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HomeMy WebLinkAboutAppendix H: Pond Design ..... Appendix H Pond Design Standards Appendix H: Pond Design Standards Local Ponds for Single Family and Multi-Family Residential Developments General Local ponds are constructed to control the rate, and sometimes the volume, of stormwater runoff from residential developments, including residential PUD' s. They should be set away from structures where they can blend in with the landscape and avoid the appearance of an unsightly pit. Slopes should be gentle to reflect the fact that they are dry most of the time. Vegetation should be different from surrounding lawns to show a clear end to the recreational character of the lawn and a beginning of a different land use. Ownership Ponds are usually owned by the City and are constructed on outlots dedicated to the public. The City may, from time to time, consent to placing these ponds on easements where the development has an association to provide perpetual maintenance. Design Criteria A. Design Storm. Ponds that have outlets shall be designed for the 100-year storm. Ponds without outlets shall be designed for two back-to-back 100-year storms. Design must include a I-foot freeboard between he high water level and the top of the pond. There must be 2 feet of elevation between the high water level and the lowest structure opening, The 100-year return period is based on 5.8 inches of rain in 24 hours and a Type II storm. B. Infiltration. Where infiltration is used to reduce the size of the pond, it must have a 2: 1 safety factor. Infiltration rates must be verified by field testing the pond bottom and/or embankment surfaces after construction has been completed. C. Rate control. The rate of discharge must be controlled so that the rate after development is equal to or less than the rate before development for the 10-year and 100-year storm events. Since the discharge is concentrated flow, provisions must be made to prevent downstream erosion. For the 2-year, or NPDES water quality event, the discharge rate must be less than 5.66 CFS per acre. D. Volume Control. Where the City determines that any increase in runoff is likely to cause damage downstream, the pond must also be designed to keep runoff volume at pre-development levels. Design Standards A. Pond Shape. To the extent possible given site constraints, ponds shall be designed so that the length is approximately 3 times the width. This can also be accomplished by creating berms or baffles in the pond to route the water to the outlet over the longest possible path. The idea is to maximize the detention time to promote settling of suspended material. B. Pond Depth. Where the pond depth exceeds 10 feet from the overflow level to the bottom of the side slope, a shelf or terrace must be provided for maintenance. The shelf must be 10 feet wide and have a 10: 1 side slope. C. Inlet Structure. Inlets should be constructed in the lowest tier of the pond to minimize potential side slope erosion. Where possible, the inlet should be brought into the pond approximately 2 feet above the pond bottom to minimize erosion potential, but still allow for sediment buildup. This can be accomplished through a drop structure where it is necessary to reduce discharge velocity. Where the inlet must enter the pond higher to maintain pipe capacity, the slope must be protected with riprap from the pond bottom to a point 2 feet above the top of the pipe. Storm Water Management Plan St. Joseph, Minnesota A-ST JOE0315 H-1 Appendix H: Pond Design Standards D. Riprap. Riprap and filter fabric must be extended out from the inlet far enough to prevent erosion due to excessive discharge velocities, occurring when the storm sewer is surcharged. Riprap protection must be carried wide enough to prevent undermining. E. Overland Flow. Where significant storm water runoff is directed to the pond by sheet flow or swales, it must be collected by a culvert or catch basin before it reaches the pond slope and conveyed to the inlet pipe. F. Outlet Structure. Outlet aprons that exceed IS-inches in diameter should be equipped with trash guards. Where there is potential for petroleum products or surface debris to accumulate in the pond, a cap-skimmer type structure should be provided at the outlet. G. Overflow. An overflow or spillway must be constructed to direct excess water safely out of the pond during runoff events that exceed the capacity of the pond. The overflow crest should be at the 100-year high water level of the pond, and should be sized to keep the backwater below the freeboard elevation. H. Pond bottom. Large ponds should have a 2 percent slope on the pond bottom directed to a pit area near the outlet that is accessible for maintenance. I. Side Slopes. Side slopes must be 6: 1 or flatter above the level of the pond outlet, and 4: 1 or flatter below the outlet elevation. J. Clearances. Ponds must be located so that the edge of the high water is at least 70 feet from the nearest structure. Ponds under public ownership must be located so that the edge of the high water is at least 20 feet from the nearest property line. K. Vegetation. Ponds that are to be operated and maintained by the City must be seeded with an approved mix of native grasses and forbs. Different mixes may be specified for the top, sideslopes, and pond bottoms. Vegetative growth must provide a clear line of demarcation between lawns and the ponding site. L. Access. Pond outlots must include a 20-foot wide access from a nearby street. The access from the street to the pond should be mowable turf to blend in with neighboring yards, but still be under City ownership to prevent complaints generated by maintenance vehicles accessing the site. In addition to access from the street, there must be a 20-foot wide access shelf around the top of the pond for maintenance vehicles. The access shelf should have a 20: 1 or flatter cross slope, and be seeded with native grasses and forbs to provide a visual break between adjacent lawns and the pond. M. City Review. All pond designs must be approved by the City. The City may approve deviations from the above standards due to site constraints on a case-by-case basis. Increasing development density is not considered a site constraint. The City may also require additional design features on a case-by-case basis. Storm Water Management Plan St. Joseph, Minnesota A-STJOE0315 H-2 Appendix H: Pond Design Standards Local Ponds for Commercial, Institutional, and Industrial Sites and Developments General Local ponds are constructed on the building site to control the rate, and sometimes the volume, of storm water runoff from commercial, institutional, and industrial developments including PUD's. They are often combined with parking lots to provide extra storage for large rainfall events. Ownership Ponds are usually owned by the private entity, and are not subject to easements unless other properties are involved. Design Criteria A. Design Storm. Ponds must be designed for the 100-year storm. The discharge or outflow rate may not exceed the capacity of the downstream storm sewer system. Parking lots and lawn areas may be used for temporary storage during the 100-year event, however, the pond must hold at least the 10-year storm event. There must be 2 feet of elevation between the high water level and the lowest structure opening. The 100-year return period is based on 5.8 inches of rain in 24 hours and a Type II storm. B. Rate Control. The rate of discharge must be controlled so that the rate after development is equal to or less than the rate before development for the 10-year and 100-year storm events. Since the discharge is concentrated flow, provisions must be made to prevent downstream erosion. For the 2-year, or NPDES water quality event, the discharge rate must be less than 5.66 cfs per acre. C. Volume Control. Where the City determines that any increase in runoff is likely to cause damage downstream, the pond must also be designed to keep runoff volume at pre-development levels. Design Standards A. Pond Shape. To the extent possible given site constraints, ponds shall be designed so that the length is approximately 3 times the width. This can also be accomplished by creating berms or baffles in the pond to route the water to the outlet over the longest possible path. The idea is to maximize the detention time to promote settling of suspended material. B. Pond Depth. Where the pond depth exceeds 10 feet from the overflow level to the bottom of the side slope, a shelf or terrace must be provided for maintenance. The shelf must be 10 feet wide and have a 10: 1 side slope. C. Inlet Structure. Inlets should be constructed in the lowest tier of the pond to minimize potential side slope erosion. Where possible, the inlet should be brought into the pond approximately 2 feet above the pond bottom to minimize erosion potential, but still allow for sediment buildup. This can be accomplished through a drop structure where it is necessary to reduce discharge velocity. Where the inlet must enter the pond higher to maintain pipe capacity, the slope must be protected with riprap from the pond bottom to a point 2 feet above the top of the pipe. D. Riprap. Riprap and filter fabric must be extended out from the inlet far enough to prevent erosion due to excessive discharge velocities, occurring when the storm sewer is surcharged. Riprap protection must be carried wide enough to prevent undermining. Storm Water Management Plan St. Joseph, Minnesota A-ST JOE0315 H-3 Appendix H: Pond Design Standards E. Overland Flow. Where significant storm water runoff is directed to the pond by shÿet flow or swales, it must be collected by a culvert or catch basin before it reaches the pond slope and conveyed to the inlet pipe. F. Outlet Structure. Outlet aprons that exceed IS-inches in diameter should be equipped with trash guards. Where there is potential for petroleum products or surface debris to accumulate in the pond, a cap-skimmer type structure should be provided at the outlet. G. Overflow. An overflow or spillway must be constructed to direct excess water safely out of the pond during runoff events that exceed the capacity of the pond. The overflow crest should be at the 100-year high water level of the pond, and should be sized to keep the backwater below the freeboard elevation. The overflow can be combined with lawn or parking lot surfaces. H. Pond Bottom. Large ponds should have a 2 percent slope on the pond bottom directed to a pit area near the outlet that is accessible for maintenance. I. Side Slopes. Side slopes must be 4:1 or flatter above the level of the pond outlet, and 3:1 or flatter below the outlet elevation. J. Clearances. Ponds must be located so that the edge of the high water is at least 20 feet from the nearest structure and at least 10 feet from the property line. K. Vegetation. Ponds may be seeded with an approved mix of native grasses and forbs, or with mowable grasses. Different mixes may be used for the top, sideslopes, and pond bottoms. L. Access. Ponds must be accessible for maintenance. M. Public Ownership. Where the pond is to be constructed on an outlot for public ownership, residential standards must be used. N. City Review. All pond designs must be approved by the City. The City may approve deviations from the above standards due to site constraints on a case-by-case basis. Increasing development density is not considered a site constraint. Storm Water Management Plan St. Joseph, Minnesota A-STJOE0315 H-4 Appendix H: Pond Design Standards Regional Ponds for Multi-Use Areas General. Regional ponds are constructed to hold the runoff from a number of smaller subwatersheds. They·will often have more than one ponding area or cell, and should have sufficient storage to accommodate the discharge from individual upstream ponds. The ponding site should be large enough to allow plantings that can screen the pond from neighboring development. Slopes should be gentle to reflect the fact that they are dry most of the time. Ownership Regional Ponds are owned by the City and are constructed on public land. Design Criteria A. Design Storm. Ponds that have outlets shall be designed for the 100-year storm. Ponds without outlets shall be designed for two back-to-back 100-year storms. Design must include a 2-foot freeboard between the high water level and the top of the pond. There must be 3 feet of elevation between the high water level and the lowest structure opening, The 100-year return period is based on 5.8 inches of rain in 24 hours and a Type II storm. In some cases, where rate control is a concern, the pond will be sized by routing the discharges of upstream ponds through the regional pond. B. Infiltration. Where infiltration is used to reduce the size of the pond, it must have a 2: 1 safety factor. Infiltration rates must be verified by field-testing the pond bottom and/or embankment surfaces after construction has been completed. C. Rate control. The rate of discharge must be controlled so that the rate after development is equal to or less than the rate before development for the 10-year and 100-year storm events. Since the discharge is concentrated flow, provisions must be made to prevent downstream erosion. For the 2-year, or NPDES water quality event, the discharge rate must be less than 5.66 cfs per acre. D. Volume Control. Where the City determines that any increase in runoff is likely to cause damage downstream, the pond must also be designed to keep runoff volume at pre-development levels. Design Standards A. Pond Shape. To the extent possible given site constraints, ponds shall be designed so that the length is approximately 3 times the width. This can also be accomplished by creating berms or baffles in the pond to route the water to the outlet over the longest possible path. The idea is to to maximize the detention time to promote settling of suspended material. B. Pond Depth. Where the pond depth exceeds 10 feet from the overflow level to the bottom of the side slope, a shelf or terrace must be provided for maintenance. The shelf must be 10 feet wide and have a 10: 1 side slope. C. Inlet Structure. Inlets should be constructed in the lowest tier of the pond to minimize potential side slope erosion. Where possible, the inlet should be brought into the pond approximately 2 feet above the pond bottom to minimize erosion potential, but still allow for sediment buildup. This can be accomplished through a drop structure where it is necessary to reduce discharge velocity. Where the inlet must enter the pond higher to maintain pipe capacity, the slope must be protected with riprap from the pond bottom to a point 2 feet above the top of the pipe. Storm Water Management Plan St. Joseph, Minnesota A-ST JOE0315 H~5 Appendix H: Pond Design Standards D. Riprap. Riprap and filter fabric must be extended out from the inlet far enough to prevent erosion due to excessive discharge velocities, occurring when the storm sewer is surcharged. Riprap protection must be carried wide enough to prevent undermining. E. Overland Flow. Where significant storm water runoff is directed to the pond by sheet flow or swales, it must be collected by a culvert or catch basin before it reaches the pond slope and conveyed to the inlet pipe. F. Outlet Structure. Outlet aprons that exceed IS-inches in diameter should be equipped with trash guards. Where there is potential for petroleum products or surface debris to accumulate in the pond, a cap-skimmer type structure should be provided at the outlet. G. Overflow. An overflow or spillway must be constructed to direct excess water safely out of the pond during runoff events that exceed the capacity of the pond. The overflow crest should be at the 100-year high water level of the pond, and should be sized to keep the backwater below the freeboard elevation. The overflow must be riprapped, and the downstream area protected from erosiOn. H. Pond Bottom. Ponds or cells should have a 2% slope on the pond bottom directed to a pit area near the outlet that is accessible for maintenance I. Side Slopes. Side slopes must be 4:1 or flatter above the level of the pond outlet, and 3:1 or flatter below the outlet elevation. J. Clearances. Ponds must be located so that the top edge of the freeboard is at least 100 feet from the nearest structure and 20 feet from the nearest property line. Additional clearance is desirable. K. Vegetation. Ponds must be seeded with an approved mix of native grasses and forbs. Different mixes may be specified for the top, sideslopes, and pond bottoms. Vegetative growth must provide a clear line of demarcation between the ponding site and adjacent property. L. Access. Pond site must include a 20-foot wide access from a nearby street. The access from the street to the pond should be mowable turf to blend in with neighboring yards, but still be under City ownership to prevent complaints generated by maintenance vehicles accessing the site. In addition to access from the street, there must be a 20-foot wide access shelf around the top of the pond for maintenance vehicles. The access shelf should have a 20: 1 or flatter cross slope, and be seeded with native grasses and forbs to provide a visual break between adjacent lawns and the pond. M. City Review. All pond designs must be approved by the City. The City may approve deviations from the above standards due to site constraints on a case-by-case basis. Increasing development density is not considered a site constraint. The city may also require additional design features in a case-by-case basis. Storm Water Management Plan St. Joseph, Minnesota A-ST JOE0315 H-6 Appendix H: Pond Design Standards Permanent Pool A. The permanent pool is important because it provides storage and treatment of runoff during and between storm events. Permanent pool volume should be greater than or equal to the volume of runoff resulting from a 2.5-inch rainstorm under complete watershed development. This value has been derived from design criteria developed in Nation-wide Urban Runoff Program (NURP), with a 25 percent increase in volume to allow for roughly 25 years of sediment accumulation. This sizing rule provides a mean hydraulic residence time of about 15 days. B. To promote settling and provide space for sediment accumulation, the mean depth of the permanent pool (volume/surface area) should be greater than or equal to 4 feet. This constraint may be infeasible for small ponds «approx. 3 acre-feet in volume, see below), where mean depths of 3-4 feet may be used. C. To prevent development of thermal stratification, loss of oxygen and nutrient recycling from bottom sediments, the maximum depth of permanent pool should be less than or equal to 10 feet. D. To promote plug flow behavior, the ratio of maximum length to maximum width (Le/We) should be greater than or equal to 3. Expected performance is less sensitive to the length/width ratio than to volume or depth. This constraint may be infeasible for some site plans or for small ponds. E. In such situations, baffles may be installed to isolate the inflow area from the remainder of the pond. A desirable alternative (for all pond sizes) is to construct two or more separate ponds in series with a total volume equal to that specified above Item (1). F. For safety purposes and to provide suitable habitat for rooted aquatic plants, the bench width should be at least 10 feet and the bench slope should not be steeper than 10:1 (horizontal vertical). The bench slope begins at the normal pool elevation and includes lower elevations until the minimum length criteria is met. G. To provide stability, the side slopes below the bench should not be steeper than 3 feet horizontal to I-foot vertical. Shallower slopes may be appropriate, depending upon soil engineering properties. Shallower slopes are more feasible for larger ponds. H. A forebay at the pond inlet will improve the settlement of sand-sized particles. The forebay also assits in pond maintenance. Storm Water Management Plan St. Joseph, Minnesota A-ST JOE0315 H-7