Greenfield Runoff & Discharge Limits

A fundamental principle of modern Sustainable Drainage Systems (SuDS) design is that post-development surface water discharge rates must not exceed the pre-development Greenfield runoff rates. This mimics the natural hydrological cycle and prevents exacerbating downstream flood risks.

Calculating Greenfield Runoff

To restrict flow appropriately, the natural greenfield runoff rate for the site must first be calculated. There are two primary methodologies accepted in the UK:

1. IH124 Method (Institute of Hydrology Report 124)

Originally developed for small catchments (< 25 $km^2$), the IH124 method is still widely used for site-level drainage design. It estimates the mean annual flood ($Q_{BAR}$) based on three variables:

$Q_{BAR} = 0.00108 \cdot AREA^{0.89} \cdot SAAR^{1.17} \cdot SOIL^{2.17}$

Where:

• $AREA$ = Catchment Area ($km^2$)
• $SAAR$ = Standard Average Annual Rainfall ($mm$)
• $SOIL$ = Winter Rain Acceptance Potential (WRAP) class index (0.15 to 0.53)

Once $Q_{BAR}$ is found, regional growth curves are applied to calculate peak discharge rates for specific return periods (e.g., 1-year, 30-year, 100-year).

2. FEH Statistical Method

The FEH Statistical method relies on the National River Flow Archive (NRFA) to perform a pooling group analysis. It matches the physical characteristics of your site to a group of similar gauged catchments across the UK to establish a much more statistically robust $Q_{MED}$ (median annual flood).

• Best Practice: The FEH method is generally considered more accurate than IH124 and is preferred by many approving authorities.

Setting Discharge Limits

Once the Greenfield rates are calculated, they dictate the maximum allowable discharge from the site for the equivalent post-development storm.

• Q_1 (1-year return period): Flow is heavily restricted to mimic everyday rainfall events.
• Q_100 (100-year return period): Flow is restricted to the 100-year greenfield rate.

Complex vs. Simple Flow Controls

To meet these varying criteria perfectly, a Complex Flow Control arrangement (e.g., an orifice sized for Q_1, combined with a high-level weir for Q_100) is sometimes required.

However, many authorities accept a Simple Flow Control strategy: using a single vortex flow control (like a Hydro-Brake) sized to limit the 100-year event to the Q_100 greenfield rate. While this may slightly exceed the Q_1 rate during smaller storms, it significantly reduces the attenuation volume required and minimises the risk of blockage associated with very small low-level orifices.

Minimum Discharge Rates

Regardless of the calculated greenfield rate, practically all flow controls have a minimum operable diameter to prevent blockages from debris. In the UK, the standard minimum orifice diameter is $75 mm$.

• If your calculated greenfield rate is so low (e.g., $1.2 L/s$) that it would require a $30 mm$ orifice, you must restrict the flow to whatever a $75 mm$ orifice provides (typically $4 - 5 L/s$ depending on head), to prevent the system from failing due to blockage.