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OnsiteStorageCalculationReivew DRAFTT TECHNNICAL MEMORANDUUM On-Site Storagge Calcuulation Revieww TO: Scott Willia ms, Public WWorks Direcctor FROM: Brian Icermman, PE; Jarrrod Hirneisee, PE DATE: October 2, 2018 SUBJECT : Onsite Storrage Calculaation Revieww Jones Edmuunds Projectt No. 085055-003-01 1 INTTRODUCTION Section 244-66 of the City of Atlanntic Beach’ss Zoning, Suubdivision, aand Land Deevelopment Regulationns requires that the diffference betwween the prre- and postt-developmeent volume of stormwateer runoff be stored on-ssite for rainffall depths uup to the 255-year/24-hhour return pperiod depth if 4000 square feeet or moree of new imppervious areea is added. The City prreviously developedd a methodoology using a Microsoft Excel sprea adsheet to calculate pree-and post- developmeent runoff vvolumes to ddetermine h ow much onn-site storagge volume iis required aand compares the requireed volume too the actual storage volume providded. The Citty has askedd Jones Edmmunds to revview the Citty’s methodoology and thhe validity/aaccuracy of the assumpptions and parammeters used in the calcuulations. Th is Technical l Memorandum summarizes the finndings and recommmendations from the rreview. 2 PREE- AND PPOST-D EVELOPPMENT RRUNOFFF VOLUMME The first sstep of the CCity’s on-sitee storage caalculator is ggenerating ppre- and poost-developmment runoff voluumes. The rrunoff volummes are calcculated by mmultiplying aan area-weigghted runofff coefficientt by the 25-year/24-hour rainfall d epth and thhe parcel areea. The diffeerence betwween the pre-and post-devvelopment rrunoff is theen calculatedd to determine the voluume of storaage that must be providedd. This methhodology is valid and wwithin commmon engineering practices for completingg this type oof analysis. We completed a detailed review of each par ameter usedd to calcula te the runofff volumes aand summarized our revieew and findiings below. 2.1 RAIINFALL DEEPTH The calcul ator assumes a rainfall depth of 9 .3 inches fo r the 25-year/24-hour event. We compared this depth to the lates t rainfall da ta source avvailable, thee National OOceanic and Atmospheeric Administtration’s (NOOAA) Atlas 14. The 25--year/24-hoour rainfall ddepth for thee City of Atlanticc Beach is 9 .2 inches baased on NOAAA Atlas 14 . The City h as consistenntly used 9..3 inches sin ce the on-s ite storage oordinance wwas passed. Hydrologicaally, the diffference betwween 08505-003-002 October 201 8 OOnsite Storage Calculation Re view 1 f DRAFTT using 9.3 inches vers us 9.2 inchees is nominaal, thereforee, using 9.3 inches for the 25-yearr/24- hour rainfall depth is valid. 2.2 RUNNOFF COEEFFICIENT The calcul ator uses ruunoff coefficcients of 0.22 for perviouus surfaces and 1.0 for imperviouss surfaces to calculate pre- and poost-developmment compoosite runoff coefficients . Using a ruunoff coefficientt of 1.0 for iimpervious surfaces is ccommon praactice and vvalid becausse the rainfaall landing onn imperviouss surfaces wwill not infilttrate and wi ll run off. Using a ruunoff coeffic ient of 0.2 ffor perviouss surfaces is s at the uppeer end of thhe range of commonlyy used valuees for lawns in flat land scapes withh slopes fromm 0 to 2 perrcent. Accorrding to the tab le in Figure 1 from the Florida Deppartment of Transportattion’s 1987 Drainage DDesign Guide, runnoff coefficieents for law ns in designn storms witth return peeriods less t han or equaal to 10 years rrange from 0.05 for sanndy soils to 0.17 for claayey soils. AAs shown, too convert th ese values to coefficients that can bee used for thhe 25-year rreturn periood the coeffiicients mustt be multiplied by 1.1, wh ich moves tthe range too 0.06 for saandy soils a nd 0.19 for clayey soilss. According to the Natuural Resourcce Conservaation Servicee’s web soil survey, a mmajority of tthe soil in the City is classified as sanndy, which mmeans that the runoff ccoefficient sshould fall o n the lower end of this rangge. Figure 1 Runofff Coefficiennt Table 08505-003-002 October 201 8 OOnsite Storage Calculation Re view 2 DRAFT Lowering the runoff coefficient used in the calculator would reduce the pre-development runoff volume and increase the amount of storage required on-site. While the values are currently close to falling within the generally accepted range, they are skewed to benefit the applicant. Lowering this value would shift some of the burden of handing the additional runoff from the City to the applicants. The degree to which the runoff value is changed will determine how much or how little of this burden is shifted. If the City chooses to use the average sandy soil value it would change the runoff coefficient from 0.2 to 0.08. 3 PROVIDED STORAGE CALCULATION Once the required on-site storage volume is determined, the provided on-site storage volume is calculated by summing the surface storage provided in the retention area on the parcel and the sub-surface storage provided in the soil column below the retention area. The provided storage is calculated and then compared to the required storage volume to determine if enough storage has been provided to retain the additional runoff volume. This methodology is valid and within common engineering practices for completing this type of analysis. We completed a detailed review of each parameter and calculation used to determine the onsite storage volume provided and summarized our review and findings below. 3.1 SURFACE STORAGE CALCULATION The surface storage provided is calculated by averaging the area at the bottom of the storage area and the area at the top of the storage area and multiplying by the depth of the storage area. The methodology assumes a constant side-slope is provided within the storage area, which is typically valid. This methodology is valid and within common practice for completing this type of storage volume calculation. 3.2 SUBSURFACE STORAGE CALCULATION The subsurface storage provided in the soil column below the surface storage area is calculated by multiplying the area of the surface storage by the estimated depth to the seasonal high water table (SHWT) and the fillable porosity/pore factor of the soil. This yields the volume of open void space in the soil column that can be used for water storage. Currently, the subsurface storage calculation is double-counting and over-estimating the amount of storage that is available in the soil column because it does not reduce the soil storage capacity under the retention area to account for the rainfall volume that immediately infiltrates into the soil and does not runoff. To account for this, we recommend using a runoff coefficient of 1.0 for the storage area instead of the 0.2 runoff coefficient that is currently being used. Using a runoff coefficient of 1.0 for the storage area will ensure that the rainfall volume infiltrating at the pond is accounted for in the storage calculation. The subsurface storage calculation assumes a soil fillable porosity/pore factor of 0.3 for the subsurface storage calculation. This means that 30 percent of the soil column volume is void and available for runoff retention. As part of the Stormwater Master Plan Update project, Jones Edmunds calculated the fillable porosities of the Natural Resources Conservation Service 08505-003-02 October 2018 Onsite Storage Calculation Review 3 DRAFT (NRCS) soil types within City limits using data from the University of Florida’s Institute of Food and Agricultural Sciences (UF IFAS) Florida Soil Characterization Data Retrieval System database. We calculated an average fillable porosity across the City of approximately 0.4. Based on our calculations using a pore factor of 0.3 is slightly conservative and likely underestimates the subsurface pore space available. The subsurface storage calculation assumes a constant SHWT elevation of 3.0 feet across the City. This elevation is used to determine the depth of the soil column available for soil storage by subtracting this elevation from the elevation of the bottom of the on-site surface storage area. Based on a desktop review of NRCS soils data, pond break line elevations in the 2007 Duval County Light Detection and Ranging (LiDAR) digital elevation model (DEM), and St. Johns River Water Management District (SJRWMD) permit data for developments within the City, the assumption of a constant groundwater table elevation across the City does not appear to be valid. According to the NRCS soils data, the SHWT depth for the City is typically within 2 feet of the surface with a majority of it being within 1 foot of the surface. This means that the water table elevation should vary as the surface elevation varies. We reviewed the water surface elevations of wet detention stormwater ponds in the 2007 LiDAR DEM. Based on the water surface elevations in the DEM, the groundwater table elevation appears to vary with the surface elevation. Ponds at higher elevations have higher water level elevations in the DEM, and ponds in lower-lying areas have lower water-level elevations in the DEM. We reviewed SJRWMD permit data obtained from the SJRWMD’s website for the new RaceTrac gas station being built on Mayport Road and the Caliber Collision on Mayport Road. These developments are on a ridge that runs through the City and have site elevations between 12 and 13 feet North American Vertical Datum (NAVD) 88 based on the DEM. According to the geotechnical borings for the RaceTrac gas station, the groundwater table was encountered at approximately 5 feet below the surface or an approximate elevation of 8 feet NAVD88 and the Geotechnical Report estimates that the SHWT for the site is at an elevation of 10 feet NAVD88. The pond normal water level for the Caliber Collision is an elevation of 9.8 feet NAVD88 and is estimated to be 1 foot below the surface. According to SJRWMD permitting criteria, orifice elevations in wet detention stormwater ponds are set at the estimated SHWT elevation. Using a water table elevation of 3 feet NAVD88 significantly benefits the applicants in a majority of the City, creating additional burden on the City’s stormwater system. Using a water table depth of 1 foot below the surface may be too conservative for some of the higher elevation lots along east of Sherman Creek. Based on these findings, we recommend that the City hire a certified soil scientist to collect soil samples throughout the City at various elevations to better characterize how the SHWT varies across the City. The soil scientist should estimate the depth to SHWT based on indicators within the soil column. We believe the City could likely then be divided into zones that would more accurately represent the depth to the SHWT. Alternatively, a depth to SHWT of 1 or 2 feet could be used as a conservative estimate across the City unless site-specific data are provided that shows a greater depth to SHWT. 08505-003-02 October 2018 Onsite Storage Calculation Review 4