Jiahe River Country Park Respone to Urban Flood Risk

• Project Statement

Opened to the public in 2014, Jiahe River Country Park focuses on building capacity for urban flood resilience as well as promoting development of its neighboring urban fringe areas. The project aims to seek design procedures to integrate urban flood management, relieve pressure on Jiahe River as a floodway, improve ecological environment and construct green space of low maintenance. With the ultimate goal of building resilience and the main strategies of "retention, integration and purification", the project conducts in-depth research and analysis on water resources management, urban flood risk mitigation, environment enhancement and ecosystem remediation. The challenge is to balance the high efficiency demand in response to urban flood risk with the cost-saving demand of a country park while providing a public green space in this urban fringe area. In the end, an innovative, integrated and practical design procedure for such projects came into being. Also, Jiahe River Country Park has been playing an crucial role in promoting the development of its neighboring urban fringe areas.

• Project Narrative

Project background

Located in Yantai City, Shandong Province, China, the site is adjacent to Outer Jiahe River. Lies in the northern part of Yantai, Outer Jiahe River act as a main floodway protecting the city.

Sits in the southern urban fringe areas of the city, Jiahe River Country Park is only a mile away from Laishan Airport. Bordering Jiahe River on the south east, the site is defined by 2 newly-built roads on the north and west. On its west side, a diversion channel of Jiahe River intersects Laishan Airport Freeway. Before, it was a wasted land with limited plantations, patches of agricultural land, shabby greenhouses, and a few scattered shacks. After, covering 48 hectares, the total construction cost of Jiahe River Country Park is merely ￥69,959,500(＄11132779.5).

Site analysis

Water resources

Despite being a coastal city, Yantai is facing severe water scarcity challenges. While its average rainfall over the years is 651.9mm, its evaporation reaches 900-1200mm annually. With its drought index at 1.5-2.4, the city falls into semi-humid climate category accordingly. As reported in July, 2017, renewable water availability in Yantai is only 435m3/cap/a, which is less than 1/5 of the national average. According to Falkenmark Water Stress Indicator, the city is considered ‘absolute water scarcity’.

Urban flood

Adjacent to Bohai Bay and the Outer Jiahe River, along with its unevenly distributed rainfall, Yantai City is very sensitive about urban floods season as well as Jiahe River in its flood.

Take year 2012 as an example. In July alone, the total precipitation in Yantai City reaches 441.9 millimeters. Over 77 districts (streets), 218,500 people and 20472.6 hectares of crops were affected. With a total 6,030 hectares area affected, 1056 houses collapsed, the direct economic toll reached ￥128.27(＄20.40) million.

Environment

Jiahe River in Yantai is facing severe regional water pollution challenges. Water at lower reaches of Jiahe River, where the river runs into the Bohai Sea, were assigned for industrial and agricultural uses. Used water was discharged into Jiahe River almost untreated. Due to domestic sewage and nitrogen loss from agricultural irrigation water in the catchment area, total nitrogen in all 6 reservoirs in Yantai exceeded the national standard by up to 13.6 times.

Ecology

Aquatic ecosystem in Yantai has been disrupted by the massive polder action alongside Jiahe River in the 1970s-1980s. Along with the reduction of wetland, its ecological functions fail. Number of aquatic animals and plants plunged, so does biodiversity.

Strategies

Urban flood treatment with high efficiency

Runoff management

The main treatment strategies of “retention, integration and purification” corresponds to the three interfaces of "city- diversion channel-Jiahe River". Jiahe River Country Park can both act as the urban runoff prevention line for Jiahe River and the resilient buffer against it.

Runoff generated in the catchment area of over 300 hectares in the urban area is introduced into the park by landform rectification, diversion and other approaches. It is then retained in the park through open field retention, waterscapes recharging, deep infiltration and other ways. The retention process reduces the urban runoff discharge pressure.

The west and south division channels are connected through inner lake system in Jiahe River Country Park. The sluices connecting 2 division channels and Jiahe River will be disconnected if the channels are considered to have the possibility of flooding. With its runoff management ability, Jiahe River Country Park will be able to take in some of the flood, and thus reduce the overall emissions and postpone peak hours.

When the water level of Jiahe River exceeds the flood control warning level, the flood discharge gate will open. The park can be act as a temporary retention pool to store the flood of the river to ensure the safety of the city.

Water purification

To mitigate the retained water pollution, approaches involving natural materials such as plant uptake, soil filtration are combined with engineering approaches such as gabions and biological retention pool.

To treat the urban runoff, multiple layers of vegetation were added to the existing woodlands. By adding biological and engineering approaches on its way to retention tools, many suspend solids and chemicals are removed.

To treat the infiltration water, soil structure is adjusted for higher porosity. By thickening planting soil, infiltration duration is increased. By setting up deep-water temperature convection wells, infiltration deepened. Thus, more pollutants were removed by the self-purification function of soil and water bodies before they reach aquifer.

Country park with low maintenance

Habitat creation

A variety of habitats were created in Jiahe River Country Park to host various plants and animals. Based on resilient ecological design strategies, large numbers of low-maintenance native tree species were introduced, the proportion of ecological revetment increased, attempting to make  created habitats resemble its wild state. With their high stress resistance and rapid growth rates, these plants enrich the stratification of landscapes, increase regional biodiversity and thus creates a more complete and stable eco-pyramid. Therefore, we may maximize the performance of natural ecological landscape, enhance the quality of regional ecological environment as well as the value of regional ecosystem services.

Recreation system construction specialized for country parks

Recreation systems in country parks engage people with a relatively wild nature habitat.

To improve its natural environment, woods dense and sparse, lawns decorated with wildflowers, aquatic plants, flower borders, are added to the plain woodlands. Together, they present a multi-layered aesthetic landscape.

A variety of specialized gardens with different themes can provide for on-site workshops and educational purpose. In addition, elements such as levees and municipal roads were transformed into a number of event sites. A complete recreation system in Jiahe River Country Park consists 10.18KM of fitness pedestrian walk, 2640m2 of wetland with plank walk, 5.64 hectares of viewing area with wild flower borders and more to provide invaluable public green space in the urban fringe area.

Landscape Performance

Ever since it’s great opening to the public, Jiahe River Country Park played an active and crucial role in supporting the regional development of this marginal area of Yantai:

Water resources

To manage water resources, the park retains 98,400 tons of runoff, reduces consumption of 98500 tons of potable water on average annually. What’s more, nearly 4% of the urban runoff reaches aquifer through deep infiltration. The collected urban runoff does not only recharge waterscape in the park but also irrigate plants on its journey. All these attempts to relieve the city of its water scarcity pressure help reduce the maintenance cost of the park by over ＄27,700 annually.

Environment

To enhance its surrounding environment, 92% of Jiahe River was restored with its meandering nature. Facilities such as wetlands, biological retention ponds and wet ponds were set up to purify urban runoff and imported river water. Taking rainfall of two-year return period as an example, the 92% of TSS, 91% of TP, 94% of TN is removed from urban runoff. The treatment process works effectively in urban runoff purification and aquifer protection.

Urban flood

In response to urban flood risk, a complete storm management system is built. Taking rainfall of two-year return period as an example, the system can retain more than 10,000m3, reduce runoff peak flow by 82% and lag its peak time for more than 84 minutes in 1 hour. When it comes to extreme rainstorms, the total amount of runoff retention can be over 60,000m3, which greatly reduces the pressure on Jiahe River as a floodway.

Ecology

The completion of Jiahe River Country Park helps improve the regional ecological environment. The annual nitrogen removal by plantations in the park is 116.9kg, the amount of carbon dioxide absorbed is 186.8 tons, while the amount of oxygen released is 130.2 tons. The park removes 256.3kg of PM10 annually as well. All was crucial when it comes to promoting the construction of regional ecological structure.