Storm water runoff is rainwater that drains from a site rather than infiltrating into the ground. Study of the existing storm water run-off pattern is an important component of site analysis since it affects optimum locations for intended functions.
Retention of existing drainage patterns boosts the sustainability of a project by minimizing the need for additional infrastructure and reducing the extent of grading required. Storm water run-off is also closely linked to increasing occurrences of urban flooding and hence several development controls now require proposals to maintain run-off discharges at or below pre-development stage. Adopting sustainable storm water management practices is a major component of urban water protection efforts.
Understanding storm water drainage patterns
The study of storm water run-off patterns involves calculations and mathematical estimates as well as a consideration of the existing site attributes which influence it. It is important to understand that the site attributes which affect storm water run-off are a part of larger level systems and should not be viewed in isolation.
A comprehensive understanding of the greater watershed and associated hydrological features forms the background of any site specific study. Adjoining areas may drain into the site either directly or through seasonal channels. The various determinants of storm water drainage are themselves closely interrelated.
Storm water run-off may be qualified in terms of volume, peak discharge and quality.
Storm water run-off: A component of the hydrological cycle
Storm water run-off is a part of the hydrological cycle, the other basic components of which are precipitation, evaporation, condensation, infiltration and abstraction. Precipitation, mainly in the form of rain generates run-off.
Rainfall is a regional phenomenon which shows variations over time. It may be categorized in terms of intensity as light, moderate, heavy and violent. Rainfall estimates are usually the start point in mathematical calculations of storm water drainage.
Geology and the water table
The penetration of water from the surface into the ground is called infiltration. Site geology affects the infiltration performance of the site and consequently the storm water run-off. If an impermeable layer of rock lies close to the surface, saturation occurs much quicker and flooding is observed sooner. Similarly the presence of a high water table offers reduced infiltration.
Aquifer recharge areas allow water to move down to the water table and replenish groundwater reserves. Unconfined aquifers are not covered by a layer of impermeable rock and are open to receive water from the land surface. Such areas are important and should be vegetated to facilitate filtration and recharge.
Soil structure and infiltration capacity
The volume of storm water run-off is inversely related to the infiltration capacity of the soil. Sandy soils allow quicker infiltration due to their texture while clayey soils have the poorest infiltration performance. Urban soils have reduced infiltration since the porous structure of the soil is often destroyed by compaction.
The moisture retained by soils after rainfall is called antecedent soil moisture. The antecedent soil moisture level also affects infiltration since it causes the soil to be saturated at a different rate at the subsequent occurrence of rainfall.
Topography
The topography of the site describes the shape and features of the site surface. The average slope of an area affects the volume and velocity of run-off generated. Shallow slopes allow rainwater to drain slowly, allowing greater infiltration than steep slopes which are also more prone to erosion. Low, lying flat sites have impaired drainage and often show flooding. Topographic features determine the existing storm water drainage pattern which if duly considered in design can bring about large savings in infrastructure costs.
Surface cover and rainfall interception
The surface cover offers the preliminary interception to rainfall. This is described numerically as the coefficient of run-off. The higher the co-efficient of run-off of a surface, the faster is the flow of water.
Impervious surfaces increase run-off both in terms of volume and peak discharge. Increase in impervious surfaces and deforestation reduce the time lag between the peak of rainfall and peak of drainage channels.
This increases the occurrence of floods. The adverse effects of impervious surfaces are mitigated to some extent when such surfaces are disconnected from the main storm water drain. Intermediate patches of pervious cover allow some infiltration and delay to the run-off. The quality of storm water running off impervious surfaces is also affected since the run-off picks up pollutants from such surfaces.
Generally complex surface covers like layered plant communities offer the best interception, since rainwater is intercepted by tree canopies and foliage and gets partially evaporated rather than falling to the ground. The vegetative cover of a site is also an indication of the soil texture and infiltration capacity.
Storm water run-off as a valued resource
While a well drained site is a fundamental requisite of any project, storm water is increasingly being looked at as a resource to be tapped for reuse and recharging ground reserves rather than being siphoned away from the site. Understanding the site determinants of run-off helps a designer to intervene in a manner which is responsive to this potential.
Sources:
- Harris Charles W, Dines Nicholas T, Time Saver Standards for Landscape Architecture, Second edition, McGraw-Hill, 2002.
- Russ Thomas H, Site Planning and Design Handbook, Mc-Graw Hill, 2002.
- Sassi Paola, Strategies for Sustainable Architecture, Taylor and Francis, 2006.
Anamika Mishra - Anamika holds a Master's degree in Urban Design with extensive experience in architectural and planning projects.
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