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. � Floodplain Information Sheet 4 Floodplain Management Series Basements & Lowest Floor Special Cases What is called a"basement"in common language usage in Minnesota is not always consistent with FEMA's definition of a"basement." In addition,there are other special situations to consider. 1. Walkout basement Lowert The typical Minnesota walkout basement is not a"basement"according to qdjacent UpperLevel s��age FENI� (Figure 4). FEMr1's basement definirion says all sides are subgrade Grde (below ground level),but the typical walkout basement has one complete �ower�e�et � ��ill side (the walkout side)with the floor at the same level as the adjouung RFPE ground level. If the walkout basement level is above the regulatory flood Nacura�soii '' protection elevauon (RF'PE),the local community may issue a standard build- rop ot�ower�e�e�noo��5 iowesc ie�ei ing permit. Figure 4.A walkout basement or lower level is permitted for a home in a floodplain if the top of A related but prohibited lay�out is shown in Figure S. Occasionally,a land- the lowest floor is above the RFPE. owner will excavate an area adjacent to one side of the basement floor. �o�n However,unless there is gravity flow drainage over the ground surface AdjacM towaxd the floodin source this is still considered a basement. Grde g � �;��._.. �.,� BfE Fi0 RFPE 2. Natural ground is above 1% annual chance flood elevation �'^e � If no fill is brought in,and the lowest adjacent grade (lowest place where � ��+� soil touches building) is above the 1%annual chance flood elevation,the 1oc or e��:roo��ww�:tr��, structure is officially out of the high-risk flood zone where floodplain man agement regulations apply(Figure 6).This structure may still be subject to Figure 5. Excavating an area adjacent to a basement severe flood damage from subsurface water pressure against the basement does not stop it from being a basement! There must be gravity flow drainage away from that lowest side. walls and standard basement construction is not recommended. Lowest lowen AdjaCeM Adjxerrt Excavated Grade Gfad2 P�;turalSoil Ma,,,;x,.�. srag Main Garage RFPE RFPE � — �� .. ' � � i �emeo� � � BF� � . . � SlE rr�r � r�r Natcual Soil Naturat So�l � Lake Top of bwest kvel flaor iz bwestfioor Figure 6. Lowest Adjacent Grade is above Base Flood Elevation Figure 7. Same situation as in Figure 6,but they excavated to make a (BFE)or the 1%annual chance flood elevation,so site is not in Zone walkout basement. They have put themselves into the high flood risk A orAE. Minimum floodplain regulations do not apply. area(Zone A orAE)and must meet the floodp/ain regulations,so now NOTE:The community may have higher standards.For example,if have a violation. the site is in shoreland district,there are still minimum e/evations. 3. Dry-floodproofed basements & "internally flooded enclosures below the lowest floor" Federal regulauons and state law give communities the authorit�•to issue conditional use permits for a few floodproofing options for the lowest floor iE: (a) the community's floodplain management ordinance includes those options, (b) the space will be used for a limited number of specific uses (usually onlj�for parking,limited stoxage,and access to up- per levels), (c) the floodpxoofing proposal meets all the other limitations for that floodproofing option (i.e.,dry floodproofing is allowed for non-residential structures only,unless the communiry is one of the eight Minnesota communities with a formal exception from FEM�1 fox dxy floodproofing of residential basements), and (d) specific technical standards and legal requirements are met. See the floodplain infornzation sheet"Conditional Uses in the Floodp/�sin"for more information. �Contact: DNR Waters in St.Paul(651)259-5700 � �� Page 2 � http://mndnr.gov/waters August 2010