DEVELOPING HYDRAULIC RELATIONSHIPS AT THE RIFFLE CREST THALWEG IN GRAVEL BED STREAMS By Gabriel Jacob Rossi A Thesis Presented to The Faculty of Humboldt State University In Partial Fulfillment of the Requirements for the Degree Master of Science in Environmental Systems: Environmental Resource Engineering Committee Membership Dr. Margaret Lang, Committee Chair Dr. William Trush, Committee Member Dr. Eileen Cashman, Committee Member Dr. Andre Lehre, Committee Member Dr. Christopher Dugaw, Graduate Coordinator December 2012 ABSTRACT Developing Hydraulic Relationships At The Riffle Crest Thalweg In Gravel Bed Streams Gabriel Jacob Rossi The alluvial riffle crest is a recurring, geomorphic feature of gravel bed rivers that has a strong influence on low flow hydraulics and ecology in streams. Riffle crest depth has been used to quantify habitat availability in the upstream pool. Recently, a quantitative relationship between streamflow (Q) and median riffle crest thalweg depth (mRCT) has been observed by researchers in gravel bed streams throughout Northern California. The mRCT is the median from a data set of riffle crest thalweg (RCT) depths measured along a channel reach of known streamflow. This thesis was undertaken to: (1) determine the strength of correlation between mRCT and streamflow in gravel bed streams, and (2) if possible, to refine the RCT depth-Q relationship into a useful tool for ecological management and restoration. A field method was developed to reproducibly locate the RCT. Riffle crests were sampled in nineteen reaches from thirteen streams across Northern California. While RCT depth was moderately correlated to Q (R2 = 0.55), mRCT was strongly correlated to Q (R2 = 0.84). The regression equation from the mRCT-Q relationship was used to predict streamflows between 1 - 155 cfs, with the best prediction (5.1 cfs median error) in ii the 12-30 cfs range and the worst prediction (12.1 cfs median error) in the 55 - 155 cfs range. Median sediment size (D50) and wetted width of the channel (WWC) had an appreciable effect on RCT depth at a given flow. In addition, an inflection in the RCT depth-Q relationship was shown to occur when streamflow reached the active channel stage. The RCT depth-Q relationship appears not to be scaled to drainage area which stands in contrast of commonly used hydraulic geometry relationships. A dimensionless variable was developed to scale the RCT depth-Q relationship to local channel morphology. The -RCT relationship produced a streamflow prediction error for individual riffle crests of 5.3 cfs based on a sample of 38 riffle crests. This thesis shows that both RCT and mRCT are correlated to Q, although the mRCT-Q correlation is much stronger. In addition, scatter in the RCT depth-Q and mRCT-Q relationships appear to be caused by variability in sediment size and wetted width of flow between riffle crests. The relationships developed in this thesis: RCT depth-Q, mRCT-Q and -RCT, have potential to become tools in ecological management applications in which depth of flow at the riffle crest is an ecological control. Such applications include fish passage through natural riffles and developing streamflow-habitat relationships. iii ACKNOWLEDGEMENTS I am grateful and fortunate to have been guided, supported, inspired and encouraged by many people throughout this project. First and foremost I am grateful to my wife Maya, for spending hours in leaky waders, more hours editing and advising, and countless hours listening to nerdy blather about rivers. Dr. Bill Trush provided the genesis of, and continued inspiration for, this project as well as my career in river restoration. Dr. Margaret Lang read, re-read and re-re-read my thesis, offering valuable direction and guidance. Drs. Eileen Cashman and Andre Lehre also read, commented on, and helped me to refine this work. Kyle Garmany spent many hours in the field, not only helping me with data collection but also providing valuable dialogue on river hydraulics (all for the price of a few tri-tip sandwiches). Lucas Walton P.E., Randy Klein, Brian Powell and Jason Bone all volunteered to provide independent testing of my field methods. Scott McBain provided time off from work, and encouragement for me to cross the finish line. Besides introducing me to the world of aquatic ecology, Dr. Robert Gearheart advised me not to “get married, get a job, or become a father…” if I wanted to get my thesis finished. Having done all three, I see now the wisdom in his words. In addition to all those mentioned by name, many professionals and academics accepted my phone calls, emails, and in some cases personal intrusions into their busy lives to answer my questions and inform the writing of this thesis. Finally I give thanks and praise to the Great Artist, and the source of all rivers. iv DEDICATION This thesis is dedicated to Luna Leopold, whose thinking was primarily responsible for the foundation of the work presented here; and to my infant son Maximo Benjamin, whom I hope to fill with the wonder of rivers. v TABLE OF CONTENTS 1 Introduction ................................................................................................................. 2 2 Background ................................................................................................................. 4 2.1 Fluvial Geomorphic Setting ................................................................................. 4 2.2 The Active Channel .............................................................................................. 8 2.3 Hydraulic Geometry ........................................................................................... 11 2.4 Weir Equations to Predict Flow over Riffles ..................................................... 14 2.5 The Ecological Significance of the Riffle Crest................................................. 15 2.6 Objectives and Tasks .......................................................................................... 16 3 Methods..................................................................................................................... 18 3.1 RCT Data Collection Methods ........................................................................... 18 3.1.1 Step 1—Visually identify the RPL ............................................................. 19 3.1.2 Step 2—Identify the thalweg within the RPL region.................................. 20 3.1.3 Step 3 – Locate the RCT ............................................................................. 20 3.2 Streamflow Data ................................................................................................. 23 3.3 Sample Size Analysis ......................................................................................... 24 3.4 Statistical Analysis of RCT Depth-Q Data ........................................................ 26 3.5 Independent Testing of RCT Depth-Q Relationship .......................................... 29 vi 3.6 Other Hydraulic and Geomorphic Controls on RCT Depth ............................... 29 3.6.1 Wetted Width Data Collection and Analysis .............................................. 30 3.6.2 Drainage Area Data Collection and Analysis ............................................. 31 3.6.3 Sediment Size Data Collection and Analysis.............................................. 32 3.6.4 Active Channel Stage Data Collection and Analysis .................................. 33 3.7 Analysis of Multiple Variables .......................................................................... 33 4 Results ....................................................................................................................... 36 4.1 Statistical Correlation between mRCT and Streamflow .................................... 36 4.2 Statistics for the RCT –Q Relationship .............................................................. 37 4.3 Statistics for the mRCT-Q Relationship ............................................................ 37 4.3.1 Variance in RCT Depths Within a Sample Reach ...................................... 39 4.4 Cross Validation of the mRCT Data Set ............................................................ 40 4.5 Independent Testing of RCT Depth-Q Relationship .......................................... 41 4.6 The Effect of Non-Hydraulic Controls on the mRCT-Q Relationship .............. 43 4.6.1 Wetted Width of Channel ........................................................................... 43 4.6.2 Drainage Area ............................................................................................. 45 4.6.3 Sediment Size.............................................................................................. 46 4.7 Analysis of Multiple Variables .......................................................................... 47 4.8 Comparing Predictive Capabilities of Three Regression Models ...................... 53 vii 4.9 Active Channel Boundary .................................................................................. 54 4.10 Sample Size Analysis Results ......................................................................... 57 5 Discussion ................................................................................................................. 60 5.1 The strength and significance of the correlation between mRCT and Q ........... 61 5.2 Refining the mRCT-Q relationship into a tool for ecological restoration ......... 65 5.3 Future Applications of the mRCT-Q Relationship ...........................................