VHH cistern response 6.12.07 Memorandum
To: Wayne Feiden, Northampton Office of Planning and Development
From: Mark B. Darnold, P.E.
Copy: Robert Kaye, Mass Development
Elizabeth Murphy, Hospital Hill Development LLC
Tom Kegelman, The Community Builders
Date: June 12, 2007
Subject: Village Hill Definitive Subdivision Community Garden Cistern & Stormwater outfall
Wayne:
Thank you for expressing your concerns regarding the outfall for the Village Hill stormwater outfall that the OPD staff would like to have resolved prior to Thursday’s Definitive Subdivision
hearing.
Following are responses, (in RED), to the e-mail which you sent on Thursday:
From: Wayne Feiden [mailto:wfeiden@northamptonma.gov] Sent: Friday, June 08, 2007 4:23 PM Subject: Community Gardens Cistern
There have been lots of independent conversations going on about the cistern at the Community Gardens, so I want to make sure that all the players are in the conversation. I understand
the issues as follows:
1. MassDevelopment is showing drainage from the north campus ending at the boundary between the land of MassDevelopment and the City owned community gardens. The slope is so steep
on the city property that even if drain water leaving MassDevelopment property has absolutely no pressure, the sheer volume of water going down such a steep slope without any mitigation
will cause unacceptable erosion on the City property.
The outfall from the project has been revised. The current proposal is to pipe the discharge from the detention basin to the” bottom” of the hill; therefore there is no longer any proposal
to discharge on the steep slope on City property. Once the water is conveyed to the toe of slope, it will be directed to an energy dissipater and a level lip spreader which will allow
the stormwater to decrease energy/velocity and then disperse the flow over a large area at a low velocity and rate.
2. The Conservation Commission and DPW Stormwater solution to the problem is for the drainage to enter the existing abandoned cistern on city property. This would provide great underground
detention and slow the velocity.
If the cistern is used as a discharge location, an outlet must be constructed on/in the cistern, which could likely affect the structural integrity of the cistern. If the outlet is
too small, the cistern will overflow and could actually concentrate the flows at a higher velocity. In order to construct an outlet in the cistern, a portion of the side of the cistern
must be removed/altered. Removing any portion of the side of the circular cistern would likely have significant impact on the integrity of the remaining portion of the structure.
The detention basin located on the Village Hill property is designed to provide for detention of stormwater and then release it at a controlled rate. The cistern would not provide “underground
detention”. An outlet must be constructed at or above the stream elevation. The volume below the outlet will provide no storage because it will never drain. The volume contained by
the cistern above the outlet is not stable; especially when/if a portion of the wall is removed to accommodate an outlet. (See the attached sketch)
3. The cistern is currently very unsafe and potentially unstable. To be reused, MassDevelopment would probably need to pull off the top and fill with stone, so that there is still
lots of water storage in the voids but less risk of collapse or of someone falling in. In the absence of MassDevelopment reuse, the city must somehow fill the cistern to avoid collapse
or serious injury or death.
The physical condition of the cistern is unknown. Whether or not the cistern is filled with stone, there will be no storage below the outlet level. There is no “cover” on top of the
cistern. If the top portion of the cistern is removed, then there will be no “storage” above the outlet elevation. Although there may be some desire to utilize the cistern as a discharge
location, the required function of the outlet will be to dissipate energy and also disperse the flow. The cistern location could be used to accomplish the energy dissipation; however
it is not conducive to dispersing the flow.
The cistern is currently not proposed to be altered by the Village Hill project
4. MassDevelopment may or may not already have the right to run drainage on this property (Beth or Alan-- can you check the terms of your deed and let us know).
The deed for the property specifically grants drainage easements across the City’s property. (Please see the attached excerpt from a portion of page 3 of the deed).
5. If MassDevelopment does NOT already have this right, the city can allow drainage onto the property with a license, although we couldn't grant an easement.
MassDevelopment apparently does have the right.
6. Either Mass Development's drainage in this area must be resolved by next Thursday, or the subdivision permit will need to be extended. The design as it is would create significant
environmental damage.
It would be very useful if anyone with relevant information or questions could weigh in on this to allow MassDevelopment information to figure out their options. Carolyn, Bruce and
I are happy to help in anyway or attend whatever meetings or calls are necessary to allow this to be resolved by Thursday. We cannot support, however, the outfall as shown on the plans
without some mitigation to prevent erosion.
In summary, the outfall design as presently proposed, (with piping to the bottom of slope, energy dissipater and level lip spreader), will provide proper attenuation of storm flows,
and minimize velocity and erosion potentials, and is in conformance with standard engineering practice.
Memorandum Page 3 of 3 June 12, 2007
Page 1 of 3
Outlet elevation
Water level
Ground
Cistern
Streamflow
SCHEMATIC CROSS SECTION OF CISTERN
PAGE 3 OF DEED