EFFECTS OF ARUNDO DONAX ON RIVER HYDRAULICS, SEDIMENT TRANSPORT, AND GEOMORPHOLOGY, SANTA MARGARITA RIVER, CALIFORNIA

Transcription

1 EFFECTS OF ARUNDO DONAX ON RIVER HYDRAULICS, SEDIMENT TRANSPORT, AND GEOMORPHOLOGY, SANTA MARGARITA RIVER, CALIFORNIA René Leclerc Geomorphologist Robert C. MacArthur, Ph.D., P.E. Principal Headwaters to Ocean (H2O) Conference Catamaran Resort San Diego, California May 26, 2011

2 Acknowledgments California Invasive Plant Council Jason Giessow, Jason Casanova, Doug Johnson U.S. Marine Corps Base Camp Pendleton Deborah Bieber, Ben Lardiere State Water Resources Control Board

3 Presentation Outline Study Objectives Arundo Donax Description Arundo Effects on Rivers Santa Margarita River Case Study Background River Hydraulics Investigation Application to other Rivers (Source: Cal-IPC 2011) Conclusions (Source: Cal-IPC 2011)

4 Study Objectives Conduct a literature review to develop an understanding of typical river response to infestation by Arundo. Summarize the geomorphic environment of the Santa Margarita River study reach. Prepare a case study of the effects of Arundo infestation on the hydraulic characteristics, geomorphology, and sediment transport capacity of the Santa Margarita River. Develop a simplified scoring system to evaluate the potential response of other southern California Rivers to their Arundo infestations.

5 Arundo in the River Environment Arundo Donax (Giant Reed) member of the grass family, native to parts of Asia and Europe Found on bare, moist substrate where water is plentiful, including the bed, banks, unvegetated bars and islands, and the floodplains of rivers Often out-competes native plants due to rapid growth rates (up to 10 cm per day in early stages) Mature stands reach heights of 6 m to 10 m (Source: Cal-IPC 2011)

6 Arundo Extent in Coastal Southern California Santa Margarita River (Source: Cal-IPC 2011)

7 Arundo Effects on Rivers Types of Impacts to River Morphology Findings from a literature review of arundo effects on large, low-gradient, braided rivers in the American Southwest: Simplification of channel form Increased lateral stability Floodplain accretion Long-term channel narrowing Bed aggradation and shallower channels in some cases, channel incision or deepening in others Important to recognize that other cause & effect processes occur concurrently

8 Arundo Effects on Rivers Some Types of Impacts to River Morphology

9 Presentation Outline Study Objectives Arundo Donax Description Arundo Effects on Rivers Santa Margarita River Case Study Background River Hydraulics Investigation Application to other Rivers (Source: Cal-IPC 2011) Conclusions (Source: Cal-IPC 2011)

10 Santa Margarita River Case Study è740 square miles in size èdrains into the Pacific Ocean near Oceanside èstudy reach located within Camp Pendleton Santa Margarita Watershed

11 Santa Margarita River Case Study Large sand-bed river in coastal Southern California Upstream urbanization altered flow characteristics over time, increasing available water and spread of Arundo Flood of record in 1993 near peak of Arundo infestation caused $100M+ in damages at Marine Corps Base Camp Pendleton Flood Magnitudes Flood damage response included levee improvements at Camp Pendleton and a widespread Arundo eradication program

12 Santa Margarita River Case Study Flow Duration Curves by Decade at Ysidora Gage Urbanization has increased flow volume and duration over time More Water = More Vegetation over time

13 Santa Margarita River Case Study Maximum extent of Arundo cover was in 1990s (Source: Cal-IPC 2008)

14 Santa Margarita River Case Study The 1993 Flood of Record

15 Santa Margarita River Case Study 1993 Flood Airfield levee breaches Note several inches of sand deposited throughout the base

16 Santa Margarita River Case Study After the 1993 Flood Arundo Eradication Program ( ) 570 acres of Arundo are mechanically removed and herbicide is applied to ensure no re-growth. NHC assists with hydraulic modeling & design efforts for a new bridge and flood protection levee at the air station. NHC develops a calibrated HEC-RAS model which includes widespread Arundo coverage present in the late 1990s. (Source: Bieber 2004)

17 Santa Margarita River Hydraulics Investigation What is HEC-RAS? HEC-RAS is a 1-D numerical model that simulates hydraulic conditions in open channels. Developed by the Army Corps of Engineers and a global standard for hydraulic modeling of river systems.

18 Presentation Outline Study Objectives Arundo Donax Description Arundo Effects on Rivers Santa Margarita River Case Study Background River Hydraulics Investigation Application to other Rivers (Source: Cal-IPC 2011) Conclusions (Source: Cal-IPC 2011)

19 Santa Margarita River Hydraulics Investigation Study Reach and Geomorphic Sub-reaches

20 Santa Margarita River Hydraulics Investigation Geomorphic Sub-reaches

21 Santa Margarita River Hydraulics Investigation HEC-RAS Model Scenarios

22 Santa Margarita River Hydraulics Investigation Manning s n Roughness Coefficient of Arundo Manning s n a coefficient used to represent flow resistance due to roughness effects in open channel flow. The higher the Manning s n coefficient, the rougher the surface. Putting it into perspective: typical gravel bed river dense willow on floodplain Initial estimates of Arundo Manning s n by NHC in 1998, based on 1993 & 1998 high water marks Arundo is !!

23 Santa Margarita River Case Study Typical Mature Arundo Stand on Santa Margarita River Floodplain in 1998

24 Santa Margarita River Hydraulics Investigation Manning s n Values for Floodplain Scenarios Scenario 1 Total Arundo Infestation Bottom 5 ft treated as obstruction due to dense thatch Area above 5 ft assigned Manning s n of 0.15 Scenario 2 Native Vegetation Manning s n = 0.10 Scenario 3 Bare Floodplain Manning s n = 0.05 Scenario Floodplain Manning s n = variable, depending on vegetation type

25 Santa Margarita River Hydraulics Investigation Water Surface Profile Results 100 yr Flow (64,000 cfs) Arundo produces a water surface elevation as much as 5 feet higher than the native vegetation scenario

26 Santa Margarita River Hydraulics Investigation Channel Flow Depth Profile 100 yr Flow (64,000 cfs)

27 Santa Margarita River Hydraulics Investigation Channel Velocity Profile 100 yr Flow (64,000 cfs)

28 Santa Margarita River Hydraulics Investigation Overbank Velocity Profile 100 yr Flow (64,000 cfs)

29 Santa Margarita River Hydraulics Investigation Differences in Hydraulic Characteristics between Scenarios

30 Santa Margarita River Hydraulics Investigation Arundo Effects on Sediment Transport Capacity (Reconnaissance-level Stream Power Approach) Stream Power: Ω = tvw t = average bed shear stress v = average sectional velocity w = channel width

31 Santa Margarita River Hydraulics Investigation Summary of Relative Differences in Stream Power by Scenario S1 = Arundo infestation S2 = Native vegetation S3 = Bare floodplain S4 = 1997 Floodplain

32 Santa Margarita River Hydraulics Investigation Arundo Effects on Sediment Transport Capacity (Reconnaissance-level Stream Power Approach) Floodplain Lower Transport Sediment deposition Vertical accretion + Channel Greater Transport Bed Scour Potential Net Effect = Short Term Adjustment Period Vertical floodplain accretion offset by some some channel bed scour Long Term Establish new equilibrium state with altered geomorphology

33 Santa Margarita River Hydraulics Investigation Summary of Key Findings Hydraulic modeling of different floodplain vegetation scenarios suggests: Arundo infestation can raise 100-year water levels by several feet Arundo infestation significantly increases the portion of flow discharge carried by the channel. Subreaches with wider floodplain exhibit greater response to Arundo infestation than narrower ones (due to greater Arundo coverage) Arundo coverage somewhere over 30% generally results in significant adjustments to the water surface profile relative to the native vegetation condition. HEC-RAS modeling results support and demonstrate the observed geomorphic response of channel narrowing and simplification of the river cross-section.

34 Presentation Outline Study Objectives Arundo Donax Description Arundo Effects on Rivers Santa Margarita River Case Study Background River Hydraulics Investigation Application to other Rivers (Source: Cal-IPC 2011) Conclusions (Source: Cal-IPC 2011)

35 Santa Margarita River Hydraulics Investigation Arundo Impact Scoring System Qualitative Method to determine potential Impact of Arundo on southern California coastal rivers using key findings from the hydraulic study.

36 Santa Margarita River Hydraulics Investigation Arundo Impact Scoring System: Application to other Rivers Santa Margarita River Santa Ana River San Luis Rey

37 Presentation Outline Study Objectives Arundo Donax Description Arundo Effects on Rivers Santa Margarita River Case Study Background River Hydraulics Investigation Application to other Rivers (Source: Cal-IPC 2011) Conclusions (Source: Cal-IPC 2011)

38 Conclusions Dense Arundo stands can dramatically increase channel roughness and flood risk if present in sufficient quantity Arundo tends to transform historically braided rivers into narrower, single-thread channels with root-stabilized banks More information and NHC s full report can be found at: