The first question project owners, investors and the Owner's Engineer ask before approving a design is: "Does this land flood, and if so, how high must the buildings be raised?" This article gives an overview of flood studies for construction, from flood types, study steps and the data to prepare, through to the results used in actual design. It suits solar farms, substations, factories, warehouses and large land development projects.
Why does a project need a Flood Study before construction?
A Flood Study is essential because a flood only a few tens of centimeters deep can damage a project's electrical equipment and critical systems. When solar panels, inverter cabinets or transformers are submerged, invisible micro-cracks can form, moisture can enter the system, and all cables and conduits may have to be inspected and replaced.
A Flood Study report is therefore a required submission before design approval on many projects, and it must answer 3 key questions:
- How deep does this area flood in a 100-year heavy rainfall event?
- How high must buildings or equipment be raised to be safe?
- After construction, will runoff leaving the project increase enough to flood the surrounding community?
What types of flooding are there, and what must be considered for each?
Floods assessed in engineering fall into 3 types by source: riverbank overflow, local rainfall flooding and storm surge. Each type behaves differently, so each needs different protection measures.
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| Type | Cause | Characteristics | What to consider |
|---|---|---|---|
| Riverbank overflow (Fluvial Flood) | Water in a canal or river overtops its banks | Flash floods (Flash Flood) arrive fast and with force, while floods from major rivers rise slowly and can persist for 10–20 days or more | Erosion, settlement and seepage through flood embankments |
| Local rainfall flooding (Pluvial Flood) | Prolonged heavy rain that cannot drain away fast enough | Runoff flows over the ground surface and ponds in low-lying areas before reaching waterways or drains | Size and capacity of the drainage system |
| Storm surge (Storm Surge) | A tropical cyclone raises the sea level | Seawater rises and floods coastal areas | Distance from the coast and ground level |
Most inland projects study only the first two types; storm surge is considered when the project is near the coast.
What does a 100-year return period (100-year ARI) mean?
The return period (Average Recurrence Interval, ARI) is the average time between occurrences of rainfall or a flood of that size. A 100-year flood therefore has about a 1% chance of being reached or exceeded in any given year. It does not mean it will happen only once in 100 years, and it can occur two years in a row.
Different parts of a project usually use different return periods:
- 25 years: for designing drains, culverts and the internal drainage system
- 50–100 years: for setting building floor levels, electrical equipment levels and flood embankments
The return periods actually used must follow the requirements of the project owner and the relevant local authorities.
What are the steps of a Flood Study?
A Flood Study has 5 main steps: field data collection, secondary data collection, rainfall analysis, flood modeling, and comparison of pre- and post-development conditions.
1. Field data collection
- Survey the site and the surrounding catchment system
- Ask local residents or site staff about the highest flood level ever recorded, and look for flood marks (Flood Mark) on bridge piers, walls or fences
- Survey discharge points (Discharge Point): their levels, flow directions and locations
- Survey cross-sections of nearby canals or watercourses (Cross-Section Survey) and hydraulic structures such as weirs, culverts and bridges
- Prepare a topographic map (Topographic Survey) of the project site
2. Secondary data collection
- Historical maximum daily rainfall from rain gauge stations of the Royal Irrigation Department or the Thai Meteorological Department, typically from at least 3 stations near the project
- Flood risk maps from the Office of the National Water Resources (ONWR) and historical flood extent data from GISTDA satellite imagery
- Land use and soil maps, to estimate how much rainfall will infiltrate the ground or run off over the surface
3. Rainfall analysis
Before rainfall data is used, it must be checked for consistency and homogeneity, e.g. with a Double Mass Curve. Frequency analysis is then carried out with statistical distributions such as Gumbel, Log-Normal or Log-Pearson Type III, and goodness of fit is checked with the Kolmogorov-Smirnov Test. The result is an IDF (Intensity-Duration-Frequency) curve, which gives rainfall intensity for each duration and return period.
Many projects also add an allowance for climate change (Climate Change Factor) based on the project's design life.
4. Flood modeling
- HEC-HMS from the US Army Corps of Engineers converts rainfall into the catchment's peak flood flow (Peak Flow, Q), commonly paired with the SCS Curve Number (CN) method
- HEC-RAS 2D simulates two-dimensional overland flow to find water depth and velocity across the site
- Rain-on-Grid (RoG) applies rainfall directly onto the terrain model. It suits local rainfall flooding but needs a detailed digital elevation model (DEM) and a Manning's n roughness value suited to shallow flow
5. Comparison of pre- and post-development conditions
The model must be run for at least two cases: pre-development (Pre-development) and post-development (Post-development), after filling, grading and building, to see how the project changes flood levels and outflow volumes for 25-, 50- and 100-year return periods.
What site data does a Flood Study need?
The most important data is accurate, detailed ground level data, because the flood depths to be assessed are often in the range of ten to several tens of centimeters. With coarse level data, such as SRTM satellite data at a resolution of about 30 m, model errors can exceed the very flood depth you are trying to find.
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| Data | Use in the model |
|---|---|
| Topographic map (Topographic Survey) | Build the DEM of the project site |
| Canal or watercourse cross-sections (Cross-Section) | Model riverbank overflow |
| Levels and locations of discharge points, culverts, weirs | Set the model boundary conditions |
| Water levels actually measured on the survey day | Sanity-check the model results |
| Soil and land use data | Estimate CN, infiltration and erosion |
How are Flood Study results used to set building and equipment levels?
The common rule is to set building floors and the underside of equipment above the 100-year flood level plus a freeboard (Freeboard) of 0.50 m. The 0.5 m freeboard comes from the Submission Checklist for Stormwater Management in Malaysia by the Department of Irrigation and Drainage Malaysia (DID), used to check designs under the MSMA manual, while the MSMA 2nd Edition (2012) manual sets the principle that building floors must be above the 100-year flood level. The values actually used depend on the project owner's requirements.
Panel underside / building floor level ≥ FL + 0.50 m FL = 100-year flood level · FGL = finished ground level · d = modeled flood depth · levels referenced to mean sea level (MSL)
🧮 Conceptual example (hypothetical values): if the model gives a 100-year flood depth of 0.30 m above finished ground level, the underside of the solar panels must be at least 0.30 + 0.50 = 0.80 m above the ground.
For solar farms, filling the entire site is usually not worth the cost or time, and it also changes the flow paths onto neighboring land. The more common approach is to use taller posts in the panel area and raise only the pads for buildings, transformers and substations. If a large area must be filled, plan the site layout and earthwork/grading at the same time.
Why control Qpost ≤ Qpre, and how do detention ponds help?
The Qpost ≤ Qpre principle means the peak flood flow leaving the project after development must not exceed that before development, for the same return period (this principle is set in the MSMA 2nd Edition manual). When soil is filled or concrete is paved, the runoff coefficient (Runoff Coefficient, C) rises, so water leaves faster and in greater volume. If it exceeds the original value, a detention pond (Detention Pond) must be built to store water temporarily and release it gradually into public drainage.
For solar farms, research by Cook and McCuen (2013) found that the panels themselves do not significantly increase runoff if the ground beneath them remains grass-covered soil. Detention ponds are therefore usually needed only for filled or paved areas, such as substations and control buildings.
Internal drainage and erosion control: what else must be designed?
Flood Study results are the starting input for drainage system design within the project. Commonly used principles are:
- Keep existing waterways and hydraulic structures as much as possible, so that flow directions do not change
- Roads raised above existing ground block water paths and need roadside drains, culverts or box culverts (Box Culvert)
- Buildings and covered walkways need gutters connected to the roadside drains
- Flow velocity in earth channels must not exceed the velocity that erodes that soil type. Bare earth channels in sand or loam may start eroding at velocities below 1 m/s, while grassed channels generally should not exceed about 2 m/s; above that, the channel should be lined.
For erosion, the USLE (Universal Soil Loss Equation) is used to estimate topsoil loss, and MUSLE to estimate sediment yield from the site. The most economical and effective measure is to establish grass cover quickly after construction and not to compact fill more than necessary, so that water can still infiltrate. Sloping areas should be considered together with landslide prevention.
Summary
A Flood Study tells you how deep the site floods, how high buildings and equipment must be raised, and whether a detention pond is needed. It should be done at the master planning stage, before earthwork and foundations are designed, because fixing problems after construction costs far more. The quality of the study depends directly on field data, especially ground levels, watercourse cross-sections and discharge points.
SPN collects field data for Flood Studies, including topographic surveys, watercourse cross-sections and soil investigation, as well as drainage and civil design. Fees depend on site size and scope of work. Contact us for a free quotation within 24 h.
Figures in this article, such as the 0.50 m freeboard, return periods and channel flow velocities, are illustrative reference values, not design recommendations for any specific project. Design levels for each project must come from a study by an engineer using actual survey data, local rainfall data, and the requirements of the project owner and relevant authorities.
References: Department of Irrigation and Drainage Malaysia, Urban Stormwater Management Manual for Malaysia (MSMA) 2nd Edition, 2012 · Department of Irrigation and Drainage Malaysia, Submission Checklist for Stormwater Management in Malaysia · US Army Corps of Engineers, Hydrologic Engineering Center, HEC-HMS and HEC-RAS · England, J.F. et al., Guidelines for Determining Flood Flow Frequency — Bulletin 17C, USGS, 2019 · Cook, L.M. & McCuen, R.H., "Hydrologic Response of Solar Farms", Journal of Hydrologic Engineering, 2013