Design Flood Estimation

Open Design Flood Estimation

The Design Flood Estimation Tool (DFET) automates the full peak-flow workflow for any catchment in South Africa — from dropping a pin on the map through automated catchment delineation, parameter extraction, design-rainfall lookup, multi-method peak-flow calculation, and professional deliverables. This umbrella guide covers the nine supported methods, the parameters they use, how to read the results, and when to choose one approach over another.

Getting Started

The Design Flood Estimation Tool (DFET) automates the process of determining peak flood discharges at any location in South Africa. It replaces the traditional workflow of manual catchment delineation, parameter extraction, and spreadsheet calculations with a streamlined, map-based interface.

To get started, you need a Delta Hydro account. Each estimation request costs 50 credits, and new accounts receive 100 credits on sign-up — enough for two full estimations. The engine returns all selected methods and return periods in a single submission, so there is no need to run the tool multiple times for a given outlet.

Before your first request

Make sure you know the approximate location of your outlet point (where water exits the catchment). You can zoom in on the interactive map to identify this point precisely, or enter coordinates directly. Satellite imagery is available as a base layer to help confirm the position relative to roads, bridges, and channels.

Prerequisites

• A Delta Hydro account (sign up free on the main app)
• Available credits (50 per request)
• Knowledge of your outlet point location (latitude/longitude or approximate map position)
• An understanding of the applicable design standard for your project (e.g. SANRAL, SANS, local municipal guidelines) so you can pick the right return periods

How It Works

The DFET follows a simple four-step workflow. From the moment you drop a pin to when you download your results, the entire process is designed for speed and accuracy. Most requests are processed within 15 minutes; you will receive an email notification when your results are ready to download.

1. Select Your Outlet Location

Navigate the interactive map and click to place a pin at your desired outlet point — the location where water exits the catchment. This is typically a river crossing, culvert location, or other point of interest for your engineering design.

The platform will verify that your selected point falls within the South African coverage area. You can zoom in to street level for precision or use the satellite imagery base layer for context. Coordinates can also be typed directly if you already know the latitude and longitude from survey data or a GIS project.

Outlet placement accuracy

The accuracy of your entire calculation depends on the outlet location. A pin placed a few metres away from the actual stream line can cause the delineation engine to trace a different sub-catchment, changing the area by tens of percent. Zoom in and confirm the point sits on the watercourse before submitting.

2. Choose Calculation Methods & Return Periods

Select one or more of the 9 available calculation methods and up to 7 return periods (1:2 through 1:200 year). Running multiple methods enables a comparative analysis that strengthens your design decision-making and gives a defensible range for your final design flood.

Tip

For a comprehensive analysis, select all 9 methods. This gives you the widest comparison and helps identify outliers or methods that may not suit your specific catchment characteristics. There is no per-method surcharge — every method ticked on submission is included in the single 50-credit price.

3. Automated Catchment Delineation & Calculation

Once submitted, the engine automatically delineates the catchment boundary from digital elevation model (DEM) data, extracts the longest watercourse, and derives all required parameters — area, slope, time of concentration, mean annual precipitation, and land cover distribution. These parameters feed into each of your selected calculation methods along with the design rainfall pulled from the nearest reliable gauges.

The engine also classifies the catchment into its drainage region, water management area, SDF basin, and Kovacs flood region so that each regional method uses the correct calibrated coefficients.

4. Review Results & Download Deliverables

When complete, your results are displayed in an interactive dashboard with tabbed sections covering catchment info, land cover, flowpath analysis, design rainfall, and method-specific results. From the same screen you can download your professional PDF report, detailed calculation log, GIS exports (.gpkg), and CAD files (.dxf) — ready to include in your engineering deliverables package.

Catchment Delineation

Automatic catchment delineation is the backbone of the DFET. Using high-resolution Digital Elevation Model (DEM) data, the system determines the drainage area contributing runoff to your selected outlet point. The delineation process identifies the watershed boundary — the ridge line surrounding the catchment — and traces the longest watercourse from the most hydraulically distant point down to the outlet.

This step is critical because the time of concentration and virtually every other hydrological parameter depend on catchment geometry. If the delineated boundary does not match your expectations, the most common cause is an outlet point placed just off the true stream alignment — try nudging the pin onto the visible channel and resubmit.

Extracted Parameters

The following parameters are automatically extracted or derived during delineation:

Parameter	Unit	Description
Catchment Area	km²	Total area of the delineated drainage basin.
Average Slope	%	Mean slope of the catchment surface, derived from the DEM.
Time of Concentration (Tc)	hours	Estimated time for water to travel from the most distant point to the outlet.
Mean Annual Precipitation (MAP)	mm	Long-term average annual rainfall over the catchment, sourced from national rainfall databases.
Watercourse Length	m	Length of the longest watercourse from headwater to outlet.
Watercourse Slope	m/m	Average slope of the longest watercourse.
Land Cover	%	Percentage distribution of land-cover classes within the catchment boundary (e.g. natural vegetation, urban, cultivated, water bodies).
Elevation Range	m	Difference between the highest and lowest points in the catchment.

Elevation

Catchment elevation statistics (minimum, maximum, mean, range) are derived directly from the DEM inside the delineated boundary. These values characterise the topographic setting of the catchment and feed indirectly into every flowpath and slope calculation. For profile-specific elevation handling — sampling, smoothing, and slope derivation from the longitudinal profile — see the dedicated Watercourse Profile guide.

Time of Concentration (Tc)

Time of Concentration (Tc) is the time required for water to travel from the most hydraulically distant point in the catchment to the outlet. It is a critical parameter because it determines the design storm duration — and therefore the rainfall intensity — used in most flood estimation methods.

Tc is calculated as the sum of overland flow time (travel across the land surface to a defined watercourse) and channel flow time (travel along the watercourse to the outlet).

Parameter	Unit	Description
Tc (corrected)	hours	Total time of concentration after applying the Kovacs area-dependent correction factor. This is the value used in design calculations.
Tc (uncorrected)	hours	Raw Tc calculated directly from flowpath geometry and velocity estimates, before any correction.
Tc Overland	hours	Travel time for water flowing over the land surface (hillslope segment) before entering a defined channel. Calculated using surface roughness and slope.
Tc Watercourse	hours	Travel time for water flowing along the main watercourse from headwaters to the outlet.
Correction Factor (τ)	—	Kovacs area-dependent multiplier that adjusts the raw Tc. For small catchments the factor is close to 1.0; for larger catchments it increases to account for the attenuating effect of channel storage.
Flow Type	—	Classification of the dominant flow regime — rural, urban, or mixed. Urban catchments have shorter Tc due to smoother surfaces and piped drainage.

Why Tc matters

A shorter Tc means higher rainfall intensity for the same return period, producing a higher peak flow. This is why urbanisation — which shortens Tc through impervious surfaces and piped drainage — increases flood peaks. Conversely, a longer Tc produces lower intensity and a flatter, more attenuated hydrograph.

For a complete treatment of the empirical Tc formulas (Kirpich, NRCS Lag, FAA, Kerby, SA SCS, FHWA 3-component), see the dedicated Tc Calculator guide.

Flowpath & Slope Methods

The flowpath is the path that water follows from the most remote point in the catchment to the outlet. It consists of an overland (hillslope) segment and a watercourse (channel) segment. The slope of the watercourse profile is a key input for Tc calculation and several flood estimation methods.

Length parameters

Parameter	Unit	Description
Total Length	km	Combined length of the overland and watercourse segments, representing the full distance from the most remote point to the outlet.
Overland Length	km	Distance that water travels over the land surface before it reaches a defined channel. Determined by the overland flow threshold.
Watercourse Length	km	Length of the main (longest) watercourse from its headwater to the outlet point.
Lc	km	Distance measured along the main watercourse from the outlet to the point nearest the catchment centroid. Used in the Unit Hydrograph catchment index calculation.

Slope calculation methods

Multiple slope methods are calculated from the watercourse longitudinal profile. Each captures a different aspect of channel steepness and is used by different flood estimation methods.

Method	Symbol	Description
10–85 Slope	S₁₀₋₈₅	Average slope between the 10% and 85% points along the watercourse profile. Excludes the steep headwater reach and the flat outlet reach, giving a representative mid-stream gradient. Recommended by SANRAL for Tc calculation.
Equal-Area Slope	Sₑq	The slope of a straight line drawn through the profile such that equal areas exist above and below the line. Provides a balanced representation of the overall watercourse gradient.
Average Watercourse Slope	Sₐᵥg	Simple elevation difference divided by watercourse length. Tends to overestimate the effective slope for profiles with steep headwaters.

Flow velocities

Estimated flow velocities along each segment of the flowpath are used to calculate travel times. Overland flow velocity is determined by surface roughness and slope; watercourse velocity is estimated from channel characteristics.

Parameter	Unit	Description
V overland	m/s	Estimated average flow velocity along the overland (hillslope) segment. Depends on surface roughness type and terrain slope.
V watercourse	m/s	Estimated average flow velocity along the watercourse (channel) segment. Depends on channel slope and hydraulic characteristics.

Drainage Regions & Classifications

South Africa is divided into a hierarchical system of drainage regions used to classify catchments and assign regional hydrological parameters. These codes are automatically identified based on your outlet location.

Classification	Description
Primary Region	The broadest division (e.g. A–Z), representing major river basins such as the Limpopo, Vaal, or Orange.
Secondary Region	Subdivisions of primary regions, typically representing major tributaries within a primary basin.
Tertiary Region	Further subdivision used for regional planning and water resource management.
Quaternary Region	The smallest standard drainage unit in South Africa (e.g. A21A). Used extensively in hydrological databases, the WR2012 study, and water resource planning.
Hydrological Zone	A zone classification used in regional flood estimation methods, particularly for assigning veld type parameters in the Unit Hydrograph and DRH methods.
Water Management Area (WMA)	An administrative region defined by the Department of Water and Sanitation for managing water resources.
SDF Basin	One of 29 statistically homogeneous flood regions defined for the Standard Design Flood method. Each basin has its own set of regionally calibrated design rainfall statistics.
Kovacs Region	Flood regions defined by Kovacs (1988) for the Regional Maximum Flood (RMF) method. Each region has an envelope coefficient (K value) representing the upper bound of observed flood magnitudes.

Climate parameters

Parameter	Unit	Description
Mean Annual Precipitation (MAP)	mm	Long-term average of total annual rainfall over the catchment. Sourced from national rainfall databases and spatially interpolated. Used in multiple methods to characterise catchment wetness and as an input to empirical flood formulas.
Mean Annual Runoff (MAR)	mm	Average annual volume of water that flows out of the catchment as streamflow. The difference between MAP and losses (evaporation, infiltration) provides an indication of runoff potential.
Probable Maximum Precipitation (PMP)	mm	The theoretical greatest depth of precipitation that is physically possible over the catchment for a given duration (typically 1-day). Used as an extreme upper bound for dam safety and critical infrastructure design.
Thunder Days (R)	days/yr	Average annual number of days on which thunder is heard at a location. Characterises the convective nature of rainfall and is used in the Rational Method to adjust the runoff coefficient via the return period factor. Regions with more thunder days typically experience more intense short-duration storms.

Design Rainfall

Design rainfall is the expected rainfall depth or intensity for a specific return period and storm duration. It is the primary meteorological input to all flood estimation methods. The DFET automatically extracts design rainfall from national databases, applies areal reduction, and feeds the result into each calculation method.

Design rainfall values are derived from statistical analysis of long-term rainfall records at gauged stations near your catchment. The platform uses Depth-Duration-Frequency (DDF) relationships to determine the rainfall depth for any combination of storm duration and return period.

Rainfall Stations & Databases

Design rainfall is extracted from nearby rainfall gauge stations maintained by the South African Weather Service (SAWS) and the Department of Water and Sanitation (DWS). The key station parameters are:

Parameter	Description
Database	The source rainfall database used (e.g. SAWS daily rainfall, design rainfall tables). Different databases may use different statistical methods to derive DDF relationships.
Method	The regionalisation or interpolation approach used to derive design rainfall at your site from surrounding station data. Common methods include distance-weighted averages and arithmetic means.
Station Weight	Each station is assigned a weight (0–1) based on its distance from the outlet point and record quality. Closer stations with longer records receive higher weights.
Record Length	The number of years of reliable rainfall records at a station. Longer records produce more statistically robust design rainfall estimates, particularly for rare events (1:100+).
MAP at Station	The mean annual precipitation measured at each individual station. Helps assess whether a station is representative of the catchment climate.

Record length matters

For reliable estimation of rare flood events (1:50 year and above), rainfall stations with at least 30–40 years of records are preferred. Short records may under- or over-estimate the true statistical distribution of extreme rainfall.

Areal Reduction Factor (ARF)

Rainfall measured at a single gauge (point rainfall) is always higher than the spatially averaged rainfall over an entire catchment for the same storm event. The Areal Reduction Factor (ARF) converts point rainfall to an equivalent areal (catchment-average) rainfall.

$$P_d \;=\; P_t \cdot \mathrm{ARF}$$

Areal reduction of point rainfall

Where:

• $P_d$ — design (areal) rainfall depth over the catchment (mm)
• $P_t$ — point rainfall depth at a single station (mm)
• $\mathrm{ARF}$ — areal reduction factor (expressed as a decimal or percentage)

The ARF depends on:

• Catchment area — larger catchments have lower ARF values because it is less likely that an entire large area receives uniformly high rainfall.
• Storm duration — shorter storms (high intensity, small spatial extent) produce lower ARFs; longer storms tend to be more spatially uniform, resulting in higher ARFs.
• Return period — rarer events are typically more localised, so ARF decreases for higher return periods.

Typical ARF values

For a 100 km² catchment with a 24-hour storm, the ARF is typically around 0.80–0.90 (80–90%), meaning the average rainfall over the catchment is 80–90% of the point rainfall. For very small catchments (< 5 km²), the ARF approaches 1.0, and point rainfall is essentially equal to areal rainfall.

Calculation Methods

The DFET supports 9 industry-standard hydrological methods. Each method takes a different approach to estimating peak flood discharge, and they may produce varying results depending on catchment characteristics. Running multiple methods provides confidence in your design values.

Method selection guidance

No single method is universally “best.” The appropriate method(s) depend on catchment size, available data, and project requirements. Many engineers use the Rational Method as a baseline and compare against SCS or Unit Hydrograph results. The SDF method is specific to South Africa and is often required by national road authorities.

Decision guide — which method to choose

Use the following rules of thumb alongside your local design standard:

• Catchment < 15 km², urban or well-defined land use → Rational Method (and Alt Rational as a check).
• Catchment 15–500 km², need full hydrograph → SCS Method or Unit Hydrograph.
• Any SANRAL national-road project in South Africa → SDF Method is typically mandated.
• Large rural catchment (> 100 km²) → Unit Hydrograph and DRH Method; compare with empirical methods.
• Dam safety / critical infrastructure upper bound → RMF Calculator for the regional envelope; QRT Calculator for statistically extrapolated peaks.
• Ungauged catchment, quick preliminary check → Index Flood Calculator (CAPA) and MIPI 7.1 for rapid cross-checks.

Rational Method

The most widely used deterministic method for estimating peak discharge from small to medium catchments. The Rational Method calculates peak discharge using the formula:

$$Q \;=\; \frac{C \cdot I \cdot A}{3.6}$$

Rational formula — SI units

Where:

• $Q$ — peak discharge (m³/s)
• $C$ — runoff coefficient (dimensionless, based on land cover and soil type)
• $I$ — rainfall intensity (mm/hr) for a duration equal to the time of concentration
• $A$ — catchment area (km²)

Best suited for catchments up to approximately 15 km². The method assumes uniform rainfall intensity over the entire catchment for the duration of the time of concentration. For the full treatment — runoff coefficients, IDF curves, composite catchments, and worked examples — see the dedicated Rational Method guide.

Alternative Rational Method

An alternative formulation of the Rational Method that uses a different approach to determine the runoff coefficient. The Alternative Rational Method retains the same fundamental equation as the standard Rational Method but derives the runoff coefficient using different empirical relationships. It accounts for return period and catchment wetness conditions more explicitly.

This provides a useful comparison against the standard Rational Method, especially for catchments where the traditional C-factor selection may be uncertain. The Alt Rational results are shown side-by-side with the standard Rational output so you can see the magnitude of the difference at a glance. See the Rational Method guide for the full comparison.

SCS Method

The Soil Conservation Service (SCS) method uses curve numbers and unit hydrograph theory to estimate runoff from rainfall. Originally developed by the US Soil Conservation Service (now NRCS), this method uses a curve number (CN) to characterise the rainfall-runoff relationship based on soil type, land cover, and antecedent moisture conditions.

Key concepts:

• Curve Number (CN) — a dimensionless parameter (0–100) reflecting soil permeability and land cover. Higher CN = more runoff.
• Initial Abstraction — the rainfall intercepted or absorbed before runoff begins.
• SCS Unit Hydrograph — a standardised triangular hydrograph shape used to convert rainfall excess to a flood hydrograph.

Widely applicable and well-suited for medium-sized catchments. Particularly useful when detailed land cover and soil data are available. See the dedicated SCS Method guide for curve number selection, the full rainfall-runoff derivation, and worked examples.

Standard Design Flood (SDF)

A South Africa-specific statistical method based on regional analysis of recorded flood data. The Standard Design Flood method was developed specifically for South Africa by the Department of Transport. It divides the country into 29 homogeneous flood regions (SDF basins), each with statistically derived flood frequency relationships.

The SDF method is often required for national road and bridge design projects in South Africa. It provides regionally calibrated flood estimates that account for the unique hydrological characteristics of each region.

Note

The SDF method is mandated by SANRAL (South African National Roads Agency Limited) for all national road drainage design. If your project involves SANRAL infrastructure, include this method. See the SDF Method guide for the regional coefficients and limits of applicability.

Unit Hydrograph Method

A synthetic unit hydrograph approach (HRU 1/72) for deriving the full flood hydrograph from rainfall excess. The Unit Hydrograph method generates the complete flood hydrograph — not just the peak — by convolving a unit rainfall excess with a synthetic unit hydrograph shape derived from catchment characteristics.

This method is valuable when you need the full hydrograph shape (e.g. for dam design, flood routing, or storage calculations) rather than just the peak discharge. The hydrograph shows how discharge varies over time, including the rising limb, peak, and recession. See the Unit Hydrograph guide for the regional coefficient tables (Ct, Ku) and the catchment index derivation.

Regional Maximum Flood (RMF)

An upper-bound flood estimation based on regional envelope curves of maximum recorded floods. The RMF method provides an upper-bound estimate of the maximum flood that could occur at a site. It uses Kovács regional envelope curves, which plot the maximum recorded unit discharge against catchment area for hydrologically similar regions.

Because the RMF represents the maximum recorded flood, it is not associated with a specific return period. It is used as a safety check — if your design flood exceeds the RMF, it warrants further investigation.

Caution

The RMF is not a design flood — it is an envelope value. Do not use it directly for sizing infrastructure without considering the appropriate return period for your design life and risk tolerance. See the RMF Calculator guide for the full methodology and regional K-value tables.

Design Rainfall Hyetograph (DRH)

Generates temporal rainfall distributions for use in hydrological modelling and flood estimation. The DRH method produces a design rainfall hyetograph — a time-series representation of how design rainfall intensity varies over the storm duration. This temporal distribution is critical for more detailed hydrological modelling.

The hyetograph is combined with a loss model and unit hydrograph (with Muskingum channel routing) to generate the design flood hydrograph. It captures the effect of storm temporal patterns on peak discharge. See the DRH Method guide for the critical-duration derivation and the routing table output.

MIPI 7.1

An empirical method for quick peak flow estimation, particularly suited to smaller catchments. MIPI 7.1 (Midgley & Pitman 1971) is an empirical peak flow estimation method that uses simple relationships between catchment area, rainfall, and discharge. It provides rapid estimates with minimal input parameters.

Best suited for preliminary estimates or smaller catchments where detailed data may not be available. Often used as a quick cross-check against more detailed methods. See the QRT Calculator guide for MIPI and the related regional peak-flow techniques.

CAPA

Catchment Area-based Peak Assessment for rapid flood estimation in ungauged catchments. CAPA is designed for rapid flood estimation where limited hydrological data exists. It relies primarily on catchment area and regional parameters to produce peak flow estimates.

Useful as an additional comparison point, especially for ungauged catchments where other methods may require calibration data that is not available. See the Index Flood Calculator guide for the index-flood formulation and frequency-factor tables used by CAPA.

Runoff Parameters & Coefficients

Each flood estimation method uses a specific set of parameters to translate rainfall into runoff. Understanding these parameters — how they are derived, what they represent, and how they influence the result — is essential for interpreting and validating your flood estimates.

Rational Method Coefficients

The runoff coefficient C in the Rational Method ($Q = C \cdot I \cdot A / 3.6$) is a composite value that represents the fraction of rainfall that becomes surface runoff. In South African practice, C is built from three weighted components based on catchment characteristics:

$$C_1 \;=\; C_s + C_p + C_v$$

Composite rural runoff coefficient

Cs — slope component

Reflects how catchment steepness affects runoff. Steeper terrain allows less time for infiltration, producing more surface runoff. The Cs value is area-weighted across the slope class distribution.

Slope Class	Gradient	Cs Range
Vleis / Pans	~0%	0.01 – 0.03
Flat	0 – 3%	0.03 – 0.08
Hilly	3 – 10%	0.08 – 0.16
Steep	10 – 30%	0.16 – 0.26
Very Steep	> 30%	0.26 – 0.30
Dolomite	Variable	0.01 – 0.03

Cp — soil permeability component

Reflects the infiltration capacity of the soils. More permeable soils (Group A) absorb more rainfall, reducing runoff. Less permeable soils (Group D) produce more surface runoff. The Cp value is area-weighted across the hydrological soil group distribution.

Soil Group	Permeability	Cp Range
Group A	Very high (deep sands, well-drained gravels)	0.03 – 0.04
Group B	Moderate (loamy soils, moderate depth)	0.06 – 0.08
Group C	Low (clay loam, shallow soils)	0.12 – 0.16
Group D	Very low (heavy clay, shallow over rock)	0.18 – 0.26

Cv — vegetation component

Reflects how vegetation cover influences runoff through interception, surface roughness, and root-zone absorption. Dense vegetation significantly reduces runoff; bare ground produces the most.

Vegetation Type	Cv Range
Thick bush / plantation	0.03 – 0.04
Light bush / farmland	0.07 – 0.11
Grasslands	0.17 – 0.21
Cultivated land (contoured)	0.17 – 0.21
Cultivated land (no contours)	0.21 – 0.26
No vegetation / bare	0.26 – 0.28

From C₁ to C₂ — urbanisation adjustment

The combined rural coefficient $C_1 = C_s + C_p + C_v$ represents natural conditions. It is further adjusted to $C_2$ to account for urbanisation (impervious surfaces increase runoff) and the presence of lakes or dams (which attenuate runoff). $C_2$ is the final coefficient used in the Rational formula for each return period.

Note

The runoff coefficient is also adjusted for return period. Higher return periods (rarer storms) produce wetter antecedent conditions, so the coefficient is increased using a frequency factor based on the number of thunder days (R) at the site.

SCS Parameters

The SCS (Soil Conservation Service) method models the rainfall-runoff relationship using a Curve Number approach. These are the key parameters:

Parameter	Symbol	Description
Curve Number	CN	A dimensionless index (0–100) characterising the rainfall-runoff potential of a catchment based on soil type, land cover, and land management. Higher CN values indicate more runoff (less infiltration). CN is area-weighted from the combined soil group and land cover distribution.
Maximum Retention	S	The maximum amount of rainfall the catchment can retain (infiltrate, intercept, or store) before all additional rainfall becomes runoff. Calculated as $S = 25\,400/CN - 254$ mm.
Initial Abstraction	Ia	The rainfall absorbed by the catchment before any surface runoff begins — includes interception by vegetation, depression storage, and initial infiltration. Calculated as $I_a = \lambda \cdot S$.
Lambda	λ	The initial abstraction ratio. Traditionally 0.2 in the original SCS formulation, but South African practice often uses 0.1 for better calibration to local conditions.
Direct Runoff	Q	The depth of rainfall that becomes surface runoff (mm), calculated using the SCS equation $Q = (P - I_a)^2 / (P - I_a + S)$, where $P$ is the design rainfall depth.
Lag Time	L	The SCS lag time (hours) — the time from the centroid of rainfall excess to the peak of the unit hydrograph. Determined from catchment length, slope, and CN.
Storm Duration	D	The effective storm duration used in the SCS unit hydrograph, typically $D = 2L$ (hours).
Time to Peak	Tp	Time from the start of runoff to the peak discharge. $T_p = D/2 + L$ (hours).

Understanding CN values

A CN of 100 means all rainfall becomes runoff (impervious surface). A CN of 30–40 indicates a highly permeable catchment with dense vegetation. Most natural South African catchments have CN values between 55 and 80. Urban areas typically exceed 85.

Unit Hydrograph Parameters

The synthetic Unit Hydrograph method (HRU 1/72) derives a flood hydrograph from catchment geometry and regional coefficients. It is widely used in South Africa for medium to large catchments.

Parameter	Symbol	Description
Catchment Index	Ic	A dimensionless index combining watercourse length, centroid distance, and slope: $I_c = (L \cdot L_c)/\sqrt{S}$. Characterises the shape and steepness of the catchment and directly determines the lag time.
Veld Type Zone	—	South Africa is divided into 9 veld type zones (1–9) based on vegetation and hydrological response characteristics. Each zone has associated regional coefficients (Ct and Ku) calibrated from observed flood data.
Lag Coefficient	Ct	A regional coefficient that, together with the catchment index, determines the lag time: $T_L = C_t \cdot I_c^m$. Varies by veld type zone and represents regional catchment response characteristics.
Unit Peak Coefficient	Ku	A regional coefficient used to calculate the unit peak flow from the lag time. Determines the peak magnitude of the unit hydrograph for a given volume of runoff.
Lag Time	TL	The time (hours) between the centroid of effective rainfall and the peak of the hydrograph. Calculated from $T_L = C_t \cdot I_c^m$, where $m$ is a regional exponent.
Storm Loss Factor	k	The proportion of design rainfall that is lost to infiltration, interception, and depression storage. Effective rainfall = $P \cdot (1 - k)$. Values are interpolated from regional tables based on veld type and return period.
Effective Rainfall	Pe	The portion of design rainfall that becomes direct surface runoff: $P_e = P_d \cdot (1 - k)$. The rainfall excess that drives the flood hydrograph.

DRH Parameters

The Direct Runoff Hydrograph (DRH) method, based on Bauer & Midgley (1974), generates routed flood hydrographs using a critical storm duration and Muskingum channel routing.

Parameter	Symbol	Description
Veld Zone	—	The veld type zone (1–9) determines the regional storm loss coefficients and temporal rainfall distribution used in the DRH method.
Storm Loss Group	—	Catchments are classified into storm loss groups (A, B, or C) based on soil permeability and vegetation. Group A has the lowest losses (most runoff), Group C has the highest losses.
Critical Duration	Dc	The critical storm duration (hours) — typically Tc rounded to the nearest whole hour. The hydrograph is also computed for ½Dc and 2Dc storms to bracket the critical response.
Muskingum K	K	A channel routing parameter (hours) representing the travel time through the reach. Calculated from $K = a \cdot A^{0.318}$, where $A$ is the catchment area and $a$ is a regional coefficient.
Routing Table	—	The DRH produces a detailed routing table showing how inflow is translated to outflow through time using Muskingum routing. Columns include time, cumulative rainfall, incremental effective rainfall, inflow, and routed outflow.

Empirical Method Parameters

Empirical methods use simplified regional relationships to estimate peak flows directly from catchment characteristics, without generating a full hydrograph. They are useful for quick estimates and cross-checks.

MIPI 7.1 (Midgley & Pitman 1971)

Parameter	Description
Veld Type Zone	Regional classification (1–9) determining the catchment coefficient C and frequency factors.
C Parameter	Regional catchment coefficient relating catchment area and rainfall to peak flow. Derived from veld zone tables.
Kt (Frequency Factor)	A multiplier applied to the mean annual flood to obtain the peak flow for a specific return period.

CAPA (Catchment Parameter Method)

Parameter	Description
M (Catchment Index)	A composite index derived from catchment area and mean annual precipitation. Used to estimate the median annual flood (Q50).
Q50 (Median Annual Flood)	The estimated median annual maximum flood (m³/s), representing the 1:2 year event. All other return period flows are scaled from this value.
Kp (Frequency Factor)	A multiplier that scales Q50 to the desired return period: $Q_p = K_p \cdot Q_{50}$.

Regional Maximum Flood (RMF)

Parameter	Description
K Value	The regional envelope coefficient from the Kovacs study. Ranges from approximately 2.8 to 5.8 depending on the flood region. Higher K values indicate regions with historically larger flood magnitudes.
Zone	Either “flood” (high flood potential) or “transition” (moderate). Determines which set of envelope curve coefficients to use.
Envelope Formula	The RMF is calculated as $Q = \text{coefficient} \cdot A^{\text{exponent}}$, where $A$ is the catchment area (km²). The coefficient and exponent are derived from the regional envelope curve of maximum recorded floods.

Caution

The RMF is an envelope value, not a design flood. It represents the maximum flood ever recorded in that region and is not associated with a specific return period. Use it as a safety check — if your design flood exceeds the RMF, the estimate warrants further investigation.

Return Periods

A return period (or recurrence interval) represents the average time between flood events of a given magnitude. A 1:100 year flood has a 1% probability of being equalled or exceeded in any given year — it does not mean it happens exactly once every 100 years.

Return Period	Annual Probability	Typical Use
1:2	50%	Minor drainage, grassed waterways
1:5	20%	Minor stormwater drainage, rural roads
1:10	10%	Urban stormwater drainage, minor culverts
1:20	5%	Major stormwater systems, suburban roads
1:50	2%	Major bridges, arterial roads
1:100	1%	Major infrastructure, flood lines, dams
1:200	0.5%	Critical infrastructure, large dams, safety checks

Choosing the right return period

The appropriate return period depends on the design life, consequence of failure, and applicable standards for your project. Always consult the relevant design guidelines (SANRAL, municipal standards, SANS codes) to determine the required return period for your specific application.

Understanding Your Results

The results dashboard presents your flood estimation data through multiple interactive sections. Each section provides a different perspective on the catchment and its hydrological response.

Peak Flow Comparison

The peak flow comparison table shows the estimated discharge (in m³/s) for each method and return period in a single view. This allows you to identify:

• Consensus values — where multiple methods produce similar estimates, increasing confidence.
• Outliers — methods producing significantly higher or lower values, which may warrant investigation.
• Trends — how peak flows scale with return period for your specific catchment.

Tip

A common approach is to adopt the median or a weighted average of the results from applicable methods, excluding any that are clearly unsuitable for your catchment characteristics. Always document the rationale for your chosen design value.

Hydrographs

For methods that generate full hydrographs (Unit Hydrograph, SCS, DRH), the results include time-discharge curves. These show how discharge changes over the duration of the flood event and are essential for:

• Flood routing through storage facilities
• Dam and detention pond design
• Understanding flood duration and volume
• Downstream impact assessment

The hydrograph volume (the area under the curve) is often just as important as the peak — for instance, when sizing a detention basin, two hydrographs with the same peak but different volumes will require very different storage.

Land Cover Analysis

The land cover section shows the percentage distribution of different land-cover classes within your catchment. This directly influences runoff coefficients, curve numbers, and infiltration parameters used by the calculation methods.

Common classes include natural vegetation, cultivated land, urban/built-up areas, water bodies, and bare ground. Understanding your catchment’s land cover helps validate the calculation parameters and identify potential sensitivity factors — for example, a catchment undergoing rapid urbanisation will produce higher peak flows in the future than the current land cover suggests.

Hydrological Soil Groups

Soils are classified into hydrological groups based on their infiltration rate when thoroughly wetted. This classification directly determines the Cp component of the Rational Method runoff coefficient and is a key input to the SCS Curve Number.

Group	Infiltration	Typical Soils	Runoff Potential
A	Very high	Deep, well-drained sands and gravels	Low — most rainfall infiltrates
A/B	High	Sandy loams, transitional between A and B	Low to moderate
B	Moderate	Moderately deep loamy soils, moderate drainage	Moderate
B/C	Moderate to low	Transitional soils between B and C	Moderate to high
C	Low	Clay loams, shallow soils with clay layers	High — limited infiltration
C/D	Low to very low	Transitional soils between C and D	High to very high
D	Very low	Heavy clay, thin soil over impervious bedrock	Very high — near-total runoff

Slope Classes

The catchment is classified into slope categories based on terrain gradient. The slope distribution directly determines the Cs component of the Rational Method runoff coefficient. Steeper slopes produce faster runoff with less infiltration opportunity, resulting in higher peak flows.

Class	Gradient	Characteristics
Vleis / Pans	~0%	Flat wetland areas and pans. Water ponds and infiltrates or evaporates — very low runoff contribution.
Flat	0 – 3%	Gently undulating terrain. Allows significant infiltration and slow overland flow.
Hilly	3 – 10%	Moderate slopes. Runoff increases noticeably as water flows faster with less infiltration time.
Steep	10 – 30%	Steep terrain producing rapid runoff. High erosion potential and short overland flow times.
Very Steep	> 30%	Mountainous or escarpment terrain. Very rapid runoff with minimal infiltration.
Dolomite	Variable	Karst terrain with sinkholes and underground drainage. Despite surface slope, much rainfall is absorbed through solution cavities — very low surface runoff.

Vegetation Types (Veld Classification)

South African vegetation is classified by veld type for hydrological purposes. The vegetation distribution determines the Cv component of the Rational Method runoff coefficient and influences surface roughness, interception, and infiltration capacity.

Vegetation Type	Runoff Impact	Description
Thick bush / plantation	Very low	Dense canopy intercepts rainfall, deep root systems promote infiltration, thick litter layer absorbs moisture. Produces the least surface runoff.
Light bush / farmland	Low to moderate	Scattered shrubs and managed agricultural land. Moderate interception and infiltration.
Grasslands	Moderate	Open grassveld typical of the Highveld and KZN midlands. Provides some interception but less than bush cover.
Cultivated (contoured)	Moderate to high	Cropland with conservation practices (contour ploughing, terracing). Contours slow overland flow and promote infiltration.
Cultivated (no contours)	High	Cropland without conservation practices. Bare soil between rows promotes rapid overland flow.
No vegetation / bare	Very high	Exposed rock, bare soil, or degraded land. Almost all rainfall becomes surface runoff. Highest Cv values.

Exports & Deliverables

Every completed estimation comes with a full suite of deliverables, ready to integrate into your project. All four formats are generated from the same underlying result set, so the numbers in the PDF, the calculation log, and the GIS/CAD files are guaranteed to match.

PDF Report

A professional, client-ready report containing catchment maps, all extracted parameters, peak flow results, methodology descriptions, and visualisations. Ready to submit as part of your engineering report or attach to project documentation. Each section mirrors what you see in the online dashboard so reviewers can follow the calculation without needing access to the platform.

Calculation Log

A detailed step-by-step breakdown of every calculation for each method and return period. Ideal for appendices, peer review, or auditing. Shows all input parameters, intermediate values, and final results — including the source of each coefficient (tables, interpolation, or direct extraction) so that the calculation is fully reproducible by hand.

GIS Export (.gpkg)

A GeoPackage file containing the delineated catchment boundary and longest watercourse as vector layers with full attribute data. Import directly into QGIS, ArcGIS, or any GIS software for further spatial analysis. Attribute tables include area, slope, Tc, and all derived parameters so you can label or classify features without needing the original report.

CAD Export (.dxf)

An AutoCAD-compatible DXF file containing the catchment boundary and watercourse geometry. Overlay directly onto your Civil 3D site plan or engineering drawing without manual digitising. The layers are pre-named by feature type (catchment, watercourse, outlet) so your standard layer mappings can be applied.

Best Practices

Place the outlet point accurately

The accuracy of your results depends on the outlet location. Zoom in to street level and use satellite imagery to confirm the point is at the correct river crossing, culvert, or point of interest. A pin a few metres off the actual stream can lead the engine to trace a different sub-catchment.

Run all applicable methods

Using multiple methods provides a range of estimates and highlights consensus values. This strengthens your engineering judgement and makes it easier to justify your design values. Because the 50-credit price is per submission (not per method), there is no cost penalty for selecting every method that is applicable to your catchment.

Review catchment parameters before interpreting results

Check that the delineated catchment area, watercourse, and parameters match your expectations. If the catchment area seems too large or small, the outlet point may be slightly misplaced. A quick sanity check against a topographic map or satellite view will catch most outlet-placement errors.

Consider land cover in your assessment

If you know the catchment is undergoing development (e.g. urbanisation), the current land cover may not represent future conditions. Factor this into your design by considering increased runoff coefficients or by manually running the Rational Method on projected land-cover scenarios.

Use the appropriate return period for your design

Always reference the applicable design standard (SANRAL, municipal guidelines, SANS codes) to determine the required return period. Over-designing is expensive; under-designing is risky. For projects with high consequences of failure (e.g. dams, critical bridges), pair the primary design return period with a safety check against a longer return period or the RMF.

Document your method selection rationale

When presenting results, explain why you selected certain methods and how you arrived at your design value. The multi-method comparison from DFET makes this straightforward — include the comparison table in your report and note which methods you weighted most heavily and why.

Other Tool Settings

The DFET uses the MERIT-Hydro DEM for its internal delineation. For other watershed sources — including the ESRI ArcGIS Online snap tolerance used by the stand-alone Watershed Delineator — see that tool’s dedicated guide. The snap-tolerance setting controls how far a pour point can be moved onto the nearest stream line before delineation runs; a larger tolerance helps when your outlet coordinates are approximate, while a smaller tolerance is safer when you know the exact stream-line position.

Glossary

ARF (Areal Reduction Factor) — A factor that converts point rainfall to spatially averaged catchment rainfall. ARF decreases as catchment area increases, reflecting the spatial variability of storm rainfall.

Catchment — The area of land that drains surface water to a common outlet point. Also known as a watershed or drainage basin.

CN (Curve Number) — A dimensionless parameter (0–100) used in the SCS method to characterise the rainfall-runoff relationship of a catchment based on soil type and land cover.

Cs, Cp, Cv — Components of the Rational Method runoff coefficient: Cs (slope), Cp (soil permeability), and Cv (vegetation). The combined coefficient is $C_1 = C_s + C_p + C_v$.

DDF (Depth-Duration-Frequency) — A relationship describing how design rainfall depth varies with storm duration and return period at a specific location.

DEM (Digital Elevation Model) — A raster dataset representing the terrain surface. Used for automated catchment delineation and slope analysis.

DRH (Direct Runoff Hydrograph) — A method for generating flood hydrographs using critical storm duration and Muskingum channel routing.

Hydrograph — A graph showing the variation of discharge (flow rate) over time at a point in a river or drainage system.

Hyetograph — A graph showing the variation of rainfall intensity over time for a design storm event.

Ia (Initial Abstraction) — The depth of rainfall absorbed by a catchment before surface runoff begins. In the SCS method, $I_a = \lambda \cdot S$.

Lc — Distance from the outlet to the catchment centroid measured along the main watercourse. Used in the Unit Hydrograph catchment index (Ic).

MAP (Mean Annual Precipitation) — The long-term average of total annual rainfall over a catchment, typically based on 30+ years of records.

MAR (Mean Annual Runoff) — The average annual volume of streamflow exiting a catchment, expressed as a depth (mm) over the catchment area.

Outlet Point — The point at which water exits a catchment. This is where the flood discharge is calculated.

Peak Discharge (Qp) — The maximum instantaneous flow rate during a flood event, measured in m³/s (cubic metres per second).

PMP (Probable Maximum Precipitation) — The theoretical greatest depth of precipitation physically possible over a catchment for a given duration.

Return Period — The average recurrence interval between flood events of a given magnitude. A 1:100 year flood has a 1% annual exceedance probability.

RMF (Regional Maximum Flood) — An upper-bound flood estimate derived from Kovacs regional envelope curves of maximum recorded floods.

Runoff Coefficient (C) — The ratio of runoff to rainfall for a catchment, used in the Rational Method. Ranges from 0 (no runoff) to 1 (total runoff).

S₁₀₋₈₅ — Average slope of the watercourse between the 10% and 85% points along its profile. The recommended slope method for Tc calculation.

SANRAL — South African National Roads Agency Limited — the body responsible for national road standards and design guidelines.

SDF (Standard Design Flood) — A South Africa-specific method dividing the country into 29 basins with regionally calibrated flood frequency relationships.

Tc (Time of Concentration) — The time required for water to travel from the most hydraulically distant point in the catchment to the outlet.

Thunder Days (R) — Average annual number of days on which thunder occurs. Used to adjust the Rational Method runoff coefficient for return period.

Veld Type Zone — One of 9 regional zones in South Africa classifying vegetation and hydrological response. Used in the Unit Hydrograph, DRH, and MIPI methods.

Frequently Asked Questions

How long does a flood estimation request take? Most requests are processed within 15 minutes. Complex catchments or high-demand periods may take slightly longer. You will receive an email notification when your results are ready.

Can I request estimates outside of South Africa? Currently, the DFET covers 100% of South Africa only. Coverage expansion to neighbouring countries is planned for the future.

What DEM resolution is used for delineation? The platform uses high-resolution DEM data appropriate for hydrological analysis. The exact resolution may vary by region but is sufficient for accurate catchment delineation at engineering scales.

Can I edit the catchment boundary after delineation? Currently, catchment boundaries are generated automatically and cannot be manually edited within the platform. If the delineation does not match your expectations, try adjusting the outlet point slightly.

How many credits does each request cost? Each flood estimation request costs 50 credits, regardless of how many methods or return periods you select. New accounts receive 100 credits on sign-up.

Which method should I use for my project? There is no single “best” method. We recommend running all applicable methods and comparing results. Refer to the Calculation Methods section above for guidance on method selection, and always consult the relevant design standards for your project.

References

• SANRAL. (2013). Drainage Manual (6th ed.). South African National Roads Agency, Pretoria. Chapter 3 — Hydrology; Section 3.3: Rational Method; Section 3.4: SCS Method; Section 3.5: Unit Hydrograph Method; Section 3.6: SDF Method; Section 3.7: Regional Maximum Flood.
• Midgley, D.C., Pitman, W.V. & Middleton, B.J. (1994, updated 2012). Surface Water Resources of South Africa (WR2012). Water Research Commission, Pretoria. The foundational regional dataset for South African hydrology.
• Alexander, W.J.R. (1990, 2002). Flood Hydrology for Southern Africa. South African National Committee on Large Dams (SANCOLD), Pretoria. The definitive reference for the SDF method and South African flood hydrology practice.
• Kovacs, Z. (1988). Regional Maximum Flood Peaks in Southern Africa. Department of Water Affairs, Technical Report TR137. The source of the Kovacs envelope curves and K-value regional classification.
• Bauer, S.W. & Midgley, D.C. (1974). A simple procedure for synthesising direct runoff hydrographs. Report 1/74, Hydrological Research Unit, University of the Witwatersrand. The basis of the DRH method.
• HRU (Hydrological Research Unit). (1972). Design flood determination in South Africa. Report 1/72, University of the Witwatersrand, Johannesburg. The Unit Hydrograph method used in South African practice.
• Midgley, D.C. & Pitman, W.V. (1969). Surface Water Resources of South Africa. Report 2/69, Hydrological Research Unit, University of the Witwatersrand. The MIPI 7.1 empirical method is derived from this series.
• USDA Natural Resources Conservation Service. (1986). Urban Hydrology for Small Watersheds — TR-55 (2nd ed.). United States Department of Agriculture, Washington DC. The SCS Curve Number and Unit Hydrograph reference.
• ASCE. (1992). Design and Construction of Urban Stormwater Management Systems. ASCE Manuals and Reports of Engineering Practice No. 77. American Society of Civil Engineers, New York.
• Chow, V.T., Maidment, D.R. & Mays, L.W. (1988). Applied Hydrology. McGraw-Hill, New York. Chapters 5 (Hydrologic Processes), 7 (Unit Hydrograph), 14 (Design Storms), and 15 (Frequency Analysis).
• US Army Corps of Engineers. (2000). Hydrologic Modeling System HEC-HMS Technical Reference Manual. Hydrologic Engineering Center, Davis CA. Industry reference for unit hydrograph theory and channel routing (including Muskingum).
• US Federal Highway Administration. (2009). Urban Drainage Design Manual — HEC-22 (3rd ed.). FHWA-NHI-10-009. Washington DC. Reference for Rational Method and Tc empirical formulas.
• Mulvaney, T.J. (1851). On the use of self-registering rain and flood gauges. Proceedings of the Institution of Civil Engineers of Ireland, 4, 1 – 8. (Original formulation of the Rational Method.)
• Kuichling, E. (1889). The relation between the rainfall and the discharge of sewers in populous districts. Transactions of the American Society of Civil Engineers, 20, 1 – 56. (Formalisation of the Rational Method for engineering practice.)

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Open Design Flood Estimation