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. 8 July 2008 Stillwater Sciences i Mad River Streambank Stabilization Project Geomorphic Evaluation 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 8 July 2008 Stillwater Sciences ii Mad River Streambank Stabilization Project Geomorphic Evaluation 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 8 July 2008 Stillwater Sciences iii Mad River Streambank Stabilization Project Geomorphic Evaluation 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). 8 July 2008 Stillwater Sciences 1 Mad River Streambank Stabilization Project Geomorphic Evaluation Figure 1. Overview of School Road erosion site area, including positions of key land management activities influencing the study site (USDA, 2005). 8 July 2008 Stillwater Sciences 2 Mad River Streambank Stabilization Project Geomorphic Evaluation 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