Flood – MAR – Living in the Land of the “Dash”

Michael Nowlan

What’s this all about? That’s probably the question you’re asking yourself in reading the title of this article. Well, let me explain. The concept of Flood-Managed Aquifer Recharge, or Flood-MAR, is a HOT item in our circles, almost like a magic pill in some people’s view. Don’t get me wrong, I am not going to “poo-poo” the idea. Far from it. What I would like to explore is the interconnections between flood management and aquifer recharge management, represented in the “-“ that connects and/or separates the terms within the name of this wonderful concept/program. Alright, I will try and stop myself from adding too many dashes and slashes from here forward. I shall resist!

There is no doubt much potential for synergy between the two concepts, but there may be some wrinkles in marrying these two things together that should be thought through. Both of these concepts deal with the control of storm runoff. Fundamentally, each approach warrants both emptying basins and filling basins at different times and for different purposes. Where and when one moves water is very important in making these concepts work in concert. Allow me to elaborate. At their most basic levels, flood management is about the management of flood volume, and managed aquifer recharge is about the management of water supply volume. At this point some of you familiar with reservoir operations may understand where I am going. The management of flood volume is all about getting the water downstream safely, but quickly. Of course, there can be some detaining aspects associated with flood management, where water must “hang out” for a bit before being released downstream. Usually this has to do with limited capacities of downstream “conveyance” and “storage” facilities. (Hey, I promised to stay away from the dashes and slashes, but quotation marks are still on the table!) In order for flood management to work, the flood facilities need to be “empty” to have the capacity that is needed before the next flood hits. Therefore, flood management needs to “drain” the system, safely and efficiently. The management of aquifer recharge is all about getting the water into the ground for future water supply, and detaining it on the surface until it can infiltrate, which can take longer periods of time. The water not only needs to occupy the surface areas, but achieving that “occupation” can necessitate building impoundment structures in the floodplain, to prevent it from draining. For aquifer recharge to really work well, the diversion of water into the floodplain needs to be more “constant” and “frequent”. Therefore, the diversion facilities need to control flow amounts in lower flow ranges. So, how can they work together? At the point(s) of diversion, the facilities need to be configured with both lower flow and higher flow controls, to allow for recharge flows to occur under a wide range of flow conditions, while also allowing very high flow releases during floods. This could include installation of movable check structures in-stream, that can be raised up under lower flow conditions to divert recharge water and lowered under higher flow conditions to allow flood waters to pass downstream safely. This could also include more elaborate stream bank facilities for off-stream facilities that involve both overflow weirs and pipes with controllable gates that are protected from erosion and debris. Lastly, the management of off-channel surface storage could include monitoring and forecasting big storms and operating drainage structures to drain the surface areas before the flood, if such facilities serve both a flood and recharge role. After all, the same volume of space cannot be both empty and full at the same time. Obviously, this article is only scratching the surface regarding these two concepts. There is much to think about in marrying these two together, but there should be no reason to call it off. We cannot know everything there is to know before moving forward. As it is with most things in life, working together involves partnership and risk. In my opinion, these two were made for each other. So, hand in hand, side by side, “Flood” and “MAR” are ready to take the plunge! (For the record, water puns were never off the table!)

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THE HIGH WATER MARK

The Newsletter of the Floodplain Management Association