Levee Certification Task 4.4 Letter Report-6.30.20June 30, 2020
File No. 01.0174343.00
Mr. David Veleta, P.E. – City Engineer
City of Northampton D.P.W.
125 Locus Street
Northampton, MA 01060
Re: Task 4.4 Interior Flooding Evaluation – Review Existing Information
Mill River and Connecticut River Systems
Northampton, Massachusetts
Dear David:
In accordance with Task 4.4 of GZA’s proposal to the City dated July 19, 2019, the
purpose of this letter report 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 requirements of 44 CFR Section 65.10. In
addition, this letter report presents GZA’s recommended scope of work for
supplementary hydrologic and hydraulic analyses to evaluate the potential for interior
flooding during the base flood.
The federal regulations pertaining to mapping of areas protected by levee systems
indicate that “An analysis 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. Coincidence is the degree to which the
interior and exterior events occur at the same time. The Coincident Frequency
analysis provides a method to compute the joint probability of interior and exterior
flooding and to determine the base flood (1%) elevation for 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.
1 U.S. Army Corps of Engineers (24 August 2018). “Hydrologic Analysis of Interior Areas”,
Engineer Manual EM 1110-2-1413. DRAFT
June 30, 2020
File No. 01.0174343.00
Interior Flooding Evaluation – Review Existing Information
Page | 2
Proactive by Design
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 and Mill Rivers);
An analysis of the 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).
The “Facility Evaluation Report” for the Hockanum Flood Control Pumping Station, prepared for the City
of Northampton by Tighe & Bond (August 2019) presents a hydrology 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 H&H
evaluation 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 frequency storm
event, based on rainfall data from the NOAA Atlas 14 – point precipitation frequency tool. 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 predicted
by the model was 350 cfs. Both the identified pumping station drainage area and design flow predicted
by this H&H evaluation were greater than values indicated by the two previous Corps of Engineers reports.
The H&H evaluation as described in the Facility Evaluation Report prepared by Tighe & Bond does not
satisfy the requirements of 44 CFR Section 65.10, because it does not include a full Coincident Frequency
analysis that considers the exterior flooding of the Connecticut River against the capacity of the pumping
station. However, based on GZA’s review of the H&H evaluation, it is GZA’s opinion that the model
developed by Tighe & Bond, 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. The model can be
expanded to simulate pumping through the pumping station against various Connecticut River stage
elevations. The results of these 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 components of the pumping station, with a
general description of the pumping station operation. The report included a recommendation to replace
2 U.S. Army Corps of Engineers (1940). “Analysis of Design Northampton Pumping Station Item N.4 – Contract”. DRAFT
June 30, 2020
File No. 01.0174343.00
Interior Flooding Evaluation – Review Existing Information
Page | 3
Proactive by Design
the existing axial lift pumps and diesel engines in kind, with newer high-efficiency axial lift pumps
equipped with newer diesel-engine technology. The report indicated that the specific sizing of the
replacement pumps would need to be developed during final design and based on actual system curve
calculations. It is GZA’s understanding that the engineering design to implement the recommendations
for pumping station improvements is currently underway.
GZA recommends the following scope of work for supplementary hydrologic and hydraulic (H&H) analyses
to evaluate the potential for interior flooding during the base flood for the Hockanum Flood Control
Pumping Station:
1.Review Tighe & Bond’s HEC-HMS model files to verify input data, calibration, and results.
2.Using USGS or other verified Connecticut River gage data, develop frequency distribution of river
stage elevations 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 limits and identify average flood depth.
The supplementary H&H analyses will require pump-head discharge curves for the pumps in the
Hockanum Flood Control Pumping Station. Given that the City is underway with the design of replacement
pumps for the pumping station, GZA recommends that the supplementary H&H analyses incorporate the
pump-head discharge curves for the new pumps, provided that they are available at the time of analysis.
The alternative is to 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, likely before
System certification is necessary.
GZA requests the following information and files from the City to complete the proposed scope of work
for the supplementary H&H analyses to evaluate the potential for interior flooding during the base flood
for the Hockanum Flood Control Pumping Station:
Tighe & Bond’s HEC-HMS model files.
Pump-head discharge curves for the proposed replacement pumps under design for the
Hockanum Flood Control Pumping Station.
GZA notes that if the pump-head discharge curves for the proposed replacement pumps under design are
used, our moving forward now with the Coincident Frequency Analysis using those assumed values would DRAFT
June 30, 2020
File No. 01.0174343.00
Interior Flooding Evaluation – Review Existing Information
Page | 4
Proactive by Design
be an excellent verification that the intended pumps—once installed--will be sufficient in capacity to
satisfy the requirements of 44 CFR Section 65.10.
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 Mill River Pumping System. We have examined and
initially assessed the following information:
AECOM, City of Northampton, Massachusetts Levee Assessment and Flood Control Alteration
Plans for the Connecticut 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 Department 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 Emergency Management Department, Local Emergency Flood Plan, March
10, 2006.
GZA & Watermark, Pre-Inspection Packet – Mill River Left Bank, Mill River Diversion, Smith College,
Northampton, Massachusetts, December 2010.
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, 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, Appraisal Report, Northampton, Massachusetts, Connecticut River,
Local Flood Protection, October 1985.
Draft Mill River Flood Control System West Street Pump Station O&M Procedures manual
Based on our review to date, we have identified the following data gaps / data needs:
The City’s most recent detailed mapping of the storm drainage system that discharges to the West
Street Pumping Station, preferably in GIS format. DRAFT
June 30, 2020
File No. 01.0174343.00
Interior Flooding Evaluation – Review Existing Information
Page | 5
Proactive by Design
Elevational information for the West Street Pumping Station (pump intake and discharge line
elevations).
The completion of the following recommendations would provide the necessary data and information to
enable a full Coincident Frequency analysis of the Mill River (West Street) Pumping Station:
Field-verification of pumping capacities (pump tests), if feasible.
Survey elevations for pumping station components (pump intake and discharge line elevations).
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.
Sincerely,
GZA GEOENVIRONMENTAL, INC.
Rosalie T. Starvish, P.E. Thomas E. Jenkins, P.E.
Project Engineer Associate Principal/Senior Engineer DRAFT