MEMORANDUM
TO:
Simajuleu Water Distribution Team
FROM:
John Chlebeck
DATE:
March 24, 2010
RE:
Implementation Planning
A.
Addressing “Actual Demand” vs. “Theoretical Demand”
1.
2.
3.
4.
Actual demand includes leakage
Actual demand measured during main line metering
Typical water use ~60,000 gpd (based on metering)
Water use per capita (including leakage, based on Jan./Feb. metering):
Ave. Gal /
Person /
Day
Water
Sector Use
1/2
68
3/4
88
5
92
Ave. Gal /
Sector /
Day
Water Use
59,764
57,713
53,079
Max. Gal /
Sector /
Day
Water Use
80,200
64,500
60,100
5. This is significantly greater than theoretical demand based on WHO estimates and the use
measured at individual residences
6. Average rate over day ~42 gpm
7. Assumed peak use in morning and evening ~85 gpm (2x average)
i. Is this reasonable?
ii. Possibly peaks are greater if lots of people filling home tanks at same
time, but if sufficient storage on system the peaks will be met without
depleting system (see tank size considerations below).
iii. However, distribution system is too small to handle this flow rate: at
50 gpm, the velocity in a 1 ¼” pipe is about 13 ft/s, and the headloss
is about a foot for every foot of pipe length! That is likely the reason
why some residents downstream are getting no water when
upstream users are filling their tanks. In a 1 ½” pipe, the velocity is
about 9ft/s, and the headloss is about ½ foot for every foot of pipe
length.
8. Should we plan for additional demand due to growth of the community over the next 20
years or so? Maybe use 2x current demand.
9. If leakage is corrected, it would correlate more with the per capita demands that were
recorded on the individual service meters: maximum observed was about 50 gallons /
person / day. If there are 572 people living in Sector 5, that is a total demand of about
30,000 gallons / day rather than 60,000 gallons per day. Maybe we should plan for 2 x
Implementation Planning
March 24, 2010
Page 2
this rate, so 60,000 gallons. If / when the population grows, the distribution system will
need to be fixed eliminate some of the leakage.
B. Phased Implementation
1. Maintain current water system management strategy (3 valve system) in short-term
2. Construct additional storage in Sector 5 as Phase I.
a. Create more reliability in supply and less overflow
b. Reduce downstream pressures
3. Do we need to also upgrade / replace the distribution system?
a. I think replacement of the line from the new tank location to the end of the line
may be beneficial, or any portion of it that we can afford to replace.
b. A 2” line should be sufficient. Likely demands on the system will be decreased
with the new line in place due to reduced leakage at service line joints.
c. This is going to be expensive and labor intensive! Do we have a cost estimate
per lineal foot of water main replacement for materials? How long will it take
(on a per lineal foot basis) to replace main and make service connections?
4. Future phases will depend upon success of Phase I.
C. Phase I: Sector 5 Tank Design Considerations
1. Tank size
a. Capture existing overflows to reduce waste
i. Water that overflows at Main Tank will instead flow down to sector tank
to be stored for other purposes.
ii. Maximum difference between supply and demand during Jan. / Feb.
metering was a surplus of 18,680 gallons.
iii. A minimum tank volume of 20,000 gallons (similar to existing main
tank) would likely handle most overflow conditions.
b. Provide buffering to absorb peak demands during the day
i. Example daily demand pattern (Pine City, MN):
Implementation Planning
March 24, 2010
Page 3
ii. The storage tank(s) needs to meet demands above the supply rate
iii. If supply rate is 40 gpm, and the demands increase to 80 gpm (assumed)
for three hours in the morning and three hours in the evening, the total
storage necessary to absorb the peaks would be:
(6 hours)x(60 min / hr)x(80 gal/min – 40 gal/min) = 14,400 gallons
c. Provide reliable supply of water when sector is offline
i. Provide for at least three days of water supply at typical actual demand
(including leakage)
ii. Actual demand is around 60,000 gpd
iii. Minimum tank size to meet this criteria would be 180,000 gallons.
iv. However, if water main is replaced, the actual demand could be
significanly reduced.
v. If 15 feet deep, the dimensions of such a tank would be about 110 ft. x
110 ft.
2. Other Design Considerations
a. Soil / geotech analysis
i. Potential for settling or foundation instability?
ii. Seismic design necessary for this area?
iii. Collect soil samples to run geotech analysis during next assessment?
b. Tank overflow provisions
i. Away from foundation
ii. Consider nearby property and potential damage
iii. Have overflow pipe discharge above grade with a downward facing
screened outlet to avoid contamination
c. Foundation / slab design / wall thickness / reinforcement etc.
i. Guatemalan design standards? Building codes?
ii. Main Tank seems to have held up well, should we follow a similar
design?
iii. Do we have a structural engineer to help out?
d. Chlorine feed capabilities
i. Erosion tablet dosing
e. Venting
i. To prevent structural damage during draining of tank due to vacuum
creation
f. Mixing
i. To prevent water stagnation or short-circuiting
ii. Inlet at top in one corner of tank
iii. Outlet at bottom in opposite corner
iv. Baffle wall necessary?
g. Drain for cleaning (and flushing after disinfection)
i. Check main tank design
ii. Grout tank bottom and slope to drain
h. Public tap stands at tank
i. Have a couple tap stands so anyone can get water just in case
ii. This would be a better option than resorting to surface water sources
iii. Could remove some of the incentive to horde if water is always available
in emergency
i. Float valve
i. Prevent overflow with float valve controlling tank influent
Implementation Planning
March 24, 2010
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jdc
c:\ewb\simajuleu\third assessment and implementation\memo.docx