Pennsylvania Handbook of Best Management Practices for Developing Areas

Appendix I -- Common Design Elements

Drop Inlets

Drop inlets are common outlet devices for sediment traps, sediment basins, dry and wet ponds, as well as constructed treatment wetlands and bioremediation facilities. The purpose of drop inlets is to allow the rapid release of water once the lip of the outlet is attained. Drop inlets are not as effective when used as components in runoff peak attenuation devices. In general, BMPs with drop inlets also will incorporate a lower-level outlet or an outlets that is designed to achieve specific attenuation or sedimentation objectives.

Drop inlet spillways usually consist of a riser structure in the reservoir area connected to a pipe or box culvert (outlet conduit) that extends through the dam embankment. Drop inlets should be designed to operated as weirs. A good practice is to arrange the inlet and emergency spillways to prevent the head over the inlet from exceeding about 33 percent of the inlet riser diameter. At greater heads, the flow may become unstable as it approaches the transition to orifice flow, leading to surging, noise, vibration, or vortex action. The equation that governs the stage-discharge rating curve for weir flow at a drop inlet spillway is:

Q = CLH_W^3/2

Where: Q = discharge (cfs)

C = weir flow coefficient, typically set at 3.0 but may vary with head
and weir shape

L = weir length (equal to 3.14 x riser diameter for circular inlets)

H_W = difference between pool elevation and inlet crest

If heads greater than 33 percent of the inlet riser diameter are required, the drop inlet spillway should be designed so that full flow is established before the orifice flow occurs at the riser top. In other words, for any Q, the water-surface elevation of the orifice flow at the riser top must be less than the water-surface elevation resulting from either weir flow at the riser rim or full flow in the outlet conduit. This will insure that orifice flow will not develop. A design that results in full flow occurring at as low a head over the riser top as practical is preferred. The designer should avoid situations where the outlet conduit flows part full and control occurs at the junction of the riser and outlet conduit, since this may result in excessive surging, noise, and vibration during operation.

At the point that full flow is established in the outlet conduit and riser, the stage-discharge rating curve will be computed from the outlet equation. The outlet conduit should be designed to flow full with control occurring at the outlet. In most cases, the following equation can be used to determine the energy loss through a principal spillway conduit operating under outlet control:

Where: H = head loss (feet)

k_e + k_b = entrance and bend loss coefficients, typically set at 0.7
but may vary depending on entrance and bend geometry

n = Manning’s roughness coefficient, typically set at 0.013 for concrete

L = conduit length

R = hydraulic radius of conduit

V = flow velocity in conduit

g = acceleration of gravity 32.2 ft/sec²

Under some cases, particularly if the outlet conduit is set at a steep slope, full flow will not occur in the pipe conduit and control may occur at the junction between the outlet conduit and riser. A rating curve with the control at this location can be calculated by assuming orifice flow into the outlet conduit and using the orifice equation or by using Federal Highway Administration ( FHWA) inlet- control nomographs (USDT, 1965.). The inlet control nomographs are not truly representative of this type of flow situation and should be used with the understanding that they were developed to predict flow through highway culverts operating under inlet control. However, depending on the size relationship between the riser and outlet conduit, the inlet control nomograph may provide a reasonable estimate.

Riser structures should be designed with a factor of safety against flotation equal to or greater than 1.3 under any flooding condition. In addition to this criterion, riser structures in wet ponds also should have a factor of safety against flotation equal to or greater than 1.5 at the normal pool elevation. When the riser is in the embankment, the buoyant weight of submerged fill over the footing projection may be considered.

Multiple Outlet Spillways

Multiple outlet spillways are used to achieve specific runoff peak attenuation goals. In general, runoff peak control will be required for several storm magnitudes. Outlets are arranged to provide the required attenuation while minimizing the overall size of the basin. Multiple outlets frequently combine a number of different control devices, including orifices, rectangular and V-notch weirs, as well as vertical risers.

The stage-discharge rating curve for the outlet system is created by superposing the discharges from each outlet at each pool elevation.

Rectangular Weir with Contractions

Q = Cw(L-0.2H)H_W^3/2

where: Cw = Weir flow coefficient, typically set at 3.0 but may vary with head
and weir shape

L = Weir length (ft)

H_W = Difference between pool elevation and inlet crest (ft)

Q = Discharge (cfs)

Figure I-1. Approaches to Multi-Outlet Design.

Q = CvH_W^5/2

where: Cv = Weir flow coefficient for V weirs

2.50 for 90^O

1.44 for 60^O

1.03 for 45^O

Orifice

where: Co = Orifice coefficient, typically set at 0.6 for sharp-edged orifices but
may vary depending on orifice geometry

A = Flow area (ft²)

g = Acceleration of gravity (32.2 ft/sec²)

H_o = Difference between pool elevation and centroid of opening (ft)

For the outlet expressions to accurately represent the outlet discharge, the no tail water is permitted. For this reason, multiple outlets incorporating weirs and orifices are usually constructed in outlet boxes served by oversized outlet conduits.

Dewatering Outlets

Dewatering outlets are required in sediment basins and sediment traps to drain the settling volume of these devices. Dewatering outlets are also used to provide the extended detention function of ponds and bioretention facilities. The optimum detention time for sediment control and water quality BMPs is generally considered to be about 48 hours. Dewatering times longer than 7 days are excessive.

The most widely used type of dewatering outlet is a vertical riser with one or more columns rows of perforations. The objective of providing an array of small orifices, instead of a single orifice, is to reduce the velocity of currents near the outlet. Perforations larger than one 1-inch in diameter are not recommended for most basins.

Perforated risers have the disadvantage that the outlet rates are greatest early in a storm event when most of the entrained sediment is still suspended. Perforated risers also draw most of the discharged water from the deepest portions of the basins where the highest concentration of suspended sediments will occur.

To avoid these problems, skimmer-type dewatering device may be considered. Skimmers operate at the surface of the ponded water and therefore will not draw sediment laden water from the submerged volume of the basin. Furthermore, for a specified dewatering period, the discharge rate for skimmers compared to perforated risers is significantly lower during the critical time when turbulence is greatest and large quantities of sediment are in suspension.

Figure I-2. Schematic Drawing of the Faircloth Skimmer Outlet.

Tests of skimmers show up to a 45 percent reduction in the mass of sediment discharged from sediment basins with skimmers compared to conventional perforated risers (Millen, J.A., A.R. Jarrett, and J.W. Faircloth, 1996, Reducing Sediment Discharge from Sediment Basins with Barriers and a Skimmer, ASAE Microfiche 96-2056)Jarrett, A.R., 1996, Communication). However, they are mechanically more complex and will require frequent inspection and maintenance in order to operate as designed.

Design guidelines for these devices are available for the Department of Agricultural and Biological Engineering at the Pennsylvania State University where research on skimmer designs is currently being carried out.

To compute the dewatering time for a dewatering device, it is necessary to determine the stage-discharge and stage-storage relationships for the basin. The rate at which water will be released through the perforations of a riser can be computed from the orifice equation shown above. For any given pool level, the discharge rates associated with each orifice can be superposed to provide the total discharge rate. Table I-1 shows the The following tabular procedure is recommended procedure for determining the time to dewater.

Definition of Columns

Elevation—Stage of reservoir

Storage Volume—Total storage volume at given elevation

Difference in Storage Volume—Difference between current and previous in storage volumes between given elevations

Discharge—Combined outletPrincipal spillway discharge at given elevation

Average Discharge—Average of current and previous discharge rate between given elevations

Time—Time required to discharge storage volume at average discharge (column 3 divided by column 5)

Accumulated Time—Summation of time from beginning of computations

Procedure for computing dewatering time:

1. Select first elevation at the crest of the primary spillway and enter in column 1. From stage-storage curve, determine total storage at this elevation and enter in column 2. From stage-storage curve, determine the discharge at this elevation and enter in column 4.
2. Select succeeding elevations at regular intervals. Include all elevations which are associated with marked discontinuities in the stage-discharge curve. Enter the total storage and discharge values for the succeeding elevations as in step 1.
3. Compute the difference in storage and average discharge values and enter in columns 3 and 5. Divide the difference in storage volume by the average discharge, convert time to hours, and enter in column 6. Add value in column 6 to value in column 7 of previous line for new accumulated time.
4. Continue steps 1 through 3 until elevation of sediment storage volume is reached.

Trash Racks

Most spillways will be subject to some degree of trash and debris brought by incoming flows, and certain spillways are more susceptible to clogging by debris than others. Before a debris control structure is designed, the anticipated debris problem should be analyzed. The type and quantity of debris will be largely affected by upstream land use, soil erodability, watershed size, and the type of stormwater management facility.

Trash racks to serve drop inlet spillways should be designed to provide positive protection against clogging of the spillway under any operating level. The average velocity of flow through a clean trash rack should not exceed 2.5 feet per second for operation during the spillway design flood. Velocity can be computed on the basis of the net area of opening through that part of the rack receiving the flow. The same criteria should apply to ports or openings along the side of a riser structure. Bar spacing should be no greater than 1/2 of the minimum conduit dimension in the drop inlet spillway. To, and, to discourage child access, bar spacings should be no greater than 5.5 inches1 foot. The clear distance between bars generally should not be less than 2 inches; however, one exception to this may be near the apex of the trash rack.

In some cases, debris control devices for dry stormwater management ponds may be required for low- level intakes at the pond bottom. In these situations, debris control structures such as those discussed in the FHA publication "Debris Control Structures" (HEC-9) should be considered where appropriate.

Figure 5.

Anti-Vortex Devices

Anti-Vortex Devices: All closed-conduit spillways designed for pressure flow should have adequate anti-vortex devices. Anti-vortex devices may take the form of a baffle or plate set on top of a riser, or a headwall set on one side of a riser. The SCS 2-way covered riser has very reliable anti-vortex and debris-control provisions inherent in the standard design.

Emergency Spillways

Vegetated Spillways

Vegetated emergency spillways should be designed to convey the spillway design flood. If a control section is used, then the grade of the exit channel should be sufficiently steep to ensure supercritical flow. A Manning’s coefficient of 0.04 will produce conservative results for vegetated spillways. Uniform flow may be assumed in the exit channel when the flow is supercritical; however, the assumption will be less accurate when the channel slope approaches or exceeds 10 percent. If the permissible velocity for the grass cover will be exceeded, the spillway section can be reinforced using turf reinforcing mesh (see Appendix E, Approach for Developing Material Specifications). or interlocking modular pavers. Unit tractive force or permissible velocity design criteria for reinforcing systems must be obtained from the vendor.

Where flow is subcritical, such as in the inlet channel, step backwater should be calculated to determine the head loss between the control section and the reservoir pool when significant head loss apparently may occur through the inlet channel. In cases where the inlet channel is very short and expands rapidly into the reservoir area, a step backwater analysis usually is not be required; the simple weir formula or direct calculation of critical depth may be sufficient for estimating the energy head upstream of the control section.

Rip RapRiprap Spillways

Rip rapRiprap emergency spillways may be considered when design velocities exceed those that are acceptable for vegetated emergency spillways. The layout of a rip rapriprap spillway should be the same as that described above for a vegetated emergency spillway. The hydraulic design of a rip rapriprap spillway is similar to that of a vegetated spillway except that roughness coefficients and permissible velocities for rip rapriprap should be used.

Combined Principal and Emergency Spillways

An emergency spillway separate from the principal spillway is advisable. However, in some cases, incorporating an overland emergency spillway may be impractical at either dam abutment. This may be due to topographic limitations (e.g., abutments too steep), land-use limitations (e.g., existing or proposed development), or some other factor (e.g., roadway embankments acting as dams). In these instances, a combined principal-emergency spillway may be considered. A principal-emergency spillway is a single spillway structure that conveys both low flows (e.g., stormwater management functions) and extreme flows (e.g., SDF). The combined spillway may take the form of a drop inlet spillway, a straight drop (free over fall) spillway, or some other spillway type. A primary design consideration for a principal-emergency spillway, particularly when in the form of drop inlet spillways, is protection against clogging. Trash racks should be designed as described above. When a drop inlet spillway is proposed as a principal-emergency spillway, then it should be routed through the impoundment as if no storage is available below the riser crest or rim, and all ports or orifices along the riser column are inoperative or clogged.

Earth Embankments

Geometry

Embankment dams typically are constructed as homogeneous (with or without internal drainage), zoned or diaphragm-type structures. Homogeneous dams without internal drainage should be constructed of select fill material with controlled placement and compaction. [Something] is not permitted for permanent impoundments because the phreatic surface will intersect the downstream surface of the dam. However, for dry detention or extended dry detention ponds where a saturated condition does not exist, a homogeneous dam without internal drainage may be appropriate. Thin diaphragms, such as soil bentonite slurry walls, also may be used within homogeneous sections to control seepage.

1. The height of an embankment dam must consider freeboard and compensation for settlement.
2. The crest of an embankment dam should be designed with the following considerations:
   1. Width: the minimum top width may be determined by the following equation but should not be less than 812 feet.

      W = (H + 35)/5

      Where: W = Width of crest

      H = Height of dam (above downstream toe
      at stream bed)

   2. Drainage: Surface drainage should be provided by either crowning or sloping towards the upstream slope. The minimum slope is 2 percent.
   3. Camber: Camber is provided to maintain the height of the dam lost by compression of the foundation soil or soil in the embankment. Camber will be based on the amount of estimated total compression and will vary from the abutments to the center.
   4. Surfacing: A grass surface is preferred unless frequent traffic or foot travel is expected, in which case a gravel, modular paving blocks, or similar surface should be installed to prevent erosion and rutting.
3. Embankment dams should be be linear. If a nonlinear where possible for aesthetic reasons. Concave embankments are inherently more stable than convex alignments. [Something is missing here] section is selected, the shape should be limited to a concave upstream geometry.

Zoning

Zoning, if required, should consist of "impervious" materials on the upstream side and pervious materials on the downstream side. If impervious material is very limited, then an impervious core or a thick blanket on the upstream embankment face may be considered.

1. Impervious Core Thickness: The design of the impervious core thickness should reflect tolerable seepage loss, minimum width that will permit proper construction, type of material available for core and shells, and design of proposed filters. Core sizes should be recommended by a geotechnical engineer. Recommendations for the core should include material type, compaction, filter requirements, width, height, and side slopes.
2. Stability Analysis: The design of slopes for embankment dams depend on the materials used for construction, foundation conditions, height of the embankment, permanent pool level, and pool fluctuations. [Something is missing here] whether the embankment is for permanent storage (wet ponds) or detention (dry or extended dry detention ponds).
3. Slope design and stability analysis should be conducted by a qualified geotechnical engineer if the site or materials are problematic.
4. Raising Height of Existing Dam: The height of the embankment dam may be raised by adding material to the upstream slope, downstream slope, or both. The material used to raise the height by adding to the slopes should be made an integral part of the existing embankment. Recommendations for material type, moisture content, compaction requirements, and side slopes should be provided. Embankment slopes should be determined by seepage and stability analyses or be based on material type., depending upon the reservoir type.

Upstream Blankets

An upstream soil blanket consisting of material similar to the homogeneous dam may be used to reduce seepage through a pervious foundation if the material is sufficiently impervious to be economical. The thickness of the blanket will be influenced by acceptable seepage loss, permeability of blanket and foundation dam material, pool depth, reservoir head, unsaturated or saturated foundation, and constructability Blankets less than 1.5 feet thick should not be considered. An low permeable upstream soil blanket also may be used for a homogeneous dam constructed of more pervious soil or in the case of a zoned embankment. The thickness, material, and compaction requirements for soil blankets should be specified and migration of fines into the foundation must be evaluated. Fillers and synthetic liners, material type, subgrade preparation, bedding, seam preparation, and overlap should be specified.

Cutoff Trench

A cutoff trench should be excavated into the foundation beneath the embankment to remove permeable soil, interrupt seepage pathways, and eliminate buried drainage conduits. The depth of the cutoff should be based on site investigations, but should always be a least 3 feet. The cutoff trench should be backfilled with relatively impervious material for homogeneous dams or the impervious core material for a zoned dam. Recommendations for backfill of the cut-off trench should include soil type, moisture content, and compaction requirements.

Compacted Fill Requirements

For embankment dams, compaction to 95 percent of the maximum dry density in accordance with ASTM D-698 or AASHTO T-99 is considered a minimum standard.

References

U.S. Department of Transportation, Federal Highway Administration, Hydraulic Charts for the Selection of Highway Culverts, Hydraulic Engineering Circular No. 5 (HEC-5), 1965.

U.S. Department of Transportation, Federal Highway Administration, Debris Control Structures, Hydraulic Engineering Circular No. 9 (HEC-9)