Mahmood Nachabe

Sampling design for water distribution network chlorine decay calibration

This paper provides a novel approach to determining optimal sampling locations for chlorine decay model calibration. Three questions are investigated: (1) What is the minimum number of chlorine sample locations needed? (2) How many combinations of sampling locations are available? (3) What is the optimal location combination? To answer the first two questions, the mathematical expressions of the chlorine concentrations between any two sampling locations are developed and sampling point relationship matrices are generated, then a mixed integer programming (MIP) algorithm is developed. Once obtained, the solutions to the first two questions are used to calculate the chlorine decay wall reaction coefficients and sensitivity matrix of chlorine concentration to wall reaction coefficients; then, sampling location combinations achieved in the second question are sorted using a D-optimality algorithm. The model frame is demonstrated in a case study.

Infiltration in Shallow Water Table Environments: Simple Two-Phase Model Accounting for Air Compression and Counterflow

AbstractA new method is introduced to simulate air compression and counterflow during infiltration into a shallow water table. The method retains the simplicity of Green and Ampt but with one added term, the air pressure resistance below the wetting front. The method differs from existing methods because it couples air compression and counterflow flux dynamically with infiltration. The new method is original and simple, yet it allows the extension of Green and Ampt to simulate infiltration into shallow water environments or a shallow impervious zone where soil-air pressurization is likely to develop. In shallow water table environments, it is shown that neglecting counterflow of air will cause a rapid decline in infiltration due to unrealistic buildup of air pressure below the wetting front. Accounting for air compression and counterflow seemed necessary to bridge between Hortonian (infiltration excess) and Dunne (saturation excess) mechanisms of runoff.

Influence of land use change on hydrology of shallow water table environments

Land development through urbanization plays a significant role in changing the hydrology of a watershed. A two-dimensional finite element variable saturation flow model is coupled with a digital terrain analysis to quantify the impacts of urbanization on the hydrology of shallow water table environments. The ground surface is sealed to infiltration for a fraction of the surface corresponding to urbanized land. A 75% increase in urbanization reduces the cumulative transpiration by 86.4%, cumulative infiltration by 76.4% and base flow by 55.5%. Simulations to assess the influences of slope on local hydrology indicated that landscapes with mild slopes have less base flow than landscapes with steep slopes. Effect of surface slope on infiltration is found to be negligible due to the presence of high porosity sandy soil at this site.