4663 Beacon Hill Rd - Memo re: DrainageMemorandum
To: Russ MATTHYS
CHAD DAVISON
PROJECT: GENERAL
FROM: PHILLIP ELKIN, PE CLIENT: CITY OF EAGAN
DATE: 9 -8 -2010
RE: 4663 BEACON HILL ROAD DRAINAGE ISSUE FILENO: 49 -10000
Russ,
Here is a follow -up to my initial findings on the Beacon Hill Road drainage memo.
I went back to the 2001 improvement projects file and found the XP SWMM model that was
constructed after the 2000 rainfall event. This model determined the peak elevations for the 1%
storm event:
Beacon Hill Beacon Hill Ridge Cliff
Prior to 2001 Improvements Backyard Street (est.) intersection
1% Event 932.50 932.13 942.00
10% Event 931.81 931.13 939.04
50% Event 928.07 927.67 935.71
8/13/10 Event 932.52 932.15 942.20
I have updated this model to show the impact of the 2001 drainage improvements using the
overflow areas created. I have also included a simulation of the August 2010 storm event.
Beacon Hill Beacon Hill Ridge Cliff
Post 2001 Improvements Backyard Street intersection
1% Event 932.41 932.09 940.91
10% Event 929.30 928.35 938.50
50% Event 927.47 926.87 934.96
8/13/10 Event 932.36 931.92 940.30
As you can see the model shows little impact to the HWL at the backyard catch basin. The drop
in HWL at the Ridge Cliff intersection is attributed to the removal of the Johnny Cake Ridge
watershed.
Based on this analysis, it appears that the August storm was relatively equivalent to a 1% storm
event; however water marks on the fence and house indicate that the water was much higher in
the backyard (estimated at 934.40). One explanation could be that the side yard fence
prevented the backyard from draining and allowed the water to rise and enter the home at 4663
Beacon Hill Road. To check this we ran the storm model without allowing overland flow from the
side yard. While the elevation rose to 933.64, enough to enter the walkout, it did not replicate
the 934.40 mark. This could indicate that intensity of the August 13 storm event had a greater
intensity than what was modeled and thus was a greater storm than the 1% event.
2010 Survey
As part of this analysis the City conducted an area survey to determine the elevations and
capacity of the side yard swales. This survey revealed two issues of concern: That there is only a
0.40 foot elevation difference between the low point of the swale and the patio walkout and that
the high point of the swale is approximately 0.5 feet higher than previously identified (933.00 in
2010 vs. 932.5 in 2001).
Bonestroo
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2335 Highway 36 W
St. Paul, MN 55113
Tel 651- 636 -4600
Fax 651- 636 -1311
www.bonestroo.com
1% Event
10% Event
50% Event
8/13/10 Event
With Blocked EOF
Post 2010 Survey
Beacon Hill Beacon Hill Ridge Cliff
Backyard Street intersection
933.13 932.09 940.91
929.30 928.35 938.5
927.47 926.87 934.96
932.36 931.92 940.304
933.96
Based on this information, my opinion of probable flooding causes are:
The Fence
Based on photographs taken after the storm, the fence on the affected property
prevented the drainage swale from working properly. Not only is this fence on the side
slope of the swale, but there is also a section of fence which completely blocks the
swale. In addition to its location, the tight construction is such that it allows very little if
any runoff to pass through. I was surprised to see the catch basin actually located on
the adjacent lot on the west side of the fence. It appears that the overflow from the CB
traveled west through back property line fence (which has spaces between the slats)
and entered the backyard of 4663 Beacon Hill Road. Based on the water levels inside
the fence on this property, it is apparent that very little water was able to move through
the drainage easement swale. Not only did the fence block drainage but backed up
water contributed to the HWL level.
The intensity of the rainfall event
Technically speaking, 4.5 inches of rainfall in four hours is classified as a storm event
with a 1% chance of recurrence, however after modeling this storm event with a
blocked overflow swale, the peak elevation within the back yard was 933.64. While this
elevation is above the walkout elevation (933.40), it is still lower than the water marks
on the fence and flower pot on the patio. This leads me to believe that while the storm
lasted 4 hours, the intensity of the rainfall was much higher dumping most of the rain in
1 or 2 hours. This intensity would classify this storm event as less probable than a 1%
event.
Recommended Solutions
• Remove the fence- It is a difficult enough area to get to drain without an
unnecessary obstruction making things worse. If the homeowners object, a fence
with openings would be a preferable option.
• Build a berm /retaining wall to protect the walk out door- A 1 -2 foot retaining wall
around the patio may be the best option to prevent future storms from flooding the
structure due to the swale backing up. Intense storm events in Eagan are
unpredictable and seem to be occurring with greater frequency and there is very
little grade to work with, therefore flood proofing the house is the best defense.
• Re -grade the swale —Based on the latest survey there appears to be enough
elevation difference to get a 1.4% slope to the street. This option is probably the
least cost effective because of the amount of disruption and the relatively little
elevation difference (<1 foot) to work with.
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