Stormwater Management Impacts Resulting from the Volumetric Abstraction of Runoff from Frequent Storms per PADEP CG-1. Geoffrey A. Cerrelli 1, P.E.

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1 Stormwater Management Impacts Resulting from the Volumetric Abstraction of Runoff from Frequent Storms per PADEP CG-1 Geoffrey A. Cerrelli 1, P.E. 1 Hydraulic Engineer USDA/NRCS, One Credit Union Place, Suite 340, Harrisburg, PA ; PH (717) ; geoffrey.cerrelli@pa.usda.gov The Pennsylvania Department of Environmental Protection (PADEP) issued stormwater management guidelines for municipalities around the state to consider enacting into local legislation in These guidelines are based, in part, on modern concepts involving bio-retention storage of high frequency storm runoff. This is intended to replace, at least in part, traditional stormwater controls involving detention storage of runoff. Questions arise as to what effect bio-retention provides on larger low frequency storms runoff and whether or not detention storage will still be required for peak rate control. Should detention storage still be required then how does this size compare to traditional detention basin storage size? This paper attempts to answer these questions based on hydrologic predictive modeling results obtained from the USDA Natural Resources Conservation Service (NRCS) Win TR-20 computer program. This paper focuses largely on Control Guideline No. 1 in PADEP s Stormwater Management Best Management Practice (BMP) Manual (Chapter 3, Stormwater Management Principles and Recommended Guidelines). The guideline specifies that the difference in runoff volume of the 2-yr, 24-hr storm from pre-development to post development shall be stored on-site. This manual directs the engineer to solve for predicted runoff by using the USDA/NRCS runoff equation and related hydrology tools (WinTR-55 and Win TR-20). Beyond that, it specifies that the peak rate of runoff shall not increase from pre to post-development for the 1-yr through 100-yr storm. The BMP Manual refers to an analysis of post development pervious areas that have been altered to the point that they do not infiltrate rainfall as readily as the undisturbed pervious areas. This disturbed pervious area could result from the removal of existing vegetation or the compaction of the soil during land development. This paper offers a potential solution for predicted runoff volume including the influence of disturbed pervious areas. A key assumption must be made in order to solve for predicted runoff volume of post development condition given that some of the pervious land has been disturbed. Specific Runoff Curve Numbers for this condition can be linked by assumption to the undisturbed pervious RCN s. This enables a calculation of the predicted post-development runoff volume (and peak rate) given any ratio of disturbed vs. undisturbed pervious land along with the impervious cover. The use of accepted assumed values for disturbed pervious RCN s (based on undisturbed pervious RCN s) simplifies the design and review process of stormwater management plans. It is imperative that assumptions for RCN s be utilized during the planning process prior to development of the land, such that runoff control practices can be designed for. There is no opportunity to gage the runoff after 1

2 development, determine RCN, and then install runoff control measures. By then it is too late. A series of assumed disturbed pervious RCN s is shown in Table-1. The basis for these values is done in part on the USDA Soil Conservation Service TR-55 manual (1986) which stated that when soils are altered during land development that the Hydrologic Soil Group (HSG) should be dropped down one grade (from A to B, B to C, or C to D). A technical paper entitled Impact of Soil Disturbance During Construction on Bulk Density and Infiltration in Ocean County, New Jersey investigated this topic and suggested that an even greater change in HSG is in order when the soils are disturbed. The assumptions for RCN s in Figure 1 are based on roughly a 1.5 downgrade in HSG when the pervious area is disturbed. Pre-Development (Undisturbed) RCN Disturbed Pervious (Post-Development) RCN Table 1. Assumed/Suggested RCN changes to be used when solving for postdevelopment runoff per PADEP Stormwater Management Manual CG-1. The BMP manual states that when calculating the 2-yr post development runoff volume when implementing CG-1, the volume from each separate land use area (specific RCN) shall be calculated independently the sum of which represents the site s total runoff. This is to be done in lieu of using a single weighted, or averaged, RCN for the entire area. This can be simplified with reasonable accuracy by calculating the runoff volume from the impervious areas (RCN 98) separately from the pervious areas, including disturbed and undisturbed areas, which are solved by using one single RCN. The potential error induced by weighting the pervious area RCN is not considered to be large when looking at the typical land areas this is applied to and the gross assumptions made in assigning the change in RCN from undisturbed to disturbed conditions. Applying the above logic and assumptions to an example 20 acre land development of ¼ acre lots (40% impervious cover) on HSG-B soils in Schuylkill County, PA (3.0 inch 2- yr, 24-hr rain) that was formerly woods in good condition can be done in the following fashion. It is estimated that 50% of the post developed pervious area will be disturbed in this example. The pre-development condition of woods (RCN 55) in good condition yields 0.19 inches of runoff for 3.0 inches of rain. This applies to the entire 20 acres. 2

3 The post-developed 2-yr runoff (from 3.0 inch rain) is estimated as 3.0 inches from the impervious area. The pervious area runoff is estimated by taking the RCN of the undisturbed area (RCN 55) and disturbed area (assumed RCN 75) averaging them according to percent of each. Since each are estimated at 50% then the resulting post development pervious RCN is 65. This yields runoff of 0.51 inches from the 3.0 inch storm. So the total runoff in the post developed condition from the 2-yr storm is (3.0 x 0.4) + (0.51 x 0.6) = 1.51 inches. The CG-1 storage required in this instance is 1.51 inches 0.19 inches = 1.31 inches (over 20 acres). If this calculation routine were repeated for the same conditions with the exception that the percent of the post developed pervious area that was considered disturbed was varied by 10% from 0% to 100% the following results are obtained. % Pervious Area that is Disturbed CG-1 Storage (watershedinches) 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Table 2. CG-1 storage volume required for yr rain with pre-developed woods (RCN 55) to post-developed ¼ acre lots (40% impervious cover) and varying amounts of pervious area that is disturbed. This can be analyzed in similar fashion for a fixed percent of pervious area as disturbed and varying percentages of impervious cover. Table 3 shows this for a 50% of pervious area considered to be disturbed on pre-developed RCN 55 land cover. % of Impervious in Post-Developed 10% 20% 30% 40% 50% 75% CG-1 Storage (watershed-inches) Table 3. CG1 storage volume required for 3 2-yr rain with pre-developed woods (RCN 55), 50% of post-developed pervious area being disturbed, and varying percent of post-developed impervious land cover. The information found in Table 3 can be solved for different pre-developed RCN conditions using the assumed changes to RCN from Table 1. This is done in Table 4 below for a fixed condition of 50% of pervious land considered disturbed. 3

4 % of Impervious in Post-Developed 10% 20% 30% 40% 50% 75% CG-1 (in) for Pre-Dev RCN CG-1 (in) for Pre-Dev RCN CG-1 (in) for Pre-Dev RCN CG-1 (in) for Pre-Dev RCN CG-1 (in) for Pre-Dev RCN CG-1 (in) for Pre-Dev RCN Table 4. CG1 storage volume (watershed-inches) required for a 3 2-yr rain with varying pre-developed RCN, varying percent of post-developed impervious land cover, and a fixed 50% of post-developed pervious area being disturbed. Table 4 is presented graphically in Figure 1 below. CG-1 Storage (in.) CG-1 Storages For Case When 50% of Pervious Cover is Disturbed (2-Yr Rain = 3.0") Pre-Dev RCN (Ia/S=0.2) 10% Imperv 20% Imperv 30% Imperv 40% Imperv 50% Imperv 75% Imperv Figure 1. Graphical representation of Table 4. This is done for assumptions of 30% and 70% post developed pervious cover considered as disturbed in Figure 2 and Figure 3. 4

5 CG-1 Storage (in.) CG-1 Storages For Case When 30% of Pervious Cover is Disturbed (2-Yr Rain = 3.0") % Imperv 20% Imperv 30% Imperv 40% Imperv 50% Imperv 75% Imperv Pre-Dev RCN (Ia/S=0.2) Figure 2. CG-1 Storage for 30% pervious cover considered disturbed. CG-1 Storage (in.) CG-1 Storages For Case When 70% of Pervious Cover is Disturbed (2-Yr Rain = 3.0") Pre-Dev RCN (Ia/S=0.2) 10% Imperv 20% Imperv 30% Imperv 40% Imperv 50% Imperv 75% Imperv Figure 3. CG-1 Storage for 70% pervious cover considered disturbed. The use of these and similarly created graphs for other fixed percentage pervious land cover allows for a simple and consistent determination of CG-1 storage volume required by stormwater management designers and reviewers. The values obtained by use of these 5

6 graphs are the basis for the size of CG-1 storage used in the analysis for the remainder of this paper. Two basic questions concerning the use of CG-1 storage is whether or not subsequent detention basin storage is needed and if so then what size in comparison to the size of traditional detention basins. The first question can be answered by use of a predictive hydrology tool like NRCS Win TR-20. The post development hydrology can be modeled to pass the outflow hydrographs of the 1-yr through 100-yr storms through a storage reach that contains the initial runoff volume equal to the CG-1 storage volumes found in the previously described graphs. This was done for various impervious land coverage and on predominant HSG B and C soils which are the two most common in Pennsylvania. The following tables represent the post development watershed response in terms of peak discharge leaving the site after flowing through properly sized CG-1 storage. A 30% disturbed pervious area is assumed throughout this analysis. Peak Q out /Q pre for Various Frequency Storms 1-yr 2-yr 5-yr 10-yr 25-yr 50-yr 100-yr Industrial (72% Impervious) 0% 0% 37% 110% 181% 414% 445% ¼ ac lots (38% Impervious) 0% 0% 28% 86% 152% 258% 284% 1 / 2 ac lots (25% Impervious) 0% 0% 30% 99% 141% 211% 227% Table 5. Ratio of pre-development peak outflow that post development peak after flowing through CG-1 only storage for various land developments on HSG-B soils. 30% of post-developed pervious land cover is considered disturbed. Peak Q out /Q pre for Various Frequency Storms 1-yr 2-yr 5-yr 10-yr 25-yr 50-yr 100-yr Industrial (72% Impervious) 0% 10% 58% 237% 268% 258% 241% ¼ ac lots (38% Impervious) 0% 14% 104% 193% 204% 194% 184% 1 / 2 ac lots (25% Impervious) 0% 20% 117% 173% 172% 163% 157% Table 6. Ratio of pre-development peak outflow to post development peak after flowing through CG-1 only storage for various land developments on HSG-C soils. 30% of post-developed pervious land cover is considered disturbed. These results indicate that CG-1 storage alone provides peak discharge control (postdevelopment peak at or below the pre-development peak) for up to about the 5-yr storm then quickly loses control for larger storms. It is apparent from this finding that subsequent detention basin storage will be necessary for peak rate control of storms through the 100-yr storm as stated in CG-1. Win TR-20 was run on sample sites to find the most efficient detention basin size after flow through CG-1 storage for all storms through the 100-yr. The resulting storage was compared to the size of a traditional detention basin with the same site conditions and storage characteristics. This was done for both HSG B and C soils and with the 6

7 assumption of 30% pervious area as disturbed. Analysis was done on five different typical percentage impervious area covers from ½ ac lots (25% impervious) to industrial land (72%). The results within each HSG type were consistently similar. The detention basins to be used with CG-1 were found to be 24-28% of the size of a traditional detention basin in HSG-B site conditions. The combined volume of CG-1 storage and subsequent detention basin volume just about matched the volume of a traditional detention basin volume that would be used for the same site conditions. The detention basins to be used with CG-1 were found to be 42-54% of the size of a traditional detention basin in HSG-C site conditions. The combined volume of CG-1 storage and subsequent detention basin volume was about 120% of the volume of a traditional detention basin volume that would be used for the same site conditions. The primary basis for the entire analysis discussed in this paper is the assumption for changes in RCN from pre-development values to those for disturbed pervious areas where applicable as shown in Table 1. Whether or not these are officially accepted it is apparent that some form of assumed values will greatly facilitate the design and review process of stormwater management plans involving CG-1. The tables and graphs used throughout this paper are based upon a 3.0 inch, 2-yr, 24-hr storm. This rainfall amount is typical for PA counties. New graphs could easily be created for counties with different 2-yr rainfall amounts or perhaps the 3.0 inch graphs could be used in conjunction with correction factors to CG-1 storage for areas with different rainfall. References Pennsylvania Department of Environmental Protection. (December 30, 2006). Pennsylvania Stormwater Best Management Practices Manual. Document Number: Ocean County Soil Conservation District, Schnabel Engineering Associates, Inc., USDA Natural Resources Conservation Service. (March 2001,Rev. 06/01/01). Impact of Soil Disturbance During Construction on Bulk Density and Infiltration in Ocean County, New Jersey. USDA Soil Conservation Service. (June 1986). Technical Release 55, Urban Hydrology for Small Watersheds USDA Natural Resources Conservation Service. (2005) Win TR-20, Version 1.0 USDA Natural Resources Conservation Service. (July 2004). National Engineering Handbook, Part 630 Hydrology, Chapter 10, Estimation of Direct Runoff from Storm Rainfall. 7