Pre-Development and Post-Development Stormwater Analysis

Development can have a significant impact on stormwater drainage in an area. The quantity of land available for water to infiltrate into the soil is decreased when natural surfaces like bare ground or grass land are converted to impermeable surfaces like highways, buildings, and parking lots. This can lead to higher runoff during rain events. Flooding, erosion, and property and infrastructure damage may result from this.

Additionally, because development alters the hydrological regime also known as amount and speed of water flowing, it may have an impact on downstream flooding. The amount and speed of water flowing into streams and rivers thus rises along due to increased runoff. As a result, flooding may occur downstream of the built region and stream flows may increase.

Downstream flooding can also be largely caused by the decrease in floodplain storage capacity as a result of development. Natural floodplains have the capacity to temporarily store floodwaters, minimising their effects downstream. Development in flood-prone areas, however, may result in the loss of these natural floodplain storage areas, reducing their capacity to lessen the effects of downstream flooding.

Stormwater management techniques like low-impact development, green infrastructure, and various types of stormwater detention ponds can be used to mitigate these effects. Through encouraging infiltration, evapotranspiration, and the natural treatment of stormwater runoff, these measures work to control the volume and velocity of runoff. By limiting direct runoff of stormwater, these strategies can reduce the likelihood of infrastructure and property damage both locally and further downstream. It can also help in prevent flooding, and protect water quality.

Increased development without mitigation measures can cause flooding in the downstream areas as we have witnessed recently during several events in Pakistan.

ESSENTIAL STEPS TO CARRY OUT STORM WATER ANALYSIS

Before carrying out stormwater analysis, it is essential to collect baseline data on the existing conditions in the area. This data can be used to assess the potential impacts of development on stormwater runoff and design appropriate stormwater management strategies.

The following are the steps to carry out stormwater analysis before and after development:

1. Collect existing data: 

Collect existing data on the area such as topography, land use, soil type, vegetation cover, hydrology, and climate.

2. Estimate rainfall: 

Estimate the rainfall characteristics for the area, such as intensity, duration, and frequency. This information can be obtained from local rainfall data or using rainfall models.


3. Develop a runoff model: 

Develop a hydrologic model to estimate the volume and timing of runoff for the existing conditions. This model can be used to identify the areas where runoff is most concentrated and prioritize stormwater management strategies.


4. Assess impacts of development: 

Using the runoff model, assess the impacts of development on stormwater runoff. This can include estimating the increase in runoff volume and velocity, changes in flow paths, and potential downstream impacts.


5. Design stormwater management strategies: 

Design appropriate stormwater management strategies to mitigate the potential impacts of development. This can include green infrastructure, low-impact development, and stormwater detention basins.


6. Model post-development conditions: 

Develop a hydrologic model to estimate the volume and timing of runoff for the post-development conditions using the stormwater management strategies designed in step 5.


7. Assess the effectiveness of the stormwater management strategies: 

Compare the results of the pre-development and post-development hydrologic models to assess the effectiveness of the stormwater management strategies in mitigating the impacts of development.


8. Adjust strategies if necessary: 

If the strategies are not effective, adjust them accordingly and repeat the modeling process until an effective set of strategies is identified.

Overall, a stormwater analysis should be a continuous process, with regular monitoring and assessment of the effectiveness of stormwater management strategies to ensure that they are meeting their intended goals.