Estimating Peak Flows using Hydrologic Modeling
Rainfall data is gathered throughout the watershed in Wisconsin and Illinois.
The majority of the rainfall data comes from observers who report measured rainfall
over the previous 24 hour period to the National Weather Service. The NWS makes
this information available around 8:30 every morning. The US Geological Survey operates
four near real time precipitation gages. Data is posted on the internet usually within
2 hours. A
map (PDF)
showing locations where precipitation are recorded is attached.
The rainfall data from the watershed is utilized in hydrologic
modeling for the Fox River. The precipitation data is entered
into the Fox River Operations Hydrologic Model. The hydrologic
modeling computes estimates of peak inflows to the Chain of Lakes.
The estimated peak inflows to the Chain of Lakes are used with
the current stage of Fox Lake to determine the gate flows from
Stratton Dam. The National Weather Service makes the official
flood predictions at the Fox River at New Munster, Wisconsin and
the Fox River at Algonquin, Illinois utilizing their own
hydrologic modeling. The NWS prediction is compared with the
OWR estimate to investigate the benefits and impacts of
alternative gate operations.
Operation Guide Development
The operation guide criteria for operating the gates were
developed with the Illinois State Water Survey. The ISWS
developed the hydrologic model. The hydrologic model was
calibrated and was utilized with the hydraulic model, Full
Equations (FEQ). The hydraulic model was calibrated then
verified by the U.S. Geological Survey by making flow measurements
and taking water surface elevations during a high water event.
These models were used to investigate 10 alternative gate
operation scenarios including the use of the proposed hinged
crest gates and analyze winter drawdown. The analysis was based
on historic storm events. After verifying the benefits of the
proposed hinged crest gates, the ISWS developed the refined
operation flow chart utilizing predicted peak inflows. This
analysis included the hinged crest gates even though the gates
were not in place at the time of the analysis.
Storage in Chain of Lakes
The Chain of Lakes is a natural
storage area
for the Fox River. As flows increase, the Chain of Lakes rises.
During a flood event, not all the flow can be released back into
the Fox River at Johnsburg since the Fox River is narrower than
the Chain of Lakes. During small flood events, the storage in
the Chain of Lakes allows for flow management so the flows on
the Fox River can be contained in bank. For these small floods,
the Chain of Lakes is not allowed to rise so high that flooding
occurs. During large flood events, peak inflows above 5000 cfs,
the gates at Stratton Dam and Algonquin Dam are open completely.
During large flood events the constraints on the flow capacity
out of the Chain of Lakes is the channel geometry and not the
setting of the gates at Stratton and Algonquin Dams.
Example - June 2008 Event
Most of the flood events are a culmination of several rainfall
events in a 10 day period. For example, the June 2008 flooding
occurred after rainfall from June 5 thru June 10 (PDF) .
The largest amounts of rainfall were recorded on the mornings
of June 6, 8 and 9.
Flows on Nippersink Creek (PDF)
near Spring Grove peaked on June 10.
Flows on the Fox River near New Munster (PDF) ,
Wisconsin peaked on June 15. The highest water surface elevation
on Fox Lake was reached on June 18. The gates were opened
incrementally based on project peak inflows. The gates were
opened on June 6, opened further on June 8, and opened completely
on June 9. The criteria (projected peak inflows and Fox Lake
stage) to completely open the gates were not reached until after
the rainfall on June 8. Note that the gates at both dams were
opened completely 9 days before the peak water surface elevation
was reached in the Chain of Lakes and 6 days before the peak
flows on the Fox River at New Munster.
Using Forecasted Rainfall for
Operation Decisions
The ISWS analysis does not recommended using predicted rainfall.
Rainfall, especially in the Wisconsin portion of the watershed,
takes 1 to 6 days before the runoff enters the Chain of Lakes.
Therefore, adequate time exists to open the gates using the recorded
rainfall. The Quantitative Precipitation Forecast (QPF), prepared by
the NWS, typically over-estimates watershed average rainfall. The
largest problem with using the QPF occurs if gates are raised to
increase flows above bankfull, i.e. flooding occurs downstream of
Stratton Dam based on predicted rainfall.
However, the impacts of predicted rainfall are analyzed. Minor
changes to gate settings that may be beneficial if the predicted
rainfall occurs may be made if these changes will not induce or
increase flooding on residences downstream. The gates will not
be opened fully unless flooding throughout the entire Fox River
and Chain of Lakes is anticipated.
Benefits of the Hinged Crest Gates
When the hinged crest gates were studied, the analysis showed
that the operation of gates would reduce flooding but not eliminate
flooding. The ISWS study of alternative operation scenarios showed
the use of the hinged crest gates reduced peak water surfaces from
the Chain of Lakes to Fox River at Johnsburg by 0.1' to 0.5' during
the historic storms. At Stratton Dam the hinged crest gates reduce
water surfaces 0.1' to 1.0' for the historic flood events. The
hinged crest gate at Algonquin Dam reduces peak water surface
elevations immediately upstream of the dam 0.3' to 1.0' for the
historic storms. The benefits of the Algonquin hinged crest
diminish as you move further upstream. The reduction in peak water
surface elevations and flooding justified the economics for
construction of the gates.
The maximum potential reduction in water surface elevations
during a flood event can be analyzed by removing the dam. The ISWS
alternative analysis showed that removal of Stratton dam would
reduce water surface elevations upstream of the Stratton dam from
0.25' to 0.5' at extremely large flood events and would have no
benefits to the Fox River downstream of Stratton Dam. Removal of
Stratton Dam would not eliminate flooding. However, removal of the
dam would reduce the recreation opportunities in the Chain of Lakes
and Fox River upstream of Stratton Dam.
Information during High Water
Events
During flood events, the County Emergency Managers and Stormwater
Managers are contacted with daily updates of gate settings and flow
at Stratton and Algonquin Dams. The National Weather Service
predictions for flows and potential qualitative impacts for the
Fox River and Chain of Lakes in Illinois are discussed. This
information should be available late morning after decisions for
gate operations are made. Gate settings are not available on the
internet.
Boating Restrictions
The Fox Waterway Agency has the authority to enact no-wake
boating restrictions during high water events.
Chapter 5 (PDF)
of the FWA ordinance specifies the water stages when these
restrictions can be enacted. The Illinois Department of Natural
Resources does not have authority to implement no-wake restrictions.
The Illinois Department of Natural Resources can designate a
body of water a "Restricted Boating Area" when conditions make
navigation of a body of water significantly dangerous to the public.
Title 17 Illinois Administrative Code Section 2030.80
describes the procedure for enacting and rescinding a Restricted
Boating Area. The determination to enact and rescind a boating
restriction is made on a case by case basis. This determination
is made based on the totality of circumstances.
Section 2030.15 (c) specifies that restricting boating for the
purposes of protecting property is prohibited. The Office of
Law Enforcement administers Restricted Boating Areas.
Operation of Stratton and Algonquin Dams (15.2 Mb PDF)
