Mad River Streambank Stabilization Project Geomorphic Evaluation
Prepared for County of Humboldt, Department of Public Works 1106 Second Street Eureka, CA 95501
Prepared by Stillwater Sciences 850 G Street, Suite K Arcata, CA 95521
8 July 2008
Mad River Streambank Stabilization Project Geomorphic Evaluation
Suggested citation: Stillwater Sciences. 2008. Mad River Streambank Stabilization Project geomorphic evaluation. Prepared by Stillwater Sciences, Arcata, California for County of Humboldt, Department of Public Works, Eureka, California.
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Table of Contents
1 INTRODUCTION ...... 1 1.1 Geologic Setting ...... 1 1.2 Recent Changes in Water and Sediment Supply...... 3 1.3 Land Management ...... 5 1.4 Prior Research on Mad River Migration...... 5 2 ANALYSIS ...... 7 2.1 Methods ...... 7 2.2 Fluvial Geomorphology...... 8 3 DISCUSSION...... 12 3.1 Erosion at School Road Bluff ...... 12 3.2 Evaluation of Potential Bluff Erosion Project...... 14 3.3 Geomorphic Evaluation Uncertainty ...... 14 4 SUMMARY ...... 12
5 REFERENCES...... 13
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List of Tables Table 1. Aerial photography years and discharge at the time of photography used in the analysis...... 7
List of Figures Figure 1. Overview of School Road erosion site area, including positions of key land management activities influencing the study site...... 2 Figure 2. The School Road erosion site on the lower Mad River, displaying the sediments composing the now-uplifted fluvial terrace...... 3 Figure 3. Annual peak series of the Mad River near the study reach based on U.S. Geological Survey (USGS) gauging station # 11481000 (Mad River near Arcata, California) annual peak flow record between 1911 and 2006...... 4 Figure 4. Migration of the Mad River mouth over time (LaValley 2002)...... 5 Figure 5. Change in active channel area over time in the lower Mad River between School Road and the river bend just upstream of the Hammond Bridge...... 8 Figure 6. Sequential channel planform in the lower Mad River...... 9 Figure 7. A comparison of sequential thalweg and bluff position for select years from 1941 to 2007 in the lower Mad River. Bluff positions were determined by LACO (2008)..... 11 Figure 8. Comparison of the Mad River planform in the vicinity of the School Road erosion site before and after the 1955, 1964, and 1996 floods...... 11 Figure 9. Google Earth view of the lower Mad River (Photo year 2006). Note the regular oscillation of the planform in this lowermost reach, with a distance of about a half-mile between each alternating bend apex...... 12 Figure 10. Comparison of survey data from 1970, 1999, and 2008 at cross-section 2 located near Tyee City on the Mad River, vertical to horizontal scale is 10:1. The 1970 data source is from U.S. Army Corps of Engineers, 1999 data are from Streamline Planning Consultants, and 2008 data from are Points West Surveying...... 13
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1 INTRODUCTION
The County of Humboldt is proposing the Mad River Streambank Stabilization Project, which consists of designing and implementing emergency erosion control measures along the lowermost and intertidal reach of the Mad River. The project site is located at the western end of School Road in McKinleyville, California (Figure 1). The erosion feature is on the western edge of an uplifted marine terrace and is about 1,200 feet long and ranges in height from about 10 feet above the low flow water surface elevation at the upstream end to 40 feet at the downstream end. Land uses along the project reach include pasture for a dairy, treated wastewater disposal, urban streets, and residential development. High winter flows that are directed at the bluffs are believed to be the cause of the bank retreat that is threatening homes and infrastructure.
The geomorphology of the lower Mad River and its estuary provides the physical framework for prospective management actions at the School Road erosion site. Although the geologic setting is the primary factor governing channel change of the Mad River, the patterns of channel planform (i.e., the shape of the channel as viewed from above) change over time and the inferred hydraulics associated with those changes are particularly important for evaluating modern erosion at the study site. In the past 60 years, the lower Mad River channel has been subjected to both natural processes and management activities that have had immediate influences on the river’s planform. Two important natural processes are episodic flooding and the deposition and erosion of sediment, which have resulted in migration of the river mouth. Important land management activities during this period include the construction of dikes, revetments, and other forms of bank protection.
The purpose of this study is to document the geomorphic changes that have occurred within the project area, and present an understanding of the geomorphic processes that affect site conditions. In addition, the evaluation will present a hypothesis regarding the potential future trajectory of the geomorphology of the project area. The focus of the evaluation is the reach extending from the Hammond Railroad Bridge to the mouth of the river (a distance of approximately 1.75 miles).
1.1 Geologic Setting The Mad River drains nearly 500 mi2 of the northern Coast Range geologic province in Humboldt and Trinity counties, a complex of Mesozoic through Cenozoic sedimentary and metamorphic rocks. The Mad River drains in a north-northwesterly direction from approximately 5,300 ft at its headwaters near Kelsey Peak to sea level where it empties into the Pacific Ocean near McKinleyville, California. Late Pleistocene and Holocene fluvial terraces composed of poorly consolidated sand, silt, and gravel are preserved along the lower mainstem Mad River between Boulder Creek and Lindsay Creek and in major tributary valleys (Berry 1981). These and other fluvial terraces in nearby coastal river valleys have formed by fluvial response to eustatic sea level change, regional uplift, active faulting, and climate change. Late Pleistocene marine terraces are also preserved near the Mad River estuary and mouth, including the School Road erosion site where the proposed bank armoring is being evaluated (Figure 2). These marine terraces are erosional remnants of uplift shore platforms formed by wave erosion during past sea- level highstands. Similar flights of marine terraces are preserved at other locations along the approximately 60-mile coastline in the southern Cascadia subduction zone (Carver and Burke 1992).
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Figure 1. Overview of School Road erosion site area, including positions of key land management activities influencing the study site (USDA, 2005).
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Figure 2. The School Road erosion site on the lower Mad River, displaying the sediments composing the now-uplifted fluvial terrace (May 2008).
These marine terraces, flanking the lowermost reaches of the Mad River, now stand many tens or even hundreds of feet above modern sea level. The distribution of elevations and ages of these marine terraces indicate long-term uplift of the earth’s crust around McKinleyville. This ongoing uplift controls the position of the Mad River, particularly near its mouth, by creating bounding terraces and bluffs that have contained the river to the north but allow it to readily migrate across the bottomlands to the south.
1.2 Recent Changes in Water and Sediment Supply The hydrology of the river has been punctuated by large, historic flood events, particularly in the 1950s and 1960s. Three large floods of record (1911 to 2006) occurred in the 1950s and 1960s. The first of this period was the 1953 flood, with a magnitude of 75,000 cubic feet per second (cfs); a flood with a peak discharge of 80,000 cfs occurred in 1955; and in 1964, the flood of record occurred, with a magnitude of 81,000 cfs (Figure 3). All of these floods had a strong impact on the morphology of the Mad River (Lehre et al. 2005). Since the 1960s there have been fewer large floods and their peak magnitudes have been much lower than the floods of the 1950s and 1960s. For example, the flood of 1997 had a peak magnitude of 44,700 cfs, and it had little impact on the reach-scale morphology of the lower Mad River, except to further undercut the pre- existing bluff at the School Road erosion site.
Human activities have strongly affected sediment supply in the lower river. Up until 1970, the Sweazy Dam (constructed in 1938 at River Mile [RM] 22) had impounded approximately 3,200,000 cubic yards of sediment (Tolhurst 1995). The dam was removed in 1970 and the sediment impounded by the dam was released into the channel. Gravel mining through the lower Mad River has likely been commonplace since Europeans moved into the area in the 1800s.
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Telltale scarring on gravel bars in aerial photographs up to 1962 shows that scalping of gravel bars in the lower river was common. Industrial gravel extraction on a large scale began in 1952; however, gravel extraction has reduced from an average harvest volume of 425,000 cubic yards annually between 1960 and 1992 (Humboldt County 1994) to currently less than 150,000 cubic yards per year (Jager et al. 2006). Annual channel cross-section surveys between the Highway 101 and Highway 299 bridges have shown bed-elevation increases since the mid to late 1990s, with these changes attributed to the reduction of gravel-extraction rates.
Mathews Dam, which impounds Ruth Lake, is located at about RM 84, well upstream of the most erosive reaches and other sediment sources of the river. Many large and small tributaries, as well as earthflows and landslides, contribute substantially to the sediment load of the river downstream of the dam. It therefore is unlikely to have much effect on sediment delivery to the channel in the project area.
Land management practices have also been shown to be a contributing factor to the sediment budget of the Mad River. The majority of the sediment delivered to the channel has been ascribed to timber harvest and road-building activities in the basin (Graham Mathews and Associates 2007).
Return Period (Year) 110100 100,000 1/17/1953
12/22/1955 12/22/1964 (cfs) 10,000 Discharge
Raw Data Log Pearson Type III 1,000 1 0.1 0.01 Exceedance Probability
Figure 3. Annual peak series of the Mad River near the study reach based on U.S. Geological Survey (USGS) gauging station # 11481000 (Mad River near Arcata, California) annual peak flow record between 1911 and 2006.
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1.3 Land Management The floodplain on both sides of the lower Mad River has been in cultivation since the late 1800s. Field observation revealed that dikes, apparently built by farmers in the past, bound the Mad River between Hammond Bridge and School Road. In the vicinity of the School Road erosion site, bank protection is extensive and easily observed: (1) old gravel levees of variable height on both banks upstream to at least Hammond Bridge, (2) revetment on the left bank near the boat ramp and immediately upstream in the vicinity of Tyee City, which reaches an elevation of 12 feet, (3) riparian planting on the right bank opposite the boat ramp and left bank revetment, and (4) revetment of the stormwater outfall on the right bank just upstream of the erosion site. These armored sites have contributed to the relative stability of the lower river, limiting lateral adjustments to the still-unarmored stretches of the bank.
Initial rock slope protection work began around 1948 in the vicinity of Tyee City and Mad River Road, which is about 0.5 mi upstream of the erosion site on the left bank (Don Tuttle 2006, personal notes). Aerial photo analysis shows that the 1955 flood removed tens of feet of the left bank between the river and Mad River Road near Tyee City. We suspect it likely that revetment was strengthened along the road at this time. A boat ramp and adjacent parking lot were constructed on the left bank in 1970 just downstream of the revetment near Tyee City. Additional revetments upstream and downstream of the boat ramp site were installed at that time. Infiltration ponds on the right bank opposite the boat ramp were built in 1983 by the McKinleyville Community Services District. Rather than rock revetment, riparian vegetation was planted along the right bank as bank reinforcement to protect the infiltration ponds. Thus, by 1983, both sides of the channel upstream of the erosion site were effectively “hardened” and resistant to further erosion or channel migration.
1.4 Prior Research on Mad River Migration The dynamic behavior and associated processes of the mouth of the Mad River have been extensively studied. The description below is summarized from Borgeld et al. (1993).
The process of river migration has been occurring along the lower Mad River for millennia. Over the long term (i.e., millennia to centuries), the drivers of channel migration are similar to other, similar settings. Delivery of watershed sediment, climate forcing, and tectonic uplift and subsidence have all influenced the rate and location of channel migration. The active channel is broadly aligned with the trace of the active Mad River Fault; lower portions of the river are immediately bounded on the north by uplifted marine terraces deformed by this structure. The broad floodplain of the Mad River, immediately south of the current river channel, has experienced net submergence.
Over short time intervals, however, the local interplay of discharge and sediment load will determine the immediate channel response. Erosion and accretion forced by large storms that lead to flooding, high surf conditions, and strong tidal currents all play important roles in shaping the dynamics and specific position of the river mouth.
Borgeld et al. (1993) divided the history of the river into several discrete periods over the past century or so. During the “Oscillation Period” (1870–1969), north-trending tidal currents were apparently of minor importance; the local and temporary balance between wave power and river discharge dictated the location of the inlet. The mouth of the Mad River migrated back and forth within just a mile’s length of coastline (Figure 4). When discharge was low, the inlet was wave-
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5 Mad River Streambank Stabilization Project Geomorphic Evaluation dominated and episodically closed. Anthropogenic modification during this time included channelization, construction of Sweazy Dam in 1938, and bed lowering in the lower river valley. Upstream channelization and bank protection increased downstream discharges that impacted the inlet during floods. Bed lowering and incision increased the tidal prism in the estuary, which in turn would have increased tidal currents and bank erosion. This period included the three record floods (1953, 1955, and 1964, with 55-, 85- and 95-year estimated recurrence intervals, respectively; Figure 3).
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The “Transition Period” (1969–1971) was governed by an apparent imbalance between wave power, discharge, and tidal currents, initiating rapid migration to the north. During these years, the mouth migrated farther north than it had during the preceding century, although monotonic northward migration began at least several years earlier. This reflected a change from a relatively stable inlet, the location of which was controlled by the balance of waves and river discharge, to one dominated by waves and tidal currents. Northward migration of the mouth appears to have been initiated by a combination of high wave energy and low to moderate discharge, consequence of (1) the longshore component of wave power, (2) concave-southward inlet mouth configuration, and (3) bank erosion caused by tidal currents.
A period of “Progressive Migration” (1971–1992) was driven by tidal currents that increased in magnitude as the inlet migrated, reinforcing continued migration. The volume of tidal prism and tidal flushing increased, and the inlet became increasingly protected from northward waves as it migrated north into the wave shadow created by Trinidad Head. The northward migration continued until rock slope protection was installed by California Department of Transportation (CDOT) between December 1991 and March 1992 in the vicinity of the Highway 101 vista point overlooking Clam Beach.
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2 ANALYSIS
2.1 Methods To characterize the changes in channel planform location over time, digitized channel planforms were generated in a Geographic Information System (GIS) by tracing the channel boundary of the Mad River depicted in digital, georeferenced aerial photography. Similarly, the inferred thalweg (i.e., the area of deepest flow with greatest velocities) for each year was digitized using general principles of open channel flow and visual evidence. For example, the deepest and swiftest portion of a meandering river is generally on the outside of a meander bend, crossing over to the opposite bank as the channel oscillates downstream.
All available aerial photographs for the study area were assembled and supplied to us by Humboldt County. A log of all the dates of the aerial photography used analyses herein is listed in Table 1.
Table 1. Aerial photography years and discharge at the time of photography used in the analysis. Photograph Year Date of Photograph Flow (cfs)1 1941 11/23/1941 not gaged 1948 6/23/1948 not gaged 1954 8/3/1954 56 1958 Unknown ? 1962 Unknown ? 1970 7/21/1970 29 1974 Unknown ? 1984 Unknown ? 1989 10/9/1989 37 1996 6/18/1996 176 1999 11/13/1999 160 2007 5/17/2007 347 1 Flow measured at USGS gage #11481000 Mad River near Arcata.
Once we received digital copies of the aerial photographs from Humboldt County, they were subsequently rectified in a GIS to account for positional differences. The aerial photography was not “orthorectified,” which is a systematic correction of the scale and relief displacement in an aerial photograph that accounts for differences in the position of the aircraft and the topography. In relatively flat terrain (as in the study area), rectification alone is typically satisfactory for linking the aerial photography to a coordinate system. Once all images were integrated through GIS, the sequence of channel planforms was digitized by tracing the active channel boundary for each year in turn. We defined the active channel as the area of the channel with flowing water and the channel margins that were generally free of riparian vegetation and presumably composed of frequently transported alluvium.
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2.2 Fluvial Geomorphology The lowermost Mad River in the vicinity of the mouth and erosion site is strongly influenced by nearshore and riverine physical processes, and the planform of the river reflects these dominant influences. Northward migration of the river mouth has had a strong influence on channel processes and planform position. In addition, since 1941 there have been four other prominent morphologic changes in fluvial processes in the vicinity of the erosion site, including: (1) a decline in the area of the active channel, (2) enlargement of the left bank bar opposite School Road, (3) vegetation encroachment along the right bank opposite Tyee City, and (4) the downstream and eastward migration of the thalweg in response to the northward migration of the river mouth.
Since at least 1941 (the date of earliest available aerial photography) there has been a steady decline in the area of the active channel in the ~1.5-mile-long reach between School Road and the river bend just upstream of the Hammond Bridge (Figures 5 and 6). The area of the active channel has declined by 32 percent in this period, which is attributed to channel narrowing and northward migration of the mouth. Narrowing of the active channel is indicated in the aerial photos by encroachment of riparian vegetation onto formerly active gravel bars.
100
95 area (acres) 90
85 s e r 80 Ac 75
70 65
60 1941 1951 1961 1971 1981 1991 2001 Year Figure 5. Change in active channel area over time in the lower Mad River between School Road and the river bend just upstream of the Hammond Bridge.
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Figure 6. Sequential channel planform in the lower Mad River.
Between 1941 and 1958 when the river mouth was located near the erosion site, the variable area of the mouth represented a significant portion of the total mapped area of the channel, and thus increased the area of the active channel relative to later time steps when the mouth was farther north. We therefore emphasize the assessment of active channel area from 1962 onward, which focuses the analysis on channel narrowing by eliminating the effects of the variable boundaries in the immediate vicinity of the river mouth. Since 1962, the area of the active channel has declined by 17 acres (20 percent). This pattern of channel narrowing is somewhat in contrast to the finding of Lehre et al. (2005) in their analysis of the channel upstream of the Highway 101 Bridge, who found that the area of the active channel area there increased after large flood events. In the lower Mad River, however, only the 1955 and 1964 floods appear to have increased the area of the active channel (Figure 5); after 1964, the area of the active channel steadily declines, unlike the river upstream of Highway 101. The decline in active channel area has been accompanied by progressive enlargement of the prominent bar opposite School Road and vegetation encroachment along the right bank opposite Tyee City.
The emergence and growth of the gravel bar opposite School Road is linked to general alluvial river behavior in response to the northward migration of the river mouth. As the channel lengthened northward, a typical sequence of channel meandering and point bar deposition developed in response. This resulted in the bar growing in both the downstream direction as well as to the east (towards the erosion site) as the meander sequence began to develop and cut into the bluffs near the erosion site.
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Between 1941 and 2007, the thalweg (i.e., the deepest part with the highest velocities) progressed eastward as the planform of the river changed in response the northward migration of the river mouth (Figure 7). Prior to about 1958, the thalweg impinged against the right bank in the field south of the erosion site, near where the present-day riparian vegetation plantings and infiltration ponds exist. Between about 1958 and 1970, the thalweg began to progress eastward in conjunction with channel meander and point bar development associated with the northward migration of the mouth and river. This progressive migration of the thalweg caused it to impinge against the bluff in the vicinity of School Road starting in about 1970, a progression that began almost a quarter-century ago and is continuing to the present day.
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Figure 7. A comparison of sequential thalweg and bluff position for select years from 1941 to 2007 in the lower Mad River. Bluff positions were determined by LACO (2008).
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Vegetation encroachment and apparent sediment deposition have taken place on the prominent bar opposite School Road. A sequential plan view of this bar depicts the progressive infilling of this once-active bar by riparian (e.g., Alnus sp. and Salix sp.) and estuarine (e.g., Carex sp.) plant species (Figure 8). Aerial photograph analysis shows that vegetation encroachment began as early as 1974 but became more prominent beginning in the 1980s. Flood deposition of fine sediments when the river submerges the bar contributes to surface elevation gain on the bar. Aeolian (i.e., wind-blown) transport of sand from the westward dunes by strong seasonal northwesterly winds has likely contributed to the deposition and growth of this bar as well. Vegetation establishment and emergence due to sediment deposition has contributed to the stability of this bar and facilitated continued eastward and northward bar growth as described previously.
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3 DISCUSSION
3.1 Erosion at School Road Bluff
The position of the mouth of the Mad River has oscillated considerably during historic time, as well as significantly migrating to the north for a period of two decades beginning around 1970. The channel began eroding into the bluffs of the School Road erosion site prior to 1958 and may well have initiated that erosion during the 1955 flood. The channel has continued to erode into the bluffs as part of the channel response to the northward migration of the mouth. Apart from these instances of significant planform change, however, the lower river in the vicinity of the erosion site has shifted its position only slightly in the past 67 years.
Erosion site at School Road
Figure 9. Google Earth view of the lower Mad River (Photo year 2006). Note the regular oscillation of the planform in this lowermost reach, with a distance of about a half- mile between each alternating bend apex.
Erosion adjacent to School Road occurs because the thalweg of the river is angled toward the bluff in this location. The bluff currently functions as the outside of a meander bend (Figures 7 and 9), which is likely developing in response to the northward migration of the mouth and subsequent emergence of meander bend and point bar sequences along the new channel course. This planform results in high flow velocities directly pointed at the base of the School Road erosion site; high flow velocities may be further elevated beyond historic conditions due to channel narrowing just upstream. Lenses of sand and gravel in the base of the bluff are vulnerable to erosion by flowing water. The finer-grained, more cohesive unit of the upper bank holds the bluff vertical until sufficient erosion occurs at the toe for the overhanging upper deposit to fail as a block. The failed block appears to break up rapidly at the toe of the bluff and so offers
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12 Mad River Streambank Stabilization Project Geomorphic Evaluation little protection to the sand and gravels below. As long as river flow continues to be directed toward the base of the bluff, there is little reason to believe that the bluff will stabilize and allow vegetation to colonize and protect it, as has happened farther to the north. Recent bluff erosion following the winter of 2006–2007 resulted in a scalloped morphology that has facilitated the development of intense eddying of flow at the base of the cliff during high water, assisting further erosion. It is probable that wave action during high tides and heavy surf contribute to undercutting of the bluff as well.
Migration in alluvial rivers is a defining characteristic and is commonplace. The planform history of the Mad River is no exception as the extensive historical review by Scalici (1993) documents many episodes of avulsion and bank erosion throughout the lower Mad River. The present-day lower Mad River, however, is a static remnant of its former condition, despite the current erosion at School Road. Despite the significant northward extension of the river mouth over the past half-century (Figure 4), the overall planform is relatively static (Figure 6) by the standards of many lowland alluvial rivers that are laterally unconstrained. The relatively static nature of the current channel is also expressed by vegetation encroachment onto formerly active bars, which is a common feature of regulated rivers—particularly those below dams that impound sediment and minimize flood peaks. However, Sweazy Dam (the only major dam in the lower watershed) was removed in 1970 and all sediment impounded was released downstream, which was likely sand and silt with a much smaller fraction of course sediment. Thus impoundments are likely not the source for the apparent recent decrease in channel processes, and are more likely attributable to bank armoring, industrial gravel extraction, and/or smaller magnitude peak floods since the 1964.
The bend in the lower Mad River just upstream of School Road is fixed in place and has not moved from its present location since at least 1958 due to the placement of rock slope protection on the outer bend and riparian plantings on the inner bend. A time series of channel cross sections between 1970 and 2008 illustrate the static nature of this bend (Figure 10). The aggradation on the inside of the bend (right margin of Figure 10) is indicative of riparian encroachment and subsequent sediment storage.
Figure 10. Comparison of survey data from 1970, 1999, and 2008 at cross-section 2 located near Tyee City on the Mad River, vertical to horizontal scale is 10:1. The 1970 data source is from U.S. Army Corps of Engineers, 1999 data are from Streamline Planning Consultants, and 2008 data from are Points West Surveying.
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The relatively fixed position of the thalweg upstream of the erosion site is similarly related to the stable channel bend along the Tyee City bend. In addition, riparian encroachment along the right bank upstream of the erosion site has narrowed the channel and further stabilized the thalweg since about 1984 (Figure 7). High flow energy carried within the stabilized and narrowed channel was then free to interact with the growing left bank bar and work on the landform at the foot of School Road. Our conclusion is that naturally occurring fluvial processes, in response to the northward migration of the mouth, initiated the meander development at the base of School Road; this process has been accentuated by upstream bank hardening and vegetation establishment, resulting in a channel pattern that causes erosion against the bluff at School Road.
In the near term, we believe that the planform of the lower Mad River upstream of the erosion site will not appreciably change, and the meander bend, which is eroding the bluff at School Road, will continue to progress eastward. The LACO Associates (2008) analysis that indicates even a moderate flood will threaten more erosion and bluff failure appears reasonable given the geomorphic context of the site.
3.2 Evaluation of Potential Bluff Erosion Project As previously discussed, under current conditions the Mad River is likely to continue developing a meander bend and eroding eastward near School Road. Thus, in order to preserve the bluff at this location, management actions are likely needed. Humboldt County has proposed a design to modify the toe of the bluff near School Road with large boulders, large woody debris, spur dikes, and willow mattresses. As the proposed project is still in a conceptual phases, detailed construction designs were not available at the time this report was prepared. A coarse-level, 2- dimensional numerical hydraulic modeling simulation was prepared in order to evaluate the potential changes in the velocity flow fields following project implementation (Stillwater Sciences 2008).
Hydraulic modeling results indicated that the proposed dikes and willow matrices will decrease the velocity at the right bank near School Road by less than 0.5 ft/s and will not change the overall flow patterns in the study reach, suggesting little significant change in the general channel morphology (e.g., the bar-pool patterns) in the vicinity of the study reach (Stillwater Sciences 2008). Assuming the proposed project is stable enough to withstand high flows, however, the project should temporarily decrease or halt erosion at the bluff site, which is anticipated as the primary morphologic impact in the immediate vicinity of the bluff.
A migration of the erosion focal point from the current School Road erosion site to the bluff position immediately downstream of the proposed management activity appears unlikely under existing conditions of flow and sediment load, for the following reasons: 1) the trajectory of the upstream thalweg that is directing high velocity flow at School Road is essentially locked in place by bank revetment and riparian vegetation; and 2) the bluff immediately downstream of the proposed project is essentially located on the inside of the current meander bend sequence (i.e., there is a point bar developed in front of the bluff) (Figure 9).
3.3 Geomorphic Evaluation of Uncertainty The study authors have high confidence in their review of the available data that led to the evaluation of the geomorphic processes within the project area under current conditions. However, there are a number of variables that could confound the geomorphic evaluation for
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14 Mad River Streambank Stabilization Project Geomorphic Evaluation future conditions. These include: (1) changes in sediment supply from upstream sources in relation to land management and hydrologic patterns, (2) adjustments of river mouth location, (3) interaction of the erosion area with the tide and wave action, (4) effect of wind-blown sand deposition in the river, and (5) lifespan of existing bank revetment and riparian vegetation within the vicinity of the project. Each of these factors are discussed in turn below.
The Mad River between the Highway 101 Bridge and the Mad River Hatchery has been subject to industrial gravel extraction activities since the mid-1900s. Between 1985 and 1991 an average of 306,536 cubic yards of aggregate were extracted from the Mad River (Knuuti and McComas 2003). Between 1992 and 2002, the average annual gravel harvest was 173,000 cubic yards (Lehre et al. 2005). The annual average volume of gravel extracted between 2003 and 2005 was 135,080 cubic yards (Jager et al. 2006). Channel cross-section surveys upstream of the project reach and below the Highway 299 Bridge suggest that the channel is aggrading in that area. Although not yet evident in the study reach, the sediment that is now accumulating in the upstream reaches should continue to translate downstream. Over the longer term (i.e., decades), that increased sediment load now being routed from upstream could combine with a large flood of a magnitude similar to the 1955 or 1964 flood to dramatically alter the planform of the lower river. This would likely reconfigure this lowermost reach of the Mad River and potentially shift, unpredictably, the focus of outer-bank erosion presently directed at the School Road site.
The river mouth has been shifting its position since the 1940’s. As stated above, the location of the mouth has significant influence on the channel planform in the vicinity of the erosion site. As the mouth continues to adjust its location the degree of its influence on the bluff will also vary. It could be expected that if the mouth of the river were to shift back to the south, changes in sediment transport and depositional processes would also occur. In addition, the bluff may become subject to increased wave impact erosion that varies with the daily tidal flux. Due to climate change, future hydrologic patterns and sea level stage represent an increasingly unknown parameter for evaluating future trajectories of channel evolution and implemented projects. Extreme flood events, or even small changes in sea level, could substantially alter the course of the lower Mad River and any associated locations of enhanced bank erosion.
The entire Mad River beach area is subject to frequent periods of relatively heavy northwest winds. These winds carry beach sand that deposits in the lee of sand dunes and vegetation, and within the river channel. The relative importance of this process was not evaluated during the course of this study, and its trajectory under future climate-change scenarios is similarly unknown.
Geomorphic evaluations presented herein assume that the bank revetments near Tyee City and the boat ramp will continue to remain in place for the foreseeable future, as will the riparian plantings and subsequent vegetation encroachment near the infiltration ponds. Removal of these elements that fix the channel planform position upstream of the erosion site would produce unpredictable results at the bluff site and proposed project, as the Mad River became free to laterally adjust.
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4 SUMMARY
• Naturally occurring fluvial processes in response to the northward migration of the mouth initiated channel meander and point bar development, shifting of the thalweg orientation, and bank erosion west of School Road. • The bend in the lower Mad River just upstream of School Road is fixed in place and has not moved from its present location since at least 1958, due to the placement of rock slope protection on the outer bend and riparian plantings on the inner bend. Hardening the banks upstream of the erosion site has contributed to channel narrowing and focusing high flow velocities directly at the School Road erosion site. • For the near term, it is likely that the planform of the lower Mad River upstream of the erosion site will not appreciably change. The meander bend that is now eroding the bluff at School Road will continue to maintain an eastward-trending pattern of migration. • The bluff near the west end of School Road is unlikely to stabilize naturally in the near future. In the absence of management action, the amplitude of the meander bend will likely grow and the bluff will continue to erode during high winter flows. • While relatively stable in the near term, over the long term the project area is subject to dynamic changes associated with fluvial and nearshore processes and seismic activity. Potentially altered future conditions, a consequence of increased sediment loads from upstream, a large flood, or sea-level rise could abruptly alter the location of the river mouth and/or the locus of most intense bluff erosion. • Hydraulic modeling results indicated that the proposed project will result only minor deviations in the current velocity flow fields (i.e., generally <0.5 ft/s) and will not change the overall flow patterns in the study reach. These results indicate that project construction will probably not cause significant change in the general channel morphology (i.e., the bar-pool patterns) in the vicinity of the study reach.
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5 REFERENCES
Berry, M. 1981. Geomorphology and relative dating analysis of quaternary fluvial terraces on the Mad River near Blue Lake, California. Master's thesis. Humboldt State University, Arcata, California.
Borgeld, J. C., M. J. Scalici, M. Lorang, P. D. Komar, and F. G. Alden Burrows. 1993. Final project evaluation report: Mad River mouth migration. Prepared for California Department of Transportation, District 1, Eureka, California.
Carver, B. A., and R. M. Burke. 1992. Late Cenozoic deformation on the Cascadia Subduction Zone in the region of the Mendocino Triple Junction. Prepared by Humboldt State University, Department of Geology, Arcata, California.
Graham Mathews and Associates. 2007. Mad River sediment source analysis. Appendix A of the Mad River total maximum daily loads for sediment and turbidity. Prepared by Graham Mathews and Associates, Arcata California for TetraTech, Inc., Pasadena, California.
Humboldt County. 1994. Program environmental impact report on gravel removal from the lower Mad River. SCH #92083049. Prepared by Humboldt County, Planning and Building Department, Eureka, California.
Jager, D., R. Klein, A. Lehre, and B. Trush. 2006. County of Humboldt Extraction Review Team (CHERT) 2005 post-extraction report. Prepared for Humboldt County Board of Supervisors, Eureka, California.
Knuuti, K., and D. McComas. 2003. Assessment of changes in channel morphology and bed elevation in Mad River, California, 1971-2000. U.S. Army Corps of Engineers, Engineer Research and Development Center, Vicksburg, Massachusetts. ERDC/CHL TR-03-16.
LACO Associates. 2008. Mad River bluffs, geologic investigation and assessment. Project no. 6869.00. Prepared by LACO Associates, Eureka, California for Humboldt County, Department of Public Works, Eureka, California.
LaValley, R. 2002. Public hearing for rock revetment at former Mad River mouth near McKinleyville.
Lehre, A., B. Trush, R. Klein, and D. Jager. 2005. County of Humboldt Extraction Review Team (CHERT) historical analysis of the Mad River 1993-2003. Prepared for Humboldt County Board of Supervisors, Eureka, California.
Scalici, M. 1993. Historical review of the events shaping the Mad River delta and estuary, northwest California: 1850–1941. Appendix to Mad River mouth monitoring report. Prepared for California Department of Transportation, Hydraulics Division, Eureka, California.
Stillwater Sciences. 2008. River2D simulation of Mad River near School Road, McKinleyville, California. Prepared for the County of Humboldt, Department of Public Works. Eureka, California.
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Tolhurst, J. W. 1995. Historical analysis of geomorphic channel changes, lower Mad River, Humboldt County, California. Master's thesis. Humboldt State University, Arcata, California.
United States Department of Agriculture (USDA). 2005. National Agriculture Imagery Program (NAIP).
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