Almost 6 million Texans, or about 20% of the population, live in an area susceptible to flooding and one-fifth of the state’s land is in a 100-year floodplain, according to first-of-its-kind data gathered as part of a statewide effort to harden Texas against floods and rising sea levels. The analysis is part of the Texas Water Development Board’s first statewide flood plan, still in development, which the Legislature required in a 2019 law passed in response to Hurricane Harvey.