The City will examine the long-term results of
each of these four monitoring elements to determine whether the new
storm water ordinance is protecting the river as new development
occurs. The project will use an “upstream/downstream” approach to
determine if storm water management practices in the Sterling Ponds
subdivision protect downstream river conditions. We will also take a
focused look at the performance of the on-site storm water management
practices that are incorporated into new developments. Our hope is that
due to the ordinance requirements, the water quality and thermal impacts
of new development will be undetectable or greatly reduced.
River Falls Precipitation:
Due to the major
influence of precipitation on river flow, temperature, and water
quality, an analysis of seasonal precipitation is conducted as a part of
this project. During the April-September 2007 monitoring period, hourly
precipitation was measured in 0.01-inch increments with an electronic
tipping-bucket rain gauge. The rain gauge, provided by the Wisconsin
Department of Natural Resources (WDNR), is located in the Sterling Ponds
subdivision at the northwest corner of the City of River Falls. This
location places the rain gauge in very close proximity to all six North
Kinnickinnic River monitoring stations. A weather station at Rocky
Branch Elementary School, on the south side of River Falls, serves as an
alternate source of daily rainfall data. This station is part of an
extensive network of local weather stations supported by KSTP-TV in
Minneapolis, MN, via the Automated Weather Source. The Rocky Branch
Weather Station also serves as a source of daily mean, minimum, and
maximum air temperatures. In addition, daily precipitation data are
available from the United States Geological Survey (USGS) Kinnickinnic
River monitoring station at County Highway F, near Kinnickinnic State
Park, approximately five miles west of River Falls.
A total of 18.36 inches
of precipitation was recorded in River Falls (at Sterling Ponds) during
the April-September 2007 period, 2.31 inches less than the normal total
of 20.67 inches for the April-September time period. Rain fell on 67
days, or 37% of the April-September 2007 period. In comparison, a
near-normal total of 19.82 inches of precipitation was recorded in River
Falls during the April-September 2004 monitoring period, an above-normal
total of 36.45 inches was measured during the April-September 2005
period, and a below-normal total of 17.16 inches was measured during the
April-September 2006 period (Figure 1). “Normal” monthly and seasonal
rainfall amounts are based upon measurements made by the National
Weather Service at the Twin Cites International Airport during the
“climate normal period” of 1971-2000.
Daily rainfall amounts
during the April-September 2007 period are presented in
Figure 2.
Monthly rainfall amounts during the April-September 2007 period, with a
comparison to normal monthly rainfall amounts, are presented in
Figure
3. Except for August and September, all months during the
April-September 2007 period were drier than normal, with monthly
rainfall deficits ranging from 0.14 inch to 2.83 inches. The greatest
rainfall deficits occurred in June and July, with the lowest monthly
rainfall amount (1.33 inches) recorded in July. August was the wettest
month (6.93 inches), exceeding the normal monthly rainfall amount by
2.88 inches. September was also a very wet month (4.76 inches),
exceeding the normal monthly rainfall amount by 2.07 inches. Until
mid-August 2007, the North Kinnickinnic River Monitoring Project Area
continued to be affected by a region-wide drought that began in early
2006 (see Figure 1).
Besides being drier than
normal, the April-September 2007 monitoring period was warmer than
normal. The mean air temperature in River Falls during the
April-September 2007 period was 65.4º Fahrenheit (F), 2.2º F higher than
the normal mean of 63.2º F for the April-September period, as measured
at the Twin Cities International Airport. Monthly mean air temperatures
during the April-September 2007 period, with a comparison to normal
monthly mean temperatures during the “climate normal period” of
1971-2000, are presented in Figure 4. With the exception of April, all
months during the April-September 2007 monitoring period were warmer
than normal. The months of May and September were nearly 4º F warmer
than normal, while the month of June was 3º F warmer than normal. The
month of April was slightly colder than normal.
The distribution of
River Falls daily rainfall amounts during the April-September 2007
period is presented in
Figure 5. Although the 2007 monitoring season
was drier than normal, it was characterized by numerous (56) days with
rainfall amounts of 0.50 inch or less. On 44 (66%) of the 67 days with
measurable precipitation, rainfall amounts were 0.25 inch or less.
These 44 days contributed only 17% of the total April-September 2007
precipitation. The majority of these 44 days occurred in the cooler
months of April, May, June, and September (Figure 6). On 12 (18%) of
the 67 days with measurable precipitation, rainfall amounts ranged from
0.26-0.50 inch. These 12 days contributed an additional 22% of the
total April-September 2007 precipitation. Three of these 12 days
occurred in May and September (Figure 6), when air temperatures were
cooler. On 3 (4%) of the 67 days with measurable precipitation,
rainfall amounts ranged from 0.51-0.75 inch. These 3 days contributed
10% of the total April-September 2007 precipitation, and occurred in
May, August, and September (Figure 6). Only 2 (3%) of the 67 days with
measurable precipitation had rainfall amounts in the 0.76-1.00 inch
range (Figure 6), contributing 10% of the total April-September 2007
precipitation. On 6 (9%) of the 67 days with measurable precipitation,
rainfall amounts exceeded 1.00 inch. These 6 days with the largest
rainfall events contributed 42% of the total April-September 2007
precipitation. Rainfall amounts in excess of 1 inch occurred on August
13, 19, 27, and 28, and September 18 and 20 (Figures
2 and
6). On 4 of
the 6 days, rainfall amounts ranged from 1.01-1.25 inches. On 2 of the
6 days, rainfall amounts exceeded 1.50 inches. All of these 6 largest
rainfall events occurred in August and September, and all but one event
(September 18) were produced by convective thunderstorm activity during
a warmer than normal summer.
To achieve the
requirements of the City’s storm water ordinance, developers must
provide on-site infiltration of post-development storm water from
24-hour rainfall events of 1.5 inches or less. Of the 67 days with
measurable precipitation during the April-September 2007 period, 65 days
(97%) had rainfall amounts less than 1.5 inches in 24 hours (a
midnight-to-midnight total). Based on that data, only rainfall amounts
on August 27 (1.72 inches) and September 18 (1.64 inches) exceeded this
criterion. Even so, some infiltration would have occurred under the
requirements of the storm water ordinance, thereby accounting for
infiltration of approximately 98% (18.00 inches) of the total rainfall
(18.36 inches) that occurred during the April-September 2007 period.
Figure 7 depicts the annual effectiveness of the River Falls Storm Water
Ordinance for infiltrating storm water runoff generated by rainfall
during the April-September period. This figure was prepared for
illustrative purposes only, and was created with the assumption that the
entire 1.5-inch event is infiltrated. This scenario essentially assumes
zero pre-development runoff, which may not necessarily be the case.
Kinnickinnic River Flow:
The flow of the
Kinnickinnic River is a reflection of strong ground water (spring)
contributions, as well as precipitation-induced storm water runoff from
predominantly agricultural and urban land uses throughout the 165-square
mile Kinnickinnic River Watershed. The United States Geological Survey
(USGS) operates a Kinnickinnic River monitoring station (number
05342000) at County Highway F, near Kinnickinnic State Park,
approximately five miles west of River Falls. The station measures
river stage (water height) and flow at 15-minute intervals, and
precipitation in 0.01-inch increments. Because accurate monitoring of
river stage and flow entails a significant investment in equipment and
labor, no continuous measurement of river flow is currently being
conducted within the North Kinnickinnic River Monitoring Project Area.
For this reason, the Kinnickinnic River flow information provided by the
USGS monitoring station is particularly valuable, as it clearly
documents when runoff events are occurring and storm water impacts may
be apparent. The City of River Falls, Kinnickinnic River Land Trust,
and Trout Unlimited provide annual cost-share funding for this USGS
monitoring station.
The daily mean (average) flow of the
Kinnickinnic River at County Highway F during the April-September 2007
period is presented as a hydrograph in
Figure 8. Daily rainfall, as
measured in River Falls at Sterling Ponds, is also presented in
Figure
8.
Precipitation patterns help explain the
changes that occur in the Kinnickinnic River hydrograph, due to runoff
events in the watershed. Prior to the start of the 2007 monitoring
season, runoff from rapid snowmelt in mid-March produced a peak daily
mean river flow of 2,024 cubic feet per second (cfs) on March 13 (not
shown in Figure 8). During the April-September 2007 monitoring period,
the highest river flow of 192 cfs on April 1 occurred as a result of
heavy rainfall (1.73 inches) on March 30-31.
Numerous small rain events (less than 0.50
inch) in April, May, June, and July had little influence on the
Kinnickinnic River hydrograph. Larger rain events on May 23 (0.94 inch)
and May 30 (0.88 inch) produced moderate increases in the Kinnickinnic
River hydrograph, with peak daily mean flows of 130 cfs and 108 cfs,
respectively.
During the August-September period,
rainfall amounts in excess of 1 inch generally had the greatest
influence on the Kinnickinnic River hydrograph. Rainfall events on
August 13 (1.04 inches) and August 19 (1.15 inches) resulted in only
moderate increases in the Kinnickinnic River hydrograph, with peak daily
mean flows of 101 cfs and 130 cfs, respectively. These moderate runoff
events, in spite of rainfall amounts in excess of 1 inch, can be
attributed to very dry antecedent conditions during the May-early August
period, and full canopy closure in the agricultural and forested areas
of the watershed. Large, back-to-back rainfall events on August 27
(1.72 inches) and August 28 (1.04 inches) produced a more significant
increase in the Kinnickinnic River hydrograph, with a peak daily mean
flow of 149 cfs. Similarly, large, nearly back-to-back rainfall events
on September 18 (1.64 inch) and September 20 (1.19 inches) also produced
a significant increase in the Kinnickinnic River hydrograph, with a peak
daily mean flow of 141 cfs.
The Kinnickinnic River hydrograph suggests
that seven significant runoff events occurred during the April-September
2007 period. One of these seven significant runoff events occurred in
late March and early April (March 30-April 3), due to early spring rains
on March 30-31. With cool air and water temperatures in late March and
early April, thermal impacts of storm water runoff are generally not a
concern, but water quality impacts can be problematic, due to frozen
soils and a lack of vegetative cover in the watershed. Two of the seven
significant runoff events occurred in May, when thermal impacts of storm
water runoff become a concern due to warmer air temperatures. On May
23, a 0.94 inch rain event resulted in a 3-day runoff event (May 23-25),
with a peak daily mean flow of 130 cfs. On May 30, a 0.88 inch rain
event produced a 1-day runoff event (May 31), with a peak daily mean
flow of 108 cfs. Four of the seven significant runoff events occurred
in August and September, during two of the four warmest months of the
year (Figure 4), when thermal impacts of storm water runoff can be a
considerable concern. On August 13, a 1.04 inch rain event resulted in
a 3-day runoff event (August 14-16), with a peak daily mean flow of 101 cfs. On August 19, a 1.15 inch rain event resulted in a 2-day runoff
event (August 19-20), with a peak daily mean flow of 130 cfs. On August
27 and 28, back-to-back rainfall events totaling 2.76 inches resulted in
a 4-day runoff event (August 27-30), with a peak daily mean flow of 149
cfs. On September 18 and 20, nearly back-to-back rainfall events
totaling 2.83 inches resulted in a 5-day runoff event (September 18-22),
with a peak daily mean flow of 141 cfs. The six runoff events in May,
August, and September should be the focus for evaluating possible storm
water impacts in the North Kinnickinnic River Monitoring Project Area in
2007, and are further analyzed in this report.
With drought conditions prevalent during
the April-early August 2007 period, Kinnickinnic River base flows
steadily decreased from approximately 90 cfs in mid April to
approximately 70 cfs in early August, as measured at County Highway F.
With higher than normal rainfall amounts in August and September, base
flows rebounded to approximately 80 cfs in early September and
approximately 90 cfs in late September.
Temperature Monitoring:
In 2007, temperature monitoring was
conducted at six City of River Falls monitoring stations (Sites 1-6) in
the North Kinnickinnic River Monitoring Project Area. To evaluate the
thermal performance of the storm water management practices at Site 5 in
the Sterling Ponds subdivision, temperature monitoring was conducted at
three locations: the wet detention pond (Site 5P), the wet detention
pond outlet to the infiltration basin (Site 5IB), and the wet detention
pond outfall to Sumner Creek (Site 5MHW).
The local Kiap-TU-Wish Chapter of Trout
Unlimited (TU) also conducted temperature monitoring at one Kinnickinnic
River station (Site 1A) within the project area, between Sites 1 and 2.
The TU monitoring station is located along Quarry Road on the northeast
edge of River Falls, just east of the WI Highway 35 bypass, and just
upstream of the Sumner Creek confluence. The TU station has been in
service during all summer periods (May-September) since 1992. In 2005,
as an additional contribution to the North Kinni Project, TU established
a temperature monitoring station in Sumner Creek (Site 4A),
approximately 100 feet upstream of the creek confluence with the
Kinnickinnic River. This station was in service during the summer
periods (May-September) of 2005-2007. The thermal impacts of Sumner
Creek on the Kinnickinnic River, including any storm water contributions
from Sterling Ponds, can be evaluated at this location.
Onset Computer Corporation’s®
HOBO Water Temp Pro Loggers are used to measure water temperature at all
City of River Falls monitoring stations (Sites 1-6). A Ryan Instruments®
RTM 2000 Temperature Logger is used to measure water temperature at the
TU monitoring station at Quarry Road (Site 1A). Onset Computer
Corporation’s® Optic StowAway Templogger is used at the TU
monitoring station in Sumner Creek (Site 4A). All Onset and Ryan
temperature loggers are programmed to record temperatures at 10-minute
intervals. Date and time stamps and the 10-minute temperature data are
electronically recorded by each logger; and all recorded information is
downloaded as necessary. The brief 10-minute time interval was selected
so that any rapid temperature increases associated with warm storm water
runoff could be documented. All temperature loggers were deployed
throughout the May-September (summer) period. The thermal impacts of
storm water runoff are most likely to occur during this summer period,
when air temperatures are highest. The summer 2007 deployment periods
(and locations) for the temperature loggers at the ten monitoring
stations were as follows:
Site:
Deployment Period: Location:
Site
1: May 4-September 30, 2007 Kinnickinnic
River
Site
1A: May 1-September 30, 2007 Kinnickinnic River
Site
2: May 4-September 30, 2007 Kinnickinnic
River
Site
3: May 4-September 30, 2007 Kinnickinnic
River
Site
4: May 4-September 30, 2007 Sumner Creek: Wet
Pool in Culvert
Site
4A: May 1-September 30, 2007 Sumner Creek: Mouth
Site
5P: May 4-September 30, 2007 Sterling Ponds: Wet
Pond
Site
5IB: May 4-September 30, 2007 Sterling Ponds:
Infiltration Basin
Site 5MHW: May
4-September 30, 2007 Sterling Ponds: Wet Pond Outlet
Site
6: May 4-September 30, 2007 Sumner Creek: Dry
Box Culvert
Kinnickinnic River
Temperature Monitoring Results:
The May-September (summer) 2007
temperature monitoring data obtained for the Kinnickinnic River at Sites
1, 1A, 2, and 3 are presented as thermographs in Figures
9,
10,
11 and
12,
respectively. Of immediate note in these thermographs is the strong
diurnal (daily) temperature pattern in the river. Although cold ground
water continually feeds the river via springs along the entire riverway,
the temperature of the Kinnickinnic River is greatly influenced by
ambient air temperature. During the daylight hours, the river gradually
warms and generally reaches a daily maximum temperature in the late
afternoon or early evening (4:30-6:30 PM). At night, the river
gradually cools and typically reaches a daily minimum temperature just
after sunrise (7:30-9:30 AM). These diurnal temperature fluctuations in
the river are natural, and the river’s residents, including
macroinvertebrates and trout, have become accustomed to a constantly but
slowly changing temperature regime.
Also of note in the 2007 Kinnickinnic
River thermographs are the relatively frequent changes in the daily
minimum and maximum river temperatures and daily temperature ranges that
are influenced by local weather patterns (cold fronts and warm fronts)
and seasonal climate changes. During the summer 2007 period, for
example, the monthly mean river temperature in the North Kinnickinnic
River Project Area (Sites 1, 1A, 2, and 3) was coolest in May (13.6
degrees Celsius (°C)) and warmest in July (16.6° C).
At Sites 1, 1A, 2, and 3, river
temperatures averaged 15.2° C and ranged from 7.7-21.6° C over the
course of the summer. Monthly and summer mean temperatures at each of
these four monitoring sites are presented in
Figure 13. These monthly
and summer mean temperatures were identical at Sites 1 and 2, slightly
warmer at Site 1A, but slightly cooler at Site 3, especially during the
June-July period.
For the third consecutive year,
higher-than-normal river temperatures probably prevailed in the North
Kinnickinnic River Project Area during the summer of 2007, since the
2007 summer average air temperature of 20.7º C (69.3º F) was noticeably
higher than the normal summer average air temperature of 19.2° C (66.5°
F). A comparison of 2004-2007 summer average air temperatures and river
temperatures (at Sites 1, 1A, and 2) can be found in the North
Kinnickinnic River Monitoring Project Indicators. Note that the 2007
summer average air temperature of 20.7º C and summer average river
temperature of 15.2º C were the highest summer average temperatures
recorded since the North Kinnickinnic River Monitoring Project began in
2004.
The most direct way to determine if any
thermal impacts occurred in the Kinnickinnic River as a result of the
Sterling Ponds subdivision is to compare the temperature monitoring data
at Site 1, located immediately downstream from Sumner Creek, to the
temperature monitoring data at Sites 1A and 2, located immediately
upstream from Sumner Creek. These two upstream sites serve as control
or reference sites, which are not impacted by Sterling Ponds storm water
discharges.
A comparison of all upstream summer
temperature data at Sites 1A and 2 to all downstream summer temperature
data at Site 1 is presented in
Figure 14. This comparison indicates
that summer temperatures were nearly identical at these three
locations. The temperature similarities at Sites 1, 1A, and 2 are even
more evident in the monthly thermographs for May, June, July, August,
and September 2007 (Figures
15-19, respectively). Figures
14-19
indicate that daily maximum temperatures tended to be slightly warmer at
Site 1A, due to less canopy cover and shading at this location. Daily
minimum temperatures tended to be slightly cooler at Site 1.
Figure 13
shows that the monthly and summer mean temperatures at Sites 1, 1A, and
2 were also nearly identical. The following should be noted concerning
aquatic life in the Kinnickinnic River:
-
Approximately 77% of all
temperatures recorded at Sites 1, 1A, and 2 during the May-September
2007 period were less than or equal to (≤) 17° C, which is considered
to be the top of the optimum temperature range for a healthy coldwater
macroinvertebrate community (Galli, 1990). A temperature of 17° C is
considered to be the physiological optimum for brown trout survival (Armour,
1994).
-
Approximately 94% of all
temperatures recorded at Sites 1, 1A, and 2 during the May-September
2007 period were ≤ 19° C, which is considered to be the top of the
optimum temperature range for brown trout growth (Armour, 1994).
-
Approximately 98% of all
temperatures recorded at Sites 1, 1A, and 2 during the May-September
2007 period were ≤ 20° C, which is considered to be the top of the
optimum temperature range for brown trout survival (Armour, 1994).
River temperatures exceeding 20º C were not recorded in May and
September. With a warmer-than-normal summer, however, daily maximum
river temperatures exceeded 20º C on 23 dates during the June-August
period. Maximum air temperatures on these 23 dates were generally
hot, ranging from 28-37º C (82-98° F) and averaging 32º C (90° F).
During eight significant rainfall events
in May, August, and September 2007, thermographs at Sites 1 and 2 can be
compared to determine if rapid temperature increases (thermal spikes),
which are characteristic of warm storm water discharges, were apparent
at Site 1.
No thermal spikes were evident at Site 1
in May (Figure 15), in spite of two significant rainfall events on May
23 (0.94 inch) and May 30 (0.88 inch). A closer examination of the
thermographs for Sites 1 and 2 during the 0.94-inch rainfall event on
May 23 (Figure 20) indicates that no thermal spike occurred at Site 1,
downstream from Sumner Creek and the Sterling Ponds subdivision. During
the same rain event, however, the thermograph for the Trout Unlimited
temperature monitoring site at Division Street (Figure 20) shows a very
prominent thermal spike, due to the thermal impacts of direct storm
water discharges from the downtown area of River Falls. Similarly, when
the thermographs for Sites 1, 2, and Division Street are compared during
the 0.88-inch rainfall event on May 30 (Figure 21), no thermal spike is
evident at Site 1, while a slight thermal spike is evident at Division
Street.
In August, no thermal spikes were evident
at Site 1 (Figure 18), in spite of multiple larger rain events on August
13 (1.04 inches), August 19 (1.15 inches), August 27 (1.72 inches), and
August 28 (1.04 inches). A closer examination of the rainfall events on
August 13 (Figure 22) and August 19 (Figure 23), and the back-to-back
rainfall events on August 27-28 (Figure 24) indicates that no thermal
spikes were apparent at Site 1, while prominent thermal spikes were
consistently evident at Division Street. During the August rainfall
events, the thermal spikes at Division Street ranged in magnitude from
1.0-3.0º C.
Finally, no thermal spikes were evident at
Site 1 in September (Figure 19), in spite of two large, nearly
back-to-back rainfall events on September 18 (1.64 inch) and September
20 (1.19 inch). A closer examination of these two rainfall events
(Figure 25) indicates that no thermal spikes were apparent at Site 1,
while prominent thermal spikes, ranging in magnitude from 0.9-1.8º C
were quite evident at Division Street.
While the presence of thermal spikes at
Division Street is attributed to the thermal impacts of untreated storm
water discharges to the Kinnickinnic River, the lack of thermal spikes
at Site 1 could be attributed to several factors, including effective
storm water management at the Sterling Ponds subdivision, or simply a
lack of Sterling Ponds storm water discharges and/or storm water
conveyance down Sumner Creek, even during the largest runoff events in
2007.
Sumner Creek and Sterling Ponds Temperature Monitoring Results:
Sumner Creek
Sumner Creek is a
low-gradient tributary of the Kinnickinnic River that exhibits only
intermittent flow for the majority of its length. Permanent flow begins
in the vicinity of the WI Highway 35 bypass, near the creek confluence
with the Kinnickinnic River (Site 4A). From this location, the creek
drainage way extends upstream to Radio Road on the far northwest corner
of River Falls. The upper portion of the Sumner Creek drainage way,
including Sites 4 and 6, conveys no flow for the majority of the year.
The headwaters area near Site 6 is a dry run. Downstream, however,
rather extensive wetland areas are apparent in the Sumner Creek drainage
way through the Sterling Ponds subdivision, and for an appreciable
distance downstream of Site 4. Anecdotal evidence suggests that flow
may occur in the upper portion of Sumner Creek during the spring
snowmelt period and perhaps during large summer rain events. During
large summer rain events, however, the wetland areas and dry portions of
the Sumner Creek channel likely provide considerable water storage,
making it very difficult to determine if and when any upstream flow is
conveyed all the way downstream to the Kinnickinnic River.
The May-September
(summer) 2007 temperature monitoring data obtained for Sumner Creek at
Site 4A are presented as a thermograph in
Figure 26. Site 4A near the
creek mouth was the only Sumner Creek monitoring location with permanent
flow throughout the summer. At Site 4A, Sumner Creek temperatures
averaged 12.5° C and ranged from 7.8-17.1° C during the May-September
2007 period. The summer mean temperature of Sumner Creek (12.5º C) was
notably colder than the summer mean temperature of the Kinnickinnic
River (15.2º C) at Sites 1, 1A, 2, and 3, reflecting strong spring
activity. Nearly 100% of all temperatures recorded at Site 4A during
the May-September 2007 period were ≤ 17° C. Temperatures exceeding 17°
C were only recorded for a brief period during the largest rainfall
event of the summer on August 27 (1.72 inches).
Based upon the summer
2007 temperature data, lower Sumner Creek may have potential as a brook
trout stream, and is regardless an important contributor of cold water
to the Kinnickinnic River. Of concern, however, are multiple thermal
spikes that occurred at Site 4A during the large rain events in May,
August, and September (Figure 26). Several prominent thermal spikes in
lower Sumner Creek, ranging from 2.7-4.2° C, occurred during the May 23,
August 27, and August 28 rain events. These Sumner Creek thermal
spikes were of even greater magnitude than those observed at the
Division Street monitoring site on the same dates. In spite of their
magnitude, none of these thermal spikes had a discernible impact on
Kinnickinnic River temperatures at Site 1, downstream from Sumner
Creek. However, thermal spikes of this magnitude and frequency may have
detrimental impacts on aquatic life in lower Sumner Creek, especially macroinvertebrates. Numerous thermal spikes were also apparent in lower
Sumner Creek (Site 4A) during the summers of 2005 and 2006. Possible
sources contributing to thermal spikes in lower Sumner Creek may
include: storm water runoff from WI Highway 35, located immediately
upstream from Site 4A; warm water from natural wetland areas in the
upper Sumner Creek drainage way; and storm water discharges from the
Sterling Ponds subdivision.
Sterling Ponds
The May-September
(summer) 2007 temperature monitoring data obtained for the Sterling
Ponds wet detention pond at Site 5P are presented as a thermograph in
Figure 27. At Site 5P, wet detention pond temperatures averaged 21.4° C
and ranged from 13.1-28.9° C during the summer period. Approximately
65% of all summer temperatures exceeded 20° C, and wet pond temperatures
continuously remained above 20° C from June 10 until August 19.
Substantial warming of small, shallow ponds such as this can be
expected, especially with no shading or canopy cover. The summer mean
temperature of the Sterling Ponds wet detention pond (21.4° C) was
substantially higher than the summer mean temperature of Sumner Creek at
Site 4A (12.5º C), clearly demonstrating the potential for thermal
impact when the pond discharges to the creek, and emphasizing the
importance of the River Falls Storm Water Management Ordinance, which
requires storm water infiltration.
Assessment of
Sterling Ponds Storm Water Infiltration and Discharge to Sumner Creek
Temperature data from
the three Sterling Ponds monitoring stations (Sites 5P, 5IB, and 5MHW)
and the two downstream Sumner Creek monitoring stations (Sites 4 and 4A)
can be used to evaluate the effectiveness of the Sterling Ponds storm
water management practices for infiltrating storm water and minimizing
warm storm water discharges to Sumner Creek. Given the warm and
relatively stable thermal regime (Figure 27) in the Sterling Ponds wet
detention pond (measured at Site 5P), pond discharges to the
infiltration basin can be readily identified when the temperature at
Site 5IB closely matches that at Site 5P. Similarly, pond discharges to
Sumner Creek can be readily identified when the temperature at Site 5MHW
closely matches that at Site 5P. Warm storm water discharges to Sumner
Creek may be detectable as thermal spikes at Sites 4 and 4A.
During the summer of
2007, the thermal performance of Sterling Ponds stormwater management
practices can be evaluated monthly by comparing the Sterling Ponds and
Sumner Creek thermographs. Performance of these stormwater management
practices during the eight significant rainfall and runoff events in
May, August, and September is of particular interest, and may help
explain the possible causes of the thermal impacts (spikes) observed in
lower Sumner Creek (Site 4A). The August 27 and September 18 events
were characterized by rainfall amounts in excess of 1.5 inches, beyond
the infiltration requirement of the River Falls Storm Water Management
Ordinance.
May
The comparative Sterling
Ponds thermographs for May 2007 are presented in
Figure 28. The May
1-22 period was warmer than normal and relatively dry, with small
rainfall events recorded on only five dates. During this time period,
no wet pond discharges occurred to either the infiltration basin or
Sumner Creek. The five small rain events, ranging from 0.01-0.27 inch
and totaling 0.55 inch, were captured in the wet pond, where the water
infiltrated or evaporated from the pond.
The comparative Sterling
Ponds and Sumner Creek thermographs for the May 23 rain event (0.94
inch) are presented in
Figure 29. As indicated by the nearly identical
temperatures at Sites 5P and 5IB, the Sterling Ponds wet detention pond
began discharging to the infiltration basin at 17:20 CDT (5:20 PM) on
May 23, shortly after the onset of rainfall at 17:00 CDT (5:00 PM). Wet
pond discharge to the infiltration basin, due to the May 23 rainfall
event and several smaller events on May 24 (0.51 inch) and May 26 (0.11
inch), continued for nearly six days, until 10:20 CDT (10:20 AM) on May
29. During this time period, no wet pond discharges to Sumner Creek
were evident, as documented by the temperature data at Site 5MHW, and no
thermal spikes were apparent in Sumner Creek at Site 4. Shortly after
the May 23 rain event began, a prominent thermal spike (3.3° C) occurred
in lower Sumner Creek at Site 4A. However, this thermal spike cannot be
attributed to a storm water discharge at Sterling Ponds, and seemed to
have a more local cause.
June
The comparative Sterling
Ponds thermographs for June 2007 are presented in
Figure 30. As
indicated by the nearly identical temperatures at Sites 5P and 5IB, the
Sterling Ponds wet detention pond was already discharging to the
infiltration basin on June 1, due to the large rainfall event (0.88
inch) on May 30 (Figure 31). Wet pond discharge to the infiltration
basin, due to the May 30 rainfall event and smaller daily rain events
during the May 31-June 7 period, began at 19:40 CDT (7:40 PM) on May 30
and continued for nearly nine days, until 18:40 CDT (6:40 PM) on June
8. Rainfall amounts during the May 31-June 7 period ranged from
0.01-0.32 inch and totaled 0.67 inch. During the May 30-June 7 time
period, no wet pond discharges to Sumner Creek were evident, as
documented by the temperature data at Site 5MHW, and no thermal spikes
were apparent in Sumner Creek at Site 4 (Figure 31). Shortly after the
May 30 rain event began, a small thermal spike (1.1° C) occurred in
lower Sumner Creek at Site 4A. However, this thermal spike cannot be
attributed to a storm water discharge at Sterling Ponds, and seemed to
have a more local cause.
The remainder of June
was very warm and quite dry, with small rainfall events recorded on only
six dates. During this time period, no wet pond discharges occurred to
either the infiltration basin or Sumner Creek (Figure 30). The six
small rain events, ranging from 0.01-0.37 inch and totaling 0.90 inch,
were captured in the wet pond, where the water infiltrated or evaporated
from the pond. Including infiltration of the May 30 rain event in early
June, the entire May rainfall amount of 3.10 inches (11 events ranging
from 0.01-0.94 inch) (Figure 6) was captured in the Sterling Ponds wet
pond or infiltrated. Similarly, the entire June rainfall amount of 1.51
inches (12 smaller events ranging from 0.01-0.37 inch) (Figure 6) was
also captured in the wet pond or infiltrated.
July
The comparative Sterling
Ponds thermographs for July 2007 are presented in
Figure 32. July was
much drier and warmer than normal, with small rainfall events recorded
on ten dates. During the month, no wet pond discharges occurred to
either the infiltration basin or Sumner Creek. The ten small rain
events, ranging from 0.02-0.33 inch and totaling 1.33 inches, were
captured in the wet pond, where the water infiltrated or evaporated from
the pond. Given the high ambient air and wet pond temperatures in July,
evaporation likely accounted for considerable water loss from the pond.
August
The comparative Sterling
Ponds thermographs for August 2007 are presented in
Figure 33. August
was the wettest month of the summer 2007 monitoring season, with 6.93
inches of rain. However, most of this rainfall amount (6.79 inches)
occurred during the August 11-28 period.
A
moderate rainfall event on August 11 (0.62 inch) and the large rainfall
event on August 13 (1.04 inches) caused no Sterling Ponds wet pond
discharges to either the infiltration basin or Sumner Creek (Figure
34). With very warm and relatively dry conditions during the June
1-August 10 period, the Sterling Ponds wet pond had adequate capacity to
capture both of these rain events. This capacity was gained due to
considerable water loss from the pond, via evaporation and possibly some
infiltration. After the August 13 rainfall event, no thermal spike was
apparent in Sumner Creek at Site 4, but a small thermal spike (1.3° C)
occurred in lower Sumner Creek at Site 4A (Figure 34). However, this
thermal spike cannot be attributed to a storm water discharge at
Sterling Ponds, and seemed to have a more local cause. Two smaller rain
events on August 14 (0.08 inch) and August 18 (0.48 inch) were also
captured in the Sterling Ponds wet pond.
Due to the large
rainfall event on August 19 (1.15 inches), seven smaller rain events
during the August 20-24 period (ranging from 0.01-0.34 inch and totaling
0.66 inch), and large back-to-back rainfall events on August 27-28 (1.72
and 1.04 inches), the Sterling Ponds wet detention pond began
discharging to the infiltration basin on August 19 and continued
discharging to the infiltration basin through the end of August (Figure
33).
The comparative Sterling
Ponds and Sumner Creek thermographs for the August 19 rain event (1.15
inch) are presented in
Figure 35. As indicated by the nearly identical
temperatures at Sites 5P and 5IB, the Sterling Ponds wet detention pond
began discharging to the infiltration basin at 15:50 CDT (3:50 PM) on
August 19, shortly after the onset of rainfall at 15:00 CDT (3:00 PM).
Wet pond discharge to the infiltration basin, due to the August 19
rainfall event and the seven smaller rain events during the August 20-24
period (totaling 0.66 inch), continued until the August 27 rain event
began. During the August 19-26 period, no wet pond discharges to Sumner
Creek were evident, as documented by the temperature data at Site 5MHW,
and no thermal spikes were apparent in Sumner Creek at Site 4. Shortly
after the August 19 rain event began, a small thermal spike (1.7° C)
occurred in lower Sumner Creek at Site 4A. However, this thermal spike
cannot be attributed to a storm water discharge at Sterling Ponds, and
seemed to have a more local cause.
The comparative Sterling
Ponds and Sumner Creek thermographs for the large back-to-back rainfall
events on August 27-28 (1.72 and 1.04 inches) are presented in
Figure
36. Due to significant rainfall (1.81 inches) during the August 19-24
period, the Sterling Ponds wet detention pond was already discharging to
the infiltration basin when the August 27 rain event began at
approximately 02:00 CDT (2:00 AM). With heavy, intense rainfall from
02:00-05:00 CDT (2:00-5:00 AM), discharge from the wet pond to the
infiltration basin increased (note the small but rapid temperature
increase at Site 5IB from 03:10-03:30 CDT). As indicated by the nearly
identical temperatures at Sites 5P and 5MHW, the Sterling Ponds wet
detention pond began discharging to the Sumner Creek drainage way at
04:20 CDT (4:20 AM) on August 27 and continued discharging until 08:00
CDT (8:00 AM). During this 4-hour period, the wet pond discharge
temperature averaged 19.8º C and ranged from 18.8-20.2º C. Some storage
of this storm water discharge likely occurred in the wetland that
comprises the creek drainage way upstream from Site 4. A small but
rapid temperature increase (1.3º C) was apparent downstream at Site 4 in
Sumner Creek by 11:00 CDT (11:00 AM) on August 27, and is likely due to
the release of warm water (including storm water) from the upstream
wetland. In spite of this warm water release, the temperature of Sumner
Creek never exceeded 20º C at Site 4 on August 27. The prominent
thermal spike (4.1º C) evident near the mouth of Sumner Creek (Site 4A)
at 06:00 CDT (6:00 AM) on August 27 cannot be attributed to the Sterling
Ponds storm water discharge, since the spike at Site 4A, located 1.5
miles downstream, occurred shortly after the storm water discharge
began, but well before the thermal spike was evident at Site 4. It
seems apparent that the thermal spike at Site 4A had a more “local”
cause, perhaps including storm water runoff from WI Highway 35 and/or
warm water flowing from natural wetland or storage areas in the upstream
Sumner Creek drainage way.
Rainfall on August 28
(1.04 inches) occurred in two “pulses”, with 0.54 inch measured from
02:00-04:00 CDT (2:00-4:00 AM) and 0.50 inch measured from 11:00-14:00
CDT (11:00 AM-2:00 PM). With the Sterling Ponds wet detention pond
already full and discharging to the infiltration basin due to the August
27 rain event, a brief wet pond discharge to Sumner Creek occurred
during the second rainfall pulse on August 28. As indicated by the
nearly identical temperatures at Sites 5P and 5MHW, the Sterling Ponds
wet detention pond began discharging to the Sumner Creek drainage way at
13:40 CDT (1:40 PM) and continued discharging until 16:20 CDT (4:20
PM). During this 3-hour period, the wet pond discharge temperature
averaged 21.5º C and ranged from 21.4-21.7º C. Some storage of this
storm water discharge likely occurred in the wetland that comprises the
creek drainage way upstream from Site 4. In spite of the storm water
discharge during the afternoon rain pulse, no thermal spikes were
apparent in Sumner Creek at Site 4 on August 28. A moderate thermal
spike (2.7° C) occurred in lower Sumner Creek at Site 4A during the
early morning rain pulse. However, this thermal spike cannot be
attributed to a storm water discharge at Sterling Ponds, and seemed to
have a more local cause.
With rainfall amounts on
August 27-28 totaling 2.76 inches, the Sterling Ponds wet detention pond
continued discharging to the infiltration basin for another four days,
until 21:30 CDT (9:30 PM) on September 1. All rainfall during the
August 1-26 period (12 events ranging from 0.01-1.15 inches and totaling
4.17 inches) was captured in the Sterling Ponds wet pond or
infiltrated. It seems likely that the majority of the August 27-28
rainfall was infiltrated. Although two brief wet pond discharges to
Sumner Creek occurred on August 27 and 28, the combined duration of
these discharges was relatively short (7 hours), compared to the
duration of discharge to the infiltration basin (138 hours) after
rainfall began on August 27.
September
The comparative Sterling
Ponds thermographs for September 2007 are presented in
Figure 37.
September was much wetter and warmer than normal, with 4.76 inches of
rain recorded on twelve dates. Most of the September rainfall amount
occurred during the September 18-30 period (4.11 inches). As indicated
by the nearly identical temperatures at Sites 5P and 5IB, the Sterling
Ponds wet detention pond was already discharging to the infiltration
basin on September 1, due to the large rainfall events on August 27 and
28. However, discharge to the infiltration basin ended at 21:30 CDT
(9:30 PM) on September 1. During the September 2-17 period, no wet pond
discharges occurred to either the infiltration basin or Sumner Creek.
Four small rain events during this period, ranging from 0.02-0.44 inch
and totaling 0.65 inch, were captured in the wet pond, where the water
infiltrated or evaporated from the pond. As indicated by the nearly
identical temperatures at Sites 5P and 5IB, the Sterling Ponds wet
detention pond discharged to the infiltration basin during the entire
September 18-30 period (Figures
37 and
38), due to eight rainfall events
ranging from 0.02-1.64 inches and totaling 4.11 inches.
A lengthy rainfall event
on September 18 resulted in the second-highest daily rainfall amount
(1.64 inches) recorded during the summer 2007 monitoring season.
Rainfall began at 07:00 CDT (7:00 AM) and continued until midnight. The
comparative Sterling Ponds and Sumner Creek thermographs for the
September 18 rain event are presented in
Figure 38. As indicated by the
nearly identical temperatures at Sites 5P and 5IB, the Sterling Ponds
wet detention pond began discharging to the infiltration basin at 13:30
CDT (1:30 PM) on September 18, more than six hours after the onset of
rainfall. During the September 18 rain event, no wet pond discharges to
Sumner Creek were evident, as documented by the temperature data at Site
5MHW, and no thermal spikes were apparent in Sumner Creek at Site 4.
Late in the evening, a very small thermal spike (0.7° C) occurred in
lower Sumner Creek at Site 4A. This thermal spike cannot be attributed
to a storm water discharge at Sterling Ponds, and seemed to have a more
local cause, perhaps warm water released from natural wetland or storage
areas in the upstream Sumner Creek drainage way.
On September 20, a
thunderstorm with brief but intense rainfall (1.19 inches) occurred from
18:00-20:00 CDT (6:00-8:00 PM). The comparative Sterling Ponds and
Sumner Creek thermographs for this rain event are presented in
Figure
38. With the Sterling Ponds wet detention pond already discharging to
the infiltration basin due to the September 18 rain event, a brief wet
pond discharge to Sumner Creek occurred due to the September 20 rain
event. As indicated by the nearly identical temperatures at Sites 5P
and 5MHW, the Sterling Ponds wet detention pond began discharging to the
Sumner Creek drainage way at 19:00 CDT (7:00 PM) and continued
discharging until 24:00 CDT (midnight). During this 5-hour period, the
wet pond discharge temperature averaged 17.6º C and ranged from
17.5-17.8º C. Some storage of this storm water discharge likely
occurred in the wetland that comprises the creek drainage way upstream
from Site 4. A very small but rapid temperature increase (0.6º C) was
apparent downstream at Site 4 in Sumner Creek by 01:00 CDT (1:00 AM) on
September 21, and is likely due to the release of warmer water
(including storm water) from the upstream wetland. In spite of this
warm water release, the temperature of Sumner Creek never exceeded 20º C
at Site 4 on September 20 or 21. The small thermal spike (1.6º C)
evident near the mouth of Sumner Creek (Site 4A) by 22:30 CDT (10:30 PM)
on September 20 cannot be attributed to the Sterling Ponds storm water
discharge, since the spike at Site 4A, located 1.5 miles downstream,
occurred shortly after the storm water discharge began, but well before
the thermal spike was evident at Site 4. It seems apparent that the
thermal spike at Site 4A had a more “local” cause, perhaps including
storm water runoff from WI Highway 35 and/or warm water flowing from
natural wetland or storage areas in the upstream Sumner Creek drainage
way.
Sterling Ponds wet pond
discharge to the infiltration basin, due to the large September 18 and
20 rainfall events and five smaller events (ranging from 0.02-0.62 inch
and totaling 1.24 inches) during the September 24-30 period, continued
through the end of September (Figure 38). During this time period, no
wet pond discharges to Sumner Creek were evident, as documented by the
temperature data at Site 5MHW, and no thermal spikes were apparent in
Sumner Creek at Sites 4 and 4A. With the exception of the brief
(5-hour) wet pond discharge to Sumner Creek on September 20, all of the
September rainfall was captured in the Sterling Ponds wet pond or
infiltrated.
Effectiveness of Sterling Ponds Storm Water
Management Practices:
During the May-September
(summer) 2007 period, the extent of storm water discharge to the
Sterling Ponds infiltration basin could be readily determined, as
temperature monitoring of the basin (Site 5IB) was conducted throughout
the summer. The extent of storm water discharge to Sumner Creek could
be directly determined via temperature monitoring at the wet pond outlet
(Site 5MHW) and/or indirectly determined by the presence of thermal
spikes in Sumner Creek (Sites 4 and 4A).
With the exception of
the three large rain events on August 27 (1.72 inches), August 28 (1.04
inches), and September 20 (1.19 inches), all summer (May-September)
rainfall events were fully infiltrated, as required by the River Falls
Storm Water Management Ordinance. 56 rain events, ranging in magnitude
from 0.01-1.64 inches, represent a total of 13.68 inches of
precipitation, or 78% of the total summer rainfall amount (17.63
inches). Of these 56 rain events, 29 events, ranging in magnitude from
0.01-1.04 inches and totaling 5.65 inches of precipitation (32% of the
total summer rainfall amount) were entirely stored in the Sterling Ponds
wet detention pond. With dry conditions and lengthy periods between
rain events in May, June, and July, the wet pond readily stored smaller
rain events, with the storm water infiltrating in the pond or
evaporating. The 27 remaining rain events, ranging in magnitude from
0.01-1.64 inches and totaling 8.03 inches of precipitation (46% of the
total summer rainfall amount), were diverted into the Sterling Ponds
infiltration basin.
All 33 rainfall events
in May, June, and July were stored in the wet detention pond or diverted
to the infiltration basin. These events ranged from 0.01-0.94 inch in
magnitude and represented monthly totals of 3.10, 1.51, and 1.33 inches,
respectively, or 34% of the total summer rainfall amount. Twelve
small-to-large rain events in August, ranging from 0.01-1.15 inches and
totaling 4.17 inches, were either infiltrated or stored in the wet
detention pond. These August rain events represented 24% of the total
summer rainfall. Eleven small-to-large rain events in September,
ranging from 0.02-1.64 inches and totaling 3.57 inches, were largely
infiltrated, with some storage in the wet detention pond. These
September rain events represented 20% of the total summer rainfall.
The Sterling Ponds wet
detention pond only discharged to Sumner Creek during the large,
back-to-back rain events on August 27 (1.72 inches) and August 28 (1.04
inches), and during the large, intense rain event on September 20 (1.19
inches). These discharges of storm water to Sumner Creek were directly
measured at Site 5MHW during all three events, and indirectly measured
as thermal spikes at Site 4 after the August 27 and September 20 events.
The August 27 rain event was the largest of the summer season, clearly
exceeding the River Falls Storm Water Management Ordinance requirement
for infiltration of a 1.5-inch, 24-hour rainfall. Given the magnitude
of this event, some storm water discharge to Sumner Creek might be
expected. Rainfall amounts on August 28 and September 20 were less than
the 1.5-inch ordinance requirement. However, the very large, antecedent
rain events on August 27 and September 18 (1.64 inches) could be largely
responsible for the August 28 and September 20 storm water discharges to
Sumner Creek. Due to the August 27 and September 18 rain events, the
Sterling Ponds wet detention pond was likely near capacity, and was
already discharging to the infiltration basin when the August 28 and
September 20 rain events began. With a limited ability to deliver more
water to the infiltration basin, the wet pond reached capacity and
discharged the excess water to Sumner Creek. The intensity of the
September 20 rain event (nearly an inch in an hour) also may have been a
factor contributing to the Sumner Creek discharge.
Although the August 27,
August 28, and September 20 rain events resulted in brief discharges of
storm water to Sumner Creek, it seems likely that the majority of storm
water from these three rain events was infiltrated rather than
discharged. On August 27 and 28, the combined duration of the Sumner
Creek discharges was relatively short (7 hours), compared to the
duration of discharge to the Sterling Ponds infiltration basin (138
hours) after rainfall began on August 27. Similarly, the duration of
the storm water discharge to Sumner Creek on September 20 was relatively
short (4 hours), compared to the duration of discharge to the
infiltration basin, which continued through the end of September. Since
the storm water volumes discharged to the infiltration basin and Sumner
Creek were not measured, it is not possible to precisely determine the
amounts of storm water infiltrated versus discharged, nor is it possible
to determine if the first 1.5 inches of the largest summer rain event on
August 27 (1.72 inches) was fully infiltrated, as required by the storm
water ordinance.
Temperature monitoring
of the Sterling Ponds storm water management practices in 2005 and 2006
indicated that warm storm water was discharged from the wet pond to
Sumner Creek during nine rain events with rainfall amounts ranging from
1.38-4.00 inches. Discharge times ranged from 4-14 hours. Rainfall
amounts for six of these rain events (1.63-4.00 inches) were greater
than the 1.5-inch ordinance requirement for infiltration, while rainfall
amounts for three events (1.38-1.49 inches) were less than the 1.5-inch
ordinance requirement.
When rainfall amounts
exceeded the 1.5-inch ordinance requirement, the wet pond began
discharging to the Sumner Creek drainage way shortly after it began
discharging to the infiltration basin, and the warm storm water
discharges likely contributed to pronounced thermal spikes in Sumner
Creek. Given the very warm storm water in the wet detention pond, it is
important to infiltrate as much pond volume as possible, thereby
minimizing warm water discharges to Sumner Creek. At a minimum, it is
especially desirable to capture the “first-flush” component of storm
water runoff, which generally conveys the greatest thermal impact and
highest concentrations of pollutants.
During the summer of
2006, rather lengthy infiltration times (1.5-8.5 days) were evident for
a variety of rainfall events (0.33-2.26 inches). An extended
infiltration time may be desirable when there is adequate time between
rain events, as it also maximizes total suspended solids (TSS) and total
phosphorus (TP) removal in the wet pond. However, it certainly limits
the available storage volume in the wet pond when the next rain event
occurs, possibly causing a premature discharge of storm water to the
creek drainage way. In 2005 and 2006, this was particularly true for
larger, back-to-back rainfall events that occurred within a 24-48 hour
period. When daily rainfall amounts exceeded one inch during these
back-to-back events, wet pond discharge to the infiltration basin was
already underway due to the first rain event, but was not yet complete
when the second rain event began. Since infiltration of the first rain
event was not yet complete, storage capacity in the wet pond was also
limited.
In early 2007, River
Falls Engineering Department staff conducted modeling of the Sterling
Ponds storm water management practices, to further investigate
performance issues and determine if any corrective action is necessary.
Modeling results suggested that the control structure for the wet pond
outlet could be raised by 6 inches and still meet performance
standards. This adjustment would provide more storm water storage in
the wet pond and allow more discharge of storm water volume to the
infiltration basin, without affecting the rate control needed to achieve
the target pollutant removal efficiencies (80%) for TSS and TP. The
modification to the control structure for the wet pond outlet was made
on June 14, 2007, midway through the 2007 monitoring season, but prior
to the six largest rain events (all exceeding one inch, with two
exceeding 1.5 inches) in August and September.
After the modification
was made to the control structure for the Sterling Ponds wet pond outlet
in mid-June, to improve infiltration performance, three rain events in
August and September still delivered warm storm water to Sumner Creek.
The largest rain event
of the summer on August 27 (1.72 inches) exceeded the 1.5-inch ordinance
requirement for infiltration, as did six rain events in 2005 and 2006
that also delivered storm water to Sumner Creek. During the 2005 and
2006 rain events, the Sterling Ponds wet pond released storm water to
Sumner Creek shortly after the onset of discharge to the infiltration
basin, with lag times as short as 10 minutes. Storm water discharges to
the creek also occurred for extended time periods ranging from 4-14
hours. In contrast, the August 27, 2007 rain event produced a longer
lag time (1 hour) and a relatively short discharge time (4 hours).
Based upon this single 2007 rain event, it seems that the modification
to the wet pond outlet structure may have provided more storm water
infiltration, including early in the rain event, when first-flush
temperature and water quality impacts are more significant.
Rainfall amounts during
the August 28 (1.04 inches) and September 20 (1.19 inches) rain events
were less than the 1.5-inch ordinance requirement, yet both events
delivered warm storm water to Sumner Creek. These discharges are
clearly due to the large, antecedent rain events that occurred on August
27 (1.72 inches) and September 18 (1.64 inches). A 21-hour period
separated the August 27 and August 28 rain events, while a 42-hour
period separated the September 18 and September 20 events. After the
first rain events occurred on August 27 and September 18, the Sterling
Ponds wet pond was still discharging to the infiltration basin when the
next events occurred on August 28 and September 20. With infiltration
of the first events still in progress, the wet pond had a reduced
capacity to store the next events, resulting in the discharge of excess
storm water to Sumner Creek. During the August 28 rain event, a time
lag of 2.5 hours occurred between the onset of wet pond discharge to the
infiltration basin and the onset of discharge to Sumner Creek. A time
lag of 1 hour was evident during the September 20 event. Durations of
discharge to Sumner Creek during the August 28 and September 20 rain
events were 3 hours and 5 hours, respectively. As was observed for the
August 27 rain event, the longer lag times and shorter discharge times
for the August 28 and September 20 rain events tend to indicate that the
modification to the wet pond outlet structure may have provided more
storm water infiltration on both the front ends (due to longer lag
times) and back ends (due to shorter discharge times) of these events.
Summary
Temperature monitoring
of the Sterling Ponds storm water practices during the 2005-2007 period
indicates that storm water discharges to Sumner Creek are common during
rain events larger than 1.5 inches, and during back-to-back rain events,
when rainfall amounts exceed one inch and time periods between rain
events are less than 48 hours. Modifications made to the control
structure for the Sterling Ponds wet pond outlet seemed to improve
infiltration capacity for these types of events in 2007. Rain events
larger than 1.5 inches exceed the intent of the River Falls Storm Water
Management Ordinance, so storm water discharges to Sumner Creek might be
expected.
Storm water discharges to
Sumner Creek during back-to-back rain events, when rainfall amounts are
less than the 1.5-inch ordinance requirement, may need further
attention. Climatological data should be analyzed to determine the
frequency of back-to-back events of this nature. Modifications to the
storm water management ordinance should be made accordingly.
While this project is
primarily focused on evaluating long-term trends, annual information is
important as well. The storm water management practices at Sterling
Ponds prevented thermal impacts on the Kinnickinnic River during the
May-September (summer) 2007 period. The following should be noted:
