Basin Element

The Basin element represents a drainage area that generates runoff from rainfall. It is the fundamental source element in every HydraLink network. Basins support three rainfall-runoff methodologies: Rational Method, Modified Rational Method (MRM), and Unit Hydrograph.

When to Use

• Every HydraLink network starts with one or more Basin elements.
• Rational Method gives quick peak flow estimates for small watersheds, storm sewer design, and inlet sizing.
• Modified Rational Method handles detention pond sizing for small watersheds using a simplified volumetric approach.
• Unit Hydrograph generates full hydrographs for detention routing, complex networks, and watersheds of any size.

Input Parameters

Every basin, whatever its methodology, carries a name, a Drains To selector that sets the downstream element, and a Share across plans option that keeps one basin's inputs identical across plans (see Plans & Scenarios).

Rational Method Parameters

Parameter	Units	Description	Notes
Area	acres	Contributing drainage area	Set from an imported boundary, or entered directly
Runoff Coefficient (C)	dimensionless	Weighted C value (0–1) representing the fraction of rainfall that becomes runoff	Computed from a land-use breakdown (built-in C tables, including several adopted municipal tables, editable and saveable) or entered directly. Entering C directly replaces any land-use breakdown. Should be based on local municipality requirements.
Tc	minutes	Time of concentration	Entered directly (Manual) or computed with the TR-55 Method; see Time of Concentration below
Frequency Factor (Cf)	dimensionless	Frequency/safety adjustment factor	Default 1.0, set per storm in the Storm Configuration dialog. Some jurisdictions require an adjustment factor that varies by storm event.

Modified Rational Method Parameters

All Rational parameters plus:

Parameter	Units	Description	Notes
Detention Method	—	Standard or Regional IDF Table (iSWM / Atlanta Regional Commission)	See MRM methodology for details on each method
Basin Roles	—	Target Flow, Design Area, Pass-Through, Bypass	Enable Use Model Elements to pick the elements filling each role, so the detention calculation follows the design as it evolves; otherwise type the per-storm flows directly
Detention Target (Qa)	cfs	Allowable release rate	Target minus bypass for each storm. When that leaves no usable release (no target selected, or bypass at or above target), type the storm’s release in the expander’s Manual column and it is used as entered
Region	—	Texas county (iSWM) or Georgia city (GSMM)	Required when using the Regional IDF method; provides region-specific a and b rainfall coefficients

Regional IDF mode also asks for the pre-development C and Tc and, per storm, the 3-hour depth P₁₈₀ and the depth at the critical duration P_td. P₁₈₀ can be fetched from Atlas 14 with one click.

MRM Basin Roles

Design Area

Post-development area detained in the pond. Its C and area contribute to the composite detained inflow.

Target

Represents pre-development conditions. Its peak flow defines the allowable release rate for the pond.

Bypass

Flow from the developed site that does not enter the detention pond. This flow reduces the allowable release from the pond, increasing the required detention volume.

Pass-Through

Off-site flow that enters the pond but is not detained. Since this is an existing flow, detention is not needed for it, but the outfall structure must be sized larger to convey pass-through flow in addition to the allowable detained release.

Unit Hydrograph Parameters

Parameter	Units	Description	Notes
Curve Number (CN)	dimensionless	SCS curve number (0–100) representing runoff potential	Computed from soil-cover breakdown or entered directly
Transform Method	—	SCS Unit Hydrograph	Determines the shape of the unit hydrograph. (Clark, Snyder, and ModClark transforms are planned but not currently active.)
Loss Method	—	SCS Curve Number or Green-Ampt	Determines how rainfall losses are computed
PRF	cfs·hr/mi²	SCS peak rate factor	Set per basin; Project Settings supplies the default for new basins

Transform-Specific Parameters

SCS Curvilinear

Uses Tc to derive lag time: tlag = 0.6 × Tc.

The peak rate factor is configurable per basin (PRF row; default 484).

Loss Method Parameters

SCS Curve Number

Uses CN to compute initial abstraction:

λ defaults to 0.20 and is editable per basin as Ia/S Ratio, which sits with CN on the panel.

Green-Ampt

Parameter	Units	Description
Hydraulic Conductivity (K)	in/hr	Saturated hydraulic conductivity of the soil
Wetting Front Suction (ψ)	in	Capillary suction at the wetting front
Initial Moisture Deficit (Δθ)	0–1	Difference between porosity and initial moisture content

Values can be looked up by USDA soil texture class.

Land Use Breakdown & Weighted C

HydraLink provides a customizable land use table where you enter the area and C value for each land cover type. The weighted C is computed as:

In the C lookup dialog, pick a land use from the table and Add Entry with its area, which you type. To give a land use everything the basin still has unassigned, press Fill instead, which adds it carrying the remainder. The unassigned figure is shown beside the buttons, and an area larger than what is left is reduced to it, so the breakdown does not overshoot the basin. When nothing is left to reduce it to, the area you typed is kept as typed and the dialog names the land use holding the area, so you can see what has to give way before the weighting is saved. The built-in tables can be edited and saved as your own.

The breakdown divides the basin’s area; it never changes it. Only a basin with no area yet takes the breakdown’s total as its area. If the entries add up to less than the basin, the dialog and the run’s validation say so, and the unassigned part is treated as having the weighted C. To change a basin’s size, edit its Area. You never have to type a surveyed area to its last digit: type what the unassigned line says is left (0.04 when it reads 0.04, or 5 when it reads 5.00) and the entry takes the exact remainder, and the dialog says so, so the breakdown adds up to the geometry. The line, the grid and this rule all use your display precision for drainage area (Project Settings), so what you see is what typing it gives you; any other figure is kept as typed. When you run, a breakdown short of its basin by more than 2% (and more than 0.01 ac) is warned about; a smaller gap is treated as complete.

When a basin comes from a Civil 3D parcel or an imported polygon, its Area is read from that geometry and cannot be typed; hover over the Area label to see where it comes from. To type an area for an imported basin instead, right-click it and choose Unlink Area from Polygon. A basin linked to Civil 3D parcels changes size only when its parcels do.

Soil-Cover Breakdown & Weighted CN

For the Unit Hydrograph method, enter the area, cover type, treatment, hydrologic condition, and hydrologic soil group (A, B, C, D) for each soil-cover combination. The weighted CN is:

CN values are from TR-55 Tables 2-2a/b/c for Antecedent Runoff Condition II (ARC II). The built-in CN lookup dialog provides the TR-55 tables, which you can edit and save as your own table. Add Entry with a typed area, and Fill for the area still unassigned, work as they do for C above.

Time of Concentration (Tc)

Choose Manual or TR-55 Method on the basin panel; Edit Flow Path… opens the TR-55 flow path calculator. The flow path is divided into segments:

1. Sheet Flow, up to a maximum of 300 ft, using Manning's n for overland flow surfaces.
2. Shallow Concentrated Flow on paved or unpaved surfaces.
3. Channel Flow, using Manning's equation with channel geometry.

Tc = sum of travel times for all segments. See the Time of Concentration methodology page for full details.

Results

Output	Units	Description
Peak Flow	cfs	Maximum runoff rate
Peak Time	h:mm	Time from start of storm to peak flow
Runoff Volume	acre-ft	Total volume of runoff (UH method only)
Hydrograph	cfs vs. time	Full runoff hydrograph (UH method only; Rational produces peak flow only)
MRM Detention Volume	ft³	Required storage volume (MRM method only)
MRM Critical Duration	minutes	Storm duration producing maximum required storage (MRM method only)

Methodology Considerations

• The Rational Method produces a peak flow only (no hydrograph). It is commonly used for storm sewer design, inlet sizing, and small watershed analysis. Its applicability depends on local jurisdictional criteria.
• The Unit Hydrograph method generates a full runoff hydrograph and is needed when routing through ponds, channels, or complex networks.
• The Modified Rational Method provides a required detention storage volume using a simplified volumetric approach. The MRM does not perform hydrograph routing; the engineer must independently design the outfall structure.
• The appropriate methodology for a given project depends on local design criteria, jurisdictional requirements, and the engineer’s professional judgment.
• For developed sites, ensure your C coefficient reflects post-development conditions.
• The SCS peak rate factor (PRF, default 484) is set per basin in the properties panel; Project Settings sets the default applied to new basins. Values other than 484 reshape the unit hydrograph per NEH-630 (see Unit Hydrograph).