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'''Problem:''' A 5-mile public transportation corridor serving light rail and low speed traffic. The corridor is on C soils (0.2 in/hr) and is 0.1 mile in width. The corridor is 100 percent impervious except for pervious areas associated with BMPs. | '''Problem:''' A 5-mile public transportation corridor serving light rail and low speed traffic. The corridor is on C soils (0.2 in/hr) and is 0.1 mile in width. The corridor is 100 percent impervious except for pervious areas associated with BMPs. | ||
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+ | This is the same problem as Exercise 1-2 except it is on C soils. Underground infiltration, used for Exercise 1-2, is not a practical BMP for this situation. Because the media depth is restricted to 0.8 feet if we want to meet the 48 hour drawdown requirement, we have to enlarge the area of the tree trench. We can also increase the porosity of the tree trench to the maximum recommended value of 0.35. The increase in porosity from 0.25 (Exercise 1-2) to 0.35 gives a 40 percent increase in water retention. |
Two workshops were held (December 5 and 7) and attended by a total of 22 people. Attendees were asked to choose from a list of problems and solve the problem in the MIDS calculator. Below are examples of approaches that could be used for each problem. These are just example solutions and you are encouraged to try different solutions.
Click on the Excel files to open and then save them on your computer. Open the MIDS calculator and open the file of interest. Included below is a discussion of BMPs and routing for each problem. If you are reviewing calculator files, [here is a link] to some common things to look for.
Problem: A 60-acre residential development on B soils (0.45 in/hr). The development includes 50 homes, each on 1-acre lots, and 10 acres of green space. The green space includes 5 acres of wooded trails and a 5-acre recreation area (e.g. ballfields). Total impervious is 25%. Meet the MIDS performance goal of 1.1 inches.
The BMPs utilized and their sequencing in a stormwater treatment train is shown in the figure to the right and summarized below.
The example utilizes several practices for illustrative purposes and does meet the 1.1 inch performance goal. Note the very high removal for phosphorus and TSS, partly because of the use of a stormwater pond that does not reduce volume but does reduce pollutant loading.
Problem: A 5-mile public transportation corridor serving light rail and low speed traffic. The corridor is on A soils (0.8 in/hr) and is 0.1 mile in width. The corridor is 100 percent impervious except for pervious areas associated with BMPs.
This site consists of 320 acres of impervious surface. The BMPs utilized and their sequencing in a stormwater treatment train is shown in the figure to the right and summarized below. Linear underground BMPs are logical BMPs for this setting.
Some other BMPs that could have been incorporated inclde planter boxes using the disconnection BMP and green roofs. Infiltration basins would be more effective than bioinfiltration but provides fewer aesthetic benefits.
Problem: A 12-acre (3 block) private development in downtown. Site is on B soils (0.45 in/hr). Buildings are being demolished and replaced by a new corporate complex that will include several new buildings and a 1-acre plaza. Roads constitute 5% of the total area. The area is 100% impervious, except for the plaza, which could be up to 25% pervious.
This site consists of 11.75 acres of impervious surface in an ultra-urban setting. There are 0.25 acres of pervious surface. BMP selection is limited at this site because the B soils do not make underground infiltration a cost-effective BMP. The following scenario was used to meet the performance goal.
Although we attain the performance goal, our phosphorus retention on an annual basis is only 61 percent. This is because green roofs do not provide phosphorus attenuation.
Problem: A 40 acre development site on B soils (0.30 in/hr). A significant portion of the site is underlain by bedrock within 3 feet of the land surface (see Figure 1). The site includes 30 acres of residential land use (30% impervious), 5 acres of commercial land use (60% impervious), and 5 acres of green space.
The proximity of bedrock to the land surface will dictate how the site is developed. The commercial area and green space will likely exist on the area with bedrock less than 3 feet from the land surface. The Construction Stormwater Permit prohibits infiltration in areas where groundwater is less than 3 feet from the land surface. The land use breakdown for the portion of the site with bedrock less than 3 feet of the land surface is as follows:
The area with bedrock greater than 6 feet consists of 6 acres of residential, with 1.8 acres of this impervious and 4.2 acres pervious. The area with bedrock from 3 to 6 feet below the land surface includes 14 acres of residential consisting of 4.2 acres of impervious and 9.8 acres of pervious. The entire site has 12 acres of impervious surface and 28 acres of pervious surface.
To conceptualize this site, consider the following BMP otions for the three land areas.
The diagram at the right illustrates the BMPs used for this exercise. Below is a short summary of some features.
The MIDS performance goal of 1.1 inches off impervious surfaces is met at this site.
Problem: A 40 acre development site. Half the site (20 acres) is on C soils (0.20 in/hr) and half is on D soils (0.06 in/hr). The site includes 30 acres of residential land use (30% impervious), 5 acres of commercial land use (60% impervious), and 5 acres of green space.
Assume half of this site is on D soils and the other half is on C soils. Using the MIDS Design Sequence Flowchart-Flexible treatment options, Flexible Treatment Option 1 should be set as the performance goal.
In the MIDS calculator, change the performance goal to 0.55 inches, resulting in a performance goal of 23958 cubic feet.
For simplicity we divided the site into two similar areas, one for C soils and one for D soils. We employed the same suite of BMPs for each soil. These are illustrated to the right and consisted of the following.
We achieve the 0.55 inch volume goal and 95 percent total phosphorus removal.
Problem: A 5-mile public transportation corridor serving light rail and low speed traffic. The corridor is on C soils (0.2 in/hr) and is 0.1 mile in width. The corridor is 100 percent impervious except for pervious areas associated with BMPs.
This is the same problem as Exercise 1-2 except it is on C soils. Underground infiltration, used for Exercise 1-2, is not a practical BMP for this situation. Because the media depth is restricted to 0.8 feet if we want to meet the 48 hour drawdown requirement, we have to enlarge the area of the tree trench. We can also increase the porosity of the tree trench to the maximum recommended value of 0.35. The increase in porosity from 0.25 (Exercise 1-2) to 0.35 gives a 40 percent increase in water retention.