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CT River Levee Systems - Interim Interior Flooding Eval Memo_03-04-2021 (2)An Equal Opportunity Employer M/F/V/H M E M O R A N D U M To: David Veleta (Northampton, DPW) Kris Baker (Northampton, DPW) From: Rosalie T. Starvish, P.E. (GZA) David M. Leone, CFM, P.E. (GZA) Date: March 4, 2021 File No.: 01.0174343.00 Re: Task 11 Interim Interior Flooding Evaluation – Hockanum Street Pumping Station Connecticut River Levee System Northampton, MA In accordance with our agreement (WF7-19-2019) dated July 19, 2019, GZA GeoEnvironmental is pleased to submit this report to the City of Northampton Department of Public Works (DPW), summarizing our interior drainage analysis for the City of Northampton’s Hockanum Street Pumping Station. This report was prepared in accordance with Task 11 of the referenced contract and is subject to the Limitations provided in Appendix A. Unless noted otherwise, elevations reported herein are in reference to the National Geodetic Vertical Datum of 1929 (NGVD29). This memorandum presents the methodology, data, and assumptions used for the interior drainage analysis for the Hockanum Street Pumping Station. The resulting estimated 1% chance interior flood elevation, total area of 1% chance interior flood, and average depth of 1% chance interior flood are presented. INTRODUCTION GZA performed the interior drainage analysis for the City of Northampton’s Hockanum Street Pumping Station in accordance with 44 CFR 65.10(b)(6), as follows: “An analysis [of interior drainage] must be submitted that identifies the source(s) of such flooding, the extent of the flooded area, and, if the average depth is greater than one foot, the water-surface elevation(s) of the base flood. This analysis must be based on the joint probability of interior and exterior flooding and the capacity of facilities (such as drainage lines and pumps) for evacuating interior floodwaters.” [44 CFR 65.10(b)(6)] GZA referred to guidance from the following sources to support the analysis: FEMA’s “Guidance for Flood Risk Analysis and Mapping for Levees” (2019), The United States Army Corps of Engineers (USACE) Engineering Circular on Certification of Levee Systems (EC 1110-2-6067), and The USACE Engineer Manual, “Hydrologic Analysis of Interior Areas” (EM 1110-2-1413). March 4, 2021 File No. 01.0174343.00 Task 11 Interior Flooding Evaluation Memorandum Connecticut River Levee System Page | 2 The following sources were consulted for information supporting the interior drainage analysis: City of Northampton, Massachusetts, Department of Public Works, Operation and Maintenance Manual for Flood Control Protection System Northampton, Massachusetts, revised August 2018. City of Northampton Emergency Management Department, Local Emergency Flood Plan, revised April 17, 2019. Morgenroth and Associates, Inc. Sewage Treatment Plant for Northampton, Massachusetts, July 1957. New England Corps of Engineers, Northampton Local Protection Project Connecticut and Mill Rivers Northampton, Massachusetts, Hydrologic Review and Analysis of Interior Drainage Facilities, September 1983. Northeast Survey Consultants, Survey: Preliminary Existing Conditions Plan of Land in Northampton, MA, Hampshire Registry, prepared for the City of Northampton, Sheets 1 to 9 of 9, 07-09-15. Tighe & Bond, Facility Evaluation Report for the Hockanum Flood Control Pumping Station, August 2019. Tighe & Bond, Massachusetts Levee Toe Drain Repairs and Improvements Updated 90% Design Plans, February 2020. U.S. Army Corps of Engineers, Record Drawings, Connecticut River Flood Control, Northampton Dike, Diversion Canal – Fiscal Year 1939 Unit, Drawings N.3a - 1 to 10 of 10, March 1939. U.S. Army Corps of Engineers, Record Drawings, Connecticut River Flood Control, Northampton Dike, Diversion Canal – Fiscal Year 1939 Unit, Drawings N.3b - 1 to 44 of 44, March 1939. U.S. Army Corps of Engineers. Analysis of Design Northampton Pumping Station Item N.4 – Contract, Connecticut River Flood Control Project, Northampton, Mass., January 1940. Whitman and Howard, Inc. Proposed Wastewater Treatment Facility Plans, Northampton, MA, Sheets 3, 4, 9, and 10., February 1978. All elevations referenced in this report are NGVD29 datum, unless otherwise indicated. SOURCES OF FLOODING The Northampton Flood Control Protection System protects populated areas of the City of Northampton (i.e., the City) from flooding along the Connecticut River and Mill River. NORTHAMPTON FLOOD CONTROL PROTECTION SYSTEM OVERVIEW The Northampton Flood Control Protection System consists of two main parts: (1) an earth dike about one mile long in the eastern part of the City affording protection against high water from the Connecticut River, and (2) an earth dike and concrete floodwall about half a mile long in the western part of the City affording protection against the Mill River and backwater from the Connecticut River Oxbow. The Hockanum Street Pumping Station evacuates interior stormwater runoff from a 940-acre drainage area by discharge to the Connecticut River via gravity flow during low March 4, 2021 File No. 01.0174343.00 Task 11 Interior Flooding Evaluation Memorandum Connecticut River Levee System Page | 3 water on the Connecticut River and by pumping when the river exceeds an elevation of 105.5 ft NGVD29. The Hockanum Street Pumping Station is located adjacent to the dike behind the Northampton Wastewater Treatment Facility off of Hockanum Road. A second pumping station was constructed in 1957 adjacent to the flood wall at West Street, to evacuate interior stormwater runoff from a 15-acre drainage area to the Mill River diversion. This memorandum describes the interior drainage analysis for the Hockanum Street Pumping Station. HOCKANUM STREET PUMPING STATION The Hockanum Street Pumping Station collects drainage from a 940±-acre drainage area. The pumping station houses one electrically-driven 16-inch volute pump and two gasoline and one diesel-driven 48-inch propeller pumps. Each of the 48-inch pumps has a rated capacity of 127.5 cubic feet per second (cfs) at 10 feet of head, and the 16-inch pump has a rated capacity of 15 cfs against a total head of 22 feet. During periods of normal river stage, interior flows are discharged via a 6 feet wide by 8 feet deep gravity flow outlet through the flood wall. When the river stage becomes elevated to necessitate pumping, a sluice gate on the intake to the wet sump is opened to direct interior flows into the 48-inch pump room. Each 48-inch pump then discharges through a 48-inch outlet under pressure flow conditions. INTERIOR HYDROLOGIC ANALYSIS The Facility Evaluation Report for the Hockanum Flood Control Pumping Station, prepared for the City of Northampton by Tighe & Bond (August 2019) presents a hydrologic and hydraulics (H&H) evaluation which included review of the USACE 1940 design assumptions regarding design flows and the contributing watershed and review of the USACE 1983 report prepared for the Hockanum Flood Control Pumping Station, Hydrological Review and Analysis of Interior Drainage Facilities. The updated H&H evaluation by Tighe & Bond included development of a model used to predict the surface runoff and anticipated flow volume (interior drainage flow rate) to the pumping station for the 10-year return frequency storm event (design storm), based on rainfall data from the NOAA Atlas 14 – point precipitation frequency tool and a drainage area of 940 acres. The H&H evaluation was performed using the USACE Hydrologic Engineering Center’s Hydrologic Modeling System (HEC-HMS). The HEC-HMS model included approximately 65 sub- catchments that represent the interior drainage area. The model hydrology was based on the Natural Resources Conservation Service (NRCS) curve number (CN) loss and unit hydrograph models to generate runoff hydrographs from each of the sub-catchments within the interior drainage area. The model used applied precipitation in the form of a hypothetical, NRCC Type C 24-hour synthetic storm distribution, and drainage area characteristics to generate runoff. Times of concentration were determined using National Engineering Handbook Part 630 Figure 15-4 and Table 15-3. Flow paths were estimated in GIS using MassGIS LiDAR data and were split based on changes in land use. The flow path length, slope, and land cover type were used to estimate the times of concentration for each sub-catchment. The model included development of stage-storage curves using MassGIS LiDAR data for three separate storage areas within the Old Mill River, as follows: (1) upstream of the Pleasant Street culvert, (2) upstream of the WWTF driveway culvert, and (3) downstream of the WWTF driveway culvert and upstream of the 8-ft diameter culvert before the pumping station. There is a small storage pond immediately upstream of the entrance conduit to the pumping station which was neglected in the model. The model was calibrated using continuous flow monitoring data. The 10-year frequency, 24-hour peak flow rate to the pumping station predicted by the model was 350 cfs. GZA modified the Tighe & Bond H&H model using HEC-HMS Version 4.6.1 to estimate interior runoff delivered to the pumping station for return frequency storm events other than the 10-year event (i.e., the 1-, 2-, 5-, 25-, 50-, 100-, and 500-year events). GZA expanded the model to simulate pumping through the pumping station against various Connecticut River stage elevations, and the results of those were used to determine if a Coincident Frequency Analysis March 4, 2021 File No. 01.0174343.00 Task 11 Interior Flooding Evaluation Memorandum Connecticut River Levee System Page | 4 would be necessary to estimate the 1% annual chance interior flood elevation. Details about the interior hydrologic analysis are described in the following sections. PRECIPITATION Precipitation was applied to each sub-catchment in the HEC-HMS model as a hypothetical, NRCC Type C 24-hour synthetic storm distribution. The depth in inches applied for each storm event frequency is summarized as follows. The precipitation depths for the 1-, 2-, 5-, 10-, 25-, 50-, 100- and 500-year frequency storm events were obtained from the NOAA Precipitation Frequency Data Server (NOAA Atlas 14, Volume 10, Version 3) for Northampton, Massachusetts. TABLE 1 - PRECIPITATION Storm Event Frequency 24-Hour Precipitation Depth (inches) 1-Year 2.45 2-Year 3.07 5-Year 4.09 10-Year 4.93 25-Year 6.09 50-Year 6.93 100-Year 7.87 500-Year 11.1 OTHER SOURCES OF PUMPING STATION INFLOW The USACE Analysis of Design document for the Northampton (Hockanum Street) pumping station includes an estimated dry weather flow of 11.1 cfs, consisting of sanitary sewage (9.4 cfs), industrial waste (0.5 cfs), and assumed infiltration (1.2 cfs). The report also states, “Dike seepage is expected to be small.” Considering the changes that have occurred since the pumping station was designed, including construction of the WWTF and industrial waste regulations, dry weather flows attributed to sanitary sewage and industrial waste discharges should no longer be present, in GZA’s opinion. The total dry weather flow may include infiltration, but it is assumed that this is negligible in comparison to flood flows. Therefore, this analysis assumed that there are no additional sources of inflow to the pumping station. March 4, 2021 File No. 01.0174343.00 Task 11 Interior Flooding Evaluation Memorandum Connecticut River Levee System Page | 5 INTERIOR HYDRAULIC ANALYSIS HEC-HMS is used to evaluate the hydraulics of discharge from the interior area to the river through the levee. During an interior storm event, interior drainage may discharge to the river via a gravity outlet through the levee, or by being pumped through the pumping station. The method of discharge will depend on the exterior river stage during the interior storm event. At the Hockanum Street Pumping Station, gravity flow ceases and pumping becomes necessary when the Connecticut River exceeds an elevation of 105.5 ft NGVD29. In HEC-HMS, the potential interior flooding area is represented by a reservoir. HEC-HMS has the capability of modeling discharge from a reservoir through gravity outlets and/or by pumping. GZA modified the Tighe & Bond H&H model for the pumping station using HEC-HMS Version 4.6.1 to incorporate both gravity outlets and pumping, as well as the ability to set a tailwater in the reservoir to represent the exterior river stage. Reservoirs are defined in HEC-HMS by a stage-storage curve or stage (elevation)-surface area curve. The Tighe & Bond HEC-HMS model included development of stage-storage curves using MassGIS LiDAR data for three separate storage areas within the Old Mill River, as follows: (1) upstream of the Pleasant Street culvert, (2) upstream of the WWTF driveway culvert, and (3) downstream of the WWTF driveway culvert and upstream of the 8-ft diameter culvert before the pumping station. The model was modified by GZA to include a small storage pond immediately upstream of the entrance conduit to the pumping station which was neglected in the original model. This small pond was defined by a stage-area curve based on the preliminary survey by Northeast Survey Consultants (2015). The stage-area data for the pond upstream of the pumping station is included in Appendix B. The pumps are defined in HEC-HMS by pump-head discharge curves, which are based on the pump capacity information provided in the USACE Analysis of Design document prepared for the pumping station. The discharge varies with the head on the pump which depends on the exterior river stage. The pump-head discharge curves are included in Appendix C. The derivation of the curve for the pumping station and the specific assumptions applied to the hydraulic model for the pumping station are described as follows: HOCKANUM STREET PUMPING STATION The full station capacity consisting of three 48-inch propeller pumps and one 16-inch volute pump was read from the capacity versus river elevation curve included in the USACE Analysis of Design for Northampton Pumping Station (January 1940). Pumping capacities were not verified by field testing. The model assumes full station capacity, which consists of three 48-inch propeller pumps and one 16-inch volute pump. The equipment in the pumping station is tested by the City by running the engines monthly. Elevations of interest for this pumping station are listed below in Table 2: March 4, 2021 File No. 01.0174343.00 Task 11 Interior Flooding Evaluation Memorandum Connecticut River Levee System Page | 6 TABLE 2 - ELEVATIONS OF INTEREST, FEET (NGVD29) Pumping Station Elevation of Gravity Outlet Pump Activation Elevation Approximate Exterior Ground Elevation at Pumping Station Location Elevation of Levee at Pumping Station Location Elevation of Riverine 100-Year Flood at Pumping Station Location (FEMA, 1978) Hockanum Street 98.5 105.5 103.7 131.7 123 ANALYSIS AND RESULTS The USACE Engineering Circular on Certification of Levee Systems (EC 1110-2-6067) states: “The analysis of interior flooding is based on a coincident analysis of exterior and interior stages that includes the capacity of gravity and blocked gravity drainage features. Coincident analysis for interior areas is explained in Chapter 4 of EM 1110-2-1413, Hydrologic Analysis of Interior Areas. For riverine levee systems, the interior analysis considers interior rainfall events during both low river stages (gravity conditions) and high river stages when the gravity outlets are closed (blocked conditions) and the performance of pumping stations as might exist.” FEMA’s “Guidance for Flood Risk Analysis and Mapping for Levees” (2019) states, “In lieu of a joint probability analysis, the engineer may evaluate several scenarios rather than identify a probability-based flood event, the most conservative combination should be used for final analysis”. Therefore, GZA performed the analysis using the following steps: 1.Establish the interior flood elevation for a range of storm event return periods. 2.Evaluate the sensitivity of the peak stage associated with each storm event return period by varying the exterior flood stage in the Connecticut River. 3.Determine if joint probability analyses of interior and exterior flooding is likely to influence the estimated maximum interior stage. GZA used the HEC-HMS model to predict the resulting interior flood elevation from various combinations of interior storm events (i.e., the 1-, 2-, 5-, 10-, 25-, 50-, 100-, and 500-year events) and exterior river stages. Ten (10) exterior river stages were selected to represent the range of potential stages on the Connecticut River at Northampton based on data from the Northeast River Forecast Center gauge for Connecticut River at Northampton, including historic crest data. Connecticut River stages at the gauge regularly vary between approximately 101 FT and 115 FT NGVD29. Peak crests include 116.9 FT (Tropical Storm Irene on 8/30/2011), 120.8 FT (5/31/1984), and the maximum recorded peak of 129.4 FT (3/19/1936). The resulting interior stage for each combination of interior storm and exterior river stage are presented below in Table 3: March 4, 2021 File No. 01.0174343.00 Task 11 Interior Flooding Evaluation Memorandum Connecticut River Levee System Page | 7 TABLE 3 – PEAK INTERIOR STAGE, FEET (NGVD29) Note: Pump activates at exterior Connecticut River stage 105.5 FT NGVD29. The results indicate that, as expected, interior stage increases with interior storm total rainfall depth. The interior stage varies depending on whether or not pumping is activated. However, the results suggest that for each interior storm event, the resulting peak interior stage is not sensitive to the exterior stage on the Connecticut River (i.e., tailwater). The peak interior stage for a given storm event is the same across the range of exterior stages evaluated in the “pump on” scenario, from 111.0 feet through 127.8 feet. CONCLUSION In GZA’s opinion, based on the results of the analysis presented above, the 1% interior stage may be estimated without performing a full coincident frequency (i.e., joint probability) analysis because the interior stage is insensitive to the exterior stage. The coincident frequency analysis would likely predict a 1% interior stage that falls between 110.9 and 112.1 FT NGVD29; thus, it may conservatively be assumed to be 112.1 FT. This elevation is presented as the 1% chance interior flood elevation, as summarized in Table 4 below. The total area and average depth of interior flooding, estimated based on elevation data from the 2015 Massachusetts LiDAR Project, is also shown in Table 4. Figure 1 presents the interior flood area and inundation depth. TABLE 4 - 1% CHANCE INTERIOR FLOOD RESULTS Pumping Station 1% Chance Interior Flood Elevation (ft, NGVD29) Total Area of 1% Chance Interior Flood (acres) Average Depth of 1% Chance Interior Flood (ft) Hockanum Street 112.1 10.65 3.9 ft (overall) 1.5 ft (upland) (PUMP OFF) (PUMP OFF) (PUMP ON) (PUMP ON) (PUMP ON) (PUMP ON) (PUMP ON) (PUMP ON) (PUMP ON) (PUMP ON) Exterior Stage (NGVD29):102.6 105.4 108.2 111 113.8 116.6 119.4 122.2 125 127.8 Interior Storm Return Period (YR) Interior Storm Annual Probability 1 1 104.7 104.7 108.1 108.1 108.1 108.1 108.1 108.1 108.1 108.1 2 0.5 105.5 105.5 108.2 108.2 108.2 108.2 108.2 108.2 108.2 108.2 5 0.2 106.9 106.9 108.2 108.2 108.2 108.2 108.2 108.2 108.2 108.2 10 0.1 107.8 107.8 108.3 108.3 108.3 108.3 108.3 108.3 108.3 108.3 25 0.04 109.0 109.0 108.2 108.2 108.2 108.2 108.2 108.2 108.2 108.2 50 0.02 110.0 110.0 111.3 111.3 111.3 111.3 111.3 111.3 111.3 111.3 100 0.01 110.9 110.9 112.1 112.1 112.1 112.1 112.1 112.1 112.1 112.1 500 0.002 112.8 112.8 114.3 114.3 114.3 115.1 115.1 115.1 115.1 115.1 Interior Stage (NGVD29): March 4, 2021 File No. 01.0174343.00 Task 11 Interior Flooding Evaluation Memorandum Connecticut River Levee System Page | 8 CLOSING We look forward to completing our interior flooding analyses for the Mill River and Connecticut River Flood Control Systems in Northampton. If you have any questions or comments concerning the contents of this letter, please contact Rosalie Starvish at 860-550-2777 or rosalie.starvish@gza.com, or Tom Jenkins at 413-563-7986 or thomas.jenkins@gza.com. Attachments: Figure 1 – Inundation Depth Appendix A – Limitations Appendix B – Stage-Area Data Appendix C – Pump Head Discharge Curves \\GZANOR\Jobs\170,000-179,999\174343\174343-00.CLB\Memos\Interior Flooding Evaluation\Task 11\CT River Levee Systems - Interim Interior Flooding Eval Memo_03-04-2021.docx Source: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CN ES/AirbusDS, U SDA, USGS, AeroGRID, IGN, and the GIS User Community JOB NO. SOURCE : THIS MAP CONTAINS THE ESRI ARCGIS ONLINE USATOPOGRAPHIC MAP SERVICE, PUBLISHED DECEMBER 12, 2009BY ESRI ARCIMS SERVICES AND UPDATED AS NEEDED. THISSERVICE USES UNIFORM NATIONALLY RECOGNIZED DATUMAND CARTOGRAPHY STANDARDS AND A VARIETY OFAVAILABLE SOURCES FROM SEVERAL DATA PROVIDERS. Data Supplied by : 4010020030040050 SCALE IN FEETUSGS QUADRANGLE LOCATION © 2021 - GZA GeoEnvironmental, Inc., J:\170,000-179,999\174343\174343-00.CLB\Figures\GIS\MXD\HockanumFloodingDepth-Formatted.mxd, 1/14/2021, 3:47:28 PM, daniel.mcgrawINTERIOR FLOODING ANALYSISHOCKANUM ROAD PUM PING STATIONCITY OF NORTHAMPTON, MA FIGURE NO.1 INUNDATION DEPTHINTERIOR FLOOD AREA 01.0174343.00PROJ. MGR.: CLBDESIGNED BY: DEMREVIEWED BY: RSOPERATOR: D EMDATE: 1/14/2021 Depth (ft) 0-1 1-2 2-3 3-4 4-5 5-6 6-7 7-8 8-9 9-10 10-11 11-12 12-13 13-14 1% CHANCE INTERIOR FLOOD EXTENTArea = 10.65-acresElevation = 112.1-FT (NGVD29); 111.4-FT (NAVD88)Average Depth Overall = 3.89-FTAverage Depth Upland = 1.46-FT(outside of old Mill River bed) Proactive by Design APPENDIX A LIMITATIONS LIMITATIONS 01.0174343.00 Interior Flooding Evaluations Page | 1 December 2020 Proactive by Design USE OF REPORT 1.GeoEnvironmental, Inc. (GZA) prepared this Report on behalf of, and for the exclusive use of the Client for the stated purpose(s) and location(s) identified in the Report. Use of this Report, in whole or in part, at other locations, or for other purposes, may lead to inappropriate conclusions and we do not accept any responsibility for the consequences of such use(s). Further, reliance by any party not identified in the agreement, for any use, without our prior written permission, shall be at that party’s sole risk, and without any liability to GZA. STANDARD OF CARE 2.Our findings and conclusions are based on the work conducted as part of the Scope of Services set forth in the Report and/or proposal, and reflect our professional judgment. These findings and conclusions must be considered not as scientific or engineering certainties, but rather as our professional opinions concerning the limited data gathered and reviewed during the course of our work. Conditions other than described in this Report may be found at the subject location(s). 3.The interpretations and conclusions presented in the Report were based solely upon the services described therein, and not on scientific tasks or procedures beyond the scope of the described services. The work described in this Report was carried out in accordance with the agreed upon Terms and Conditions of Engagement. 4.GZA's evaluation was performed in accordance with generally accepted practices of qualified professionals performing the same type of services at the same time, under similar conditions, at the same or a similar property. No warranty, expressed or implied, is made. The findings are dependent on numerous assumptions and uncertainties inherent in the review process. The findings are not an absolute characterization of operations and maintenance preparedness, but rather serve to evaluate minimum standards of performance provided by the documentation reviewed. RELIANCE ON INFORMATION FROM OTHERS 5.In conducting our work, GZA has relied upon certain information made available by public agencies, Client, and/or others. GZA did not attempt to independently verify the accuracy or completeness of that information. Any inconsistencies in this information which we have noted are discussed in the Report. COMPLIANCE WITH CODES AND REGULATIONS 6.We used reasonable care in identifying and interpreting applicable codes and regulations necessary to execute our scope of work. These codes and regulations are subject to various, and possibly contradictory, interpretations. Interpretations with codes and regulations by other parties are beyond our control. ADDITIONAL INFORMATION 7.In the event that the Client or others authorized to use this Report obtain information on conditions at the site(s) not contained in this Report, such information shall be brought to GZA's attention forthwith. GZA will evaluate such information and, on the basis of this evaluation, may modify the opinions stated in this Report. ADDITIONAL SERVICES 8.GZA recommends that we be retained to provide services during any future investigations, design, implementation activities, construction, and/or property development/ redevelopment at the Site(s). This will allow us the opportunity to: i) observe conditions and compliance with our design concepts and opinions; ii) allow for changes in the event that conditions are other than anticipated; iii) provide modifications to our design; and iv) assess the consequences of changes in technologies and/or regulations. Proactive by Design APPENDIX B STAGE – STORAGE DATA Proactive by Design APPENDIX C PUMP HEAD DISCHARGE CURVES River elevation at station outlet (NGVD29) River elevation at station outlet (NAVD88)Pump Head (ft) Discharge (cfs) - 4 PUMPS TOTAL Discharge (cfs) - 16" pump Discharge (cfs) - 3- 48" pumps 106 105.33 7.5 440 29 411 108 107.33 9.5 439 27 412 112 111.33 13.5 430 26 404 116 115.33 17.5 418 25 393 120 119.33 21.5 395 20 375 124 123.33 25.5 370 19 351 127 126.33 28.5 345 10 335