Modeling the Flood Modification Effects of a Mid-Sized Unregulated Wetland

Abstract

In contrast to the extensive literature on hydrologic modeling of agricultural and urban land classes, relatively little attention has been focused on the prediction ofthe storm flow response from wetland ecosystems. If required to predictthe rate of outflow from a vvetland system, the hydrologist or engineer has few numerical tools and little available data to assist in the prediction. This chapter presents the application of a numerical wetland model using synthetic precipitation events. The wetland model consists of a field hydrology model fully coupled to a stream routing model. The field hydrology model incorporates two distinct layers, one representing the surficial hummock terrain common to many wetlands and the other representing the organic layer characteristic of all wetland sites. The hydrology model includes process representations for the quickflow responses associated with overland flows through the surficial hummock terrain and longterm subsurface interflow mechanisms within the wetland organics. Antecedent saturation states dictate the amount of storage available for temporarily storing meteorologic inputs. Under low saturation levels, the stormflow response is controHed by subsurface stonnflow mechanisms. Under high saturation levels, overland flow mechanisms shape the response. Over the past few years, the integration of adjacent urban la11d use has been a focus of discussion. The results of this study illustrate the complexity and variability of the hydrologic response at a wetland site, even to a single precipitation input. McKillop, R.C., N. Kouwen and E. Soulis. 1999 (Mitsch and Gosselink, 1986). Wetlands improve water quality, protect shorelines from erosion, provide a measure of flood control, and offer social and economic benefits. The importance of protecting and preserving these wetland systems is being increasingly recognized by the general public, water resources engineers and regulators. It is estimated that wetlands currently comprise 1,270,000 km 2 or 14% of Canada's land surface (National Wetlands Working Group, 1988). In Ontario, wetlands occupy approximately 33% of the land area or 290,000 km 2 , predominantly within the northern regions ofthe province. In southern Ontario, it has been estimated that over 75% of the wetland sites have been lost since European settlement (Ministries of Natural Resources and Municipal Affairs, 1992). In the past, the dominant cause of wetland loss in southern Ontario has been the reclamation of agricultural land use (Bardecki, 1981).At any wetland site, the hydrologic response is influenced by numerous factors. These include: the size and shape of the wetland, the microtopography of the wetland, the properties of the organic sediments, the interaction between the wetland and the channel network, the nature ofthe water inputs and amount of storage space available yvithin the organic sediments. It is recognized that the storm flow response of wetlands is strongly influenced by seasonal variations in both water inputs and evapotranspiration demand....