Flood Hydrology

Floods demand our attention. The first priority is, of course, to get people out of harm’s way, and to secure property to the extent we can. There are too many sad stories about people who underestimate the power of our rivers when they are in flood, and are swept away by the force of the current (even when in their cars). These dangerous conditions will persist for days to come. Addressing electrical systems in flooded areas, and threats to the integrity of dams, bridges and buildings are major priorities as well.

The next priority is water quality, which is often a serious problem during big floods. Sewers often “surcharge” due to huge inflows of stormwater, causing overflows of raw sewage into our rivers. In fact this has happened at the Town Wharf in Ipswich for the past several days, and in other locations this week. When this happens, it’s important to avoid contact with the water if possible, as it carries pathogens that can cause illness or infection.

Pollutants on the ground and roadways also wash into waterways, and sometimes floods tip over or inundate fuel tanks and other sources of hazardous pollutants. Also, a single major storm like this can cause erosion from unprotected land, resulting in major soil loss and sedimentation. Mud and gravel that has covered sections of roadways marks some of these areas.

Another issue is the role of people in this “natural disaster.” Humans don’t cause floods and droughts, but the way we live on the land may make these natural events more extreme. The loss of wetlands leaves less area to soak up and store the rain. Building on low land near waterways makes more property vulnerable to flooding. Development changes drainage patterns, flooding basements that didn’t previously have water problems.

Paved surfaces of suburban subdivisions, roads, parking lots and buildings are impervious to rainwater. So, instead of slowly percolating into the ground, the water rushes off these hard surfaces, surcharging our streams and rivers. The flip side of all this runoff is that less water is stored in the ground. This becomes a problem in the summer, when households consume more water from the Ipswich River and its connected groundwater aquifers to irrigate lawns. The result is lower low flows later in the summer, along with the higher high flows that we are seeing now. (In fact, despite the then-record flows of March 2001 at South Middleton, the Ipswich River experienced the lowest flows ever recorded later that same year.)

The “too much or too little” water problem is only growing worse. The four most severe floods ever experienced in the Ipswich River have occurred in the past twenty years. Meanwhile, the river has dried up for much of its length during five of the past ten summers. The ecosystem is at risk, and our native fish and wildlife are suffering. This week’s flooding shows that we humans are suffering too, for our pattern of development places our homes and water supplies at risk from more extreme floods and droughts.

What can we do? We can build our homes and communities in ways that maintain the natural hydrology, so that rain can slowly percolate into the ground and recharge the river. We can retrofit existing developments, and new developments can be compact and clustered around existing downtown areas, and preserve remaining forests and wetlands. We can use “low impact development” techniques like rain gardens, porous pavement and constructed wetlands to infiltrate rain into the ground where it falls. Green roofs can minimize runoff and reduce energy costs too. We can minimize lawn area, avoid over-irrigating, and use native plants to create beautiful natural landscapes. Many of these techniques are already in use in the Ipswich River watershed.

For more information on actions that residents and communities can take to reduce flooding AND low-flow problems, please see Programs & Projects. IRWA’s new handbook, Water Wise Communities: A Handbook for Municipal Managers in the Ipswich River Watershed, outlines 20 key tools that communities can use to effectively manage water resources and restore the Ipswich River.