Attch_9_-_Final_Design_Report_0001.pdf

Minden, Nevada

Reno, Nevada

1603 Esmeralda Avenue

Minden, Nevada 89423

775.782.2322 www.ROAnderson.com

Y:\Client Files\2862\2862-015\Documents\100% Submittal\Technical Memo - Sewer Analysis.docx

T e c h n i c a l M e m o r a n d u m

Date: April 4, 2024

To: Carl L. Bush, PE

NATIONAL PARK SERVICE, LAKE MEAD NRA

From: Robert O. Anderson, PE, CFM, WRS

R.O. ANDERSON ENGINEERING, INC.

Subject: Replace Boulder Beach Main Lift Station

Lake Mead National Recreation Area, Nevada

PMIS #311964, Task Order No. 140P8523F0040

Title II – Design Development and Final Construction Documents

100% Design Submittal

Introduction and Background The project that is the subject of the following analysis is a proposed sanitary sewer lift station to replace the existing Main Lift Station and Redundant Lift Station serving the

Boulder Beach Campground at Lake Mead NRA in Clark County, Nevada. The Main Lift

Station now in use at Boulder Beach was constructed in 2002. The corrosive environment together with operating conditions within the wet well have caused deterioration of components that dictate the need to replace the lift station. This lift station is a critical component to pumping raw sewage generated from all Boulder

Beach facilities, including National Park Service (NPS) facilities and housing, concessioner commercial facilities and employee housing, to the existing wastewater treatment lagoons. Sewage from the Main Lift Station at Boulder Beach is pumped through an 8-inch diameter PVC sewer force main a distance of about 6,217 feet to the existing treatment lagoons.

The construction documents propose removing the existing Redundant Lift Station and equipment and constructing a new wet well and pumping system at this location. This will allow the Main Lift Station to remain operational during construction of the new lift station. Upon completion of the new lift station, the existing Main Lift Station and associated components will be removed.

The new lift station will be constructed as a wet well configuration but utilizing three submersible pumps and motors rather than vacuum pumps, which are currently in use.

The existing emergency overflow tank will be removed, and a new emergency overflow

Replace Boulder Beach Main Lift Station – Technical Memorandum April 2024 tank will be constructed with valves allowing the force main to fully drain to facilitate repairs. A bypass pumping connection is included in the design to allow for pumping sewage directly into the force main, bypassing the lift station if necessary during repairs.

Further details of the existing and proposed systems can be seen in the attached improvement plans.

The purpose of this report and analysis is to evaluate and size the sewer lift station necessary to serve the existing forementioned facilities that are serviced by the collection system that when complete will allow the existing Main Lift Station to be removed from the site. The proposed lift station has been designed in accordance with the guidelines set forth in Nevada Division of Environmental Protection (NDEP) Bureau of

Water Pollution Control’s (BWPC) Technical Document #WTS-14, Pumping Station

Design & Submittal Criteria, and the Nevada Administrative Code (NAC).

Estimate of Sewer Flows

Wastewater usage rates were estimated based on daily flow records provided by representatives of NPS. Daily flow records were provided for the periods of: February 1, 2019, through July 31, 2021; and, July 1, 2022, through June 2023. That is a total of 1,276 daily flow records. No flow data was provided for the intervening period of August 1, 2021, through June 30, 2022.

Based on these data, the average daily flow was calculated to be 17,694 gallons per day (GPD), or about 12.3 GPM. The maximum recorded daily flow of 90,559 gallons (63

GPM) occurred in August 2019. The second highest daily flow, 63,387 gallons (44 GPM), occurred in September 2019. Based on the two recorded maximum daily flow values the maximum day to average day flow ratio would range from 5.1 and 3.6, respectively. The flow histogram reflecting the variation in daily flow ranges is provided below.

Figure 1 – Summary of Daily Wastewater Flows

Hydraulic Design Criteria

Pursuant to NDEP BWPC #WTS-14, a detailed analysis has been performed to size and configure the sewer lift station for the proposed modifications to the existing disposal system. The distance from the new lift station to the discharge manhole at the existing lagoons is about 6,360 feet, more or less, and the difference in elevation from the lowest water level in the new lift station to the discharge point is approximately 156 feet. The force main was constructed in 2008 of approximately 6,228 feet of 8-inch diameter pipe

(PVC, AWWA C900 DR14) and 132 feet of 6-inch diameter, fusion welded, high-density polyethylene (HDPE) pipe.

Based on the average diameter of the existing 8-inch PVC force main (7.758 inches), the minimum flow rate to achieve a two feet per second (fps) cleansing velocity is 295

GPM. This flow rate exceeds the measured maximum daily flow rate at the existing lift station and hydraulic calculations were performed utilizing this discharge rate to estimate total dynamic head (TDH)in the system. To establish a hydraulic system curve, TDH was estimated for discharge rates ranging between 295 to 400 gallons per minute

10,000

20,000

30,000

40,000

50,000

60,000

70,000

80,000

90,000

100,000

G a llo n s p e r D a y

Summary of Daily Wastewater Flows from Boulder Beach

Daily Wastewater Flow Average Daily Flow Maximum Daily Flow

(GPM). FlowMaster software1 was used as an aide in performing multiple calculations to develop a system curve and this system curve was used to select a pump that would accommodate pumping a minimum of 295 GPM against 172 feet of TDH. These hydraulic conditions are summarized in the table below.

Table 1 – Summary of System Curve

Boulder Beach Lift Station - System Curve

Discharge

(GPM)

Pipe

Dia.

(IN)

Length

(FT)

Static

Head

(FT)

Dynamic

Head

(FT)

Headloss in

6-inch Pipe

(FT)

Total

Dynamic

Head (FT)

Velocity

8"

(FPS)

Velocity

6"

(FPS)

295 7.758 6228 156.3 170.3 1.1 171.4 2.00 3.45

305 7.758 6228 156.3 171.2 1.2 172.4 2.07 3.56

341 7.758 6228 156.3 174.6 1.4 176.0 2.33 4.02

380 7.758 6228 156.3 178.6 1.8 180.4 2.50 4.31

400 7.758 6228 156.3 180.8 1.9 182.7 2.71 4.67

Pump Selection

Pumps are generally sized for a design duty point consisting of an expected peak flow rate and an estimation of TDH. The design process for the lift station began with an assessment of the site conditions, including historic flow conditions and the total dynamic head that the pump will need to overcome to deliver sewage to the discharge manhole near the lagoons. Given that the type of water is untreated wastewater it is expected to contain solids, a chopper/grinder pump with solids handling capacity was selected.

In addition to the hydraulic conditions summarized above, it was also a consideration that the selected pump should be operable when power is supplied by GGNRA’s existing 40 kilowatt (kW) portable generator.

The proposed wet well diameter, 8 feet, will accommodate three pumps arranged in parallel and will operate in an alternating fashion. The submersible chopper pump selected for this lift station is the Flygt NP 3171 SH 3~275 with an estimated operating point of 380 GPM at 178 feet TDH. At this operating point, the specified pump is expected to operate with an overall efficiency of approximately 57.8%. The manufacturer’s pump curve with system curve is attached for reference.

1 FlowMaster ® CONNECT Edition Update 3, Haestad Methods Solution Center, 27 Siemon Company Drive, Suite 200W, Watertown, CT 06795

Lift Station Design

Wet well sizing is primarily a factor of reducing pump starts per hour, with most manufacturers recommending no more than 4 to 6 pump starts per hour. This criterion implies that the wet well should, at a minimum, provide storage volume sufficient to allow for a maximum of 6 pump starts per hour. Theoretically, the minimum pumping duration (and subsequently the greatest number of pump-starts per hour) occurs when the influent rate is equal to half of the pump’s discharge capacity. At the design discharge rate of 380 GPM, this occurs when the influent flow rate is about 190 GPM.

Sizing the wet well volume for this condition is based upon the following equation:

Where Flowrateout is the pump flow at 380 GPM, and the minimum time between pump starts, Timemin, is 10 minutes to prevent the pump from starting more than 6 times per hour. By solving the equation, a minimum operating volume of 950 gallons is recommended. The specified wet well has a liquid capacity below the inlet of more than 2,500 gallons. The lift station specifies float elevations that create an operating volume for the primary pump cycle of about 1,692 gallons and provides a pump runtime of 4.5 minutes. A schematic of the wet well is provided below for reference.

Figure 2 – Wet Well Schematic

For employee safety and ease of maintenance operations, the new lift station will be equipped with a permanent jib crane with a rolling electric hoist.

The lift station design necessarily to accommodate influent flows from two upstream manholes, one for each of the two incoming gravity sewer lines.

Discharge piping from the wet well includes a valve vault and a meter vault, which will accommodate an electromagnetic flow meter. Additionally, downstream of the meter vault, a bypass pumping connection with quick connect has been provided to allow the operators to by-pass the lift station if needed in an emergency.

The control panel will be fitted with an audible and visible alarm and a remote readout from the flow meter.

The existing 5,000-gallon emergency overflow storage tank will be removed and replaced with a 15,000-gallon precast concrete storage tank. The wet well is designed with a 10-inch overflow pipe to the emergency storage tank with an 8-inch drain back line and isolation valve. Further details of the design can be seen on the project’s improvement plans. The storage tank was sized to safely store the full volume of the force main.

Operation and maintenance manuals will be provided by the contractor prior to contract close-out.

Conclusion and Recommendations

Based on this analysis, the following conclusions and recommendations have been made:

1. The design flow rate of 380 GPM exceeds the maximum recorded daily flow of 63

GPM (90,559 GPD). A single pump will sufficiently handle the maximum flow rate with a peaking factor of 6 without engaging the lag pump.

2. The 15,000-gallon emergency overflow storage tank has sufficient capacity to support approximately 4 hours of down-time at the maximum recorded daily flow of 63 GPM in the event of power loss or a system failure.

3. The wet well and valve vault shall have lockable, traffic rated aluminum access covers with fall protection.

4. The modifications to the existing sanitary sewer collection system include the following primary components:

a. A totalizing flow meter with remote readout located at the control panel.

b. An emergency bypass pumping connection with quick connect to allow for pumping directly into the force main.

c. Gravity mains and laterals shall be PVC (SDR 35).

d. Lift Station discharge piping through the meter vault shall be epoxy lined

& coated (AWWA C213) ductile iron Class 51 with flanged joints unless specified otherwise.

e. An automatic Air-Vacuum Valve shall be located prior to the flow meter and has been properly sized to prevent build-up of air or gases.

f. The force main shall be buried a minimum of 36 inches below grade.

Maintain a minimum of 12 inches of clearance beneath water laterals and mains.

As designed, the new Boulder Beach Sewage Lift Station at this project site meets the requirements of NDEP BWPC and Clark County design standards for sewage collection.

Attachments:

• Flygt NP 3171 Pump Performance Datasheets

• Construction Drawings randerson 24x36 Topo randerson Text Box 4/4/24