07/07/98 Hansz/Southfield Park SubdivisionJuly 7, 1998
Mr. Thor Jackola
Jackola Engineering
317 First Avenue East
Kalispell, MT
59903
Reference: Southfield Park subdivision
Dear Mr. Jackola:
I want to follow up on our discussions of June 26' regarding the storm drainage design you have
proposed for Southfield Park. As Dick Amerman and I mentioned, we are very concerned that
the design as submitted will not function as intended because of the very high ground water
conditions of the area. In general, drain trenches are a frequent source of disappointment because
their proper operation depends on so many variables; soil percolation rates, ambient ground
water changes, overall size, rainfall intensities and changes in the originally assumed
development plan must all be carefully managed to ensure functionality of the system. To date
we have not received any further information from you regarding the items we discussed. So
there is no misunderstanding let me review our concerns one more time.
With respect to Southfield Park, the information you have provided clearly indicates that the
present ground water elevation is well above the pipe and as a result the pipe will act as a
collector of ground water rather than as a conduit for water to drain away. The system as
designed will, in the opinion of this Department, be immediately non-functional. In order to
work, a drain trench needs to have the drain pipe above the water table most of the time in order
to provide the hydraulic gradient necessary to permit water flow into the surrounding permeable
soil strata. As presently designed, we believe the system will flood above the surface throughout
the developed portions of the project until sufficient head is achieved to permit proper flow into
the water table. This assumes the capacity of the trench and pipe work are correctly sized for the
anticipated storm intensity, which we are unable to determine from the data you have submitted.
In addition, the submitted plan does not provide any elevation data to show the relationships of
surrounding features to the proposed improvements. The relief pipe connecting to the City
system is assumed to be properly located and sized but this is not verifiable. Further, the City's
Extension of Services Plan stipulates that storm discharge from a development is limited in rate
and quantity to the amount resulting from pre -development conditions. Excess amounts are to be
dealt with onsite by the developer. I have enclosed a copy of this requirement for your
Post Office Box 1997 • Kalispell, Montana 59903-1997
Telephone (406) 758-7700 • FAX (406) 758-7758
information. This design requirement is frequently difficult to accommodate hence our concern
that the proposed solution for Southfield Park be carefully designed and constructed so as to
ensure proper operation for the owners, and to avoid situations where the City may be called
upon in the future to assist in correcting a problem. The intense thunderstorm'of this past
weekend and the many local flooding problems it generated should caution all of us to be
extremely prudent in each aspect of storm drainage design.
Your reference to the possibility of a future connection to a new storm drain, part of the US 93
improvements, may be feasible provided the improvements are installed as anticipated. But, you
should be aware that the design of these facilities is based on the existing runoff conditions. The
requirement for onsite retention of flows in excess of predevelopment quantities will remain. In
any event, these improvements would not be available for use for at least two years or more.
Other items of this development, the access points, roadway improvements, water and sewer
extensions appear to be satisfactory and are as we previously discussed.
From our discussions we understood that you would revise the storm drainage design for
Southfield Park to provide a higher margin of operating safety, and to relocate the drain pipe
above the water table to give it the best chance of working without problems. To date we have
received no new information that reflects the changes we discussed. We are ready to work with
you to resolve these concerns and look forward to receiving your latest plans.
James C. Hansz, P.E.
Director of Public W rks / Jty Engineer
cc: City Attorney / Interim City Manager