Enhanced grass swales

This article is about installations designed to capture and convey surface runoff along a vegetated channel, whilst also promoting infiltration.
For underground conveyance which promotes infiltration, see Exfiltration trenches.
For conveyance along planted channels, on both surface and underground, see Bioswales.

Overview

The fundamental components of an enhanced grassed swale are:

• Graded channel
• Resilient turf grass
• Check dams, to facilitate short term ponding

Additional components may include:

• Amended soil, to increase infiltration to soils below
• Impermeable membrane, to prevent infiltration to soils below
• Turf reinforcement, to prevent scour

Planning considerations

Best X-section

Both sections a)triangular and, b)trapezoidal, are constrained within ratios of 8:1 H:V

Enhanced swales aim to both reduce the flow rate and retain a portion of the conveyed water. For these purposes the best X-section is that which maximizes the wetted perimeter. The trapzoidal channel has the marginally greater hydraulic radius, but also the greater area.

Examples worked without units, using the ratios given

Section	Wetted perimeter	Area } Check dams Check dams are small dams or weirs constructed across a drainage ditch, swale, or channel to lower the speed of concentrated flows for a certain design range of storm events and to promote infiltration. Check dams: may be constructed of any resilient and waterproof material, including: rock gabions, earth berms, coarse aggregate or rip-rap, concrete or metal. Stone used in check dams should have minimum median diameter 25 - 75 mm. for enhanced swales may be up to 0.6 m in height; the maximum design depth of ponded water should be ≤ 0.6 m. designed for higher flow velocities should have spillways incorporated into their profile, to direct water to the centre of the swale. are usually installed between 10 - 20 m along the swale. The spacing of dams should not exceed the horizontal distance from the toe of the upstream dam to the same elevation on the downstream dam. should have energy dissipation and erosion control measures installed in the 1 - 2 m downstream. Examples include large aggregate or turf reinforcement Sizing and spacing of check dams Check dams are a feature of enhanced grass swales. They promote infiltration and evaporation by promoting limited ponding. To design check dams into a swale: The height of each dam is determined by the depth of ponded water that will infiltrate in a specified period (often 48 - 72 hours). The infiltration may be through the turf grass or other plants directly into the native soil or some soil amendments may be proposed in the design. The gradient between the top of the lower check dam and the bottom of the upper one is called "compensation gradient" which is the future or final effective gradient of the swale. It is formed when material carried by flowing water fills the check dams to spillway level. Dams are usually installed between 10-20 m along the swale. The spaces between check dams can be determined according to the compensation gradient and the effective height of the dams. They are distributed such that the crest of each dam is at approximately the same elevation as the toe of the upstream dam. If the slope along the swale varies, so should the distance between the dams. The compensation gradient of enhanced swales must be < 1 % (0.5 % preferred). The objective of these design recommendations are to maximize the distribution of ponded water along the whole BMP. Detailed design may require iteration of the dam heights and distances along each section of a long swale. To estimate the depth of water that can be infiltrated into a surface within a given time: $y=f'\times t$ Where: y = the depth of ponded water in mm, f' = the design infiltration rate in mm/hr (after correction, where required). t = time in hrs After surveying the longitudinal profile of the swale, the number of check dams for the swale can be calculated by using the following equation: $Number\ of\ dams={\frac {L\left(S_{i}-S_{e}\right)}{h}}$ Where: L: Length of swale (m) Si: Initial existing slope ratio of the swale (rise/run) Se: Desired effective slope of the enhanced swale (between 0.005 - 0.01, rise/run) h: The average effective height of the check dams in m (excluding foundations)(suggest you use y, calculated above) The first check dam should be constructed on a stable point in the gully such as a rock outcrop, the junction point of the gully to a road, the main stream or river, lake or reservoir. If there is no such stable point, a counter-dam must be constructed. The distance between the first dam and the counter-dam must be at least two times the effective height of the first check dam. The points where the ensuing check dams are to be built are determined according to the compensation gradient and the effective height of the check dams. The effective height of the second check dam is determined by taking into account the depth of the swale, the depth of the spillway and the maximum height of the check dam. In this way, all the other proposed check dam points can be calculated. When spacing check dams, give preference to the narrowest parts of the swale in order to reduce construction costs. In this case, to establish the compensation gradient between the proposed check dams, proportionately increase the foundation depth of the upper check dam when the space between the lower and upper check dam is extended. When the space is shortened, decrease the foundation depth. As the foundation depth is increased, the total height of the check dam (effective height plus foundation depth) should not exceed the permissible, maximum total height. Example Calculation An enhanced swale of 42 m has an existing longitudinal slope of 6 %. The underlying soil has been amended to infiltrate at 16 mm/hr (after safety correction applied); the regulatory authority requires that ponded water must not remain for over 48 hours. The maximum height of the check dams (h) to prevent extended ponding is:: $h=16\ mm/hr\times 48\ hrs=768\ mm$ But the maximum recommended depth of check dams is 0.6 m (600 mm). (As an aside, these will drain in ${\frac {600\ mm}{16\ mm/hr}}=37.5\ hrs$) The current vertical distance along the swale is:: $42\ m\times \left({\frac {6}{100}}\right)=2.52\ m$ The desired effective slope to slow flow is 0.5%. The vertical distance along the swale with compensation gradient is:: $42\ m\times \left({\frac {0.5}{100}}\right)=0.21\ m$ The number of check dams required is:: ${\frac {\left(2.52\ m\times 0.21\ m\right)}{0.6\ m}}=3.85$ This is rounded up to 4 dams, creating 5 ponding zones:: ${\frac {42\ m}{5}}=8.4\ m$ So the four check dams will be 0.6 m high and spaced every 8.4 m along the swale. These are fairly close together, but this construction is achievable and the dimensions are not remarkable, given the change in gradient is an order of magnitude. Design Swales should be designed to meet the following criteria: Minimum residence time of 5 minutes. Maximum flow velocity 0.3 m/s Bottom width between 0.6 - 2.4 m Minimum length 30 m Slope variations < 2 % Use underdrain to distribute water for infiltration 2 - 5 % Design swale for surface flow, consider check dams 5 % Check dams required to reduce effective slope to 2 – 3 % Maximum depth of flow 50% height of grass for regularly mown swales, to maximum of 75 mm. 33% height of vegetation for infrequently mown swales. Modeling Swale element in TTT menu Weir elements may be incorporated as check dams for detailed design It is recommended that grass and enhanced grass swales be modelled using the 'Swale' element in the TTT. A 'swale' has to connect two existing elements within the TTT Bioswales or dry swales, which have amended filter media, should be modelled as bioretention cells. The alternative is to use the 'enhanced swale' within the LID toolbox, but this incorporates fewer design parameters (and doesn't account for infiltration). A 'swale' as a conveyance element in the TTT (key parameters) General Info Upstream Node Name of node on the inlet end of the swale (higher elevation) Downstream Node Name of node on the outlet end of the swale (lower elevation) Manning's Roughness Lower numbers indicate less surface obstruction and result in faster flow. Suggested range for mown grass (dependent on density) 0.03 – 0.06 Upstream Invert (m) Depth of swale invert above node invert at inlet end of the swale Downstream Invert (m) Depth or elevation of the swale invert above the node invert at the outlet end of the swale Cross section Maximum Depth (m) Depth of the swale Bottom Width (m) Bottom width of the trapezoidal swale For a triangular channel, enter 0 Left Side Slope (m/m) Left side slope (run/rise). Suggested value of 3 or 4 if design permits. Right Side Slope (m/m) Right side slope (run/rise). Suggested value of 3 or 4 if design permits. Seepage (mm/hour) Infiltration rate of native (or amended) soil Parameters for 'enhanced swales' in the LID toolbox of the TTT Surface Berm height (mm) This is the height of the curb which constrains the overland sheet flow of water. Where the bottom of the slope discharges directly into another LID facility without impedance, the value is 0. Surface roughness (Manning’s n) Lower numbers indicate less surface obstruction and result in faster flow. Suggested range for mown grass (dependent on density) 0.03 – 0.06 Surface slope (%) If the slope > 3%, use Check dams to create temporary ponding, increase infiltration, and slow flow to reduce erosion. Swale side slopes (run/rise) Suggested value of 3 or 4 if design permits. Materials

The portion of water precipitated onto a catchment area, which then flows as surface discharge from the catchment area past a specified point.

Water from rain, snow melt, or irrigation that flows over the land surface.

The slow movement of water into or through a soil or drainage system.

Penetration of water through the ground surface.

A shallow constructed channel, often grass-lined, which is used as an alternative to curb and channel, or as a pretreatment to other measures. Swales are generally characterized by a broad top width to depth ratio and gentle grades.

A shallow constructed channel, often grass-lined, which is used as an alternative to curb and channel, or as a pretreatment to other measures. Swales are generally characterized by a broad top width to depth ratio and gentle grades.

Structures constructed of a non-erosive material, such as suitably sized aggregate, wood, gabions, riprap, or concrete; used to slow runoff water. Can be employed in practices such as bioswales and enhanced grass swales.

A long narrow trench or furrow dug in the ground, as for irrigation, drainage, or a boundary line.

A broad category of particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates, and available in various particulate size gradations.

Structures constructed of a non-erosive material, such as suitably sized aggregate, wood, gabions, riprap, or concrete; used to slow runoff water. Can be employed in practices such as bioswales and enhanced grass swales.

Includes the protection of soil from dislocation by water, wind or other agents.

Abiotic transfer of water vapour to the atmosphere.

The natural ground material characteristic of or existing by virtue of geographic origin.

Structures constructed of a non-erosive material, such as suitably sized aggregate, wood, gabions, riprap, or concrete; used to slow runoff water. Can be employed in practices such as bioswales and enhanced grass swales.

Best management practice. State of the art methods or techniques used to manage the quantity and improve the quality of wet weather flow. BMPs include: source, conveyance and end-of-pipe controls.

Vegetated open channel, with check dams; designed to convey, treat and attenuate stormwater runoff.

A shallow constructed channel, often grass-lined, which is used as an alternative to curb and channel, or as a pretreatment to other measures. Swales are generally characterized by a broad top width to depth ratio and gentle grades.

A perforated pipe used to assist the draining of soils.

A shallow constructed channel, often grass-lined, which is used as an alternative to curb and channel, or as a pretreatment to other measures. Swales are generally characterized by a broad top width to depth ratio and gentle grades.

Linear bioretention cell designed to convey, treat and attenuate stormwater runoff. The engineered filter media soil mixture and vegetation slows the runoff water to allow sedimentation, filtration through the root zone, evapotranspiration, and infiltration into the underlying native soil.

The engineered soil component of bioretention cell or dry swale designs, typically with a high rate of infiltration and designed to retain contaminants through filtration and adsorption to particles.

Low Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

The rate at which stormwater percolates into the subsoil measured in inches per hour.

A compacted earthen wall that diverts runoff or creates shallow ponding of runoff. In some cases, runoff ponds behind the berm and gradually flows through it or is infiltrated.

(1) The wearing away of the land surface by moving water, wind, ice or other geological agents, including such processes as gravitation creep; (2) Detachment and movement of soil or rock fragments by water, wind, ice or gravity (i.e. Accelerated, geological, gully, natural, rill, sheet, splash, or impact, etc).