Hydrological and Hydraulic Assessment of Source Control Sustainable Drainage Systems and Regional Attenuation in the Hartbeespruit Catchment, South Africa

Introduction:

Urban densification in the Hartbeespruit catchment, City of Tshwane, has increased impervious surfaces, increasing stormwater runoff, overloading drainage infrastructure, and elevating flood risk in both the stormwater network and receiving watercourse. While Sustainable Drainage Systems (SuDS) and regional attenuation are recognised stormwater management strategies, quantitative comparisons of their hydrological and hydraulic performance in retrofitted South African urban catchments remain limited. This study compares distributed source control SuDS and regional attenuation for improving infrastructure capacity and reducing flood risk.

Methods:

The catchment was modelled using EPA SWMM (hydrology and stormwater network hydraulics) and HEC-RAS (watercourse hydraulics). Inputs were derived using the Rational Method and design rainfall from the RLMA&SI regional frequency analysis procedure per the Tshwane SIA Guidelines (2016). SWMM was parameterised by adjusting catchment imperviousness until simulated design flows matched Rational Method estimates, verified against the guideline methodology. Twenty-four-hour design storms (2-, 20-, 50- and 100-year return periods) were analysed for a baseline (existing network) and two intervention scenarios: source control SuDS and regional attenuation. Performance metrics included peak runoff reduction, surcharged pipe length, maximum water surface elevation (WSE), and flow velocity.

Results:

Source control SuDS reduced peak runoff by 21.0%, 6.9%, 11.0% and 6.9% (2-, 20-, 50-, 100-year), versus 6.0%, 14.8%, 17.0% and 15.3% for regional attenuation. Source control reduced surcharged pipe length by 65.2%, 32.6%, 21.2% and 15.6%, while regional attenuation gave negligible improvement. In the watercourse, source control reduced 100-year maximum WSE by only 0.014 m and velocity by 2.9%; regional attenuation achieved larger reductions in bigger storms: 0.054 m WSE and 4.9% velocity reduction (100-year).

Conclusions:

Source control SuDS best reduces runoff and network pressure during frequent storms; regional attenuation better mitigates watercourse hydraulic impacts during larger storms. The approaches are complementary, supporting integrated strategies combining distributed source control with strategically located regional attenuation for resilient, densifying urban catchments.