Catchments & Creeks guidelines

Description                                           Special Requirements
Level spreaders consist of a level, grassed,          Outlet area must be free of depressions
side-flow weir (i.e. water discharges at 90           that may concentrate the outflow.
degrees to the inflow direction) constructed
                                                      Extra erosion protection using jute mesh,
along the contour.
                                                      Erosion Control Mats, turf, rock etc. may be
                                                      required at the sill (Figure 4).
Purpose
                                                      Generally constructed by bozers no larger
Used to allow concentrated inflow to be
                                                      than D5 or equivalent.
released as sheet flow down a stable,
vegetated slope.                                      Extreme caution must be exercised when
                                                      attempting to discharge sheet flow down a
Can be used as an outlet for Catch Drains
                                                      steep gradient (>10%) to ensure that the
and Flow Diversion Banks.
                                                      sedimentation or damage to the outlet sill
Level spreaders are commonly used in rural            does not concentrate the outflow.
areas to discharge stormwater from
roadside table drains into an adjacent                Site Inspection
property (Photos 1 & 2).
                                                      Check for sediment build-up on the sill, or
                                                      the concentration of outflow.
Limitations
                                                      Check for erosion down-slope of the sill.
Minimum sill length of 4m.
Maximum sill length of 25m.                           Installation
                                         3
Maximum discharge of around 0.85 m /s.                1. Refer to approved plans for location,
                                                         dimensions and construction details. If
Must only be used where the outflow can
                                                         there are questions or problems with
be discharged to an undisturbed, stable,
                                                         the location, dimensions, or method of
grassed surface.
                                                         installation contact the engineer or
Construction traffic should be prohibited                responsible     on-site   officer   for
from the area of the level spreader.                     assistance.
Not suitable for highly erosive soils,                2. Wherever practical, locate the level
dispersive soils, or soils with poor                     spreader on undisturbed, stable soil.
vegetation cover.
                                                      3. Ensure flow discharging from the level
                                                         spreader will disperse across a properly
Advantages
                                                         stabilised slope not exceeding 10:1
Inexpensive to construct and maintain.                   (H:V) and sufficiently even in grade
                                                         across the slope to avoid concentrating
Disadvantages                                            the outflow.
Can be difficult to construct the outlet sill to      4. The outlet sill of the spreader should be
the required precision.                                  protected with erosion control matting
                                                         to    prevent     erosion    during   the
May require a considerable           width   of          establishment of vegetation.         The
undisturbed land.                                        matting should be a minimum of
May require the land to be free of trees,                1200mm wide extending at least
shrubs and other surface irregularities to               300mm upstream of the edge of the
avoid local erosion problems.                            outlet crest and buried at least 150mm
                                                         in a vertical trench. The downstream
Common Problems                                          edge should be securely held in place
                                                         with closely spaced heavy-duty wire
The most common problems result from                     staples at least 150mm long.
damage to the outlet sill either from erosion,
sedimentation, or stock.                              5. Ensure that the outlet sill (crest) is level
                                                         for the specified length.
Other problems can result from water flow
concentrating below the level spreader due            6. Immediately after construction, turf, or
to the existence of a concave surface,                   seed and mulch where appropriate, the
vehicular tracks, or uneven vegetation                   level spreader.
cover.




© Catchments & Creeks Pty Ltd                 V2 February 2010                               Page 4