Final Report - Flathead County, Montana

Average daily sewage flow for the 16-year period from 1996 to 2011 has been
446,400 gallons per day. Peak flows are influenced significantly by inflow and
infiltration during times of wet weather and high groundwater in the shallow Evergreen
water table aquifer. During years with considerable runoff, flows increase during spring
runoff. The three highest daily flows of 1,195,400 gpd; 999,900 gpd and 730,932 gpd
were observed during 2011; 1997 and 2006, respectively. There was some limited
flooding from the Whitefish and Stillwater Rivers in Evergreen during these years and
the flow in the Flathead River remained high for an extended period of time.
Some additional future growth in Evergreen is possible on the remaining vacant
property or on currently underutilized parcels, but the potential for additional population
and increased sewage flows is not great when compared to the amount of developed
land that already exists within the sewer service area. The number of vacant parcels
within the District was counted in 2010 during a periodic review of sewer capacity
allocation. A total of 254 vacant parcels were noted. Projected flow from these parcels,
assuming 1 dwelling per parcel, is 47,752 gpd. At the unit population density used for
the design of the sewer system, 2.806 persons per dwelling, these vacant parcels would
have 713 additional persons living on them at full build out. However, these projections
are at the lower end of the range of possibility since many of the vacant parcels are
large enough to support more than one dwelling. Actual flows will likely be higher.
According to the Supplemental 201 Facilities Plan for the Kalispell Vicinity, which
was written in 1989 by Stahly Engineering & Consultants, the Evergreen sewage
collection system was designed for a 2010 population of 6,473. Population and flow
projections for the Evergreen collection system related are tabulated as follows:
Table EG-3 - Evergreen Population and Flow Projections from 1989 Facilities Plan
Year
Equivalent
Population
Ave Daily Flow
MGD GPM
Peaking
Factor
Peak Hourly Flow
MGD GPM
1990 6,018 0.4032 280 3.17 1.278 888
2010 6,473 0.4337 301 3.14 1.361 946
Ultimate 10,179 0.6820 474 2.95 2.010 1,396
Unit residential flow used to generate the numbers in the above table is 67 gpcd, which
was taken from the Evergreen sewer system design criteria developed in 1992.
The average daily flow for the two years 2009 and 2010 is 433,425 gpd, which is
eerily close to the 2010 flow predicted for the system back in 1990 (433,700 gpd). The
2010 population predicted for Evergreen in the facility plan (6,473) is not so close to the
2010 population reported by the census (7,616). The discrepancy probably is due to
the difference between the census boundary for Evergreen and the boundary of the
Evergreen sewer service area.
Sewage Treatment in the Flathead Basin 29