146 N C ana l S t , Su i t e 111 • Sea t t l e , WA 98103 • w ww .con fenv .c om

To: Ron Jepson, Jepson and Associates

From: Kathy Ketteridge, Ph.D., P.E., and Jessica Cote, P.E.

Date: 5/10/2018

Re: Sandy Point Maintenance Dredging – Coastal Processes Evaluation

1.0 PURPOSE The purpose of this memorandum is to provide a partial response to the Notice of Additional Requirements (NOAR) provided by Whatcom County as part of the Shoreline Substantial Development Permit process for the Sandy Point Maintenance Dredging Project; specifically, the Geohazards (WCC 16.16) comment provided by Andy Wiser on page 3 of the NOAR dated August 29, 2017. The full text of the comment is provided below:

This memorandum provides an “analysis of specific site and down-drift impacts” to coastal processes because of the proposed channel dredging. Mitigation measures required to offset identified impacts, if any, will be developed and discussed by others, including beach nourishment.

Also, the comment in the NOAR refers to a “new/expanded rip rap jetty” in addition to proposed channel dredging. The proposed project does not include construction or expansion of any rip rap structures at the site. Therefore, this analysis will discuss impacts to sediment

Ron Jepson, May 10, 2018

w w w . c o n f e n v . c o m page 2 of 7

transport processes associated with channel dredging only, which is the focus of the proposed project.

2.0 SITE DESCRIPTION The project location is at the southern end of Sandy Point peninsula, which is an approximate 1 mile long sand and gravel spit extending from the mainland to the north to the south to the end of the barrier spit (see Figure 1). Sandy Point is exposed to wind waves from the north-west over a fetch distance of over 40 miles, but semi-sheltered from wind waves from the south and south-west by Lummi Island (see Figure 1). Net littoral drift is from the north to the south along the western side of the spit and from west to east along the southern side of the spit (Washington Department of Ecology).

Figure 1: Vicinity Map of Sandy Point (taken from Bing Maps)

Prior to the late 1950’s, the Sandy Point spit was primarily agricultural land, diked along the eastern side with beaches along the western and southern sides, as shown in Figure 2a. Sandy Point was excavated in the 1950’s to construct a basin in the center of the spit connected to open water through a relatively large entrance channel constructed 1958 (Johannessen, 2002), as shown in Figure 2b. The constructed channel was about 300 feet wide and had a dredged elevation between -10 and -12 feet MLLW (all depths based on Bellingham Bay tidal datum).

Ron Jepson, May 10, 2018

w w w . c o n f e n v . c o m page 3 of 7

a) 1942 b) 1961 c) 2017

Figure 2: Aerial Photos of Sandy Point (not shown at the same scale). Source: Whatcom County Conservation District.

Construction of the entrance channel interrupted transport of sediment (via littoral drift) from the north to the south along the Sandy Point spit. Since that time, sediment that would have historically been transported to the southern tip of the spit has been accumulating north of the entrance channel forming a spit locally referred to as Cape Horn (see Attachment 1). This spit has continued to grow in a south-south-easterly direction narrowing the dredged entrance channel as shown in Figure 2c. Attachment 1 shows the existing condition of the entrance channel to Sandy Point, which is now about 50 feet wide with a controlling depth of about -2 feet MLLW.

Average accretion volumes for Cape Horn over the last 30 years were estimated to be about 800 to 1,000 cubic yards per year1 and the channel is anticipated to close off by the year 2030 (Johannessen, 2013).

3.0 PROPOSED WORK Ocean Properties LLC/Sandy Point Dredge Committee proposes to conduct maintenance dredging of the Sandy Point Harbor entrance channel to retain safe navigation into the harbor. The proposed dredge design consists of a 50 to 75 wide channel at a depth of -10 feet MLLW. This template is significantly smaller than the 1958 dredged channel for the site as shown in Figure 2b.

Attachment 2 shows the design plans for dredging at the site. Specific elements proposed for the work include:

Begin dredging at the mouth of the entrance channel starting at the BLM Tideland Boundary. Dredge from existing grade to -10 feet MLLW using a 5H:1V slope. This excavation will include removal of a gravel bar, as shown on the design plans, that has formed across the lower half of the entrance channel at the mouth.

1 Transport volumes listed in this memorandum were taken directly from Johannessen, 2013. No independent verification of these transport rates was conducted by the author(s) as part of this work.

Ron Jepson, May 10, 2018

w w w . c o n f e n v . c o m page 4 of 7

Dredge an approximate 450-foot channel at -10 feet MLLW from the mouth to the

interior of the Sandy Point Harbor. The width of the channel at -10 feet MLLW will vary from 50 feet at the mouth to 75 feet inside the harbor

Dredge side slopes for the channel at 5H:1V slope. Excavation of the side slopes to the north of the channel will result in excavation of lower Cape Horn.

No channel stabilization structures are proposed as part of this project. All rip-rap identified on the drawings provided in Attachment 2 refers to existing rip-rap structures at the site.

4.0 IMPACTS TO COASTAL SEDIMENT TRANSPORT PROCESSES This section provides analysis of potential impacts to erosional and deposition processes due to the proposed dredging work outlined in Section 3 and shown in Attachment 2. This analysis conforms with relevant sections of the WCC for a site located within a coastal erosion hazard area (WCC 16.16.310 (C.5.b), .355, and .360). Specific processes evaluated as part of this study include the following:

Net longshore drift processes, including sediment accretion/erosion at Cape Horn and South Cape

Channel migration waterward of the BLM Tideland Boundary (2013), as identified on the design drawings in Attachment 2

Sediment transport and deposition processes within the harbor entrance channel

4.1 Net Longshore Drift

4.1.1 Historical and Existing Conditions

Prior to initial dredging of the entrance channel in 1958, sediment accumulating on Cape Horn would have been transported to South Cape. However, the initial dredging of the entrance channel interrupted the littoral drift processes at the site.

Existing (2018) littoral drift processes at the site include two defined drift cells along the shoreline of Sandy Point separated by the entrance channel to the Sandy Point harbor. One drift cell extends from the northern extent of the Sandy Point peninsula to Cape Horn with net transport direction from the north to the south. The second drift cell extends form the southern side of the harbor entrance channel to the south and east along the southern side of Sandy Point. As mentioned previously, sediment being transported from the north accumulates on Cape Horn causing the spit to grow in a south-south-easterly direction (see Figure 3).

In the 1942 and1961 aerial photographs, there are large sand bars on the south side of South Cape. These sand bars are commonly observed at the end of a littoral drift cell and in this case, are the result of material from the north being deposited off the end of the point. However, these sand bars are much less prevalent in the 2017 aerial photo

Ron Jepson, May 10, 2018

w w w . c o n f e n v . c o m page 5 of 7

providing further evidence of a reduction in volume of sediment being supplied to South Cape after the dredging of the channel. Sediment that would have reached South Cape prior to 1958 is currently directed onto Cape Horn.

4.1.2 Impacts from Proposed Maintenance Dredging

The proposed dredging will not measurably alter the existing net longshore drift processes along the western (including Cape Horn) and southern (including South Cape) shorelines of Sandy Point. Construction of the entrance channel in the late 1950’s divided the original single drift cell into two separate drift cells, as described above. The proposed maintenance dredging will not further interrupt this process. Sediment accumulation and growth of the spit at Cape Horn will continue at a similar rate as it has in the past, as long the sediment source to the north along the shoreline remains the same as in past years.

Littoral drift is driven by sediment transport in the intertidal zone by wind-waves shoaling and breaking across the changing bathymetry along the shoreline. This project is not proposing any dredging waterward of the entrance channel mouth (BLM tideland boundary) in the littoral zone. So, there will not be any direct alterations of the nearshore bathymetry or water depths in the intertidal areas just offshore of the entrance channel. Therefore, no changes to the wave conditions along the intertidal areas of the site are expected and therefore no changes to littoral drift processes are anticipated.

In summary, the proposed deepening and widening of the existing entrance channel at Sandy Point will not alter existing littoral drift processes at the site. Cape Horn will continue to grow to the south, as material is transported along the shoreline (due to waves) from sediment sources to the north of the site. Existing (2018) coastal sediment transport processes at South Cape will remain the same following the planned dredging work. Therefore no additional erosion of South Cape (compared to existing 2018 conditions) is anticipated to occur following dredging of the existing entrance channel.

4.2 Channel Migration Waterward of the BLM Tideland Boundary The approach channel to the harbor entrance (waterward of the BLM Tidal Boundary) is currently located adjacent to the shoreline along the northern side of the entrance to the harbor. The north-west/south-east orientation of the entrance channel and the existing gravel bar located on the south side of the entrance channel most likely force the approach channel to remain in that current alignment. Excavation of the gravel bar and the slight straightening of the entrance channel mouth (see Attachment 2) increases the likelihood of migration of the approach channel to a location south of its current alignment. However, the wider channel post-project will significantly reduce average current velocities in the approach channel and entrance channel. This reduction in current velocities will minimize potential for erosion within the approach/entrance channels and subsequent channel migration to the south.

Ron Jepson, May 10, 2018

w w w . c o n f e n v . c o m page 6 of 7

Figure 3: Existing Net Littoral Drift Directions (north is towards the left in the photo)

As discussed in Section 4.2 above, the existing littoral drift processes will not be significantly impacted by the proposed maintenance dredging work. Therefore, after the planned excavation/dredging is completed, Cape Horn will continue to grow to the south narrowing and shallowing the entrance channel until it resembles current channel conditions (if there is a source of nearshore sediment to the north). The continual movement of Cape Horn to the south will cause the orientation of the entrance channel mouth towards the north-west, which will minimize the opportunity for the entrance channel to migrate to the south.

4.3 Sediment Transport and Deposition within the Harbor Channel The dredged entrance channel inside the harbor, once complete, will continue to migrate to the south as Cape Horn grows due to net littoral drift of sediment from the north. Sediment accumulating on Cape Horn will move into the dredged entrance channel both narrowing the channel width and shallowing the channel depth. This is the same physical process that has caused the existing channel to infill and narrow since it was originally dredged in 1958. The proposed maintenance dredging work will not alter this process.

In addition to sediment deposition in the channel due to accretion at Cape Horn, sediment can also be transported into the entrance channel and harbor due to tidal currents in the channel. Existing bathymetry provided in Attachment 1 shows an area of deposition where entrance channel meets the interior of the harbor (labeled “Flood Shoal Area” in Attachment 1). This deposition area is where the relatively higher tidal currents in the entrance channel will begin to slow down as the channel constriction widens. The proposed maintenance dredging will both widen and deepen the existing entrance channel. This will reduce constriction of tidal currents in the channel compared to existing conditions. This will reduce the capacity of the entrance channel to transport sediments into the harbor. Sediments may accumulate more widely in the channel itself, due to decrease in constriction of tidal currents. Over time, however, as Cape

Ron Jepson, May 10, 2018

w w w . c o n f e n v . c o m page 7 of 7

Horn migrates to the south, the channel will again become constricted and more localized accretion of sediments at the eastern end of the entrance channel will occur.

5.0 SUMMARY Proposed maintenance dredging of the Sandy Point entrance channel will not alter existing (2018) physical processes associated with net littoral drift along the shoreline, sediment erosion/deposition patterns within the entrance channel, erosion at South Cape or sediment accumulation on Cape Horn.

Excavation of the exiting gravel bar (see Attachment 2) as part of the maintenance dredging work is a concern for potential migration of the approach channel to the harbor waterward of the BLM Tideland Boundary. However, the channel widening due to proposed maintenance dredging actions will significantly lower average tidal velocities in the approach/entrance channel minimizing potential for erosion and channel migration.

6.0 REFERENCES Jepson and Associates, 2017. Sandy Point Navigation Channel Dredging Design Drawings, Prepared for the US Army Corps of Engineers.

Johannessen, J.W. 2011. Sandy Point: South Cape Historical Shore Change Analysis, Prepared by Coastal Geologic Services, Prepared as a Memorandum for Jeremy Freimund, Lummi Water Resources Manager.

Johannessen, J. W. 2002. Western Sandy Point, Lummi Indian Nation: Coastal Processes, Prepared by Coastal Geologic Services, Prepared for Lummi Indian Business Council, Office of the Reservation Attorney.

Washington Department of Ecology, 2002. Net Shore-Drift in Washington State. Enclosures: Attachment 1: Existing Site Conditions Attachment 2: Proposed Dredging Design

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VERTICAL DATUM

TIDAL DATUM BASED ON MEAN LOWER LOW WATER (MLLW) DERIVED

FROM NOAA PRIMARY BENCH MARK:

STAMPING: BM 1 1974

DESIGNATION: 944 9292 TIDAL 1

MONUMENTATION: TIDAL STATION DISK VM#: 9296

THE PRIMARY BENCH MARK IS A DISK SET FLUSH IN THE CONCRETE BASE

OF A RETAINING WALL ON THE EAST END OF THE CONCRETE SLAB

SUPPORTING THE OVERHEAD BOAT LAUNCH OF THE MARINA, 13 M (42 FT)

SE OF THE GAS PUMPS LOCATED ON THE NW CORNER OF THE CONCRETE

SLAB, AND 1 M (3 FT) BELOW THE LEVEL OF THE CONCRETE SLAB.

PUBLISHED ELEVATION : 11.217

BASIS OF BEARINGS

WASHINGTON STATE PLANE NORTH ZONE (NAD 83/98).

HELD N 01°27'19" E - 415.07' ALONG CENTERLINE OF MAYNE

LANE BETWEEN BRASS DISKS FOUND (AS SHOWN).

STATE OF WASHINGTON

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SOUTH CAPE AT SANDY POINT - PHASE 1, AS FILED UNDER

WHATCOM COUNTY AUDITOR'S FILE NO. 2150500568.

LEGEND

2" BRASS DISK SURFACE MONUMENT FOUND.

CONCRETE MONUMENT FOUND. (NOAA BM AH6114)

PK NAIL & FLASHER FOUND (P.L.S. 9631)

UPLAND AREA

STATE SUBMERGED AREA

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THE USE OF THESE PLANS AND SPECIFICATIONS SHALL BE RESTRICTED TO THE ORIGINAL SITE FOR WHICH THEY WERE PREPARED AND PUBLICATION THEREOF IS EXPRESSLY LIMITED TO SUCH USE, REPRODUCTION, OR PUBLICATION BY ANY METHOD, IN WHOLE OR IN PART, IS PROHIBITED. TITLE TO THE PLANS AND SPECIFICATIONS REMAINS IN THE ENGINEER WITHOUT PREJUDICE. VISUAL CONTACT WITH THESE PLANS AND SPECIFICATIONS SHALL CONSTITUTE PRIMA FACIE EVIDENCE OF THE ACCEPTANCE OF THESE RESTRICTIONS.

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