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7. Flood Plain and Surface Water Management0 CITY OF CHNVHASSEN 7700 Market Boulevard PO Box 147 Chanhassen, MN 55317 Administration Phone: 952.227.1100 Fax: 952.227.1110 Building Inspections Phone: 952.227.1180 Fax: 952.227.1190 Engineering Phone: 952.227.1160 Fax: 952.227.1170 Finance Phone: 952.227.1140 Fax: 952.227.1110 Park & Recreation Phone: 952.227.1120 Fax: 952.227.1110 Recreation Center 2310 Coulter Boulevard Phone: 952.227.1400 Fax: 952.227.1404 Planning & Natural Resources Phone: 952.227.1130 Fax: 952.227.1110 Public Works 7901 Park Place Phone: 952.227.1300 Fax: 952.227.1310 Senior Center Phone: 952.227.1125 Fax: 952.227.1110 Web Site www.ci.chanhassen.mn.us MEMORANDUM TO: Planning Commission FROM: Robert Generous, Senior Planner DATE: April 5, 2011 SUBJ: Surface Water Management and Flood Protection The purpose of this report is to provide information on surface water management and flood protection in the City of Chanhassen. No action is required of the Plan ning Co mmission o n this item. BACKGROUND The City adopted the Floodplain Overlay District on December 15, 1986 to provide for the protection and preservation of water channels and those portions of the adjoining floodplains which are required to carry and discharge a regional flood, and are subject to inundation by regional floods. It is the intent of the district to be applied to those areas which, if left unrestricted, could result in loss of life and property, health and safety hazards, disruption of commerce, utilities and governmental services, extraordinary public expenditures for floodplain protection and relief and impairment of the tax base. On April 15, 1968, the Village of Chanhassen adopts standards for Planned Industrial Districts, Ordinance Number 26. One section of the ordinance requires a grading plan providing analysis of the adequacy of surface drainage and erosion control. In Ordinance Number 33 adopted on February 17, 1969, the Village of Chanhassen adopts platting authority for subdivisions within the community. As part of the design features are requirements for drainage facilities based on review of the City Engineer. As part of Ordinance Number 33A adopted on January 22, 1973, the City adds requirements for erosion control. Soil and water conservation areas for the control of drainage, erosion and siltation are required with Ordinance 33C adopted on September 12, 1979. With Ordinance Number 33D adopted on February 25, 1985, the City expanded on the requirements for storm drainage requiring drainage plans, erosion and sediment control plan and specifying the protection of water bodies. Chanhassen is a Community for Life - Providing for Today and Planning for Tomorrow Planning Commission April 5, 2011 Page 2 On February 12, 1990, the City adopted Ordinance Number 119 providing site plan review standards, including requiring grading and drainage plans. On May 24, 1990, the City adopts Ordinance Number 128 creating standards and requirements for excavating, mining, filling and grading. This ordinance requirespersons engaging in earthwork activities to obtain a permit from the City. The City adopted ordinance Number 132 establishing a surface water management utility on September 24, 1990. With Ordinance Number 178 adopted on October 26, 1992, the City requires development to conform to its Construction Site Erosion and Sediment Control Best Management Practices Handbook. In February 1994, the City adopts the Surface Water Management Plan. On October 24, 1994, the City adopts ordinances requiring all development to comply with the Surface Water Management Plan. In August 2006, the City adopts the Second Generation Surface Water Management Plan. The City is located within four watershed districts: Minnehaha Creek Watershed District, Riley, Purgatory, Bluff Creek Watershed District, Carver County Watershed Management Organization and Lower Minnesota River Watershed District. APPLICABLE REGULATIONS Many of the City's ordinances have be adopted to achieve the goals and policies set forth in the city's surface water management plan in order to preserve, protect and improve the city's surface water resources. Chapter 7, Article II, Building Code Chapter 7, Article III, Excavating, Mining, Filling and Grading Chapter 18, Subdivision, Articles III and IV, Design Standards and Easements, Dedication of Land or Cash Contribution for Public Purpose and Required Improvements Chapter 19, Article VII, Surface Water Management Chapter 20, Article V, Floodplain Overlay District Chapter 20, Article VI, Wetland Protection Chapter 20, Article VII, Shoreland Management District Chapter 20, Article XXIII, Division 2, Performance Standards Chapter 20, Article XXXI, Bluff Creek Overlay District City of Chanhassen Surface Water Management Plan (2006). ht!p://www.ci.chanhassen.mn.us/serv/cip/swmv/introduction.htm Planning Commission April 5, 2011 Page 3 Table 1. Plan Goals and Goal Statements Goal Goal Goal Statement Number 1 Water Quantity Provide at least 100 -year (1% chance) flood protection for all structures. 2 Water Quality Achieve water quality standards in lakes, streams, and wetlands consistent with their designated uses and established classifications. 3 Wetlands Protect and rehabilitate wetlands to maintain or improve their function and value. 4 Erosion and Sediment Minimize soil erosion and sedimentation. Control 5 Financing Establish and maintain funding sources to finance activities of this Plan. Regulatory Maintain primary responsibility for managing water resources at the 6 Responsibility local level but continue coordination and cooperation with other agencies and organizations. Public Education and Provide information and educational resources to improve 7 Information knowledge and promote an active public role in management of water resources. DISCUSSION Surface Water Management The City's surface water management plan (SWMP) was developed in order to preserve, protect and improve the city's surface water resources. The City's SWMP addresses the following issues: Quantity, Quality, Rate Control and Volume. Quantity deals with the total amount of surface water that will be generated in an area. Quality deals with the treatment of surface water to remove pollutants. Rate control deals with the speed at which of the surface water leaves an area. And volume deals with the amount, measured in inches, of storm water runoff over a specified area. According to the 1996 National Water Quality Inventory, stormwater runoff is a leading source of water pollution. Stormwater runoff can harm surface waters such as rivers, lakes, and streams which in turn cause or contribute to water quality standards being exceeded. The National Urban Runoff Program (NURP) was begun to investigate storm water pollution issues and prevention since little had been done to address the issue. The overall goal of NURP was to provide information that would help local decision makers with a rational basis for determining if urban runoff is causing water quality problems and postulating realistic control options and developing water quality management plans. The preliminary results of studies were provided in 1982 by the Environmental Protection Agency (EPA). "EPA conducted a broad analysis of storm water runoff characteristics in its Nationwide Urban Runoff Program between Planning Commission April 5, 2011 Page 4 1979 and 1983. During the 1980's the Agency made several attempts to promulgate regulatory controls for storm water runoff under the statutory framework of the 1972 Clean Water Act. Following enactment of the Water Quality Act of 1987, EPA began development of a more comprehensive regulatory program. During the course of these actions, the use of best management practices (BMPs) in addressing runoff problems was frequently identified, however it was known that additional research on the performance of BMPs was also needed." (Preliminary Data Summary of Urban Storm Water Best Management Practices, United States Environmental Protection Agency, August 1999) Stormwater runoff can change natural hydrologic patterns, accelerate stream flows, destroy aquatic habitats, and elevate pollutant concentrations and loadings. Development substantially increases impervious surfaces thereby increasing runoff from city streets, driveways, parking lots, and sidewalks, on which pollutants from human activities settle. Common pollutants in runoff include pesticides, fertilizers, oils, metals, pathogens, salt, sediment, litter and other debris which are transported via stormwater and discharged - untreated - to water resources through storm sewer systems. The Stormwater Program for municipal separate storm sewer system (MS4s) is designed to reduce the amount of sediment and pollution that enters surface and ground water from storm sewer systems to the maximum extent practicable. Stormwater discharges associated with MS4s are regulated through the use of National Pollutant Discharge Elimination System (NPDES) permits. NPDES permits are legal documents. Through this permit, the owner or operator is required to develop a stormwater pollution prevention program (SWPPP) that incorporates best management practices (BMPs) applicable to their MS4. Many stormwater best management practices (BMPs) aim to reduce runoff by evaporation, infiltration, detention and retention. This approach can help maintain onsite water balance. Infil- tration systems recharge groundwater, filter pollutants out of stormwater and irrigate plants. Detention and retention systems slow or eliminate the release of stormwater, protecting downstream water bodies from erosive flows and can provide an aesthetic amenity as well. Flood Protection The City participates in the National Flood Insurance Program, NFIP. The NFIP offers flood insurance to homeowners, renters, and business owners if their community participates in the NFIP. Participating communities agree to adopt and enforce ordinances that meet or exceed FEMA requirements to reduce the risk of flooding. The regulatory flood protection elevation shall be an elevation no lower than two feet above the elevation of the regional flood (Section 20 -329, Chanhassen City Code). Under the City's surface water management plan, the City requires flood protection greater than that required under the Flood Protection ordinance. The facility design shall provide adequate live storage to provide protection from the design storm, consistent with lowest building elevation standards. Lowest building floor elevation is defined as the lowest slab elevation for a home or building, including basements and crawl spaces. The lowest building floor elevation for Planning Commission April 5, 2011 Page 5 structures adjacent to wetlands and water bodies shall be an elevation three feet above the 100 - year high water level (Section 19 -144 (a) (2), Chanhassen City Code). The City of Chanhassen is located within three maps provided by the National Flood Insurance Program: Flood Insurance Rate Map (FIRM) panels: 270051 0005, 270051 0010 and 270159 0005 GLOSSARY Surface Water Management: BMP's: Best Management Practices. These are structural, non - structural and managerial techniques that are recognized to be the most effective and practical means to prevent and/or reduce point source and non -point source pollution, in order to promote stormwater quality and protection of the environment. A combination of land use, conservation practices, and management techniques, which when applied to a unit of land will result in the opportunity for a reasonable economic return with an acceptable level of water quality or water quantity improvements. Design Storm: A rainfall event of specified size and return frequency that is used to calculate the runoff volume and peak discharge rate to a BMP. Detention: The temporary storage of runoff from rainfall and snowmelt events to control peak discharge rates and provide an opportunity for physical, chemical and biological treatment to occur. Erosion: The wearing away of land surface and soil by the action of natural elements (wind and/or water). Erosion Control: Anything that will keep soil on site and out of streets and storm drains. This may include silt fencing, berms (barriers), properly placed hay bales, grass and swales. Exfiltration: The downward movement of runoff through the bottom of an infiltration BMP into the subsoil. Extended Detention: A storm water design feature that provides for the gradual release of a volume of water (typically 0.25 to 1.0 inches per impervious acre) over a 12 to 48 hour time period. With proper design, the extended detention period allows for an increased settling of pollutants, and can protect channels from frequent flooding or scour. Extended Detention (ED) Ponds: A conventional ED pond temporarily detains a portion of storm water runoff for a period of 12 to 48 hours after a storm using a fixed orifice. Such extended detention allows urban pollutants to settle out. ED ponds can be designed to be "dry" between storm events and thus do not have any permanent standing water or "wet" with a permanent pool of water. An enhanced ED pond is designed to prevent clogging and re- suspension and provides greater flexibility in achieving target detention times. It may be Planning Commission April 5, 2011 Page 6 equipped with plunge pools near the inlet, a micropool at the outlet, and utilize an adjustable reverse - sloped pipe at the ED control device. See also "wet pond" definition for diagram. Extended Detention Wetland: A storm water wetland design alternative in which the total treatment volume is equally split between a shallow marsh and temporary detention of runoff above the marsh. After a storm, the normal pool of the shallow marsh may rise by up to two feet. The extra runoff is stored for up to 24 hours to allow pollutants to settle at, before being released downstream. Filtration Basin: A treatment area designed to treat storm water by a process that physically removes particles from the water. Forebay: An extra storage area provided near an inlet of a pond or BMP to trap incoming sediments, reducing the amount that accumulates in a pond or BMP. Groundwater: Water that flows below the ground surface through saturated soil, glacial deposits or rock. Impervious Surface: The portion of the buildable parcel which has a covering which does not permit water to percolate into the natural soil. Impervious surface shall include, but not be limited to, buildings, all driveways and parking areas (whether paved or not), sidewalks, patios, swimming pools, tennis and basketball courts, covered decks, porches, and other structures. Open, uncovered decks are not considered impervious for the purposes of this ordinance. The use of patio blocks, paver bricks or class 5 gravel material are considered impervious surfaces as a majority of water runs -off the surface rather than being absorbed into natural soils underneath. Some exceptions to these conditions may include paver blocks or pavement systems engineered to be permeable with the underlying soils suitable for infiltration. Infiltration Basin: An impoundment where incoming storm water runoff is stored until it gradually infiltrates into and through the soil of the basin floor. Infiltration Trench: A conventional infiltration trench is a shallow, excavated trench that has been backfilled with stone to create an underground reservoir. Storm water runoff diverted into the trench gradually exfiltrates from the bottom of the trench into the subsoil and eventually into the water table. An enhanced infiltration trench has an extensive pretreatment system to remove sediment and oil. It requires an on -site geotechnical investigation to determine appropriate design and location. Infrastructure: Public facilities and services, including transportation, storm water pipes, structures and ponds, water and sewer pipes and structures, telecommunications, recycling and solid waste disposal, parks and other public spaces, schools, police and fire protection, and health and welfare services. NURP: Nationwide Urban Runoff Program, a study by the U.S. Environmental Protection Agency. A key component of this program was to assess the effectiveness of urban runoff detention /retention basins (e.g., ponds) in removing pollutants from storm water runoff. Planning Commission April 5, 2011 Page 7 Rate Control: Rate control refers to controlling the rate of runoff using structures to simulate pre- construction conditions. Retention: The permanent storage of runoff from rainfall and snowmelt events with volume reduction coming from infiltration, evaporation or emergency release. Runoff: Water from rainfall or snowmelt, which flows across the ground surface into drainage facilities, rivers, streams, ponds, lakes and wetlands or shallow groundwater. Runoff (Storm Water): The overland and near surface flow from storm water and snowmelt. Runoff Conveyance: Methods for safely conveying runoff to a BMP to minimize disruption of the stream network, and promote infiltration or filtering of the runoff. Runoff Pretreatment: Techniques to capture or trap coarse sediments before they enter a BMP to preserve storage volumes or prevent clogging within the BMP. Examples include forebays and micropools for pond BMPs, and plunge pools, grass filter strips and filter fabric for infiltration BMPs. Stormwater: Rainwater, snowmelt runoff or precipitation that accumulates in stormwater storage systems during and immediately following a storm event, that enters the storm drain system and empties into lakes, rivers and streams. Storm Water Treatment: Detention, retention, filtering or infiltration of a given volume of storm water to remove pollutants. Storm Water Treatment Pond: Any water body that has been specifically created to remove sediment and nutrients and "treat" surface water runoff. Storm water ponds that were created from existing wetland are still regulated as jurisdictional wetlands. Storm water ponds created from upland areas are not wetland and are exempt from regulatory jurisdiction. Surface Runoff: The portion of rainfall that moves over the ground toward a lower elevation and does not infiltrate into the soil. Surface Water: Water that remains on the surface of the ground, including rivers, lakes, ponds, reservoirs, streams and wetlands, whether natural or artificial, public or private. Upland: General term to describe any area that is not a wetland. Vegetated Filter Strip: A vegetated section of land designed to accept runoff as overland sheet flow from upstream development. It may adopt any natural vegetated form, from grassy meadow to small forest. The dense vegetative cover facilitates pollutant removal. A filter strip cannot treat high velocity flows; therefore, they have generally been recommended for use in agriculture and low- density development. A filter strip can also be an enhanced natural buffer, whereby the removal capability of the natural buffer is improved through engineering and maintenance activities such as land grading or the installation of a level spreader. A filter strip differs from a grassed swale in that a swale is a concave vegetated conveyance system, whereas a filter strip has a fairly level surface. Planning Commission April 5, 2011 Page 8 Wetland: Transitional land between terrestrial and aquatic systems where the water table is at or near the surface or the land is covered by shallow water. For purposes of the plan, wetlands must have a predominance of hydric soil, be inundated or saturated to the surface or groundwater at a frequency and duration sufficient to support a prevalence of hydrophytic vegetation typically adapted for life in saturated soils; and under normal circumstances supports a prevalence of hydrophytic vegetation. Wet Pond: A conventional wet pond has a permanent pool of water for treating incoming storm water runoff and a live storage component for flood storage and additional water quality treatment detention (see typical cross section) Flood Protection: Flood: A temporary rise in stream flow or stage that results in inundation of the areas adjacent to the channel or water body. Flood Frequency: The average frequency, statistically determined, for which it is expected that a specific flood stage or discharge may be equaled or exceeded. Flood Fringe: That portion of the 100 -year floodplain outside of the floodway. Flood Obstruction: Any dam, well, wharf, embankment, levee, dike, pile, abutment, projection, excavation, channel rectification, culvert, building, wire, fence, stockpile, refuse, fill, structure or matter in, along, across or projecting into any channel, watercourse or regulatory flood hazard area which may impede, retard or change the direction of the flow of water, either in itself or by catching or collecting debris carried by such water, or that is placed where the flow of water, either in itself or by catching or collecting debris carried by such water, or that is placed where the flow of water might carry the same downstream to the damage of life or property. Floodplain: Floodplains are lowland areas adjoining lakes, wetlands, and rivers that are susceptible to inundation of water during a flood. For regulatory purposes, the floodplain is the Planning Commission April 5, 2011 Page 9 area covered by the 100 -year flood and it is usually divided into districts called the floodway and flood fringe. Areas where floodway and flood fringe have not been determined are called approximate study areas or general floodplain. Floodplain means the land adjacent to a body of water which has been or may be hereafter covered by floodwater, including that land covered by the regional flood. The floodplain consists of the floodway and the flood fringe area. (20) Floodplain (General) Area: The general floodplain area is determined using the best available data, in lieu of performing a detailed engineering study. These data may be from soils mapping, experienced high water profiles, aerial photographs of previous floods, or other appropriate sources. There are no associated published 100 -year flood elevations with general floodplain delineations, unlike detailed study areas. General floodplain area is synonymous with approximate study area and unnumbered A -Zone. Floodplain Forest: Wooded area adjacent to stream or river that is periodically flooded. Within this plan, floodplain forests have been specifically identified as a separate wetland category due to their unique ecology and protection needs. Flood Proofing: A combination of structural provisions, changes or adjustments to properties and structures subject to flooding primarily for the reduction or elimination of flood damages to properties, water and sanitary facilities, structures and contents of buildings in a flood hazard area in accordance with the Minnesota State Building Code. Floodway: The floodway is the channel of a river or other watercourse and the adjacent land areas which must remain open in order to discharge the 100 -year flood. Floodway means the minimum channel of a watercourse and those portions of a floodplain adjoining the channel that are reasonably required to discharge the regional flood. (20) Freeboard: A factor of safety usually expressed in feet above a certain flood level. Freeboard compensates for the many unknown factors (e.g., waves, ice, debris, etc.) that may increase flood levels beyond the calculated level. Regional Flood: A flood which is representative of large floods known to have occurred generally in Minnesota and reasonably characteristics of what can be expected to occur on an average frequency in the magnitude of the 100 -year recurrence interval. Regional flood is synonymous with the term "base flood" used in the Flood Insurance Study. Flood, regional means a flood which is representative of large floods known to have occurred generally in the state and reasonably characteristic of what can be expected to occur on an average frequency once every 100 years. (20) ATTACHMENTS 1. Chanhassen Watersheds Map. 2. MN DNR Floodplain Management Series — Residential Uses and Standards in a Floodplain. 3. MN DNR Floodplain Management Series — Basement and Lowest Floors for Homes in a Floodplain. g: \plan \city code\issue paper\floodplain and surface water management.doc ti IIMI J CITY OF CHANHASSEN Subwatershed Boundaries Purgatory Creek Mitchell Lake Rice Marsh Lake Minnehaha Creek Lake Lucy Lake Susan O Lake Riley Lower MN River O Lake Minnewashta Lot- Lake Lake Susan Rice Marsh La .7 Lake Riley i Ali 'I __. C 7 M Rice Lake - .7 7 Chanhassen Watersheds Lake Hazeltine Lake Ann Christmas Lake Bluff Creek Miles 0 0.5 1 Lake Susan Rice Marsh La .7 Lake Riley i Ali 'I __. C 7 M Rice Lake - .7 7 Chanhassen Watersheds Floodplain Information Sheet 3 Floodplain Management Series Residential uses and standards in a floodplain W UP1L fEAtS'<S When property owners discover their property is in a floodplain, a common reaction is to assume they cannot use or develop it. In some parts of the floodplain or in some communities, options for land use are very limited. In other situations, most uses are allowed, but the structures must be elevated or flood - proofed to maintain public safety and minimiz risk of property damage during a flood. Two key questions must be answered that affect what can be done to property in a floodplain: 1. Floodway Or flood fringe? Is the property in a floodplain shown on a Flood Insurance Rate Map (FIRM) prepared by the Federal Emergency Management Agency (FEMA)? If so, is the property in the floodway or the flood fringe? (See "What are the floodmay and the flood fringe" information sheet #2.) Property in a floodway is subject to more use restrictions than property in a flood fringe. 2. Local zoning? What uses are allowed by the local zoning authority (city zoning if your property is within city limits; county zoning if it is in a rural area)? The standards discussed below are based on state and federal minim standards. Your community will have zoning regulations according to zone, including low- density residential, high- density residential, commercial, industrial, and open space. The community can specify the allowable uses in each zone. In addition to the zoning use guidelines, your community may have building standards that are more restrictive than the state and federal minimums. What uses are allowed in the floodway and flood fringe of the floodplain? Floodway uses • Open space uses and limited grading and earth moving may be permitted if they do not create an obstruction or cause any increase in the flood levels. Uses such as gardens, farming, parks, trails, or golf courses may be allowed depending on your zoning district. • New structures, additions to existing structures, and substantial improvements to existing structures are prohibited. • A structure built before the publication date of your community's first FEMA floodplain map (pre -FIRM) is "grand - fathered" with the following conditions: — A structure may continue to be used, repaired, and maintained, but no addition may be constructed. — If a structure is damaged (e.g., by fire, flood, tornado) by more than 50 percent of the pre - damage value, the structure may not be reconstructed. Flood fringe uses • The use must be allowed by the underlying zoning of the community. • New structures and additions to existing structures must be elevated on fill (Figure 1). • New structures and additions, after a conditional use permit is obtained, may use alternative elevation methods, if allowed by the regulations in that community. (See "Conditional Uses in the Floodplain " information sheet #6). • Accessory structures (e.g., garages, sheds) may be constructed if properly elevated on fill or flood - proofed (for smaller, "minimal invest- ment" structures). (See "Residential Accessory structures in the flood fringe" information sheet #5). (continued on back...) Contact: DNR Ecological & Water Resources in St. Paul (651) 259 -5700 Page 1 http: / /miidnr.goN February 2011 Figure 1. Houses may be constructed in the flood fringe but must be properly elevated to maintain public safety and minimize damage risk during a flood. Floodplain Information Sheet 3 What are the key building standards for residential structures in flood fringe? • A structure built before the publication date of your communit}r's first FEMA floodplaui map (pre - FIRM) is "grandfathered" with the following conditions: — A structure mg continue to be used, repaired, and maintained, but additions must be elevated or properly flood - proofed in compliance with current community regulations. — If a structure is damaged (e.g., by fire, flood, tornado) by more than 50 percent of the pre - damage value or if the cumulative additions and improvements are more than 50 percent of the original structure's inflation- adjusted value, the entire structure would have to be reconstructed to comply with the community's floodplain regulations (thus losing "grandfather" status). If possible, build outside the floodplain, with your lowest floor (including basement) above the regulatory flood protection elevation (RFPE). This reduces your risk of flooding, and you will not be required to buy flood insurance. Also, if you build outside the high flood risk zone (the 1- percent chance or 100 -year flood zone), preferred risk policy (PRP) flood insurance is available at a significantly reduced premium. If you must build in the flood fringe, work with your community to ensure that all local regulations are addressed in your proposed building plan. NOTE: Local ordinances may be more restrictive. The key building standards that meet state and federal law (Figure 1) include: • No placement of fill is permitted in the floodway. • Top (i.e., walking surface) of the lowest floor (including basement or crawl space) is at or above the RFPE. • Fill is at 100 -pear flood elevation plus floodway stage increase, or higher, extending at least 15 feet horizontally from all sides of the structure. • An "as built" survey is submitted to the zoning authority to verify that the development was built at the permitted elevation. • Must meet all local ordinance requirements, including setback* requirements (i.e., from lot lines, and for shoreland man- agement or wild and scenic rivers ordinances). Many communities also require that the access (driveway and access roads) elevation is no lower than 2 feet below the RFPE. *Setbacks are typically measured from the ordinary bigb -water (OHW) level, whicb is the top of the bank of the channel on watercourses. Ae OHW is not the sayne as the 100year floodplain elevation, serves a different purpose, and is rtsually at a lower elevation. RFPE Formula 100 year flood elevation • stage increase due to floodway • freeboard (state requires 1 foot minimum) = regulatory flood protection elevation (RFPE) ' >� 15 ft i RFPE ® ® j 1 ► 1 foot ® ® freeboard (minimum) _ 10 0' r i l l Floodplain Management Series Equivalent Terms and Definitions Base Flood Elevation (BFE) — Same as the 1 percent annual chance flood elevation or the 100- year" flood elevation. Regulatory Flood Protection Elevation (RFPE) —The RFPE is the 100 -year flood (1 percent annual chance flood) elevation + stage increase due to establishing floodway + freeboard (Minnesota's minimum is 1 foot). The lowest floor must be elevated so that the walking surface of that lowest floor is at the RFPE or higher. a c hance floo el (in_ludinS sr,:,e increase) 4r-- OHW Natural Soil Typical Neater level l Flood Fringe 0 -- Floodway -0 Figure 2. Key standards for structures in the flood fringe (picture not drawn to scale). Contact: DNR Ecological & Water Resources in St. Paul (651) 259 -5700 http: / /nindnr.gov /waters Page 2 Februdry 2011 Floodplain Information Sheet 4 Floodplain Management Series Basement & Lowest Floors for Homes in a Floodplain Determining the "lowest floor" of a structure in a floodplain is important because the top (i.e., walking surface) of the lowest floor must be at or above the Regulatory Flood Protection Elevation (RFPE). Lowest floors include basements and crawl spaces of any size. An improperly elevated lowest floor can result in a significant increase in flood insurance premiums!! Related Definitions Lowest Floor— Lowest floor of the lowest enclosed area (including basement or crawl space). The lowest floor level must meet minimum elevation requirements (i.e., the Regulatory Flood Protection Elevation). Basement —Any area of a structure, including crawl space, having its floor subgrade (below ground level) on all sides. RF luwrbt upwtevel Adjacent Girad: �. f�rel Top of lowvvi level floor is lowest flaar Figure 1. A concrete slab on grade is permitted for a home in a floodplain if the top (walking surface) of the lowest level floor is above the RFPE. Regulatory Flood Protection Elevation (RFPE) — The RFPE is the 100 -year flood (1 percent annual chance flood) elevation + stage increase due to es- tablishing floodway + freeboard (Minnesota's mini- mum is 1 foot). The lowest floor must be elevated so that the walking surface of that lowest floor is at the RFPE or higher. Base Flood Elevation (BFE) —Same as the 1 percent annual chance flood elevation or the "100- year" flood elevation. The floodplain regulations apply to all enclosed spaces; they RFPE Formula do not differentiate between finished versus unfinished, habit- 100 -year flood elevation + stage increase due to floodway able versus not habitable, or a space that is more than a certain + freeboard (state requires 1 -foot minimum) minimum height versus a space that is only a couple feet high. = regulatory flood protection elevation (RFPE) i.rseargat Mangy 6aage RFPE t yJ Tops of bewment fbor is kX%V t floor Figure 2. This example is not permitted for a home in a floodplain because the basement is the lowest level of the home and it is below the RFPE. Lowest twain RFPE Level T. IN Top of crawl space floor is lowest fkxn Figure 3. A below grade crawl space or utility space is considered a basement and it is the lowest floor. It is not permitted for a home in a floodplain because it is below the RFPE. Myths and Faces of Basement Definitions Myth: It is not a basement if I cannot stand up in it. Fact: It does not matter if a basement is 2 feet or b feet high, or if it is "habitable" based on state building code. It is a basement if it is below grade on all skies. Myth: It is not a basement if the floor is unfinished or dirt. Fact: It does not matter if a basement floor is concrete or unfinished. It is a basement if it is below grade on all sides. Contact: DNR Waters in St. Paul (651) 259 -5700 Page 1 http: / /mndnr.gov /waters August 2010 Floodplain Information Sheet 4 Floodplain Manage Se 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 The typical Minnesota walkout basement is not a "basement" according to FEMA (Figure 4). FEMA's basement definition says all sides are subgrade (below ground level), but the typical walkout basement has one complete side (the walkout side) with the floor at the same level as the adjoining ground level. If the walkout basement level is above the regulatory flood protection elevation (RFPE), the local community may issue a standard build- ing permit. A related but prohibited layout is shown in Figure 5. Occasionally, a land- owner will excavate an area adjacent to one side of the basement floor. However, unless there is gravity flow drainage over the ground surface toward the flooding source, this is still considered a basement. 2. Natural ground is above 1% annual chance flood elevation 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 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 severe flood damage from subsurface water pressure against the basement walls and standard basement construction is not recommended. Lowest Adjacent Grade Main � RFPE _ l3-m FE en :1!1!1. B �� "' Lake Figure 4. A walkout basement or lower level is permitted for a home in a floodplain if the top of the lowest floor is above the RFPE. trrlrft Lowest Excwated Grade RFPE —lam Figure 6. LowestAdjacent Grade is above Base Flood Elevation (BFE) or the 1 % annual chance flood elevation, so site is not in Zone A orAE. Minimum floodplain regulations do not apply. NOTE: The community may have higher standards. For example, if the site is in shoreland district, there are still minimum elevations. Tap d b.-t h-11bW it . fkw Figure 7. Same situation as in Figure 6, but they excavated to make a walkout basement. They have put themselves into the high flood risk area (Zone A or AE) and must meet the floodplain regulations, so now have a violation. 3. Dry - floodproofed basements & "internally flooded enclosures below the lowest floor" Federal regulations and state law give communities the authority to issue conditional use permits for a few floodproofing options for the lowest floor if: (a) the community's floodplain management ordinance includes those options, (b) the space will be used for a limited number of specific uses (usually only for parking, limited storage, and access to up- per levels), (c) the floodproofing proposal meets all the other limitations for that floodproofing option (i.e., dry floodproofing is allowed for non - residential structures only, unless the community is one of the eight Minnesota communities with a formal exception from FEMA for dry floodproofing of residential basements), and (d) specific technical standards and legal requirements are met. see the floodplain information sheet "Conditional Uses in the Floodplain "for more information. Contact: DNR Waters in St. Paul (651) 259 -5700 Page 2 http: / /mndnr.gov /waters August 2010 Top of lower level flow Is bereft krd Figure 5. Excavating an area adjacent to a basement does not stop it from being a basement! There must be gravity flow drainage away from that lowest side.