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1 Storm Sewer Report - Folen • 2 *the engineer's computer modeling indicates St. Anthony's storm 3 sewer system was designed to handle the flow from a storm which has a 4 50% chance of occurring in a given year without adversely affecting 5 residential property (also referred to as a 2 year storm) ; 6 *the focal point of the study turned out to be the exceptions 7 on Murray and St. Anthony Boulevard where there are two or three 8 homes which catch the overflow from low spots in the alley between 9 those streets and problems on Wilson Street N.E. where the storm 10 sewer lines have a slightly lower capacity than some of the upstream 11 and lower lines; 12 *the other more significant spot of flooding during last 13 summer's 100 or 500 year rainfall, was at the intersection of 29th 14 Avenue N.E. and Silver Lake Road which affected access but the 15 surface water didn't rise high enough to damage any homes; 16 *the general cause of surface water flooding in St. Anthony is 17 because the City has a steep rise in the northern watershed and 18 shallower slopes that don't drain water rapidly in the south; 19 *it's technically possible, but not economically feasible or 20 reasonable to try to protect communities from every type of rainfall; • 21 *the City has to make economic choices based on the damage 22 caused by surface water flooding. 23 Mr. Folen described the sizes of the pipes in various parts of the 24 City, saying the outlet for the whole City is in Highway 88 where the 25 City's storm water flows down to the Minneapolis storm sewer conduit 26 and eventually into the Mississippi River. The constraint on that 27 outlet prevents increasing St. Anthony's line size. He pointed out, 28 however, that if it were possible to redesign St. Anthony's system to 29 provide for a five year storm (which has a 20% chance of happening in 30 any one year) , it would cost about $2 .5 million. To protect from a 31 ten year storm with a 10% chance of happening in any year, would cost 32 the City about $3.8 million. And, even if St. Anthony residents were 33 willing to pay those costs, the Minneapolis outlet couldn't handle 34 the flow anyway. The outlet along Highway 88 is designed to accept 35 only the capacity which goes into it now and if the flow were 36 increased, it would flood the shopping centers. Bigger pipes would 37 only mean the City would be transferring the flooding down to the 38 southern sections faster. 39 Ponding in the St Anthony Center Would Require an Acre of Land and 9 40 Foot Pond Depth 41 The areas behind the homes on Silver Lake Road between 30th and 31st 42 Avenues and another east of Silver Point Park wouldn't be feasible • 12