Storage capacity
The required storage capacity and normative runoff can be set based on the so-called Storage Discharge Frequency relations. These SDF-curves can be used to estimate the required gross storage demand in a district in view of the existing or future stormwater discharge capacity. The figure below shows an example of SDF curves.
Figure 1. Example of SDF curves for different return periods
The first thing you have to agree on is the acceptable return period of exceedance of a runoff event (i.e., a runoff event with a certain discharge or more). The lines in the graph represent the relation between storage capacity and discharge capacity of the water system that is exceeded once every X years. One of these curves is selected.
Next step is to find the discharge capacity of the district. That is the capacity of the pumping station that drains this district to pump out the excess water. This capacity is expressed in inches per day (over the total drainage area of a pumping station). The existing pumping capacity is assessed, or, if reconstruction will take place in the near future, the projected pumping capacity. The selected curve in the graph can be used to find out how much storage capacity (in inches) is to be available given the capacity of the pump. As can be seen from the graph, the larger the discharge capacity the less storage capacity is required to achieve the same probability of exceedance.
This storage capacity (inches) is multiplied by the Project area size to find the storage capacity target value (cubic feet).
In case the project area is draining by gravity the capacity of the discharge drain is used instead of the capacity of the pumping station.
It must be noted that installing a larger pumping station is in general not sufficient as the hydraulic capacity of the stormwater drainage system that leads the runoff towards the pumping station must be increased as well. Thus, any plans made with the AST should be considered approximations until engineers and planners can assess the effectiveness of conveyance to the pump station.
Peak flow reduction; normative runoff
The next issue is to control the peak flow of stormwater runoff to this discharge point. We can estimate the reduction of return time of a specific runoff volume as an estimator for the normative runoff. Setting a target for the normative runoff is by far not as critical as the storage capacity target value assessment. But green infrastructure detains or retains runoff and reduces peak flows so that flow velocities in canals are reduced. Hence this reduces bank erosion, sediment wash-off and sediment transport. Default value for the normative runoff is considered the event with the 2-year return interval. Depending on the erosion sensitivity of the area target values for the normative runoff could then be set events that occur every 5 or 10 years every five or ten years.
Evaporation (Evapotranspiration)
Evaporation means cooling. The more water evaporates, the less energy is available to heat the air and the less hot it gets. In hydrology we use the term evapotranspiration for the combination of (1) evaporation that occurs from surface water and intercepted water – that is water that is stored temporarily on surfaces - and (2) the water that is transpired by plants. So, the greener and bluer surface in our project area, the more water evapotranspires and the more cooling occurs.
Groundwater recharge
Groundwater recharge is relevant in areas sensitive to land subsidence because increased groundwater recharge enhances groundwater levels which in its turn can slow down land subsidence. Next to that, groundwater recharge enhances the availability of water for vegetation during dry periods.
Target values for groundwater recharge depend on the existing and the desired groundwater levels that result from overall groundwater recharge, groundwater abstraction and groundwater drainage next to local subsurface conditions and surface water levels. Setting a target value for groundwater recharge requires a groundwater system analysis using a detailed groundwater model would be required.
Construction costs
If the available total reconstruction budget is known this figure can be used as a ‘target value’ in the sense that this budget should not be exceeded. Construction cost estimates of the AST show considerable uncertainty because these estimates are based on generic unit cost prices and local conditions in the project area are not taken into consideration, if at all known. Moreover, costs often increase when making detailed and final designs due to the fact that extra functionalities are added to the first conceptual designs that are made during our workshops. In order to take this into consideration it is our recommendation to use only 60 – 75 % of the available reconstruction budget as target value in the AST.
Later on, in the planning session attention could be paid to the distribution of construction costs (and benefits) over the stakeholders, being both public authorities and private land and property owners.
Maintenance costs
If the available annual maintenance budget is known this figure can be used as a ‘target value’ in the sense that this budget should not be exceeded. Maintenance cost estimates of the AST show considerable uncertainty, because these estimates are based on generic unit cost prices; moreover, local conditions in the project area are not taken into consideration, if at all known. In order to take this uncertainty into consideration it is our recommendation to use only 60 – 75 % of the available maintenance budget as target value in the AST.
As with a fair distribution of construction costs, attention should be paid to the distribution of maintenance costs over the stakeholders, during the planning of adaptation measures.
