Van Tuyl: Controlling Where the Water Goes

After a week full of rain and flooding, it was entirely fitting to have a Van Tuyl over the challenges of designing storm-water control measures. The talk was given by Holly Piza, who is a senior project engineer for the Urban Drainage and Flood Control District on the front range. While there was some anticipation of discussion over the recent deluge of water that assaulted the Front Range, Piza was frank with the audience, “I guess it is a great time to talk about rainfall and flooding, but that’s not what I planned.” Luckily for the audience, there was much more at stake than large floods in the design for storm-water control measures.

In the Denver region, the annual precipitation comes to around 15 inches of rain. Depending upon where that rain falls, it may find its way into the ground where it can be used, or it may turn to runoff. Storm-water control measures are designed to best minimize the amount of runoff from a given system so that the water can be used effectively. “We want to reduce the amount of runoff from a site,” revealed Piza, “we can do this by detaining and slowly releasing storm-water.” According to Piza, one of the primary challenges to this extent is establishing a set of best management practices, or BMPs, that is suitable for the region, since every region is unique in what challenges are present.

Since it is impractical to use any random site, one of the first phases in establishing BMPs is site selection. Piza walked the audience through several different sites, discussing both their strengths and weaknesses for determining BMPs. The first that was brought up was the the Denver Wastewater Management Building alongside I-25. Different pavers were used in the entryway design so that their effectiveness could be tested. “Unfortunately,” said Piza, “the only problem is that the reference site is ‘too clean’ of a site.” On the flip side, a site can be undesirable for being too dirty. Piza regaled the audience with one of the sites that had been chosen, “we monitor a sand filter at a city maintenance yard, it is a dirty site […] it clogs almost every time it rains.

Another major design consideration that was hit upon was the idea that all aspects of the site must be documented. Since any non-rain water can be disastrous to model building, it is crucial to find out what drains into storm-water management sites. Piza also hit upon the need to establish a site that can be free from degradation, “we try to hide things as much as we can, but they usually get damaged.”

While having a good design and background is a good first step, Piza also noted that it is important to actually get out and observe the sites from time to time. At one site in particular, the automatic sampler was picking up strange flow. “We had a guy with a sprinkler,” revealed Piza, “you don’t want to sample water that isn’t rain.” Another factor that can cause issues is wind. Most rain gauges are on the ground, but in cities it can be tempting to put them on buildings where they will not be vandalized.

Before launching into actual design specifications, Piza reinforced one of the critical ideas, “The biggest challenge,” she said, “is getting accurate measurements in the field.” This thought has guided design considerations and approaches in the field. Since there is a strong relationship between amount of rain, and calculated volume, Piza revealed that she can use models to work through whether or not a design is working correctly. For the basic storm-water control measure experiments, V-notch weirs are used to drain slowly over time in a way that can be measured. In one case, a site was working bad, though the reason was not apparent in the data. A site visit revealed that the flow in the measurement device was not quiescent. Piza and her team modified the collection device to be more tranquil, “now it is much better.” Another example of a bad design was brought up by Piza. In a case of a permeable pavement site, the sampling tube was left in the wrong location for a decade. This coupled with an image of the site inundated with sediment was used to hone in on the idea that was repeated by Piza, “you really have to spend a lot of time at the site.”

In cases where storm-water control methods are implemented correctly, there are still necessary limitations and considerations. Piza used the example of the new green roof at the Denver Botanic Gardens. In the 2011 season, there had been 17 rain events. While most of these rain events were reliable, how big an event is can have an effect on how accurate the data can be. “We need to pull a sample at least four times,” said Piza, on top of that “we want them evenly spaced through the hydrograph.” In small events it can be difficult to get the minimum four samples and in large events, the sampling events may not be as spaced out as they should be. Several different examples of hydrographs were put up by Piza to emphasize the point.

One of the duties of Piza and her group has been to establish a volume of best management practices. Since the Front Range hydrological area has its own unique challenges, this volume doesn’t necessarily reflect what is in normal manufacturing specifications. A major issue in the area is the behavioral differences between what pervious pavements and asphalts should do and what they do in the field. To combat this Piza’s team worked to come up with a new concrete mix and a new design. Instead of pervious concrete, the new method involves cutting deep cuts into the concrete so it becomes slotted. For the trials it intercepted the water, unfortunately Piza admitted, there is no way to know how it will hold up.

Another area where the Colorado standards are diverging is in bioretention media. Piza elaborated that there are “thousands of magical mixes,” unfortunately most areas use peat, which is not sustainable or local. for the Colorado area. Instead of peat Piza revealed that her team is in the process of spreading a mix that focuses on using recycled shredded paper and sand. Since it slows down infiltration temporarily and keeps undesired nutrients out, it should be ideal. Unfortunately, the eureka moment did not transition well from the drawing board to the field. Piza’s plan was set back since recycled paper is hard to come across, most document destruction companies have agreements that the paper will be pulped. Luckily there was a way around that by means of old phonebooks. With the phonebooks, sand, and a mechanized process, the mixture was able to come together correctly.

Piza finished off the presentation with one case study, or as she put it, “the most exciting one, it is what is going on.” Her group has been working with the Denver Green School to come up with a larger scale storm-water control measure. The main focus of the project is a large cistern which captures water for use on the site. Piza’s group also uses this cistern to get an idea of the storm-waters that come. “Typically when rain comes and you need to capture it, the cistern will already be full,” Piza explained, “what is special here is that the cistern will purge itself to collect more rain.”

The system is important in that it could be used to create a new set of BMPs in communities where there is a combined sewer systems. If water can be purged, it will alleviate flooding in those scenarios. Since many urban areas in the Eastern United States have combined sewer systems, it is a solution that is much needed. When asked about what happened to purged water, Piza let out the best part of the design. “We can set up other BMPs in series,” she explained excitedly, “then we can have a rain guard downstream with a smaller footprint.” By putting some of these systems in one whole network, more water can be retained for future use. The Denver Green School system is also significantly more high tech than other similar systems. The cistern is linked to a NOAA weather forecast which allows it to purge if it calls for rain. The site is also linked up by a hotspot to Piza’s office so she can monitor what is going on. To hit home the strength of the BMP as a whole, Piza put up a screenshot of the dashboard from the rainy week to show just how accurate it could be. As was revealed in the questions portion, the system is expensive, but within the past two years it has worked nearly perfectly, with the only issues coming this year with the rain. To highlight the effectiveness of the cistern, Piza revealed that in the prior year, the school did not have to use water from Denver Water for the purpose of the cistern and this year the system was used even more.