Discussion # 4 (10 July 2009)

Article Reference:
Kristin L Gilroy, Richard H McCuen, Spatio-temporatl effects of low impact development practices, 2009, Journal of hydrology

Introduction:

Many hydrologists are taking up the option of Best Management practices which is majorly coz of their cost effectiveness. As it is known that with the increase of urbanization, impervious surface increases which prevents water from infiltrating. This prevention of infiltration reduces the ground water recharge, lowers the water table, increasing the surface runoff and reducing the base flow during the dry periods. These add to degrading the stream channels and increasing the transport of pollutants. With storm management plans like detention pond, wet pond and infiltration basin which though they reduce the downstream peak discharge, issue of increased runoff into the stream continue which harms the aquatic life in the stream. Author points out as to the dependency of finding out the factors like optimal location, quantity of BMP and land use types needs to be investigated to achieve the optimal effectiveness of the LID’s. According to study carried out by Schneider et.al (2006) cisterns have showed to be more effective for the small storms when compared to large storms. Various studies concluded that infiltration technique is beneficial depending on the soil texture and the rainfall event size. According to Brander et. Al (2004) using numerous infiltration technique is more effective than few conventional practices with equivalent volume. Hood et. Al (2007) carried out the comparative study with LID development and traditional development which showed that the peak discharge, runoff volume and lag time is reduced and especially for smaller storms and shorter durations. The paper covers the study carried out on a spatio-temporal micro watershed which is used to study the effectiveness of BMP with respect to the location and the volume.

Methodology:
Spatial Temporal micro watershed model was developed in Matlab taking into consideration both spatial and temporal characteristics of rainfall and runoff processes of any return period on the lot sized watershed. The study was carried out for 1 yr (2.2 cm) and 2 yr (3.6 cm) events and for 4 watershed layouts (i.e. forested and three varying development patterns – townhome with 35% impervious land, residential with 32.5% impervious cover, and commercial land use with 72% impervious cover). In the study they have considered bio retention and cisterns to deal with the increased peak discharge and runoff volume. Bio retention was used majorly as the infiltration where as cisterns was above surface storage which is used to manage the runoff from the rooftop. From the long term precipitation records in Middle Atlantic region 50% of the storm events are 2 yr or less and of 1 hr duration.

Each lot was assumed to be of 0.1 ha. The model created grids of 20 x 12 cells. For the residential and townhome development, runoff from the roadways is directed onto the grass. Surface runoff of each cell is calculated differently which contained bio retention facility and with cisterns. Cisterns are used only to intercept the rooftop runoff. Outflow from the cistern is drained through the orifice. Once the storage of the cistern is reached, the outflow occurs through the cistern. In the bio retention cells once the depth of the stored water exceeds the storage it overflow where the overflow is equal to total volume of water in storage minus the maximum storage capacity of the facility and this is directed to the down gradient cell.

Results:

Onsite effects on the development
The comparison was made between the type of land pattern (forest, residential, townhome, and commercial) with respect to the two hydrologic metrics i.e. peak discharge and the total volume. It was observed that 1 yr storm was totally infiltrated in the forest pattern i.e. pre development condition. The development patterns have reduced the infiltration potential by increasing the impervious cover.

Effectiveness of cisterns
Effectiveness of cisterns was studied by analyzing the different land patterns with 1 and 2 yr storm events. It was observed that cisterns were more effective with smaller storm, there was about 30% reduction of peak discharge for single family lot for 1 yr storm where as it was only 10% reduction for 2 yr rainfall. If cistern is the only type of BMP in the site then they are ineffective for 2 yr storm, where as they provide some control for smaller events. Volume of cistern should be based on the volume of the runoff at the time of maximum rainfall intensity.

Effectiveness of bio retention location
For the study they have considered the location of bio retention near the cistern overflow, around the outlet of the micro watershed, along the roadway in residential/townhome lot and along the parking lot in commercial lot. There was effective reduction in the runoff volume when bio retention intercepted the roadway runoff and cisterns intercepted the rooftop runoff. But considering the bio retention at the overflow of the cisterns was more effective in reducing the peak discharge, as cisterns storage is reached before the highest storm intensity with bio retention facility there would be storage to control the peak. Location and volume of BMP is dependent on the time of distribution of the rainfall event. Location of bioretention did not make much of a reduction in the total runoff volume as the impervious land cover was greater when compared to the bio retention storage. Whereas peak discharge was decreased with the cisterns storing the runoff and overflow from the cisterns was drained to bio retention facilities.

Effect of storage volume
It was observed that effect of storage volume depends majorly on the land use type. For 1 yr storm event for commercial lot, it was noticed that when the facilities were doubled the peak discharge was reduced by 10% and volume to 30%. Author makes an observation that number of additional facilities is dependent on each watershed which might improve the hydrology.

Varying effects on hydrologic metrics
The two metrics considered to measure the effectiveness of the LID are peak discharge and total runoff volume. Effect of changing the volume of the storage on these two metrics vary as runoff volume can be controlled with the increased storage

Discussion:
I personally found this article interesting as they have tried to check the effectiveness of the LID’s considered, which would be similar to what I am trying to do in my thesis. Check the effectiveness of the RHS and PP in improving the hydrologic condition of the study area. This paper would be helpful when I take up the option of optimizing the location of the LID’s I am considering. For my thesis I would like to measure the two metrics they have mentioned and use Marcio’s HFR metric to analyze the impacts.