100-year floods more frequent, say development foes 11/24/99
    A proposed apartment and office complex on the Shawsheen River has prompted residents to call for redefinition of the area's flood plain, saying that current guidelines don't accurately reflect how much the Shawsheen watershed floods.
      "We believe these numbers are dated," says Patrick Robbins, who spoke against the development of the Powder Mill Square project at a recent Conservation Commission meeting. Robbins and neighbors of the proposed project say if the guidelines for flooding in the area used by engineers of the project and town officials aren't updated, they will allow a project to be built that would create more flooding.
      "We believe there will be an increase in impervious cover, which causes an increase in flooding," says Bob Rauseo, president of the Shawsheen River Watershed Association.
      The project must be designed to accommodate flood waters up to a "100 year-flood," or the amount of flooding from a storm likely to occur once every 100 years.
      As proposed, the office buildlng would be built over the river, and the bottom floor of its  parking  garage would be designed to hold flood water during such a storm.
      Residents  say guidelines that define the 100 year-flood aren't  accurate because the amount of impervious  surface around the watershed in surrounding towns has increased since the maps were produced 10 years ago. Since there is less unpaved ground along the river to absorb water,more  water flows into the river during heavy storms, flooding areas down stream, residents say.
      Linda Cleary, Andover conservation agent, says development could be a factor in how much the area floods.
      "It just makes sense that  development upstream in the flood plain  area  could change what happens downstream,"   she says.
        But to analyze how much  things  have changed would be a major  undertaking, river experts say.
        The FEMA (Federal Emergency  Management Agency) map that local insurors use for  flood  insurance information and conservationists  use  in evaluating  proposal's impact on the wetlands was last updated June 5, 1989, Clearv says.
       "These are the only maps available to us. The thought may be that the area needs to be studied," she says.
        Michael Goetz, regional hydrologist for FEMA's Boston office says  adjusting  the guidelines is a question of cash flow as much as water flow.
       The Boston office handles FEMA maps for all of New England, and has a limited budget each year to restudy or update maps.
        For the Shawsheen River watershed to be updated or restudied, residents and town officials would have to make a case showing evidence of flooding problems and flood damage.
        A map update costs FEMA ahout $40,000 to study a 3 or 4 mile area of stream, Goetz says. A more involved restudy of a watershed area could take up to a year and a half; he says.
       "We basically have to see how much the property is at risk, and whether the cost (of the study) would equal the benefit," he says.
        "Increased development may add pressure in those aaeas that would lead a community to ask us to do a study, but we have to prioritize our information," he says.
       The application process puts other watershed areas in competition with coastal communities that show repetitive losses from floods over the years, Goetz says.
"It's up to the community to make as good a case as it can to get us the benefit/cost ratio," he says.
        A new FEMA program allows watershed communities to help pay for studies and donate expertise, a method that helps projects get done much more quickly, Goetz says, but communities with many experts are likely to be found in the Midwest, where county planners can be made available to work with FEMA, he says.
       A restudy of an area could show that the amount of rainfall that once produced a 100 year-flood, or a flood that has a 1-percent chance of occurring every year, may under current conditions create a 50 year flood, Goetz says.
      "As development increases, you get more runoff in the entire basin, especially if communities don't practice good storm-water management. All the water goes right into the stream," he says.
Andover Conservation Commission Chairman Paul Finger says it's the incremental changes in the watershed over time that have compounded into today's situation.
     "There's no question at all that the flood plain (maps) are not necessarily that accurate. But it's a very long and detailed study that won't be happening in the immediate future That's why we encourage in new regulations to bring water back into the ground," Finger says.
      State wetlands bylaws, which took effect in the mid '80s, were the state's first attempts to begin undoing the damage done by watershed-related development up to that point.
     "Andover, I think, was in the forefront of requiring more from the Conservation Commission, but many communities didn't," he says.
     In addition to handling changes all over the watershed by enforcing state and local bylaws, Finger says he believes the commission needs to examine the bylaws reviewed by the Planning Board and the Conservation Commission.
     Not all proposed projects require approval from the Planning Board and the Conservation Commission, he says. If a project is not proposed for a wetland area, it is only reviewed by the Planning Board, Finger says.
     The Planning Board requires that new development does not increase the rate of runoff produced by a new project, but does not address any increase in the voume of water, Finger says.
     A storm-water management plan would address both, Finger says.
     Andover's storm water management plan, is very detailed, but is more than 35 years old.