Site Level Flood Risk Assessment and Integrated Catchment Approach

Site Level Flood Risk Assessment and Integrated Catchment Approach

June 1, 2022

Site level flood risk assessment is the process of understanding flooding sources, identifying mitigation measures, and providing advice for action before and during a flood event. Per the National Disaster Management Authority (NDMA) of the Government of India, due to changing climate and more intense rainfall events, flooding is the most frequent cause of damage to property and life.

The development of international urban areas without planning for high rainfall intensities is one of the major reasons for flooding. Additionally, the lack of maintenance to urban drainage systems is the primary issue  elevating flood risk. Hence, localized flash flooding due to smaller rain events pose even more risk to communities than flooding from nearby rivers.

Developing an understanding of an integrated catchment for commercial, residential, hotel, or airport sites is more practical and appropriate for risk evaluation to ensure the design is resilient to extreme rain events. An integrated catchment approach starts with an understanding of site level drainage and offsite catchment runoff to the site during a rain event.

It is critical to understand the groundwater level and soil characteristics because they are helpful in gauging infiltration opportunities. Furthermore, understanding the overall watershed behavior draining to a property outfall helps identify other external constraints such as backwater potential, topography, impervious cover, vegetation, and other watershed characteristics that influence the rate of storm water runoff. In many cases, the study site drainage functions properly but the offsite runoff is not collected and routed by the catchment drainage system. This leads to flooding on or around the site. 

The time of concentration for drainage system is always shorter than the drainage area for river. Hence the timing of offsite runoff peak is going to be faster than the backwater inundation from river flooding.

During flash flood events, the community or city is prone to flood. As part of design or evaluation process flash flooding potential should be considered, and the sizing of the entire drainage network should be designed accordingly. Additionally, if onsite approaches such as detention ponds or underground storage tanks are used to store the runoff—perhaps for rainwater harvesting—it is important to understand the impact of offsite backwater. The holistic approach helps understand limitations, capacity, and impact on the site and for the drainage system within the catchment.

At the assessment level, items to be addressed include:

  • Local data (rainfall, flooding, inundation issues, etc.)
  • Location of the final stormwater outfall
  • Site topography and topology 
  • Offsite public drainage capacity
  • Groundwater level data
  • Geotechnical information on permeability of the surrounding soils
  • Existing utility infrastructure around the site
  • River floodplain information

Once the appropriate information is available, the analysis can be performed using different hydrologic methods or sophisticated models. Tools such as GIS are used for data analysis, catchment delineation, and mapping. The solutions are not limited to structural defense but also awareness and stakeholder engagement to ensure the site operation and maintenance staff are well equipped and prepared for a flood event. The outcome of a flood risk assessment helps stakeholders ensure the design is resilient for long-term. Involving the stakeholders is critically important during site selection stage.

In the end, this integrated catchment approach also helps guide how excess runoff from the site can be utilized for any onsite water demand.
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Hrushikesh Sandhe, PE LEED AP, is principal and head of the Infrastructure Group in Walter P Moore’s Pune, India office. He can be reached at hsandhe@walterpmoore.com.

Additional information on flood protection is available by downloading An Introduction to Flood Protection: What Owners Need to Know to Protect Their Properties.