Copy of Monroe Water Intake Technical Memorandum 01-10-2022_RFP 2026-01_Appendix B.pdf

Technical Memorandum

DATE: January 10, 2022

TO: Dana Dedrick & Jed Kaul Long Tom Watershed Council

FROM: Scott Wright, PE, PMP Pete Gruendike River Design Group, Inc.

SUBJECT: City of Monroe water intake retrofit concepts in response to potential modifications of

Monroe drop structure.

1 INTRODUCTION

The Long Tom Watershed Council (LTWC) is an independent, non-governmental organization (NGO) working to support co-sponsors City of Monroe (City) and Confederated Tribes of Siletz Indians (CTSI) on a U.S. Army Corps of Engineers (USACE) Section 1135 Ecosystem Restoration Project. The Project seeks to improve fish passage at the existing Monroe Drop Structure on the Long Tom River. The drop structure is located at approximately river mile (RM) 7 in the City of Monroe, Oregon. The USACE Section 1135 Project is considering alternatives and modifications to the drop structure that could result in changes to the channel bed elevations, water surface elevations, and water velocities at the City’s municipal water intake structure located upstream of the drop structure. The current intake consists of a pipe extended into the Long Tom River with a perforated screen and a 60-horsepower electric pump. LTWC selected River Design Group (RDG) to develop conceptual schematics for modification of the municipal water intake that will maintain the City’s ability to withdraw 350 gallons per minute (gpm), equivalent to 0.78 cubic feet per second (cfs), from the Long Tom River. For this Technical Memorandum, we are considering two potential scenarios: 1) lowering the drop structure elevation by removing a portion of the structure and constructing a rock ramp for fish passage and 2) the full removal of the drop structure.

2 EXISTING CONDITIONS

A brief overview of the existing site conditions is provided based on our past knowledge of the Long Tom River within the project area and from existing data sources. The primary points of interest are river dynamics in and around the existing water intake and potential changes to river flows.

2.1 Existing Water Intake

The City’s existing water intake structure is located approximately 100 ft downstream from the Oregon Department of Transportation (ODOT) Highway 99W bridge as illustrated in the aerial photo (Figure 1).

The Monroe Drop Structure is located downstream from the water intake approximately 600 ft. The drop structure backs water upstream to the water intake and maintains a constant water surface elevation during low summer flows. The Long Tom River primarily transports fine grained sediment (i.e., sand and silt) and floating material (i.e., leaves and wood) in this area. Sediment deposition within the area upstream of the drop structure appears to consist of fine gravel, silt, and sand based on visual observations. ODOT soil borings at the bridge show sandstone overlain with greater than 5 ft of clay on the banks of the Long Tom River.

Figure 1. View looking downstream showing water intake on right side of river and drop structure downstream during typical summer flows in August 2021.

2.2 Existing Long Tom River Hydrology

The Long Tom River watershed drains 410 square miles of land at the southwestern end of the Willamette Valley. The lower Long Tom River is heavily regulated with the dominant proportion of discharge contributed by releases from Fern Ridge Dam. Fern Ridge Dam, a USACE facility, was completed in 1941 and impounds the upper Long Tom River watershed to provide water storage for winter flood abatement and for irrigation and recreational purposes throughout the summer. Below Fern Ridge Dam, several tributaries contribute additional flow to the Long Tom River. Substantial tributary inputs between Fern Ridge Dam and the Willamette River include Bear Creek, Amazon Creek, and Ferguson Creek. Most of the floodplain along the lower Long Tom River has been converted from floodplain forest and valley bottom woodlands to residential and agricultural land uses.

Two U.S. Geological Survey (USGS) river gages were used to evaluate recent hydrologic conditions in the lower Long Tom River watershed. The USGS gage located at Alvadore, OR (#14169000) is located near RM 24 (252 sq. miles drainage area) directly below Fern Ridge Dam and provides a reliable measurement of the controlled flow released into the lower Long Tom River from Fern Ridge Reservoir. The Long Tom River at Monroe, OR (391 sq. miles drainage area) gage (#14170000) has been operational since 1921 and is located at RM 7.5 near the Highway 99W bridge in Monroe. Flows recorded at the Monroe gage include the contributions from the three major tributaries listed above. Prior to regulation of flows at Fern Ridge Dam, peak flows often exceeded 10,000 cfs. Figure 2 includes peak flows from the historical and regulated periods illustrating the significant reduction in peak flood flows at the City’s water intake location. Figure 3 shows a comparison of mean daily flows for the historical and regulated periods at the City’s water intake and is useful to understand the typical flows throughout the year. Compared to the regulated hydrograph, the historical hydrograph typically had higher flows from late winter through spring and

Water Intake

Drop Structure lower flows in the fall. Summer mean daily flows are similar or higher for the regulated period and rarely drop below 20 cfs, easily providing adequate water for the City’s water intake and water right.

Figure 2. Annual peak flows for the historical and regulated periods on the Long Tom River at Monroe, Oregon gage.

Figure 3. Historical (1921-1941) and regulated (1942-2016) mean of mean daily flows at Monroe, Oregon.

To examine existing water depths in the project area, RDG used a bathymetric dataset created by Christina Appleby as part of her 2016 graduate thesis completed at the University of Oregon entitled “Modeling Historical Meander Bend Reconnection on the Lower Long Tom River in Lane Co. and Benton Co, OR”.

During the summer of 2015, approximately 6.5-miles of the mainstem Long Tom River, between Ferguson Road and the City of Monroe, were surveyed as part of the research. Approximately 40,000 points were collected using a combination of RTK GPS units and an acoustic doppler current profiler (ADCP) mounted on a hydro-board towed behind a kayak. The bathymetric survey results were merged with existing 2009 LiDAR data available for the reach to create a continuous terrain model. RDG used the resulting surface model unmodified except for a transformation to a local state plane coordinate system and conversion of units from metric to standard for engineering purposes. Figure 4 shows the surface model contours and location of water intake structure.

Figure 4. Plan view of existing drop structure (dam) in relationship to the City’s water intake located approximately 600 ft upstream of drop structure.

Typical water depths during summer low flow in the area adjacent to the intake are approximately 2 - 5 feet, while depths upstream at the bridge tend to be deeper, greater than 5 ft. Feedback from City staff indicates that the intake screen routinely needs cleaning to maintain adequate flow throughout the year and especially during low summer flows.

3 DROP STRUCTURE MODIFICATION SCENARIOS

The USACE is using a comprehensive stakeholder engagement process to develop and evaluate several alternatives for fish passage at the drop structure. Modifications to the existing drop structure that include lowering the crest elevation, or removing the structure, will have impacts on the City’s water intake due to the proximity of the intake structure as shown in Figure 4. The primary impacts that are likely to occur include lowering the water surface elevation that will influence pump performance and changes to sediment transport dynamics that could increase siltation around the intake. Below we discuss the two scenarios for drop structure modification, and potential impacts and solutions for the water intake structure. Returning the river to a more free-flow condition will have positive effects on water quality by reducing solar gain through elimination of the reservoir, reducing algal growth, and reducing stagnation and foul smell by increasing stream velocities.

3.1 Scenario 1: Drop Structure Lowering

One scenario being considered to improve fish passage at the drop structure is to reduce the overall height of the structure and construct a rock ramp to facilitate fish passage over the structure. At this time, it is unknown how much the overall structure height would be lowered. However, the resulting effects at the City’s water intake will be similar regardless of the elevation change and would likely include lower water surface elevations during water withdrawals and increased sedimentation. Figure 5 illustrates how the water surface is lowered due to the drop structure being modified and shows how the effects translate upstream to the water intake at STA 10+77. A representative cross-section of the Long Tom River is shown in Figure 6 and highlights how the lower water surface will impact potential availability of water at the existing intake suction inlet. Current City operations require cleanout of sediment periodically at the intake and this scenario would certainly exacerbate the periodic maintenance requirement.

Figure 5. Longitudinal profile of Long Tom River showing potential change to water surface due to modifications of drop structure.

Figure 6. Long Tom River cross-section (looking downstream) showing approximate location of water intake.

3.2 Scenario 2: Removal of Drop Structure

To maximize fish passage and to restore the Long Tom River in the Project area to more natural conditions, complete removal of the drop structure is being considered as a solution that meets Project objectives.

Complete removal would allow the natural flux of stored sediment upstream of the drop structure, and would likely change the river thalweg elevation and channel form at the water intake structure. Channel modifications and stabilization would likely be required at the existing bridge to ensure protection of the bridge piers and foundation. Typical stabilization measures that may be required if the drop structure is removed would likely include channel shaping and placement of angular riprap.

After the drop structure is removed, a more natural river channel elevation and velocity condition would exist at the water intake that would result in significantly lower water surfaces throughout the year.

Preliminary hydraulic models developed by USACE (2017) estimate the standing pool elevation would lower by approximately 8 ft under the removal scenario. As a result, the water intake structure would require substantial modifications to be operational as described in the following section.

Approximate location of existing intake pipe invert Elev. ±274.6

4 WATER INTAKE MODIFICATION CONCEPTS

The two scenarios presented above will create similar issues and effects at the existing water intake structure. Specifically, the water surface elevation at the intake will be lowered and the channel thalweg (i.e., lowest river cross-section elevation) will be lower and further away from the intake. Both conditions create a situation that will reduce or eliminate the ability for water to be withdrawn from the Long Tom River at the current location of the water intake structure. To resolve this potentially detrimental condition for the City, there are two alternatives that should be considered for the intake structure as described below.

4.1 Alternative 1: Relocate and Build New Water Intake

The existing water intake is located approximately 100 ft downstream from the existing bridge. This location currently works with the drop structure in place, however, once the drop structure is modified or removed, the channel and water depth will change at the water intake. Since the current water intake will no longer function properly, it is recommended that the location of the intake be moved to take advantage of the anticipated river dynamics. Specifically, the channel width will narrow down and deepen making the existing bridge an ideal location for water intake relocation. Figure 7 shows a potential revised location for the new water intake that will minimize maintenance, protect it from floating debris, and maximize the likelihood for long-term stability as it sits in the hydraulic shadow of the bridge constriction.

The intake could also be located upstream of the bridge to minimize the potential for contamination from roadway pollutants; however, this would add substantial cost and the river hydraulics upstream of the dam are not as favorable and floating debris would become a greater problem.

Figure 7. Alternative location for water intake structure to improve conditions after lowering drop structure.

Relocation of the water intake serves another purpose in minimizing the potential down time of the water intake system. The new intake structure could be built and put into service while the existing system is still in place to maintain the City’s water supply. Furthermore, the existing bridge provides certainty of river flow characteristics and stability. Relocating the intake next to the bridge provides greater confidence of likely flow conditions and location of river thalweg (i.e., lowest thread or flowline elevation of the riverbed) over time to ensure access to water during extreme low river flows.

The City’s water intake facility withdraws up to 350 gpm (0.78 cfs) from the Long Tom River for approximately 8 hours per day according to City staff. This withdrawal rate allows for a passive screen system based on Oregon Department of Fish and Wildlife (ODFW) and National Marine Fisheries Service (NMFS) fish screening criteria. The new diversion screen would have to meet fish screening parameters such as approach velocity, screen opening size, and effective screen area to ensure compliance with state and federal requirements. For this relatively small diversion rate, several fish screen examples exist that could be used on the Long Tom River. Figure 8 shows a set of gravity water intake screens on the Rogue River during construction. These screens consist of stainless-steel profile bar with openings that meet fish screen criteria. The water that passes through the screens is conveyed into a wet well where vertical shaft turbine pumps pressurize and transport the water. This type of water intake allows for the intake screens to be placed further into the river and ensures water levels are adequate during low summer flows.

Figure 8. Construction photo of river water intake screens (left) on the Rogue River along with headwall structure and gravity piping manifold (right photo, prior to backfill over pipes) that transports river water to a wet well for pumping.

An example of a simple gravity water intake system is a precast concrete box structure with a screened opening that allows water to be diverted as illustrated in Figure 9. This diversion on Whychus Creek has been functioning for over 10 years and has a withdrawal rate of 2 cfs, more than double the City’s current diversion rate. This type of intake could be configured into the bridge abutment area and integrated along the riverbank to create a smooth transition that maintains water flow and suitable levels for withdrawal.

Figure 9. Precast concrete box structure (left) with fish screen and installed location on Whychus Creek (right).

Another standard municipal gravity water intake example is illustrated in Figure 10. This standard configuration provides a concrete structure and wet well where water is pumped using a vertical turbine pump and logic control system. Configurations usually consist of duplex systems with variable frequency drives (VFD) for redundancy due to the critical nature of a municipal water supply. Backup generators are also provided to ensure power is always available for water pumps.

Figure 10. Typical cast-in-place concrete structure with fish screen and vertical turbine pump system regularly utilized for municipal water intakes throughout Oregon.

These example water intakes structures provide several viable concepts for improving the City’s water withdrawal to accommodate the potential changes being considered at the drop structure. The primary function of water withdrawal from the Long Tom River can be achieved through extending the intake further out into the river to compensate for the lower water surface elevation. Figure 11 provides a concept for a new water intake that compensates for expected water surface and channel thalweg changes. This concept utilizes various techniques described in the example water intakes above. The proposed fish screens would be oriented parallel to the river flow to minimize the potential for debris and sediment to clog the intake.

Figure 11. Gravity water intake concept for City’s water supply located near existing bridge.

An opinion of probable costs for the design and construction of a new water intake was developed in accordance with the American Association of Cost Engineers (AACE) recommendations. For studies where the definition of alternatives is at the “planning” or less than 1% maturity level, AACE guidance recommends a Class 5 cost estimate (DOE 2018). Our class 5 estimate utilizes a combination of parametric (statistical relationships from historical data), analogous (past projects), and actual cost estimates from suppliers to create a robust opinion of probable cost. The estimated cost is provided in Table 1 and includes a 35% contingency estimate for this level of planning.

Table 1. Class 5 cost estimate for new water intake system for City of Monroe.

Item Estimate

General Requirements: Includes items such as insurance, bonds, contractual obligations, temporary facilities, job trailer, mobilization, and demobilization of equipment.

$90,000

Construction: Equipment, labor, and materials to build the project including site access, utilities, erosion control, dewatering, demolition, concrete work, pumps, valves, fish screen, ancillary structures, site stabilization, and site restoration.

$900,000

Engineering: Includes design plans and contract documents, permit assistance, contractor bidding and selection, on-site construction administration, compliance monitoring, stakeout during construction, and as-built surveys.

$250,000

Contingency: Approximately ±35% of construction cost based on complexity of project and concept level of detail.

$300,000

Total $1,540,000

4.2 Alternative 2: Extend Existing Water Intake

As an alternative to relocating the water intake to a more suitable location upstream, the existing intake could be retrofit to work in the current location. An intake retrofit would maximize the use of existing piping and infrastructure by extending the intake pipe further into the river to a location that would maintain adequate water flow after dam removal. This type of configuration would be similar to the example in Figure 11, where a pipe is connected to the existing intake and extended to the anticipated low flow river level. This configuration would consist of a new pipeline segment constructed out of steel, and adequate anchorage to ensure the pipeline can withstand river hydraulic forces. The pipeline would extend to the anticipated low river flow and have an end-of-pipe fish screen sized appropriately for the intake. A new VFD pump and controls would be installed as part of the retrofit to modernize the overall water intake system and allow for remote monitoring.

Operation and maintenance for both Alternative 1 and Alternative 2 would be similar to the existing water intake. The fish screen on the end of the pipe would require cleaning during the summer months due to algal growth and the frequency would be similar to the existing system. Occasional floating debris and sediment may also need to be removed around the screen and could likely be done without mechanized equipment.

Table 2. Class 5 cost estimate to modify existing water intake system for City of Monroe.

Item Estimate

General Requirements: Includes items such as insurance, bonds, contractual obligations, temporary facilities, job trailer, mobilization, and demobilization of equipment.

$50,000

Construction: Equipment, labor, and materials to build the project including site access, utilities, erosion control, dewatering, demolition, steel pipe, pumps, valves, fish screen, ancillary structures, site stabilization, and site restoration.

$270,000

Engineering: Includes design plans and contract documents, permit assistance, contractor bidding and selection, on-site construction administration, compliance monitoring, stakeout during construction, and as-built surveys.

$90,000

Contingency: Approximately ±35% of construction cost based on complexity of project and concept level of detail.

$100,000

Total $510,000

5 SUMMARY

Lowering or removing the existing Monroe drop structure will affect the City’s existing water intake structure and require modifications. Constructing a new water intake structure upstream of the existing intake would be the best solution to ensure that the City has a low-maintenance, dependable water supply intake. Moving the water intake structure upstream, next to the bridge, creates ideal conditions for a new water intake that protects it from floating debris and maintains adequate scour based on the bridge constriction. Another potential option is to keep the existing infrastructure and extend the water intake pipeline further into the river channel to allow for water diversion at a lower water surface elevation. This option would be cheaper and would meet the City’s water intake requirements with minimal changes to operation and maintenance.

Both alternatives rely on a gravity water intake configuration similar to the City’s current water intake.

Dam removal does not impact river flows and the USACE operates Fern Ridge Reservoir upstream of Monroe that releases stored water and targets a low flow of 30 cfs at Monroe. The USACE has assured the City that this flow rate can be maintained, even during extreme drought conditions, with reservoir water storage and release. Prior to Fern Ridge flow regulation, the extreme low flow was 7 cfs in 1939 (USGS 2022) and the water intake alternatives would function at this extreme low flow. Operation and maintenance costs are expected to be similar to the City’s current water intake system. Planning level cost opinions were developed for each alternative and provide costs for comparison purposes.

6 REFERENCES

Appleby, Christina. 2016. Modeling Historical Meander Bends Reconnection on the lower Long Tom

River in Lane Co. and Benton Co., OR. Master of Science Thesis presented to the Department of Geography, University of Oregon. September 2016.

https://scholarsbank.uoregon.edu/xmlui/bitstream/handle/1794/20717/Appleby_oregon_0171N_11710.pdf

U.S. Army Corps of Engineers, 2017. Memorandum for Record, “Potential Removal of Monroe Drop

Structure – Initial Review Modeling”. Paul Sclafani and Jeffrey Ballantine. August 1, 2017, 4 pp.

U.S. Department of Energy, 2018. Cost Estimating Guide. U.S. Department of Energy document DOE

G413.3-21A, June 6, 2018, 153 pp.

U.S. Geologic Survey, 2022. Access to stream gage records, 14170000 Long Tom River at Monroe, OR, https://waterdata.usgs.gov/or/nwis/wys_rpt/?site_no=14170000&agency_cd=USGS