Discussion # 1 (12 June 2009)

Article Reference:
Evan Shane Williams and William R. Wise, Hydrologic Impacts of Alternative Approaches to Storm Water Management and Land Development, April 2006, Journal of the American Water Resource Association

Introduction:
The paper discusses of the hydrologic impacts caused by different alternatives of storm management and land development options. The study is carried out based on four alternatives-
1. Traditional land development (full lot size as specified by county rules) with conventional (i.e. pond) storm management system
2. Cluster development i.e. reducing the lot size to half with conventional (i.e. pond) storm management system
3. Traditional land development with LID technique i.e. emphasizing on vertical storage
4. Cluster land development with LID technique i.e. emphasizing on vertical storage

Along with these alternatives they studied how the hydrology is affected with connected (runoff from impervious cover being discharged into the conveyance system directly without any infiltration i.e. implying no vertical storage) and disconnected impervious cover (allowing infiltration i.e. rooftop runoff discharged into lawns and roads draining to swales).

Above mentioned different alternatives were compared with the existing conditions i.e. without the land development.

Results:
Peak flow: Scenarios studied- Existing, without depression storage and with depressions storage. It was observed that two year peak timing change was minimal when compared to 25 yr storm event. LID designs could not meet the peak flow control without the depression storage.
Runoff Volume: Reduction in the runoff volume was observed when the footprint was reduced and using the directly connecting systems. Further reduction of volume was noticed with the usage of infiltration.
The results show that when they ran a continuous model (4 months data) which showed that LID did not perform as expected especially during tropical storms as the soil moisture content is greater than the average condition.

Conclusion:
Infiltration based storm management might perform worse when the soil moisture are above the average value thereby reducing the infiltration or percolation rate. Author mentions about using LID to control the peak upto a certain event beyond which depression storage need to used to control the larger events with this both the quality and quantity concerns are met. This study basically illustrates the impact of using connect and disconnected impervious cover, different types of developments (traditional and clustered type) and usage of conventional pond in addition to infiltration media.

Discussion:
There have been many limitations associated with the study such as using lumped modeling approach, but then this study gives a idea of how the different land developments and different storm management techniques could affect the hydrology of the watershed. This study illustrates how conventional pond system would be required even along with LID (infiltration based) to meet the peak flow control requirement for larger storms.

This paper though doesnot speak about different types of LID’s which could be used, it explains about how the hydrology is differed with different planning methods. In the similar grounds I would like to compare the behavior of the catchment hydrology based on the LID strategies I am going to consider. Through this paper I got the idea of what are all the components of comparison I can make between the scenarios or alternatives I run.