For many years, Alexandria's Arlandria section was, paradoxically, both depressing and useful for the city and some of its residents. It was one of the most deprived neighborhoods, marked by failing businesses and regular flooding along Four Mile Run. But for the poor -- blacks, whites and refugees from many countries -- these same conditions meant low, affordable rents close to Washington.

Now that a federally funded flood project has reduced the flooding risk, Arlandria will be useful and depressing in a new sense. Assessments have soared and developers have already begun to rebuild the area. That's progress, but it is likely to price many poor residents out of their homes.

A group of developers is prepared to buy more than 1,000 low-to moderate-income apartments in Arlandria and turn them into more luxurious and costly residences. There is no place in Alexandria for a large number of displaced renters.

What can be done to prevent at least some of those residents from being displaced and to find affordable apartments close to the District for those who must move? New applications for subsidized housing, the fastest growing federal subsidy program during the Carter administration, have been cut sharply in the past five years. Section 8 housing construction programs have dried up. Meanwhile, the quality of the existing housing stock is declining, and an estimated 500,000 low-rent units have been removed from that stock through conversions, demolitions and expensive renovations.

In Alexandria, officials say the best way to avoid complete displacement is to offer tax-exempt financing to developers in exchange for their setting aside 20 percent of purchased housing for low-and moderate-income residents. But even that scheme, in light of the Washington area's high median income, might be too expensive for Arlandria's current residents. The city's ability to offer more tax- exempt bonds -- it has issued $20 million for housing projects in each of the past two years -- could also be curtailed. The tax bill reported out by the House Ways and Means Committee would put new limits on those bonds.

The Reagan administration has proposed the issuance of vouchers to eligible people in need of housing assistance. People would find their own homes under that plan, and federal funds would pay the difference between 30 percent of a family's income and the fair market rent for that area. But that would not speak to the housing supply, the issue in places such as Arlandria. There has to be more thought on how to provide less bleak housing choices to the residents of the whole country's Arlandrias. Where will the poor live?