The following article was originally published in North Dakota Water magazine’s March 2019 issue.
The relationship between surface drainage and flooding is often misunderstood. Logic would dictate that drains move water and therefore cause floods. As with most any water-related topic, reality is far more complex. The purpose of this article is to explain the basics of surface drainage and the true correlation between drainage and flooding.
I begin this discussion with a few watershed facts that relate to drainage. First, water moves downhill due to gravity regardless of the presence of drains. Second, the rate of water movement will increase when aided by drainage (for example, channelized flow). Third, the volume of water moved downhill can increase due to drainage. Fourth, water movement is slowed by road embankments and culverts that meter flow along the course of drains. Fifth, the size of a flood event depends heavily on the timing of runoff from various locations within the watershed. Finally, agriculture in the Red River Basin as we know it is not possible without adequate surface drainage. Understanding these facts is a precursor to any responsible drainage design.
Proper drainage design begins with an understanding of the existing condition and future “with-project” condition. In other words, what will change if the project is built? Current modeling technology allows for an accurate assessment of potential impacts (positive and negative) that any proposed water management project may have, including surface drainage. These tools allow the engineer to identify and manipulate design features that alter both the rate and volume of runoff, which are the fundamental principles of water management. Second, it is broadly accepted that a certain “minimum level” of drainage is required for modem production agriculture. This level has been identified as the 10-year 24-hour rainfall design event by the 1998 Mediation Agreement between representatives of watershed districts in the Minnesota portion of the Red River Basin and Minnesota state and federal agencies. The goal of this design level is to provide “adequate” drainage that minimizes frequent crop losses but accepts that crops will be lost when larger rainfalls occur.
Back to rate and volume of runoff, the principal objective of the engineer is to provide a design that results in adequate drainage and also minimizes adverse downstream impacts. More specifically, the goal is to design surface drainage projects in a way that accepts the fact that the rate and, in some cases, volume of runoff will increase for smaller storm events but controls the rate and volume of runoff for larger storm and snowmelt events that are the cause of flooding. Contrary to what the average citizen understands, this can be done and is being done. A great example of drainage projects that control the increase in runoff volume that results from their construction are closed basin outlets. These drains possess a gate that only allows water to pass when downstream conditions allow. This is relatively easy to understand. But what about the more typical scenario of drains that are built to simply move water faster, (increase the rate of runoff)?
Controlling the rate of runoff is accomplished by balancing the size of the channel with the size of the road-crossing culverts and elevation of the adjacent spoil bank or berm. Again, the concept of adequate drainage accepts that the rate of runoff will increase for smaller nuisance rainfalls. However, the rate of runoff for larger events can be controlled by the project design elements, such as a channel, culverts and berm. The design begins with the channel, which is sized to provide the adequate flow capacity (convey the 10-year 24-hour event). Next, the culverts are sized to be slightly less efficient than the channel. Typically, what this means is inserting a culvert into the open channel will drive the water surface elevation up between 0.5 and 1.0 feet. In other words, the road-crossing culvert is restricting the channel flow slightly. Finally, the berm adjacent to the channel is lowered such that when the water surface rises due to the culvert restriction the water leaves the channel and floods the adjacent field. In other words, flows that exceed the channel capacity are restricted by the culvert and the excess water is forced out of the channel and into floodplain storage in adjacent fields. Therefore, properly designed drainage systems slow the flow of water, through temporary storage, for floods larger than the 10-year 24-hour event. This is a self-mitigating design. The design concept should be employed along the entire length of the channel thereby distributing both the benefits and the flooding when runoff exceeds the capacity of the system. Red River Basin engineers have the advantage of utilizing published guidance which, when followed, results in responsible drainage designs that mitigate flood damages through proper culvert and channel sizing (River of the North Basin Technical and Scientific Advisory Committee [BTSAC] Briefing Paper No. 3—Water Management Options for Surface Drainage, September 15, 2014).
Watershed timing is also an important consideration. Moving water more quickly, or slowing it down, can be beneficial in one area and detrimental in another. A watershed can typically be divided into three “timing zones” (Red River Basin Flood Damage Framework—Technical Paper No. 11, May 2004). The most downstream region located near the outlet can be thought of as the “early” zone, typically the center of the watershed can be thought of as the “middle” zone, and the furthest upstream portions can often be thought of as the “late” zone. If the outlet of the watershed is the point of interest, flow at the outlet is being generated by runoff from early, middle and late “timing zones.” It is the “middle” zone of the watershed that contributes the most to the flood peak or crest at the outlet. Accelerating the “late” water so that it joins the “middle” water will exacerbate flooding at the outlet. Likewise, slowing the “early” water near the outlet so that it coincides with the “middle” water will also exacerbate flooding. Therefore, accelerating water in the “early” zone so that it is evacuated ahead of the “middle” water will often reduce the flood peaks. The typical tributary to the Red River is flattest and in most need of drainage near the outlet in the “early” zone. This can result in a win-win situation as the area in most need of drainage is also the area where accelerating water typically will be beneficial for reducing floods near the tributary outlet and within the “early” zone.
One caveat to the early-middle-late discussion above is that the location of the point of interest may change the observation. That is to say, if the point of interest were to change from the outlet of the tributary to a location further downstream along the mainstem of the Red River, the conclusions could be different. Typically, accelerating flow in the “early” zone of a tributary will also benefit the Red River mainstem in the vicinity of the tributary outlet. However, this may not be the case further downstream. This is where understanding watershed timing can become very complex. Regardless, engineers can play it safe by simply following the self-mitigating design guidance cited above.
Public drainage systems typically sponsored by water resource districts are developed and managed under Title 61 of the North Dakota Century Code. These projects are carefully planned and considered, and subject to review and approval by the State Engineer through a permit. Public drainage is an important element of overall watershed management. It has proven to be the most cost effective means of minimizing the adverse impacts associated with smaller, more frequent, floods. Utilizing drainage to manage smaller floods and other projects like dams and levees, and diversions to manage larger floods, is the basis for sound watershed management. These various types of flood risk reduction projects complement each other; they work together to solve the overall flood equation. It is safe to say that without properly designed drainage, the cost of flooding to society would be exponentially higher. Furthermore, without drainage, agriculture as we know it would cease to exist in the Red River Basin.
Chad Engels is a Senior Project Manager with Moore Engineering. He has 19 years of experience serving as the designated engineer for numerous water resources organizations in Minnesota and North Dakota. Throughout his career, Chad has managed the study, development, design and construction of watershed improvement projects from simple best management practices to complex flood risk reduction and water quality improvement infrastructure.