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International Journal for Service Learning in Engineering
Vol. 9, No. 1, pp. 64-78, Spring 2014
ISSN 1555-9033
64
Gutter Design and Business Development for Domestic
Rainwater Harvesting Systems: A Case Study
Jillian Zankowski
Community Environment & Development,
Environmental Resource Management
College of Agricultural Sciences
The Pennsylvania State University
jdz5021@gmail.com
Yixin Sun
Engineering Science
College of Engineering
The Pennsylvania State University
yix5207@gmail.com
Abdalla Nassar
Applied Research Laboratory
The Pennsylvania State University
arnassar@gmail.com
Khanjan Mehta*
Humanitarian Engineering and Social Entrepreneurship
The Pennsylvania State University
khanjan@engr.psu.edu
*Corresponding Author
Abstract - Rainwater harvesting is a simple and effective tool to collect and store water for
domestic and institutional use. In developing countries, captured rainwater can be used to
replace or supplement government-supplied or manually-transported water. A rainwater
harvesting system consists of a catchment area, gutter, and storage tank. Gutters typically
have a V-shaped, trapezoidal or rectangular cross-section. This work presents a case study
on the design and performance analysis of three conventional and one novel, “wrapped”
gutter cross-section along with the implementation of a novel gutter design in the
developing world. A Team of undergraduate students performed the design and analysis
and, though a service-learning experience in May 2013, investigated barriers to
implementing rainwater harvesting in central Kenya. It was found that while gutters can
be easily fabrication and installed using locally-available materials and skill-sets, for
consumer, the potential return on investment was low and the cost of implementation was
high. For producers and installers, non-uniform roof designs and conditions was a major
obstacle.
Index Terms - Rainwater harvesting; Catchment; Domestic; Roof Runoff; Water; Business
Model
INTRODUCTION
Within water-scarce areas, specifically those that have underdeveloped water infrastructure,
rainwater collection is a creative means of sustaining water sources and improving the quality of
life1. One of the most common practices is the use of rainwater harvesting (RWH), a technique
that aims to collect water running off roofs and other surfaces1. A rainwater harvesting system
normally consists of the catchment area, an attached gutter with mounting system, and a storage
area2. Systems vary most greatly in the cross-sectional design of their gutters. In a previous work3,
we experimentally assessed the performance of four gutter cross-section designs based on their
ability to intercept and convey water to a collection tank. We showed that, under heavy rainfall
conditions, a cross-section that wraps around the ends of the catchment area reduces losses due to
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water overshooting the gutter. But, due to leaks associated with its more complex connections, the
wrap design was second best under rainfall of less than 7 inches per hour. It was asserted that
using more watertight connections, the performance of the wrap design could have been
improved. This work seeks to explore issues associated with in-the-field implementation of this
and other RWH systems in the developing world, within a service-learning framework.
The service learning experience took place during May 2013 in Nyeri, Kenya, as part of a
humanitarian engineering and social entrepreneurship program at Penn State University. A team
of undergraduate students, majoring in both engineering and agricultural sciences, and under the
supervision of engineering faculties, sought to understand if and how novel gutter systems could
be implemented in the developing world; what barriers to entry exist for this and other gutter
designs; and, where would implementation of rainwater harvesting be most beneficial to users?
To answer these questions, students built multiple prototypes and installed them to typical roofs at
a children and youth center in the area, gathering data to determine the costs and tradeoffs
associated with rainwater harvesting.
Prior to detailing field-work activates and results, a brief overview of the laboratory work, used
to determine gutter performance is given in Section I. Following this, in section II, a description
of the fieldwork is provided, together with an analysis of barriers to implementing rainwater
harvesting in the developing world. This work serves as both as a case study of a project-based,
service-learning experience, which complemented classroom and laboratory study, to investigate
broader issues associated with technological innovation.
SECTION I
DESIGN CRITERIA FOR GUTTER CROSS-SECTIONS
Material Selection
Rainwater harvesting gutter systems work successfully through the interaction of three basic
components: the catchment, delivery system, and storage reservoir4. Most critically, the
interaction between the catchment and the delivery system must convey water from the roof of a
home into the gutter system with minimal water lost. The total conveyance of a gutter system can
be calculated using Manning’s formula, shown in equation (1). This flow of water from the roof
(Q) is a function of the roughness coefficient of the gutter material (n), the cross-sectional area of
the gutter, (a), hydraulic radius of the gutter, (r), and the slope of the gutter following the roof,
(S)2.
(1)
Equation (1) can be used to compare the conveyance of different potential gutter materials,
using their roughness coefficients. Sheet metal is typically used due to its low roughness
coefficient (<0.015) and the ease with which it can be shaped. Moreover, sheet metal is readily
available in most of the developing world, including Kenya, where our fieldwork was conducted.
For these reasons, we used galvanized sheet metal to construct the gutters described here.
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Cross Section Design
While roughness dictated the materials used, cross-sectional area, a, had a greater impact on the
flow capacity of a gutter system, and thus the amount of water that could be collected and stored
within the storage tank. Cross-sectional area also directly impacts the performance of using a
gutter to collect rooftop runoff.
By compiling various data regarding flow rates and capacities of several gutter designs2,5, three
gutter designs, V-Shaped, Square, and Trapezoidal, were chosen for further examination. Through
the use of a rainwater simulator, we were able to determine the best gutter design for varying rain
intensities3. Along with V-Shaped, Trapezoidal, and Square gutters, a “wrap” design was
implemented during the second round of test trials. All cross-sections are shown in FIGURE 1,
copied, with permission, from Zankowski, et al., 2013.
FIGURE 1
CROSS SECTIONS (FROM LEFT TO RIGHT) OF V-SHAPED, SQUARE, TRAPEZOIDAL, AND WRAP
GUTTER DESIGN, WITH DASHED LINES REPRESENTING EXTENSION OF ROOF
Methodology
In order to compare cross-sections, each cross-section prototype was installed onto a test roof and
placed under a rainwater simulator modeled after that described by Miller and Fennessey6.
Experiments were conducted to measure conveyance, conductance and interception characteristics
of the four different gutter designs. Interception refers to the act of water flowing off the roof
being caught by the gutter. Gutters were tested under rainfall intensities of 1 inch (2.54 cm) per
hour, 3 inches (7.62 cm) per hour, 7 inches (17.78 cm) per hour, and 9 inches (22.86 cm) per
hour, to replicate the variable rainfall of Nyeri, Kenya, where fieldwork took7. Each experiment
lasted 2 minutes, used to replicate the first wave of intense downpour during a storm8. Rainfall
caught within the gutters flowed into a cylinder, which was compared to the amount of rain which
fell onto the roof, or catchment area. Overshoot and percentage of rainfall caught could be
calculated in this way. Under 1 inch per hour, gutters were subjected to 0.5 gallons (1.9 L) of
water, 1.5 gallons (5.7 L) under 3 inches per hour, 3.5 gallons (13.2 L) under 7 inches per hour,
and 4.5 gallons (17 L) under 9 inches per hour. A 5 foot (1.5 m) high test roof, with a catchment
area of 24 ft2 (2.16 m2) and an angle of 15 degrees (FIGURE. 2), was used to mimic rooftops
within Kenya. The V-Shaped, Square and Trapezoidal cross-sections were hung from the jig,
while the wrap-design was attached directly to the roof. Further experimental details are discussed
in the authors’ previous work3.
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FIGURE 2
FRONT IMAGE OF TEST JIG
To compare the cross-sections, overflow, overshoot, the volume of standing water left in the
gutters, volume caught, and rate of draining were taken into account. Standing water refers to
water remaining inside the gutter after the rainfall. The amount of standing water was influenced
by the gutter’s longitudinal slope along the side of the building, as well as the length of the
building. To create a slope along the building, the distance from the roof to the bottom of the
gutter’s cross-section increased from one end of the gutter towards the downspout; longer
buildings thus would require flatter slopes. Volume caught refers to water, which fell onto the
roof, which was collected by the downspout during the rainfall, while rate of draining was
measured by the amount of time that has passed between the end of the rainwater simulation and
the cessation of water flow through the gutter. Data points in each category were weighted so as
to be comparable. TABLE I shows the weights assigned to each category, adding up to 1, to assess
total gutter performance. Weights were subjectively determined by the authors based on the
perceived importance of those criteria for the Kenyan context3. For full explanation of weighted
points and reciprocal evaluation, please see Appendix 1
TABLE I
SCORING MATRIX FOR GUTTER PERFORMANCE
Design Criteria
Weight
Standing Water
0.125
Overflow
0.250
Overshoot
0.250
Caught vs. Lost
0.250
Rate of Draining
0.125
Total Score
1.000
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Results
Laboratory data were collected for each cross-sectional design under 1 inch, 3 inches, 7 inches,
and 9 inches per hour. Within a typical year in Kenya, rainfall intensity is 1 inch per hour or
less9. Thus for the remainder of the paper, data collected at a rainfall intensity of 1 inch per hour
are used in evaluating performance and economy. Weighted performance data for this intensity,
including overflow, drainage, standing water, overshoot and total rain caught, can be found in
TABLE II. To be clear, these are not raw data, but weighted (dimensionless) values based on the
weights assigned in TABLE I. Raw data are available in the authors’ previous work3.
TABLE II
WEIGHTED GUTTER PERFORMANCE UNDER 1 INCH OF RAINFALL PER HOUR
Raining
intensity
Overflow
Draining
Rate
Standing
water
Overshoot
Rain
Caught
V Shape
1
1.72
4.29
1.24
1.05
Wrap
1
1
4.29
1.68
1.11
Trapezoid
1
1.66
1
2.8
1.18
Square
1
1.54
1.58
1
1
While overflow was not observed in any instance, overshoot was a common problem.
Overshoot of the gutter increased with rainfall intensity. Though the wrap design (shown in
FIGURE. 3) did surround the edge of the roof, gaps between the gutter and the roof caused
leakage. This water loss was attributed to overshoot, since it was a measure of the gutter’s ability
to intercept the water flowing from the roof. Watertight connections would significantly enhance
the gutter’s overshoot performance.
FIGURE 3
CROSS-SECTIONAL VIEW OF WRAP DESIGN, IN LAB
The weighted performance data, presented in TABLE II allows comparison of each gutter
across our design criteria. A sum of performance values across all criteria was used to compare
overall performance of gutter cross-sections under different rainfall intensities. Final results
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comparing performance of gutter cross-sections for different rainfall intensities are shown in
TABLE III.
TABLE III
BEST-FIT GUTTERS FOR PARTICULAR RAINFALL INTENSITIES
Rainfall Intensity
Square
V-Shape
Trapezoidal
Wrap
1 in/hr
X
3 in/hr
X
7 in/hr
X
9 in/hr
X
Though a trapezoidal gutter cross-section was determined to be the best fit for rainfall
conditions at, or below, 3 inches per hour, the wrap design was chosen for field-testing in Kenya
for two reasons. One, the wrap design’s underperformance with respect to overshoot was viewed
as improvable with watertight connectivity. Two, although rare, rainfall intensity above 3 inches
per hour does occur and results in a significant rainfall that can be captured9. Field research was
focused on determining the feasibility of using this design on non-uniform roofs in the Kenyan
context and the feasibility of implementing a more complex, though more efficient, gutter design.
SECTION II
COST-BENEFIT ANALYSIS AND ECONOMIC IMPLICATIONS
During May 2013, fieldwork was done in Kenya to test both the feasibility of local construction of
the rainwater harvesting system (RWH), as well as to investigate the economics of rainwater
harvesting in Nyeri, Kenya. Due to limited time and resources, only the wrap design was field
tested in Nyeri. This section will discuss the barriers to adoption of RWH technologies within
Kenya, with particular emphasis on feasibility of a cost-efficient system, the value proposition to
both producer and consumer, and the scalability of such a RWH business venture. Barriers to
entry were found specifically with regard to return on investment due to higher installation costs,
compared to locally available options, as well as the availability of government supplied water.
These issues are discussed through the use of an efficiency analysis, with regard to water caught
over time, and a cost analysis, taking given efficiencies and the time until the gutter system
become profitable.
Cost and efficiency analysis
Efficiency analysis
Rainwater harvesting systems tend to be sold by the foot, or meter, within the Nyeri market, as is
common within the developing world context. In order for a rainwater harvesting system to be
cost-effective, the system must cost less than, or equal to, the cost of water, over its lifetime.
Thus, determination of cost-effectiveness requires knowledge of average rainfall, material cost
and calculation of gutter efficiency.
First, the efficiency of the gutter, with regard to price, can be calculated using raw material
costs as found in Nyeri. The efficiencies of each gutter can be determined through Equation (2).
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(2)
TABLE IV compares the efficiency of four gutter designs assuming a rainfall of 1 inch per
hour, and a 400 ft2 (37 m2) roof. This roof area is comparable to ones found in small schools,
strip malls, and similar structures in Kenya. For example, the manager of the Children and Youth
Empowerment Center (CYEC) in Nyeri, Kenya, estimated that nearly 2000 gallons (7570 L) of
water was purchased by the center from the government each month. Based on this, the hours of
rainfall needed was calculated using Equation (3), with M representing the number of hours of
rainfall needed in order to match the 2,000 gallons used per month. At a rainfall intensity of 1
inch per hour, 33.3 cubic feet, or 249.33 gallons fall on the roof each hour. The hours of rainfall
necessary, per month, to meet the total consumption needs of the CYEC are given in TABLE IV.
(3)
TABLE IV
HOURS OF RAINFALL PER MONTH NEEDED, AT A RAINFALL INTENSITY OF 1 INCH PER HOUR, TO
COLLECT 2000 GALLONS BASED ON EFFICIENCY
Efficiency
Hours of continuous
rainfall to reach 2000
gallons
V Shape
0.82
10.10
Trapezoidal
0.92
9.00
Square
0.78
10.62
Wrap
0.87
9.52
Based on the values in TABLE IV, the ranges of rain needed for each system to store enough
water to replace that purchased from the government is between 9.00 and 10.62 inches. While
seemingly small, the wettest month of the year, April, only produces 7.7 inches of rain on
average9. It is therefore impossible to collect sufficient rain to meet monthly needs. Hence,
harvested rainwater is unlikely to replace purchased water. A rainwater harvesting system may
however be used as a supplemental water delivery system, and can save money over time by
decreasing the need to rely on purchased water.
Cost analysis
Consumers want to buy the least expensive and most durable product for their home or business.
Within the developing world context, cost is a primary driving factor. TABLE V compares the
prices of different gutters per 6-foot sections, based on raw materials and tools needed. In
calculating the values shown in TABLE V, it was assumed that the gutters are attached to a 20 ft x
20 ft roof, typical of large institutions surveyed in Nyeri, Kenya, requiring 80 feet of gutter. It
may be noted that 80 ft is an over estimate, as many institutions use a gable or skillion roof rather
than a pyramid-shaped roof. It was also assumed that 650 Kenyan shillings (KES) will be spent
for a tank--this figure is based on a survey of shop owners within the market of Nyeri, Kenya.
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The cost of a full 80-foot system can be estimated by calculating the cost of 80 feet of gutter and
then adding the cost of the tank. These prices were used to determine how many hours of
rainfall, at a relatively strong rainfall intensity of 1 inch per hour9, would be needed to recoup the
capital costs (i.e. return on investment). Payback periods can be seen in TABLE V.
TABLE V
RETURN ON INVESTMENT EXPRESSED IN HOURS
Gutter Type
Price per 6
feet (KES)
Price per
system
(KES)
Hours of rain until Break
Even
Trapezoid
657.33
9414.4
38.99
V Shape
429
6370.0
28.68
Wrap
570
8250.0
43.81
Square
450
6650.0
31.66
Assuming that water costs 2,000 KES per month for an average local institution, it would take
over a day of continuous rainfall or more than 28 inches of rain for the gutter systems to make
back their investment in full. Even when spread out over the course of a month, it is highly
unlikely that sufficient rainfall events would occur in an arid or semi-arid environment,
particularly in the dry season. Simply put, there are cheaper options available to obtain clean
water other than a RWH system.
Barriers to Entry
Standardization Barrier
One of the fundamental objectives of this rainwater harvesting project was to standardize gutter
design and installation. However, our fieldwork demonstrated that this approach is inherently
problematic. In Nyeri, Kenya, and much of the developing world, roofs are not standardized
(FIGURE. 4), and thus anything attached to them has to be altered to account for unique roof
designs.
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FIGURE 4
CLOSE-UP OF A RUSTED AND NON-UNIFORM ROOF IN NYERI, KENYA
For one, materials used to make roofs vary considerably. In cities, most rooftops are
constructed with corrugated metal sheet in flat, skillion (or lean-to), and hipped shapes. In rural
areas, where people do not have access to sheet metal, rooftops are constructed with lumber,
leaves, straw, and newspaper. Practically, metal-gutter-based rainwater harvesting systems can
only be attached on the skillion or fascia board and hipped roofs made of sheet metal. This
implies that rural areas, where the need for water is greatest, cannot be supplied easily. Secondly,
there is no standard for the slope of the roof and the distance between the edge of the roof and
the wall. Since our wrap design is dependent on the slope of the roof and the distance between
the edge of the roof and the wall, customizing the gutter design is crucial and inevitable. Thus,
mass manufacturing, which can drastically reduce overhead costs cannot be easily employed.
Moreover, lack of maintenance is a common problem in both rural and urban areas, contributing
to rusted and fragile roofs that are not strong enough to attach gutter systems.
Value proposition challenges
To justify the expense of the gutter, its primary purpose of collecting and storing rainwater has to
be the main goal of the consumer. Within the parts of Kenya in which government water delivery
is available, there is no need to collect rainwater for domestic use. Government-supplied water is
also widely available in the more developed areas, which coincide with areas that receive periods
of heavy rainfall and have many available bodies of water. Bodies of water, including streams,
rivers, and lakes, are used when there is a need for more water than what the government is
providing through taps. Thus, the need for an additional water source is nonexistent. Informal
interviews with locals indicated that these regions tend to use gutters to keep water from running
down exterior building walls to prevent them from decaying. In this instance, a wrap design
would provide the most benefit, since, when installed correctly, no gaps occur between the gutter
and the wall of the building, allowing minimal water to escape.
In other contexts, such as Northern regions of Kenya, where water is scarcer, drought is a
common occurrence. Schools and households in Kenya struggle to continually have enough
water to keep their facilities running. Throughout Kenya, while there are 55 water service
providers, only 7 of them offer continual service, including Nyeri, where our research took
place10. For the areas under the other 48 water service providers, rainwater harvesting would be
beneficial. However, medium and large institutions require much larger amounts of water than
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what can be provided by domestic rainwater systems. In order to meet 2000 gallons of water
needed based on the previous assumption, massive catchment systems would have to be
installed. Unfortunately, such massive systems require the institution to have access to
significant financial capital and long-term maintenance.
While not all areas of Kenya have access to government-supplied water, those that do not tend
to be in arid areas that have less than less than 14 inches of rain over the year11. In these regions,
water bodies are the primary sources of water. Though residents may need to travel long
distances to get to these bodies of water, there is a lack of other employment; so wasted time
does not mean wasted money. Thus, the rainwater harvesting system can save time for local
people, but they do not see the value, and the value proposition fails. While there is a need for an
additional water source within the area, domestic rainwater harvesting does not provide the right
answer, since there is little rainwater to be harvested.
The value proposition for the entrepreneur is fundamentally connected to the value created by
the product for potential customers. Within the central Kenyan market, customers we
interviewed did not find inherent value in installing rainwater harvesting systems on their roofs.
The value of the product is hard to justify. At the same time, due to the non-uniformity of each
roof, each gutter needs to be customized based on the slope of the roof and the distance between
the edge of the roof and the wall. This leads to higher costs and makes the product even less
marketable. Thus, mass manufacturing is not possible and the installation process would need to
be customized to every roof, requiring a trained cohort of workers. In essence, a business that
installs domestic rainwater harvesting systems is not a lucrative venture under current conditions
and municipal water costs.
CONCLUDING REMARKS
Initially, the student team set out to design an efficient domestic rainwater harvesting systems to
address the need for affordable, reliable and sustainable water sources in developing countries
likes Kenya. Towards this end, a three-tiered approach was used. First, a laboratory analysis of
the performance of four gutter cross-sections was conducted to assess the performance of three
conventional and one novel gutter designs under rainfall between 1 and 9 inches per hour. Next,
an analysis of the costs, benefits and challenges to implementations of a rainwater harvesting
system, based on fieldwork in central Kenya, was conducted. Finally, economic and laboratory
results were compared to determine the value and feasibility of a rainwater harvesting venture in
Kenya.
While the ability to procure material, fabricate and install gutter systems was easily validated
in Nyeri, Kenya, economic barriers and value proposition challenges emerged as main obstacles.
A key challenge is the cost of the rainwater harvesting system. In areas where rainfall intensities
are sufficient to enable consumers to collect enough water to meet their needs, inexpensive
(utility-supplied) or free (manually-transported) water is available. Furthermore, conversations
with consumers revealed that they purchased gutters to protect exterior walls from water damage,
not for rainwater harvesting. Coincidentally, the communities in need of sustainable and reliable
water delivery lacked access to capital and were not capable of recouping their capital costs in a
short term due to the lack of rain in their area. While the laboratory results behind our design
holds strong, it does not serve as a viable platform for a successful business.
Potentially, one can still implement the proposed wrap design in a different context, with a
unique entrepreneurial approach, to help alleviate water scarcity. Non-profits can use the design
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to enhance the efficiency of the rainwater harvesting system donated to help local communities.
For-profit businesses can implement the design in urban areas, where people have more
disposable income to invest in a rainwater harvesting system.
Ultimately, however, this work serves as a case study that illuminates the challenges
associated with implementation of rainwater harvesting systems in the developing world. Though
it may be argued that some implementation challenges, such as inconsistent roof designs, should
have been obvious to the student team without fieldwork, the fieldwork proved to be the critical
factor in understanding the complexities of applying engineering solutions in the developing
world. Informal interviews with locals allowed generation of realistic pricing and water
consumption data. First-hand experience installing gutters allowed students to understand the
difficulties associated with installation of a gutter on rusty, non-uniform roof. Soliciting the
views of local vendors, homeowners and managers of institutions allowed students to better
appreciate that though the design may work, individuals will not purchase an item they don’t see
as cost efficient or beneficial. Through this service learning experience, students were able to
better appreciate engineering design and its real-world considerations, as well as the broader
impacts.
ACKNOWLEDGMENTS
We would like to acknowledge Dr. James Hamlett for generously allowing us to use his
rainwater simulator and assisting us in data collection and analysis. Thank you to Chiyan Poon
and Emily Passauer, who helped in data collection. We would also like to thank Tara Sulewski,
Liz Kisenwether, Susan Beyerle, Andras Gordon and Irene Mena for mentoring several freshmen
design teams that collectively inspired our team to explore various gutter designs and installation
methods. Lastly, thank you to those that made our fieldwork possible through scholarship and
awards, particularly Penn State’s Undergraduate Research Exhibition judges, who awarded us
the Gerard A. Hauser award, contributing to our research funds.
REFERENCES
1 Sazakli, E., Alexopoulos, A., & Leotsinidis, M. (2007). Rainwater harvesting, quality
assessment and utilization in Kefalonia Island, Greece. Water Research, 2039-2047.
2 Still, G., & Thomas, T. (December 2002). The Optimum Sizing of Gutters for Domestic
Roofwater Harvesting. Research Report, University of Warwick, School of Engineering.
3 Zankowski, J., Sun, Y., Poon, F., Passauer, E., Nassar, A., & Mehta, K. (2013). Gutter Design
and Selection for Roof Rainwater Catchment. IEEE Global Humanitarian Technology
Conference. San Jose, CA.
4 Worm, J., & van Hattum, T. (2006). Rainwater Harvesting for Domestic Use. Wageningen, The
Netherlands: Agromisa Foundation.
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5 Hatibu, N., Mutabazi, K., Senkondo, E., & Msangi, A. (2006). Economics of rainwater
harvesting for crop enterprises in semi-arid areas of East Africa. Agricultural Water
Management, 80(1-3), 74–86.
6 Fennessey, L. A., Miller, A. C., & Hamlett, J. M. (2001, August). Accuracy and precision of
NRCS models for small watersheds. Journal of the American Water Resources Association,
37(4), 899-912.
7 The World Factbook. (2013, April 9). (Central Intelligence Agency) Retrieved April 16, 2013,
from https://www.cia.gov/library/publications/the-world-factbook/geos/ke.html
8 Homebuilding, F. (2005). Roofing: Flashing & Waterproofing. Newtown, Connecticut, USA:
The Taunton Press.
9 World Weather Online. (2013). World Weather Online. Retrieved November 18, 2013, from
http://www.worldweatheronline.com/Nyeri-weather-averages/Central/KE.aspx
10 WASREB. (2009). Water Service Quality - WASREB Impact Report 2009. Kenya: WASREB.
11 Community Adaptation and Sustainable Livelihoods. (n.d.). Arid and Semi-Arid Lands:
Characteristics and Importance. Retrieved November 11, 2013, from International Institute for
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APPENDIX
Methodology of rainfall simulation, from Gutter Design and Selection for Roof Rainwater
Catchment 3
METHODOLOGY
Experiments were conducted on a rainwater simulator that was modeled after the simulator
described by Miller and Fennessey.6 The simulator was used to test conveyance, conductance and
interception characteristics of the four different gutter designs (FIGURE. 1). Rainfall intensities
were described as “light”, “moderate”, “heavy”, and “violent”, with rainfalls of 1 inch per hour, 3
inches per hour, 7 inches per hour, and 9 inches per hour, respectively. Note that use of these
descriptors of rainfall intensities are only intended to be descriptive for the purposes of this paper
and not standardized as in Still and Thomas.2 The duration of each experiment was 2 minutes.
Such a short experimentation time was used to mimic the first wave of intense downpour during a
storm.8 This normally occurs within the first 5 minutes, but due to time constraints, 2 minutes was
used. During this time, rainfall was captured in a calibrated graduated cylinder. Rainfall intensity
was determined by dividing the total amount captured in the cylinder by the duration of the
experiment (1/30) hours. It may be noted that increasing the duration of the experiment from 2 to
3 minutes did not affect the gutter performance results.
A 5 foot high test roof (Fig. 2), angled at 30 degrees, and with a 4 ft by 6 ft corrugated metal
sheet secured on top was used to simulate a roof catchment. The total catchment area was 24 ft2.
The ‘V’ Shaped, Square and Trapezoidal gutters were suspended from the roof catchment using
steel wire hung from an iron rod. The wrap gutter, however, was attached using nuts and bolts
every 1.2 meters. During the light, moderate, heavy, and violent rainfalls, each gutter was
subjected to 1.89L, 5.66L, 13.21L, and 16.99 L of water over 2 minute duration of each
experiment, respectively. This volume was calculated based on roof area, roof angle and rainfall
intensity.
International Journal for Service Learning in Engineering
Vol. 9, No. 1, pp. 64-78, Spring 2014
ISSN 1555-9033
77
FIGURE 5
FRONT IMAGE OF TEST JIG
Collected data reflected the amount of water lost through overflow, rate of water drainage,
amount of standing water remaining in the gutter, amount of water lost via overshoot and the total
amount of rain caught by the gutters. Overflow was defined as the volume of water that exceeded
the volume of the gutter, and spill out of the gutter, measured by the liters of water collected from
the gutter’s downstream. Rate of drainage was defined as the elapsed time after rainfall had
stopped and the flow from the gutter stopped. Standing water was the volume of water remaining
in the gutter after all flow had ceased, measured in milliliter. These measured values characterized
the conveyance of each gutter system. Once raw data were collected for each gutter and rainfall
intensity, they were ranked against each other and then multiplied by their weighted rates shown
in Table VI. Weights in Table VI were determined based on our assessment of criteria important
for success of a gutter system in and around Nairobi, Kenya. Readers are encouraged to design
their own scoring matrix based on their specific context.
TABLE VI
SCORING MATRIX FOR GUTTER EFFICIENCY.
Design Criteria
Weight
Standing Water
0.125
Overflow
0.250
Overshoot
0.250
Caught vs. Lost
0.250
Rate of Draining
0.125
Total Score
1.000
International Journal for Service Learning in Engineering
Vol. 9, No. 1, pp. 64-78, Spring 2014
ISSN 1555-9033
78
Ranking was accomplished by normalizing the reciprocals within a given data set (e.g.
overflow, rate of draining, etc.), with 1 being equal to the smallest reciprocal. For instance, if A,
B, C and D are measured, scalar values of some property, each respective reciprocal was scaled
such that
c × min{1/A, 1/B, 1/C, 1/D}=1
Next, the scaled quantities, {c/A, c/B. c/C, c/D}, where c is the scaling coefficient, were
multiplied by the weight for the measured data set, given in Table VI. The resulting data was
termed weighted data.
Radar plots juxtaposed weighted data for each gutter design. This allowed determination of the
best gutter for each rainfall intensity. Note that the scoring matrix shown in Table VI is
subjective and based on the importance of each criteria within the context in which the RWH
system is expected to be used. Additionally, it should be noted that experiments were conducted
under laboratory conditions, where environmental factors such as wind, rusting and roof
deformation were not accounted for. These factors may ultimately prove important in real-world
implementation.
