Welcome to the Duplicable City Center water catchment hub. As part of our global transformation approach to Highest Good housing, we are using this page to share the complete design, construction, storage, and ongoing maintenance and upkeep details of the Duplicable City Center water catchment system we are designing.
The Duplicable City Center will be a totally sustainable and eco-friendly building. One of the tools that we can use to make that possible is implanting a system of rainwater harvesting and reusing water as much as possible. Water will be collected off of the entire Duplicable City Center. Water catchment for this area has been calculated using the following color coordinated zones: Pink for the domes and Blue for the central area. There is a 4th-floor cupola that will cover the central area but water will still be collected from this cupola so we have used the zones you see below for simplicity.
The Duplicable City Center footprint for each of the domes (area of the circle) was calculated for rainwater catchment. The area of a 74′ diameter circle equals 4,300 feet x 3 domes = 12,900 sq ft. Using a (conservative for our location) 10-inch annual rainfall* and applying the formula for calculating water harvesting (catchment area x rainfall x runoff) yields: (12,900) X (.833) X (7.48) = 80,485 gallons of water harvested per year from the domes.
The area of the central water collection zone (blue) for the Duplicable City Center includes the area shown on the map above (topped with the cupola that will cover much of this area) equaling roughly 3,010 sq ft. of rain collecting space. Using a (conservative for our location) 10-inch annual rainfall* and applying the formula for calculating water harvesting (catchment area x rainfall x runoff) yields: (3,010) X (.833) X (7.48) = 18,754 gallons of water harvested per year from the Duplicable City Center Hub.
*Note: For calculating cistern and pond sizes we used the average annual rainfall for our location of 13.6 ~ 15 in = average of 368.3 mm. For calculating pipe size we used the Daily Critical Rainfall (the maximum daily amount of rainfall in our location) of 1.3″ / 33 mm.
Here is an overview of the piping for the collection, transport, and storage of rainwater from the City Center:
To design the system we started with the maximum amount of rain that could be experienced for our location. Figure 1 below shows a diversity of different areas:
For our our location, the number we’re concerned about is 1.3 in.
Next we needed to size the pipes and choose the slope. Referencing the International Code Council, this is the table we used for drainage pipe installation.
The slope of ¼ was chosen because a 2.5″ pipe will be sufficient for rainfall in our area.
Next, we used the information above to calculate the discharge rate:
The material with the best cost x benefit ratio is PVC because it is light, durable and not expensive. The roughness (friction loss) coefficient of the PVC is 150.
Using factors such as flow rate or discharge, Hazen Williams’ friction coefficient, and hydraulic grade line slope, it was possible to calculate the diameter of the pipes. For the initial catchment water, we will use a total of 8 drains with a diameter of 2.5 inches to guarantee that no water will be wasted.
Referencing this Uniform Plumbing Code table with our 1.3″ maximum anticipated rainfall within a 5 minute period (Figure 1 above), and knowing the total area of the water collection is 15,910 sq ft., we chose 4 downspouts to cover the areas not covered by ground-based gutters. Calculating these areas meant each downspout will be responsible for an area of approximately 1,989 sq ft. The rest of the water will flow off the domes and be transported via ground-based gutters to drains.
In our case, a 3-inch downspouts will be totally sufficient for our purposes. We will only use a total of 4 downspouts because the gutters installed around the base of the domes will handle most of the water. Here is a picture of the downspouts and piping from the drains:
Length of Downspouts:
Here are the specific pipes diameters and lengths:
Considering that the water flowing inside the gutters can be considered as an open channel flow, the best option to calculate the best size for the gutters is using the Manning Equation. This equation is a function of the channel velocity, flow area and channel slope. The following table shows Manning’s Roughness Coefficient:
By applying the Manning Equation in our project we conclude that the best solution is use 6” k-style seamless PVC guttering.
Total Length of Gutters:
182′ + 162′ + 181′ + 116′ + 192′ = 833 ft
For health reasons, it is important to address the first flush of water. When it rains, water slowly builds up in the roof gutter system before it exits through the downpipe. The first flush of water from the roof can contain amounts of bacteria from decomposed insects, lizards, bird and animal droppings and concentrated tannic acid. It may also contain sediments, water borne heavy metals and chemical residues, all of which are undesirable elements to have in a water storage system.
There are many systems to flush off the first flush of water, but one of the smartest and economical systems to use is a dependable ball and seat system. This is a simple automatic system that does not rely on mechanical parts or manual intervention. As the water level in the diverter chamber rises, the ball floats, and once the chamber is full, the ball rests on a seat inside the diverter chamber preventing any further water entering the diverter. The subsequent flow of water is then automatically directed along the pipe system to the tank. Here’s what this looks like:
A cistern should have sufficient storage capacity to carry the household through extended periods of low rainfall. A three-month supply of water, or one-fourth of the annual yield of the catchment area, is generally adequate in areas where the rainfall is distributed fairly evenly over the course of the year.
The figure below illustrates this idea. For example, if you have determined your annual domestic water needs to be 40,000 gallons (and, most importantly, you have enough catchment area and annual precipitation to supply this amount of water), then you should design and build a cistern with a 10,000-gallon storage capacity.
Considering that our location has fairly even rainfall distribution over the course of the year, and knowing that the total water catchment for the Duplicable City Center is 99,240 gallons of water for the year, we chose to adopt a cistern of 25,000 gallons (3,345 cubic feet).
In this situation the best solution would be to split this by constructing two cisterns whose dimensions are:
Cistern volume: π x 5.5² x 18 x 2 = 3,421 cubic foot
Final Design of Rainwater Harvesting System: