Hydraulic and hydrologic stormwater modeling in the early stages of city planing: Case Study with investigation of future changes

Sammanfattning: The recent development of modeling tools on the market has opened up for water and wastewater
related applications in many respects. Stormwater modeling in urban planning process is deemed to
provide important information that can form the basis for decision making in the early stages of
urban plans.

The purpose of this study is to examine the effects of future changes on the hydraulic capacity of an
urban stormwater system. Future changes referred to are: (1) Climate change - such as increased
rainfall intensity, (2) Urbanization - the increase in the proportion of imperviousness surfaces, and (3)
Change in pipe network conditions – reduced hydraulic capacity.

The goal is to make suggestions about how this type of information / survey can be used early in the
planning and thereby increase the ability to create and maintain a sustainable urban environment.
Through a literature review identified five areas of application of storm water modeling in the urban
planning process. As part of this work applied two of these focus on applications in dense areas. A
stormwater model is constructed in the modeling program MIKE FLOOD (1D/2D) for a catchment
with a total area of 10 hectares and with an existing storm water system with a total of 71 manholes.
The description of the pipe model based of physical data where contributing areas to the gridchecked
with high-resolution elevation data. Surface runoff above ground is described with a
resolution of 2 x 2 meters. The model is verified against the control measurements of physical data in
the field but also by a visual inspection of the pipe system conditioning and instrumental surfaces.

A model description of the current drainage situation is the basis for a scenario analysis with a total
of eight simulations. The impact of relative changes in the basin, in the form of urbanization, climate
change and condition in the pipe system is analyzed through the individual and combined effects.

Urbanization is described in two steps, today's hard surfaces and establishment of a fictitious area
with urbanization rate of 80%. Climate change is described by applying a climate factor of 20% on a
design block rain event. Besides this is also tested runoff situation at a more extreme rainfall event,
by simulation of a CDS rainfall with return period 100 years (according to today's climate). The pipe
network condition is described in three steps by changing the flow conditions of the pipe network.
First, by increasing the roughness based on existing conditions but also by changing the geometry in
the network by reducing the cross sectional area. The result shows that urbanization causes greater
hydraulic impact on the stormwater system in relation to climate related impacts and the pipe
network conditions.

Though this work focuses on city planning in combination of future changes in stormwater systems
the study's conclusions are addressed to two of the stakeholders in planning process, the city
architect and stormwater engineer.

From a technical perspective the stormwater system conditioning in some cases may exceed the
effects of climate change. Capacity related actions in the pipe system and renewal leads to positive
effects on the runoff situation in the urban city environment. In this manner, margins are created to
enhance the safety dissipation as the catchment develops and also due to climate change.

From a city planning perspective it concludes that urbanization and climate change increase the
runoff situation in urban areas regardless of the storm water network capacity and condition. As an
aid in city planning, stormwater modeling tools can describe and provide visualization result of