sign in sign up
<<
Home , Armor (hydrology), Feeder bluff

Marine Shoreline Design Guidelines: New performance assessment and guidance document from Puget , WA Johannessen, J.1, A. MacLennan1, A. Blue1, J. Waggoner1, S. Williams1, W. Gerstel2, R. Barnard3, R. Carman3, and H. Shipman4 for the Washington Department of Fish and Wildlife, Olympia, Washington. Download the complete document at: http://wdfw.wa.gov/publications/01583/ 1Coastal Geologic Services Inc., 2Qwg Applied Geology, 3Washington State Department of Fish and Wildlife, 4Washington State Department of Ecology

Impacts of Armor (continued) Cumulative Risk Model. Fetch = whichever is greater: maximum fetch from southern Bluff or Barrier Sites Design quadrant or half of maximum from other aspects. Setback distance = measured dis- • Altered wave action. At higher water levels, waves can reflect off of tance from bluff crest (or OHWM for no-) to most waterward infrastructure. The Salish Sea are characterized as complex with a high degree structures, possibly increasing and scour and in some case in- of variability and therefore site-specific analysis and design is always fluencing longshore transport patterns (Griggs 2010, Ruggiero CUMULATIVE RISK MODEL required for successful project design. The region has a wide range of 2010). Engineers have long been aware of localized end effects adja- EROSION POTENTIAL beach conditions, wave fetch distances, drift cell lengths, and littoral cent to wall ends associated with bulkheads and other coastal structures Shoretype Score Fetch Score transport rates which must be considered along with other factors in the No Appreciable Drift (NAD) - Planting native vegetation Large wood used to en- Rock used to (Kraus and McDougal 1996). 0 development of a final design. The design sequence for nourishment to manage bank erosion hance storm berm prevent bank erosion Bedrock/Low Energy 0-1 mile 1 projects would include these elements: Modified, Accretion Shoreform, 1 Examples reflecting a range of management approaches. Site Assessment Approach NAD-Delta • Determine risk to structures and establish project goals Backshore linear logs, minor nourishment Complex log pattern to trap logs (NW NAD-Artificial, Transport Zone, • Determine general extent and placement approach The site assessment takes into account the site-scale or “footprint” of 2 1-5 miles 2 for backshore (Oak ) Whidbey) Description of each design technique covered in Chapter 7. the subject property and erosion area to determine the mechanisms, • Evaluate design parameters and develop design details Feeder Bluff 3 5-15 miles 3 potential causes, and driving of erosion at the site. Evaluating Sediment supply degraded Littoral drift reduced up- Net drift cell shortened: Design Technique Description site conditions in the field and through background research includes: Feeder Bluff Exceptional 4 15+ miles 4 up-drift of site: drift of site: - Caused by major near- - Caused by cumulative - Caused by fill and struc- fill completely Erosion Potential Score = Shoretype Score + Fetch Score Beach Nourishment Beach nourishment is the addition of and gravel to • An evaluation of existing conditions and processes occurring at and impacts of numerous ture into the nearshore blocking drift (BN) build the beach to mitigate . immediately adjacent to the site hard armor structures at - Not completely blocking - Bifurcating historical drift • Locations and potential causes of erosion and/or mass wasting INFRASTRUCTURE THREAT historical feeder bluffs littoral drift, but reducing cell and reducing volume volume of littoral drift Large Wood (LW) Placement of large wood to retain beach materials and • Human infrastructure at the site, including existing erosion control Setback Score Infrastructure Type Score dissipate wave energy (may contain some rock to pro- measures >60 ft 1 Property without structures 1 vide ballast but not in excess of 20% areal density). • Risks to infrastructure, property, and public safety Septic drainfield or unattached resi- Potential impacts of sea level rise and climate change on nearshore processes (from 36-60 ft 2 2 • The extent and types of nearshore habitats and shoreforms dential infrastructure, not lived in Clancy et al. 2009). Reslope / Resloping is lowering the slope of the bank to increase 21-35 ft 3 Home or residential building 3 Revegetation (RE) stability. Revegetation is planting a bank with native Backshore placement at east end Hood Logs interlocking with (East 0-20 ft 4 Major infrastructure 4 Process Anticipated Impacts bridge Dungeness) riparian vegetation to create a root network (and ulti- Is ANY erosion acceptable? East Dungeness, Clallam County; East Lummi , Whatcom County; mately, shrub or tree canopy) that reduces erosion. It Infrastructure Threat Score = Setback Score + Infrastructure Type Score Sediment Increased sediment supply from bluff erosion and , backshore/dune placement MHHW-backshore placement can be applied to the existing bank or one which has • Erosion may not threaten a house or improvements if there is an supply and increased littoral drift rates, likely loss of sediment sources been resloped. This technique is exclusively applied to adequate setback from the beach or bluff crest (depending on transport because of new shore protection. the upland (above MHHW and the OHWM). site). Cumulative Total Risk = Erosion Potential x Infrastructure Threat • Landslides typically do not recur in one place for 25-50 years. Beach erosion Exacerbated erosion along erosional and generally stable Bulkhead Removal A beach restoration design technique applied along • Erosion may be easier to live with than costly permitting and in- and accretion shores and likely shifted areas of accretion. Overall landward Examination of the cumulative risk model results (Appendix A) allowed shift (transgression) of shore features and associated habi- (BR) shores where coastal erosion is not substantial or where stalling hard or soft shore protection. armor serves solely as a feature of landscaping. The for determination of different risk classes (e.g., low, medium, high) as tats. beach may be restored while infrastructure remains un- follows: affected. When is erosion “control” NOT recommended? chan- Channels may accrete with rising sea levels and have greater • On-site evidence suggests a coastal structure will not abate land- nel migration tendency for migration in response to altered freshwater input. Hard Armor-Rock Placement of stationary sloping rock, e.g., “”. slides due to upland processes. • Low risk scores between 0–15 Tidal Tidal channels may accrete and processes may become less Marine Park, Bellingham; lower intertidal North Beach, Orcas Island; lower inter- Revetment (RV) • Moderate risk scores between 16–36 ABSTRACT: Shore armor – the construction of bulkheads and seawalls • Shore change analysis indicates only a few feet of erosion over formation and predictable in response to altered freshwater input. to backshore placement tidal to backshore placement Anchored and placed high on the beach Minimizing end erosion at the ends of – has become a significant environmental issue in the Puget Sound 50 years, the site is relatively stable. • High risk scores greater than 36 maintenance for erosion control immediately adjacent bulkheads and transitioning into a soft region. Shore armor (also referred to as hard armor) has been continu- Hard Armor-Vertical Vertical face structure constructed of concrete, sheet to the Northwest Maritime Center in Port shore protection project. Bulkhead (VB) pile, rock, or wood. Freshwater Freshwater input predicted to become more variable, with Townsend. ously constructed over the past 50 years or more for a variety of rea- input more flooding (winter-) and drought conditions Legend Vegetation-dunegrass Profiles X Coastal Processes Assessment (summer-fall). sons including, but not limited to, protecting the shore from coastal ero- Pile Large woody debris ? Back Sight Examples of backshore log installations for different intents at erosion control sites in /" Sed photo Wrack sion. Years of scientific study has led to the determination that hard *# Survey setup the Puget Sound region.Arrows point at anchored logs where not obvious. Case study Pipe Creek bank The objective of the coastal processes assessment is to understand the D Tidal Greater inundation and tidal flows into semi-enclosed sys- armor profoundly influences coastal processes, alters coastal ecology, Toe of beach Concrete bulkhead Downspouts sites indicated in parenthesis; see Appendix A (photos J. Johannessen). 0 100 natural range of conditions at the site and identify landscape-scale tems, increased saltwater incursion. and reduces the resilience of the to rising sea level (Shipman et Feet causes of erosion. Results of the site visit and from the coastal pro- al. 2010, Schlenger et al. 2011, Johannessen and MacLennan 2007). cesses assessment should be used to identify the most appropriate Detritus recruit- Likely greater detritus recruitment due to overall greater wave ment and reten- energy reaching marine . Likely increased The Marine Shoreline Design Guidelines (MSDG) provide assesment management technique which will achieve project objectives with the tion storminess and storm surges (including more frequent and and design guidence for alternatives to hard armor for managing risk to least possible impact to nearshore processes, structure, and function. intense El Niño storms) and increased input from due to increased peak flows. Snakelum Point, Penn , Whidbey Tolmie State Park, Thurston County; structures and infrastructure posed by coastal erosion in the Puget D D Soft shore design techniques integrate and work with coastal pro- Island; MHHW-backshore placement lower intertidal to backshore placement Sound area. Guidence includes the use of best management practices, cesses. A thorough understanding of historical and conditions Exchange of Reduced productivity of threatened salmon stocks due to structure relocation, and implementation of “soft shore protection” pro- * # and their relationship to coastal processes provides the foundation for aquatic organ- increased winter flooding, decreased summer and fall Examples of the range of different beach nourishment projects surveyed for this study ject designs. Soft shore protection projects contrast from hard armor by (Appendix A), with placement approach noted in captions. ? isms flows, and elevated warm season and . / " appropriate design. The coastal processes assessment entails: preserving natural coastal shoreline dynamics that are immobilized Loss of biological diversity/localized extinctions of marine and / " Before (left, 2004) and after construction (right, 2007) at Marine Park in Bellingham, X / " freshwater species if habitat shifts outpace ability of species to X along an armored shore. Successful soft shore protection project de- Washington. This project is an example of multiple design techniques employed at one / " • First, the net shore-drift cell, shoretype, and general coastal pro- migrate or adapt to changing conditions. a) Pre-case concrete deadman anchor b) Auger type / " signs must be informed by a thorough understanding of specific site site: bulkhead and partial fill removal, with beach nourishment and a rock drift sill to / " cesses (such as change rates) are evaluated as a means of better un- with center hole (ecology block) conditions and work within the range of current and historical coastal contain sediment (partially visible in far end of beach), and adjacent revetment derstanding the natural range of physical conditions at the site. For maintained to protect railway (on right). Solar radiation Altered solar patterns due to hotter summers, colder winters. processes. Science and engineering principals are combined to develop example, does the drift cell naturally have a high volume of sediment MHHW MHHW and monitor designs. in transport? What is the tidal range at the site? Wind and Overall greater wave erosion and potential accretion due to waves SLR. Geomorphic setting of Puget Sound MLLW Profile MLLW Lower intertidal to backshore • The second element includes an assessment of how intact or de- CANADA Vancouver UNITED STATES Puget Sound has approximately 4,000 km (2,500 miles) of shore, much Fra graded nearshore processes are, and the potential repercussions of ser Example site plan created following a residential site assessment conducted by CGS of it consisting of and coastal bluffs subject to chronic erosion. Figure Location St current processes on site conditions. For example, is sediment supply Phase 1 Case Study Assessment results rait (Oak Bay Case Study site).The features on this map reflect items in the Site Visit of Georgia Many segments of this shoreline are heavily developed, with roads, British Columbia . degraded due to armored feeder bluffs up-drift? WASHINGTON STATE Washington R homes, and industry along the water’s edge. Other shoreline areas MSDG began with a case study assessment of 25 previously con- k iv c e MHHW MHHW a r s k o • Finally, the assessment should include an evaluation of how the site o remain relatively unaltered, but are under increasing . structed applications of the 6 major design techniques commonly imple- N Identifying Site-Level Causes of Erosion Bellingham will respond to sea level rise and other implications of climate change. mented for erosion control on Puget Sound shores (Table 5-3, Figure San Juan MLLW Upper intertidal to backshore MLLW MHHW to backshore c) Earth anchor d) Pinned in place with rock r This has raised concerns about the long-term impact of erosion control e v Vancouver Island i 5-5). and vertical bulkhead projects were later combined R BARRIER BEACH SITES S ish practices on shoreline dynamics, coastal ecosystems, and public re- am Skagit River into a single technique named “hard armor”, resulting in 5 design tech- Anacortes sponsibilities for managing the coast (Macdonald et al. 1994, Broad- Flattery nique categories. Detailed field surveys and case study assessments o Victoria hurst 1998). Erosion is not just a threat to shoreline property but is also Neah Bay f rk were performed for each project site and adjacent shores to evaluate Juan North Fo de Fuca an important natural geomorphic process that builds beaches and main- S Hokover tillaguamish site characteristics, design specifications, and the relative benefits and Ri MHHW MHHW River S out tains coastal habitats (Johannessen and MacLennan 2007). Eroding r h F e or impacts relevant to project goals and performance. A successful shore-

v k Port i

R coastal bluffs are the primary source of beach sediment on most Puget

s

Angeles s El e line design project should integrate the following key elements: MLLW MLLW

S n Backshore/dune wha n Everett OHWM 2 e H B ª g o Sound beaches (Keuler 1988, Johannessen and MacLennan 2007). ` ¢ \ K h n o u m RANGE River i ish R D s Skykom ive h r OHWM 1 Do R S • Use of appropriate design techniques for the site conditions at the se . n MHHW MHHW MHHW w o Beach nourishment placement approaches for Puget Sound application from most a OUND ll q Mixed grain-size beaches, such as those on Puget Sound, exhibit com- i S u ps Rive OLYMPIC r a landscape (coastal process unit) scale and site-specific (parcel) scale l m extensive in profile to least. Note that the limited-width approaches sometimes have ckabush Ri Seattle u ve i MOUNTAINS D r e plex patterns of both cross-shore and longshore R • Adherence to general design standards excavation into the beach (as shown by hatching in MHHW to Backshore profile) for a Ham iv m m Duwam e PUGET Entire shore eroded: Area(s) of wave overwash: Specific area eroded: am er a r (Adams et al. 2007, Curtiss et al. 2008, Warrick et al. 2009). Like other Riv nal e) Interlocking in a dune (no anchors) f) Concrete ballasted log SkokomishH Bremerton River - Caused by natural / pas- - Intermittent only, influ- - Caused by adjacent bulk- • Provides maximum benefits and minimizes negative impacts keying in the toe of gravel at sites where erosion control is critical. ish Ceda od Ca r estuarine beaches, those on Puget Sound are characterized by a Ho River sive erosion or coastal enced by site conditions head end-effects; persis- River CASCADE Example of different anchoring approaches and hardware used in Puget Sound G veneer of mobile beach sediment and low longshore transport rates. r ee ver processes impacts (gaps in vegetation here) tent n Ri This Design section is organized to assist the designer to assess and projects. Credits: a–c, f) Coastal Geologic Services; d) Warren Demetrick; e) Four W - Caused by natural / pas- - High water events affect- - A site-based cause $+ MARINE SUBBASINS h Seasons Engineering. ite The irregular shape of the shoreline, combined with the fetch-limited ^_%, evaluate key design parameters. The goal is to balance project ele- Strait TacomaP River

u sive erosion, not a site- ing areas lacking vegeta- y a Whidbey Basin l l wave environment, leads to the division of the coast into hundreds of ments to achieve a functioning and resilient beach, without unnecessary u based cause of erosion tion for erosion control p Central Puget Sound D e N R Whatcom County s is iv discrete littoral cells, each with its own sources and sinks of sediment c q e changes or impacts to coastal processes or on- or off-site habitats. This h u r South Puget Sound Olympia u a Reslope-Revegetation Design: Slope Grading t ll Bellingham es y R BLUFF SITES Hood Canal R iv (Schwartz et al. 1989). $+ section is organized as follows: iver er *# Puget Sound watershed *# k *# The designed and constructed slope gradient must be based on the re- boundary 0 MILES 30 • Placement approach and project length sults of a thorough site assessment to determine the causes of slope %, k 0 KILOMETERS 50 Impacts of Armor k k • Wave energy and volume density instability. Where erosion of the slope toe is the trigger for transferring San Juan County • Site geometry These regional studies suggest a broad range of potential effects of ero- instability upslope, other treatments such as large wood and beach The Puget Sound basin, showing major rivers, oceanographic subbasins, k • Drift sills sion control structures on Puget Sound shores. In general, these can be Landslide Skagit County nourishment might be considered. However, slope instability is often and selected locations. scarp • Berm elevation categorized as follows: ^_ caused by upslope land-use practices such as concentrated surface OHWM 2 Sediment supply degradation in northeast Jefferson County from PSNERP Strategic • Sediment size selection run-off, excessive groundwater recharge (from irrigation, septic systems, • Loss of upper beach and backshore. Even when built high on the OHWM 1 Range of approaches, design techniques, key elements, and negative impacts to near- MHHW MHHW MHHW Needs Assessment process evaluation framework (Schlenger et al. 2011, Simenstad k • Beach habitat considerations beach profile, seawalls typically eliminate a narrow zone of the high concentrated or inappropriate water ), or yard waste disposal. shore processes. et al. 2011). • Site grading beach. On Puget Sound, this may result in the absence of accumulated *# For this reason, resloping is more commonly applied to the upper por- Bluff subjected to toe Specific bluff area eroded Specific bluff area eroded Island County tions of a bluff or bank. drift logs and beach wrack and the loss of dry beach at high , which erosion and intermittent at toe, which leads to land- where *# Port Type of Impacts to Townsend Design Technique Key Elements may in turn reduce the area available for forage fish spawning (Penttila landslides: slides: runoff concentrated: Identifying Coastal Processes Causes of Erosion Approach Processes Clallam County Large Wood Design Because grading work generally exposes sediment that is subject to 2007) and for recreation. - Caused by natural / pas- - End erosion caused by - Impervious surfaces ^_ Snohomish County Legend erosion, subsequent planting is an important component of this stabiliz- Restoration Bulkhead removal Removal of structures to restore Improve- • Armor modifies the natural transi- sive erosion, not a site- adjacent up-drift rock draining to one part of Bluff or Barrier Beach Sites Everett Large wood placements are not as well understood as beach nourish- Aquatic-terrestrial connectivity. Project Type ing treatment. Vegetation improves soil cohesion, improves soil drain- the natural beach profile ment tion between terrestrial and aquatic ecosystems. This can affect move- based cause bulkhead the bluff crest *# ment, hard armor, or other techniques described in these guidelines. - A site-based cause - A site-based cause Beach Nourishment (BN) age through aeration and evapotranspiration, and reduces surface ero- ^_ Large Wood Placement (LW) The interaction of logs with waves and determining the forces acting on Passive Best mgmt. practices Nonengineered management None ment of materials and organisms between systems, reduce the quality ^_ sion. The planting configuration should be based on the slope gradient, $+ Reslope/Reveg (RE)Jefferson County Techniques Vegetation mgmt. practices such as planting of riparian functions, and introduce discontinuities to this narrow eco- Examples of common site based causes of erosion/mass wasting at barrier beach and logs by waves in particular are examples of this. %, Bulkhead Removal (BR) nature of the sediment (or substrate), type of erosion being addressed, Relocation native vegetation and managing tone and ecological corridor. Structures also tend to result in alterations bluff sites. 2 k Rock Revetment (RV) This section describes elements of feasibility for installing large wood and plant species being installed. Native plants are strongly encour- surface and groundwater 2 to the pattern of natural drainage to the beach. k Vertical Bulkhead (VB) aged. Kitsap County and the specific design elements unique to large wood placement pro- Preservation/enhancement of 0 30 • Passive erosion. Most shores in Puget Sound are naturally eroding. jects including: the specific location within the beach profile that the natural processes How to Measure Cumulative Risk 1 A seawall or revetment may effectively stabilize the area landward of the 1 Miles Infrastructure unaffected, relo- 1 2 Basemap Sources: GEBCO, NOAA, National King County large wood will be placed (elevation and orientation), large wood size cated, or removed structure, but does nothing to address the underlying retreat of the A cumulative risk model was developed for these guidelines which inte- Geographic, DeLorme, and ESRI. and type to be used, whether it is placed atop the current beach grade beach face or shoreline, which will continue on the seaward side of the grates natural conditions with risk to infrastructure (Table 3-4). The or buried (and to what depth if buried), and the type of anchoring Soft Shore Beach nourishment Preservation of natural pro- Low structure (Fletcher et al. 1997, Griggs 2005). model was calibrated and refined using data from the 25 case study Mason County %, mechanism, if anchoring is necessary. This guidance is divided into the Protection Large wood cesses and coastal dynamics • Sediment delivery and transport. Seawalls on coastal bluffs stop sites, along with some additional site data from Shipman et al. (unpub.) Reslope/revegetation Use of natural materials following sections: Slowing rather than eliminating the natural erosion of feeder bluffs, thereby reducing the delivery of and unpublished data from other sites. The first step of the risk model Channel migration: Northward migration of Migrating sand wave: Grays Harbor County erosion sediment to the littoral system and reducing the overall budget of the entails quantifying the erosion potential of the site by evaluating its - Caused by tidal channel shoreform: - Caused by a pulse of %, • Feasibility assessment ^_ *# Pierce County 1711 Ellis St, Suite 103 local littoral cell. Bulkheads that encroach across the beach, either be- shoretype and maximum fetch. The sites with the greatest erosion po- or subtidal river channel - South shore recedes as sediment migrating in the kk • Large wood placement elevation Hard Revetments Halting natural processes, creat- Moderate- cause of their original construction, or because of subsequent erosion of tential are high bluffs with considerable wave exposure, while bedrock migrating closer to shore entire migrates north direction of net shore-drift Thurston County • Large wood placement orientation Bellingham, WA 98225 Armor Vertical bulkheads ing a static shoreline to-high adjacent shores (passive erosion), may act as groins, impeding long- and low energy shores (with no appreciable drift – NAD-LE), get the - Waves and currents ex- - Predominant southerly - The bluff toe erodes with- • Large wood characteristics acerbate erosional pro- waves and net northward out the lobe of sediment coastalgeo.com (“seawalls”) Lost beach area and substrate • Anchoring Attempts to eliminate erosion shore transport of sediment and leading to localized erosion on down- lowest score. The shoretype score is then added to the fetch score to cesses at the site sediment transport to buffer wave attack drift properties. give the resultant erosion potential score. Sites assessed as part of the MSDG case study assessment. • Cost