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In a natural, undeveloped setting, pervious ground cover allows water, including stormwater runoff, to <br />infiltrate the soil. Land development and increased impervious areas alter natural drainage patterns and <br />increase the rate and volume of stormwater runoff. The additional volume of runoff can increase water <br />levels in ponds, lakes, streams, and wetlands, which increases the potential for erosion and flooding. It <br />also causes large, flashy flows in storm sewers, which increases the potential for flooding and property <br />damage. Increased precipitation also results in high water tables and increased groundwater flow to <br />springs, potentially threatening the stability and capacity of downstream structures. <br />Managing the risk of flooding is a focus of the LRRWMO and its cities due to the potential threat to public <br />health and safety, infrastructure, and the environment. In addition to property damage, flooding may <br />cause other impacts that are harder to quantify, including the following: <br />• Flooding of roads making them impassable to emergency vehicles and residents <br />• Shoreline erosion <br />• Destruction or alteration of riparian habitats <br />• Restricted recreational use of waterbodies, trails, and adjacent lands <br />• More strain on budgets and personnel for repairing flood -damaged facilities and controlling <br />public use of facilities during flooding events <br />The Federal Emergency Management Agency (FEMA) has identified areas prone to flooding during <br />100-year flood events to assist cities and residents in managing flood risk. FEMA-mapped floodplains <br />within the LRRWMO are generally limited to areas surrounding lakes, ponds, and streams and may not <br />reflect localized flood risk related to stormwater conveyance systems (see Figure X). <br />During plan development, member cities were polled and did not identify significant flood risk issues for <br />which LRRWMO assistance is requested. Minor local flooding issues (e.g., temporary street flooding) are <br />described in the local water management plans of Andover, Anoka, and Ramsey. <br />While there are few existing flood risk issues, precipitation patterns are trending towards larger, more <br />intense storms (see Section 2.1.2). NOAA's 2013 assessment of climate trends for the Midwest found that <br />precipitation amounts are predicted to increase significantly over what is historically used in floodplain <br />assessments and infrastructure design. Median estimates of mid-21 St century 24-hour precipitation events <br />with a 1% chance of occurring in a given year (i.e., 100-year event) exceed 10 inches, a significant increase <br />over current design values (7.44" 100-year Atlas 14 event). Understanding the hydrologic response of the <br />watershed to large precipitation events is critical to identifying areas of flood risk and evaluating <br />strategies to reduce flood risk or damages. The CAC identified flood risk and water quality impacts (e.g., <br />increased erosion) resulting from increased precipitation as high priority to be addressed in the LRRWMO <br />Plan. <br />3-5 <br />