Aim
Identify the distribution, management and behaviour of runoff within the study area.
Description
One of the main characteristics of green infrastructure is its role in regulating the urban hydrological cycle. Understanding the water dynamics in the study area allows for the efficient location of green infrastructure in space, including the determination of potential points for hydraulic monitoring and modelling, which allows for better sizing of the elements. Although the hydrological cycle comprises several processes, runoff is defined in this guide as the most relevant and easiest process to understand. However, in some locations without an adequate wastewater collection system, runoff may include both stormwater and wastewater. In addition, the runoff process can occur either on the surface or underground through the sewer system.
Steps
1. Delimitation of the catchment area within the study area
Based on maps or field visits analysing the terrain and drainage infrastructure, delimit the catchment area (runoff basin) including water inlet and outlet points in the area of interest. These are usually located along roadways. The use of a Geographic Information System (GIS) for the digitisation of the information is recommended.
2. Delimitation of micro-watersheds within the study area
An urban micro-watershed can be delineated by the area of stormwater runoff to a culvert. Therefore, the location of the culverts is a first step in its delineation.
To locate the catchment area (hydrological watershed) the urban topography can be followed to understand the direction and origin of runoff. This includes domestic stormwater runoff into the public area. A high degree of detail is obtained by field observations; topographical surveys allow approximation of the boundaries, but the results will depend on the resolution of the topographical plans.
3. Characterisation of an existing drainage system
Plans of the sewerage system can be obtained from the responsible local institutions. However, a more direct way is manual dimensioning. In any case, it is recommended to check the initial design against the existing construction in order to identify possible alterations or adaptations of the system. The sizing should include:
- Location of sewers, manholes, drains.
- Geometry and measurements of diameters, depths, as well as sewer material.
- Type of sewerage: separate or combined.
- Slope of the pipes.
- Topography of the study area.
4. Identify the current management of the drainage system
a. Identifying local dynamics with the drainage system
Interviews (see guide “Gathering information through qualitative interviews with citizens or experts“) and informal conversations with residents make it possible to find out about critical points in the drainage system, i.e. those that generate discomfort or dissatisfaction. The causes of the problem may be various, and the interviews can help to understand them, whether they are natural or social processes. For example, both the blockage and the cleaning of sewers may involve different parties responsible (informal dumping of solid waste), forms of management (residents or municipal workers) and local consequences (flooding, bad smells).
b. System maintenance
Identify the actors responsible for the maintenance of the system, both cleaning and repair. The form of local communication between residents and authorities responsible for events requiring such maintenance should also be defined.
It is also suggested to identify the frequency of maintenance and the main causes of problems.
Field activities include periodic review of the functioning of the system to understand possible recurrent problems and critical areas within the system. The photographs show monitoring and sizing activities of the drainage system by a municipal official and a student.
Results
Geometry, type and structure of the drainage system within a study area as a basis for identifying the potential for the implementation of green infrastructure.
Rules
- In case technical drawings of the drainage system are available, a field verification is necessary to identify possible changes to the system.
- Characterisation of the drainage system may require time-consuming and attention-grabbing work for residents. It is necessary to be clear and concise during communication with the neighbours and to have the approval of the local representatives. Especially during the sizing of the sewerage system, local traffic may be disturbed, so safety measures and necessary permits are required in advance to avoid creating a nuisance.
Scopes and challenges
Access to the underground system must be made with personal safety measures as unexpected unsanitary conditions can be a risk factor. Sizing the system may require the assistance of more than one person. Determining additional features such as slopes and topography requires special equipment and previous experience. In addition, lack of existing information and available human resources limit field activities.
The dimensioning of a drainage system makes it possible to define the local urban water cycle in great detail and to plan construction measures adapted to the existing systems.
References and Additional Information
The document presents an example of the sizing and characterisation of urban spaces as a basis for the location of green infrastructure elements.
Fluhrer Tanja, Fernando Chapa, Jochen Hack, 2021. A Methodology for Assessing the Implementation Potential for Retrofitted and Multifunctional Urban Green Infrastructure in Public Areas of the Global South. Sustainability.