Science & Environmental Poicy
Coastal Geomorphology and Policy Applications along Southern Monterey Bay, California part of Vignettes:Vignette Collection
Introduction Global sea level stood between 100 m and 130 m lower than today's sea level 18,000 years ago at the peak of the latest ice age. Since that time, melting ice and thermally expanding oceans have gradually drowned the continental margins of the world. Vertical sea level rise translates to horizontal inland migration of the shoreline, or "coastal retreat." Locally complicating coastal retreat rates are the local rock types and attendant rates of coastal erosion, the movement of coastal sediment, and the pre-existing coastal topography that either invites or resists inundation. "Coastal geomorphology" analyzes this dynamic system, and can inform coastal management policies and plans. This paper describes the coastal erosion processes of southern Monterey Bay, California in the context of coastal management (Fig. 1). Do We Need Coastal Management Policies and Plans? Why not let coastlines retreat under the natural process of sea level rise and coastal erosion? While unmanaged retreat is one end-member of a whole suite of options available to regional planners and cities, it is rarely the chosen option in California where countless poor land-use decisions have placed homes, hotels, highways, sewer lines, and businesses directly in harm's way (Griggs et al., 2005). Rather, the default response has been local coastal armoring with a seawall. Armoring is typically the result of "emergency" conditions that exist when individual structures or roads are under imminent threat of destruction, and when little time remains for rational evaluation of other options (Fig. 2). The installation of a great number of local, emergency seawalls has the cumulative effect that seawalls gradually become the de-facto regional "solution." While armoring has its place, it has been historically misused as a local band-aid on a regionally hemorrhaging wound. The typical result of seawall installation is the slowing of coastal retreat, but with an attendant loss of beach width in front of the structure (Hall and Pilkey, 1991; Kraus and McDougal, 1996). Cities need preemptive planning efforts that lead to longer-term regional coastal management plans that include a spectrum of tools. Coastal Retreat–How Is It Measured and How Bad Is It? Coastal retreat rates vary greatly through space and time. A quick analysis of southern Monterey Bay shows that the average long-term retreat rate is about 0.8 m/yr where the seacliffs are poorly-lithified Quaternary sand dunes (Fig. 1). The retreat rate is far less where the coastline comprises granodiorite (Fig. 1). Short term rates for the past few decades can be deduced by tracing the seacliff in a sequence of georeferenced historic aerial photos spanning 30 years. These modern average rates vary monotonically from close to 0 m/yr near the southern end of the bay to a stunning maximum of 1.5 m/yr near Marina (Fig. 1). More precise measurements using recent airborne LiDAR support the rates derived from aerial photos, and a pair of LiDAR flights provide the data required to estimate the volume of sand produced from the eroding coastline (Thornton, et al., 2006). In addition to erosion rates and sand volumes, the data and direct observation show that decadal-scale winter storm waves combined with high tides account for most of the long-term average rates of coastal retreat (Fig. 3; Cambers, 1976; Griggs et al., 2005; Thornton, 2006). Systems Approach to Coastal Processes and the Sand Budget Coastal processes and sand budgets can be developed at a variety of scales. We will take a systems approach to analyze a typical 100 m long section of sea cliff near CSU Monterey Bay (Fig. 4). For this work, a "system" is a conceptual model of reality where transfer rates and storage of sand are identified and quantified. The goal is to develop a sand budget, just like a bank account with inputs and outputs. We have identified the major storage areas and significant inputs and outputs of sand to this section of beach (Fig. 5). The "sand bank" in our example is the beach width (e.g., Hall and Pilkey, 1991), which is a reasonable proxy for volume, and is easier to measure on aerial photographs. Historical aerial photos indicate that the beach width is in approximate steady-state equilibrium (constant average width) over a decadal time-frame. Thornton et al. (2006) was able to quantify parts of the sediment budget. Surprisingly, seacliff erosion is by far the dominant source of beach sand. If we assume that littoral inputs and outputs are reasonably balanced along our short section of coast, then the steady-state condition leaves us with a startling conclusion. The great volume of beach sand sourced from seacliff erosion resides in the beach for a while, providing the constant average width, but is ultimately carried away by rip currents to very long-term storage on the continental shelf. In other words, Monterey's beautiful wide beaches exist because of coastal erosion; this anti-intuitive result is critical for regional management decisions. Applications–Do You Want Wide Beaches or Stable Coastlines? Each environmental management decision is a value judgment, hopefully made in the context of science. Our analysis indicates that coastal armoring along southern Monterey bay will result in narrower or missing beaches by shutting off the chief sand supply. Now the science-based question can be asked of city planners or concerned citizens, "Do you want seawalls or wide beaches?" If "wide beaches" is the preferred long-term goal, a spectrum of other coastal management options can be considered, including synthetic surfing reefs, emergent breakwaters, beach nourishment, and a variety of "softer" actions that honor the eventual loss of ground to the sea. The sand mine listed in the systems diagram (Fig. 5) removes 200,000 yds3yr-1 of sand from the littoral zone up-current from the seacliffs of southern Monterey bay. Compare this volume with the volume derived from seacliff erosion (Fig. 5)! So, one clear goal for regional coastal management is to work with the miners to rethink their mining strategy. Perhaps mining cessation would increase the littoral drift input to the system, providing a steady-state beach condition that demands less seacliff erosion (Thornton, 2006). Summary Sea level is rising, and coastlines are retreating. Imminent economic losses in California and around the world are driving a need for local geomorphologic studies that can help plan thoughtful regional responses to coastal retreat.
Marine Geomorphology: Geomorphic Processes, Hazards, and Paradoxes in Monterey Canyon part of Vignettes:Vignette Collection
Introduction While traditional geomorphic studies focus on the geologic processes and products that shape our terrestrial topography, there are equally exciting opportunities to explore the other 71% of the planet lying below sea level. The study of seafloor topography (bathymetry) was originally used to aid navigation by mapping shifting sand shoals and treacherous reefs. In the 1950's, sonar (sound wave) technology revealed Earth's mid-oceanic ridges and deep-sea trenches–the global-scale geomorphic features that eventually led to plate tectonic theory and its many refinements (Heezen et al., 1959; Severinghaus and McDonald, 1988). Recent advances in both sonar resolution and GPS positioning now allow us to study seafloor morphology in ever-sharper detail (e.g., McAdoo et al., 2000). Bathymetric data are commonly displayed as digital shaded relief views depicting seafloor morphology as clearly as if the oceans had been drained away. This paper describes modern studies of Monterey Canyon, which cleaves the central California Coast (Figs. 1 and 2). Giant Sand Waves in Monterey Canyon The head of Monterey Canyon almost reaches the beach at Moss Landing Harbor, so it captures all the beach sand moving either north or south toward Moss Landing Harbor. According to rough estimates over 200,000 m3 of sand enters the canyon each year (e.g., Paull et al., 2005), so there is plenty of sediment available to generate interesting geomorphic features. Our first high-resolution bathymetric survey of the canyon (2002) revealed that this huge volume of sand generates giant sand waves as it moves down the first 4 km of the canyon (Smith et al., 2005). The waves range from 1 m to 3 m tall and have a wave-length of about 50 m (Fig. 3). In each of the semi-annual surveys since 2002, the sand wave crests were in new positions, so we postulated that the sand wave field somehow records the processes of down-canyon sand transport. Sand Wave Interpretations: From Bad to Worse The sand wave shapes are asymmetrical, with a steep face on the down-canyon side and a gentler slope on the up-canyon side. Sand waves in modern rivers and desert dunes have similar asymmetry, suggesting that these submarine sand waves probably migrate down canyon maintained by unidirectional, down-canyon water currents (Smith et al., 2005; Xu et al., 2007). If all we had was one survey, we might not look beyond that explanation, but there are paradoxes when we look closer. There are three problems with the simplistic "uniformitarian" interpretation. First, migrating sand waves leave deposits bearing a fingerprint "cross-bedding" pattern, but shallow cores taken from the sand waves in Monterey Canyon do not show the pattern! Second, Smith et al. (2007) compared sand wave crests from two surveys taken 32 days apart. If the crest correlations are correct, at least some of the sand wave crests migrate up-canyon as antidunes! Third, when we compare surveys taken months apart, there is evidence of strong currents, but current meters deployed in the canyon in this area have not detected the "required" currents! The evidence for strong currents include 8 m to 10 m deep scour holes forced at the outsides of tight turns in the canyon bottom (Fig. 4), erosion of about 100,000 m3 of sand from the channel walls and floor, and net transport of at least 300,000 m3 of sand each year through a 10 m wide slot canyon ("narrows" in Fig. 2) without net aggradation! Other workers believe that the sand waves have a structural rather than sedimentological origin. So far no satisfactory hypotheses about the sand waves have been published. Landslides and Tsunamis? One common application of geomorphic theory is the reduction of landslide risk through mapping and monitoring potentially unstable hill slopes (e.g., Cannon et al., 2009). While submarine landslides will never bury a poorly planned housing subdivision, they can generate deadly tsunamis by pulling a deep trough into the sea surface as they fail. Precise surveys of canyon slopes can reveal the locations and dimensions of past submarine slope failures, and prehistoric tsunamis can be modeled from the slide geomorphology of ancient slides and a few assumptions (e.g., McAdoo and Watts, 2004). We caught a modern canyon wall landslide "in action" by comparing the seafloor elevations from March 2003 and September 2003 surveys (Fig. 4). Sometime in that narrow time window 70,000 m3 of canyon wall slid to the canyon floor. While this slide was too small to generate a dangerous tsunami, the fact that a significant slope failure occurred during the brief time since our monitoring began indicates that canyon wall failures may be more common than previously thought. Slide probabilities might eventually be developed if enough small slides are captured and analyzed. The Canyon that Swallowed Moss Landing? Given the dynamic nature of Monterey Canyon, it is reasonable to wonder if the position of the canyon head is stable through time (Fig. 5). If the head is filling in, the canyon lip would gradually migrate seaward. If the head were eroding headward like a giant gully, Moss Landing Harbor, a great number of buildings, and California's largest gas-fired electrical power plant might be at risk of falling into the canyon! The canyon lip is defined as the break in slope between relatively horizontal continental shelf and the steep canyon walls. Tracing the lip position through time reveals that the lip moves as much as 50 m inland and back to sea on a sub-annual basis, but it is apparently in steady-state equilibrium between sediment buildup at the lip, and slope failure sending the sediment down canyon, so Moss Landing is safe for now. As these analyses are based on only a few years of data, it is unclear if the apparent equilibrium is just an illusion based upon too few years of record. Summary The field of marine geomorphology is seeing an exciting revolution driven by ever-improving technology. New data sets indicate that peri-coastal submarine canyons are poorly understood, large-scale, dynamic geomorphic systems that strongly impact coastal sediment budgets. They may pose risks to coastal communities through headward advance and tsunamigenic landslides.