**4. How do wetlands provide flood control?**

Severe flood events can be distinguished as a natural disaster as its effect includes damages to properties and agricultural lands and in some cases loss of life. Flooding may be caused by dam failures, snow melts and when a large amount of rainfall occurs, and the natural waterways do not have enough capacity to convey excess water and result in overland flow. Overland flow can result from two hydrological processes: the first process is through a big storm event, where the rainfall intensity is large and it cannot infiltrate into the soil and the other process is when the soil is oversaturated - where there is no more capacity for the soil to hold extra rainwater [5]. Rain that falls onto the surface can either go through the evaporation process, get infiltrated into the soil, run along the impervious surfaces or get captured in hollows surface of the ground or wetlands. In the last 150 years, we have lost almost 70% of the capacity of the soil to hold water due to developments being built and more impervious areas installed [24]. This capacity needs to be restored to reduce the amount of overland flows and the risk of properties and people getting inundated. To minimize the risk of flooding and to protect assets and properties downstream, engineers have manipulated the use of constructed wetlands for flood mitigation and control.

In the past, engineers used structural restraints such as levees, which is not always the optimal solution to prevent flooding. Unfortunately, levees tend to hold up water which can significantly increase the level of the river stage and increase the flood velocity. There are a few flood cases where flood water would surge and overtops levees like the flood event in 1993 along the Mississippi River and Missouri River [24]. The flooding in mid-July 1993 from the Mississippi River and Missouri River exceeded the 100-year average recurrence interval (ARI) and had caused major properties damage cost of between US\$ 12 billion–US\$ 16 billion and approximately 32 losses of life [25]. As more areas are being inundated by overland flow and more properties are damaged, engineers have re-evaluated the situation and have considered other alternatives for flood mitigation plans and developing effective floodplain management programs. The aftermath of the great flooding of the Mississippi and Missouri River, wetlands have been considered as part of the flood management and flood mitigation process [26]. Wetlands have been installed and restored in many areas by the US federal government and the US Army Corps of Engineers in the United States and flooding at the downstream of the rivers was reported to have been reduced [27].

Constructed wetlands and the restoration of wetlands have come into wide practice as they have the potential to act as an effective water treatment basin as well as providing essential flood control. Wetland can alter flooding in many ways such as reducing the peak flood water level, the timing of flood water can be delayed, or the flows can be reduced by providing flood storage [5]. The location of wetlands is important for implementing flood protection. Wetlands that are located at an upstream location, the wetland will mainly be affected by headwater from rainfall, whereas wetlands that are located at the downstream location will mostly be affected by river flow. Constructed wetland at the downstream locations is often dry before a storm event therefore it has the potential to store more water during a storm event [5].

The sizing of wetlands is also equally important to the location of wetlands where continuous hydrologic modeling must be undertaken to simulate wetland storage

### *Understanding the Role of Constructed Wetlands in Stormwater Management DOI: http://dx.doi.org/10.5772/intechopen.102912*

during major storm events. Moreover, the time of concentration that a large flood needs to reach the wetlands must be taken into consideration. With the right computed concentration time, it can help reduce flooding at the downstream location just before the flood peaks [26]. Like a retarding basin, flood water that is stored in wetlands helps to delay the peak time of downstream hydrograph by releasing its water slowly and in a controlled manner. According to [26], the drainage of wetlands can influence flood levels where the storage of water attenuates and potentially delay downstream flood peaks.

Constructed wetlands are designed to replicate natural wetlands such as meadows, saltwater marshes, forested wetlands, and bogs. Aquatic or wetlands plants are used in constructed wetlands not only to reduce the amount of pollutants for stormwater quality but also to offer an ecological habitat to a wide range of wildlife species. Moreover, wetlands plants can create great landscape features and recreational amenities for the community [28]. Wetlands not only act as a flood storage but also have the capability to reduce the velocity of flood water with the influence of wetlands plants or vegetation. According to [29], velocities of flood water through wetlands are usually delayed by a friction factor which is influenced either by the slope, depth and/ or the vegetation type (density and height of plants). This friction factor also known as the Manning's roughness coefficient developed by Chow in 1959 is widely applied by engineers to calculate the resistance of flow in open channels.

Velocity rate can be altered depending on the surface roughness where a higher Manning's value can significantly reduce the velocity of water. This can be seen in Manning's equation where velocity is respectively proportional to the roughness coefficient [29]. Wetland's riparian or vegetation have higher Manning's value ("*n*") in comparison with "*n*" value of a concrete path (Vegetation, *n* = 0.06 and concrete path, *n* = 0.02). Therefore, the vegetation of wetlands can influence flows in which the roughness of wetlands channel bed can potentially reduce the velocity of flood water and reduce the peak flow discharge [5]. According to [30], with the presence of wetland plants, the velocity profile is uniform in the vertical direction. This is illustrated in **Figure 4**. The velocity of flows through wetlands may be reduced significantly depending on the vegetation density and the height of the wetland plants increase friction and drag by the vegetative stems [30].

#### **Figure 4.** *Schematic elevation view of a wetland emergent plant (adapted from [30]).*

The importance of wetlands for flood mitigation is now recognized and understood widely. Ogawa and Male [31] performed a hydraulic simulation on evaluating the flood mitigation potential of wetlands for the "Charles River, Neponset River and Ten Mile River in Massachusetts". From the simulation, the results implied that both upstream and downstream wetland locations altered peak flows and reduced flooding. Another study of wetland's role for flood mitigation was done for the Red River Valley major flood in 1997 and the damage cost by the flood that year was US\$ 3.5 billion. The study concluded that a 5% increase in wetland area would significantly reduce flood volume by 5.6% for the 1997 flood event and also reduce the amount of damage cost [32]. Moreover, the restoration of wetlands within the Devils Lake basin of North Dakota could potentially store 72% for the 2 Year ARI storm event and 41% for the 100 Year ARI storm event of total runoff and the US Corps of Engineers calculated that flood damage cost (Approximately US\$ 17 million) can be prevented each year with the use of wetlands for flood mitigation [5]. Wetlands are not only used to treat stormwater but also play an important part in reducing flood peaks and flooding at the downstream locations.
