1 Prepared for: Prepared by: Mauna Kea Soil and Water AECOM Conservation District Honolulu, HI Kamuela, HI January 2012 and Stream Stabilization Preliminary Investigation
2 Prepared for: Prepared by: Mauna Kea Soil and Water AECOM Conservation District Honolulu, HI Kamuela, HI January 2012 and Stream Stabilization Preliminary Investigation Prepared By Stephen Blanton, PE, CFM Reviewed By Ardalan Nikou, PE
3 EXECUTIVE SUMMARY The Waipi o Valley and Wailoa Stream are repeatedly exposed to rapid influxes of sediment from frequent landslides in the upper valley. As sediment is transported downstream into the lower valley where channel slopes are less, the larger bedload material is deposited within the channel. The result is channel migration as the stream adjusts to new flow paths. The November 2006 earthquake caused several large slope failures in the upper valley. The amount of sediment, gravels and large boulders entering the stream due to the earthquake has caused an increase in the instability of the valley. The project is intended to develop alternatives at the Kawashima Waterhead and Linda Beech Crossing that will provide solutions to stabilize the Wailoa Stream channel. At both locations, deposition and channel migration are adversely affecting the local taro farmers ability to supply adequate irrigation to the taro fields. Additionally, bank erosion is affecting levees designed to protect the fields during high flow events. Existing site conditions at the Kawashima Waterhead include a widening of the stream channel, which results in multiple flow paths and deposition of gravel. The original auwai (irrigation canal) has breached downstream of the waterhead, requiring repeated repairs, which consist of placing rocks to hold flow in the auwai. The Linda Beech Crossing site is used by local farmers to cross the Wailoa Stream in order to access fields on the west side of the valley. Historically, the stream channel in the area migrated across a wide swath. Recently, the stream migrated approximately 200-feet to the east, creating a new channel with a much steeper slope, which is capable of transporting the increased sediment load. The concern lies with what happens when the river corrects itself, perhaps by creating a new channel and adversely affecting adjacent taro fields and abandoning waterheads/ auwai. For both locations, multiple design alternatives were developed to establish a stable channel geometry, which will transport the sediment load through the project reach, while also providing reliable diversion of flow in the auwai. Using field-identified, stable stream sections as a blue print, bank full geometry for the two project sites were estimated, and design alternatives were developed to reconstruct stable channel geometry. To address the streambank erosion concerns at the two sites, the alternatives use rock vanes to redirect the erosive velocities of the stream away from the bank and into the center of the river. The vanes are also used to develop flow conditions within depositional zones to increase the sediment transport through the reach. Stakeholders reviewed the initial alternatives for the two sites. Following review of the alternatives, the stakeholders selected hybrids of the alternatives as the final site design, which involved elements of multiple alternatives. Table ES-1 below contains the estimated costs associated with each of the alternatives for the two project sites as well as the final selected designs. As presented, each alternative provides additional elements to the project site. ii
4 EXECUTIVE SUMMARY Table ES-1: Estimated Costs Associated with Each Alternative Alternatives Location Total Construction Cost Total for Design and Construction 1 Stable Channel Design Kawashima Waterhead $620,000 $790,000 2 Waterhead Re-design Kawashima Waterhead $820,000 $1,040,000 3 Including Rock Vanes Kawashima Waterhead $642,000 or $842,000 $870,000 or $1,070,000 4 Stabilize Current Channel Linda Beech Crossing $1,170,000 $1,490,000 5 Plug Side Channel Linda Beech Crossing $440,000 $560,000 6 Restore Old Channel Linda Beech Crossing $1,420,000 $1,800,000 Selected Design for Kawashima Kawashima Waterhead $830,000 $1,050,000 Selected Design for Linda Beech Linda Beech Crossing $870,000 $1,110,000 iii
5 CONTENTS EXECUTIVE SUMMARY ii ACRONYMS AND ABBREVIATIONS vii 1.0 INTRODUCTION BACKGROUND Kawashima Waterhead Linda Beech Crossing Previous Studies STAKEHOLDER INPUT EXISTING CONDITIONS INVENTORY AND ANALYSIS Bankfull Identification Kawashima Waterhead Field Observations Planform Particle Size Distribution Longitudinal Profile and Cross-section Linda Beech Site Field Observations Planform Particle Size Distribution Longitudinal Profile and Cross-section Stability Analysis Sediment Capacity Sediment Competency Hydrology Flood Frequency Estimates Flow Duration Hydraulic Modeling Downstream Boundary Condition Wailoa Stream Mouth Blockage HEC-RAS Modeling at Kawashima Waterhead HEC-RAS Modeling at Linda Beech Crossing ALTERNATIVES ANALYSIS Kawashima Site Sediment Transport and Streambank Stabilization Streambank Erosion Protection Linda Beech Site Sediment Transport and Streambank Stabilization Alternatives Determination and Description Kawashima Waterhead Linda Beech Site Comparison of Alternatives Work on Other Areas of Wailoa Stream PROJECT COSTS Considerations Site Access and Equipment Mobilization Material Availability Construction Estimates Potential Cost Impacts Estimated Costs for Design and Construction 6-2 iv
6 Contents 6.2 Estimated Costs for Planning and Permitting SELECTED ALTERNATIVES Kawashima Waterhead Linda Beech Crossing Final Design Complete Plans and Specifications Design-Build PERMITTING REQUIREMENTS NEPA Requirements National Pollutant Discharge Elimination System (NPDES) Permit for Storm water Associated with Construction Activities NPDES General Permit for Construction Activity Dewatering Effluent Department of the Army Permit Water Quality Certification, Section Stream Channel Alteration Permit Conservation District Use Permit Endangered Species Act Section 7 Consultation National Historic Preservation Act Section 106 Consultation PROJECT IMPLEMENTATION AND FUNDING OPPORTUNITIES Federal U.S. Department of Agriculture Natural Resources Conservation Service U.S. Fish and Wildlife Service U.S. Army Corps of Engineers U.S. Environmental Protection Agency National Oceanic and Atmospheric Administration Hawai i Congressional Delegation State of Hawai i Department of Health Department of Land and Natural Resources Coastal Zone Management Program Department of Agriculture Mauna Kea Soil and Water Conservation District State Legislature County of Hawai i Non Governmental Organizations Big Island Resource Conservation and Development Hawaii Community Foundation Other Foundations Implementation Process Water Committee Plan Preparation Environmental Review Design Permitting CONCLUSIONS REFERENCES 11-1 APPENDICES A B Stakeholder Meeting Notes Alternative Designs v
7 Contents C Conceptual Plans FIGURES 1 Vicinity Map Lo i in the Waipi o Valley Waipi o Valley Site Locations (Aerial Date 2002) Kawashima Waterhead with Manuwai Flow Split at Linda Beech Crossing Lane s Diagram Bare Valley Walls Resulting from Landslide Looking Upstream at the Kawashima Waterhead where Flows Reconverge Facing Towards Bank Erosion Area on the Left Bank above the Waterhead Looking Upstream from a Pool Located Just Upstream of the Flow Split Kawashima Waterhead Pebble Count Section Upstream of Kawashima Waterhead Section at the Flow Split of Kawashima Waterhead Looking Upstream from the Flow Split in the Active Channel Looking Downstream at the Right Split Channel Pebble Count Upstream at the Linda Beech Site Pebble Count Downstream at the Linda Beech Site Cross-section Upstream of Flows Split at Linda Beech Site Cross-section of Left Split Channel at Linda Beech Site Cross-section of Right Split Channel at Linda Beech Site Kawashima Site, Unit Stream Power (lb/ft/sec) vs. Discharge (cfs) Linda Beech Site, Unit Stream Power (lb/ft/sec) vs. Discharge (cfs) The Wailoa Stream Watershed Island of Hawai i Hydrologic Regions Flow Duration Curve for the Wailoa Stream Waipi o Valley Hydraulic Model Layout Kawaihae Tide Prediction for Late May Accumulated Material at Wailoa Stream Mouth Wailoa Stream Accumulated Material Modeled in HEC-RAS Wailoa Stream Water Surface Profile HEC-RAS Results at Kawashima Waterhead Site HEC-RAS Results at Linda Beech Crossing Site Stable Cross-section Upstream of Kawashima Waterhead Site Existing Cross-section at Kawashima Waterhead Site 5-3 vi
8 Contents 36 Typical Cross-section of Proposed Conceptual Cross-Section at Kawashima Waterhead Unit Stream Power versus Discharge Relationship of Upstream Stable Section Compared with Proposed Typical Section Detail of Soil Layer Lifts Method of Streambank Stabilization (Source - VDEQ 2004) Typical Vane (J-Hook type) Configuration (Source - NRCS 2007) Typical Cross-section for Implementation on Left Branch of Linda Beech Site Unit Stream Power versus Discharge Relationship of Upstream Stable Section Compared with Proposed Typical Section Example Detail of a Permanent Ford 5-12 TABLES 1 Additional Studies Related to the Waipi o Valley Planform Characteristics for the Two Project Sites Geomorphic Properties of the Wailoa Stream through the Kawashima Waterhead Site Planform Characteristics for Linda Beech Site Geomorphic Properties of Wailoa Stream through the Linda Beech Site Shear Stress Analysis of the Wailoa Stream at the Linda Beech and Kawashima Properties USGS Wailoa Stream Flow Gage Summary Wailoa Stream Estimated Peak Flows Tidal Elevations for Kawaihae HEC-RAs Results for Kawashima Waterhead Project Reach HEC-RAS Results for Linda Beech Project Reach HEC-RAS Hydraulic Parameter Results at Kawashima Waterhead Flow Split Evaluation at Linda Beech Crossing Alternative Selection Evaluation for the Kawashima Waterhead Site Alternative Selection Evaluation for the Linda Beech Site Estimated Cost for Alternative No Estimated Cost for Alternative No Estimated Cost for Alternative No Estimated Cost for Alternative No Estimated Cost for Alternative No Estimated Cost for Selected Design at Kawashima Waterhead Site Estimated Cost for Selected Design at Linda Beech Site 7-3 vii
9 ACRONYMS AND ABBREVIATIONS % percent ARMD Agricultural Resources Management Division BIRC&D Big Island Resource Conservation and Development BKF bankfull BMP Best Management Practice CDUP Conservation District Use Permit cfs cubic feet per second Crest Coastal Resilience Networks CWA Clean Water Act CWB Clean Water Branch CWRM Commission on Water Resource Management CY cubic yard CZM coastal zone management DA Department of the Army DBEDT Department of Business, Economic Development and Tourism DLNR Department of Land and Natural Resources Dmax largest measured particle size DOA Department of Agriculture, State of Hawai i DOH Department of Health, State of Hawai i DPW Department of Public Works, County of Hawai i EA Environmental Assessment EIS Environmental Impact Statement EQIP Environmental Quality Incentives Program EWP Emergency Watershed Protection FONSI Finding of No Significant Impact fps feet per second ft feet foot ft/ft feet per foot ft 2 square feet HCF Hawaii Community Foundation HEC Hydrologic Engineering Center Hrs Hawaii Revised Statutes JD jurisdictional determination lb/ft 2 pounds per square foot LHD Lower Hamakua Ditch LiDAR light detection and ranging Lm meander length mi 2 square mile MKSWCD Mauna Kea Soil and Water Conservation District mm millimeter MOU memorandum of understanding NEPA National Environmental Policy Act NHPA National Historic Preservation Act NMFS National Marine Fisheries Service NOAA National Oceanic and Atmospheric Administration NOI Notice of Intent NPDES National Pollutant Discharge Elimination System NRCS National Resource Conservation Service NWP Nationwide Permit OCCL Office of Conservation and Coastal Lands PAS Planning Assistance to the States viii
10 Acronyms and Abbreviations RAS RC Rc/Wbkf SCAP SHPD SMP SSCBMP U.S. USACE USDA USFWS USGS WARSSS Wblt WHIP WO WQC WSE WSP River Analysis System radius of curvature ratio of radius of curvature to bankfull width Stream Channel Alteration Permit State Historic Preservation Division Stream Management Plan Site-specific Construction Best Management Practices United States United States Army Corps of Engineers United States Department of Agriculture United States Fish and Wildlife Service United States Geological Survey Watershed Assessment of River Stability and Sediment Supply belt width Wildlife Habitat Incentives Program Watershed Operations Water Quality Certification water surface elevation Watershed Surveys and Planning ix
11 1.0 INTRODUCTION Waipi o Valley, on the northern coast of the Island of Hawai i (Figure 1), is an important cultural and economic resource to native Hawaiians, the County of Hawai i, and the State of Hawai i. Native Hawaiian cultural traditions place Waipi o as the wellspring of the political dynasties that consolidated power in the islands, and as the source of the traditional kalo-based society and economy. The Wailoa Stream system, which winds through Waipi o Valley and outlets at Waipi o Bay, is one of the most managed stream segments in the state. Hawaiians have cultivated kalo for over 1,000 years in Waipi o, sustaining populations in the thousands. Management of the dynamic stream processes and equitable allocation of water for irrigation required intensive and sophisticated management to successfully continue kalo production for a millennium. The decline of traditional cultural practices and conversion to modern forms of kalo cultivation came with the transition to single farmers with less communal involvement in farming. The traditional methods to manage the stream and to make and implement water allocation decisions also have suffered. A number of damaging floods in the past 50 years have made farmers keenly aware of the need to manage the river system to protect their fields against severe flood effects and to recover quickly from inevitable flood events. Floods in 1979, 1986, and 1989 caused considerable damage to taro lo i by avulsion of the stream and deposition of bedload in the lo i. During much of the 20th century, farmers used heavy equipment, such as bulldozers, in the river system to remove gravel bars and to repair streambanks following a storm for flood protection of the taro lo i. In the 1990s, complaints to the United States (U.S.) Army Corps of Engineers (USACE), under provisions of the Clean Water Act (CWA), halted the ability of farmers to manage the river with heavy equipment. Since the cessation of unpermitted mechanical stream maintenance, the natural stream tendencies for braiding in some reaches and meander in other reaches have increased the flooding and erosion problems for many taro farmers. The Waipi o kalo farmers are attempting to organize themselves and develop modern environmental principles to manage the streams in Waipi o to support kalo cultivation, while being mindful of traditional practices. They have requested stream analysis, sustainable solutions to flooding and bank erosion, and assistance to complete permit applications and execute the terms of the permits. The U.S. Department of Agriculture (USDA) Natural Resources Conservation Service (NRCS), in partnership with other organizations, such as Friends of the Future and the Mauna Kea Soil and Water Conservation District, has assisted the Waipi o farming community to progress toward their stream management goals. In 1999, at the request of the Waipi o Taro Farmers Association, Waipi o Valley was included in the project area for the NRCS Lower Hamakua Ditch (LHD) Watershed project. NRCS offered technical assistance to the Waipi o taro farming community for stream management and maintenance activities. In 2006, NRCS, with significant stakeholder input, completed the Waipi o Valley Stream Management Plan (SMP), which provides a framework for stream management to increase resilience of the taro lo i and farm improvements to flood damage. The SMP provided the contacts and processes for acquiring permits for maintenance activities and discussed the community organization s need to implement the management plan. The SMP identified the major problem areas for Waipi o farmers. The problem areas included: The blockage at the rivermouth, which increased flooding in the lower part of the valley. Gravel bars and islands in the midstream, approximately one mile upstream of the rivermouth, are reducing flood capacity and are causing bank erosion, including bank erosion near the Kawashima farm. 1-1
12 The unstable road crossing that requires constant maintenance. The intake to the major auwai system at the Kūnaka split is unstable and is threatening to direct the river into the auwai system. The meandering stream reach near the Linda Beach Crossing, which has caused considerable damage due to streambank breaches that pour floodwater and bedload into neighboring taro lo i. In 2009, NRCS assisted the Waipi o community in acquiring the necessary permits and approvals to clear the sand and rocks from the rivermouth to reduce flood damage and lower the stream surface elevation by as much as 5 feet (ft). This effort was conducted in partnership with the State of Hawai i Civil Defense, which was able to extend the State s Emergency Declaration for the October 2006 earthquake. The present study focuses on two of the most severe problem areas identified in the SMP. The study will develop conceptual solutions or alternatives for the stream problems near Kawashima farm and at the Linda Beach Crossing. 1-2
13 January 2012 Figure 1: Vicinity Map 1-3
14 2.0 BACKGROUND The is concerned with providing a reliable source of irrigation water to the valley taro farmers from the Wailoa Stream, while also addressing approaches to reduce damages to property resulting from flooding events. The production of taro requires a steady flow of water to pass through the lo i (Figure 2), supplying needed nutrients to the growing plants. Flow from the Wailoa Stream is diverted to the fields throughout the valley using auwai (irrigation canals). The diversion point of each of the auwai is referred to as a waterhead. In the Waipi o Valley, waterheads are generally cut through the existing stream bank to connect the river and irrigation system. Flow diverted from the river into the fields is returned to the river after passing through the fields. Figure 2: Lo i in the Waipi o Valley As identified in the Request for Proposal, the efforts associated with this project will investigate alternatives for reducing flood damage and stream erosion, with the goal of adding protection for the water supply system in the valley. For this project, efforts will focus on two sites: the Kawashima Waterhead and Linda Beech Road Crossing. Figure 3 illustrates the location of the two sites within Waipi o Valley. 2-1
15 Figure 3: Waipi o Valley Site Locations (Aerial Date 2002) 2.1 KAWASHIMA WATERHEAD The Kawashima Waterhead site is experiencing channel migration and bank erosion within the stream reach. Channel migration has resulted in multiple channels and both vegetated and gravel islands. The multiple channels and islands have directly affected the waterhead s ability to maintain adequate irrigation levels to the taro farmers. Current mitigation efforts require manual placement of a manuwai (rock weir) in the channel to maintain the water level at the waterhead. Figure 4 shows the hand-placed rock material. 2-2
16 Figure 4: Kawashima Waterhead with Manuwai Downstream of the waterhead, bank erosion is threatening the stability of levees and the stream bank that protect the taro fields during high flow events. Large trees have grown along the levees and the natural bank. As the banks continue to erode, many of the trees are being undercut, threatening failure of the bank and resulting in the tree falling into the river. Trees falling into the river may result in localized flow conditions that increase the likelihood of bank erosion. The project goal for the Kawashima site is to return the stream to a historic condition so that the waterhead functions appropriately without continual placement of rock in the channel, and to protect the small levees downstream of the waterhead. 2.2 LINDA BEECH CROSSING The Linda Beech Crossing site refers to a ford used to drive across the stream to access farmland on the other side. The channel in the area is unconfined and historically meandered across the valley. As the river abandons channels and creates new ones, flows are divided between two or more channels and the river channel invert adjusts. The changes in the river channel location and elevation adversely affects the ability of local auwai to divert flows. Taro farmers in the area are currently concerned that the river migration increases the chance that high flows may be directed toward existing lo i. Past flooding events that have reached lo i have resulted in crop loss and damage to farm facilities. Currently upstream of the crossing, a manuwai (Figure 5) has been placed by farmers. 2-3
17 Figure 5: Flow Split at Linda Beech Crossing The project goal at the Linda Beech Crossing site is to develop an approach to maintain the stream in a control zone in order to provide greater protection of property and crops as well as providing a reliable access location across the Wailoa Stream. 2.3 PREVIOUS STUDIES The issue of maintaining the Wailoa Stream and protecting taro production has been ongoing for many years. Previous efforts have been conducted to investigate these issues and provide approaches for addressing the problems (Table 1). One of the goals of this project is to build off previous efforts related to the Waipi o Valley and the Wailoa Stream. Two recent studies, described below, have addressed the erosion and channel instabilities facing the valley. In these two studies, actions were prepared to remedy the site-specific issues. Since the two reports were published, the 2006 earthquake occurred, which affected the sediment volume and size distribution entering the Wailoa Stream system. Waipio Valley Planning Assistance to States Study Waipio, Hawaii (USACE 2002). This report provides technical support in the development of alternative plans for stream maintenance and flood control in the Waipi o Valley. The Kawashima and Linda Beech Crossing locations were identified in the study report. For the Kawashima site, the study identifies gravel bar development and streambank erosion as issues adversely affecting the site. At the Linda Beech Crossing location, the study also identifies the presence of gravel bars and channel instability. The study provides a list of proposed actions that generally focus on removal of the accumulated gravels. 2-4
18 Waipio Valley Stream Management Plan (NRCS 2006). The management plan was developed and finalized in 2006 by the NRCS. The plan s intended purpose is to assist the taro farmers and residents of the valley in maintaining the river and complex irrigation system in a manner that meets the needs of the valley community, while also addressing environmental and legal limits of available actions. The plan is an excellent resource on the history of the valley from both a cultural and environmental resources approach. Table 1: Additional Studies Related to the Waipi o Valley Year Document Title 1946 Stearns, H. T. and G. A. MacDonald Geology and Ground-Water Resources of the Island of Hawaii. Hawai i Division of Hydrography, Honolulu Lennox, C. G A report to the Trustees of the Bishop Museum on the resources of the Waipio Valley, Island of Hawaii, their past and present uses and an analysis of the problems facing their fuller use in the future. Honolulu; Bishop Museum (unpublished) 1970 MacDonald, G. A., A. T. Abbott, and F. L. Peterson Volcanoes in the Sea, The Geology of Hawaii. 2nd ed. Honolulu: Univ. of Hawai i Press 1975 Yoshimura, Phillip A Master Plan proposal for Waipio Valley, Hawaii. Hamakua District Development Council Cordy, Ross A Regional Synthesis of Hamakua District, Hawaii Island 1997 Englund, R. A. and R. Filbert Native and Exotic Organisms Study in the Kaiwainui, Alakahi, Koiawe, and Lanakea Streams, Lower Hamakua Ditch Watershed Project, County of Hawaii. Pacific Aquatic Environmental, Inc United States Department of Agriculture, Natural Resources Conservation Service (USDA NRCS) Lower Hamakua Ditch Watershed Plan and Final Environmental Impact Statement Englund, R. A. and D. J. Preston Biological Assessment of the Lower Hamakua Ditch on the Hawaiian Stream Fly and other Aquatic Insects. Hawai i Biological Survey, Bishop Museum. Honolulu: USDA NRCS. February 1999 Dizol, L., D. Hegger, H. Keehne, K. Kinjo, M. Le Maitre, K. McKeague, S. Prasai, A. Resture, C. Shen, and X. Xing. (1999). Waipi o Valley: Towards community planning and ahupua a management. Planning Practicum, Department of Urban and Regional Planning, University of Hawai i United States Department of Agriculture, Natural Resources Conservation Service (USDA NRCS) Wailoa River Trip Report Waipio Valley: Stream Overlaying Methodology 2001 The Waipio Valley Restoration Project: Hydrological and Biological Assessment of Wailoa River and Hi iawe Stream 2001 R. A. Englund, C. Imada, D. J. Preston, N. L Evenhuis, R. H. Cowie, C. Puttock, K. Arakaki, and J. Dockall Native and Exotic Organism Study: Lower Wailoa River, Waipi o Valley, County of Hawai i. Hawai i Biological Survey, Bishop Museum. Honolulu. November CWRM Stream Channel Alteration Permit for Removal of Gravel Bars 2002 Waipio Stream Cross Section Survey (2 parts) 2002 Department of Urban and Regional Planning (DURP) Waipi o Valley: Towards Community Planning and Ahupua a Management, Phase II. University of Hawai i. Honolulu Source: NRCS
19 3.0 STAKEHOLDER INPUT Recognizing and attempting to address the concern of all stakeholders to the greatest extent possible is key to developing successful projects. Due to the cultural, economic, and environmental importance of the Waipi o Valley, many groups are included as stakeholders for this project. These groups include federal, state, and local agencies, local farmers and landowners. A stakeholder s meeting was held prior to the beginning of major efforts associated with the Waipi o Valley project. The meeting was held on November 16, 2010 near the mouth of the Wailoa Stream. The AECOM project team, NRCS and Mauna Kea Soil and Water Conservation District (MKSWCD) personnel, and taro farmers attended the meeting. The purpose of the initial meeting was to obtain input from the people who may be directly affected by the project and to discuss the issues, history and concerns of the stakeholders. The outcome of the November 16, 2010 meeting was a list of concerns that would need to be addressed in the development and assessment of the project alternatives. The major project issues are listed below: Clearing of the rivermouth to reduce backwater flooding. Stakeholders are interested in keeping the approach simple. Sediment will be stored within the floodplain and used for road maintenance. The meeting minutes and list of attendees for the meeting is located in Appendix A. Stakeholder involvement also included a review of the interim project report as well as a decision on selected alternatives for the two project sites (Section 7.0). 3-1
20 4.0 EXISTING CONDITIONS INVENTORY AND ANALYSIS In order to understand the interaction between the river and the land within the Waipi o Valley and the impacts on the farming community, it is important to understand the natural dynamics of a river system. All river systems exist in a state of dynamic equilibrium, which refers to a system s ability to maintain a generally consistent balance related to a set of characteristics. Lane (1955) defined this balance in a river system as a relationship between sediment load, sediment size, stream slope, and discharge (Figure 6). Any change in one of these parameters will result in a natural adjustment in one or more of the other parameters to balance out the system. If the adjustment is not made, the result will be either degradation or aggradation of the river. Figure 6: Lane s Diagram For the Wailoa Stream, a major variable in the Lane relationship is sediment load and size. Because of the Waipi o Valley s steep valley walls, events of mass wasting and landslides can abruptly alter the volume and size of sediment entering the system. Figure 7 illustrates a typical slide within the valley. As the sediment from these slides work through the Waipi o Valley, the river system adjusts to allow the increased material load to be transported to the ocean. The ability of the river to transport the material being delivered from upstream can be described by its capacity and competency. Competency is a measurement of a river s ability to transport the largest particle delivered from upstream, while capacity is a measurement of the total volume or load of sediment that can be carried by the stream. Capacity can be evaluated based on a channel s unit stream power, while competency can be measured based on its shear stress. When the size and/or load of material delivered from the watershed exceeds the channel s current capacity and competency, the channel will seek an equilibrium by increasing its shear stress and unit stream power, which it does by adjusting its hydraulic geometry. These adjustments are accomplished through phenomenon such as avulsion to increase the slope and aggradation of the floodplain to increase the channel depth. The adjustments tend to have adverse consequences of bank erosion due to mid-channel and transverse 4-1
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