Hydraulic Efficiency in Pond Design

Doctoral thesis, 1999

Today there is a growing interest in constructed ponds to treat agricultural runoff, sewage water and stormwater. But ponds not only remove nitrogen, phosphorus and pollutants such as heavy metals; they can also have ecological, social and esthetic values. Many researchers claim that hydraulic design of surface flow wetland (SFW) systems and ponds cannot be overemphasized, and that short-circuiting is one of the greatest hindrances to successful design. The research questions asked in this thesis are: (i) Is a two-dimensional vertically integrated model able to describe the hydraulic performance of ponds? (ii) How does the design affect hydraulic performance? (iii) What is hydraulic efficiency and how should it be measured? (iv) Which factors determine pond design? By raising these questions the thesis follows a tradition of research on the hydraulic processes in ponds and surface flow wetlands, and on how design can be improved. The results show that two-dimensional vertically integrated numerical models can, although not without restrictions and experience, be used to simulate hydraulic performance in ponds. Results from simulations of hypothetical ponds indicate, for instance, that a subsurface berm or an island placed in front of the inlet improves the hydraulic performance as regarding short-circuiting, effective volume and amount of mixing. In this thesis it is suggested that hydraulic efficiency can be defined as a product of the effective volume ratio and the amount of mixing. In a case study, seven explicit factors were identified that determine stormwater pond design: site-specific factors, financial resources, esthetic values, demand for pollutant removal and detention, ecological values, and social values. There were also indications that preferences based on academic tradition may act as an implicit factor, affecting the design. Particularly in stormwater pond design, it is suggested that landscape architects should focus on the pondís outer space, while the inner space is primarily a task for engineers and biologists. The shape is, however, of common interest.