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Bromirski, PD, Miller AJ, Flick RE, Auad G.  2011.  Dynamical suppression of sea level rise along the Pacific coast of North America: Indications for imminent acceleration. Journal of Geophysical Research-Oceans. 116   10.1029/2010jc006759   AbstractWebsite

Long-term changes in global mean sea level (MSL) rise have important practical implications for shoreline and beach erosion, coastal wetlands inundation, storm surge flooding, and coastal development. Altimetry since 1993 indicates that global MSL rise has increased about 50% above the 20th century rise rate, from 2 to 3 mm yr(-1). At the same time, both tide gauge measurements and altimetry indicate virtually no increase along the Pacific coast of North America during the satellite epoch. Here we show that the dynamical steric response of North Pacific eastern boundary ocean circulation to a dramatic change in wind stress curl, tau(xy), which occurred after the mid-1970s regime shift, can account for the suppression of regional sea level rise along this coast since 1980. Alarmingly, mean tau(xy) over the North Pacific recently reached levels not observed since before the mid-1970s regime shift. This change in wind stress patterns may be foreshadowing a Pacific Decadal Oscillation regime shift, causing an associated persistent change in basin-scale tau(xy) that may result in a concomitant resumption of sea level rise along the U.S. West Coast to global or even higher rates.

Bromirski, PD, Flick RE.  2008.  Storm surge in the San Francisco Bay/Delta and nearby coastal locations. Shore & Beach. 76:29-37. Abstract

California’s San Francisco Bay/Sacramento-San Joaquin Delta (bay/delta) estuary system is subject to externally forced storm surge propagating from the open ocean. In the lower reaches of the delta, storm surge dominates water level extremes and can have a significant impact on wetlands, freshwater aquifers, levees, and ecosys- tems. The magnitude and distribution of open-ocean tide generated storm surge throughout the bay/delta are described by a network of stations within the bay/delta system and along the California coast. Correlation of non-tide water levels between stations in the network indicates that peak storm surge fluctuations propagate into the bay/delta system from outside the Golden Gate. The initial peak surge propa- gates from the open ocean inland, while a trailing (smaller amplitude) secondary peak is associated with river discharge. Extreme non-tide water levels are generally associated with extreme Sacramento-San Joaquin river flows, underscoring the po- tential impact of sea level rise on the delta levees and bay/delta ecosystem.

Bromirski, PD, Flick RE, Miller AJ.  2017.  Storm surge along the Pacific coast of North America. Journal of Geophysical Research-Oceans. 122:441-457.   10.1002/2016jc012178   AbstractWebsite

Storm surge is an important factor that contributes to coastal flooding and erosion. Storm surge magnitude along eastern North Pacific coasts results primarily from low sea level pressure (SLP). Thus, coastal regions where high surge occurs identify the dominant locations where intense storms make landfall, controlled by storm track across the North Pacific. Here storm surge variability along the Pacific coast of North America is characterized by positive nontide residuals at a network of tide gauge stations from southern California to Alaska. The magnitudes of mean and extreme storm surge generally increase from south to north, with typically high amplitude surge north of Cape Mendocino and lower surge to the south. Correlation of mode 1 nontide principal component (PC1) during winter months (December-February) with anomalous SLP over the northeast Pacific indicates that the dominant storm landfall region is along the Cascadia/British Columbia coast. Although empirical orthogonal function spatial patterns show substantial interannual variability, similar correlation patterns of nontide PC1 over the 1948-1975 and 1983-2014 epochs with anomalous SLP suggest that, when considering decadal-scale time periods, storm surge and associated tracks have generally not changed appreciably since 1948. Nontide PC1 is well correlated with PC1 of both anomalous SLP and modeled wave height near the tide gauge stations, reflecting the interrelationship between storms, surge, and waves. Weaker surge south of Cape Mendocino during the 2015-2016 El Nino compared with 1982-1983 may result from changes in Hadley circulation. Importantly from a coastal impacts perspective, extreme storm surge events are often accompanied by high waves.

Bromirski, PD, Flick RE, Cayan DR.  2003.  Storminess variability along the California coast: 1858-2000. Journal of Climate. 16:982-993.   10.1175/1520-0442(2003)016<0982:svatcc>;2   AbstractWebsite

The longest available hourly tide gauge record along the West Coast (U. S.) at San Francisco yields meteorologically forced nontide residuals (NTR), providing an estimate of the variation in "storminess'' from 1858 to 2000. Mean monthly positive NTR (associated with low sea level pressure) show no substantial change along the central California coast since 1858 or over the last 50 years. However, in contrast, the highest 2% of extreme winter NTR levels exhibit a significant increasing trend since about 1950. Extreme winter NTR also show pronounced quasi-periodic decadal-scale variability that is relatively consistent over the last 140 years. Atmospheric sea level pressure anomalies (associated with years having high winter NTR) take the form of a distinct, large-scale atmospheric circulation pattern, with intense storminess associated with a broad, southeasterly displaced, deep Aleutian low that directs storm tracks toward the California coast.

Bromirski, PD, Cayan DR, Flick RE.  2005.  Wave spectral energy variability in the northeast Pacific. Journal of Geophysical Research-Oceans. 110   10.1029/2004jc002398   AbstractWebsite

The dominant characteristics of wave energy variability in the eastern North Pacific are described from NOAA National Data Buoy Center ( NDBC) buoy data collected from 1981 to 2003. Ten buoys at distributed locations were selected for comparison based on record duration and data continuity. Long- period ( LP) [ T > 12] s, intermediate- period [ 6 <= T <= 12] s, and short- period [ T < 6] s wave spectral energy components are considered separately. Empirical orthogonal function ( EOF) analyses of monthly wave energy anomalies reveal that all three wave energy components exhibit similar patterns of spatial variability. The dominant mode represents coherent heightened ( or diminished) wave energy along the West Coast from Alaska to southern California, as indicated by composites of the 700 hPa height field. The second EOF mode reveals a distinct El Nino-Southern Oscillation (ENSO)-associated spatial distribution of wave energy, which occurs when the North Pacific storm track is extended unusually far south or has receded to the north. Monthly means and principal components (PCs) of wave energy levels indicate that the 1997 - 1998 El Nino- winter had the highest basin- wide wave energy within this record, substantially higher than the 1982 - 1983 El Nino. An increasing trend in the dominant PC of LP wave energy suggests that storminess has increased in the northeast Pacific since 1980. This trend is emphasized at central eastern North Pacific locations. Patterns of storminess variability are consistent with increasing activity in the central North Pacific as well as the tendency for more extreme waves in the south during El Nino episodes and in the north during La Nina.

Bromirski, PD, Flick RE, Graham N.  1999.  Ocean wave height determined from inland seismometer data: Implications for investigating wave climate changes in the NE Pacific. Journal of Geophysical Research-Oceans. 104:20753-20766.   10.1029/1999jc900156   AbstractWebsite

Knowing the wave climate along the California coast is vital from the perspectives of climatological change and planning shore protection measures. Buoy data indicate that the wave climate is very similar along much of the California coast. We show that elements of the wave climate can be accurately reconstructed using near-coastal inland broadband seismometer data. Such reconstructions are possible because swell approaching the coast generates pressure fluctuations that are locally transformed into seismic waves at the seafloor that propagate inland and are detectable by land-based seismometers. Buoy and seismometer data show that most of the microseism energy recorded inland near the coast is generated from wave events at nearby coastal locations. A site-specific, empirically derived seismic-to-wave transfer function is demonstrated to be applicable to seismic data from the same location for any year. These results suggest that ocean wave heights estimated from near-coastal broadband seismometer data are sufficiently reliable for monitoring the coastal wave height when buoy data are unavailable, provided that adequate simultaneous nearby buoy measurements are available to calibrate the seismometer data. The methodology presented here provides an important tool that allows the investigation of potential wave climate changes from reconstructions using archived seismic data collected since the 1930s.

Cayan, DR, Bromirski PD, Hayhoe K, Tyree M, Dettinger MD, Flick RE.  2008.  Climate change projections of sea level extremes along the California coast. Climatic Change. 87:S57-S73.   10.1007/s10584-007-9376-7   AbstractWebsite

California's coastal observations and global model projections indicate that California's open coast and estuaries will experience rising sea levels over the next century. During the last several decades, the upward historical trends, quantified from a small set of California tide gages, have been approximately 20 cm/century, quite similar to that estimated for global mean sea level. In the next several decades, warming produced by climate model simulations indicates that sea level rise (SLR) could substantially exceed the rate experienced during modem human development along the California coast and estuaries. A range of future SLR is estimated from a set of climate simulations governed by lower (B1), middle-upper (A2), and higher (A1fi) GHG emission scenarios. Projecting SLR from the ocean warming in GCMs, observational evidence of SLR, and separate calculations using a simple climate model yields a range of potential sea level increases, from 11 to 72 cm, by the 2070-2099 period. The combination of predicted astronomical tides with projected weather forcing, El Nino related variability, and secular SLR, gives a series of hourly sea level projections for 2005-2100. Gradual sea level rise progressively worsens the impacts of high tides, surge and waves resulting from storms, and also freshwater floods from Sierra and coastal mountain catchments. The occurrence of extreme sea levels is pronounced when these factors coincide. The frequency and magnitude of extreme events, relative to current levels, follows a sharply escalating pattern as the magnitude of future sea level rise increases.

Elwany, MHS, Flick RE, Aijaz S.  1998.  Opening and closure of a marginal southern California lagoon inlet. Estuaries. 21:246-254.   10.2307/1352472   AbstractWebsite

Over the past 50 yr, direct observations of the inlet status (open or closed) of San Dieguito Lagoon, a typical southern California lagoon located in Del Mar, California, have shown that river flooding is the major natural determinant of inlet conditions on time scales longer than a few years. River flooding is strongly dependent on rainfall in the San Dieguito River watershed and on the influences of two water storage reservoirs in the area. Rainfall fluctuates on yearly and longer time scales and undergoes cycles of wet and dry periods. Over short time periods, ranging from a few months to several years, inlet status is primarily determined by the available tidal prism and littoral sand transport. Recognition of these factors is crucial in order to correctly evaluate the probability that a small lagoon will remain open naturally. A probability approach is essential because the variables controlling inlet conditions are random in nature. The results of our study show that the inlet will remain open naturally 34% of the time. The tendency to remain open is vastly smaller during years of dry weather (12%) versus times of above-average rainfall (66%).

Elwany, MHS, Flick RE, Hamilton MM.  2003.  Effect of a small southern California lagoon entrance on adjacent beaches. Estuaries. 26:700-708.   10.1007/bf02711981   AbstractWebsite

This paper considers the effects of natural and artificial openings of a typical, small, southern California coastal estuarine lagoon on the adjacent barrier beach. A detailed history of beach profiles and lagoon entrance transects before and after flood-induced and artificial openings of San Dieguito Lagoon in Del Mar, California, has been analyzed. The results suggest that there is no statistically significant erosional effect on the adjacent beach when the lagoon inlet is artificially opened to tidal flow.

Elwany, MHS, Flick RE.  1996.  Relationship between kelp beds and beach width in Southern California. Journal of Waterway Port Coastal and Ocean Engineering-Asce. 122:34-37.   10.1061/(asce)0733-950x(1996)122:1(34)   AbstractWebsite

The relationship between the width of kelp beds and the width of the beaches inshore was examined in the San Diego region of Southern California. Two statistical approaches were used. The first simply determined the correlation between kelp-bed width and adjacent-beach width. A small (0.3), but statistically significant, positive correlation was found in the 20% of shoreline that had both a nonzero beach width and an offshore kelp bed; however, no correlation was found when the entire shoreline was considered. The second method examined differences in width between beaches inshore of the kelp beds and those immediately to the north and south. No statistically significant differences were found. The overall conclusion is that there is no clear correlation or consistent pattern indicating that offshore kelp beds have any direct influence on adjacent-beach width.

Elwany, MHS, Oreilly WC, Guza RT, Flick RE.  1995.  Effects of Southern California Kelp Beds on Waves. Journal of Waterway Port Coastal and Ocean Engineering-Asce. 121:143-150.   10.1061/(asce)0733-950x(1995)121:2(143)   AbstractWebsite

The effect of a Macrocystis kelp forest on shoreward propagating surface gravity waves was measured. Observations were made over a 67-day period at four locations around a 350-m-wide kelp bed off Carlsbad, California. Instruments were located directly offshore and onshore of the kelp bed at depths of 13 m and 8 m, respectively, and at control stations at the same depths, but displaced 750 m alongshore, away from the kelp bed. The bathymetry between the offshore and onshore sites was gently sloping and featureless. The measured spectra, significant wave height, mean wave direction at peak frequency, and total radiation stress differed only slightly between the offshore kelp and control stations and were similar at the onshore sites. The similarity of the wave field at the onshore kelp and control sites shows that this typical southern California kelp bed, with an average density of about 10 plants per 100 m(2), does not have a significant effect on waves. These measurements can be used to place upper bounds on drag coefficients in numerical models of the effect of kelp on waves.

Ewing, L, Flick RE, Synolakis CE.  2010.  A review of coastal community vulnerabilities toward resilience benefits from disaster reduction measures. Environmental Hazards-Human and Policy Dimensions. 9:222-232.   10.3763/ehaz.2010.0050   AbstractWebsite

The coast has always been an area of significant hazards. In situations of community self-sufficiency, consequences of coastal hazards might be isolated to regions directly affected by the hazard. But, in the current global economy, fewer and fewer communities are isolated; damage to one location frequently has consequences around the globe and coastal community resilience can have broad-reaching benefits. Hazard responses for the built coastal environment have typically been resistance: constructing stronger buildings, enhancing natural barriers or creating artificial barriers. These approaches to hazard reduction through coastal engineering and shoreline defence efforts have been crucial to sustained coastal development. However, as coastal forces continue or magnify and resources become scarcer, resistance alone may be less effective or even unsustainable, and interest in resilience has grown. Resilience is a community's ability either to absorb destructive forces without loss of service or function, or to recover quickly from disasters. Community resilience encompasses multiple elements, ranging from governance to structural design, risk knowledge, prevention, warning systems and recovery. This paper focuses on hazards of coastal communities, and provides a review of some recent engineering efforts to improve the resilience elements of risk knowledge and disaster warnings for coastal disaster reduction.

Flick, RE, Guza RT, Inman DL.  1981.  Elevation and Velocity-Measurements of Laboratory Shoaling Waves. Journal of Geophysical Research-Oceans and Atmospheres. 86:4149-4160.   10.1029/JC086iC05p04149   AbstractWebsite

Measurements of wave elevation and orbital velocity in the shoaling, breaking, and bore regime of single-frequency laboratory waves show that third-order Stokes theory, when energy flux is conserved, predicts the wave height change and harmonic growth in the regime where the Ursell number Ur = (H/ h)/(kh)2 is 0(1) or less. Shoreward of the Stokes region and up to the breakpoint, harmonic amplitudes are well described by the cnoidal theory. It is shown theoretically that a smooth transition regime exists between Stokes and cnoidal regions for waves which eventually break by plunging. The wave profile asymmetry about the vertical plane observed in near-breaking waves and bores is due to slow changes of phase of the harmonics relative to the primary wave as the wave train shoals. By contrast, only asymmetry about the horizontal plane is possible in the Stokes and cnoidal wave theories, since these classical solutions allow no relative phase shifts between harmonics. Velocity measurements made with hot-film anemometers show that ‘unorganized’ fluctuations at the bottom under breaking waves are of the order of half the rms amplitude of the wave-induced ‘organized’ flow. The correlation between surface elevation and bottom velocity under breakers and bores suggests that turbulence contributes more strongly to the unorganized flow at the bottom under plunging than under spilling waves.

Flick, RE, Bromirski PD.  2019.  Ham Radio from McMurdo Station and Yesterday Camp, Ross Ice Shelf, Antarctica. QST Journal of the American Radio Relay League. 103(2):65-67.
Flick, RE, Guza RT.  1980.  Paddle Generated Waves in Laboratory Channels. Journal of the Waterway Port Coastal and Ocean Division-Asce. 106:79-97. AbstractWebsite
Flick, RE, Murray JF, Ewing LC.  2003.  Trends in United States tidal datum statistics and tide range. Journal of Waterway Port Coastal and Ocean Engineering-Asce. 129:155-164.   10.1061/(asce)0733-950x(2003)129:4(155)   AbstractWebsite

Yearly tidal datum statistics and tide ranges for the National Oceanic and Atmospheric Administration/National Ocean Service long-term stations in the United States tide gauge network were compiled and used to calculate their trends and statistical significance. At many stations, significant changes in the tide range were found, either in the diurnal tide range [mean higher high water (MHHW)-mean lower low water (MLLW)], or mean tide range [mean high water (MHW)-mean low water (MLW)]. For example, at San Francisco, the diurnal tide range increased by 64 mm from 1900 to 1998, while at Wilmington, N.C., the mean tide range increased at a rate of 542 mm per century from 1935 to 1999. This analysis suggests that any studies concerned with present or future water levels should take into account more tidal datum statistics than just mean sea level (MSL). For example, coastal flooding and storm damage studies should consider trends in high water levels, since it is the peak values that cause flooding and determine the design of coastal structures. For habitat restoration planning, mean low water and tide range changes should be considered.

Flick, RE, Knuuti K, Gill SK.  2013.  Matching Mean Sea Level Rise Projections to Local Elevation Datums. Journal of Waterway Port Coastal and Ocean Engineering-Asce. 139:142-146.   10.1061/(asce)ww.1943-5460.0000145   AbstractWebsite

A method is presented to consistently tie future mean sea level rise (MSLR) scenario projections to local geodetic and tidal datums. This extends the U. S. Army Corps of Engineer (USACE) guidance for incorporating the effects of future MSLR into coastal projects. While USACE relies on the National Oceanic and Atmospheric Administration (NOAA) 19-year National Tidal Datum Epoch (NTDE) for its datum relationships, the approach proposed herein generalizes this guidance by choosing the appropriate 19-year epoch centered on the start year of the MSLR scenario under consideration. The procedure takes into account the local annual sea level variability, which confounds the matching to any given single year while generalizing and preserving the 19-year averaging long used by NOAA to calculate the NTDE. Examples of the MSLR scenario matching procedure are given using actual data and projections for La Jolla, California, and Sewells Point (Hampton Roads), Virginia. DOI: 10.1061/(ASCE)WW.1943-5460.0000145. (C) 2013 American Society of Civil Engineers.

Gallien, TW, Sanders BF, Flick RE.  2014.  Urban coastal flood prediction: Integrating wave overtopping, flood defenses and drainage. Coastal Engineering. 91:18-28.   10.1016/j.coastaleng.2014.04.007   AbstractWebsite

Flood extent field observations collected following a wave overtopping event are used to evaluate the accuracy of two urban flood prediction models: a static ('bathtub') model that simply compares water level to land elevation, and a hydrodynamic model that resolves embayment dynamics, overland flow, concrete flood walls, and drainage into the storm water system. Time-dependent overtopping rates were estimated using empirical models parameterized with survey data and local wave heights transformed to the nearshore using Simulating Waves Nearshore (SWAN) and subsequently input to the hydrodynamic model. The hydrodynamic model showed good agreement with field observations, whereas the static model substantially overpredicted flooding suggesting that urban backshore flood depths do not equilibrate with shoreline water levels in transient events. In the absence of a high backwater condition, storm system drainage attenuates wave overtopping flooding. Hydrodynamic model simulations suggest that bay side flood defenses may exacerbate flooding by restricting drainage and that temporary flood mitigation berms can significantly reduce backshore flooding. This study points to a promising urban coastal flood prediction and management framework. (C) 2014 Elsevier B.V. All rights reserved.

Gallien, TW, O'Reilly WC, Flick RE, Guza RT.  2015.  Geometric properties of anthropogenic flood control berms on southern California beaches. Ocean & Coastal Management. 105:35-47.   10.1016/j.ocecoaman.2014.12.014   AbstractWebsite

Coastal flood riskfrom coincident high tides' and energetic waves is concentrated around low-lying urban areas. Municipalities construct temporary sand berms (also known as sacrificial dunes) to manage potential flooding, however the relationships between berm geometry (e.g., height, width and length) and performance are not understood. Concomitant pressures of sea level rise and urbanization will increase active beach berming. Effective future coastal flood risk management will depend upon optimizing berm efficacy relative to geometry, placement, and water levels. Here, 34 individual berms at seven southern California locations are characterized using 18 LiDAR datasets spanning nearly a decade. Three berm classifications emerged based on deployment duration: event, seasonal and persistent. Event berms, deployed to manage specific storms or high water events, are triangular in cross-section, relatively low volume (similar to 4 m(3)/m) and low crest elevation (similar to 5 m NAVD88). Seasonal berms are larger, volumes vary from 6 to 28 m(3)/m, and average crest elevations are between 5.3 and 6.4 m. A persistent berm, captured in all LiDAR data for that area, is the largest (48 m(3)/m), longest (1.2 km), and highest mean crest elevation (7 m NAVD88) of all study berms. Total water levels, estimated using observed tides and a regional wave model coupled with an empirical runup formula, suggest that overtopping is rare. Currently, event berms are vulnerable to wave attack only a few hours per year. However, even with modest sea level rise (similar to 25 cm) or El Nino conditions, exposure increases significantly, and substantial nourishments may be required to maintain current flood protection levels. (C) 2014 Elsevier Ltd. All rights reserved.

George, R, Flick RE, Guza RT.  1994.  Observations of Turbulence in the Surf Zone. Journal of Geophysical Research-Oceans. 99:801-810.   10.1029/93jc02717   AbstractWebsite

Turbulence generated by waves breaking on a natural beach is examined using hotfilm anemometer data. Turbulence intensity is estimated from dissipation rates determined from wavenumber spectra of short (1/8 s) hotfilm time series. The resulting Froude-scaled turbulence intensities are relatively uniform between the seabed and the wave trough level and are similar in vertical structure but lower in magnitude than in existing laboratory studies. The magnitudes of the turbulence intensities observed in both the field and laboratory are consistent with an existing macroscopic model of bore dissipation in the surf zone. Scaling by this bore model relates turbulence intensities generated by monochromatic waves in small-scale laboratory experiments to those generated by random waves in the natural surf zone.

Graham, NE, Cayan DR, Bromirski PD, Flick RE.  2013.  Multi-model projections of twenty-first century North Pacific winter wave climate under the IPCC A2 scenario. Climate Dynamics. 40:1335-1360.   10.1007/s00382-012-1435-8   AbstractWebsite

A dynamical wave model implemented over the North Pacific Ocean was forced with winds from three coupled global climate models (CGCMs) run under a medium-to-high scenario for greenhouse gas emissions through the twenty-first century. The results are analyzed with respect to changes in upper quantiles of significant wave height (90th and 99th percentile H-S) during boreal winter. The three CGCMs produce surprisingly similar patterns of change in winter wave climate during the century, with waves becoming 10-15 % smaller over the lower mid-latitudes of the North Pacific, particularly in the central and western ocean. These decreases are closely associated with decreasing windspeeds along the southern flank of the main core of the westerlies. At higher latitudes, 99th percentile wave heights generally increase, though the patterns of change are less uniform than at lower latitudes. The increased wave heights at high latitudes appear to be due a variety of wind-related factors including both increased windspeeds and changes in the structure of the wind field, these varying from model to model. For one of the CGCMs, a commonly used statistical approach for estimating seasonal quantiles of H-S on the basis of seasonal mean sea level pressure (SLP) is used to develop a regression model from 60 years of twentieth century data as a training set, and then applied using twenty-first century SLP data. The statistical model reproduces the general pattern of decreasing twenty-first century wave heights south of similar to 40 N, but underestimates the magnitude of the changes by similar to 50-70 %, reflecting relatively weak coupling between sea level pressure and wave heights in the CGCM data and loss of variability in the statistically projected wave heights.

Inman, DL, Nordstrom CE, Flick RE.  1976.  Currents in Submarine Canyons - Air-Sea-Land Interaction. Annual Review of Fluid Mechanics. 8:275-310.   10.1146/annurev.fl.08.010176.001423   AbstractWebsite
Kochnower, D, Reddy SMW, Flick RE.  2015.  Factors influencing local decisions to use habitats to protect coastal communities from hazards. Ocean & Coastal Management. 116:277-290.   10.1016/j.ocecoaman.2015.07.021   AbstractWebsite

Coastal hazard mitigation policy in the US has historically focused on construction of hardened, or gray, infrastructure. Recently, there is increased public interest and policy supporting the use of habitats, or natural infrastructure (NI), following decades of increasingly supportive ecological, engineering, and economic evidence. This trend suggests that behavioral and institutional factors may also be important for mainstreaming NI. To understand what factors affected decisions to use NI, we conducted semi-structured interviews with a total of 16 individuals associated with three NI cases: Ferry Point Park Living Shoreline, Maryland (MD); Surfer's Point Managed Retreat, California (CA); and Durant's Point Living Shoreline, North Carolina (NC). Our grounded theory analysis of the interview transcripts revealed four common themes across the decisions: 1) perception of benefits (N = 45) and costs (N = 31), 2) diffusion of innovation led by innovators (N = 34), 3) local champions (N = 46), and 4) social networks and norms (N = 30). This grounded theory suggests that the decisions to use NI were driven by innovators (citizens, local non-governmental organization (NGO) staff, and/or state government resource managers) who were influenced by seeing NI successes implemented by trusted experts and perceived NI benefits beyond protecting coastlines (e.g., maintaining coastal heritage and sense of place). Innovators also acted as local champions, getting others "comfortable" with NI and connecting to local interests. In addition, our analysis shows the role of regulatory permitting requirements in perpetuating or controlling biases against innovations like NI. In 2008, MD passed a policy that helped address biases against NI by changing NI from a preferred option to the required option except in places where scientific analysis suggested that gray infrastructure would be needed, while in CA and NC gray infrastructure remains only a preferred option. These results suggest an opportunity to harness heuristics, such as visual demonstrations and messaging from trusted persons, in addition to policy tools to mainstream NI in places where there is evidence that it would be effective. These results also suggest that heuristics could result in biases that not only lead to underuse but also to inappropriate use of NI; and, policies, similar to the policy in Maryland, are needed to control these biases. (C) 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (