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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 Page / of 2 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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