DEPENDENCIAS POSTALES INGLESAS

The advantages of soakaway trenches have been recognised in Doha as a means of overcoming the problem of locally impermeable ground. Because the jointing that increases permeability is sub-vertical, a horizontal soakaway is likely to encounter and

connect otherwise separate zones of higher permeability, thereby facilitating the dispersal of the stormwater. Also for a given stored volume, they have a larger internal surface area for dispersion of the water onto the ground.

They can also be used as a conveyance system and converted to positive drainage systems by future interconnection

The design principles of trench-type soakaways are given in BRE digest 365xxx _{referred to earlier.}

Detailed design will need to take into account site- specific conditions and the requirements of a detailed hydrogeological and geotechnical site investigation are given in Chapter 3 of Volume 1.Several different types of design are accepted in Qatar, eg culverts, plastic modules, and perforated pipes. Choice will be dependent upon local conditions

Infiltration trenches are normally constructed parallel to the edge of a carriageway or other liner feature to be drained. They generally comprise a trench, up to 1m deep and 500mm wide, filled with single sized stone. The stone may be surrounded with suitable fine-textured geotextile.

Normally water is allowed to run off the edge of the carriageway directly into the infiltration trench. The water should not be allowed to flow across open ground before entering the trench as it may pick up soil and sand which, if deposited in the trench, will reduce its effective life.

When constructed adjacent to carriageways, the top layer of stone may be bound together by the application of a bituminous spray, at application rates sufficient to bind the stone but insufficient to seal the top of the trench. This treatment is intended to reduce the risk of the stones being displaced and causing an accident should a vehicle inadvertently veer off the carriageway and onto the infiltration drain. The highway authority should be consulted on the size of single-sized stone that is acceptable adjacent to the carriageway.

1.13

Storage Facilities

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There are several types of storage which can be designed and the nomenclature is often used interchangeably.

Storage facilities can be described as attenuation areas, detention basins, or retention ponds or tanks. All of these have similarities in that their purpose is to attenuate peak flows into the drainage system and store a percentage of flood water for a predetermined time. Ponds used for storage of floodwater in Qatar are commonly called EFA’s. This is a similar concept to detention basins used in other countries. Types of storage area which might be designed for Qatar are: • EFA’s i.e. detention basins (see section 1.10.4),

which will be formed in low lying areas and can be subjected to an acceptable level of surface flooding during rainfall;

• Permanent wetlands/Constructed Wetlands (see section 1.10.4), are retention ponds which permanently contain water either naturally or by design, but accommodate flood peaks by varying water level during rainfall events; • Combined EFA’s with constructed storage

tanks. These may benefit by enhancing the natural amenity of the EFA (normally arising from the original topography of the area) with engineered storage structures to provide a permanent land feature. This method of flood attenuation is preferred by DA as it optimises the use of land in Qatar;

• Storage Tanks (see section 1.13.2 below). There are no set procedures for designing such facilities, but major considerations which must be addressed by designers include the following: • Soil conditions and geology;

• Environmental factors (see section 1.10.4 above, and Volume 1);

• Health and safety; • Land availability; • Required storage volume;

• Detention period. This will typically be taken as 24 hrs for initial sizing, but precise determination of the detention period will depend upon the available reserve in the system and the storm size under consideration, all of which will be determined by a modelling exercise, and agreed with the DA;

• Rates of evaporation;

• Flow controls, hydraulic conditions, inlet and outlet structures;

• Accessibility for operation and maintenance; • Operability as a storage facility in conjunction

with other uses, e.g. how siltation will be dealt with in sports pitches and playgrounds. From the above considerations, it will be apparent that CW’s will not generally be viable in Qatar, as the health and safety requirement for a maximum depth of 1m is less than the rate of evaporation. This means that all such ponds are likely to dry out between the infrequent rainfall events. This leads towards provision of EFA’s as more practical. However, CW’s may be considered appropriate in certain areas, and at present there are two in operation (at Abu Nakla and Messaimeer Lake, both of which are associated with wastewater treatment). They may be included as elements of permanent landscaping, where appropriate measures will be required to control depth and retain water during dry periods. This may involve compartmentalising and use of pumping.

1.13.2

Tanks

Layouts

Tank arrangements fall into two main categories, namely on-line and off-line, of which there are many further sub-classes. Figure 1.13.1 shows schematic layouts.

On-line tanks are storages constructed along the route of the pipe in question, and share the same hydraulic gradient. On-line tanks (with perhaps the exception of emergency storage) always drain flows to the downstream drain by gravity. On-line tanks would normally be preferred to off-line from an operational point of view, but require certain hydraulic conditions to be satisfied in order to present a viable option. All storage tanks, are generally equipped with flow control devices on their outlet to limit peak flows from the tank, unless the flow control is provided by downstream constraints.

Off-line tanks are constructed along a route separated from the main drain, and may return flows to the main drain by gravity or pumping, again depending upon the hydraulic conditions.

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There are many different possible arrangements for such tanks, each design being dependent upon required level of service and local topography. Materials and Construction

Materials for tank construction may be concrete, GRP, plastic or coated steel. In-situ reinforced concrete is the most obvious choice for construction of specific designs, but certain applications will lend themselves to the use of proprietary products, e.g. large diameter pipes, precast concrete box-culverts and modular thin-walled plastic or GRP tanks with mass concrete surrounds. Designs using plastics should ensure adequate resistance to jetting pressures. All underground structures should have adequate resistance against uplift due to groundwater pressures.

On-line Storage

This is the simplest type of arrangement, and should be used wherever possible. Hydraulic conditions will determine the viability. The tank will need to operate within the hydraulic regime of the existing system – on-line tanks of any size will not be practical in very flat drains or culverts, due to the large surface area requirement. On-line tanks become more practical with increased gradient, but on extreme slopes, due consideration will need to be given to the greater pressures developed at the downstream ends, e.g. at pipe joints. In such cases, consideration may be given to the use of backdrops and cascades of tanks. An on-line tank will operate by surcharging as the flow approaches the predetermined pass-forward flow. This flow may be the capacity of the downstream drain or pumping station, as in Case 1, Figure 1.13.1, or a lesser value to prevent downstream flooding, as in Case 2, whereby a flow control is required to limit the pass-forward flow. In both of the above cases, care should be taken to ensure an adequate self-cleansing velocity, to prevent sediment build up. In large diameter tanks with low base-flows, this may be difficult. In such cases, a dry weather flow channel should be provided. It is recommended (Sewerage Detention Tanks – A Design Guide, WRc, 1997xxxi_{) that the}

longitudinal slope of the tank be kept to a minimum of 1:100 in on-line tanks, and that sidewall slopes into the centre channel are a minimum of 1:4. Care should be taken with benching in on-line and off-line tanks - this should be steel trowel finished with granolithic topping to prevent accumulation of solids.

Off-line Storage

Off-line storage with gravity return is shown in Cases 3 to 6, Figure 1.13.1. This would typically be preferred where construction could proceed without the need for over-pumping, or insufficient length is available for on-line storage. The storage may be provided in a single tank, an over-sized pipe/box-culvert or groups of pipes. Care should be given to flow distribution at the upstream end, and the order of preference in filling. As the tank may not be 100% filled on a regular basis, selection of a preferential flow channel will reduce the need for desilting operations.

Operational Issues

Operation and maintenance of such underground structures present particular health and safety issues for access and maintenance. These aspects include:

• Blockage of flow control devices: access needs to be provided to safely enter the structure and for clearance tools and removal of debris. Where a blockage has resulted in water being retained for some time, clearing the blockage suddenly may have an unacceptable impact on downstream facilities, such as pumping stations and outfalls. Designs therefore need to consider facilities for gradual emptying or removal of flows;

• Removal of sediment: access needs to be provided to safely enter the structure and for clearance tools and removal of debris; • Design to optimise removal of sediment: to

minimise time and effort needed inside underground structures. Modifications to the structure of the tank to allow sediment to be removed from ground level; use of low friction coatings to discourage accumulation of sediment; modification of inlet design to increase scour; steepening of benching and installation of dry-weather flow channels to encourage self-cleansing; use of mechanical plant and flushing mechanisms to periodically remove sediments.

A checklist of typical design considerations is included in Table 1.13.1 below. The designer should note that this is an aide memoir, but not exclusive, as each application will have its own issues which require resolution.

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1st _{Edition June 2005 -  Copyright Ashghal} 1. On-line Storage

2. On-line Storage + Flow Control

Storage Tank

Storage Tank

3. Off-line Storage + Gravity Return

Storage Tank 4. Off-line Storage + Screened Overflow + Gravity Return 5. Off-line Storage + Screened overflow + Gravity Return 6. Off-line Storage + Gravity Return + variable flow control

7. Off-line Storage + Pumped Return + screened overflow Storage Tank Flow Control Storage Tank Flow Control Non-return Valve Overflow & Screen Overflow & Screen Flow Control Storage Tank Outfall Outfall Outfall Outfall Overflow & Screen Overflow & Screen Storage Tank Flume Flow Control Flume Pump Figure 1.13.1 Alternative Tank Layouts

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Table 1.13.1 - Storage Tank Design Checklist

Consider maintenance & cleaning operations

Consider the erection/removal of falsework in confined spaces during construction (use false soffits or pre-cast slabs for roof sections)

Design benching to be self-cleansing

Ensure sufficient access of adequate size are incorporated (NB can plant be removed once constructed) Consider type of covers (think about manual handling, and security of access)

Incorporate a sufficient number of davit sockets What telemetry is required?

On-line or Off-line tank? Are welfare facilities required?

Is a gravity discharge achievable? Otherwise pumps will be required. Is a power supply needed?

Is a water supply needed for washing down?

Planning permission is required for all control kiosks and permanent accesses to the site

Is a standby generator required?

DA and RA Discharge consents for emergency overflow What is required in the way of control kiosks/buildings Ensure that access for a tanker is possible

Place screens on inlet to tanks on off-line tanks Consider the type of screen required

Design out any possible maintenance hazards Ensure adequate ventilation is achieved Is odour control required?

Consider retention times of the tank

How long does it take to empty the tank? Consider follow on storm events

Provide a facility for overpumping of the tank

Are overflows required?

Provide penstocks on the tank inlets/outlets to enable flows to be diverted or isolated What return period is tank designed to (1 in x year)?

Provide a penstock protected bypass pipe

Is a flow control required on the tank outlet/bypass pipe? Reinstatement of area, consider future access requirements Does the site need to be purchased?

HARAS complete? EIA complete ?

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1.14

Groundwater

Control

1.14.1

Groundwater Levels

It is generally desirable to achieve certain critical groundwater levels to ensure successful operation of urban infrastructure. As a general guide, the following are recommended.

Table 1.14.1 – Guideline Depths of Infrastructure and Minimum GW Levels

Facility Depth (MBGL) Minimum Depth to ground water Septic tanks and

soakaways Formation level 3.0-4.0 0.5 below formation level Telephone cables and chambers 0.4-1.5 2.0 Power cables and chambers 0.4-1.5 2.0 Potable water system; pipes and chambers 0.9 1.4 TSE system, pipes and chambers 1.0 1.5 Roads; formation level of base course 0.3-1.0 0.5 below formation level Buildings foundation level 1.0-1.5 0.5 below formation level Buildings basements 4.0-4.5 0.5 below formation level