SHELLEYS FIELDS THERMAL IMPACT ANALYSIS

Transcription

1 SHELLEYS FIELDS THERMAL IMPACT ANALYSIS Prepared By Dr. Edward Bouwer Department of Geography-Environmental Engineering Johns Hopkins University 3400 North Charles Street 307 Ames Hall Baltimore, Maryland and Richard D. Klein COMMUNITY & ENVIRONMENTAL DEFENSE SERVICES 811 Crystal Palace Court Owings Mills, Maryland Fax: Web Page: At The Request Of The FREELAND ASSOCIATION February 28, 2007

2 INTRODUCTION Shelleys Fields is proposed for a 154-acre site located in northern Baltimore County. The project would consist of 19 single-family detached homes along Cotter Road and a recreational complex between Cotter Road and Middletown Road. The applicant s Hydrology & Stormwater Management Report (see Attachment J), revised 1 January 2007, shows that four extended-detention stormwater ponds are proposed for the Shelleys Fields project. The ponds are designed to manage the one-year storm, which is also known as the channel-protection volume. The applicant is not required to manage larger storms, such as the 10- or 100-year event. Three of the four ponds will be preceded by Surface Sand 2 3 Filters to meet the Water Quality Volume requirement. The fourth (Pond #1) will use a 4 micropool for water quality. The proposed stormwater ponds will discharge into the headwaters of an unnamed tributary to Prettyboy Reservoir. This tributary is designated Class III-P (Nontidal Cold Water/Public Water Supply) in the Code of Maryland Regulations (COMAR J(4)) (see Attachment A). According to the Maryland Department of Natural Resources (DNR) report Management of 5 Maryland s Coldwater Streams, the unnamed tributary supports brook trout (see Attachment B). This finding was based upon sampling conducted in the unnamed tributary by DNR fishery biologists. The most recent sampling occurred in August, 2000 and confirmed the continuing presence of brook trout (see Attachment C). Brook trout are highly sensitive to thermal impact. Brook trout prefer a temperature of 66 F or less and the lethal temperature for this species ranges from 75 F to 79 F (see page 91 in the Appendices of Attachment D). Maryland Department of the Environment (MDE) regulations (COMAR E(2)(a)) call for a temperature of no more than 68 F or ambient, whichever is higher (see Attachment E). Data from recording thermometers installed in four headwater streams in Baltimore County shows that July and August are the months when stream temperature is highest and therefore most restrictive for brook trout. The recording thermometer data shows July-August ambient 1 The specifications for an extended-detention stormwater pond can be viewed in Chapter 3 of the 2000 Maryland Stormwater Design Manual: 2 The specifications for a Surface Sand Filter can also be viewed in Chapter 3 of the 2000 Maryland Stormwater Design Manual: 3 The Water Quality Volume is the first inch of runoff from impervious surfaces, which is equivalent to 90% of all runoff from these surfaces. 4 Ibid. 5 Management of Maryland's Coldwater Streams, Annual (2000) and Final ( ) Performance Report, Survey and Management of Maryland's Fishery Resources, Federal Aid Project: F-48-R-10, Maryland Department of Natural Resource, Fisheries Service, Freshwater Fisheries Division, Tawes State Office Building, Annapolis, MD

3 water temperature is 62.1 F in headwater streams, such as those located immediately downstream of the four ponds proposed for Shelleys Fields (see Attachment F). Several studies show that stormwater ponds discharge detained runoff at a maximum temperature of 80 F to 87 F. 2 6 Bahr (1996) studied the temperature regime of three stormwater management ponds receiving runoff from MD Route 32 in Anne Arundel County. The three ponds were extended-detention facilities with micropools. They were designed for drawdown periods of 6-, 12-, and 24-hours. The study was conducted for three months during the summer of Bahr found that the discharge from the pond with the 12-hour drawdown (Basin B) attained a maximum temperature of 87 F (see July 24, 1995 spreadsheet in Appendix 4 of Attachment G). Bahr also found that the daily peak temperature averaged F. 7 Galli (1990) monitored the temperature of stormwater discharged from an extendeddetention wetland pond, a dry extended-detention pond, and a wet pond from April through September in Galli documented a maximum temperature of 80.8 F to 82.6 F (see Table 1, page xiii, in Attachment D). 8 Data gathered by the Baltimore City Bureau of Water Supply shows that the surface temperature of Prettyboy reservoir averaged 80.2 F in July and August from and the maximum July-August temperature averaged 81.1 F (see Attachment K). Prettyboy reservoir lies less than a mile from Shelleys Fields. Both Bahr and Galli concluded that it is the temperature of the air overlying a stormwater pond which determines the temperature of the pond discharge. If the overlying air is in the upper 80 F range while runoff is discharging from a pond, then the discharge will be in the upper 80 F range, even if the pond is shaded from direct sunlight. The data from Bahr, Galli, and Prettyboy reservoir show that the surface of impounded water heats to a temperature range of 80 F to 87 F. Of the three data sources, the Bahr study is the most comprehensive and relevant to Shelleys Fields. The applicant has proposed the same type of pond studied by Bahr: an extended-detention pond designed for a 12-hour drawdown. Because of the particular relevance, two of the summary statistics from the Bahr study will be 6 Bahr, R.P., A temperature study of discharges from three extended detention/wetland stormwater management basins in Maryland. Chesapeake Biological Laboratory, University of Maryland, Solomons, MD Galli, J Thermal impacts associated with urbanization and stormwater management best practices Department of Environmental Programs, Metropolitan Washington Council of Governments, 777 North Capitol Street, N.E., Washington, D.C (202) Data was provided by Mr. William Stack, Baltimore City Bureau of Water Supply, Ashburton Water Treatment Plant, 3001 Druid Park Drive, Baltimore, MD,

4 used in the following analysis of the effect of the four Shelleys Fields ponds on the temperature of the receiving waters; 87 F maximum temperature measured by Bahr in the discharge from the pond with a 12- hour drawdown; and the average of daily maximum temperatures measured by Bahr in the discharge from the pond with a 12-hour drawdown. 3 Thermal Impact Analysis Nine tables will be found at the end of this document. The first four tables are numbered 1a to 4a corresponding to proposed Ponds #1 through #4. In Tables 1a to 4a it is assumed that the pond will discharge at the maximum temperature (87 F) observed by Bahr (1996). In Tables 1b through 4b the average daily peak temperature of F from Bahr is used as the pond discharge temperature. Letters from A to R appear along the left hand side of all eight tables. Following is a description of each line and how these variables are used to model thermal impact. Receiving Water Computations The purpose of the receiving water computations is to determine the volume of streamflow present at the point where the pond discharge mixes during July and August - the months most critical for stream temperature. A. Drainage Area at a Springhead or a Head of Stream Below a Pond The computations for volume of the receiving water begin by determining the drainage area at the point where the pond discharge would enter a springhead or head of stream. The drainage area for ponds 1, 2, and 4 was measured from the applicant s Existing Conditions Drainage Area Map. The drainage area for Pond No. 3 was obtained from the existing conditions worksheet in the applicant s report Shelleys Fields Hydrology & Stormwater Management Report (see Attachment J). B. Existing Impervious Area The extent of existing roads, buildings, and other impervious areas was determined in two ways. For Ponds 1, 2, and 4 existing impervious area was measured from the applicant s Existing Conditions Drainage Area Map. The impervious area for Pond No. 3 was obtained from the existing conditions worksheet in the applicant s report Shelleys Fields Hydrology & Stormwater Management Report (see Attachment J). Existing impervious area would reduce the volume of groundwater entering the receiving waters. Therefore, the July-August discharge is reduced accordingly. It is assumed that measures 9 required to meet the Recharge (Re ) would compensate for new impervious surfaces v 9 In Appendix D of Shelleys Fields Hydrology & Stormwater Management Report it is assumed that soils affected by proposed impervious surfacings belong to Hydrologic Soil Group A and B where it is required that the equivalent of and 0.26-inches, respectively, of runoff from impervious surfaces be recharged into the soil via

5 4 created by the project within the drainage area, which is why the added imperviousness is not factored in to the analysis. C. Existing Pervious Area for Springhead or Head of Stream Drainage Area The existing pervious area (woods, cropfield, lawn, pasture, etc.) was computed by subtracting Existing Impervious Area from Drainage Area at Springhead or Head of Stream Below Pond. The water present at the springhead or head of stream would be composed of groundwater which originated as precipitation that infiltrated into the pervious areas within the drainage area. D. July-August average monthly stream flow for the nearest U.S. Geological Survey gaging station (Graves Run at Beckleysville) To run the thermal impact computations one needs to know the volume of flow at the springhead or head of stream. This was determined using the proportional area method developed by the US Geological Survey (USGS). This method is applied to streams where flow records are unavailable. One computes stream flow in cubic feet per second per acre of drainage area for the nearest USGS stream flow gaging station. This value is then multiplied by the drainage area at the mixing point to determine flow. The nearest gaging station is on Graves Run at Gunpowder Road, 2.5-miles southwest of the site. July-August average monthly discharge data for the Graves Run gage was downloaded from the USGS real-time data website ( July-August flow data was selected to reflect receiving water volume at the time when pond discharge temperature will be the highest. A print-out will be found in Attachment H of monthly streamflow statistics for the six-year period of record ( ) for the Graves Run gage. These statistics show that July-August discharge averages 7.25 cubic feet per second (cfs). E. Drainage area for nearest gaging station The drainage area at the Graves Run gage is 7.68 square miles (see Attachment H). F. Drainage area converted to acres Drainage area was converted to acres by multiplying square miles by 640. G. July-August average discharge converted to cfs/acre July-August average monthly discharge was converted to cfs/acre by dividing the discharge (7.25 cfs) by the drainage area in acres (4,915) at the Graves Run gage. H. July-August average discharge at springhead or head of stream The discharge at the springhead or head of stream was computed by multiplying cfs/acre, from Row G, times the Existing Pervious Area, from Row C. infiltration measures. On page 10 of the Shelleys Fields Hydrology & Stormwater Management Report it is proposed that the recharge requirement be met with stone trenches intercepting runoff discharged from the sand filter or pond forebays.

6 I. July-August ambient water temperature Using July-August water temperature measurements made at four other headwater streams in Baltimore County, the ambient water temperature was determined to be 62.1 Fahrenheit (see Attachment F). 5 J. Temperature x Discharge This part of the analysis concluded by multiplying the Springhead or Head of Stream temperature (62.1 F) by the Springhead or Head of Stream discharge ( cfs) to obtain a dimensionless value to be used in computing the temperature after mixing with the pond discharge. Proposed Stormwater Pond Computations The purpose of the stormwater pond computations is to determine the volume of discharge released at the spring head or head-of-stream. K. Channel Protection Volume The Channel Protection Volume (CP v), in cubic feet, was obtained from Table 4. Water Quantity Design Summary on page 8 in Shelleys Fields Hydrology & Stormwater Management Report (see Attachment J), prepared by KCI Technologies, Inc. L. Discharge with 12-hour drawdown The four ponds will be designed with a 12-hour drawdown. This means that the peak of discharge from the pond will be designed to occur 12 hours after the inflow peak during a one-year storm event. Since the springhead and head of stream volume computations (above) were based upon cubic feet per second (cfs), the extended-detention storage volume was divided by the number of seconds in 12 hours (60 x 60 x 12 = 43,200) to determine the average pond discharge volume in cfs. M. Maximum and Daily Peak Temperature For his masters thesis, Raymond Patrick Bahr studied the temperature regime of three stormwater management ponds receiving runoff from MD Route 32 in Anne Arundel County. The three ponds were extended-detention facilities with micropools. The ponds were designed for drawdown periods of 6-, 12-, and 24-hours. The study was conducted for three months during the summer of Mr. Bahr found that the discharge from the pond with the 12-hour drawdown (Basin B) attained a maximum temperature of 87 F (see July 24, 1995 spreadsheet in Appendix 4 of Attachment G). The Daily Peak temperature of F was determined by averaging the highest temperature on each day that Pond B was discharging. While Pond No. 2, 3, and 4 will not have a micropool, this does not negate the relevancy. The volume of water stored in the micropool of the ponds studied by Bahr was quickly displaced by inflow during each storm event. Bahr found a temperature of 87 F occurred for up to two hours after the 12-hour basin began receiving inflow. Water residing in the micropool prior to a runoff event would have been displaced in the first half-hour or so.

7 6 N. Distance from pond outfall to the springhead or head of stream Galli (1990) found that stormwater cools as it flows along well-shaded channels. Galli s report will be found in Attachment D. Figure 15, on page 47 of Attachment D, shows how stormwater pond discharge cooled from 77.2 F to 74.1 F after traveling for about 1,200 feet along a channel with 95% canopy. This data shows an average cooling rate of F per foot of well-shaded channel. The distance from the outfall of the four ponds ranges from 115- to 790-feet. It is assumed that the entire length will be well shaded. O. Anticipated cooling between outfall and springhead or head of stream To compute cooling, the distance from the pond outfalls (in feet) to the springhead or head of stream was multiplied by F per foot. P. Temperature of the discharge upon arrival at Springhead/Head of Stream The cooling (in degrees) resulting from flow along the shaded channel to the springheadhead of stream was subtracted from the pond outfall temperature. Q. Temperature x Discharge The temperature of pond discharge upon reaching the springhead or head of stream is multiplied by the pond discharge volume to get a dimensionless value used in computing the mixed temperature. R. MIXED TEMPERATURE Here the dimensionless values from J and Q (temperature x discharge) are totaled then divided by the combined volume of the discharge (L) and the springhead/head of stream flow (H). This results in the temperature after the springhead/head of stream flow mixes with the incoming pond discharge. Instantaneous, complete mixing is assumed, which is reasonable given the small size at the springhead/head of stream. As shown in Table 5, the resulting temperature, after mixing, ranges from 77.2 F to 85.4, which is far in excess of the 62.1 F ambient stream temperature, the 68 F MDE water quality standard for Class III-P streams, and the 75 F lethal temperature for brook trout. SUMMARY The proposed stormwater ponds will discharge water at a temperature of 81 F to 87 F. The discharge from the pond will travel 115- to 790-feet before reaching a springhead or a head of stream. The discharge will cool by 0.3 F to 2.3 F by the time it reaches a springhead or a head of stream. After mixing with the stream flow the pond discharge will cause the ambient temperature at the springhead or head of stream to increase to 77.2 F to 85.4 F, which is 15.1 to 23.3 above the ambient temperature. Therefore, all four pond discharges under both scenarios will not only exceed ambient temperature, but the 68 F water quality standard, and the 75 F lethal temperature for brook trout. The applicant could also resolve the thermal impact by reducing the proposed impervious area sufficiently that the one-year discharge does not exceed 2.0 cfs, which would eliminate the need for ponds.

8 7 Compliance with 2000 Maryland Stormwater Design Manual Baltimore County is required to verify that stormwater management plans comply with the 2000 Maryland Stormwater Design Manual. Specifically, Section , of Title 4. Stormwater Management states: (1) Except as provided in this title, a person may not develop or redevelop any site without providing for appropriate stormwater management measures that control or manage stormwater runoff from the site. (2) The stormwater management measures shall be: (i) Consistent with the Design Manual; Section (h), of the Baltimore County Stormwater Management regulations, defines Design Manual as: means the 2000 Maryland Stormwater Design Manual, Volumes I and II, which is incorporated by reference in COMAR By causing the thermal impacts documented above, the Shelleys Fields development plan fails to comply with the following specific requirements of the Design Manual (see Attachment I): Page 1-13 and 1-14: Section 1.2 General Performance Standards for Stormwater Management in Maryland To prevent adverse impacts of stormwater runoff, the State of Maryland has developed fourteen performance standards that must be met at development sites. These standards apply to any construction activity disturbing 5,000 or more square feet of earth. The following development activities are exempt from these performance standards in Maryland: 1. Additions or modifications to existing single family structures; 2. Developments that do not disturb more than 5000 square feet of land; or 3. Agricultural land management activities. The following performance standards shall be addressed at all sites where stormwater management is required: Standard No. 8 Stormwater discharges to critical areas with sensitive resources [e.g., cold water fisheries, shellfish beds, swimming beaches, recharge areas, water supply reservoirs, Chesapeake Bay Critical Area (see Appendix D.4)] may be subject to additional performance criteria or may need to utilize or restrict certain BMPs. [Emphasis Added] Page Pond Feasibility Criteria

9 8 The use of stormwater ponds on coldwater streams capable of supporting trout (Use III and IV) may be prohibited. Stormwater ponds located in Use III and IV watersheds should be designed to significantly reduce and/or eliminate thermal impacts (See Chapter 4, Section 4.1). [Emphasis Added] Page 4-3 Section 4.1 Watershed Factors The design of urban BMPs is fundamentally influenced by the nature of the downstream water body that will be receiving the stormwater discharge. Consequently, designers must determine the Use Designation of the watershed in which their project is located prior to design (see COMAR and Appendix D.9). In some cases, higher pollutant removal or environmental performance is needed to fully protect aquatic resources and/or human health and safety within a particular watershed or receiving water. Therefore, a shorter list of BMPs may need to be considered for selection within these watersheds or zones. The areas of concern include: Coldwater Streams (Use III and IV). Cold and cool water streams have habitat qualities capable of supporting trout and other sensitive aquatic organisms. Therefore, the design objective for these streams is to maintain habitat quality by preventing stream warming, maintaining natural recharge, preventing bank and channel erosion, and preserving the natural riparian corridor. [Emphasis Added] Techniques for accomplishing these objectives may include: Minimizing the creation of impervious surfaces, Minimizing surface areas of permanent pools, Preserving existing forested areas, Bypassing existing baseflow and/or springflow, or Providing shade-producing landscaping Some BMPs can have adverse downstream impacts on cold water streams and their use is highly restricted. [Emphasis Added] Page B MDE Dam Safety Division Small Pond Review and Approval Criteria The following criteria are established for the MDE Dam Safety Division small pond review and approval: 1) If any of the following apply, a permit is required from the MDE Dam Safety Division: a) Drainage area of the pond is greater than 640 acres. b) Dam embankment height is greater than 20 feet (top of dam to lowest point on the upstream toe).

10 9 c) Pond is an intermediate or high hazard structure the failure of which is likely to cause damage to homes, public transportation, loss of life or property (NRCS Class b & c). 2) If the pond is in a USE III watershed, a permit is required if the pond will be: a) Capturing a flowing stream (stream with a base flow*), b) Capturing any spring, or c) A wet pond, or d) Located within 100 feet of a flowing stream, or e) Proposes extended detention for the one year storm longer than 12 hours. Requirement 2)e) above, which calls for a 12-hour drawdown in Use III watersheds is often cited as the only specific requirement contained in the Design Manual relevant to the protection of coldwater streams. The other sections quoted above show that designing a pond for a 12-hour drawdown is merely a first step. If further analysis shows that stream warming will still occur then other options must be explored. Specifically, the portions of the 2000 Maryland Stormwater Design Manual cited above, require that a stormwater management plan be designed to prevent stream warming by reducing or eliminating thermal impacts and discourages the use of stormwater ponds when they will cause warming in a coldwater stream, such as that draining the Shelleys Fields site. The thermal impact analysis shows that the ponds proposed for Shelleys Fields have not complied with these requirements. Therefore, the Hearing Officer is obligated to deny approval for the Shelleys Fields development plan.

11 TABLE 1a: ANALYSIS OF THE THERMAL EFFECT OF SHELLEYS FIELDS STORMWATER FACILITIES POND NO. 1 FORMULA VALUE UNIT Receiving Water Computations A Drainage Area at the head of stream (from Existing Conditions DA Map) acres B Existing Impervious Area (from Existing Conditions DA Map) acres C Existing Pervious Area for the head of stream Drainage Area A - B acres D July-August average monthly stream flow for the nearest U.S. Geological Survey 7.25 cubic feet/second (cfs) gaging station (Graves Run at Beckleysville) E Drainage area for nearest gaging station 7.68 square miles F Drainage area converted to acres E 640 4,915 acres G July-August average discharge converted to cfs/acre D F cfs/acre H July-August average discharge at head of stream C G cfs I July-August ambient temperature (See footnote 2) 62.1 degrees Fahrenheit J Temperature x Discharge H I 1.60 dimensionless Proposed Stormwater Pond K Channel Protection Volume POND NO. 1 (see footnote 3) 24, cubic feet L Discharge with 12-hour drawdown K ( ) 0.56 cfs M Maximum Temperature of Two-Hour Duration (from Bahr 1996 footnote 1) 87 degrees Fahrenheit N Distance from POND NO. 1 Outfall to the head of stream 330 feet O Anticipated cooling between outfall and the head of stream (see footnote 4) N degrees Fahrenheit P Temperature of the discharge upon arrival at the head of stream M - O 86.0 degrees Fahrenheit Q Temperature x Discharge L P dimensionless R MIXED TEMPERATURE (J + Q) (H + L) 85.0 degrees Fahrenheit Footnotes 1 Bahr, R.P., A temperature study of discharges from three extended detention/wetland stormwater management basins in Maryland. Chesapeake Biological Laboratory, University of Maryland, Solomons, MD Water temperature measurements made at other headwater streams in Baltiomore County show an ambient July-August temperature of 62.1 Fahrenheit. See Headwater Temp Data worksheet 3 The Channel Protection Volume was obtain from Table 4. Water Quantity Design Summary in Shelleys Fields Hydrology & Stormwater Management Report, prepared by KCI Technologies, Inc. 4 Anticipated cooling is degrees Fahrenheit per shaded foot of distance between the stormwater pond outfall and Study Point A. This rate of cooling is based upon Figure 15, in Thermal Impacts Associated with Urbanization and Stormwater Management Be

12 TABLE 1b: ANALYSIS OF THE THERMAL EFFECT OF SHELLEYS FIELDS STORMWATER FACILITIES POND NO. 1 FORMULA VALUE UNIT Receiving Water Computations A Drainage Area at the head of stream (from Existing Conditions DA Map) acres B Existing Impervious Area (from Existing Conditions DA Map) acres C Existing Pervious Area for the head of stream Drainage Area A - B acres D July-August average monthly stream flow for the nearest U.S. Geological Survey 7.25 gaging station (Graves Run at Beckleysville) cubic feet/second (cfs) E Drainage area for nearest gaging station 7.68 square miles F Drainage area converted to acres E 640 4,915 acres G July-August average discharge converted to cfs/acre D F cfs/acre H July-August average discharge at head of stream C G cfs I July-August ambient temperature (See footnote 2) 62.1 degrees Fahrenheit J Temperature x Discharge H I 1.60 dimensionless Proposed Stormwater Pond K Channel Protection Volume POND NO. 1 (see footnote 3) 24, cubic feet L Discharge with 12-hour drawdown K ( ) 0.56 cfs M Daily Maximum Temperature (from Bahr 1996 footnote 1) degrees Fahrenheit N Distance from POND NO. 1 Outfall to the head of stream 330 feet O Anticipated cooling between outfall and the head of stream (see footnote 4) N degrees Fahrenheit P Temperature of the discharge upon arrival at the head of stream M - O 80.1 degrees Fahrenheit Q Temperature x Discharge L P dimensionless R MIXED TEMPERATURE (J + Q) (H + L) 79.3 degrees Fahrenheit Footnotes 1 Bahr, R.P., A temperature study of discharges from three extended detention/wetland stormwater management basins in Maryland. Chesapeake Biological Laboratory, University of Maryland, Solomons, MD Water temperature measurements made at other headwater streams in Baltiomore County show an ambient July-August temperature of 62.1 Fahrenheit. See Headwater Temp Data worksheet 3 The Channel Protection Volume was obtain from Table 4. Water Quantity Design Summary in Shelleys Fields Hydrology & Stormwater Management Report, prepared by KCI Technologies, Inc. 4 Anticipated cooling is degrees Fahrenheit per shaded foot of distance between the stormwater pond outfall and Study Point A. This rate of cooling is based upon Figure 15, in Thermal Impacts Associated with Urbanization and Stormwater Management Be

13 TABLE 2a: ANALYSIS OF THE THERMAL EFFECT OF SHELLEYS FIELDS STORMWATER FACILITIES POND NO. 2 FORMULA VALUE UNIT Receiving Water Computations A Drainage Area at Springhead Below Pond (from Existing Conditions DA Map) acres B Existing Impervious Area (from Existing Conditions DA Map) acres C Existing Pervious Area for Springhead Drainage Area A - B acres D July-August average monthly stream flow for the nearest U.S. Geological Survey 7.25 gaging station (Graves Run at Beckleysville) cubic feet/second (cfs) E Drainage area for nearest gaging station 7.68 square miles F Drainage area converted to acres E 640 4,915 acres G July-August average discharge converted to cfs/acre D F cfs/acre H July-August average discharge at Springhead C G cfs I July-August ambient temperature (See footnote 2) 62.1 degrees Fahrenheit J Temperature x Discharge H I 1.63 dimensionless Proposed Stormwater Pond K Channel Protection Volume POND NO. 2 (see footnote 3) 20, cubic feet L Discharge with 12-hour drawdown K ( ) 0.47 cfs M Maximum Temperature of Two-Hour Duration (from Bahr 1996 footnote 1) 87 degrees Fahrenheit N Distance from POND NO. 2 Outfall To Springhead 115 feet O Anticipated cooling between outfall and Springhead (see footnote 4) N degrees Fahrenheit P Temperature of the discharge upon arrival at Springhead M - O 86.7 degrees Fahrenheit Q Temperature x Discharge L P dimensionless R MIXED TEMPERATURE (J + Q) (H + L) 85.4 degrees Fahrenheit Footnotes 1 Bahr, R.P., A temperature study of discharges from three extended detention/wetland stormwater management basins in Maryland. Chesapeake Biological Laboratory, University of Maryland, Solomons, MD Water temperature measurements made at other headwater streams in Baltiomore County show an ambient July-August temperature of 62.1 Fahrenheit. See Headwater Temp Data worksheet 3 The Channel Protection Volume was obtain from Table 4. Water Quantity Design Summary in Shelleys Fields Hydrology & Stormwater Management Report, prepared by KCI Technologies, Inc. 4 Anticipated cooling is degrees Fahrenheit per shaded foot of distance between the stormwater pond outfall and Study Point A. This rate of cooling is based upon Figure 15, in Thermal Impacts Associated with Urbanization and Stormwater Management Be

14 TABLE 2b: ANALYSIS OF THE THERMAL EFFECT OF SHELLEYS FIELDS STORMWATER FACILITIES POND NO. 2 FORMULA VALUE UNIT Receiving Water Computations A Drainage Area at Springhead Below Pond (from Existing Conditions DA Map) acres B Existing Impervious Area (from Existing Conditions DA Map) acres C Existing Pervious Area for Springhead Drainage Area A - B acres D July-August average monthly stream flow for the nearest U.S. Geological Survey 7.25 gaging station (Graves Run at Beckleysville) cubic feet/second (cfs) E Drainage area for nearest gaging station 7.68 square miles F Drainage area converted to acres E 640 4,915 acres G July-August average discharge converted to cfs/acre D F cfs/acre H July-August average discharge at Springhead C G cfs I July-August ambient temperature (See footnote 2) 62.1 degrees Fahrenheit J Temperature x Discharge H I 1.63 dimensionless Proposed Stormwater Pond K Channel Protection Volume POND NO. 2 (see footnote 3) 20, cubic feet L Discharge with 12-hour drawdown K ( ) 0.47 cfs M Daily Maximum Temperature (from Bahr 1996 footnote 1) degrees Fahrenheit N Distance from POND NO. 2 Outfall To Springhead 115 feet O Anticipated cooling between outfall and Springhead (see footnote 4) N degrees Fahrenheit P Temperature of the discharge upon arrival at Springhead M - O 80.7 degrees Fahrenheit Q Temperature x Discharge L P dimensionless R MIXED TEMPERATURE (J + Q) (H + L) 79.8 degrees Fahrenheit Footnotes 1 Bahr, R.P., A temperature study of discharges from three extended detention/wetland stormwater management basins in Maryland. Chesapeake Biological Laboratory, University of Maryland, Solomons, MD Water temperature measurements made at other headwater streams in Baltiomore County show an ambient July-August temperature of 62.1 Fahrenheit. See Headwater Temp Data worksheet 3 The Channel Protection Volume was obtain from Table 4. Water Quantity Design Summary in Shelleys Fields Hydrology & Stormwater Management Report, prepared by KCI Technologies, Inc. 4 Anticipated cooling is degrees Fahrenheit per shaded foot of distance between the stormwater pond outfall and Study Point A. This rate of cooling is based upon Figure 15, in Thermal Impacts Associated with Urbanization and Stormwater Management Be

15 TABLE 3a: ANALYSIS OF THE THERMAL EFFECT OF SHELLEYS FIELDS STORMWATER FACILITIES SWM POND NO. 3 FORMULA VALUE UNIT Receiving Water Computations A Drainage Area at Study Point A (from Existing Conditions DA Map) acres B Existing Impervious Area (from Worksheet 2 - Existing Conditions) acres C Existing Pervious Area for Study Point A Drainage Area A - B acres D July-August average monthly stream flow for the nearest U.S. Geological Survey 7.25 gaging station (Graves Run at Beckleysville) cubic feet/second (cfs) E Drainage area for nearest gaging station 7.68 square miles F Drainage area converted to acres E 640 4,915 acres G July-August average discharge converted to cfs/acre D F cfs/acre H July-August average discharge at Study Point A C G cfs I July-August ambient temperature (See footnote 2) 62.1 degrees Fahrenheit J Temperature x Discharge H I 3.08 dimensionless Proposed Stormwater Pond K Channel Protection Volume POND NO. 3 (see footnote 3) 14, cubic feet L Discharge with 12-hour drawdown K ( ) 0.33 cfs M Maximum Temperature of Two-Hour Duration (from Bahr 1996 footnote 1) 87 degrees Fahrenheit N Distance from POND NO. 3 Outfall To Study Point A 450 feet O Anticipated cooling between outfall and Study Point A (see footnote 4) N degrees Fahrenheit P Temperature of the discharge upon arrival at Study Point A M - O 85.7 degrees Fahrenheit Q Temperature x Discharge L P dimensionless R MIXED TEMPERATURE (J + Q) (H + L) 82.6 degrees Fahrenheit Footnotes 1 Bahr, R.P., A temperature study of discharges from three extended detention/wetland stormwater management basins in Maryland. Chesapeake Biological Laboratory, University of Maryland, Solomons, MD Water temperature measurements made at other headwater streams in Baltiomore County show an ambient July-August temperature of 62.1 Fahrenheit. See Headwater Temp Data worksheet 3 The Channel Protection Volume was obtain from Table 4. Water Quantity Design Summary in Shelleys Fields Hydrology & Stormwater Management Report, prepared by KCI Technologies, Inc. 4 Anticipated cooling is degrees Fahrenheit per shaded foot of distance between the stormwater pond outfall and Study Point A. This rate of cooling is based upon Figure 15, in Thermal Impacts Associated with Urbanization and Stormwater Management Best Management Practices.

16 TABLE 3b: ANALYSIS OF THE THERMAL EFFECT OF SHELLEYS FIELDS STORMWATER FACILITIES SWM POND NO. 3 FORMULA VALUE UNIT Receiving Water Computations A Drainage Area at Study Point A (from Existing Conditions DA Map) acres B Existing Impervious Area (from Worksheet 2 - Existing Conditions) acres C Existing Pervious Area for Study Point A Drainage Area A - B acres D July-August average monthly stream flow for the nearest U.S. Geological Survey 7.25 gaging station (Graves Run at Beckleysville) cubic feet/second (cfs) E Drainage area for nearest gaging station 7.68 square miles F Drainage area converted to acres E 640 4,915 acres G July-August average discharge converted to cfs/acre D F cfs/acre H July-August average discharge at Study Point A C G cfs I July-August ambient temperature (See footnote 2) 62.1 degrees Fahrenheit J Temperature x Discharge H I 3.08 dimensionless Proposed Stormwater Pond K Channel Protection Volume POND NO. 3 (see footnote 3) 14, cubic feet L Discharge with 12-hour drawdown K ( ) 0.33 cfs M Daily Maximum Temperature (from Bahr 1996 footnote 1) degrees Fahrenheit N Distance from POND NO. 3 Outfall To Study Point A 450 feet O Anticipated cooling between outfall and Study Point A (see footnote 4) N degrees Fahrenheit P Temperature of the discharge upon arrival at Study Point A M - O 79.8 degrees Fahrenheit Q Temperature x Discharge L P dimensionless R MIXED TEMPERATURE (J + Q) (H + L) 77.5 degrees Fahrenheit Footnotes 1 Bahr, R.P., A temperature study of discharges from three extended detention/wetland stormwater management basins in Maryland. Chesapeake Biological Laboratory, University of Maryland, Solomons, MD Water temperature measurements made at other headwater streams in Baltiomore County show an ambient July-August temperature of 62.1 Fahrenheit. See Headwater Temp Data worksheet 3 The Channel Protection Volume was obtain from Table 4. Water Quantity Design Summary in Shelleys Fields Hydrology & Stormwater Management Report, prepared by KCI Technologies, Inc. 4 Anticipated cooling is degrees Fahrenheit per shaded foot of distance between the stormwater pond outfall and Study Point A. This rate of cooling is based upon Figure 15, in Thermal Impacts Associated with Urbanization and Stormwater Management Be

17 TABLE 4a: ANALYSIS OF THE THERMAL EFFECT OF SHELLEYS FIELDS STORMWATER FACILITIES POND NO. 4 FORMULA VALUE UNIT Receiving Water Computations A Drainage Area at Head of Stream Below Pond (from Existing Conditions DA Map) acres B Existing Impervious Area (from Existing Conditions DA Map) acres C Existing Pervious Area for Head of Stream Drainage Area A - B acres D July-August average monthly stream flow for the nearest U.S. Geological Survey 7.25 gaging station (Graves Run at Beckleysville) cubic feet/second (cfs) E Drainage area for nearest gaging station 7.68 square miles F Drainage area converted to acres E 640 4,915 acres G July-August average discharge converted to cfs/acre D F cfs/acre H July-August average discharge at Head of Stream C G cfs I July-August ambient temperature (See footnote 2) 62.1 degrees Fahrenheit J Temperature x Discharge H I 1.20 dimensionless Proposed Stormwater Pond K Channel Protection Volume POND NO. 4 (see footnote 3) 7, cubic feet L Discharge with 12-hour drawdown K ( ) 0.18 cfs M Maximum Temperature of Two-Hour Duration (from Bahr 1996 footnote 1) 87 degrees Fahrenheit N Distance from POND NO. 4 Outfall To Head of Stream 790 feet O Anticipated cooling between outfall and Head of Stream (see footnote 4) N degrees Fahrenheit P Temperature of the discharge upon arrival at Head of Stream M - O 84.7 degrees Fahrenheit Q Temperature x Discharge L P dimensionless R MIXED TEMPERATURE (J + Q) (H + L) 82.5 degrees Fahrenheit Footnotes 1 Bahr, R.P., A temperature study of discharges from three extended detention/wetland stormwater management basins in Maryland. Chesapeake Biological Laboratory, University of Maryland, Solomons, MD Water temperature measurements made at other headwater streams in Baltiomore County show an ambient July-August temperature of 62.1 Fahrenheit. See Headwater Temp Data worksheet 3 The Channel Protection Volume was obtain from Table 4. Water Quantity Design Summary in Shelleys Fields Hydrology & Stormwater Management Report, prepared by KCI Technologies, Inc. 4 Anticipated cooling is degrees Fahrenheit per shaded foot of distance between the stormwater pond outfall and Study Point A. This rate of cooling is based upon Figure 15, in Thermal Impacts Associated with Urbanization and Stormwater Management Be

18 TABLE 4b: ANALYSIS OF THE THERMAL EFFECT OF SHELLEYS FIELDS STORMWATER FACILITIES POND NO. 4 FORMULA VALUE UNIT Receiving Water Computations A Drainage Area at Head of Stream Below Pond (from Existing Conditions DA Map) acres B Existing Impervious Area (from Existing Conditions DA Map) acres C Existing Pervious Area for Head of Stream Drainage Area A - B acres D July-August average monthly stream flow for the nearest U.S. Geological Survey 7.25 gaging station (Graves Run at Beckleysville) cubic feet/second (cfs) E Drainage area for nearest gaging station 7.68 square miles F Drainage area converted to acres E 640 4,915 acres G July-August average discharge converted to cfs/acre D F cfs/acre H July-August average discharge at Head of Stream C G cfs I July-August ambient temperature (See footnote 2) 62.1 degrees Fahrenheit J Temperature x Discharge H I 1.20 dimensionless Proposed Stormwater Pond K Channel Protection Volume POND NO. 4 (see footnote 3) 7, cubic feet L Discharge with 12-hour drawdown K ( ) 0.18 cfs M Daily Maximum Temperature (from Bahr 1996 footnote 1) degrees Fahrenheit N Distance from POND NO. 4 Outfall To Head of Stream 790 feet O Anticipated cooling between outfall and Head of Stream (see footnote 4) N degrees Fahrenheit P Temperature of the discharge upon arrival at Head of Stream M - O 78.8 degrees Fahrenheit Q Temperature x Discharge L P dimensionless R MIXED TEMPERATURE (J + Q) (H + L) 77.2 degrees Fahrenheit Footnotes 1 Bahr, R.P., A temperature study of discharges from three extended detention/wetland stormwater management basins in Maryland. Chesapeake Biological Laboratory, University of Maryland, Solomons, MD Water temperature measurements made at other headwater streams in Baltiomore County show an ambient July-August temperature of 62.1 Fahrenheit. See Headwater Temp Data worksheet 3 The Channel Protection Volume was obtain from Table 4. Water Quantity Design Summary in Shelleys Fields Hydrology & Stormwater Management Report, prepared by KCI Technologies, Inc. 4 Anticipated cooling is degrees Fahrenheit per shaded foot of distance between the stormwater pond outfall and Study Point A. This rate of cooling is based upon Figure 15, in Thermal Impacts Associated with Urbanization and Stormwater Management Be

19 Table 5: Summary of the Thermal Impact of Stormwater Facilities Proposed for Shelleys Fields Temperature of Receiving Waters Stormwater When Facility Discharge Is At: Management Maximum Daily Peak Pond Temperature Temperature Discharge Temperature Ambient Temperature 62.1 Water Quality Standard Lethal Temperature Maximum Temperature is the highest temperature recorded in the discharge from extendended-detention ponds during the threemonth Bahr study period. Bahr, R.P., A temperature study of discharges from three extended detention/wetland stormwater management basins in Maryland. Chesapeake Biological Laboratory, University of Maryland, Solomons, MD Daily Peak Temperature is an average of the highest temperature recorded on each day the three extended-detention ponds were discharging during the three-month Bahr study. Ambient Temperature is the normal temperature in July-August at the spring head or head-of-stream before it receives runoff discharged from the proposed ponds. Water Quality Standard is the Maryland Department of the Environment temperature standard for Class III Nontidal Cold Water, which appears in the Code of Maryland Regulations at D(3)(a) Lethal Temperature is the temperature at which mortality begins among brook trout (Salvelinus fontinalis) based upon Galli (1990). Thermal impacts associated with urbanization and stormwater management best practices. Department of Environmental Programs, Metropolitan Washington Council of Governments, 777 North Capitol Street, N.E., Washington, D.C (202)