The Rapid Pace of Urbanization has Increased the Impact of Severe Rainfall Events in Qatar.
The State of Qatar in the Arabian Gulf is one of the most arid regions in the world. With average annual rainfall of around 80 millimetres per year1, it is not often thought of as being a place where flooding is of great concern. However, due to the rapid pace of urbanization across the country, particularly in the capital city, Doha, rainfall generally and particularly severe rainfall events now have a greater impact on populated areas as a consequence of the associated increase in impervious surfaces (see figures 1 and 2). This now requires designers to consider carefully how to protect infrastructure from flood damage, ensure strategic facilities continue to function, and protect lives. Parsons Brinckerhoff worked with Ashghal, the Public Works Authority of Qatar, on the Local Roads and Drainage Programme (LR&DP) in Qatar to develop better guidance for consultants involved in the design of surface water drainage systems.
Qatar Design Standards
The Qatar Sewerage and Drainage Design Manual (QSDDM) provides guidance on the level of flood protection to be applied and the design standards that are to be used in the design of surface water drainage systems in Qatar. The design approach taken in the QSDDM is to design the drainage system around the maximum capacity of the system to convey stormwater without exceedance flow being generated for a particular design flooding return period. It does not provide guidance on the appropriate design storm return period to be used in the design of the minor systems (i.e., guidance for pipe full flow) as is traditionally used in design practice in the U.K. The choice of which level of service for flood protection is to be used in a drainage design is based on land use, and this is defined in the QSDDM as shown in Table 1.
It is necessary for a design check to be carried out by the designer to ensure that adequate flood protection is provided within a drainage catchment appropriate to the designated land use. The designer must be aware that most catchments are composed of different land uses. For example, parks and playgrounds may be found next to government institutional and other official developments, and the designer has to address this mix of land use by running multiple design iterations to confirm that each area within a catchment receives the appropriate level of flood protection.
Parsons Brinckerhoff’s Role
Parsons Brinckerhoff in its role as the programme management consultant for the Local Roads and Drainage Programme is responsible for ensuring that design standards are correctly followed by the general engineering consultants (GECs) working on the programme. This role involves reviewing current design standards and, where necessary, revising them or writing clarifications to ensure the intent of the standard is correctly understood by the users. Following a review of surface water design submissions received from the GECs by Parsons Brinckerhoff and the Public Works Authority (PWA) in Qatar (Ashghal), it was concluded that an advice document was needed to ensure consistent interpretation of the surface water design standards by designers. Parsons Brinckerhoff undertook this task in collaboration with the PWA.
When interpreted correctly the use of the design manual produces a design that meets the needs of the PWA and provides a satisfactory level of flood protection and service to residents. However, a lack of understanding by some designers of the design approach used in the reference standards from which the design manual was derived, resulted in an inconsistent approach to design. For example, the choice of design storm event for residential areas was a source of confusion due to the lack of definition over the risk factor the client was willing to accept. In residential areas a 1 in 5 year event or the 1 in 10 year event could be used. This had resulted in different design storms being used on
Other parts of the design manual covering exceedance flows, storm durations, and sizing of trench soakaways also caused some confusion amongst designers and it was determined that more explicit guidance was required.
Parsons Brinckerhoff reviewed the current design standards in collaboration with the PWA and prepared an interim advice note (IAN 013) that gave guidance on the use of the current PWA design standards for the design of surface water and ground water drainage systems. Attention was given to known areas of uncertainty within the standard which were presenting difficulty to designers; these areas were:
- Selection of the design storm event;
- Freeboard distance from ground level to hydraulic gradient2 or top water surface;
- Range of storm durations to be used in design;
- Soakaway design; and
- Exceedance flow.
Selection of the Design Storm Event
The first area to be dealt with was the selection of the appropriate design storm. The guidance was updated to reflect the new functional classification of roads and to make clear to the designer the appropriate design storm to be used in residential areas; the design guidance now clearly stated that the appropriate level of service to prevent flooding on the local roadway network in residential catchments is a 1 in 5 year storm event. Previously the designer only had to consider rainfall events up to a 1 in 25 year storm. The opportunity was now taken to formalize guidance given for the level of flood protection provided at highway underpasses, which require a 1 in 50 year rainfall event to be considered by the designer. This level of flood protection, although not previously given in the QSDDM, is consistent with design advice followed by expressway designers in Qatar and the European Standard BS EN 752:2008 - Drain and sewer systems outside buildings. The revised guidance is illustrated in Table 2.
Freeboard Distance From Ground to Hydraulic Gradient
The second area to be addressed was the distance from ground level to the hydraulic gradient. In Qatar there has been a requirement that the designer provide a freeboard between the top of the hydraulic gradient and ground level of 300 millimetres (mm) within the drainage network. This is not an unusual requirement; the Ministry of Public Works in Kuwait has a similar requirement written into its surface water design specification, although it uses a distance of 500mm to ensure that gully outlets are not restricted by surcharging within the drainage network for the design storm event. However, the PWA wished to increase this distance to 1000mm from the 300mm specified in the QSDDM to account for uncertainties in the estimation of catchment areas.
Range of Storm Durations to Be Used in Design
Clarity was provided on the range of storm durations to be used in design and when carrying out the level of service check, which is required to ensure flooding does not occur for a range of storm durations up to 24 hours. The range of durations that designers are recommended to use are 15mins, 30mins, 60mins, 2hrs, 6hrs, 12hrs, and 24hrs.
No explicit guidance was given in the QSDDM on the required emptying time of soakaways such that there is sufficient volume to accept a follow-on or sequential storm event. A specific clause was written into the advice note stating that infiltration systems should discharge from full to half volume within 24 hours.
Within the QSDDM there is a requirement to identify flood plans and routes. This is not well developed within the manual and required further clarification. It should be noted though that this requirement is also not explicitly covered in the reference documents for the QSDDM, either BS EN 752:2008 and Sewers for Adoption (WRc, 2006), which do not provide guidance on the requirements for the storm return period for which exceedance flows and overland flood routes are to be assessed. To assist designers, a requirement was included in the advice note that the impact of a 1 in 50 year return period storm on the catchment was to be examined specifically to confirm that flooding of underpasses does not occur and to identify areas where flooding can be predicted to occur. The designer was specifically directed to identify flood routes based on this storm event.
In preparing this advice document, Parsons Brinckerhoff collaborated with the PWA Drainage Networks Design Department to produce an interim advice note that met their requirements and provided clarity to designers, thus eliminating overdesign and ensuring consistency in design approach across Qatar.
The interim advice note (IAN 013) was accepted by the PWA and now forms part of Qatar’s national design standards. Since its implementation, improvements in the design approach used by designers have resulted in better and more cost effective surface
1Reference the Qatar Integrated Drainage Master Plan
2The hydraulic gradient is defined as a line joining the points of highest elevation of water in a series of vertical open pipes (in this case the vertical open pipes are the manholes) rising from a pipeline in which water flows under pressure, the pipeline is surcharged. If the pipeline is not surcharged and water is flowing freely along the pipeline the hydraulic gradient will be the water surface and is more or less parallel with the line of pipes