Geotechnical Environmental Water Resources Ecological Woodcliff Lake Dam Study Hillsdale, New Jersey Submitted to: Borough of Hillsdale 380 Hillsdale Avenue Hillsdale, NJ 07642-2794 Submitted by: GEI Consultants, Inc. 1 Greenwood Avenue, Suite 210 Montclair, NJ 07042 May 2013 Project 132029-0 ___________________________ Sean T. DiBartolo, P.E., LEED AP Project Manager WOODCLIFF LAKE DAM S TUDY REPORT BOROUGH OF HILLSDALE M A Y 2 0 1 3 Table of Contents Executive Summary iv ES.1 Overall Summary of Findings iv ES.2 Summary Response to Requested Scope Items from the Borough’s Request for Qualifications v 1. Introduction 1 1.1 Scope of Work 1 1.2 Authorization 1 1.3 Project Personnel 1 1.4 Limitations 2 2. Background 3 2.1 General 3 2.2 Elevation Datum 3 3. Hydraulics and Hydrology Reports and Models 4 3.1 Existing Documentation 4 3.1.1 Review of “Woodcliff Lake Dam PMF Determination” (GFI, 2006) 4 3.1.2 Review of “HEC-HMS, HEC-RAS Hydrology and Hydraulic Analysis” (BS&J, 2010) 5 3.2 Conclusions Related to Hydraulics and Hydrology 7 3.2.1 Conclusions and Recommendations for the Inflow Hydrology and Spillway Sizing Calculations 7 3.2.2 Conclusions and Recommendations for Stream Routing 8 4. New Jersey Rules and Regulations for Dam Safety 9 4.1 General 9 4.1.1 Safe Dam Act 9 4.1.2 Dam Safety Standards 9 4.2 Operation Practices 10 4.3 New Jersey Department of Environmental Protection Dam Application Permit 11 4.4 Conclusions Related to NJDEP Rules, Regulations and Permitting 12 4.4.1 Conclusions and Recommendations Related to Rules, Regulations, and Operational Procedures 12 4.4.2 Conclusions and Recommendations Related to the Construction Permit 12 i WOODCLIFF LAKE DAM S TUDY REPORT BOROU GH OF HILLSDALE M A Y 2 0 1 3 5. Flooding Exposure during Construction 13 5.1 General 13 5.1.1 Background on Water Control during Construction 13 5.2 Auxiliary Spillway Construction 13 5.3 Rehabilitation of the Low-Level Outlet 14 5.3.1 Flood Exposure during Drawdown and Cofferdam Construction 14 5.3.2 Embankment Excavation/Backfill and Pipe Repair 15 5.3.3 Reservoir Refilling 15 5.4 Raising the Low-Lying Embankment Areas 15 5.5 Conclusions Related to Flood Exposure during Construction 16 6. Flood Protection Alternatives 17 6.1 General 17 6.2 U.S. Army Corps of Engineers Flood Protection Feasibility Study 17 6.3 Off-site Flood Storage Alternative 18 6.4 Floodwater Conveyance Alternative 18 6.5 Conclusions Related to Flood Protection Alternatives 19 6.5.1 Conclusions and Recommendations Related to Downstream Flood Protection Alternatives 19 6.5.2 Conclusions and Recommendations Related to Rehabilitation Alternatives Impact on Flood Protection 19 7. Recommendations for Future Actions to Evaluate Pascack Brook Flood Mitigation 21 7.1 Request a Review of the Inflow Hydrology and Spillway Design 21 7.2 Proceed with Dam Rehabilitation Efforts 21 7.3 Develop a Water Resource System Model 22 References 23 ii WOODCLIFF LAKE DAM S TUDY REPORT BOROU GH OF HILLSDALE M A Y 2 0 1 3 Table of Contents (cont.) Tables 1 USACE Flood Protection Feasibility Study Results 2 Historic Event Precipitation and Peak Inflow (BS&J, 2010) Figures 1 Cross Section 1.11 (from BS&J HEC-RAS Model) 2 Cross Section 1.11 with Obstructions and Revised Manning’s “n” 3 Significant Structures along Pascack Brook 4 Flood Control via Water Transfers Alternatives 5 Profile from Woodcliff Lake to Wood Dale County Park 6 Profile from Woodcliff Lake to Musquapsink Brook Appendices A Construction Activities and Temporary Water Control B Specifications for Cofferdam and Reservoir Lowering, Fish Salvage, and Relocation \\GTB1V-FS01\Data\WPROC\Project\Borough of Hillsdale\Woodcliff Lake Dam Study Report1320290\Woodcliff Lake Dam Study Report 2013-05-08.doc iii WOODCLIFF LAKE DAM S TUDY REPORT BOROU GH OF HILLSDALE M A Y 2 0 1 3 Executive Summary ES.1 Overall Summary of Findings This report presents an independent evaluation performed by GEI Consultants, Inc. (GEI) for the Borough of Hillsdale of the improvements to the Woodcliff Lake Dam located in Hillsdale, New Jersey, proposed by United Water New Jersey, Inc. as designed and submitted by Buck, Seifert & Joist, Inc. (BS&J). Based on our review of the information provided, we have developed the following overall opinions concerning the design of the safety improvements proposed for the dam and their potential impacts on downstream flooding conditions. As it is currently constructed, the Woodcliff Lake Dam and spillway cannot safely pass the current Spillway Design Flood (SDF) without overtopping of the dam. The NJDEP has issued a compliance order to United Water mandating the dam undergo rehabilitation to safely pass the SDF. The rehabilitated spillways should be large enough to pass the SDF and prevent the dam crest from overtopping. If the spillways are not capable of passing the SDF, the dam is at risk of failure and while the risk of failure is low, it is important to correctly rehabilitate the dam in a timely manner. Based on our review of the inflow hydrology and spillway sizing calculations, we think the design flood may be larger than is currently estimated and the spillway capacity to convey floodwater may be less than currently estimated. This implies the dam may be undersized to pass the SDF without overtopping the dam. We recommend the magnitude of the SDF and capacity of the spillways be verified by United Water and NJDEP to ensure the current proposed design provides adequate protection against failure. It is our opinion that there will not be a significant change in flood scenarios once the dam is reconstructed and the areas downstream will still have flooding conditions during significant storm events. The flooding of Pascack Brook under smaller, more frequent events was qualitatively analyzed to the extent possible. We have reviewed the HEC-RAS (Army Corps Hydrologic Engineering Center - River Analysis System) model prepared by BS&J and found that it is unsuitable for evaluation of flooding of Pascack Brook downstream of the dam because it lacks sufficient site-specific detail to reliably predict flood impacts. In its present configuration, the model cannot be used effectively to evaluate downstream flooding. Significant modifications to the HEC-RAS model would be required in order to quantitatively analyze various flooding scenarios. However, we have reviewed the available data and it is GEI’s general opinion that the primary cause of the downstream flooding is iv WOODCLIFF LAKE DAM S TUDY REPORT BOROU GH OF HILLSDALE M A Y 2 0 1 3 related to the limited channel capacity of Pascack Brook, constrictions along its length, and encroachment into the floodplain. Without significant modifications of these elements, Pascack Brook will continue to flood regardless of the dam configuration. ES.2 Summary Response to Requested Scope Items from the Borough’s Request for Qualifications GEI was contracted by the Borough of Hillsdale to address nine specific tasks for our evaluation. Our summary responses for these tasks are as follows. More detail discussion is presented in the body of this report. 1. Review NJDEP Dam Permit issued by NJDEP for improvements plan. We reviewed the dam permit issued by NJDEP and recommend that Hillsdale request the NJDEP Bureau of Dam Safety review the final construction water control plan and inspect water control facilities upon construction. 2. Review the hydraulic model used for maximum precipitation/flood determination. Determine if the project will increase downstream flooding of the Pascack Brook. We have reviewed the HEC-RAS (Army Corps Hydrologic Engineering Center - River Analysis System) model prepared by BS&J and found that is unsuitable for evaluation of flooding of Pascack Brook downstream of the dam because it lacks sufficient site-specific detail to reliably predict flood impacts. In its present configuration, the model should not be used to evaluate downstream flooding and significant modifications to the HEC-RAS model would be required in order to complete this task. Note that it was not a part of our scope to develop a suitable HEC-RAS model to evaluated downstream flooding. 3. Review dam improvements plan and consider design alternates to affect similar, but better possible outcome. As stated above, it is GEI’s opinion that the primary cause of the downstream flooding is related to the limited channel capacity of Pascack Brook, constrictions along its length, and encroachment into the floodplain. Without significant modifications of these elements, Pascack Brook will continue to flood regardless of the dam configuration. Also, development of a more comprehensive HEC-RAS model would be required to evaluate possible dam operation scenarios that may help to mitigate some downstream flooding issues for some of the more minor flooding events. 4. Identify adverse effects of reconstruction, if any, during typical flood scenarios (i.e. 5-, 25-, 50-, 100-year type storm frequencies) to the Borough both at dam and downstream in Hillsdale. v WOODCLIFF LAKE DAM S TUDY REPORT BOROU GH OF HILLSDALE M A Y 2 0 1 3 As described in Item 2, the existing HEC-RAS model is not sufficiently developed for flood routing analyses purposes and we were not able to address this item. 5. Identify adverse effects to stream corridor performance, if any, during 2-year anticipated reconstruction period i.e. reservoir surface lowering and fixation of bascule gates. Under all likely scenarios, it is our opinion that the proposed construction would not increase flood exposure in the downstream reach of Pascack Brook above what would be observed under current conditions or after construction is completed. 6.