets outlined in this report. Three of the outfalls will be impacted by the roadway widening and will be relocated to the new toe of slope. These outfalls will receive standard NDOT riprap aprons to dissipate energy and will continue to naturally infiltrate to the wooded area of the park. The fourth drop inlet is outside of the construction footprint and will not be modified as part of this project. Lastly, the drainage from the new access road will be allowed to sheet flow off the roadway surface and will also infiltrate within the wooded portion of the park.

REFERENCES

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

Federal Highway Administration (FHWA).

FHWA (2006) “HEC‐14, Hydraulic Design of Energy Dissipators for Culverts and Channels, Third Edition.”

Federal Highway Administration (FHWA)

FHWA (2009) “HEC‐22, Urban Drainage Design Manual, Third Edition.” Federal Highway Administration

(FHWA)

FHWA Hydraulic Toolbox v. 4.4 (July 2018).

Nevada Department of Transportation. “Drainage Manual, 2nd Edition” December 2006.

Nevada Department of Transportation. “Standard Plans for Road and Bridge Construction” 2020.

Nevada Department of Transportation. “Standard Specifications for Road and Bridge Construction” 2014.

Tahoe Regional Planning Agency (2020) “Code of Ordinances” Amended Sept 30, 2020.