Dispersal and Soakaways

Water that has been collected from runoff or from sub-surface drainage systems has to be disposed of. The simplest means is to route it to a naturally occurring surface waterbody (stream or lake). Often a retention pond (see Section 13.4.8.1) is interposed between the water coming out of the highway and the outflow into the surface water body. The retention pond reduces peak flow rates (therefore making the outflow easier for the environment to receive without flooding) and may have environmental benefits, too (see Chapter 12, Section 12.4 and Section 13.3.8 in this chapter). However, such surface water aspects are beyond the scope of this book and interested readers are referred to one of the many texts dealing with surface water drainage. Alternatively, the runoff or seepage flows can be introduced into the ground via a soakaway. This is particularly attractive when there would be lit­tle fall between an outlet and the receiving surface water body. Sometimes water can be dispersed into a wetland area. Only purpose-built wetlands (see Chapter 12, Section 12.3.1 and Section 13.3.8 in this chapter) should be used and they should be designed to take this water although, in the past, it has not been unusual for wetland areas to develop around points where sub-surface water seeps to the surface. Areas handling sub-surface seepages should expect low flows over long periods compared with the short, “peaky” hydrographs associated with runoff.

Permitting water to soakaway to the ground is only permissible where regulation, or regulatory authorities, allow. In particular, the use of soakaways in areas where the groundwater is used for drinking water is very restricted or, in some countries, not permitted at all.

Water collected from embankment grips will usually be of acceptable quality for disposing by soakaway as it came from the natural subsurface and is returning to it. The issue is not, therefore, likely to be only one of quality (unless the cutting intercepts an already contaminated groundwater), but maybe more one of volume. Can the disposal soakaway disperse the water supplied to it without surcharging and without causing problems to the receiving groundwater levels? Whether seepage water from the road drainage system can be disposed in the same way is less certain. In many cases the water collected from seepage started as rain that fell onto the road construction, collecting contaminants as it did so. By the time it arrives at the potential disposal route it will have travelled through many pavement layers and a sub-surface drainage system during which sorption and natural attenuation processes may have removed much or all of the contaminant that it once contained (Dawson et al, 2006).

According to answers to the WATMOVE questionnaire (see www. watmove. org), approximately half the countries in Europe that use soakaways have requirements concerning the water that flows from pavements into them. Often the water has to go through some kind of treatment, i. e. as a minimum sedimentation and oil separation, or a minimum vertical infiltration/percolation time must be ensured by the design. Soakaways need to be positioned to meet several criteria:

• that they encounter a substantial zone of porous ground. For this reason they cannot be used in clay soils. In moderate permeability soils, soakaways may be made more efficient by providing radiating “branches” from a central water store so that slow seepage over a wider area can provide sufficient rate of seepage;

• that they have sufficient capacity to hold most[30] of the water supplied to them through the connected drainage system until it has percolated into the ground. For this reason a soakaway must have void space that is above the maximum groundwater level; and

• that the soakaway space is kept open — either by filling the soakaway hole with coarse aggregate that has a high voids content and lining the sides to prevent wash in of fines, or by supplying a solid wall with openings (see Fig. 13.28).

For a successful design the volume of water to be drained; the return period for this amount of water to come into the soakaway again; and the percolation rate of the soil/sub-soil must all be assessed. Then a water balance calculation can be

Fig. 13.28 Schematic of a walled soakaway

performed — for example by calculating the volume of water arriving at the soak­away in each hour, m, (from a knowledge of the rainfall intensity, P (mm/h), and the area being drained to the soakaway, A (m2), and calculating the amount soaking into the ground, IR (m3/h/m2) (Eq. 13.1). The excess generated during the storm of length n (h) must never accumulate to a volume larger than the storage capacity, V (litres), of the construction.

m=n

(Pm — IRm) X A < V (13.1)

m=1

Swales (see Figs. 1.10 and 1.11) are a form of linear soakaway, with water being able to soak into the ground but, hopefully, leaving contaminants behind in the lining and vegetation of the swale. As a system for dealing with surface runoff they are beyond the scope of this book, but the water that they introduce into the ground does need to be considered by the designer of drainage of subterranean waters. They should not be so positioned that they will raise the groundwater levels in areas where this would decrease the stability of the surrounding slopes or where it would feed water into the pavement foundation, thereby reducing the bearing capacity of the pavement. For these reasons, the positioning of a swale for disposal of surface runoff presents the designer with a dilemma — too close to the pavement and it is likely to result in reduced pavement performance, too far away and it will not be easy to make it useful for its primary purpose.