Mitigating 100 Year Floods: Catchments And Watershed Management

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A River Runs Through It, Over It, and Around It

Competition is usually a good thing. Except when the contest is over water management and the competitors are commercial interests, agriculture, consumer interests, government, and the environment. Until recently the biggest short term loser in that contest has been the environment. But as the information age continues to unfold, the message from Mother Nature is becoming clearer than ever. Altering watersheds and catchments without long term vision and total environmental planning inevitably generates unforeseen and usually dire consequences, and everyone loses.

Since first observing natural or animal-made dams and levees, it is likely humans have been attempting to control watersheds for their own purposes. Early recorded history documents some successful projects such as appropriating annual flooding of the river Nile in Egypt to irrigate crops. This especially notable example has been in existence for nearly 5,000 years in spite of the usual political, hydrological, population, and institutional changes of the region. The success of this system was largely achieved by utilizing and enhancing the river’s hydrological cycles as opposed to blocking or disrupting them. Recognizing the value of flood induced nutrient deposition and distributed water percolation into the soil by the early Egyptians was a watershed moment that has withstood the test of time.

When a Levee Breaks

History also records dismal failures in attempting to control water’s destiny. Levees and dams built on rivers can enhance shipping traffic, create reservoirs, and provide some predictability for flood events. But they can also lead to dysfunctional flood plains and raise average water levels by concentrating sediment deposition onto the river bottom. Dredging and raising the height of levees may appear to solve the problem until the next hundred year flood event (or failed containment) becomes a thousand year event due to the artificially increased volume of entrained water. Flooding of China’s Huang He (Yellow River) in 1887 and the United States’ Mississippi River in 1993 are two notorious examples of failed watershed transformations. The Huang He flood is said to have claimed over a million lives at the time. The economic impact of the Mississippi flood is estimated at over $10 billion. Both of these events could have been prevented if hydraulic flow dynamics and natural hydrological cycles were better understood. Clearly a more balanced approach in utilizing these watersheds was required.

Dam Nations

China, at over 80,000 and the United States, at over 75,000, lead the world in constructed dams over 25 feet tall. No other country comes close to these numbers. And while much of China’s dam construction has occurred in the past 40 years, many of the dams in the United States are much older. This has led to great concern about their viability and lack of proper maintenance. In fact the American Society of Civil Engineers graded the current state of U.S. dams at “D” for lack of care and funding for repairs. Even more disconcerting is that some of these dams are privately owned and not under direct civil authority. One such structure, the Ka Loko Dam in Kilauea, Kauai breached on March 14, 2006. Originally constructed in 1890 this earthen dam created a reservoir for a sugar cane plantation and was subsequently modified and changed ownership several times prior to 2006. Miscommunication, lack of inspection and enforcement funding, and questionable engineering expertise over the years left the condition of the reservoir structures in doubt. With inevitable consequences; seven fatalities were recorded from this relatively small breach. Multiply this event by the several hundred flood control structures currently in similar condition across the U.S. and the magnitude of the problem becomes obvious.

A Little More Consideration, Please

Water usage and management is one of the most complicated yet important projects in civil engineering. The list of variables and interests is long and getting longer: hurricanes, monsoons, record rainfalls, fast snow melts, lack of infrastructure funding, hydro-political vulnerabilities, hydro-diplomacy, hydrodynamics, recreation, shipping, fishing, irrigation, bathing, drinking, and so on. Water has probably been the most contentious resource tackled since constructing that first radical arch support on drainage ditch #0000001 circa way back when. With ever increasing population pressure on existing sources, water fronts, and flood plains, adequate consideration of balanced watershed use is more critical than ever.

Glass of water, anyone?