The phrase "littoral environment" is often taken to mean "low threat environment" when in fact it is the highest threat environment a warship could find itself in.
This is what littoral combat looks like:
The Americans seem to be particularly confused in this regard.
The United States Marine Corps bases it's entire amphibious doctrine around Over The Horizon (O.T.H.) assault in order to keep it's troop packed ships beyond the reach of littoral threats.
Simultaneously and quite perversely the United States Navy is also building dedicated Littoral Combat Ships (L.C.S.).
The latter have weapons and sensors suited only to low threat environments and hulls and machinery suited to nothing more naval than waterskiing,yet they cost more than a full capability frigate.
In "blue water",beyond the weapon/sensor horizon of the coast (typically about 25 miles offshore),ships face threats from submarines,warships,aircraft and mines.
In "green water",within the weapon/sensor horizon of the coast,ships face these same threats but with the addition of land based threats from hostile ground forces.
This "green water" environment poses a number of challenges.
Firstly,the submarine threat is increased both because it is more difficult to detect submarines in the sonar conditions close to shore and because the warships will be exposed to "submarine ambush".
The limited submerged speed and endurance of diesel electric submarines precludes chasing surface ships which can simply run away from them in open ocean.
However,if the warship must enter the enemy littoral,the submarine may lie in wait or it.
Secondly,threats from surface ships increase because enemy vessels can use the coast for concealment and protection and receive support from shore based weapons and sensors.
The doctrine of the Baltic navies and others is based around such tactics.
Thirdly,the air threat is increased,the proximity of land limits the range of the ships sensors and thus reduces engagement times.
Also,air power is inversely proportional to the range at which it is applied.
As the warship approaches the hostile coast it also approaches enemy air fields and hence the combat power of the enemy air force increases in the littoral.
Fourthly,the mine threat increases in the littoral as many types of mine are more effective in shallow water,particularly pressure mines,and also because the more confined waters make approach routes more predictable and hence mineable.
All of the above threats demand much the same kind of warship as might be needed in "blue water" but the threat of attack by land based "army" type weapon systems is unique to the littoral.
These threats can be subdivided into "green water" threats and "brown water" threats,which are a subset of "green water" threats.
On rivers and close to the coast ships will face the most common land based weapon systems ranging from small arms fire up to tank and mortar fire.
Usually only small boats and landing craft would be exposed to these "brown water" threats which rarely extend beyond 5 miles from the coast.
During the Falklands War a more substantial warship,the corvette Guerrico was exposed to "brown water" threats and did not come off well,see here:
Ironically,she is just the sort of vessel many regard as a "littoral combatant".
Beyond "brown water" and out to the edge of "green water" the land threat consists of heavy and field artillery.
These are major threats to even large ships and although surface combatants may conduct counter battery fires,it is not easy for them to detect and engage concealed artillery observers or to hit mobile artillery systems concealed by the terrain.
Surface combatants are similarly at high risk in this environment.
Consequently the most useful ships in this littoral environment are the air attack ship and the air assault ship,both of which can dilute the threat in the littoral without exposing themselves to "green water" threats.
Combat aircraft and assault troops with surface to surface and surface to air artillery reduce the threat level in the littoral,allowing surface combatants to come in to "green water" extending the range of naval gunfire further inland until the environment is secure enough for landing ships to put army heavy units ashore.
The design requirements for a littoral surface combatant then are much the same as those for a blue water warship only yet more demanding.
The full range of antiship,anti submarine and antiaircraft capability is needed with the addition of a land attack capability.
All of these capabilities must be first rate for a ship to survive in the littoral environment.
A larger hull is particularly helpful with regard to survivability,it also permits a taller radar mast which allows targets to be detected at longer ranges.
There are very few areas of sea where a warship of less than 10,000 tonnes cannot get within weapon/sensor range of the coast.
There are areas where such a ship cannot physically get near to the coast but there is little need for it to do so if it is equipped with adequate large boats.
Small rigid hulled inflatable boats cannot independently stand up to threats such as this:
Large boats can operate in higher threat environments and have greater range,endurance,seakeeping and payload capabilities.
Such boats can operate with a high degree of independence while performing tasks such as boarding,landing,piqueting,mine hunting,mine laying,surveying,patrolling and even sub-chasing.
Big hulled frigates can carry,support and protect large boats.
The constitution of a littoral navy then is similar to a blue water navy but with a need for more capable and survivable surface combatants,aircraft carriers which perform land attack missions in addition to air defence and antisubmarine roles and a requirement for amphibious air assault ships.
A littoral navy can also perform the tasks of a "blue water" navy.
Such a fleet is not very different to what the Royal Navy is at present trying to become.