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Hydrological study reports are the most important engineering studies that examine the rainfall-runoff relationship of a water basin and form the basis for the design of water structures such as dams, ponds, bridges, or flood protection facilities. In Turkey, these reports must be prepared in accordance with the standards of the State Hydraulic Works (DSİ).

Main Stages of the Report

1. Data Collection and Basin Analysis: The boundaries of the basin where the project area is located are drawn, and physical features such as slope, vegetation, and soil structure are determined.

2. Rainfall and Runoff Analyses: Recurrent flood discharges (Q10, Q50, Q100, Q500) are calculated using extreme rainfall data obtained from the nearest meteorological stations in the region. Appropriate hydrological methods such as Rational, Mockus, or Synthetic Unit Hydrograph are selected.

3. Hydraulic Modeling: How the found discharges will change the water levels (water surface profiles) in the river or stream bed is modeled with software such as HEC-RAS.

4. Evaluation and Reporting: The obtained results are turned into a report in the template and layout specified in the DSİ technical specifications, supported by plans and maps.

Incomplete or incorrectly prepared reports are frequently returned by the institution, extending the project schedule by months. With our professional team, we are by your side to get your reports approved on the first submission.

HEC-RAS software, used in flood risk analysis, can be run as 1-dimensional (1D) or 2-dimensional (2D) depending on the nature of the project and the topographic complexity of the terrain. DSİ and other official institutions now directly mandate 2D analysis in many projects.

Why Do We Need 2-Dimensional (2D) Analysis?

1D analysis assumes that water flows in only one direction (usually along the riverbed). It can be sufficient in steep, narrow, and non-irregular channels. However, when water overflows its bed and spreads to wide plains, urban areas, or complex topography, 1D analysis falls short.

2D analysis, on the other hand, calculates the movements, velocities, and depths of water in both X and Y axes (right, left, backward). In this way:

• How floodwaters will pass between buildings,
• Where water will accumulate at intersections or flat areas,
• The structural damage that flow velocity will cause to structures is determined with much higher precision.

ArchSu Hydraulic Engineering perfectly executes the 1D and 2D hydraulic modeling your project needs by using the latest HEC-RAS versions and high-resolution digital elevation models (DEM).

In highway and railway projects, the sizing of bridges and culverts built at points where the road crosses waterbeds is of critical importance in terms of the safety and cost of the project. An incorrectly sized culvert can cause the road to flood or the structure to collapse completely.

What are the Critical Design Criteria?

The sizing of hydraulic structures in infrastructure projects is based on specific steps:

• Determination of Flood Discharge: Recurrent flood discharges of Q50, Q100, or Q500 are taken as basis according to the class of the road (Highway, State Road, Village Road, etc.).
• Extracting Water Surface Profile: Using past rainfall data and basin characteristics, the maximum water elevation (Qmax) that water will reach while passing through the structure is calculated.
• Determination of Span and Clearance: A free clearance margin must be left above the calculated flood water elevation to allow the passage of debris (tree branches, stones, etc.). According to DSİ standards, this margin should generally be a minimum of 1.0 - 1.5 meters depending on the bridge span.
• Scour Analyses: Bed scour that may occur due to the velocity of water at the bridge piers must be calculated, and the foundation depth must be designed to remain below this scour elevation.

Secure your projects by working with the ArchSu team for the hydraulic sizing and reporting of your bridges and culverts in accordance with DSİ norms.

Extreme weather events and sudden floods experienced in recent years reveal the impact of climate change on engineering designs. Traditional hydrological calculations predict the future based on past meteorological data. However, changing climate conditions bring the reliability of these past data into question.

Is Q100 Now Becoming Q50?

The 100-year recurrence discharge (Q100) statistically represents a large flood with a 1% probability of occurring in any given year. But due to climate change, the frequency of severe rainfall is increasing, and floods that we previously said "happen once in 100 years" are now starting to be seen every 50 or 20 years.

For this reason, while making large-scale infrastructure projects, industrial facilities, and urban plans:

• Not only past data but also climate projections (RCP scenarios) should be examined.
• Design criteria should be determined on the safer side (for example, by basing on Q500 instead of Q100 or adding a 15-20% climate change multiplier to discharge values).
• Nature-based solutions (sponge cities, permeable surfaces) should be integrated with classical concrete channels.

ArchSu protects your projects from future flood risks today with advanced stochastic hydrology and up-to-date scientific approaches for sustainable and climate-resilient infrastructures.