B08_Attachment_06__CD_FINAL_Design_Supplemental_Hydraulic_Analysis_0001.pdf

MEMORANDUM

DATE January 12, 2023 JOB NO. 2022‐0214

TO Nils Wiberg, Chief Water Resources Engineer Fuss & O’Neill

(401) 787‐1709

FROM Matt Shultz, PE, Arden KT Herrin, EIT Direct Phone: (508) 495‐6259 mshultz@woodsholegroup.com

CC Cara Soh, Project Manager, EYP Kirk Bosma, PE

Supplemental Review and Analysis of Hydraulic Design Conditions for Mill Creek Water Control Structure, Wellfleet, MA

Woods Hole Group is pleased to provide Fuss & O’Neill (F&O) this Technical Memorandum detailing our work to review the latest design guidance, projections for sea level rise (SLR), and provide updated design conditions for the Mill Creek Water Control Structure (WCS) in Wellfleet, MA. The following summarizes the work and results for each Task in accordance with the approved Scope of Work detailed in our Proposal dated October 4, 2022.

Introduction Mill Creek is a tidally influenced stream located in Wellfleet, MA that provides drainage to approximately 200 acres including both undeveloped wetlands, residences along Hampton Farm Road, and the Chequessett Yacht and Country Club. Water drained from the Mill Creek basin discharges into the Herring River via a network of shallow, sinuous channels approximately 0.75 river miles in length. Because Mill Creek is hydraulically connected to the Herring River, proposed changes at the Chequessett Neck Road (CNR) crossing are expected to increase the tidal exchange in Mill Creek. In order to regulate the amount of tidal exchange, a dike at the confluence of Mill creek and the Lower Herring River is being proposed. The dike will consist of a 25‐foot‐wide adjustable height opening to physically separate Mill Creek from the Herring River and allow adaptive control of the tidal exchange into the Mill Creek sub‐basin.

Prior modeling and design assessment work was completed for Mill Creek between 2013 and 2016. Since that time there have been multiple design guidance documents released and SLR projections for the Commonwealth have been updated. This work is to ensure the latest hydraulic design guidance and parameters are accounted for in the design of the Mill Creek WCS.

A. Review of Recent Design Guidance and SLR Projections Woods Hole Group reviewed the recent design guidance issued after completion of the prior studies to confirm design parameters used for the CNR Bridge. The following guidance documents were reviewed: 1) MassDOT LRFD Bridge Manual, updated 2020, 2) FHWA Hydraulic Engineering Circular No. 25 Third Edition, dated January 2020,

3) NCHRP 15‐61 Final Report, dated March 22, 2019, and 4) NCHRP 15‐61 Design Practices Guide, dated March 15, 2019.

Additionally, the latest SLR projections were reviewed and compared with SLR accounted for in prior studies.

Specifically, extreme coastal storm water levels were extracted from the Massachusetts Coast Flood Risk Model (MC‐FRM) for the year 2070 for comparison with previous extreme storm elevations used in design of the Mill Creek WCS.

Review of the recent FHWA and NCHRP guidance documents indicates that use of probabilistic, coupled hydrodynamic‐wave modeling approach to look at future projected design conditions is recommended. This is based on the FHWA award‐winning approach that MC‐FRM established, and FWHA HEC‐25 lists the MC‐FRM as an example analysis approach that should be followed for the highest Level 3 vulnerability assessment. Therefore, review and use of the MC‐FRM results to define potential design conditions is appropriate. Table 1 below lists the results from the MC‐FRM within Wellfleet Harbor at the entrance to Herring River.

Table 1. Extreme water levels for Wellfleet Harbor extracted from MC‐FRM

Annual Exceedance Probability

Return Period

Water Surface Elevation (present)

Water Surface Elevation (2030)

Water Surface Elevation (2050)

Water Surface Elevation (2070)

(%) (yrs) (Ft‐NAVD88)

0.2 500 10.8 12.0 14.6 16.6

1 100 9.8 11.0 13.3 15.2

2 50 9.4 10.6 12.7 14.6

5 20 8.8 10.0 12.0 13.9

10 10 8.4 9.5 11.4 13.2

B. Supplemental Modeling and Analysis to Support Design Development

The extreme water levels extracted from MC‐FRM (Table 1) indicate the 1% storm water level in Wellfleet Harbor is 9.8 feet NAVD88 in present‐day conditions, and that may increase to 15.2 feet with projected SLR over the next 50 years. Woods Hole Group reviewed the prior model simulations conducted for Mill Creek and a model run was conducted for a storm surge level of 11.9 feet NAVD88 in Wellfleet Harbor. This is the maximum surge level that does not exceed the Chequessett Neck Road (CNR) dyke. To confirm whether this should be considered the design event for the Mill Creek WCS, the latest topography surrounding Mill Creek and potential flood pathways were reassessed using the latest USGS Topo bathymetric Digital Elevation Model (DEM) (Danielson & Tyler, 2017).

Figure 1 shows the landforms surrounding Mill Creek, the entrance to Herring River at the CNR dyke, and color contoured elevations from 0 to 15 feet relative to NAVD88. Coastal storm surge in Wellfleet Harbor up to 12 feet can only enter Mill Creek through the opening(s) in the CNR dyke. For coastal storm surges between 12 and 15 feet, water can enter Mill Creek through the CNR dyke openings, through overtopping of the CNR dyke, or through a flood pathway located further east along CNR at the intersection with Mill Creek Lane.

Figure 1. Landforms surrounding Mill Creek and Chequessett Road Dike with color contours of elevation (USGS, shown between 0‐15 ft

NAVD88)

Potential overtopping at the CNR dyke and the additional flood pathway to Mill Creek for storm surges between 12 and 15 feet were further assessed by estimating the overflow rates that would occur during a coastal storm event, considering the duration over which overtopping would occur. Storm surge hydrographs were developed for storms having peak elevations up to 15 feet, and the amount of overflow that would occur over the CNR dyke was computed using the equation for a broad‐crested weir. This calculated amount of overflow was shown to not be significant given the size of the upstream basin, and there was no appreciable effect on the water level upstream of the CNR dyke. Therefore, the storm with a peak storm surge level of 11.9 feet NAVD88 in Wellfleet Harbor can be considered the design event for the Mill Creek WCS.

Table 2. Peak design water level for Mill Creek WCS

At Wellfleet Harbor

Downstream of Mill Creek

WCS

Upstream of Mill Creek

WCS

Max Surge before Overtopping CNR Dyke

(Ft‐NAVD88)

11.9 8.8 8.5

To assist with the structural design of the Mill Creek WCS, locally generated wind‐wave conditions were calculated using the Automated Coastal Engineering System (ACES) shallow water, fetch‐limited model for the design water level presented in Table 2. This analysis using assumed 1% and 0.2% winds (60 mph and 80 mph, respectively based on FEMA’s Flood Insurance Study for Barnstable County) which produced maximum wave heights (Hmo) of

1.8 feet with a period (Tp) of 2.2 seconds. The wave conditions are provided in Table 3.

Source: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AEX, Getmapping, Aerogrid, IGN, IGP, swisstopo, and the GIS User Community

Wellfleet Harbor

CNR Dike

Mill Creek WCS

Herring River

Flood pathway at CNR

Table 3. Design wave conditions for Mill Creek WCS

Peak WL Conditions (1% & 0.2% winds)

Maximum Wave Conditions

60 mph Wind Speed 80 mph Wind Speed

Wave Height (Hmo, ft) 1.2 1.8

Wave Period (Tp, sec) 1.9 2.2

Wave forcing calculations and wave pressure distributions for the Mill Creek WCS were developed for the design water level and newly calculated wave conditions. Wave pressure diagrams are included in Attachment A.

To support other design analyses for the Mill Creek WCS including subdrainage, seepage, and long‐term settlement assessments as well as overtopping analyses including determination of structure height and scour countermeasures, the model scenarios conducted for the Mill Creek system were reviewed considering both for inland flooding and coastal surge conditions. The modeling results for the scenarios listed in Table 4 were collected from prior efforts or run in the Mill Creek sub‐basin model, as necessary.

Time series of water level were also extracted on either side of the Mill Creek to support the additional design analyses. These are provided in MS Excel format in Attachment B.

Table 4. Model Scenarios and Water Levels (WLs) upstream & downstream of the Mill Creek WCS

Model Scenario

CNR

Opening (w x h)

Mill Creek Opening (w x h)

Peak WL Downstream of Mill Creek

WCS

Peak WL Upstream of Mill Creek

WCS

(Ft‐NAVD88)

Spring Tide w/ 100‐yr rainfall 165’ x 10’ 6’ x 1’ 5.2 3.7

Spring Tide w/ 100‐yr rainfall 165’ x 10’ 25’ x 1’ 5.2 4.4

100‐year coastal event Fully open 25’ x 1’ 8.7 4.8

100‐year coastal event Fully open 25’ x 6’ 8.7 7.6

Max surge = 11.9 ft at Wellfleet Harbor 165’ x 10’ 25’ x 1’ 8.8 5.8

Max surge = 11.9 ft at Wellfleet Harbor 165’ x 10’ 25’ x 6’ 8.8 8.5

100‐year coastal event w/ 10‐yr rainfall Fully open 25’ x 1’ 8.7 4.9

100‐year coastal event w/ 10‐yr rainfall Fully open 25’ x 6’ 8.7 7.6

10‐year coastal event w/ 100‐yr rainfall Fully open 25’ x 1’ 7.8 4.6

10‐year coastal event w/ 100‐yr rainfall Fully open 25’ x 6’ 7.8 6.9

Spring Tide w/ 100‐yr rainfall 165’ x 10’ Closed 5.2 3.3

C. Wave Overtopping Assessment for the Mill Creek WCS

Wave overtopping for the Mill Creek WCS was assessed for the design maximum surge event and wave conditions detailed above. Wave overtopping discharge rates were calculated using the empirically based formulations detailed in the EurOtop Manual (Van der Meer, et al., 2018). The Mill Creek WCS consists of a series of combination sluice‐flap gates at the main channel and an earthen dyke/sheet‐pile wall extending out from the sides of the main channel. The sheet‐pile wall extends above the crest of the dyke to an elevation of 9.5 feet NAVD88. The slope of the dyke embankment on both the upstream and downstream sides is proposed at 4:1 (h:v) and the embankment slope is blanketed in vegetated articulated concrete block (ACB) matting.

For a vertical wall with a foreshore slope and on‐impulsive wave conditions, equation 7.6 from the EurOtop Manual (shown below) can be used to determine the mean overtopping rate, q:

𝒒 𝒈𝑯𝒎𝒐𝟑

𝟎. 𝟎𝟔𝟐𝒆 𝟐.𝟔𝟏

𝑹𝒄 𝑯𝒎𝒐 where Hmo is the wave height, g is the acceleration due to gravity, and Rc is the relative freeboard.

Table 5 shows the computed overtopping rates calculated for the Mill Creek WCS for the design water level and wave conditions. The calculated overtopping rates at the Mill Creek WCS were compared with established limits to prevent damage to the structure (see Figure 2 from (Van der Meer, et al., 2018)) and the rates are well below the limits for a grass covered slope (with Hmo being < 1 m and > 0.3 m). With the embankment slopes already being protected by vegetated ACB matting, there is no additional concerns or mitigation needed to prevent damage from overtopping.

Table 5.Wave overtopping discharge at Mill Creek WCS for the Design WL and waves

Overtopping for the Design WL

Design wave Conditions

1% 0.2%

Mean Discharge (q, l/s/m) 0.002 0.011

Figure 2. Table 3.1 from EurOtop Manual – Limits for wave overtopping for structural design of breakwaters, seawalls, dikes, and dams.

D. Updated Scour Assessment and Mitigation for Mill Creek WCS Scour, or the localized acute removal of sediment around a hydraulic structure’s components, is a potential failure mechanism which needs to be accounted for during the design process. In 2016, Woods Hole Group conducted a scour analysis of the water control structure between Mill Creek and Herring River using a suite of rainfall and tidal surge events impacting the system (Woods Hole Group, 2016). The proposed hydraulic connection between Mill Creek is a system of five (5) sluice gates each measuring five (5) feet in width for a combined opening 25 feet wide. Each of the gates can be raised or lowered to allow for adaptive management of the Mill Creek section of the Herring River estuary. For the scour analyses described in this section, all five (5) gates were modeled with tow (2) variations in gate opening height: a one (1) foot gate height and a six (6) foot gate height. Time series of flow rates for each of the simulations were extracted upstream and downstream of the flow control structure and were used to calculate the potential depth of scour should countermeasures not be implemented. As in the 2016 scour analysis, two (2) approaches for determining potential scour depth were calculated to determine the most conservative estimate of depth to be used in the geotechnical analysis for design of the structure foundation.

The design cases evaluated include the six (6) cases detailed in the 2016 scour analysis and an added Case 7 which is the max surge event before overtopping the CNR dyke (peak surge = 11.9 ft at Wellfleet Harbor, 165’ x 10’ CNR opening, and 25’ x 1’ opening at Mill Creek WCS).

1. Contraction Scour

Contraction scour is calculated either as live bed scour, which occurs when the velocity in the channel is of sufficient magnitude to actively transport sediment in the water column upstream (or downstream, depending on direction of flow) or clear water scour, when the bed is not in motion. The type of contraction scour is determined by comparing the velocity in the channel with the critical velocity given as:

𝒗𝒄 𝑲𝒖𝒚𝟏 𝟔⁄ 𝑫𝟏 𝟑⁄

Where 𝒗𝒄is the critical velocity, 𝑲𝒖 is a unit correction factor equal to 11.17 for U.S. customary units (6.19 for SI units), y is the depth of flow in feet, D is the sediment size of the bed material of which 50% is smaller (d50) given in feet.

If the observed velocity is lower than the critical velocity, the contraction scour is expected to occur under clear water conditions, and is given as:

𝒚𝟐 𝑲𝒖𝑸𝟐

𝑫𝒎 𝟐 𝟑⁄ 𝑾𝟐

𝟑 𝟕⁄ 𝒂𝒏𝒅 𝒚𝒔 𝒚𝟐 𝒚𝟏 where y2 is the average equilibrium depth in the contracted section after contraction scour, Q is the discharge, Dm =1.25*d50, W is the bottom width at the channel contraction, y0 is the average existing depth, and Ku is a constant equal to 0.0077 in U.S. customary units (0.025 SI).

If the observed velocity is greater than the critical velocity, the contraction scour is expected to occur under live bed conditions, and is given as:

𝒚𝟐 𝒚𝟏

𝑸𝟐

𝑸𝟏

𝟔 𝟕⁄ 𝑾𝟏

𝑾𝟐 𝒌𝟏 𝒂𝒏𝒅 𝒚𝒔 𝒚𝟐 𝒚𝟏 where y2 is the average depth in the contracted section after scour occurs, y1 is the average depth in the main channel, Q2 and Q1 are, respectively, the volumetric flows in the contracted section and the main channel, and W1 and W2 are the channel widths of the main channel and the contracted section.

Results of the contraction scour analyses are listed in Table 6 with a maximum scour of 5.5 feet occurring under Case 1 during ebb tides. For flood tides, the maximum scour of 8.3 ft occurs on the downstream side of the flow control structure under Case 7. Due to the short duration of surge events, however, it is likely that the calculated scour depths are conservative.

Table 6. Scour results, in feet, for each flow scenario using the contraction scour equations.

Ebb Flood

Case y0 ys ynew y0 ys ynew

1 3.8 5.5 9.3 2.1 3.7 5.8

2 3.8 5.5 9.2 2.0 2.4 4.3

3 0.7 0.4 1.1 1.6 0.0 1.6

4 1.5 1.0 2.5 1.8 3.2 5.0

5 1.8 1.3 3.2 2.3 4.2 6.5

6 1.2 0.8 2.0 2.2 0.1 2.3

7 2.8 4.0 6.8 4.6 8.3 12.9

2. Clear Water Scour for Open Bottom Culverts

An open‐bottom culvert is a structure that has sides and a top but uses the natural stream bed as the bottom of the culvert and can either be designed with or without wing walls. There are separate equations depending on whether wing‐walls are present, but both are only for use in clear water conditions as described above. For the Mill Creek dike, where wing‐walls are to be used, the scour at the front entrance corner is calculated using:

𝑦 𝐾 𝑄 . 𝑄

𝑊 𝐷 ⁄ 𝑎𝑛𝑑 𝑦 𝑦 𝑦 where ymax is the flow depth at the culvert entrance corner including contraction and local scour, QBl is the amount of discharge that is blocked on one side of the dike, Q is the total discharge through the structure, Wc is the width of the opening, D50 is the median grain size of the sediment. QBl for this scour analysis was calculated using the following equation:

𝑄

𝑄𝑊

2𝑊 where Wc is the width of the Mill Creek flow control structure and W is the width of flow downstream of the structure. Table 7 lists the starting depth, maximum depth, and the depth of scour. For the open bottom culvert calculations, the largest scour occurred during Case 7 for both ebb and flood tides. Again, these scour estimates are conservative due to the short duration of coastal storm events.

Table 7. Scour results, in feet, for each flow scenario using the open bottom culvert scour equations.

Ebb Flood

Case y0 ymax ys y0 ymax ys

1 3.8 13.0 9.2 2.1 13.2 11.1

2 3.8 13.0 9.2 2.0 9.7 7.7

3 0.7 5.8 5.1 1.6 2.2 0.6

4 1.5 11.0 9.5 1.8 11.1 9.4

5 1.8 10.4 8.6 2.3 11.2 8.8

6 1.2 6.1 4.9 2.2 4.5 2.4

7 2.8 22.8 20.0 4.6 25.3 20.7

3. Scour Countermeasures

Woods Hole Group used the flow rate in the constriction to calculate the median size rip rap to armor the approach to the dike opening, the wing‐walls, and the Mill Creek dike based on guidelines detailed in the Federal Highway Association’s Hydraulic Engineering Circular‐23 (HEC‐23) using the equation:

𝑑 𝐾 𝑦

𝑆 1

𝑉 𝑔𝑦 where d50 is the size riprap of which 50% is finer, Kr is a sizing coefficient (0.68), y0 is the average flow depth before scour, Sg is the riprap specific gravity (2.65), g is the acceleration due to gravity (32.2 ft/s2) and VAC is the velocity of the flow at the entrance to the structure (Federal Highway Administration, 2009). The armor sizing was calculated for each of the seven flow scenarios during ebb and flood and the results are listed in Table 8. In addition to the stone riprap, a filter fabric should be installed under the scour protection to prevent the loss of fine sediments over time.

Table 8. Riprap median stone sizing for each flow scenario

Case Ebb Flood d50 (ft) d50 (ft)

1 0.74 0.73

2 0.74 0.52

3 0.31 0.08

4 0.67 0.59

5 0.62 0.60

6 0.33 0.20

7 1.46 1.63

Using the most conservative estimate of stone sizing with a calculated D50 of 1.63 ft (19.6 in), the recommended riprap is a Class VI design sizing. Table 9 lists the maximum and minimum gradations for the riprap scour protection.

Table 9. Minimum and maximum particle sizes for Class VI riprap as specified in HEC 23. (Federal Highway Administration, 2009)

Nominal Riprap Class by Median Particle Diameter (D50)

Particle of which 15% is finer than (D15)

Particle of which 50% is finer than (D50)

Particle of which 85% is finer than (D85)

D100

Class Size Min Max Min Max Min Max Max

VI 21 in 13.0 in 18.5 in 20.0 in 24.0 in 27.5 in 32.5 in 48 in

VI 750 lb 170 lb 500 lb 650 lb 1150 lb 1650 lb 2800 lb 6000 lb

Figure 3 shows an updated version of the recommended riprap scour countermeasure layout. Because the Mill Creek water control structure is subject to flows in both directions, both the upstream and downstream should have both 45o wing walls as well as scour countermeasures extending 15 feet (minimum) along the dike. In the channel bed, the riprap should extend a minimum of 20 feet both upstream and downstream of the gate opening.

The recommended riprap layer thickness is 48 inches.

Figure 3. Modified Figure 18.9 from HEC 23 (Volume 2) showing extents of riprap scour protection (Not to Scale).

References Danielson, J., & Tyler, D. (2017). Topobathymetric Model for the New England Region States of New York, Connecticut, Rhode Island, and Massachusetts, 1887 to 2016. U.S. Geological Survey.

Federal Highway Administration. (2008). Highways in the Coastal Environment, Second Edition; Hydraulic

Engineering Circular No. 25. Springfield, VA: National Technical Information Service.

Federal Highway Administration. (2009). Bridge Scour and Stream Instability Countermeasures: Experience, Selection, and Design Guidance‐ Third Edition; Hydraulic Engineering Circular No. 23. Arlington, VA:

National Highway Institute.

Van der Meer, J., Allsop, N., Bruce, T., De Rouck, J., Kortenhaus, A., Pullen, T., . . . Zanuttigh, B. (2018). Manual on wave overtopping of sea defences and related structures. An overtopping manual largely based on European research, but for worldwide application. EurOtop. Retrieved from www.overtopping‐ manual.com

Woods Hole Group. (2013). Herring River Restoration Project: Final Dike Control Structure Hydrodynamic Modeling. Falmouth, MA: Submitted to the Friends of Herring River.

Woods Hole Group. (2013). Task 1.2 ‐ Reconstruct the 100‐ and 500‐year still water levels. Falmouth, MA: Woods Hole Group.

Woods Hole Group. (2016). Technical Memorandum: Mill Creek Dike Scour Analysis. E. Falmouth, MA: Woods Hole Group.

ATTACHMENT A: Mill Creek WCS Wave Pressure Diagrams

ATTACHMENT B: Model Output:

Time Series of Water Levels Upstream & Downstream of Mill Creek WCS

(Electronic MS Excel File)