LOWER ROOSTER RIVER FLOOD CONTROL The following update to the previous study report of 2013 was funded by a state bond. It should be noted that while this study covers improving the capacity of the lower Rooster River and Ash Creek to for flood water flows it indirectly affects the upper river flooding as was experienced in September, 2018 due to severe, short duration rain storms Flood & Erosion Control Board March, 2019 Rooster River Flood Control Project Phase 3 Report Update March 15, 2019 Town of Fairfield Table of Contents 1.0 INTRODUCTION ..........................................................................................................1.1 2.0 ORIGINAL STUDY BACKGROUND ............................................................................2.1 2.1 ORIGINAL STUDY GOALS ..........................................................................................2.1 2.2 HYDROLOGIC AND HYDRAULIC STUDY OVERVIEW...............................................2.1 2.3 ORIGINAL STUDY CONCLUSION ...............................................................................2.2 3.0 FLOOD STUDY UPDATE ............................................................................................ 3.1 3.1 WATERSHED HYDROLOGIC ANALYSIS ................................................................... 3.1 3.2 HYDRAULIC ANALYIS ................................................................................................. 3.2 3.3 FLOODPLAIN EVALUATION .......................................................................................3.4 3.4 OPINION OF PROBABLE COST .................................................................................3.5 4.0 CONCLUSION .............................................................................................................4.1 LIST OF APPENDICES HYDRAULIC AND HYDROLOGIC BACKUP DATA.................................. A.1 HEC-RAS INFORMATION ......................................................................... B.1 ORIGINAL REPORT INFORMATION ........................................................ C.1 i 1.0 INTRODUCTION Stantec Consulting Services Inc. has been retained by the Town of Fairfield to update the Rooster River Flood Control Study - Phase 3 (including Addendum) report dated February 28, 2003 and updated July 10, 2006. This study was conducted in association with a contract provided by the Connecticut Department of Energy and Environmental Protection (CTDEEP). The updates include the following items which are detailed in this report: • Update all cross-section data in the hydraulic model with new surveyed data and include additional cross sections in the Feroleto Steel Bridge area. • Calculate peak flows based on the revised current rainfall data published by NOAA. • Update the starting water surface elevation downstream of the Brewster Street Bridge based on current tidal information. • Review the watershed for potential significant development which may affect the hydrology/hydraulics within the Rooster River watershed. • Review as-built drawings for scour countermeasures installed at the Metro North Bridge and update the hydraulic sections accordingly. • Update the previously submitted property impact matrix based on the model revisions noted above outlining the impacted properties for the revised existing conditions and the proposed conditions after the implementation of the Phase I and Phase II improvements. • Update the opinion of probable cost for the Phase I and Phase II improvements. 1.1 2.0 ORIGINAL STUDY BACKGROUND 2.1 ORIGINAL STUDY GOALS The Rooster River and its floodplain north of I-95 have been studied many times beginning in 1958 with the original construction of the highway. The prior Phase 3 study prepared by FGA on April 1990 was based, in part, on an earlier study completed by Anderson-Nichols & Co., Inc. (ANCO) in 1980. Both previous studies included numerous measures to increase flood protection in the Town of Fairfield, with the FGA report progressing as far as contract plans and specifications. However, a lack of funding and public support served to undermine any significant implementation of either of the study’s recommendations. The intent of the original 2003 Rooster River Flood Control Study (by Stantec – formerly Vollmer Associates) was to develop flood mitigation alternatives which are cost effective and might be endorsed by the public. The goals of the study were summarized as follows: • Provide new insights to flooding factors through extensive field investigations and computer analysis so that realistic flood water flows and elevations can be determined; and • Evaluate alternative flood mitigation measures that are feasible, economical and likely to gain public support. Since the purpose of the study was to provide new flood mitigation alternatives which have not been previously proposed, any measures which have been found to be infeasible from previous studies were not reexamined. This included the raising of King’s Highway (Route 1) to act as a dike and the construction of a pressure relief conduit in Coolidge Avenue. The basis of the original study was to evaluate the hydraulics and hydrology of the watershed including improvements for the 100-year flood based on to the funding source and direction from DEEP at that time. 2.2 HYDROLOGIC AND HYDRAULIC STUDY OVERVIEW For the original Stantec hydrologic study, drainage boundaries were determined from existing USGS maps, existing Town storm drainage maps, and extensive field investigation. For purposes of evaluation, the drainage basin was divided into four major sub-basins as follows: Sub-basin A: Upper watershed flows which converge at King’s Highway East (Route 1) where the river flow gauge is located. Sub-basin B: Local flows in the area of immediate concern which outlet into the Rooster River between Route 1 and I-95. Sub-basin C: Local flows from Bridgeport which outlet directly upstream of I-95. 2.1 Sub-basin D: Lower watershed flows which outlet south of I-95 at Brewster Street. Hydrographs were developed in HEC-HMS using standard Soil Conservation Service (SCS) methods as outlined in TR-55 for computing run-off and time of concentration. Time of concentration was computed at a flow depth equal to 25% of conduit height. Curve numbers were based on existing development within each drainage sub-area as determined from field investigations, and from soil types as determined from the SCS Soil Survey for Fairfield County. Weighted curve numbers were derived for each subarea and were taken as the average of the Type I and Type II Antecedent Moisture Condition value. Storm events were based on 24-hour rainfall depths as noted in “Rainfall Frequency Atlas of the United States” Technical Paper No. 40 by U.S. Department of Commerce, Weather Bureau. Rainfall distribution was assumed to be a ‘Type III’ hypothetical storm as determined by SCS. A hydraulic study of the Rooster River was performed between Brewster Street and Route 1 to determine water surface profiles for the various storm events with emphasis on the 100-year flood. Using the flow rates determined from the hydrologic study, the hydraulic model takes into account various channel characteristics such as channel size, slope and roughness, overbank flow, bridges and other constrictions to determine depth of flow along the river. Thirty-six (36) cross sections of the Rooster River were surveyed between Brewster Street and Route 1 for use in the hydraulic study. Existing bridge openings were measured, and parapet heights recorded for use in bridge modeling. Channel bank and overbank characteristics were noted for use in selecting channel roughness coefficients. The U.S. Army Corps HEC-RAS program (v. 3.0.1) was used to create a hydraulic model of the river and to determine the resulting water surface profiles. Channel roughness coefficients for in-bank and overbank flow were taken as 0.035 and 0.06 respectively. Where overbank flow occurred in a parking lot or other paved area, a coefficient of 0.015 was used. Ineffective flows areas were assumed at building walls or other obstructions to flow. Bridge railings were assumed to be solid (no openings) as they are likely to clog with debris during high flows. Water surface profiles were first created for the existing conditions and ‘natural’ conditions for the river. The natural conditions model removed any artificial constrictions to river flows such as bridges and channel filling. Subsequent profiles were then created for various proposed changes to the river, such as the removal of bridges, channel improvements and storm water pumping so as to evaluate the impact of each feature separately on channel hydraulics. Appendix C contains the original matrix of the models that were produced along with the reduction in water surface elevations upstream of I-95. 2.3 ORIGINAL STUDY CONCLUSION The recommendations outlined in the preliminary edition of this report (dated October 7, 2002) were presented by CT DEP officials to the public at a public informational meeting held at Sherman Elementary School in Fairfield on October 10, 2002 and were generally well received by the attending public. Subsequent to the public meeting, the Owner of the Feroleto Steel Company expressed concern that removal of the steel bridge in front of his property (a study recommendation) would severely impact his ability to conduct business, and that he is only able to use the adjacent Scofield Avenue bridge during emergency cases. The final recommendation in the addendum for the original report was divided