100-YEARS OF SHOAL EVOLUTION AT THE MOUTH OF THE COLUMBIA RIVER: IMPACTS ON CHANNEL, STRUCTURES, AND SHORELINES

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1 100-YEARS OF SHOAL EVOLUTION AT THE MOUTH OF THE COLUMBIA RIVER: IMPACTS ON CHANNEL, STRUCTURES, AND SHORELINES Hans R. Moritz, 1 Heidi P. Moritz, 2 Jessica R. Hays, 3 and Heather R. Sumerell 4 Abstract: The deep draft navigation project at the Mouth of the Columbia River, USA (MCR) consists of a dredged navigation channel 8 km (5mi) long that extends through a jettied entrance between the Columbia River and the Pacific Ocean. During , three rubble mound jetties and a series of pile dikes were constructed at MCR to stabilize the inlet, confine flow within the 2 mile-wide entrance of MCR, and prevent encroachment of shoals into the navigation channel. The MCR rubblemound jetties (north, south, A ) were built on flood/ebb tidal shoals. The shoals protected the jetties from excessive waves and currents. The re-direction of currents through the jettied entrance of MCR resulted in the discharge of million cubic meters ( mcy) of sand from the estuary to the ocean and re-orientation of the tidal inlet. Much of the present-day Peacock Spit and Clatsop Spit were formed by sand discharged from MCR during/after jetty construction, however, these spits and tidal shoals have been eroding since jetty construction. The erosion trend of sand spits/shoals at MCR is problematic for both the longterm stability of the jetties and the cost-effective sustainability of the MCR project. The key to preventing additional jetty deterioration will be to prevent scour of the jetty toe; i.e. shore up the sand shoals upon which the jetties were built. Using up to 3 million cubic meters (4 mcy) per year of dredged sand to shore-up the tidal shoals and forego jetty re-construction appears to offer a superior method (economically) for maintaining MCR navigation. INTRODUCTION The deep draft navigation project at the Mouth of the Columbia River, USA (MCR) consists of a dredged navigation channel 8 km (5mi) long that extends through a jettied entrance between the Columbia River and the Pacific Ocean (Figure 1). The MCR is situated between the states of Oregon and Washington. Consistent and safe navigation through the mouth of the Columbia River (MCR) is facilitated by 5 separate federal project features: 1) MCR navigation channel, 2) North Jetty, 3) South Jetty, 4) Jetty A, and 5) Sand Island pile dikes. Each project feature was constructed to fulfill a specific hydraulic function, while minimizing the overall maintenance needed to provide safe navigation through the MCR. Figure 1 shows project features, the current configuration of the shoals, and the location and orientation of the navigation channel. During , three rubble mound jetties and a series of pile dikes were constructed at MCR to stabilize the inlet, confine flow within the 2 mile-wide entrance of MCR, and prevent encroachment of shoals into the navigation 1) U. S. Army Corps of Engineers, Portland District, P.O. Box 2946, Portland, Oregon, 97208, USA, hans.r.moritz@usace.army.mil. 2) U. S. Army Corps of Engineers, Portland District, P.O. Box 2946, Portland, Oregon, 97208, USA,. heidi.p.moritz@usace.army.mil. 3) U. S. Army Corps of Engineers, Portland District, P.O. Box 2946, Portland, Oregon, 97208, USA, jessica.r.hays@usace.army.mil 4) U. S. Army Corps of Engineers, Portland District, P.O. Box 2946, Portland, Oregon, 97208, USA, heather.r.sumerell@usace.army.mil.

2 channel. The sediment budget at MCR affects coastal locations within the Portland (NWP) and Seattle Districts (NWS). meters, ngvd Washington Baker Bay Sand Island pile dikes Peacock Spit North jetty Jetty A MCR channel 5 miles long x 2640 ft wide x 55 ft deep (1) 2 (2) (3) Columbia River South jetty Pacific Ocean Clatsop Spit Oregon MCR ENVIRONMENT Figure 1. Mouth of the Columbia River and Navigation Structures The tidal range at MCR is 2.6 m (8.5 ft) and average wind-wave height during the winter is 3.7 m (12 ft). Extreme significant wave heights can range from 5 m to 14 m (16 to 46 ft). Instantaneous flow through the MCR can exceed 40,000 cms (1.5 mcfs) during ebb tide and currents within the navigation channel typically attain 2.6 m/s (5 knots). The 3.4 million m 3 (4.5 mcy) of sediment annually dredged from MCR is classified as fine-medium sand (mean grain size = 0.22 mm). In coastal waters immediately adjacent to the MCR, the dynamics of the Columbia River plume exerts its influence, tending to draw (denser) bottom marine waters into the estuary while discharging low salinity waters (the plume) at the surface. Figure 2 shows how the MCR plume affects coastal waters far from the MCR. The asymmetry of tidal flow, coastal currents, and wave action in vicinity of MCR is caused by the orientation of bathymetric contours offshore of Peacock and Clatsop spits. The orientation of the shoals/spits at MCR has arisen due to waves and currents; the morphology and coastal processes at MCR are linked. The littoral dynamics near MCR are drastically different than at open coast areas away from the entrance. 2

3 Willapa Bay MCR CR Estuary Columbia Circulation due to MCR Plume 15 miles Tillamook Head Tillamook Bay Figure 2. Magnitude of MCR plume circulation Historically, the Columbia River has been a major source of sediment to the northwest coast. On the oceanside of MCR, the mouth of the river is flanked by broad sandy beaches. To the immediate south, lies Clatsop spit where the beach is backed by substantial dunes. To the north, lies Peacock spit and a rocky headland (Cape Disappointment) which anchors the shore. The sandy sediment on the southwest Washington and northeast Oregon coastal beaches originated from the Columbia River. EBB TIDAL SHOAL EVOLUTION Erosional impacts to ebb and inner shoals include equilibrium adjustments subsequent to jetty construction, progressive navigational channel deepenings, and several significant El Nino/La Nina decades. Positive impacts to shoal maintenance are achieved through predominant ebb tidal shoal dredged material disposal. Successive channel deepenings at the MCR entrance modified the navigation channel depth from 7.6 m (25 ft) MLLW to 16.8 m (55 ft) MLLW. Figure 3 shows the MCR entrance in 1839 (46 years prior to jetty construction) and 1885 when the south jetty construction was initiated. Figure 4 illustrates shoal changes in 1916, at the time of the 3

4 Figure 3. Historical Shoal Configurations 1839 and 1885 (Hickson et al, 1950) 4

5 Figure 4. Historical Shoal Configurations 1916 and 1950 (Hickson et al, 1950) 5

6 north jetty construction and in 1950, 33 years after its completion. Looking at figure 3, in 1839, the middle sands shoal can be seen lying south of the predominant north channel which funnels through Baker Bay. A large southerly Clatsop Spit is also shown. The first phase of the south jetty was constructed in This entrance configuration shows a relatively symmetrical ebb tidal shoal and a straight, westerly-directed primary channel. The shoal that the south jetty was constructed on is clearly shown. Looking at the 1916 shoal configurations in figure 4, the wide underwater shoal on either side of the north jetty is clearly visible at the time of construction. The strong westerly flow is also shown as the river follows a predominant flow path through the north side of the estuary. In 1950, the inner shoal of Peacock Spit, south of the north jetty, has been dispersed and the growth of the inner Clatsop Spit northward is illustrated. Both of these results push the channel north toward the north jetty. Adjacent shorelines have experienced adjustments up to 3050m (10,000 ft). Figure 5 illustrates 40- ft (12.2 m) contour ebb tidal shoal evolution since pre-project surveys. The 1885 distribution of the 40 ft contour displays a fairly symmetrical shape about the inlet. The recession from 1930 (the furthest offshore) to 1993 (a 63-year period) appears to have occurred at a fairly gradual rate, while the recession from 1993 to 2000 (a 7-year period) exhibits rapid recession. Figure 6 illustrates along shore volume changes for key time periods in the vicinity of MCR. Figure 7 presents sediment budget differences for the ebb tidal shoal and the MCR entrance for two time periods, 1868 to 1958 and 1958 to (Gelfenbaum et al, 2001) Map of 40 ft contours around 5 time periods The ebb tidal shoal is receding at an accelerated rate between 1993 and MCR jetties were built on tidal shoals that are now eroding. Stability of jetties is compromised due to scour-based failure. Figure 5. Ebb tidal shoal 40 ft contour recession over time 6

7 Jetty Construction flushes huge volumes of sand to coast- --rapid beach accretion near jetties Sediment supplied to coast due to jetty construction begins to disperse alongshore Endgame of surplus sediment --- alongshore dispersal will continue --- accreted shorelands will recede; to stable Figure 6. Volume change in m3/yr/m for sections of the Long Beach Peninsula and Clatsop Plains for the time periods 1870 s 1926, s, and 1950 s (Gelfenbaum et al, 2001)

8 Accretion (m) Erosion (m) Figure 7. Bathymetric change maps from (left) and (right) in Mm 3. Accumulation of sand is positive and erosion is negative. (Gelfenbaum, et al.)

9 As expected, the largest volume changes occurred subsequent to jetty construction, with the ebb tidal shoal accreting by over 250 Mm 3. Losses during this time period are shown south of the entrance and between the jetties. The volume change comparisons for 1958 to 1998 illustrate material moving northerly from the ebb tidal shoal with losses on the ebb tidal shoal itself and continued significant losses south of the south jetty on Clatsop Spit. (Gelfenbaum et al, 2001) Excluding the period of jetty construction, there is not a firm consensus regarding how sand has been (and is presently) discharged through the MCR to the ocean; is sand continually discharged through MCR to the ocean (like a conveyor belt), or is sand discharged to the ocean only under extreme flow events (floods/freshets/jetties)? In fact, there is evidence that suggests sand is now being transported into the MCR channel/columbia River estuary from the ocean, rather than the sand being discharged out to the ocean. It is likely that much of the sand that the U.S. Army Corps of Engineers (USACE) now dredges at MCR (4.5 million cy/yr) is coming from the spits on the oceanside of the inlet. STRUCTURAL INTERACTION WITH SHOALS AND CHANNEL The MCR rubblemound jetties (north, south, A ) were built on flood/ebb tidal shoals. The jetty is a long, thin, narrow backbone of solid material, resting upon a very doubtful foundation, against which the forces in action at the locality have accumulated large quantities of the shifting sands. These in turn have been able to break the force of the waves and protect the jetty from destruction. Its safety and the permanence of the present favorable condition of channel over the bar depend upon the amount of this sand that can be accumulated. (USACE, 1903) The shoals have protected the jetties from excessive waves and currents. Due to the evolution of tidal shoals at MCR, the sand foundation of each jetty is receding. As the tidal shoals recede the water depth near the jetties gets deeper and the jetties unravel. The cost of rebuilding Pacific NW jetties is prohibitive: $5,000-$70,000 per foot length, depending on location on structure and exposure environment. If only 20% of each existing MCR jetty was to be rebuilt, it would cost $140-$260 million. Combined jetty head loss of more than one mile of structures has impacted the sand shoals at MCR, dredging requirements, jetty stability, and adjacent shoreline erosion. Figure 8 identifies areas of moderate to severe damage along the MCR rubblemound structures. All of the structures exhibit loss of head. Other key losses and areas of concern are connected to foundation failure or increased exposure to wave energy. HYDRODYNAMIC AND SHOAL RESPONSE TO STRUCTURES AND CHANNEL During , MCR jetty construction facilitated the discharge of million cy from the estuary to the ocean/nearshore regions north and south of MCR. Since 1917, the sand discharged from MCR, due to jetty construction, has been dispersed by waves/currents to areas offshore and onto nearshore areas north and south of MCR. This process resulted in rapid shoal and landform accretion. This pulse of sediment discharge to the coast is equivalent to 1,000 years of natural deposition and threw the littoral system of MCR out of balance. Now, the surplus of sand is beginning to run its course; the surplus is turning to deficit. Figure 9 illustrates the navigation channel location and bathymetric changes along cross sections in the estuary from 1912 to Cross sections are all approximately perpendicular to the south jetty and section locations are shown

10 Pacific Peacock Spit WA Ilwaco North Jetty N South Jetty Jetty A Columbia River Estuary Clatsop Spit Ocean 2 miles OR Figure 8. Moderate to Severe Damage Areas Along MCR Structures in figure 1. Cross section 1 connects the seaward tips of the north and south jetties. Section 2 connects the south jetty to the landward connection of the north jetty. And Section 3 connects the south jetty approximately to the Jetty A location. These sections illustrate a northerly movement of the channel toward the north jetty and increasing depths in the vicinity of Jetty A. DREDGE MATERIAL DISPOSAL PRACTICES The mouth of the Columbia River (MCR) is the ocean gateway for navigation access to/from the Columbia Snake River navigation system. The MCR channel is 805 m (2640 ft) wide and nominally 17m (55 ft) deep (below MLLW). To maintain the MCR channel at authorized dimensions, about 4.5 million cy/yr of sand is dredged from the channel and placed in ocean dredged material disposal sites (ODMDS). An ODMDS is typically sized to have minimum area. Exceeding ODMDS capacity can result in local changes to waves and currents, that may adversely affect navigation, erosion/deposition, and jetty stability. For certain ODMDSs, it is desirable to have placed dredged material transported (by waves/currents) out of the site, to benefit areas away from the navigation channel and restore ODMDS capacity. Consequently, long-term management of MCR ODMDS has been subject to competing issues. Ever since the Corps began dredging the mouth of the Columbia River (MCR) in 1904, the effort has been made to place as much of the dredged sand within the littoral zone as possible. This was 10

11 Depth (ft MLLW) Distance North of South Jetty (ft) N o r t h Depth (ft MLLW) N o r t h J e t t y Depth (ft MLLW) Auth. Channel Figure 9. Navigation channel and bathymetry change in estuary from 1912 to

12 done mostly for the purpose of increasing dredging efficiency (it s easier to place the dredged material at a shallow water site located 1-2 miles from the site of dredging than to place the material 3-6 miles offshore in deep water). Between 1904 and 1997, about 60% of all sand dredged from the MCR had been placed within the littoral zone (depth less than 18 m (60 ft)). This means that out of 208 million cubic yards (cy) of sand dredged from MCR, 125 million cy was placed in the littoral zone. (Oceanographic data collection activities in at MCR indicates that the active littoral zone extends out to depths up to 18 m (60 ft) at MCR.) Throughout the 1990 s, the Corps has been working with Washington Department of Energy (WDOE) to facilitate placing dredged sand specifically within the littoral zone. Site E is located just offshore of the north jetty along side Peacock Spit in water depth 14 to 18 m (45-60 ft) and is highly dispersive (dredged sand placed at Site E is quickly transported out of the site by waves and currents). To allow for increased use of Site E, the site was expanded in Since 1997, over 90% of all sand dredged from MCR has been placed within the littoral zone (over 12 million cy has been placed in Site E or along the south side of the north jetty since 1997). About 80% of the sand placed within Site E has been transported out of the site and presumably into the littoral zone. These nearshore sites are used to the maximum extent possible, keeping sediment in the littoral system and helping to protect the north jetty from potential undermining. Use of Site E and the north jetty site are limited to avoid impacting small boat navigation safety, and any remaining material is placed at Site F, in deeper water offshore. As this scenario indicates, selection and use of disposal sites at the MCR is complicated by the need to balance conflicting objectives and uses of the ocean. Figure 10 illustrates bathymetric change on Peacock Spit from 1958 to Dredged sand placed at ODMDS E appears to be feeding Peacock Spit as intended. Currently, most of the sand dredged from the 5-mile long MCR navigation channel originates from the ebb tidal shoal or from deposits within the river mouth. After several decades of waves and currents transporting sand off the ebb tidal shoal, the western edge of the shoal has migrated to the north and offshore. The crest of the ebb tidal shoal, known as Peacock Spit, has been reduced in elevation. The migration to the north and reduction of the ebb tidal shoal at MCR has resulted in the process of shoreline migration (re-distribution of accreted sand). Sediment is being transported off Peacock Spit at a rate of 3 to 5 Mcy per year. Material appears to be moving north, onshore and offshore. CONCLUSIONS Construction of the MCR jetties during 1885 to 1917 pushed a huge volume of sand offshore resulting in large ebb tidal deltas known as Peacock Spit and Clatsop Spit. These spits immediately supplied a large volume of sand to adjacent shore areas. The result was long-term accretion of shoreland to the north and south of MCR. Now, the long-term accretion is giving way to shoreline recession as waves and currents act to establish equilibrium with the displaced ebb tidal shoal. What appears to be widespread erosion along the southwest coast of Washington is a localized redistribution of accreted sand due to dynamic estuary entrances and a volatile Pacific Ocean storm climate. In addition, sediment supply to the coastal areas has historically been related to extreme pulses of events from the watersheds rather than a consistent annual supply. 12

13 Northing (ft) Bathymetry Change on Peacock Spit between 1958 and Oct 2001 Contour lines show Bathymetry for Aug-Oct 2001 Recent Accumulation of Dredged Material Placed at ODMDS E Contours = elevation ft MLLW, interval = 5 ft ODMDS E # 7 Peacock Spit Seabed Change Deposition in feet Erosion -25 # 9 Clatsop Spit Grid coords in NAD 27, OR N Geo. Coords in NAD Easting (ft) Figure 10. Bathymetric Change on Peacock Spit, 1958 to 2001 The primary reason for recent shoreline recession along the southwestern coast of Washington is due to natural processes and is two fold: (1) increased storm climate over the time period of 1993 to 2002 and (2) coastal sediment budgets that were thrown out of balance due to jetty construction now attaining a degree of equilibrium. Construction of jetties at the MCR as well as other inlets in the Northwest resulted in huge volumes of sand being transported from the estuaries to the ocean. The abundant supply of sand at the coastal entrances resulted in long-term accretion of shoreland adjacent to the estuary entrances. The injection of sand due to jetty construction has run its course and is now being dispersed. Recently accreted shoreland is now receding; the sediment is moving north, south, or offshore. The erosion trend of sand spits/shoals at MCR is problematic for both the longterm stability of the jetties and the cost-effective sustainability of the MCR project. Combined jetty head loss of more than one mile of structures has impacted the sand shoals at MCR, dredging requirements, jetty 13

14 stability, and adjacent shoreline erosion. The key to preventing additional jetty deterioration will be to prevent scour of the jetty toe; i.e. shore up the sand shoals upon which the jetties were built. Erosional impacts to ebb and inner shoals include equilibrium adjustments subsequent to jetty construction, progressive navigational channel deepenings, and several significant El Nino/La Nina decades. Positive impacts to shoal maintenance are achieved through predominant ebb tidal shoal dredged material disposal. Due to the evolution of tidal shoals at MCR, the sand foundation of each jetty is receding. As the tidal shoals recede the water depth near the jetties gets deeper and the jetties unravel. Ever since the Corps began dredging the mouth of the Columbia River (MCR) in 1904, the effort has been made to place as much of the dredged sand within the littoral zone as possible. Dredged sand placed at ODMDS E appears to be feeding Peacock Spit as intended. Improved management of dredged material placement can stabilize sand shoal erosion, defer expensive jetty repair, feed the littoral system, and optimize the dredging disposal program at MCR. Using up to 3 million cubic meters (4 mcy) per year of dredged sand to shore-up the tidal shoals and forego jetty reconstruction appears to offer a superior method (economically and holistically) for maintaining MCR navigation. REFERENCES Byrnes, M.R., and Li, F Regional Analysis of Sediment Transport and Dredged Material Dispersal Patterns, Columbia River Mouth, Washington/Oregon, and Adjacent Shores (Draft). Prepared for US Army Corps of Engineers Waterways Experiment Station, Coastal and Hydraulics Laboratory. Examination and Survey of the Mouth of the Columbia River, Oregon and Washington th Congress, 1 st Session, Document No. 94. Gelfenbaum, G., Buijsman, M.C., Sherwood, C.R., Moritz, H.R., and Gibbs, A.E Coastal Evolution and Sediment Budget at the Mouth of the Columbia River, USA (Draft). Prepared as a part of Southwest Washington Coastal Erosion Study. Hickson, R.E. and Rodolf, F.W Case History of Columbia River Jetties. US Army Corps of Engineers, Portland District. Improvement at Mouth of the Columbia River Oregon and Washington US Army Corps of Engineers, Portland District. Lockett, J.B Phenomena Affecting Improvement of the Lower Columbia Estuary and Entrance. Proceedings of the Federal Interagency Sedimentation Conference. USDA. Moritz, H Observing Large Waves Using Bottom-Mounted Pressure and Current Meters, Waves 2001 Conference, ASCE, San Francisco. Sherwood, C.R., Jay, D.A., Harvey, R.B., Hamilton, P., and Simenstad, C.A Historical Changes in the Columbia River Estuary. Progress in Oceanography, 25: US Army Corps of Engineers, Portland District Physical Processes and Geological Resources, Columbia River Channel Improvement Study and supplemental EIS. Appendix H, exhibit B. US Army Corps of Engineers Chief s Reports US Army Corps of Engineers, Portland District. 14