Design Guide for Community Water Systems - The Water, Sanitation ...

parameters: average winter (December, January, February) daily flow intoand out of the tank; winter diurnal flow pattern into and out of the tank;water temperature into the tank under the winter design conditions; heattransfer between the water layer and air layer in the tank; heat transfer ratethrough the tank wall and any insulating layer; heat transfer from theoutside tank surface to the atmosphere based on wind speed and airtemperature from the winter design conditions; solar energy input to thetank during daylight under the winter design conditions; and radiation heatloss from the tank to the sky during night time under winter designconditions. If winter design conditions are not available, a reasonablyconservative estimate can be made using the high and low temperaturefrom the record low temperature day from the nearest U. S. weatherstation with 100 years of record for one of the seven days, using theaverage coldest winter day from the nearest U. S. Weather station with100 years of record for the other six days, using sunrise/sunset times andsun angle for the seven days centered on the winter solstice, using averagewind speed for December – February from the nearest U. S. Weatherstation with 100 years of record, and assuming 50% cloud cover for skyconditions. This energy balance should be used to set the winter tankturnover rate, winter pump ON/pump OFF elevations, inlet and outletdesigns to ensure mixing to minimize ice formation, insulationrequirements (if appropriate) and heater requirements (if appropriate).c. The tank should be designed to turn over a sufficient percentage of thestored water daily to minimize aesthetic water quality problems. Thispercentage may vary with local condition, but a 25% daily turnover issuggested as a default value. Note that additional turnover may berequired for winter operations to minimize freezing. Separate inlet andoutlet lines should be provided.7.0.3. Locationa. The bottom of reservoirs and standpipes and footings for elevated tanksshall be above the 100 year return frequency flood level and shall beabove the highest known historic flood elevation. Top of footings forelevated tanks shall be at least one foot above the finished grade.b. The bottom of reservoirs and standpipes and footings for elevated tanksshall be above the true ground water level.c. The bottom of reservoirs and standpipes should be placed above thenormal ground surface. When the bottom must be placed below groundsurface, sewers, drains, standing water and similar sources ofcontamination must be kept at least 50 feet from the reservoir except thatspecially constructed gravity sewers may be located no closer than 20 feetfrom the reservoir. These specially constructed gravity sewers shall bemade of water main pipe pressure tested in place to 50 psig pressurewithout leakage. The top of the reservoirs shall not be less than two feetabove the normal ground surface except that clear wells under filters mayAug. 29, 2003Page 134