By Dr Tim Ball – Re-Blogged From http://www.WattsUpWithThat.com
Petula Clark sang, “Don’t sleep in the subway, darling. Don’t stand in the pouring rain.” More helpful advice would urge, “Don’t live in the floodplain, darling. Don’t you know it’s pouring rain?” It’s called a floodplain for a reason. The dangers of flooding mostly involve people living in dangerous places. Why are people allowed to live in these regions without being forced to accept full responsibility for their actions? They are encouraged by governments and insurance that enable bad practices, questionable, and unnecessary behavior.
There was a time when living near a river was important for transport, water supply, waste removal, and even food supply. We don’t need to live close to rivers or at least within the area identified as the floodplain. If you live there, flooding is inevitable, even if flooding protection is in place. In fact, the protection creates a false sense of security. Inevitably the protection will fail through neglect, accident, or water levels that exceed the design capacity.
Engineers design flood control structures based on the frequency of events. Usually, it is for the one in 100-year event. Most think this means if you have such an event then another one will not occur for another 100 years. It is known be a few names the return period, recurrence interval, repeat interval, or expected frequency. It is defined by
Where n = number of years: m = number of occurrences of flood events
There are several problems with this approach, many now producing headlines about global warming and climate change.
The first involves the changing patterns of precipitation. There are very few records of precipitation, and most are less than 100 years long. The greatest range of variation of precipitation occurs in the middle latitudes in association with the changes in the latitude and amplitude in Rossby Waves along the Polar Front. Much longer time periods of precipitation are now occurring. These are not because of man-made climate change but the natural mechanism. The IPCC don’t consider these because of the restrictions of the definition of climate change to only human causes.
Second are the ongoing changes to the river as flows vary with changing precipitation. These are superimposed on the natural changes in a river as it evolves from youth to maturity to old age.
Third are man-made changes in the river basin that alter the pattern of runoff spatially and temporally.
All these issues confronted the Assiniboine River Management Advisory Board I was appointed to Chair. We were charged with creating a total basin management strategy. (Figure 1)
The first challenge was to come to grips with the great range of flow. Like most Great Plain’s rivers, it varied considerably. Fortunately, what triggered the demand for a management strategy was a swing in 6 years from the lowest to the highest flow in the then 94-year record. Figure 3 only covers from 1906 to 1973 but illustrates the range of maximum and minimum flows. Sedimentary evidence indicates much longer and larger scale wetter cycles. For example, Figure 2a, 2b, and 2c are cross-sections through a dune located half way along the current Assiniboine River near Brandon. They show three distinct well-formed paleosols formed through prolonged wetter spells.
Figure 2a (Source: Author).
Figure 2b Top of Dune – Two paleosols visible
Figure 2 c Middle dune paleosol.
The flow rate was important along the river, but also because the Assiniboine is the major tributary of the Red River of the North. The Red consistently causes urban flooding problems at Minot, Fargo in North Dakota, and Winnipeg. The City of Winnipeg built a massive diversion channel called the floodway that takes water out of the River south of the city and returns it north of the city (Figure 4). It effectively doubles the river capacity over the length of the diversion channel. It was built based on the modern record of flooding. The recurrence frequency considered a one-in-100-year flood including the 1950 event that triggered demand for flood control.
In fact, precipitation pattern changes much more frequently and widely than any 100-year record could accommodate. They ignored the historical evidence of the 1826 flood that was three times larger and reports of an even bigger flood in 1776, which coincides with the Little Ice Age. The 1826 flood was approximately a one-in-400-year event. Another flood of this magnitude occurred in 1996, and the floodway was inadequate, and though it alleviated to some extent, it forced them to expand the floodway.
There is a reason government provide flood insurance in the US. Private insurance companies won’t get involved because living in flood prone areas is asking for trouble, a self-inflicted wound, and governments often create or aggravate the problems by such actions as changing the surface.
In climate, most are aware of the urban heat island effect (UHIE), but that tends to focus on the temperature. An important cause of the temperature change is the changed surface and altered rates of runoff and evaporation.
The greatest surface changes are in the city center: an area called the Central Business District (CBD) with almost 100% impervious surface. Even the suburbs are at least 50 percent impervious surface. Figure 5 shows an average suburban lot and impervious surfaces.
Figure 5: Average suburban lot and impervious surfaces
Extensive drainage systems are designed to carry water away quickly. The water stays around in the countryside and evaporates slowly or is used by plants and transpired slowly, both processes creating cooling.
Figure 6. Rates of Runoff Urban/Rural
Figure 6 shows how these changes alter the peak at which the water reaches the river channel. The channel develops to accommodate a certain runoff rate so when water arrives too quickly flooding potential is increased.
Most rivers flood. The channels they create are for average natural flow, but if precipitation increases the channel will fill. The first stage is “bank full” when water reaches the top of the banks. Once water flows over the bank, it is in “first flood stage” and covers an area called the first flood plain. (Figure 7)
Figure 7: Thalweg is deepest part of a channel. Levees form from silt deposited during flooding.
Nobody should be allowed to build in that floodplain. Dikes to contain the river should not be allowed either because when breached catastrophic flooding occurs. Also, they prolong the flood because they prevent water returning to the river.
It is an event that occurs naturally in the old age stage of river development as a broad flat floodplain develops. Once the first flood stage occurs, water flowing along the edges of the river is slowed, and sediment is deposited creating levees. They deflect small tributaries from entering the river, so they flow parallel in a distinctive pattern known as a yazoo stream (Figure 8).
The precipitation patterns over long periods change much more than anything measured in the modern instrumental record. There is no need to build in the areas flooded shown in the following pictures of Calgary (9) and High River (10) Alberta, and Tewkesbury (11), UK.
Figure 11 shows the church builders knew long ago where the dry, as well as the moral high ground was located. Now with our faith in engineering, we are more arrogant and think we can ignore the long term patterns of nature. So, we need a new song, “Don’t live in the floodplain darling”.
Convert those floodplain areas within all cities, especially in the centers. They need more parks to give people access to nature, ameliorate the impact of the UHIE, and save the costs of dealing with the loss of lives and damage to property that floods inevitably bring. It doesn’t matter if the record is inadequate or if you only built for a 100-year flood to save money. When the 100-year event aggravated by changes to the urban area or the inevitable 400-year event occurs, it overwhelms and traps people who don’t understand recurrence frequencies. You can use the floodplain, but only with the ability to let nature use it for its designed purpose when she chooses.