HomeMy WebLinkAbout1991-12-04 Barr Engineerig Memo To VBWD (As Received 08-19-99)BARR ENGINEERING CO.
SUBJECT: Effect of New Stillwater High School and Ultimate Development on Cloverdale and McDonald
Lake Water Levels end the Rate of Runoff to Cloverdale Lake
.... DATE. December 4, 1991
Th purpose of emorandum is to summarize the results of analyses regarding the possible impact
of develop e Cloverdale Lake watershed. At their November 14, 1991 meeting, the Managers directed
that the following concerns be studied:
1. The effect of develops pent on the normal and flood levels of Cloverdale Lake and McDonald
Lake.
2. The effect of development on the rate of runoff to Cloverdale Lake.
The two analysis results are discussed separately in the following pages.
1. Effect on water Levels
The Managers previously adopted an approximate method for determining the loo -year flood level for
landlocked basins such as Cloverdale and McDonald Lakes. Because there is no surface outlet, runoff collecting
in these depressions is removed only by seepage and evaporation. Therefore, a detailed flood level analysis
should include the effects of seepage and evaporation. An analysis of this type can be very complex.
To determine approximate flood levels for lakes such as Cloverdale and McDonald, a simpler method
of analysis was devised. with this method, the approximate 100 -year flood level and change in normal level'
can be determined using the annual runoff volumes shown in the following table (which is also in the District's
Rules and Regulations ):
100 -year
Average
Net I00-
Annual
Annual
Year
Runoff
Runoff
Annual
Volume,
Volume,
Volume,
Land Use
Inches
Inches
Inches
Impervious
40
10
30
Turf6d
18
8
10
Water Surface
12
-6
18
For Cloverdale and McDonald Lakes, the average year's runoff volume is assumed to be dissipated by
the average seepage and evaporation from the lakes and the normal water level is a reflection of the average
runoff. The additional runoff for a year with above normal precipitation is assumed to be stored in Cloverdale
and McDonald Lakes above their normal level. The normal water levels for these lakes are assumed to be the
water levels shown on the most recent LUSCS quadrangle map of the area. The Managers previously set
100 -year flood levels for Cloverdale and McDonald Lakes by assuming ultimate land use conditions. Concerns
have been raised that land use changes in the lakes' watershed may affect not only flood levels, but also affect
normal water levels. The approximate method outlined above was used to assess the impacts of land use
changes on the water levels of McDonald and Cloverdale Lakes. The results are shown in Table 1,
2. Effect on Runoff Rate
Concern has been expressed regarding how fast stormwater runoff will reach Cloverdale Lake as a result
of the Stillwater High School and other development. To date, the best available information concerning the
effect of development on stormwater discharge rates is from the preliminary stormwater analysis performed by
the City of Oak Park Heights' engineers, In this analysis, peak flow rates and times were estimated for the
100 -year, -hour storm for stormwater leaving the School District's southern property boundary. This analysis
was performed for three different scenarios, as follows:
Existing conditions.
For this condition, the peak flow rates and times were estimated by the Watershed District
engineer. The discharge point is the drainage swale on the west end of the School District's
southern property boundary.
® Interim conditions.
Under these conditions, the school site is developed and the northwest stormwater pond and the
south pond are in place. The discharge point is the south pond outlet pipe (close to the school's
south property boundary).
Ultimate conditions.
Under these conditions, the property north of the school site and the school property are fully
developed, and the pond north of the school property, the northwest pond and the south pond are
in place. The discharge point is the south pond outlet pipe (close to the south property
boundary).
The results are summarized in Table 2.
These results show the importance of providing stormwater storage in developing watersheds. The
impacts of the ponding will be to delay the time of the peak discharge and to reduce the peak discharge during
storm events.
A- .\WATERLEV.WP AKMH 2
1. l eis
Reduced 1 foot from 100-year flood level shown in District's water Management Plan as a result of
removing the Kern Center drainage area from the watershed.
* * Estimated overflow elevation from USGS quadrangle map - requires field verification.
Ao \wATERLEV.wP 1 MH 3
LAKE LEVELS
(FT MSQ
CLOVERDALE LAKE
IVMcDONALD LAKE
AVERAGE
100 -YEAR
AVERAGE
100 -YEAP.
LAND USE CONDITION
Existing Conditions (including
899.0
908 **
885
595.5
Blackwoods Development)
Interim Conditions - Addition of
899.1
908 **
886
896.1
Stillwater High School
Development
Future Conditions (Ultimate
899.3
908 **
886
898.5*
Planned Land Use)
Reduced 1 foot from 100-year flood level shown in District's water Management Plan as a result of
removing the Kern Center drainage area from the watershed.
* * Estimated overflow elevation from USGS quadrangle map - requires field verification.
Ao \wATERLEV.wP 1 MH 3
TABLE 2
PEAK DISCHARGE RATES AND 'TIMES
PEAK TIME
(HRS FROM BEG Il�MNG OF
PEAK DISCHARGE (CFS) STORM)
Existing Conditions 95 13.1 hours
Interim Conditions 10 18.7 hours
Ultimate Conditions 10 24.4 hours
A:\W ATER.LEV.WP 1\KMH