Ellicott City, the county seat of Howard County Maryland, has a long and tragic history of flooding. Starting in the mid-18th century followed by the Great Flood of 1868, which reportedly took 42 lives, (according to Preservation Maryland) there were floods in 1817, 1837, 1866, 1868, 1901, 1917, 1923, 1942, 1952, 1956, 1972 , 1975, 1989, 2011, 2016, and 2018. Ellicott City was a mill town strategically located on the Patapsco and Tiber Rivers. Like so many other flood-prone communities Ellicott City and Howard County have frequently studied the flooding problem and had numerous reports prepared. For a variety of reasons, infrastructure change necessary to address the causes of flooding is slow to come.
On May 31, 2018, Ellicott City experienced, for the second time since 2016, historic flooding, property damage, and loss of life. Words cannot adequately describe the pain and suffering being experienced by the Ellicott City community but an evaluation of the causes and effects of the flooding is of interest to our clients. Media reports of the Ellicott City flooding tend to be dramatic, depicting city streets with millions of gallons of rushing water, and cars being carried away in the deluge. Careful analysis is necessary to determine the causes of the flooding of individual properties. We have undertaken such a study. The starting point in any flood investigation is a careful analysis of the watersheds. A watershed is generally considered (according to the USGS) to be an area of land that drains all the streams and rainfall to a common area. Ellicott City has multiple watersheds and sub-watersheds. If we know the contours and characteristics of a particular watershed we can evaluate, determine, and predict how a storm will affect a specific property in that watershed.
While adjacent watersheds should be jointly evaluated to develop a comprehensive stormwater management plan, to determine the cause of flooding at a particular property, conditions such as rainfall, stream flow, and ground conditions should be considered separately from adjoining watersheds. Because rainfall can vary drastically from one watershed to another it is important to determine as precisely as possible how much rain fell in a particular area and in what period of time. Rainfall in one watershed does not necessarily accurately reflect the rainfall or conditions in an adjacent watershed. Rainfall can be determined by reference to rain gages but if they are not in the specific watershed the hydrologist will have to interpolate the data. The flow of water in streams (as measured by stream gages) can also be used to determine rainfall but the location of the stream is critical. It is also important to understand what the soil conditions are in a particular watershed. Rainfall will run off a watershed that is intensely developed with impervious paved parking lots and box stores. Rainfall tends to be absorbed into a watershed that is composed of soil, grass, and woods that is not already saturated. Runoff contributes to flooding. We have found that difference in rainfall or in soil conditions can mean the difference between a historic storm and a storm that should have been anticipated. All of these variables have to be considered in determining if a storm and flood were foreseeable.
After the 2016 flooding, Howard County commissioned a comprehensive study (The Ellicott City Hydrology and Hydraulic Study) of the causes and potential solutions to Ellicott City flooding (the 2016 and 2018 floods affected the same general neighborhoods of Ellicott City and resulted from similar rain events). Among other things, the study identified three sub-watersheds and numerous sub-sub watersheds. The sub-watersheds include the Hudson, Tiber, and New Cut watersheds. Our preliminary investigation indicates that the rainfall, while certainly considerable in each watershed, was different in each. What may have been a 1000-year storm in one watershed may have been a much smaller storm in another. Little can be done to prevent or mitigate the effects of a 1000-year storm and resulting flood but infrastructure is supposed to be designed to accommodate 100-year storms and improvements to the stormwater system will inevitably improve the system's response to all storms. In fact, the report indicates that implementation of the proposed changes might decrease the water flow in the study area by up to 60 percent. The recommendations in the 2016 report have been incrementally adopted due, in part, to limited resources and time constraints.
The 2016 report makes specific recommendations regarding flood prevention and mitigation such as storage and gradual release of stormwater and improvements to stormwater infrastructure to enhance the system’s ability to safely convey stormwater. According to Samantha Montano reporting in the City Lab May 31, 2018 edition, “Rather than being a “natural” disaster or an act of God, Ellicott City’s flooding was caused by a combination of torrential rainfall, a vulnerable location, development that did not account for a changing flood risk, and other policy decisions.” The effect of environmental change also should be considered. Maryland’s Tort Claims Acts require that notice of tort claims against local and state agencies be made in a timely manner (one year) and there are limitations on how much can be recovered.