GB2038387A

GB2038387A - A Method and Apparatus for Storm Water Drainage

Info

Publication number: GB2038387A
Application number: GB7938104A
Authority: GB
Original assignee: ISOAHO M
Current assignee: ISOAHO M
Priority date: 1978-11-07
Filing date: 1979-11-02
Publication date: 1980-07-23
Also published as: FI56716B; FI56716C

Abstract

In an infiltration-percolation basin for draining storm water into the ground a main sewer or drain 2 Fig. 1 (not shown), is connected to a number of laterally extending discharge channels 3/5. Each discharge channel 5 is formed of a coarse filter medium 6 and a concrete cover 4. Storm water, either pumped or by kinetic energy runs into the channels 5, filters through the filter medium 6 and outwards and upwards through filter cloth 12 into the ground 9. <IMAGE>

Description

SPECIFICATION A Method and Apparatus for Storm Water Drainage The invention relates to a method and apparatus for draining storm water by infiltrationpercolation of the water into the ground.

In the conventional infiltration system water to be percolated is flowing downwards while the water level during the storage reservoir filling rises up. Thus the water energy head of the inflowing water is lost during the infiltration, so the effects of particulate matter contained in the water cannot be controlled. The solids try to drain into the ground or cause the storage media bed to clog and jam. If the inflow is downwards the energy spent in the storm water infiltrating into the filtering material is also lost, because the ground water rises up while the reservoir storage fills and partially carries back up to the surface the accumulated sediments, solids, and impurities.

Storm water run-off is always a temporary event. That is why the infiltration-percolation basin can be designed to act with the two directions, but with only one direction for solid movements.

In the conventional percolation method, a coarse filling material allows the deposit and particulate matter to descend with a slow velocity of flow to the bottom of tiie basin but it prevents storm water from percolating into the ground rapidly. Correspondingly, if the filling material of the conventional infiltration-percolation basin is of fine grain size, the filling medium becomes clogged at the surface and so water cannot percolate into the ground.

The aim of the invention is to produce a method and apparatus by which storm water runoff can be efficiently infiltrated and percolated into the ground.

According to the present invention there is provided a method for storm water infiltrationpercolation into the ground via underground discharge channels wherein water flowing in a discharge channel is allowed to discharge through a porous invert of the channel into storage filling media of an infiltration-percolation basin mainly as upflow current by using the kinetic energy developed by the elevation difference between the water surface or grade line of the inflow sewer and the ground water table or the bottom of the storage reservoir basin and then the water that is so infiltrated unto the storage reservoir basin is allowed to slowly percolate into the ground mainly as downflow current. In this way, an adequate flow rate upwards in the porous substance is achieved.The solids and impurities can be caused to move in the ground at a slower velocity than in the sewer pipe.

The invention also provides apparatus for storm water infiltration-percolation into the ground including under-ground channels located in an infiltration-percolation basin wherein the discharge channel is a tunnel having an invert formed of a filter medium with the cover thereover, from which tunnel water discharges through the invert laterally to the sides of the tunnel.

Advantageously the bottom of the discharge channel is substantially at the level of the ground water table.

By such a discharge channel construction the kinetic energy of the storm water carries the solids upwards to the surface by feeding the water into the basin as low as possible through the coarse invert of the discharge channel(s).

The principle of the method according to the invention is as follows. During the inflow of storm water while filling the storage reservoir the flow velocity upwards in the basin is greater than the one downwards in which the water percolation into the ground while emptying the storage reservoir. Coarse gravel is used as the filling material of the basin. Fine particulate matter and impurities carried in the water flow penetrate unto the upper part of the bed basin during infiltration while supplying the storage reservoir.

When storm flow diminishes the infiltrated water starts to flow down due to percolation into the ground, but with such a low velocity of flow that deposition of solids doesn't occur any more. This is why the infiltration-percolation basin continuously remains in the proper condition of action.

The inflow water surface or grade line in the discharge channel is higher than the level of the natural ground water. The construction of the discharge channel is so dimensioned that the flow rate in the coarse layer from the channel into the basin is sufficient to prevent the invert from plugging. The flow velocity in the coarse layer ranges approximately from 0.40 to 0.70 cm/s.

The velocity of inflow from the main distribution sewer to the discharge channel(s) is approximately 0.80 m/s and the velocity in the discharge channel(s) approximately 0.20 m/s, respectively. While water is filling the storage reservoir the velocity of flow in the basin ranges approximately from 0.05 to 0.40 cm/s whilst the rate of percolation into the ground depends on the permeability of the ground, but typically has the rate about 0.01 cm/s. The percolation rate into the ground can be slow, because the basin acts partially or totally as a storage reservoir, and clogging doesn't cause any head loss changes in the percolation flow rate.

Maintenance is needed only for the discharge channel(s) and not for the whole basin. Floatable material and particles possibly accumulated at the invert of the channel can be removed through the inspection chambers built at junctions of the channels.

Due to the underground structure the basin remains in action all around the year. Freezing of surface layers has little or no effect on the function of the basin. Neither does any unfavourable biological growth appear because of the location of the basin.

An enbodiment according to the invention will now be described by way of example, with reterence to the drawings, in which: Figure 1 shows an infiltration-percolation basin according to the invention with its underground discharge channels, and Figure 2 shows a cross section of a discharge channel.

In Figure 1 there is shown an infiltrationpercolation basin which is a field under which there is located a main distribution sewer 2 with discharge channels 3 at spaced intervals therealong extending laterally from the sewer 2 in opposite directions. Manholes 10 are located, for inspection purposes at the junctions between the main sewer 2 and the discharge channels 3.

Figure 2 shows a cross section of a channel 3 which consists of an inverted open channel 5 and a concrete cover 4 above it. The inverted open channel 5 is made of a coarse filter material 6 and the channel 5 and material 6 is buried in a filling material 1 of the field. The lower-most part of the channel 5 is located approximately at the depth of the ground water table 8. A filter cloth 12 is located in the filling material 1 a short distance below the field surface 9. Influent is able to flow in the channel with minimum head loss under pressure, too, and water is discharged through a coarse filter bottom 6 of the channel into the filling material 1 of t e i the infiltration-percolation basin 1. The water is discharged through the invert of the channel, but outside the channel it flows sideways and upwards.The potential energy of inflow water is changed to the dynamic energy while the water is discharged through the invert into the basin. This is the way to achieve adequate flow rate to transfer solids and fine particulate matter into the proper location in the porous filling media.

The filling material of the basin 1 allows solids movement with a sufficient flow rate, but it also prevents the solid matter from depositing out with a slow flow rate.

The variable flow rate has been taken into consideration in the discharge channel construction design. As can be seen from Figure 2, the coarse layer 6 is widest at the lowest part.

At the initial stage of absorbing storm water flow the head differential is highest and velocity of flow into the infiltration-percolation basin 1 most rapid. That is why the inflow efficiently scours the invert and releases impurities possibly accumulated during low flow rates and then carries them to the upper part of the basin. The velocity increases, however, smoothly so that deposits are not released suddenly and so do not cause clogging of the filling media. The infiitration-percolation basin acts as a separator of different grain sizes of solids lifting finest particulate matter to the upmost position.

The hydraulic dimensioning of the constructions can be based on the 2-year storm i.e. the worst storm likely to occur in a two year period, so that the maximal backwash and scour of the basin media is achieved with a frequency of 2 years intervals. For dimensioning the infiltration-percolation basin for instance the 10 min. duration of the 2-year storm flow may be used.

An application to the detention reservoir storage is a low cost structure. Thus the infiltration-percolation basin can be compared with a concrete equalisation basin but having the difference from the solid depositing. The dewatering of the basin also can be accelerated with an outflow orifice at the bottom of the distribution channel.

Mean urban storm run-off has a solid concentration from 100 to 300 ppm. Calculating the solid load according to precipitation and catchment areas, and dimensioning the basin as stated above, it is estimated that a basin will gather a solid layer with the thickness ranging from 3 to 10 mm annually. Consequently the effective-function life of the infiltrationpercolation basin may vary from 50 to 100 years because of storage volume reductions, without continuous maintenance.

The central distances of the discharge channels 3 are dimensioned according to the estimated watering time of the basin and the desired lateral velocity of flow.

The percolation into the ground is slow before a sufficient head of water develops, so that the water surface starts to rise. The influent flows through the filling material 1, 6 twice. The aeration effect of storm water in the porous substance is reached also in the upflow direction.

This contributes to the water purification in the ground.

The filter cloth 12 on the top of the storage filling media prevents surface materials from being disturbed during the maximal water level situation. The system cannot be surcharged, because the weight of surface materials prevents the structures from moving. The method allows wide variability in hydraulic loading.

The method for storm water infiltrationpercolation based on the underground reverse inflow discharge is a competitive solution to urban drainage problems because of its simple structure.

Claims

1. A method for storm water infiltrationpercolation into the ground via underground discharge channels wherein water flowing in a discharge channel(s) is allowed to discharge through a porous invert of the channel into storage filling media of an infiltration-percolation basin mainly as upflow current by using the kinetic energy developed by the elevation difference between the water surface or grade line of the inflow sewer and the ground water table or the bottom of the storage reservoir basin and then the water that is so infiltrated into the storage reservoir basin is allowed to slowly percolate into the ground mainly as downflow current.

2. A method according to claim 1, wherein water discharging through the invert of the channel outside the channel flows sideways and upwards.

3. A method according to claim 1 or 2, wherein the channels is/are mainly at the level of the ground water table.

4. A method according to claim 1, 2 or 3, wherein water flows in the channel(s) under pressure.

5. Apparatus for storm water infiltrationpercolation into the ground including underground channels located in an infiltrationpercolation basin wherein the discharge channel is a tunnel having an invert formed of a filter medium with the cover thereover, from which tunnel water discharges through the invert laterally to the sides of the tunnel.

6. Apparatus according to claim 5 wherein the bottom of the invert of the discharge channel is substantially at the level of the ground water table.

7. Apparatus according to claim 5 or 6, wherein a filter cloth is located over the discharge channel on the top of the storage filling media.

8. A method for storm water infiltrationpercolation substantially as described herein with reference to and as illustrated in the accompanying drawings.

9. Apparatus for storm water infiltrationpercolation substantially as described herein with reference to and as illustrated in the accompanying drawings.