A sediment retention pond (SRP) is a temporary pond formed by excavation into natural ground, or by the construction of an embankment. SRPs incorporate an outlet device to dewater the pond at a rate that allows a high percentage of suspended sediment to settle out within the pond.
The purpose of an SRP is to detain runoff flows so that deposition of transported sediment can occur through settlement.
Due to the detention provided, SRPs also attenuate flows thereby reducing downstream channel erosion effects.
Conditions where practice applies
SRPs should be used:
- Where treatment of sediment-laden runoff is required
- Where concentrated flows of sediment-laden runoff occur.
They are typically the most appropriate control measure for catchments greater than 0.3 ha.
Limitations of SRPs are:
- Specific geotechnical design input may be required
- Location needs to be considered in relation to on-going maintenance (particularly during winter) and decommissioning at the completion of earthworks
- Specific design details may be required, including drawings, to ensure correct construction
- Catchment areas should be restricted to 5 ha. This limits the length of overland flowpaths, reduces maintenance, and limits the size of flocculant treatment devices.
Key design criteria
An SRP is an impoundment area formed by excavation or filling to form embankments. The embankments provide the required impoundment volume and shape. In practice, most SRPs are formed from a combination of excavation and filling. The maximum height of any filled embankment should not exceed 2.6 m. This height accommodates a maximum 2.0 m pond depth (base of pond to primary spillway), 300 mm freeboard from the primary spillway to the emergency spillway, and a further 300 mm depth of spillway. Exceeding this 2.6 m maximum height will increase the overall footprint of the SRP.
The following design criteria apply to SRPs (refer also Figures 64, 65, and 71 to 73):
- Size SRPs based on the contributing catchment area and slope length.
- On earthwork sites with slopes less than 18% and less than 200 m in length, design SRPs with a minimum volume of 2% of the contributing catchment area (200 m3 for each ha of contributing catchment).
- On earthwork sites with slopes greater than 18% or greater than 200 m in length, design SRPs with a minimum volume of 3% of the contributing catchment area (300 m3 for each ha of contributing catchment).
- The above calculation defines the total storage volume, which is measured from the base of the pond to the top of the primary spillway.
- The slope angle is determined by the slope immediately (within 20 m) above the SRP, or by the average slope over the contributing catchment, whichever is greater. The slope angle should also be the greater of the pre- or post-construction slope.
- Maximise the distance between the inlet and the outlet (including the emergency spillway) to reduce the risk of short circuiting and to promote quiescent (inactive) conditions. If this cannot be achieved by correctly positioning the inlet and outlets, install baffles to achieve the appropriate length to width ratio design.
- Ensure the length to width ratio of the SRP is no less than 3:1 and no greater than 5:1. The length of the SRP is measured as the distance between the inlet and the outlet (decant system).
- The length to width ratio is measured at the height of the primary spillway.
- Ensure the SRP has a level invert as described below to promote the even and gradual dissipation of the heavier inflow water across the full area of the SRP.
- The construction of the SRP invert with a reverse slope can aid maintenance by promoting heavier sediment to drop out and accumulate at the inlet end of the device.
- For external batter steepness refer Figures 72 to 74. For internal batters, a 2:1 ratio is recommended, subject to available space and ground conditions.
- SRP depths may be 1 – 2 m, but no deeper than 2 m. This depth is measured from the invert to the top of the primary spillway. Deeper ponds are more likely to cause short circuiting problems during larger storm events and require specifically designed floating decant systems.
- The decant design in this guideline operates through a maximum live storage range of 1.5 m.
Dead storage (permanent storage)
- Dead storage is the component of impoundment volume that does not decant and remains in the SRP. It is important for dissipating the energy of sediment-laden inflows.
- Dead storage should be retained at 30% of the total SRP storage by positioning the lowest decant 0.4 - 0.8 m above the invert of the SRP.
- The decant design detailed in this guideline allows the lower decant arm to be raised as sediment deposition increases, thereby maintaining the percentage volume of dead storage.
Live storage (decant storage)
- Live storage is the volume between the lowest decant outlet level and the crest of the SRP primary spillway.
- The live storage volume capacity should be 70% of the total SRP storage.
- The decant design detailed in this guideline allows the decant arms to be raised as sediment deposition increases, thereby maintaining the percentage volume of live storage.
- The forebay should extend the full width of the pond, be a minimum of 1 m in depth, and be located upstream of the level spreader.
Decanting/outlet dewatering device
- The SRP decant/outlet dewatering device should be designed to remove water within the upper water column without removing any of the settled sediment, or any appreciable quantities of floating debris. Either a 100 mm or 150 mm diameter decant can be used.
- The floating T-bar dewatering device described in this guideline allows decanting of the cleaner surface water from the top of the water column.
- The recommended decant rate from an SRP is 3 L/second/ha of contributing catchment. This rate ensures that appropriate detention times are achieved.
- A standard T-bar design is detailed in Figure 66. This standard T-bar decant design provides a decant flow rate of 4.5 L/second. Decants are either added incrementally to accommodate catchments greater than 1.5 ha or the number of holes in the decant is restricted to maintain the decant rate of 3 L/second/ha of contributing catchment.
- T-bars should be able to float to the top of the primary spillway at all times.
- To achieve a decant rate of 4.5 L/second per decant, six rows of 10 mm diameter holes should be drilled at 60 mm spacings (200 holes) along the 2 m long decant arm.
- For catchments of less than 1.5 ha, the appropriate number of holes should be sealed off to achieve a 3 L/second/hectare discharge rate.
- Single T-bar decants must be able to operate through the full live storage depth of the SRP.
- If two decant systems are required, the lower T-bar decant must operate through the full live storage depth of the SRP. The upper T-bar decant should operate through the upper 50% of the live storage depth of the SRP only.
- If three decant systems are used, then the lower T-bar decant should operate through the full live storage depth and the second T-bar decant through the upper two thirds of live storage depth of the SRP. The upper T-bar decant should operate through the upper one-third of live storage depth of the SRP as detailed in Figure 67.
- For contributing catchments:
- Up to 1.5 ha, use a 150 mm (minimum) outlet pipe
- Between 1.5 ha and 3.0 ha, use a 150 mm (minimum) outlet pipe
- Between 3.0 ha and 5.0 ha, use a 300 mm (minimum) outlet pipe.
- The primary spillway is the vertical upstand or riser pipe to which the decant is connected. All SRPs require a piped primary spillway.
- For contributing catchments:
- Up to 1.5 ha, use a 150 mm (minimum) upstand as a primary spillway
- Between 1.5 ha and 3.0 ha, use a 150 mm (minimum) upstand as a primary spillway
- Between 3.0 and 5.0 ha, use a 1050 mm concrete manhole riser as a primary spillway.
- The primary spillway should be a minimum 600 mm lower than the top of the SRP embankment and a minimum 300 mm lower than the emergency spillway crest. Ensure the riser and the discharge pipe connections are all completely watertight.
- An emergency spillway is essential for all SRPs.
- Emergency spillways must be capable of accommodating the 1% AEP event without eroding.
- The emergency spillway level should be a minimum 300 mm lower than the top of the SRP embankment.
- The emergency spillway crest and downstream batter require a very high standard of stabilisation with well-compacted fill material.
- When using geotextile for emergency spillway stabilisation purposes, the batter face must be smooth and all voids eliminated. If geotextile is used, a soft needle punch geotextile is covered with a strong woven low permeability geotextile. Ensure the geotextile is pinned at 0.5 m centres over the full area of the emergency spillway.
- Design the emergency spillway as a stabilised trapezoidal cross-section, with a minimum bottom width of 6 m, or the width of the pond floor, whichever is the greater; unless specific design calculations have confirmed a smaller emergency spillway will accommodate the 1% AEP event.
- Incorporate baffles (refer Figure 67) into the SRP design if the recommended pond shape cannot be achieved. Extend baffles the full depth of the SRP and place them to maximise dissipation of flow energy.
- Generally, baffles are in the form of a wing to direct inflows away from the outlet and maximise the stilling zone. A series of compartments within the pond can be used to achieve this, although care must be taken to avoid creating in-pond currents and re-suspension of fine sediment.
- Incorporate a level spreader (refer Figures 68 and 69) into the inlet design to reduce inflow velocities and rapid dissipation of inflow energy. The inlet batter downstream of the level spreader must be well compacted and smoothed (no steeper than a 3:1 gradient), and stabilised over its entire area. It is essential to ensure the level spreader is level, non-erodible and spans the full width of the SRP.
- To ensure even inflows, install a trenched and pegged 150 mm x 50 mm timber weir or similar across the full width of the inlet. Secure the ends of the timber weir with compacted earth and a concrete cover to prevent flows outflanking the weir. Install a concrete haunch along the edges of the level spreader to provide added structural strength. This timber weir also serves to toe in any geotextile protection that may be required. Sediment accumulated upstream of the level spreader may require periodic removal.
- Position the top of the level spreader weir 100 – 200 mm above the invert of the emergency spillway.
- The discharge pipe should be laid at a 1 – 2% gradient and surrounded by compacted fill. Anti-seep collars (refer Figure 70) should be installed around the pipe with a spacing of approximately 10 m to increase the seepage length along the pipe. The vertical projection of each collar should be 500 mm. All anti-seep collars and their connections around the pipe must be watertight. Figure 71 provides a schematic of the anti-seep collar. Site-specific constraints may preclude certain features of this design.
SRPs can be a safety hazard if not appropriately fenced and if safety rules are not followed. Low gradient pond batters provide an additional safety measure (for access/egress). Check the safety requirements of Worksafe NZ. Refer Section C1.8.2 for further discussion on safety considerations.
The majority of SRPs will require flocculant treatment.
The details of various flocculant treatment options are provided in Section F6.0 and Auckland Council’s Technical Publication TP227 - The Use of Flocculants and Coagulants to Aid the Settlement of Suspended Sediment in Earthworks Runoff : Trials, Methodology and Design, June 2004.
SRP potential modifications
The development of the SRP design promoted in this guideline has been supported by field trials that have confirmed the sediment retention efficiencies now adopted as the assumed minimum performance for the Auckland region. A number of additional measures and modifications have been developed and implemented by the earthworks industry since those original trials. The additional measures include:
- Baffles at dead water level (refer Figure 74 below)
- Geotextile stabilisation of the internal batters of the SRP
- Floating booms to reduce wave propagation and reduce maintenance to address blockage of T-bar holes by floating (organic) debris
- The use of additives other than PAC.
At the time of preparation of this guideline, the above measures had not been subject to any documented comparative testing to verify their benefit. However, they are generally supported by the industry as additional means to achieve or exceed the SRP efficiencies assumed by the guideline. Design specifications for these measures or modifications are not currently included in this guideline.
1.1.2 Construction, operation and maintenance
Construction and operation
For constructing and/or operating SRPs, follow the following general steps:
- Form clean water diversion bunds to isolate the SRP construction area
- Install a silt fence or other appropriate sediment control below the SRP construction area
- Clear areas under proposed fills of topsoil or other unsuitable material
- Large fill embankments may need to be keyed in. Ensure that the embankments are constructed to appropriate engineering design standards
- Use only certified fill
- Place and compact fill in layers as per the engineering specifications
- Construct the base of the SRP with a reverse slope so heavier sediment drops out and accumulates at the inlet end of the device. This will assist with maintenance, specifically during regular desilting
- Do not place pervious materials such as sand or gravel within the fill material
- Construct fill embankments approximately 10% higher than the design height to allow for settlement of the material. Install appropriate pipe work and anti-seep collars during the construction of the embankment and compact around these appropriately (refer Figure 75). Where possible, install the discharge pipes through the embankment once the embankment fill height provides sufficient cover over the pipe to continue filling once the discharge pipe has been installed
- Install the emergency spillway with a minimum of 300 mm freeboard height above the primary spillway. Where possible, construct emergency spillways in well vegetated, undisturbed ground (not fill) and discharge over long grass. If the emergency spillway is to be constructed on bare soil, provide complete erosion protection by measures such as grouted rip-rap, asphalt, erosion matting/ geotextile or concrete. When using geotextile for emergency spillway stabilisation purposes, the batter face must be smooth and all voids eliminated. If geotextile is used, a soft needle punch geotextile is laid first and then covered with a strong woven low permeability geotextile. Ensure the geotextile is pinned at 0.5 m centres over the full area of the emergency spillway
- Install and stabilise the level spreader (refer Figure 76)
- Construct the forebay.
- For the decant system:
- Securely attach the decant system to the horizontal pipework with steel strapping directly on top of the decant arm (refer Figure 77). This should be weighted to keep the decant arm submerged just below the surface through all stages of the decant cycle. This will also minimise the potential for blockage of the decant holes by floating debris. The most successful method is to weigh the decant arm by strapping a 1.8 m long waratah between the float and the decant (approximately 4 kg of weight). Make all connections watertight
- Position the T-bar decant at the correct height by tying 5 mm nylon cord through decant holes at either end of the decant arm and fastening it to waratahs driven in on either side of the decant. T-bars should be able to float to the top of the primary spillway at all times
- Use a flexible thick rubber coupling to provide a connection between the decant arm and the primary spillway or discharge pipe. To provide sufficient flexibility (such as is required for the lower decant arm), two couplings should be installed. Fasten the flexible coupling using strap clamps and glue. Self-tapping screws will provide added strength and robustness to the joint. Steel bands should be tightened with a socket rather than screwdriver and care should be taken to leave space between the ends of the PVC pipes within the coupling to allow the T-bar to flex.
- Where a concrete riser decant system is used, ensure the lower decant connection is angled upwards so that it bisects the angle that the decant operates through. This will reduce the deformation force on the coupling used
- Where a concrete riser is used, ensure it is incorporated within the pond embankment to prevent it floating. Where this is not possible, use a suitable volume of concrete ballast in the base of the manhole to prevent flotation
- Decants should include a mechanism to allow outflows from the SRP to be temporarily stopped. This is to facilitate flocculant treatment via batch dosing and as a contingency in the event of spill or discharge of contaminants. It will allow contaminated runoff to be retained and prevent it from discharging from the site. A rope and pulley system, to lift the decants above the SRP water level, is the preferred mechanism; however other options such as plumbing bungs, valves or screw on end caps can also be used subject to the specific details of each SRP
- Place any manhole riser on a firm foundation of impervious soil
- Lay the discharge pipe at a 1 – 2% gradient. The fill material should be compacted using a machine compactor and must incorporate anti-seep collars around the pipe to increase the seepage length along the pipe with a spacing of approximately 10 m. The vertical projection of each collar should be 500 mm, and they must be watertight including their connections
- Do not place pervious material such as sand or scoria around the discharge pipe or the anti-seep collars
- Install baffles if required
- Fully stabilise the external batter face, by vegetative or other means, immediately after construction, in accordance with the site’s approved ESC Plan
- Provide an all-weather access track for maintenance. Consider future maintenance. Construct a muck-out bund adjacent to the forebay if space for future desilting maybe an issue
- Install and commission flocculant treatment devices
- Certify the SRP to confirm all design criteria have been met. Rectify any deficiencies as required
- Install sediment-laden diversions to direct runoff to the SRP.
For maintenance of SRPs:
- Inspect SRPs daily and before and after each rainfall event
- Clean out SRPs before the volume of accumulated sediment reaches 20% of the total SRP volume. To assist in gauging sediment loads, clearly mark the 20% volume height on the decant riser
- Clean out SRPs with high capacity sludge pumps, or with excavators (long reach excavators if needed) loading onto sealed tip trucks or to a secure bunded area where the sediment can dry
- Maintain access to the forebay at all times to allow removal of accumulated sediment. Clean out the forebay after each runoff event if there is any evidence of sediment deposition
- The ESC Plan should identify disposal locations for the sediment removed from the SRP. Deposit the sediment in a location that avoids direct discharge to receiving environments. Stabilise all disposal sites as required and approved in the site’s ESC Plan. Provide all weather access for the desilting and secure bunded areas if the SRP is to operate throughout winter
- Immediately repair any damage to SRPs caused by erosion or construction equipment.
The decommissioning of an SRP should only occur once the contributing catchment has been fully
stabilised or alternative appropriate sediment retention devices have been installed.
The following steps should be followed:
- Dewater pond (refer section G1.0)
- Remove and correctly dispose of all accumulated sediment
- Remove fabric, concrete, pipe and other construction materials
- Backfill the pond, compact soil, and re-grade as required
- Stabilise all exposed surfaces.