Scope of Work Lower Arroyo Grande Creek Flooding Analysis

Transcription

1 Scope of Work Lower Arroyo Grande Creek Flooding Analysis Overview Please note that the limits of work for the hydrologic and hydraulic analysis include the following: Detailed topographic surveys: flood control portion of Arroyo Grande and Los Berros Creek and adjacent areas that would be impacted during a 100-year runoff event. See Figure 1 for details. Minor adjustments, such as including additional areas upstream on Los Berros Creek, can be made during the project initiation meeting and prior to scheduled aerial topographic survey flights. Hydrologic analysis: Hydrologic data will be developed for all areas of the Arroyo Grande Creek watershed downstream of Lopez Reservoir with the intent of providing necessary input parameters to the hydraulic model. Hydraulic analysis (HEC-RAS Model): The HEC-RAS model will be developed for the flood control portions of Arroyo Grande and Los Berros Creeks. The limits of work for the sediment source assessment and sediment transport analysis include the following: Sediment Source Assessment: The sediment source assessment will be developed for all areas of the Arroyo Grande Creek watershed downstream of Lopez Reservoir with the intent of providing necessary input parameters to estimate sediment supply, transport, and deposition conditions within the flood control portions of Arroyo Grande and Los Berros Creeks. Sediment Transport Analysis: Estimates of sediment flux, transport, and deposition using Parker (1990), or other appropriate methods, will be developed for the flood control portions of Arroyo Grande and Los Berros Creeks. As part of the proposed scope we have solicited several bids from aerial survey firms to develop a high-resolution topographic map for the project area. Cannon Associates, a local San Luis Obispo County firm, provided the most complete and cost effective bid. A description of the services they will provide to the project is included. We are also pursuing a bid from Kellogg Aerials of Sacramento for aerial topographic mapping. Page 1 of 9

2 Task Descriptions Task 1 Topographic Data Collection Subtask 1.1: Aerial Topographic Data and Photos This task will be completed by an aerial survey firm to be determined at a later date following evaluation of additional cost estimates (Note: The bid listed in the cost estimate was submitted by Cannon Associates). The primary objective will be to develop high resolution aerial topographic data of the study area, including lands to the north and south of the channel where flooding may result during a 100-year event. The study area also includes portions of the historic Los Berros Creek channel in the event that restoration of the old channel is considered as a component of one of the proposed alternatives. The aerial survey will consist of placing aerial panels on the site to provide aerial control. Once the panels are set, the site will be photographed. A minimum of two overlapping photos will be taken. These photos will then be used to derive contours, features, roadways, etc. This information will be the basis for an AutoCAD drawing and 3-D model of the site. The aerial survey will be compiled according to the following specifications: Elevations will be represented with 10-foot index contours and 1-ft intermediate contours (accuracy of +/ feet). Contours will be accurate to National Mapping Standards. Mapping will be produced at a scale of 1 = 40. Coordinates and elevations will be based upon local county coordinates. Ortho-Rectified photos of the defined project area will be provided with a resolution of 0.3ft pixels, scanned at microns and burned to a DVD or CD. Subtask 1.2: Refine topographic data Additional ground-based surveys will be conducted by SH+G engineers to refine the aerial-based survey around bridges, below water surfaces, and within thick vegetation. Supplemental information provided by the RCD and NRCS along the levees and within heavily vegetated areas will also be used. All information available from the ground-based surveys will be integrated into the aerial topo to produce a final topographic product that will be used to develop the HEC-RAS model. Task 2 Define Flood Reduction Alternatives Subtask 2.1: Define Water Surface Elevation (WSE) Flood Reduction Actions A set of actions will be compiled to reduce flood stage or reduce impacts associated with flooding on Arroyo Grande Creek. These actions may include, but are not limited to, levee raises, levee setbacks, vegetation management, replacing bridges, or developing overflow weirs. The proposed actions will be developed in cooperation with stakeholders such as the RCD, NRCS, County of San Luis Obispo, landowners, residents, and permitting agencies. The initial set of potential actions will be based on a brainstorming meeting and will be comprehensive. Following compilation of a comprehensive list, SH+G will analyze each action item and reduce the list to those action items that are the most feasible in terms of engineering feasibility, cost effectiveness, and permitability. Project costs will be estimated based on preliminary concepts of the proposed action using similar projects as examples. The final list will be submitted to the RCD for review and comment by stakeholders. Page 2 of 9

3 Subtask 2.2: Sediment Reduction Management Actions Assuming that the results of our sediment budget and transport analysis (Task 4) suggest that sediment supply has accelerated due to anthropogenic influences, possibly requiring a long-term commitment to maintenance dredging, actions that will achieve a reduction in the supply of sediment to the flood control reach of Arroyo Grande Creek will be evaluated. A range of sediment reducing actions will be developed during a brainstorming meeting with stakeholders. This comprehensive list will be evaluated and streamlined by SH+G staff based on engineering feasibility and the likelihood that the action would significantly reduce the supply of sediment. Rough cost estimates will be made for each item on the streamlined list of actions. The final list will be submitted to the RCD for review and comment by stakeholders. Subtask 2.3: Select Preferred Flood and Sediment Reduction Actions A Technical Advisory Team (TAT) meeting will be held to evaluate the results of the hydraulic and sediment transport analysis for each proposed action developed in Tasks 2.1 and 2.2. Each action will be evaluated based on the level of flood protection the action provides, the cost of the action, the engineering feasibility of the action, and the likelihood that the action could be implemented given budgetary and permitting constraints. Based on the TAT s evaluation of each action a ranking will be assigned to each action defining a priority of implementation as high, medium, or low. The meeting will also include a discussion of potential funding sources and a phasing schedule to implement each action by priority ranking. Task 3 Hydraulic Modeling Subtask 3.1: Compile Site Hydrology Through a combination of existing data compilation (i.e. Habitat Conservation Plan, CCSE Watershed Assessment, USACE HEC-RAS model) and analysis of discharge data for Arroyo Grande Creek, the hydrology for the site will be defined including the 2.5-, 5-, 10-, 25-, 50-, and 100-year recurrence discharges. Hydrographs will be developed for each of these flood intervals as input to the HEC-RAS unsteady flow model. Where feasible, similar hydrologic parameters will be developed for streams tributary to the reach of interest (Los Berros channel). Subtask 3.2: Develop Existing Conditions HEC-RAS Model Both a steady and unsteady HEC-RAS model will be developed for Arroyo Grande Creek between Los Berros and the mouth and include all model elements including topography, bridge geometry, roughness characteristics, and input flow conditions. Where feasible, the model will be calibrated using known discharge and stage conditions. The HEC model that is developed will be linked with Geo-RAS, an Arcview GIS based product that allows for 3-dimensional visual representation of flow depths and stage within the study area. SH+G will build on an existing HEC-RAS model developed for the Arroyo Grande Creek channel by the USACE in 2001, where feasible. The USACE model utilized historic cross-section and topographic data for development of the 2001 model, therefore, the topographic data will be replaced with the updated information generated from the aerial flights. Where feasible, model parameters such as roughness and discharge values will be carried over from the USACE model and comparisons will be made between Existing Conditions parameters between the two models. The USACE model will be a valuable tool to assess how the hydraulic character and sediment transport conditions have changed within the flood control channel in the intervening period of the two topographic datasets. Page 3 of 9

4 Subtask 3.3: Existing Conditions Flooding Analysis The HEC-RAS model will be used to assess flood stage and volumes associated with the 2.5-, 5-, 10-, 25-, 50-, and 100-year recurrence discharges. GIS layers will be generated to assess relationships between flooding during different events and land use and will include an analysis of flood impacts, tolerance, and risk. The following components will be included: Floodplain Subareas: Areas that will be subject to flooding will be identified and linked to their land use and level of development. Land use and level of development GIS layers will be provided by the RCD through San Luis Obispo County or appropriate city agencies. The floodplain subareas will be compiled onto GIS layers for each flooding scenario. Flooding Impacts: For each sub-area identified above, an estimate of the financial impact on adjacent properties will be compiled into a GIS layer for each flooding scenario. Data detailing crop values, property values (with land and structure values broken down), and the potential for lost income associated with business closings or access to property will be provided to SH+G on a per parcel basis by the RCD through San Luis Obispo County or appropriate city local agencies. Flooding Tolerance and Risk: For each sub-area and flood scenario SH+G will document the tolerance to flooding (frequency, magnitude and duration) and convert the cumulative potential impact of flooding in each subarea to a ranking scale for each flood scenario. The purpose of the ranking scale is to prioritize mitigation project to reduce the extent of flooding based on frequency, magnitude, duration, and overall cost of potential damages. Subtask 3.4 & 3.5: Develop HEC-RAS Model and Conduct Flooding Analysis for Proposed Actions This stage of the hydrologic and hydraulic analysis will consist of an iterative process that will result in a preliminary action plan to reduce flood impacts on Arroyo Grande Creek. Items remaining on the action list will be analyzed within HEC-RAS either as single runs, combined, or in series to allow for an assessment of the cost-benefit of each item as funding allows for its implementation. The focus of the analysis will be to arrive at a feasible plan for reducing flood risk and flood impacts on Arroyo Grande Creek through a combination of projects, actions, or programmatic items. Task 4 Sediment Budget and Transport Modeling Subtask 4.1: Erosion Source and Supply Identification Preliminary work is being conducted as part of the Arroyo Grande Watershed Assessment to identify the dominant sources of erosion and the degree to which each source (e.g. bank erosion, roads, etc) contributes sediment to the mainstem of Arroyo Grande Creek. The California Conservation Corps are also identifying discrete bank erosion sources along the mainstem of Arroyo Grande Creek as part of comprehensive aquatic habitat surveys. Our study will build on the data collected as part of the Watershed Assessment by identifying and quantifying discrete sources of sediment on the tributaries and in upland areas for the portion of the Arroyo Grande Creek watershed below Lopez Reservoir. The approach will closely follow the methodology outlined by Reid and Dunne, Rapid evaluation of sediment budgets (1994), with adjustments made based on existing data and watershed conditions. Bank erosion surveys will be conducted on tributaries not assessed through the habitat surveys. SH+G will require assistance from CCSE, RCD, and NRCS staff to obtain landowner permission where necessary. Erosion sources will also be identified through the use of aerial photos and along accessible roads. A single sediment sample will be taken from approximately 3 to 5 source types within each subwatershed to characterize the grain-size of material available. Each source will then be characterized based on its Page 4 of 9

5 expected delivery efficiency to adjacent stream channels and the likelihood that the material will be available for transport downstream. Higher delivery efficiency values will be given to sources that are directly connected to the stream network and have a high likelihood that the delivered material would reach the mainstem of Arroyo Grande Creek. Subtask 4.2 and 4.3: Sediment Budget Development and Sediment Flux Estimates This task consists of estimating a sediment budget for the flood control portion of Arroyo Grande Creek. Development of the sediment budget will closely follow the methodology outlined by Reid and Dunne, Rapid evaluation of sediment budgets (1994), with adjustments made based on existing data and watershed conditions. The sediment budget will consist of both the bedload and suspended load fractions though the focus will be on the bedload fraction due to its potential impact on bed aggradation and flooding. A sediment budget will be estimated through a variety of techniques including GIS analysis of the watershed using published erosion rates, data collected as part of the erosion source surveys, USGS sediment sampling data from the Arroyo Grande or other nearby watersheds, published erosion rates in adjacent watersheds, sedimentation studies for Lopez Reservoir or other nearby impoundments, and an aerial photo analysis. The estimated sediment budget will include a breakdown of supplied sediment by source within each primary subwatershed to prioritize sediment source reduction programs. The Reid and Dunne approach will provide an estimate of the amount of sediment being transported from hillslopes to channels. To estimate the rates at which sediment is moving through channels at key locations in the watershed we will use the Parker bedload transport equation (Parker, 1990). The Parker equation utilizes local cross-section, slope, and surface bed material information to calculate a sediment flux per unit time. This calculation can be made for a range of discharges to produce a rating curve for bedload transport. Cross-sections and grain size data collected as part of the Arroyo Grande Watershed Assessment will be supplemented by data collected at addition sites throughout the watershed. Suspended load estimates at key locations in the watershed will be estimated from existing USGS data within the Arroyo Grande watershed or other adjacent watershed through a comparative analysis of drainage area and basin attributes. Subtask 4.4 and 4.5: Sediment Transport Analysis Existing and Proposed Action Conditions The focus of this analysis will be on bedload movement. The underlying assumption through the flood control reach is that coarse sediment deposition has reduced flood capacity and increased the risk of flooding. We will evaluate whether the current channel is at equilibrium with current sediment supply or if additional aggradation, and consequently an increase in the flood risk, should be expected. The movement and deposition of sediment within a river system is a direct function of the channel geometry, slope, grain size characteristics, and hydraulic influences such as constrictions or backwatering. HEC-RAS output variables can be used to calculate a longitudinal profile of stream power or shear to determine where sediment deposition would be expected to occur. Such products, combined with field surveys of depositional areas or geomorphic mapping from aerial photos and topographic information, have been shown to be a cost effective approach to understanding cause and effect relationships between aggradational areas and channel hydraulics. In addition, calculations of bedload flux through use of sediment transport equations such as Parker (1990) is an alternative to development of costly sediment transport models. Existing conditions will be described based on an analysis of the HEC-RAS results and field verification (e.g. mapping of significant bar deposits, grain size description). The Parker equation (1990) will be used to quantify sediment flux at key points in the channel where HEC-RAS and field data suggest there are Page 5 of 9

6 hydraulic transitions from transport to aggradation and visa versa. Sediment reduction measures proposed in the watershed to reduce sediment delivery to the flood control reach will be evaluated based on published sediment reduction efficiency estimates available for a range of BMP s, and their delivery efficiency to stream channels based on location and connectivity to the channel network. Based on these calculations, flux estimates will be adjusted. The Parker equation will be used directly to reevaluate sediment flux in areas where channel geometry has been modified to improve the sediment transport regime. Task 5 Flood Management Report Subtask 5.1: Draft Report A comprehensive report will be prepared that describes all the work completed on the project. The focus of the report will be to describe the existing hydrology, hydraulics, and sediment transport conditions through the flood control reach of Arroyo Grande Creek, proposed actions to reduce flooding and sedimentation, and how those proposed actions perform within the hydraulic and sediment transport modeling environment. In addition, the report will include appropriate background information, modeling assumptions, input variables, maps, and HEC-RAS output in support of a description of the proposed actions. Based on the results of the modeling, a concise process will be described to implement the most cost-effective actions with phasing proposed based on expected future funding. A draft will be prepared for Technical Advisory Team and Stakeholder review. Following submittal of the draft report a meeting will be held with the TAT and Project Team to discuss the report and provide comment. SH+G will accept written comments from all interested parties following a review period of one month following submittal of the Draft Report. To reduce reproduction costs, only five hard copies of the Draft Report will be submitted with one copy on file at the RCD and/or NRCS office for public and stakeholder review. Additional copies will be available by request as a PDF and distributed either via or on a CD. Subtask 5.2: Final Report SH+G will review and incorporate, where appropriate, written comments on the Draft Report into a Final Report. A total of 10 hard copies and 10 PDF copies on CD s will be submitted to the RCD for distribution. Task 6 CEQA Compliance Subtask 6.1: Background Review Existing sources of information about the study area and vicinity will be reviewed, summarized, and cited as appropriate. Other sources of information relevant to the project vicinity will also be reviewed. Examples of such information include aerial photographs of the project site, U.S.G.S. topographic maps, Soil Survey of San Luis Obispo County, U.S. Fish and Wildlife Service National Wetland Inventory Maps, County Sensitive Species maps, the California Natural Diversity Database (CNDDB), other technical literature related to the biotic resources of the project vicinity such as the existing Habitat Conservation Plan, regional planning documents (general plan policies, EIR's from the region, etc.), species data compiled by the California Native Plant Society, the National Audubon Society, or other public interest groups, and resource agency data (U.S. Fish and Wildlife Service, California Department of Fish and Game, etc.). Subtask 6.2: Checklist on Proposed Actions The primary focus of this task for SH+G will be to evaluate the expected permitting process for each proposed flood control and habitat enhancement actions developed as part of the Flood Management Plan and prepare an Initial Study checklist to evaluate potential impacts of each proposed action. This will act as a preliminary review of each action in terms of the expected level of CEQA analysis and environmental Page 6 of 9

7 review that may be required. This information will be used to assess potential costs and benefits associated with the project. Much of this information will be taken from existing biological and environmental documents prepared for the flood control reach. A more in-depth CEQA analysis will be conducted for the preferred actions. Subtask 6.3: Habitat and Wildlife Surveys SH+G biologists will conduct field surveys of the project site and its immediate vicinity. These surveys will allow our biologists to put into context the information generated in Task The habitats present on the site and in the immediate vicinity of the study area require that a botanist/wetland ecologist and wildlife biologist visit the site. Identification of biotic habitats. The biotic habitats of the project site will be surveyed and delineated on a map of appropriate scale. The dominant tree, shrub, and herbaceous flora will be described. This information will be generated both for the purpose of characterizing the botanical resources of the site, but also for the purpose of characterizing wildlife habitat values (see below). Included in this task will be the identification of biotic habitats suitable for the occurrence of plant species of special status (state and/or federally threatened or endangered, federal candidate species, and California Native Plant Society List 1B species). Protocol-level surveys for special-status plant species are not included within this scope of work; however, a separate contract can be provided if such surveys are warranted. Assessment of wildlife use of project site. The information generated during the botanical assessment will be utilized to characterize wildlife habitat values. Field surveys will also document general wildlife use of the project area including an assessment of the federally listed steelhead known to occur in Arroyo Grande Creek on site. Habitats appropriate for other state- and federallylisted threatened and endangered terrestrial vertebrate species, and such species of special status will be identified. Protocol-level surveys for special-status wildlife surveys are not included within this scope of work; however, a separate contract can be provided if such surveys are warranted. Reconnaissance Survey for Regulated Habitats (Jurisdictional Waters and Riparian Habitats). SH+G will conduct a survey (not sufficiently detailed for submittal to the U.S. Army Corps of Engineers) of Arroyo Grande Creek and surrounding area that may meet the technical criteria of Waters of the United States (waters) and/or under the authority of the California Department of Fish and Game. These potentially regulated areas will be described and their approximate limits will be shown on a map of appropriate scale. Subtask 6.4: Preparation of Biotic Section of Initial Study SH+G will prepare the biotic section for the Initial Study in order to discuss our findings. This report will include an existing conditions section, description of habitats, special-status species table, and description of regulated habitats. Graphics will include a site/vicinity, biotic habitat, CNDDB special-status species occurrence, and potential regulated habitat maps. Impacts to biotic resources resulting from the proposed chosen action will be clearly identified and discussed. Mitigation of all potential significant environmental effects on biotic resources of the project site shall be described and will include recommendations for species surveys, if warranted. A draft report will be submitted to you for review. This scope assumes that revisions will be minimal and that a final report will be prepared based upon one iteration of revisions. Page 7 of 9

8 Task 7 Meetings and Public Outreach Subtask 7.1: Project Team Meetings The Project Team consists of SH+G, CSLRCD, and NRCS staff who are directly involved in the day-to-day goals and objectives of the study. As part of this proposal, the Project Team will meet up to four times to discuss project progress, coordinate public outreach and TAT meetings, review key products such as selection matrices, cost estimates, and conceptual drawings associated with proposed flood reduction actions. The Project Team makes up the core group of researchers and project coordinators. Subtask 7.2: Technical Advisory Team Meeting The Technical Advisory Team (TAT) will consist of Project Team members, agency staff, project funders, and key stakeholders. The TAT will meet three times during the course of the project at the following project mileposts: Project Development Meeting: Once the project has been contracted and the topographic data are available, SH+G will meet with the TAT to discuss the proposed scope of work, define the goals and objectives of the project and discuss the range of potential project actions that could be implemented to reduce flood impacts. Selection of Preferred Actions Meeting: Following SH+G s detailed analysis of proposed project actions a meeting will be held with the TAT to complete items discussed in Subtask 2.3. The outcome of the meeting will be a prioritized list of proposed actions. Discuss of Draft Report: Following submittal of the Draft Report, SH+G will meet with the TAT to accept, review, and discuss comments. SH+G will make a presentation of the report to the TAT at this meeting as a way to stimulate discussion of each element of the Plan. Subtask 7.3: Public Workshops Two public workshops will be conducted over the course of the project to educate and encourage input from the general public on the proposed project approach, goals and objectives, and values of the community. The first public meeting will be held following the first TAT meeting (Project Development Meeting). This meeting will discuss the process to date, introduce members of the Project Team and TAT, discuss the goals and objectives of the project, and solicit input from the general public regarding their concerns and/or interest in seeing the project move forward. The second meeting will be held following completion and review of the Draft Report. At the second meeting SH+G will present the findings of the study to provide a basis of discussion in an open forum format. The goal of the discussion would be to determine if the goals and objectives, in the public s view, have been met and to solicit feedback on the project prioritization matrix and the public s interest in pursuing each project given the potential cost to the community. At this stage, we see SH+G providing a support role as the primary technical clearinghouse. Organization of the meeting, invitations and announcements, and meeting coordination will be managed by RCD and/or NRCS staff. At each meeting SH+G will make a Powerpoint presentation to describe the project and provide the necessary display tools to allow for focused discussion. Page 8 of 9

9 References Reid, L. M. and Dunne, T Rapid construction of sediment budgets for drainage basins. Catena- Verlag, Cremlingen, Germany. 160 pp. Parker, G Surface-based bedload transport relation for gravel rivers. Journal of Hydraulic Research, Volume 28. Page 9 of 9