May 21, 2008 The following was provided to County staff as internal guidance for reviewers in applying the adequate outfall requirements of the Public Facilities Manual (PFM). A number of private sector engineers have requested copies of the document to aid them in preparing plan submissions. It is being provided with the caveat that the guidance in this document does not represent a formal interpretation of the PFM provisions referenced in the various questions posed and is subject to change. The document does not address all possible situations nor does it include a thorough discussion on how the determinations were arrived at. The guidance should only be applied to the specific scenarios described herein based on a thorough review of conditions in the field. Particularly with respect to the extent of downstream review, it is noted that under 6-0203.2E, the Director may require analysis farther downstream when the submitted narrative described in 6-0204 and all related plats and plans are insufficient to show the true impact of the development on surrounding and other lower lying properties, or if there are known drainage problems downstream. If you have any questions, please contact a stormwater engineer in the Stormwater and Geotechnical Section of the Environmental and Site Review Division at 703-324-1720. LDS Technical Guidance Adequate Outfall (PFM 6-0200) March 1, 2008 The following is provided to County staff as an internal guidance document in an attempt to clarify interpretations of portions of the PFM pertaining to application of the outfall requirements on a development plan. This is to be considered a living document, and will be updated in the future as warranted.
Page 2 of 11 1. Where does the outfall analysis start? The starting point for the outfall analysis is where the proposed storm system meets an existing receiving pipe or channel/stream. Any portion of the receiving pipe or channel/stream that is replaced or stabilized as part of the project of development is considered to be part of the existing receiving pipe or channel/stream (i.e., the starting point would not move downstream). A natural watercourse or channel/stream has bed and banks, PFM 6-0201.2. A pipe system (see Diagram 1) is a man-made enclosed conveyance system, and the confluence occurs at manhole structures. Diagram 1 2. A point of confluence is where at least two concentrated flows come together. Sheet flow or shallow flow cannot be considered as a point discharge unless it first enters a storm system through a man-made structure, such as an inlet, or collectively in an open channel/stream. Shallow flow that is not within a bed and banks conveyance system or a man-made channel (excluding minor swales) is considered to be sheet flow. Small discharges, like those from an underdrain (4 to 8 diameter pipe), are not considered to be concentrated point discharges since the flow quickly dissipates and alone does not create a bed and bank situation. MH2 & Inlet 3 are Points of Confluence in Diagram 1.
Page 3 of 11 3. How do we measure the first drainage area for applying the 90% rule (PFM 6-0203.2A)? Determine the starting point. The first drainage area is the total drainage area to the starting point (including the drainage area of the receiving pipe or channel). The First Drainage Area is 10 acres in Diagram 1 above. 4. Define development site. Development site is the area within the limits of disturbance of the development. It may not necessarily follow the property boundary. It does include areas (even off-site), which are disturbed to construct a new outfall system to an existing receiving pipe or channel/stream. However, it does not include areas which are disturbed to replace and improve an existing receiving pipe or channel/stream solely to make it adequate. 5. When a channel/stream or storm sewer pipe crosses a development site, should the outfall be considered concentrated at the property line, even though the runoff from the development site is sheet flow to the channel/stream or existing inlet structure? No. If the development is not concentrating flow (pre-existing watercourse) and the flow is over vegetated (undeveloped) areas outside the limits of clearing and grading, the flow from the site should be considered as sheet flow. The channel/stream or storm sewer pipe crossing the site is considered as bypassing concentrated flow as the development site did not cause the concentration of flow (e.g. an INF grading plan for a lot where there is large stream in the back yard that will not be disturbed during construction). See Diagram 2.
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Page 5 of 11 However, for an addition or improvement to previously developed land with sheet flow from the addition or improvement, the downstream end of the existing storm sewer system must be checked if there are drainage complaints on file, to ensure any increase will not aggravate any existing drainage problem, PFM 6-0202.6B(2) (e.g. a site plan for a portion of an existing shopping center that shows sheet flow to existing drop inlets). See Diagram 3.
Page 6 of 11 6. Does a driveway culvert concentrate flow? Driveway culverts placed within an existing roadside ditch are not considered to be concentrating flow. Driveway culverts placed to convey sheet flow from one side of the driveway to the other are not considered to be concentrating flow unless the flow rate is greater than 2 cfs during a 10-year storm or the culvert is diverting the direction of flow. 7. When determining the extent of review in accordance with PFM 6-0203.2A (90% rule), if more than one channel/pipe is entering at the same point of confluence (common in piped systems), should the drainage areas contributing to that point of confluence be added together rather than evaluated separately? Added together. The comparison drainage area contributing at a specific point is the drainage area exiting at the point of confluence minus the drainage area entering the point of confluence from the development site s direction (which may not be the same as the first drainage area ). If, in a piped system, any additional flows entering the structure, either through a pipe or an inlet, other than the pipe containing the runoff from the project of development s flow is counted toward the drainage area contributing at the point of confluence. Inlet 3 in Diagram 1 has three points of inflow, from MH2, from the storm pipe with 8 acres of drainageshed, and the 1-acre of sheet flow through the inlet at the top. The total drainage area is 24 acres. The drainageshed area from the direction of the development site is 15 acres. Subtracting 15 from 24 we get 9 acres. Nine acres is at least 90% of the First Drainage Area of 10 acres. Therefore, the extent of review is 150 feet downstream of IN3 if the lowest 150 is adequate for stability and capacity. 8. A pond should be evaluated as a point of confluence, as runoff enters from several directions and by a combination of flow regimes (sheet flow and concentrated flow). A pond should be handled as a point of confluence since the outfall of the pond does not distinguish whether the flow entering the pond is sheet flow or concentrated, or whether a pipe or channel discharges into the pond at the top of the pond or closer to the control structure. The pond is a man-made system. When using the 90% rule to determine the extent of review, to determine the drainage area for comparison, subtract the drainage area from the development site s direction from the total drainage area to the pond s control structure. If the darinage area meets the 90% criteria, the extent of review will be at least 150 feet
Page 7 of 11 below the daylight point of the principal spillway or at least 150 feet below the first structure below the dam if discharging into a closed system. If the entire drainage area is 100 times greater than the contributing drainage area of the development site, the extent of review is at the pond s control structure. Diagram 4 The drainage area at the Starting Point is 8 acres. The drainage area to the pond from the development site s direction is 9 acres. The drainage area to the pond is 17 acres. Subtract 9 from 17 to get 8 acres as the drainage area for comparison. 8 acres is at least 90% of the drainage area at the Starting point of 8 acres. Therefore the extent of review is at least 150 feet below the end of the principal spillway pipe. 9. Which areas are used to determine the proportional improvement under PFM 6-0203.4A (shear stress) and 6-0203.4B (capacity method) when there is a drainage diversion? If the downstream system is inadequate and the designer chooses to show no adverse impact with a proportional improvement, the proportional improvement is determined using the existing (pre-development, i.e. pre-diversion) drainage area to the subject cross section and the post development (i.e. post-diversion) drainage area to the subject cross section. 10. Which areas are used to determine the proportional improvement under PFM 6-0203.4C (detention method) when there is a drainage diversion?
Page 8 of 11 If the downstream system is inadequate and the designer chooses to show no adverse impact with a proportional improvement using the detention method, the proportional improvement is determined using the development site s runoff volume in a good forested condition computed from the existing drainage area to the outfall and the runoff volume of the post development condition from the post development drainage area to that particular outfall. 11. Providing the Storm Sewer Design Computations on a plan is an acceptable method to demonstrate capacity at a cross-section along the extent of review). The typical computations on a plan are for the proposed storm sewer system to be constructed with the plan of development, PFM 6-0905. To satisfy the outfall requirements, one option is to include For Information Only those sheets from the approved design plan from which the system was approved, or the other option is to extend the chart with the notations that they system is existing through labeling, ensuring that the actual flows, capacity flows and velocities are included. PFM Sections verbatim 6-0201.2 Adequate drainage of surface waters means the effective conveyance of storm and other surface waters through and from the development site and the discharge of such waters into a natural watercourse, i.e., a stream with a defined channel (bed and banks), or man-made drainage facility of sufficient capacity without adverse impact upon the land over which the waters are conveyed or upon the watercourse or facility into which such waters are discharged. 6-0202.6 (91-06-PFM) The owner or developer may continue to discharge stormwater which has not been concentrated (i.e. sheet flow) into a lower lying property if: 6-0202.6A (91-06-PFM) The peak rate after development does not exceed the predevelopment peak rate; or 6-0202.6B(1) (91-06-PFM) The increase in peak rate or volume caused by the development will not have any adverse impact (e.g. soil erosion, sedimentation, duration of ponding water, inadequate overland relief) on the lower lying property as determined by the Director; and 6-0202.6B(2) (91-06-PFM) The increase in peak rate or volume caused by the development will not aggravate any existing drainage problem or cause a new drainage problem on the downstream property. 6-0202.7 (91-06-PFM) Increases in peak rates or volumes of sheet flow that may cause any adverse impact on lower lying properties shall be discharged into an adequate existing drainage
Page 9 of 11 system or the developer shall provide an adequate drainage system satisfactory to the Director to preclude any adverse impact upon the adjacent or downstream property. 6-0203.2 The extent of the review of the downstream drainage system shall be: 6-0203.2A To a point that is at least 150 ft (46 m) downstream to a point where the receiving pipe or channel is joined by another that has a drainage area that is at least 90% of the size of the first drainage area at the point of confluence; or 6-0203.2B To a point at which the total drainage area is at least 100 times greater than the contributing drainage area of the development site; or 6-0203.2C To a point that is at least 150 ft (45 m) downstream of a point where the drainage area is 360 acres (1.46 km 2 ) or greater. 6-0203.2D When using 6-0203.2A and 6-0203.2C for the extent of review, the analysis must be to a point where all the cross-sections are adequate in the farthest downstream reach of 150 feet. A minimum of three cross-sections shall be provided in the 150 foot reach. If the detention method described in 6-0203.4C is used, the three cross-sections in the farthest downstream reach of 150 feet shall be limited to showing a defined channel or a man-made drainage facility and checking for flooding as described in 6-0203.4C(3) and 6-0203.5. 6-0203.4A Critical Shear Stress Method 6-0203.4A (1) If the outfall is inadequate due to erosive velocities along the extent of review, which is described in 6-0203.2, the critical shear stress method may be used to show no adverse impact due to erosive velocities. The erosive work on the channel for the postdevelopment conditions shall be reduced to a level below the erosive work on the channel under pre-development conditions by the required proportional improvement. The required proportional improvement of the downstream system at each inadequate cross-section is the ratio of the post-development C times A (see 6-0803 for a description of C times A) for the contributing drainage area of the site to the existing development C times A for the entire drainage area at that cross-section. The required proportional improvement is computed as follows: P i = [C d A d / C cs A cs ] X 100 where, P i = Required Proportional Improvement (%) C d = Runoff Coefficient for the Contributing Drainage Area of the Site in a Post-development Condition A d = Contributing Drainage Area of the Site C cs = Runoff Coefficient for the Contributing Drainage Area to the Cross-section in a Existing Development Condition A cs = Contributing Drainage Area to the Cross-section
Page 10 of 11 6-0203.4B Channel Capacity Method 6-0203.4B (1) If the outfall is inadequate due to inadequate capacity along the extent of review, which is described in 6-0203.2, the channel capacity method may be used to show no adverse impact due to overtopping. The largest storm that does not exceed the actual channel, pipe, or culvert capacity under pre-development conditions shall be determined for the cross-section that is most frequently over its capacity. The post-development peak flows for the above storm and the 2-year and 10-year storms shall be reduced to a level below the pre-development conditions by a percent equal to the required proportional improvement. See 6-0203.4A (1) for a description of the required proportional improvement. 6-0203.4C Detention Method 2 6-0203.4C(1) It shall be presumed that no adverse impact and a proportional improvement will occur if on-site detention is provided as follows and the outfall is discharging into a defined channel or man-made drainage facility: 6-0203.4C (1) (i) Extended detention of the 1-year storm volume for a minimum of 24 hours. If extended detention of the BMP volume (see 6-0400 et seq.) also is provided, the 24 hours shall be applied to the difference between the 1-year storm volume and the BMP volume; and 6-0203.4C(1)(ii) In order to compensate for the increase in runoff volume, the 2-year and 10- year post-development peak rates of runoff from the development site shall be reduced below the respective peak rates of runoff for the site in good forested condition (e.g., for NRCS method, a cover type of "woods" and a hydrologic condition of "good"). This reduction results in a proportional improvement and is computed as follows: R i = [1 - (V f / V d )] X 100 where, R i = Reduction of Peak Flow Below a Good Forested Condition (%) V f = Runoff Volume from the Site in a Good Forested Condition V d = Runoff Volume from the Site in a Post-Developed Condition 2 Because of the long detention times resulting from this method, consideration shall be given to hydrology, soils and extended detention when choosing the appropriate landscaping for the detention facility. 6-0905 Closed Conduit Design Calculations. In general, design calculations required for submittal to the Director are as follows: 6-0905.1 A copy of the drainage plan showing drainage divides, contributing areas and adopted Comprehensive Plan recommendation or existing zoning, whichever is higher. 6-0905.2 Stormwater runoff quantities.
Page 11 of 11 6-0905.3 Pipe design calculations: 6-0905.3A For storm sewer systems or portions of systems designed for pressure flow, a storm sewer profile with energy and hydraulic gradients drawn on it shall be submitted for the portion of the system that experiences pressure flow. 6-0905.3B Energy and hydraulic gradients do not need to be submitted for non-pressure systems. 6-0905.4 Energy loss calculations at storm sewer junctions.