Mill River _ CT River Levee Systems - Interior Flooding Review Memo_01-13-2021-finalAn Equal Opportunity Employer M/F/V/H
M E M O R A N D U M
To: David Veleta, P.E. (Northampton, DPW)
Kris Baker, P.E. (Northampton, DPW)
From: Thomas E. Jenkins, P.E. (GZA)
Rosalie T. Starvish, P.E. (GZA)
David M. Leone, CFM, P.E. (GZA)
Date: January 13, 2021
File No.: 01.0174343.00
Re: Task 4.4 Interior Flooding Evaluation – Review Existing Information
Mill River Levee System
Connecticut River Levee System
Northampton, MA
In accordance with Task 4.4 of GZA’s proposal to the City dated July 19, 2019, the purpose of this
memorandum is to present GZA’s review of the prior interior runoff evaluation for the Hockanum
Flood Control Pumping Station completed for the City by Tighe & Bond in 2019, and to present
GZA’s evaluation of the suitability and completeness of that evaluation relative to the requirements
of 44 CFR Section 65.10. In addition, this memorandum summarizes GZA’s review of available in-
formation regarding the Mill River Levee System’s West Street Pumping Station and presents GZA’s
recommended scope of work for supplementary hydrologic and hydraulic analyses for both the Mill
River Levee System and the Connecticut River Levee System to evaluate the potential for flooding
interior to the levee systems, relative to requirements of 44 CFR Section 65.10.
The federal regulations pertaining to mapping of areas protected by levee systems indicate that
“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)]
The U.S. Army Corps of Engineers’ Engineer Manual, “Hydrologic Analysis of Interior Areas” (EM
1110-2-1413)1 provides guidance for a “Coincident Frequency Method” of analysis which computes
the percent chance exceedance frequencies of various interior flooding elevations based on the
probabilities of exceeding given exterior river stages during different interior storm events. “Coin-
cidence” is the degree to which the interior and exterior events occur at the same time. The Coin-
1 U.S. Army Corps of Engineers (24 August 2018). “Hydrologic Analysis of Interior Areas”, Engineer Manual
EM 1110-2-1413.
January 13, 2021
File No. 01.0174343.00
Task 4.4 Interior Flooding Evaluation Memorandum
Mill River Levee System
Connecticut River Levee System
Page | 2
cident Frequency analysis provides a method to compute the joint probability of interior and exterior flooding and to
determine the base flood (1% annual chance) elevation for interior areas. For riverine levee systems, the interior
drainage 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 interior pumping stations are in operation.
The Coincident Frequency Method requires the following components:
An evaluation of the frequency distribution of river stage elevations on the exterior of the flood control system
(i.e., river stage – frequency curves for the Connecticut River and the Mill River);
An analysis of the stormwater runoff delivered to each pumping station from the interior drainage area, for
multiple return frequency storm events (e.g., 2-, 5-, 10-, 25-, 50-, 100-, and 500-year storms) based on the
selected hydrologic data and precipitation frequency estimates (e.g., NOAA Atlas 14);
Pump-head discharge curves and understanding of pumping station hydraulics (pipe sizes, elevations, storage
chambers, wet well, gates, etc.); and
An understanding of the storage available at each pumping station (stage-storage relationship).
CONNECTICUT RIVER LEVEE SYSTEM (HOCKANUM FLOOD CONTROL PUMPING STATION)
The “Facility Evaluation Report” for the Hockanum Flood Control Pumping Station, prepared for the City of Northamp-
ton by Tighe & Bond (August 2019) presents a hydrologic and hydraulics (H&H) evaluation which included review of
the Corps of Engineers’ 19402 design assumptions regarding design flows and the contributing watershed and review
of the Corps of Engineers’ 1983 report prepared for the Hockanum Flood Control Pumping Station, Hydrological Review
and Analysis of Interior Drainage Facilities. The Corps of Engineers’ 1940 design assumed a runoff design flow of 400
cfs based on the 10-year frequency rainfall and a drainage area of 770 acres. The pumping station was designed to
discharge a maximum flow of 300 cfs, with excess inflows to be stored in an 18 acre-feet storage pond. The Corps of
Engineers’ 1983 hydrologic review concluded that the 10-year frequency peak runoff rate would not exceed 200 cfs
for a drainage area of 700 acres and recommended a minimum pumping capacity of 250 cfs at the pumping station.
The updated H&H evaluation by Tighe & Bond included development of a HEC-HMS model used to predict the surface
runoff and anticipated flow volume (interior drainage flow rate) to the pumping station for the 10-year return fre-
quency 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 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 WWTP driveway culvert, and (3) upstream of the flood pumping station. 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. The identified pumping station drainage area predicted by the Tighe &
Bond H&H evaluation was approximately 30% greater than values indicated by the two previous Corps of Engineers
reports. The predicted runoff flow rate of 350 cfs fell between the values indicated by the 1940 and 1983 Corps of
Engineers reports (400 cfs and 200 cfs, respectively).
2 U.S. Army Corps of Engineers (1940). “Analysis of Design Northampton Pumping Station Item N.4 – Contract”.
January 13, 2021
File No. 01.0174343.00
Task 4.4 Interior Flooding Evaluation Memorandum
Mill River Levee System
Connecticut River Levee System
Page | 3
Based on GZA’s preliminary review of the H&H evaluation as described in Tighe & Bond’s “Facility Evaluation Report,”
it is GZA’s opinion that the HEC-HMS model developed for that evaluation, with modification, can be used to estimate
interior runoff delivered to the pumping station for return frequency storm events other than the 10-year event. As
needed, the model can be expanded to simulate pumping through the pumping station against various Connecticut
River stage elevations, and the results of those simulations can then be used to perform the Coincident Frequency
analysis in accordance with EM 1110-2-1413.
The “Facility Evaluation Report” also includes a summary of the pumping station components, with a general descrip-
tion of the pumping station operation. The report included a recommendation to replace the existing axial lift pumps
and gasoline (2) and diesel (1) engines in kind, with newer high-efficiency axial lift pumps equipped with newer diesel-
engine technology. The report indicated that the existing pumping equipment has a combined pumping capacity of
approximately 348 cfs and suggested that specific sizing of the replacement pumps would be developed during final
design and final pumping capacities would be based on actual system curve calculations for the replacement equip-
ment. It is GZA’s understanding that the engineering design for replacement of the pumps will occur as Phase 3 of a
three-phase design effort. The Phase 1 engineering design for the pump station is currently underway. Funding for
Phase 3 is not yet determined.
GZA recommends the following scope of work for supplementary hydrologic and hydraulic (H&H) analyses to evaluate
the potential for interior flooding for the Hockanum Flood Control Pumping Station:
1.Review Tighe & Bond’s HEC-HMS model files in further detail to verify input data, calibration, and results.
2.Using USGS or other verified Connecticut River gage data, develop frequency distribution of river stage eleva-
tions for the Connecticut River.
3.Expand the HEC-HMS model to update or refine input data as needed, to estimate interior runoff delivered to
the pumping station for return frequency storm events other than the 10-year event (i.e., 2-, 5-, 25-, 50-, 100-
, and 500-year storms); and to simulate pumping through the pumping station against various Connecticut
River stage elevations.
4.Perform Coincident Frequency analysis to identify the 1% annual chance interior flood elevation based on the
joint probability of interior and exterior flooding.
5.Prepare mapping of the 1% annual chance interior flood extent and identify average flood depth.
The supplementary H&H analyses will require pump-head discharge curves for the pumps in the Hockanum Flood Con-
trol Pumping Station. Replacement pumps for the pumping station will not be selected or installed prior to the com-
pletion of the supplementary H&H analyses. The new pumps will have equal or better performance to the existing
pumps; thus, GZA will use the pump-head discharge curves for the existing pumps, which are available as Plate No. 14
in the Analysis of Design document for the Northampton Pumping Station, Item N.4 – Contract, Connecticut River Flood
Control Project, Northampton, Mass., prepared by U.S. Army Corps of Engineers, January 1940. Typically, GZA would
recommend the field-verification of design pumping capacities, but we do not feel this is advised because the pumps
are going to be replaced.
GZA requests the following information and files from the City to complete the proposed scope of work for the sup-
plementary H&H analyses to evaluate the potential for interior flooding during the base flood for the Hockanum Flood
Control Pumping Station:
January 13, 2021
File No. 01.0174343.00
Task 4.4 Interior Flooding Evaluation Memorandum
Mill River Levee System
Connecticut River Levee System
Page | 4
Tighe & Bond’s HEC-HMS model files (Note: This data need was identified in June 2020 and has subsequently been
provided to GZA by the City).
MILL RIVER LEVEE SYSTEM (WEST STREET PUMPING STATION)
GZA’s Contract No. WF7-19-2019 for Levee Certification Support was recently amended to include further evaluation
of the City’s Flood Control Pumping Stations. Accordingly, GZA has also preliminarily reviewed the existing information
available to us regarding the West Street Pumping Station. We have examined and initially assessed the following
information:
AECOM, City of Northampton, Massachusetts Levee Assessment and Flood Control Alteration Plans for the Con-
necticut River Flood Control System, Mill River Levee, Drawing M-1, 12/15/15.
AECOM, Memorandum to James R. Laurila, P.E., “Engineering Assessment of Connecticut River and Mill River
Flood Control Levees, West Street Pump Station Risk Analysis Technical Memorandum (Task 2)”, January 11,
2016.
AECOM, Mill River Levee and Flood Control System Assessment Summary Report, May 19, 2016.
CDM, Stormwater and Flood Control System Assessment and Utility Plan, Northampton, Massachusetts De-
partment of Public Works, May 2012.
City of Northampton, Massachusetts, Department of Public Works, Mill River Left Bank, Mill River Diversion
Status Report, August 1, 2016.
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.
GZA & Watermark, Pre-Inspection Packet – Mill River Left Bank, Mill River Diversion, Smith College, Northamp-
ton, Massachusetts, December 2010.
New England Corps of Engineers, Northampton Local Protection Project Connecticut and Mill Rivers Northamp-
ton, Massachusetts, Hydrologic Review and Analysis of Interior Drainage Facilities, September 1983. [GZA
notes that this reference included no information for the West Street Pumping Station.]
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, 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. [GZA notes that this reference in-
cluded no information for the West Street Pumping Station.]
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. [GZA notes that this reference
included no information for the West Street Pumping Station.]
January 13, 2021
File No. 01.0174343.00
Task 4.4 Interior Flooding Evaluation Memorandum
Mill River Levee System
Connecticut River Levee System
Page | 5
U.S. Army Corps of Engineers, Appraisal Report, Northampton, Massachusetts, Connecticut River, Local Flood
Protection, October 1985.
Reportedly, the West Street Pumping Station is rarely activated. 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”.
Given that the pumping station is rarely activated, GZA recommends the following simplified approach for hydrologic
and hydraulic (H&H) analyses to evaluate the potential for interior flooding during the base flood for the West Street
Flood Control Pumping Station:
1.Develop HEC-HMS model (SCS UH/TR-55 methodology) to estimate interior runoff delivered to the pumping
station for return frequency storm events including the 2-, 5-, 10-, 25-, 50-, 100-, and 500-year storms.
2.Evaluate the potential head at the manhole upstream of the pumping station (DMH 5214) resulting from the
runoff flows for 2 scenarios: (1)- discharge freely through the gravity outlet and (2)- if the gravity outlet is
submerged by high water in the Mill River.
3.If the results indicate that there is less than a 1 ft average depth of flooding in the interior area, then the
analysis is complete.
4.If the results indicate that there is more than a 1 ft average depth of flooding in the interior area, proceed with
full coincident frequency analysis to identify the water surface elevation of the 1% annual chance interior flood,
per 44 CFR 65.10(b)(6):
a.Using USGS gage data, develop frequency distribution of river stage elevations for the Mill River.
b.Perform Coincident Frequency Analysis to identify the 1% annual chance interior flood elevation based
on the joint probability of interior and exterior flooding.
c.Prepare mapping of the 1% annual chance interior flood limits and identify average flood depth.
Based on our review to date, GZA requests the City’s most recent detailed mapping of the storm drainage system that
discharges to the West Street Pumping Station, preferably in GIS format. This information is required to complete our
interior flooding evaluation (Note: This data need was identified in June 2020 and has subsequently been provided to GZA by the
City).
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.jen-
kins@gza.com.
Attachments:
Appendix A – Limitations
\\GZANOR\Jobs\170,000-179,999\174343\174343-00.CLB\Memos\Interior Flooding Evaluation\Task 4.4-Review Existing Information\Mill River _ CT River Levee Systems - Interior
Flooding Review Memo_01-13-2021.docx
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.