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1 Appendix A Sizing Storm Drainage Systems 2009 National Standard Plumbing Code 219

2 A.1 Rainfall Rates for Cities The rainfall rates in Table A.1, RAINFALL RATES FOR CITIES, are based on U.S. Weather Bureau Technical Paper No. 40, specifically Chart 14: 100-YEAR 1 HOUR RAINFALL (inches) and Chart 7: 100-YEAR 30-MINUTE RAINFALL (inches). The data in Chart 7 were multiplied by 0.72 to determine the rainfall for a 15-minute period, then multiplied by 4 to establish the corresponding rainfall rate in inches per hour. The flow rates in gallons per minute (gpm) were established by dividing the inches per hour by 12 to determine cubic feet per hour per square foot, then multiplying by 7.48 gallons per cubic foot to determine gallons per hour per square foot, then dividing by 60 minutes per hour to determine the equivalent gallons per minute. A.2 Roof Drainage Primary roof drainage systems are sized for a 100-year, 60-minute storm. Secondary roof drainage systems are sized for a more severe 100-year, 15-minute storm. The rainfall rates in Table A.1 should be used for design unless higher rates are established locally. A.3 Sizing by Flow Rate Storm drainage systems may be sized by stormwater flow rates, using the appropriate GPM/SF of rainfall listed in Table A.1 for the local area. Multiplying the listed GPM/SF by the roof area being drained (in square feet) produces the gallons per minute (gpm) of required flow for each drain inlet. The flow rates (gpm) can then be added to determine the flows in each section of the drainage system. Required pipe sizes can be determined from Table and Table A.4 Sizing by Roof Area Storm drainage systems may be sized using the roof area served by each section of the drainage system. Required pipe sizes can be determined from Table and Table Using this method, it may be necessary to interpolate between the various listed rainfall rates (inches per hour). To determine the allowable roof area for a listed size pipe at a listed slope, divide the allowable square feet of roof area for a 1" rainfall rate by the listed rainfall rate for the local area. For example, the allowable roof area for a 6" drain at 1/8" slope with a rainfall rate of 3.2 inches/ hour is 21400/3.2 = 6688 square feet. A.5 Capacity of Rectangular Scuppers Table A.5 lists the discharge capacity of various width rectangular roof scuppers with various heads of water. The maximum allowable level of water on the roof should be obtained from the structural engineer, based on the design of the roof National Standard Plumbing Code

3 Table A.1 RAINFALL RATES FOR CITIES ALABAMA Birmingham Huntsville Mobile Montgomery ALASKA Aleutian Islands Anchorage Bethel Fairbanks Juneau ARIZONA Flagstaff Phoenix Tucson ARKANSAS Eudora Ft. Smith Jonesboro Little Rock CALIFORNIA Eureka Lake Tahoe Los Angeles Lucerne Valley Needles Palmdale Redding San Diego San Francisco San Luis Obispo COLORADO Craig Denver Durango Stratton CONNECTICUT Hartford New Haven DELAWARE Dover Rehobeth Beach DISTRICT OF COLUMBIA Washington FLORIDA Daytona Beach Ft. Myers Jacksonville Melbourne Miami Palm Beach Tampa Tallahassee GEORGIA Atlanta Brunswick Macon Savannah Thomasville HAWAII Hawaiian Islands (1) 2009 National Standard Plumbing Code 221

4 Table A.1 RAINFALL RATES FOR CITIES, continued IDAHO Boise Idaho Falls Lewiston Twin Falls ILLINOIS Chicago Harrisburg Peoria Springfield INDIANA Evansville Indianapolis Richmond South Bend IOWA Council Bluffs Davenport Des Moines Sioux City KANSAS Goodland Salina Topeka Wichita KENTUCKY Bowling Green Lexington Louisville Paducah LOUISIANA Monroe New Orleans Shreveport MAINE Bangor Kittery Millinocket MARYLAND Baltimore Frostburg Ocean City MASSACHUSETTS Adams Boston Springfield MICHIGAN Cheboygan Detroit Grand Rapids Kalamazoo Traverse City MINNESOTA Duluth Grand Forks Minneapolis Worthington MISSISSIPPI Biloxi Columbus Jackson National Standard Plumbing Code

5 Table A.1 RAINFALL RATES FOR CITIES, continued MISSOURI Independence Jefferson City St. Louis Springfield MONTANA Billings Glendive Great Falls Missoula NEBRASKA Omaha North Platte Scotts Bluff NEVADA Las Vegas Reno Winnemucca NEW HAMPSHIRE Berlin Manchester NEW JERSEY Atlantic City Paterson Trenton NEW MEXICO Albuquerque Carlsbad Gallup NEW YORK Binghamton Buffalo New York Schenectady Syracuse NORTH CAROLINA Ashville Charlotte Raleigh Wilmington NORTH DAKOTA Bismarck Fargo Minot OHIO Cincinnati Cleveland Columbus Toledo Youngstown OKLAHOMA Boise City Muskogee Oklahoma City OREGON Medford Portland Ontario National Standard Plumbing Code 223

6 Table A.1 RAINFALL RATES FOR CITIES, continued PENNSYLVANIA Erie Harrisburg Philadelphia Pittsburg Scranton RHODE ISLAND Newport Providence SOUTH CAROLINA Charleston Columbia Greenville SOUTH DAKOTA Lemmon Rapid City Sioux Falls TENNESSEE Knoxville Memphis Nashville TEXAS Corpus Christi Dallas El Paso Houston Lubbock San Antonio UTAH Bluff Cedar City Salt Lake City VERMONT Bennington Burlington Rutland VIRGINIA Charlottesville Richmond Roanoke Norfolk WASHINGTON Seattle Spokane Walla Walla WEST VIRGINIA Charleston Martinsburg Morgantown WISCONSIN La Cross Green Bay Milwaukee Wausau WYOMING Casper Cheyenne Evaston Rock Springs Rainfall rates in Hawaiian Islands vary from 1.5 in/hr to 8.0 in/hr depending on location and elevation. Consult local data National Standard Plumbing Code

7 WATER HEAD (Inches) Table A.5 DISCHARGE FROM RECTANGULAR SCUPPERS - GALLONS PER MINUTE WIDTH OF SCUPPER - (Inches) NOTES: 1. Table A.5 is based on discharge over a rectangular weir with end contractions. 2. Head is depth of water above bottom of scupper opening. 3. Height of scupper opening should be 2 times the design head. 4. Coordinate the allowable head of water with the structural design of the roof National Standard Plumbing Code 225