Dams, dikes, and levees are engineered structures with an impervious core (which, in most cases, makes the difference with vernacular dams) that support flood control and the protection of built and agricultural areas. They are usually located along or across rivers, deltas, or seashores (see also Measure [06]). Dams: Dams represent engineered, mostly large barriers for water control, storage, and supply during drought. Dams can also impound other liquids, such as wastewater. They often come with complex control systems (e.g., spillways or control gates). This compendium does not refer to dams for hydroelectricity production. Dikes and levees: Dikes and levees (also: embankments) aim to act as a barrier for diverting, redirecting, or confining flood waters. In contrast to dams, dikes, and levees usually do not have complex water control mechanisms. Some non-engineered dikes and levees do not have an impervious core (see Measure [02]). The impervious core, which is built deeper than the dike base, aims to avoid water infiltration through the soil. The crest and the inner wall must be designed (at least in some strategic places) to withstand the submersion, avoiding a total collapse of the dike during a flood. Moreover, geotextile containers and tubes come increasingly into use as a hybrid form of embankment or to support the structure of dams, dikes, or levees (see Measure [03]). Note that the goal of the dike contradicts the drainage necessities (see Measure [07]) and therefore must be carefully designed. Check Dams: Check dams represent a simpler type of dams and describe barriers across channels or rivers. They aim to reduce erosion and sediment accumulation and fix the stream axis during a flood event. However, in contrast to other dams and dikes, check dams continuously operate and do not only come into effect during flood events. Wooden structures and gabion retention walls can also be used as check dams (see Measure [05]).

Next to engineered floodwalls (see Measure [01]), there are simpler dams, dikes, and levees made from local materials and without an impervious core. These can include piles of soil, earth, sand, wood, vegetation, stones, or rocks. Vernacular dams are a specific type of such nature-based dams. They describe structures created from locally available materials and use context-specific traditional knowledge and construction techniques. Dikes and levees can also occur entirely based on geological processes. For example, naturally occurring dikes describe a body of rock blocking water flow, often originating from volcanic action. Natural levees form due to accumulated sediments (sand, gravels, silts, clay) after repeated flooding. Combining vernacular and natural dams with engineered structures (including an impervious core) can be particularly efficient regarding the environmental impact, risk protection, durability, and affordability of a dam, dike, or levee.

Geotextile tubes and containers can function as a special form of dike or support other structures like seawalls, dunes, and breakwaters. The flexible containers are filled with solids from the local site. The local filling materials usually comprise of a sand and water slurry that is then filled into the container by a pump, dredger, or funnel. Once the geotextiles are filled with the slurry, the water dissipates through the flexible and synthetic fabrics. The sand then prevails as the tube’s main composition.

Protecting riverbanks and coasts aims to decrease the overall water velocity and reduce (soil) erosion. It stabilizes the slopes by covering unconstrained and angular rocks or stones along channels, rivers, or waterbodies, often called “riprap”. Riprap can also be installed at slopes exposed to weathering and where it is not possible to plant vegetation. They can be built with natural materials (e.g., stones) or artificial (e.g., concrete blocks) and can be either graded or uniform. The first includes stones of mixed size, while uniform riprap uses only one stone size. Graded riprap is usually preferred to uniform stones because it is easier and less costly to install. Bank protections can also be achieved with other construction methods, such as gabion walls (see Measure 05).

While bank protection (riprap) reduces erosion along the embankments, sometimes retention walls are necessary to avoid landslides from the terrain above the water body. These can be built from different materials, including wooden or metallic planks and gabion walls. Gabion walls describe galvanized mesh boxes filled with rocks that are stacked in the form of closed cages. The purpose of the permeable gabion walls is to stabilize soils. Besides embankment stabilization, they also protect from flooding and reduce the water flow.

Seawalls are large and engineered installations to protect the coast and shoreline from the impact of the sea. Most commonly, they are constructed as vertical armors along the coast. However, seawalls can also be built perpendicularly from the shore (in some contexts called “groins”) to manage the sediment budget of beaches. In addition, they can be freestanding in the sea at a distance from the shore (also called “breakwaters”). The structures can be combined with tetrapods and geotextile containers, among others. Due to their reverse risks, scale, and costs, seawalls should not be prioritized in the context of refugee settlements.

Drainage systems require a global planning at the different scales of a settlement, and a comprehensive understanding of the surrounding areas and (existing) drainage networks (see introduction “Surface Water Management”). Drainage networks can take many forms and include several techniques and materials such as : 1) Natural drains (primary canals) are based on restorations of riparian vegetationand planting of trees and other shrubs. This nature-based measure comprises natural canals with low gradients where the water runs slowly. 2) Low-tech drainage systems can include interventions like bamboo drains (primary, secondary, tertiary drainage) and geotube/geobag embankment drains (primary, secondary, tertiary canals). 3) Medium and high-tech drainage systems describe measures such as masonry and precast concrete drains (secondary, tertiary canals). When using concrete ditches, re-infiltration through holes in the drainage bottom should be used wherever it is possible. 4) Other interventions include ridgeline and cascade drains, silt and waste traps, and microsoak pits.

Bioswales: Bioswales (also Vegetated Swales) describe low-lying, vegetated, or mulched channels with gentle slopes. As a nature-based alternative to engineered gutters or sewers, they can treat, reduce, decelerate, and absorb stormwater runoff. The intervention is particularly efficient in the event of less heavy but frequent precipitation. In larger stormwater events, bioswales still play a significant role in the overall runoff reduction and the removal of pollutants. However, a bioswale acts more like a corridor for the rainwater, leading it to another point (e.g., into a rain garden or infiltration basins). That is why bioswales are often used in combination with rain gardens and infiltration basins. Rain gardens and infiltration basins: Rain gardens and infiltration basins mitigate the runoff during (heavy) rainfall by infiltrating the water flow. While both interventions have the same function and are characterized by highly permeable soils, rain gardens are smaller than infiltration basins. Rain gardens are primarily implemented at the plot, community, and block scales. The water is collected from the roofs close by or the water channeled through a bioswale. The infiltration basins tend to be of greater extent and mitigate direct stormwater runoff. As a result, rain gardens and infiltration basins serve as simple and sustainable measures to prevent the nearby shelter, public spaces, and pathways from being flooded. At the same time, they support groundwater recharge.

Rainwater harvesting (RWH): Rainwater harvesting (RWH) describes the process of collecting and storing rainwater. The practice enables the storage of stormwater runoff from rooftops, courtyards, greenhouses, reservoirs, retention ponds or other built infrastructure. This is also the first element of most drainage systems. Water can be directly channeled to drains in the ground or harvested for further use. There are several possibilities to harvest rainwater such as by using water tanks, rain barrels, and cisterns. Cisterns can be as simple as large containers located on rooftops for rainwater storage. Commonly, the harvested water comes into use for irrigation, firefighting, toilets, sinks, showers, or laundry making. RWH is particularly useful in the context of humanitarian settlements, in rural areas, bothin (semi-) arid or tropical climatic conditions. Retention basins: Retention basins (also: wet ponds) are a special type of rainwater harvesting and/or infiltration basins (see Measure [08]). They show a permanent water level which, during heavy rain events, can store further amounts of stormwater runoff while improving the water quality based on natural processes. Mostly, the collected water in wet ponds is used for irrigation or watering livestock.

Using permeable material for the surface of roads, pathways or other open spaces diminishes the overflow by increasing the overall pervious surface, allowing water to infiltrate the ground to limit runoff. Multiple materials can be used such as compacted gravel and sand for roads, or pavement systems with holes or large, permeable joints between the pieces. Pavements can be produced with different materials and promote the use of locally available materials whenever possible. Combined with geotextiles, the PPS increase their porous capacity and cut the charges for operating sewer systems. Geotextiles remove pollutants from the water before it enters the ground, making it an eco-friendly solution for the downstream surroundings. This leads to the co-benefit of groundwater recharge. Overall, PPS is most effective in the context of short but heavy rainfall.

A common and comparatively simple practice against flooding is to elevate the foundation of assets (e.g., buildings, latrines, entire plots, cattle sheds). Originating from indigenous knowledge, raising the ground of structures takes place through piles, landfills, or stilts. When using stilts to elevate buildings, the structural soundness is primordial, especially in areas that are prone to heavy winds, landslides or earthquakes. Proper calculation of the structural design is necessary before building on stilts. A special type of elevated architecture are amphibious constructions (see Measure [12]).

Amphibious constructions are a valuable alternative to elevated buildings (see Measure [11]) which tend to be at risk of damage due to strong wind. Since temporary elevation to stay above water during flood events is the key concept of amphibious housing, theses constructions are less vulnerable to wind damage. In the case of flooding, the foundation of amphibious houses rests on the ground. The remaining parts of the house have a structural frame that floats depending on the intensity and depths of the flood event.

The consolidation of structures on the shelter/block level is directed at a single building or housing cluster. Where the elevation of shelters is not possible, consolidating interventions can mitigate the impact of floods on the built infrastructures. Such small-scale interventions include dry and wet floodproofing as well as permanent flood walls and levees. Dry and wet floodproofing: Dry floodproofing focuses on protecting the walls and openings of a building against the impact of incoming water. The goal is that the water cannot enter the structure. In the context of dry floodproofing, the use of sacrificial layers is essential. They are designed to be ‘sacrificed’ or replaced after flood events. The goal is to protect and mitigate the damage to the more critical parts of the shelter while reducing repair costs. Sacrificial layers include interventions such as outer wall layers, door-protection gates, flood-resistant coatings, watertight doors, walls, or temporary flood barriers such as sandbags (See also measure [14]). The process of wet floodproofing means allowing floodwater to enter the built infrastructure without the risk of damage. Such interventions include using flood-resistant materials, protected utilities, or openings in the structure. Permanent floodwalls and levees: Small, permanent floodwalls (made from concrete or steel) and levees (made from earth) are placed along the riverbanks to protect the adjacent built infrastructure from flooding. Levees can also be erected around the shelter or block where the structures are most prone to flooding or ponds. Watertight materials for these barriers include clay, mud, concrete, masonry, or steel.

Temporary flood barriers describe pre-installed and removable flood protection systems placed at building entries, yards, pathways, or roads, among other locations. They come into use when immediate responses are needed and/or permanent flood protection measures do not suit the context-specific technical, economic, or environmental resources. The temporary barriers or floodwalls can come in the form of panels, containers, or tubes filled with earth and sand, among other fillings.

Green roofs are vegetated installations on top of a building or another built structure. Depending on the building type, size, and strength of the building structure, the installations can range from simple, low-cost green roofs to high-cost and complex roof gardens. Similarly, green walls are vertical, vegetated installations along any kind of wall. They are particularly favored in areas where there is limited space for planting on the ground. Green roofs and walls can support flood mitigation by slowing down the waterflow on the roof and, for example, by avoiding the gutters to spill over. They are best combined with other measures such as rainwater harvesting (see Measure [09]).

Wetlands show moist or saturated surface conditions throughout the year or during parts of it. Mostly linked to groundwater-, stream- or coastal systems, wetlands infiltrate, clean, store, and slowly release water. Wetland types range from upland rain-fed wetlands and wet grasslands to peatlands. They also include coastal and river-fed floodplains. Restoring wetlands is, therefore, closely linked to floodplain restoration (see Measure [19]). Due to their capability to store and manage water, wetlands are also measures for surface water management (see Category II).

Trees reduce the volume of storm-water surface runoff in three steps: First, the leaves, branches, and trunks catch and intercept the raindrops. The water then trickles off into the ground or evaporates into the air. The rate of the interception, infiltration, surface runoff reduction and, therefore, the flood response increase with the forested area in relation to the catchment size. In this regard, the measure is also linked to surface water management (see Category II). Mangrove forests are a specific type of tree for flood risk mitigation. They are multi-functional ecosystems that mainly occur along sheltered tropical and subtropical coasts. As for flood risk mitigation, mangroves reduce the wind height, incoming waves, and the level of storm surges. In addition, they protect the coastline and control its erosion. Figure below: Example of a Miyawaki forest built by the SUGi team in Buea, Cameroon, 2.5 years after the tree planting. SUGi 2022.

Dunes are natural flood barriers that protect the inland from the brunt of coastal floods and storm surges. The sandy ridges develop in parallel to the shoreline. Dunes change their size and shape due to tides, winds, storms, or heavy sea. In case of flooding, the health of the vegetation can decide upon the effectiveness of the dune’s mitigation capacity. The restoration of dunes includes the recovery of eroded areas and dune stabilization based on vegetation (e.g. dune-forming perennial grasses) and fences. In general, the interventions that join the restorations of dunes should not disrupt the natural forming processes and the dune ecosystems. A careful assessment of the site before the implementation of measures is highly recommended.

A floodplain is a low-lying, flat area of nutrient-rich sediment along the river. A river and floodplain together describe an integrated system. The floodplain enables the water body to transport floodwaters. As a result, upstream (and downstream) retention and expansion areas benefit flood risk mitigation and reduce water logging. However, floodplains face continuous degradation due to permanent flood barriers (see Category I), urban and agricultural development, or river channelization. These changes have significantly affected the efficiency of floodplains as aquatic and terrestrial habitats, water qualifiers and natural providers of flood protection. The rehabilitation or conservation of effective floodplains is, therefore, an essential intervention for flood risk mitigation in humanitarian settlements. Floodplain restoration is closely linked to wetland restoration (see Measure [16]).

Relocation: One of the four main strategy types for flood risk mitigation (see 1.2) is the partial or full relocation of the refugee settlement. If the flood risk or (expected) damages turn unmanageable, a settlement or parts thereof can be shifted to another location. This strategy implies a long process of securing adequate land, equipping it, and organizing the move of population. When planning new zones or settlements, refer to the principles and guidance described in UNHCR’s Masterplan approach. While planning the relocation, a possible intervention for limiting flood risk in the new location is to add buffer zones along the areas at flood risk. Buffer Zones: Buffer zones designate protective areas between particularly flood-prone locations and the refugee settlement. These buffers should be based on comprehensive flood risk assessments, the local topography, and climatic conditions. Usually, the designated areas prohibit any form of (residential) buildings. They foster the absorption or diversion of floodwaters and can include measures like floodplains, wetlands, flood resilient agriculture or tree planting (see Measures [16-19]); bioswales and infiltration basins, or permeable pavements (see Measures [08,10]); as well as engineered barriers such as dikes and levees (see Measure [01]). Informing the population about the risk is crucial to avoid the planned buffer zones being used for construction at a later stage. Awareness-raising campaigns (see Measures [21] and [22]) should be repeated regularly so that newcomers are informed as well. Figure below: Buffer Zones in Cox’s Bazar. Nadia Carlevaro, UNHCR n.d.

The preparedness phase combines the prevention of, the preparation for, and the individual precaution against hazards. Capacity building and raising risk awareness among the community strengthens the humanitarian settlements in their preparedness and responses to hazardous events. Preparedness measures include early warning systems, the planning of escape routes and refuge, the individual preparation of emergency bags, and the proper storage of emergency supplies and tools such as pumps or latrine pit emptying equipment. The stocks need to be controlled regularly to ensure materials are sufficient and functional. Awareness raising campaigns need to convey simple key messages and be repeated regularly to ensure best comprehension of all members of the community. Community trainings on reacting to early warning systems, using escape routes and safe refuges are primordial to ensure the effectiveness of the preparedness measures. Inclusive and long-term capacity building can derive from educational efforts based on the combination of local, indigenous, and scientific knowledge systems. For this purpose, a dedicated disaster management cell or emergency operation center could be institutionalized within the humanitarian settlement, where selected people are trained on monitoring stocks, warning systems and other preparedness mechanisms. Local communities and humanitarian organizations can also prepare for disasters via land use plans, hazard maps, and (GIS-based) risk assessments (see Measure [22] and GIS Add-In).

In the context of flood risk management, participation promotes the interaction among the stakeholders that are responsible for and affected by the implementation of the mitigation measures. Stakeholder engagement allows the (public, private, and local) stakeholders to come together for a dialogue on the interventions before and after their implementation. Mapping the risk is one of the first steps to know what strategies would be best adapted to respond to a flood event. Hazard maps can be prepared using global and local data to draw a model of the probable extent of potential floods. Risk assessments will highlight the assets in need of protection and help prioritize mitigation actions. The process can involve participatory mapping. After identifying the essential stakeholders, the project initiator should actively listen and document the diverse perspectives. Then, the stakeholders’ ideas and wishes should become part of the overarching goal and a common agenda for flood risk mitigation (and its monitoring) in the refugee camp. The present project on risk mitigation strategies also includes guidelines for participatory mapping including semi-guided interview templates and a proposal for organization of mapping workshops.