Compare the effectiveness of wetlands vs reservoirs in flood mitigation
1H.M.A.G Herath, 2N. K Gunasekara
ABSTRACT: Nowadays in Sri Lanka, small size wetlands located in urban environment are developed as the useful and profitable area for the community. For the example, wetlands are developed as urban parks, jogging tracks with small reservoir, relaxing area, grounds, commercial developments etc. With this kind of develop-ment flood retention function of the wetland is completely changed or lost. So, small size reservoir is added to the development for implement the flood retention function properly as before. But any size of reservoir cannot provide the same effect for flood mitigation as relevant wetland. So, comparison of the the flood mitigation func-tion of wetland and reservoir separately is essential and important. Then it can be determined the proper dimen-sion for the reservoir and implement the same flood retention function after the developments. In this research, compared the flood mitigation function of wetland and reservoir separately for Rampalawatta reservoir using the HEC-RAS, HEC-HMS and ARC-GIS computer software.
Key words: Flood, Wetlands, Reservoirs
Wetlands are area where ground water table is on the ground surface or near the ground surface. Wetlands are beneficial to the people in many ways. Some of them are, provision of effective protection from flooding, reducing extreme air temperature and making the city more livable, the wetlands soils help to mitigate global climate change, they con-tribute to a city rich in biodiversity and support an endangered animals and plant species. This re-search mainly focusses on the flood retention func-tion of wetlands. Wetlands can keep water for long time at the flood time. Density of grass, woods also reduce the velocity of flow. ergo wetlands can effec-tively reduce downstream flooding.
In contrast reservoir are man-made structures for storage of water for various purposes such as for agricultural purposes, for hydropower generation, for flood mitigation etc.
Hence it also has capability to reduce the floods by storing some portion of water at flooding. Flood mitigation function of the reservoir can be changed by changing the dimension of main components of the reservoir at the time of construction.
This study is based on the Rampalawatta reservoir which was constructed removing a wetland area few years ago. This reservoir is located near the Bat-taramulla-Pannipitiya road at Nagahamulla. The extent of this reservoir is about 7 ha. It Has a 100m long bund and a 6m wide spillway. Thera is a road parallel to the bund and culvert across the s road. Downstream of this lake connects with the Di-yawanna Oya.
Flooding is one of the largest environments haz-ard in Sri Lanka. It causes to the severe damage for human being and other structures. Lot of wetlands is situated in flat area in Sri Lanka. In the wetlands, water particle is come slowly to downstream be-cause of the presence of the trees, infiltration and other losses. But in the reservoir water come quickly to downstream. So, flood level of downstream is high and it occurs quickly. When wetlands are presence, there are many advantages except flood mitigation. It helps to degraded harmful materials. As example when some contaminators come to the wetland, it is degraded naturally with time because wetland has lot of factors cause for degradation. But there is reservoir, contaminators are not degraded naturally without harm to surrounding. therefore, degradation of harmful contaminators is one prob-lem we have to face. As well as increasing the tem-perature near the constructed reservoir is another problem form with constructing reservoirs. Those some problems we have to face with constructing reservoirs
This research will be focused to identify their contribution in flood mitigation separately and compare them. And this is the HEC RAS software-based model analysis on the comparison of flood attenuation at wetlands and reservoirs. It will be modeled the wetland and reservoir in HEC RAS software in separately considering the effected fac-tors and get the output on them separately. And fi-nally, it will be given the comparison between them. This research will help to the government, construc-tors and environmentalists for deep study the situa-tion before do this kind of construction.
Reduce the flood effect in Colombo area and give instructions to the people to how to do their con-struction in the wetlands. And also identifying the challenges which is going to be face in near future
Compare the peak discharge through the wetland and reservoir separately at the flood occasion and identify the risk of flooding downstream from these two cases separately.
3 LITERATURE REVIEW
Wetland is the place where exists permeant or tem-porary accumulation of water with floral and fau-nal communities. In the past, urban wetlands have used as dump sites for urban waste and people considered it as a wasteland. So it was developed to usable area for human community. With the time and increasing knowledge people realized that wet-land is valuable and vulnerable environment on which a variety of plants, animals and human community depend(Karunathilake, 2003).
wetlands help the human being in many ways. Most of the time those are positive impacts to peo-ple. One of the major functions done by wetland is flood retention. Floods are the major natural disas-ter causes huge damaged for people and properties and are most frequent natural disaster in many re-gions. Wetlands and floodplains have considerable potential to reduce flooding. Both of them act as the green infrastructure to mitigate flooding keeping water some time and releasing slowly to down-stream. Among them wetlands are the most effec-tive in reducing small, frequent flood events(Watson et al., 2016).
Also, wetland is excellent at multitasking. It pro-vides various types of other services too. Those are filtering and purifying water, storing carbon, re-plenishing ground water, reducing erosion, provid-ing wildlife habitat and providing recreational op-portunities. Restoration and conservation of wet-land is highly cost effective when compared with other infrastructures for flood mitigation like dry dams. cost of restoration of wetland is between 3.5 dollars to 6.7 dollars per m3 and maintenance cost of dry dam is between the 1.4 dollars and 7 dollars per m3(Flood Mitigation & Basin n.d.).
The report prepared by English nature, the envi-ronment agency the department for environment, food and rural affairs and the forestry commission. 3rd chapter of the report clearly indicates on the wetland creation options for flood mitigation. Those are as follows (English Nature et al., 2003), Crea-tion of wetlands on land previously protected from flooding to a high standard where an acceptable change in land use to a wetland allows the stand-ard of protection to the land to be reduced then ad-ditional floodwater storage can be gained that pro-vides flood management benefit.
Creation of wetlands in existing or new wash lands, flood detention reservoirs and flood bypass channels maintaining a high-water table and/or standing water in such areas reduces the storage available for flood management and will require the storage area to be “over-designed” to compensate for this
Wetlands created alongside river channels by removing or setting back existing defenses or dur-ing widening of channels unless a very large area is involved this will have a neutral impact on storage processes within the floodplain but can increase the capacity of the channel to pass flood flows.
Wetlands created by land management changes
These include measures such as removing/not maintaining under drainage, restoring ditches, re-ducing stock levels, restoring arable land to rough pasture and creating woodlands. Given these con-ditions wetlands will naturally re-establish them-selves where they are sustainable. Land manage-ment changes that increase storage are nearly al-ways beneficial to flood management.
The important function of flood retention reservoir is to decrease the flood peak by retention. This is oc-curred by keeping some portion of water and releas-ing it in controlled manner. Therefore, peak flow is reduced at downstream places to be protected. The flood retention function of reservoir is not necessary to reduce the total volume of runoff but need to re-duce the peak flood by redistributing the inflow hy-drographs(Harmanciogiu, 1994).
Generally, a reservoir is constructed for multi-purpose performance. Among the water storage is the major one. Except that reservoir is used to water supply for drinking, irrigation, hydropower genera-tion and flood controlling. In chapter 3.1.1 of this report it has been described on flood moderation. During peak flood, the reservoir stores the all or por-tion of water within the reservoir and release it to downstream slowly. Small and medium floods ful-ly captured by the reservoir and large floods are on-ly partially attenuated. As well as, reservoir is need-ed for flood mitigation only for few days or weeks in any particular year(Flood and Tools, 2015). Dams and reservoirs have been classified as four groups considering their role in flood mitigating as follows (European group of flood 2010).
1. Reservoirs with the single purpose of regulation (water supply, irrigation, hydropower, navigation or environmental flows), in which the incidence in flood mitigation could be small.
2. Multipurpose reservoir in which flood mitigation is an important objective, but secondary to purposes associated with water storage.
3. Multipurpose, reservoir in which flood mitiga-tion is the principle objective, combined with other secondary objectives.
4. Reservoirs with the single purpose of flood miti-gation and reduction of downstream flood impacts and damages.
5. And furthermore, flood mitigation purpose is needed only for flood season which is small time period of particular year. So, reservoir operated for other purposes during rest of the year.
3.3 Modelling parameters for HEC-RAS, HEC-HMS, ARC-GIS software
HEC RAS is an integrated system of software. It can be used to design multi-tasking and multi-network environment using this software. The modeling ap-proach of this software is simple to use and require minimum amount of input data and computer power. Every model geometry is same approximate-ly to the real geometry. therefore, value of effective roughness parameter is very important factor. So it is difficult to choose the correct roughness value for modeling(Pappenberger et al., 2005).
When it is modelling wetland in HEC-RAS com-puter software, mannings roughness co-efficient is essential factor. Flow velocity or resistance to flow is governed by this roughness co-efficient. therefore, it is very important to determine realistic value for roughness co-efficient. There are several factors which are used to determine the mannings rough-ness co-efficient. Those factors are soil type of wet-land, surface irregularity, variation of flood-plain cross section, effects of obstructions, amount of veg-etation and degree of meander. According to these factors it can be easily calculated the mannings roughness coefficients (G.J.Arcement, 1984).Hydrological soils group of the area is also important factor for modelling the runoff hydro-graph using HEC-HMS. According to sand and clay content of the area it can be chosen the hydrological soil group(NRCS, 2007) &(Soil Conservation Service United States Department of Agriculture, 1989).
1. Carry out the literature review of related research-es and other publications.
2. Obtain the available related data of the different wetland and reservoir around the Colombo area.
3. Using above data, it will be modelled the reser-voir and wetlands in HEC RAS software separately.
4. Then, run the model in different conditions by changing characteristics of the wetlands and reser-voirs.
5. Next obtain the hydrograph from the program as output separately.
Estimation of hydrological and hydraulic pa-rameters. The selected area has been divided into six are-as considering above factors. Then calculat-ed mannings roughness coefficients as follows
Then plot the hydrographs obtained from wetland and reservoirs in together in particular condition.
Region Mannings roughness coef-ficients(n)
Hydrological soil group is group C
SCS curve number = 89 (for all catchment).
HEC-HMS and HEC RAS modelling was done and results were got for 10- and 50-year storms
Using that graph, it can be checked whether reser-voirs are better solution for wetlands or not. If it is not, it can be decided, how it is improved up to bet-ter state, what factors it should be changed. Etc.
5 COMPUTER MODELLING, RESULTS AND ANALYZING
In this research, it was modelled the two 2D models using HEC-RAS, HEC-HMS and ARC-GIS comput-er software. Digital Elevation Modal (DEM) of rele-vant area was used for modelling works. Those two 2D models are existing reservoir and wetland before construct the reservoir at 10-year return period flood and 50-year return period flood events. ARC-GIS was used for find the flow paths and catchment ar-eas. Inflow hydrograph was modelled using the HEC-HMS and flow simulation for study area was done using HEC-RAS computer software.
Water surface elevation
In RAS mapper water surface elevation has taken at the point just above the existing culvert through the existing road. This point is chosen due to easiness to obtain the water elevation near existing road. Then it is easy to compare with water surface eleva-tion and existing road elevation. Then it can be found whether water go over the road and culvert or not
The modal has been simulated for two different rainfall events which are 10-year return period and 50-year return period. There are two major outputs were checked after simulation. First one was check whether existing culvert through the road was blocked or not due to flood for every modal. Second one was getting the flow hydrograph with respect to the reference line for two models for different rain-fall events.
Outflow hydrograph for wetlands and reservoir for different rainfall events as follows.
According to above graphs (Time vs WSE) for wet-lands, it can be clearly seen most of the time for both 10-year return period and 50-year return period wa-ter surface elevation of upstream of culvert is higher than that of downstream.it means at that time water flowed over the culvert and flooded. For reservoir Most of the time upstream water level is lower than that of downstream. Its means with reservoir water is not flowing over the culvert. Flood risk is reduced with reservoir.
According to outflow hydrograph peak flow of the flood has been reduced significantly for whole time period and for both 10-year return period rainfall and 50-year return period rainfall. As well as, out flow hydrograph of reservoir for two rainfall events is almost same. It means reservoir is controlled the flood of the downstream significantly.
Peak flow for 10-year return period(m3/s) Peak flow for 50-year return period(m3/s)
Total Inflow 8.8 11.5
Outflow (Wet-land) 6.1 7.8
Outflow (Res-ervoir) 0 2.5
Wetland is also provided some resistance to flood and reservoir gives more resistance to the flood than wetland. Therefore, it appears that the best option than a wetland. Therefore, at the study site, the new development (construction of a reservoir) has actu-ally increased the flood retention and help to miti-gate the flood at downstream reach compared to the wetland.
Finally, it can be clearly seen this reservoir has re-duced the flood risk of downstream significantly.
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Watson, K. B. et al. (2016) Quantifying flood mitigation services: The economic value of Otter Creek wetlands and floodplains to Middlebury, VT, Ecological Economics. Elsevier B.V., 130, pp. 1624. doi: 10.1016/j.ecolecon.2016.05.015.