HomeMy WebLinkAboutAttachment F, Southfork HydrologySOUTH FORK HYDRO, LLC RENEWABLE ENERGY FUND GRANT APPLICATION
ATTACHMENT F SOUTH FORK HYDROELECTRIC PROJECT
OCTOBER 8, 2008 PAGE 1 OF 5
Southfork Hydrolgy
Southfork’s stream flow has been individually measured a number of times. The only consistent
flow measurements over a longer period have been made specifically for this project. The
longest period was when a gage was installed and operated from January 1994 to April 1995.
The results are shown on the attached Southfork Flows Chart. It should be noted the gage
reading provided a negative reading on March 11, 1995. This is not possible. When
consideration that the lowest recorded flow at this time about 15 cfs the information as a whole is
likely 15 cfs greater. There are also anomalies that occurred in the late winter that are due to
icing. These values were ignored in the power calculations.
Some spot readings were taken above the upstream culvert were Hiland road crosses Southfork
during the low flow periods which occur at the beginning of snow melt. A flow of 12.6 cfs was
obtained on April 18, 1998. This is typical of other low flow measurements taken over a period
of time. On April 11, 2008 reading of 23.3 cfs was obtained above the power house. This
reading used the Salt Method as the bed configuration on this rocky stream makes it difficult to
gage.
Flows of the Southfork and Ship Creek were compared. They correlate. Ship Creek and the
Southfork are in parallel basins that have similar orientation. The differences are that the
Southfork is not quite as close to Prince William Sound. Southfork has a fairly large lake up
river and has a small glacier at its headwaters. Ship Creek does not. This has the advantage of
evening the Southfork’s flows.
Ship Creek has a drainage area of 90.5 square miles above the dam, whereas the Southfork’s
drainage is 26 square miles. This corelates to 29 % of Ship Creek’s area. Gage records for Ship
Creek go back as far as October 1946. Ship Creek diverts water for domestic use by the City and
the military bases. This diversion is reported as an average flow each month. The consequence
is that daily records are likely biased by these figures. This average monthly flows will tend to
smooth the gage readings.
In 1985 the flow duration was calculated based on the factored Ship Creek values. A graph
depicting these calculations is included. The calculation indicates the amount of time when
certain flows can be expected. Any flows greater than 50 cfs will not count as this is all the
equipment can use and is near the limit permitted. It should be clear that 35% of the time, 4
months of the year, significant amounts of water will flow in the stream between the intake to the
powerhouse. The flow duration chart provides a probabilistic calculation the takes into account
yearly variations. Therefore, it accurately predicts the likely generation based on past experience
but not the specific year. As a general rule based on McRoberts Creek there will be some yearly
variation in flows. Most of these will occur during the shoulder months which are not as
consequential as the duration of maximum flows over the summer.
The area under the flow duration curve can be used to calculate the power output of the plant.
Each flow can be multiplied by its plant output and added together for each time period.
SOUTH FORK HYDRO, LLC RENEWABLE ENERGY FUND GRANT APPLICATION
ATTACHMENT F SOUTH FORK HYDROELECTRIC PROJECT
OCTOBER 8, 2008 PAGE 2 OF 5
Southfork Flow Chart
January 1994 through April 1995
SOUTH FORK HYDRO, LLC RENEWABLE ENERGY FUND GRANT APPLICATION
ATTACHMENT F SOUTH FORK HYDROELECTRIC PROJECT
OCTOBER 8, 2008 PAGE 3 OF 5
Southfork Spot Flow Readings
April 18, 1998
Southfork Salt Method Flow Measurment
May 11, 2008
SOUTH FORK HYDRO, LLC RENEWABLE ENERGY FUND GRANT APPLICATION
ATTACHMENT F SOUTH FORK HYDROELECTRIC PROJECT
OCTOBER 8, 2008 PAGE 4 OF 5
Southfork Flow Duration Curve
SOUTH FORK HYDRO, LLC RENEWABLE ENERGY FUND GRANT APPLICATION
ATTACHMENT F SOUTH FORK HYDROELECTRIC PROJECT
OCTOBER 8, 2008 PAGE 5 OF 5
Southfork Average Power Output Graph